GPRS Description

Mobile Communication System
GSM900

GSM -R

GSM1800

Phase 1

Phase 2

Phase 2+

GSM1900/24

GSM1900/30

BR5.5
SF10420 – V2.1
Base Station Subsystem
General Packet Radio Service (GPRS)
Long Descrip tion

BTS

BSC

LMT

TRAU

OMC-B

NMC

Tool Set

In General
Today a number of wireless data services are available, but none are as innovative as the data
service for GSM networks called General Packet Radio Service (GPRS).
GPRS refers to a high -speed packet data technology. The most important aspects of GPRS are:
•
•

Data transmission speeds of up to over 100 Kbps
Packet-based technology

•

Support of the world's leading Internet communications protocols, Internet Protocol (IP)
and X.25

•
•

Always online
Volume-based billing

Example
• Packet data technology provides a seamless and immediate connection from a mobile PC
to the Internet or corporate Intranet allowing all existing Internet applications such as Email and Web browsing to operate smoothly without even nee ding to dial into an Internet
service provider via a fixed line (see Figure 1).
The advantage of a packet-based approach is that GPRS only uses the medium, in this case the
radio link, for the duration of time that data is being sent or received. This means that multiple us ers can share the same radio channel very efficiently. Since many applications have idle periods
during a session, with the packet data technology users will only pay for the amount of data they
actually transfer, and not the idle time. In fact, with GPRS, users could be "virtually" connected for
hours at a time and only incur modest connection charges.

Figure 1: From a mobile PC to the Internet v ia GPRS
While packet-based communication works well with all types of communications applications, it is
especially well-suited for the frequent transmission of small amounts of data ("bursty" data trans BR5.5 - SF10420 – V2.1
1/2
Siemens AG 2004
Technical modifications possible.
Issued by the Information and Communication Networks Technical specifications and features are binding only insofar as they are
Hofmannstraße 51, D- 81359 München
specifically and expressly agreed upon in a written contract.

fer), such as E-mail. But packet transmission is equally well-suited for large batch operations, and
other applications involving large file transfers.

Customer Benefits
GPRS is a service for both bursty and bulky data transfer. The main benefits for the operator are
as listed below:
•

new business opportunities on the market with tremendous growth potential

•

better cost to bandwidth ratio

•
•

effective use of radio resources
direct access to packet data networks, e.g. the Internet (with faster access)

•

optimized transfer media for frequent point -to-point transmission of small data volumes

•

reliability

•
•

scalability
integrated services, Operation and Management

•
•

future-proof technology, GPRS principles as the basis of future networks
higher data transfer rates,

• cost reduction due to volume -dependent charging,
• new applications enabling real plug and play.
With the new GPRS, the customer is able to go an evolutionary step towards UMTS, the third
generation of mobile communications. Siemens GPRS hardware is suitable for the new UMTS
networks.
Improvement of Quality of Service
The following features are designed to improve the QoS:
Delay Class
Quality profiles varying over a wide range have been defined for data transmission (GSM
Rec.02.60). For the QoS, the Delay Class is an important attribute. In BR5.5, the “Best Effort”
class is supported.
Operation and Maintenance
The O&M functionalities, as offered for the SBS, are supplemented with the management of the
BSS part of the GPRS network. The management principles for switch-oriented GSM services
are equally valid for packet-oriented GPRS services.
The O&M functions for GPRS are equally available from an LMT and from the OMC -B. The individual network elements of the BSS are accessed from the Radio Commander for management
purposes via the existing O interface and the BSCs.
With the introduction of GPRS, the network provider is able to:
•
•

verify the correct dimensioning of the GPRS network (for short- and long-term planning)
check the parameters of the GPRS network planning

• provide fine-tuning of the GPRS network configuration parameters
• determine the QoS and performance of the GPRS network
To this end, new performance measurements have been introduced. This measurement set supplies information on the following functionalities and may be used to confirm and optimize the
GPRS confi guration parameters in the network:
• GPRS radio access
•
•

radio resource usage
radio resource reassignment

•
•

dynamic allocation and de -allocation of GPRS radio resources
packet queuing

BR5.5 - SF10420 – V2.1

2/33

•

used coding schemes

Performance measurements and counters
GPRS's flexibility concerning transfer rates and radio assignment influenced and improved performance measurements. The PPCU (new board for the Gb interface between the BSC and the
SGSN) is responsible for all GPRS traffic -related tasks in the BSC, e.g. access control, radio
channel management or PDCH scheduling, and is also responsible for carrying out the GPRS
performance measurements. The following measurements are available for GPRS:
• Dynamic allocation / de-allocation of GPRS radio resources
There is no fixed number of radio resources allocated to GPRS traffic. Furthermore, the
distribution of packet -switched and circuit-switched traffic is made dynamically. The number of PDCHs can increase to an operator-defined maximum of 8 and decrease to an operator-defined minimum (0 is also possible as minimum). Additional PDCHs can be allocated if the number of mobile stations on a single PDCH exceeds a given operator defined threshold (two thresholds, one for uplink and one for downlink), or if the number of
PDCHs allocated within the cell is not sufficient to fulfill the MS capability in terms of the
number of channels that can be handled in uplink and downlink. A PDCH is de-allocated if
CS needs resources in order to serve new calls or if the PDCH is not used for an operator
defined timer.
To provide the operator with information on how to set these thresholds appropriately, the
following measurements are available:
NAVPDTCH - ‘Min/max/mean number of available (configured) PDTCHs per cell’ (3
counters, counter types: real)
AALPDTT - ‘All available PDTCH fully allocated time per cell' (1 counter, counter type: duration, unit: seconds)
•

Initiation of a GPRS connection by a mobile station
To supervise the establishment of a GPRS connection, the following measurements are
available:

•

TASAGPRS - ‘Number of attempted GPRS accesses per cell’ (1 counter, counter type: integer)
TASUGPRS - ‘Number of successful GPRS accesses per cell’ (1 counter, counter type:
integer)
Resource reassignment
The BSS can change the radio resources of an existing GPRS connection. This change
can be an addition of uplink resources while downlink is running (concurrent TBF) or vice
versa. Handling concurrent TBF can result in an addition of PDCHs, ending a concurrent
TBF can result in the removal of PDCHs. The resource reassignment is carried out by a
message on PACCH. The measurements related to these reassignment procedures are:
NATPRRE - ‘Number of attempted packet resource reassignment procedures per cell’ (1
counter, counter type: integer)
NSUPRRE - ‘Number of successful GPRS accesses per cell’ (1 counter, counter type: integer)

•

•

Packet retransmission
In case a GPRS packet (i.e. Packet Data Unit PDU) is not received correctly (uplink by the
BSS or downlink by the mobile station), the sender is notified by a message. The sender
is then able to retransmit the failed PDUs. (There is no similar mechanism in circuitswitched connections.) The measurement for counting these retransmissions is:
NRETPDU - ‘Number of retransmitted PDUs (uplink / downlink)‘ (2 counters, count er type:
integer)
Paging
In GPRS, paging is possible in three different ways. It can be performed by using subchannels of CCCH, PCCCH or PACCH. The measurements related to paging are:
NATGPPAG - ‘Number of attempted GPRS paging procedures’ (1 counter, counter type:
integer)

BR5.5 - SF10420 – V2.1

3/33

•

NSUGPPAG - ‘Number of successful GPRS paging procedures’ (1 counter, counter type:
integer)
Throughput measurements
Throughput measurements provide the operator with the appropriate information transmitted through an interface, in Kbytes/s. The result of these measurements can easily be
taken to check whether the system is close to its upper level of capacity. The following
throughput measurements are available:
MUTHRF - ‘Mean user data throughput (uplink / downlink) per cell on the RF interface’ (2
counters, counter type: mean throughput)
MSTHRF - ‘Mean signaling data throughput (uplink / downlink) per cell on the RF interface’ (2 counters, counter type: mean throughput)
MUTHBS - ‘Mean user data throughput (uplink / downlink) per cell on the BSSGP interface’ (2 counters, counter type: mean throughput)
MSTHBS - ‘Mean signaling data throughput (uplink / downlink) per cell on the BSSGP interface’ (2 counters, counter type: mean throughput)

•

Allocated PDTCH
This measurement gives the mean numbe r of PDTCHs for a GPRS connection per cell.
(There is no analogous measurement for TCHs.)
MEALPDCO - ‘Mean number of allocated PDTCHs per GPRS connection per cell’ (1
counter, counter type: calculated mean)

•

BSC processor load
The existing BSCPRCLD measurement will be extended in order to contain not only the
processor load on the MPCC and TDPC boards but also on the PPCU boards. This
means that this measurement now contains two more counters for each board (4 new in
total), one for the prime time and one for the total time. Just like in the case of the MPCC
and the TDPC, the values are provided by the operating system.
Just like in the case of the MPCC and the TDPC, the values are provided by the operating
system.

Improvement of Resources Management
GPRS enables more efficient frequency usage on the air interface: radio resources are used on
demand only. It offers shared use of physical radio resources, thus increasing the number of subscribers per channel. By channel combining and the use of new coding sche mes, GPRS offers
higher user data rates.
Timeslot combining
GPRS enables high data rates by combining several timeslots. According to the recommendations (GSM Rec. 02.60, 03.60, 03.64) up to 8 timeslots may be combined for one user. The
PDCH distribution is dynamically managed depending on instantaneous traffic conditions and
service requests, in order to serve traffic spots and traffic peaks when and where necessary.
Up to 7 timeslots may be allocated on a BCCH carrier or as well up to 8 timeslots using dif f erent carriers than BCCH.
Channel coding
Channel coding was strongly modified for GPRS (GSM Rec.03.64). ETSI defined 4 new channel coding schemes (CS1 to CS4).
In BR5.5, the CS1 and CS2 channel coding schemes will be used. Theoretically, CS1 allows
the transmission of 9.05 Kbps and CS2 allows 13.4 Kbps. In practice, given excellent radio
conditions, CS1 enables data transmission rates of up to 8 Kbps, and CS2 up to 12 Kbps. CS1
is especially suited for safe coding of the RLC / MAC data and control blocks.
Channel coding starts with splitting the digital information into blocks to be transferred. These
so-called radio blocks, i.e. the blocks before coding, consist of:
•
•

MAC header
RLC / MAC signaling block or RLC data block

BR5.5 - SF10420 – V2.1

4/33

• Block Check Sequence (BCS)
Data loss of the radio blocks is prevented with convolutional coding. Furthermore, channel
coding comprises interleaving, i.e. that the radio blocks are interleaved to a certain number of
bursts / burst blocks.
New logical channel types
The packet-switched traffic provided by GPRS is much more suited for data transfer than the
circuit-switched traffic. Its flexibility concerning transfer rates and assignment of radio resources meets the requirements of the bursty nature of data traffic.
In GPRS, the physical radi o resource is called PDCH. Each PDCH consists of 52 TDMA
frames. A mobile station may use more than one PDCH and there is no exclusive use of a radio resource by a mobile station. Whereas in circuit-switched traffic each connection occupies
one TCH, in GPR S downlink traffic up to 16 MS may share one physical radio resource
(PDCH), and, in uplink traffic up to 8 mobile stations can be handled at a time, since one TCH
is required by the PRACH. This is implemented by the definition of different logical channels
(PDTCHs) in one PDCH.
Thus, in addition to the existing TCH and SDCCH, GPRS introduces new logical channel
types in the connection between the mobile station and the BSS (Um interface):
•

PBCCH group:
Packet Broadcast Control Channel (PBCCH) to transmit sy stem information to
all mobile stations in a cell (downlink)
PCCCH group:

•

-

Packet Random Access Channel (PRACH) to initiate packet transfer or to answer to paging messages (uplink)

-

Packet Paging Channel (PPCH) to page an MS prior to downlink packet transfer
(downlink)
Packet Access Grant Channel (PAGCH) to send resource assignment in packet
transfer establishment (downlink)

•

PTCH group:
-

Packet Data Traffic Channel (PDTCH) to transfer data (uplink / downlink)
Packet-Associated Control Channel (PACCH) to transf er signaling information
(uplink / downlink)

Additional Business
New subscriber groups can be reached, since GPRS allows completely new applications. It
opens the way for the operator to participate in the tremendous growth of Internet -based services
(Internet access, establishing Intranets).
Furthermore, GPRS provides a means for new mobile applications and services (e.g. Telematic,
E-commerce, etc.)
Example:
• For business users, GPRS enables a data connection with the office wherever they go, so
that they can have access to E-mail, the Internet, their files, faxes and other data wherever and whenever it is needed, giving them a competitive advantage and more flexible
lifestyles.
GPRS is expected to profoundly alter and improve the end -user experience of mobile data com puting, by making it possible and cost-effective to remain constantly connected, as well as to
send and receive data at much higher speeds than today.
GPRS will complement rather than replace the current data services available through today ’s
GSM digital cellular networks, such as circuit-switched data and Short Message Service (SMS).
Additional Revenue
Additional revenue may be obtained through new applications and new data subscribers.

Functionality

BR5.5 - SF10420 – V2.1

5/33

GPRS is an integral part of GSM Phase 2+. It provides a direct high -speed radio access to
Packet Switched Data Networks (PSDN). It defines four new Coding Schemes (CS1 to CS4; in
BR5.5, the BSS handles CS1 and CS2) and uses channel combining to enable higher data rates
and more network efficiency .
As a packet-oriented service, GPRS is suited for all applications requiring both bulky and bursty
data transfer. Thus, GPRS is an ideal solution for Internet applications, e.g. E-mail.
GPRS Features
The SBS design introducing GPRS, as implemented in the GPRS initial phase (BR5.5), took special care to:
•
•

keep SBS flexible and modular,
avoid resource waste,

•
•

minimize the operators' investments,
provide overnight service, and

•

ensure good performance.

In line with this concept GPRS offers the following feat ures:
New logical channel types
In GPRS, the physical radio resource is called Packet Data Channel (PDCH). A mobile station
may use more than one PDCH, and there is no exclusive use of a radio resource by a mobile
station. Up to 8 mobile stations may share one PDCH. This is implemented by the definition of
different logical channels, so-called Packet Data Traffic Channels (PDTCHs) in one PDCH.
Thus, in addition to the existing Traffic Channel (TCH) and Stand -alone Dedicated Control
Channel (SDCCH), GPRS introduces new logical channel types in the connection between
the mobile station and the BSS (Um interface):
•
•

Packet Broadcast Control Channel (PBCCH) group:
PBCCH to transmit system information to all mobile stations in a cell (downlink)
Packet Common Control Channel (PCCCH) group:
-

Packet Random Access Channel (PRACH) to initiate packet transfer or to answer to paging messages (uplink)

-

Packet Paging Channel (PPCH) to page a mobile station prior to downlink
packet transfer (downlink)

Packet Access Grant Channel (PAGCH) to send resource assignments in packet
transfer establishment (downlink)
Packet Transfer Channel (PTCH) group:

•

-

Packet Data Traffic Channel (PDTCH) to transfer data (uplink / downlink)
Packet-Associated Control Channel (PACCH) to transfer signaling information
(uplink
/
downlink)
One PACCH is associated with one or more PDTCH(s) concurrently assigned to a
mobile station and is allocated to one of the physical channels of the related
PDTCH(s). In the case of half duplex / fixed allocation mobile stations, a PACCH block
downlink is sent during a three (optionally two) timeslot gap in the uplink allocation on
the PACCH; with this type of mobile station no data will be sent downlink in the timeslot preceding (optionally following) and during uplink PACCH timeslots.

Support on CCCH and PCCCH
To provide GPRS services, the present GSM radio interface supports new logical and physical
channel types. The functionality is similar to that required by normal GSM traffic: GPRS
Common Control Channels (CCCHs) are among those logical resources whose similarity to
normal GSM CCCHs allows the compatibility with actual physical CCCHs. It is therefore possible to support GPRS common signaling either on already existing CCCHs (shared CCCHs)
or on GPRS-dedicated CCCHs (PCCCHs).

BR5.5 - SF10420 – V2.1

6/33

Shared CCCH
There are no GPRS -dedicated control signaling channels, so that GPRS common control signaling packets access a CCCH in accordance with its mapping rules. This mechanism is
mandatory whenever a dedicated CCCH is not allocated.
The mes sages are carried in the LAPD related to the BTSE. The channel is routed via switching matrix to a PPLD where the LAPD protocol is processed. The extracted messages are
read by TDPC via Telephonic Bus from the PPLD Dual Port RAM. In the TDPC, the messages
are analyzed: GPRS -related messages are written by TDPC via Telephonic Bus to the Dual
Port RAM of the Peripheral Packet Control Unit (PPCU), where they are processed.
Dedicated CCCH (PCCCH)
PCCCH is mapped in the multiframe of a PDCH. In this case the common control signaling is
carried on a logical channel dedicated to GPRS traffic.
The messages are carried in a TRAU frame of the 16 Kbps timeslot related to the physical
PDCH where the dedicated CCCH is mapped. The timeslot is routed via switching matrix directly to the PPCU, where the channel is processed.
To avoid GPRS signaling load on "normal" CCCHs, it is recommended to use PCCCHs as
soon as GPRS traffic increases, so that GPRS signaling traffic has no influence on normal
signaling and the overall traffic capacity is improved.
The advantages of using PCCCHs are straightforward:
•

On the air interface, CCCH performance for normal GSM traffic is not reduced because of
GPRS messaging.

•

On the Abis interface, the capacity of the LAPD link is not shared betwe en GSM and
GPRS traffic.

•

The TDPC does not waste real time to route GPRS messages toward PPCUs and to multiplex in LAPDs the messages received from the PPCUs.
• The Telephonic Bus is relieved in both directions from the message exchange between
PPLD, PPCU and TDPC.
On the other hand, shared CCCHs are supported to provide the first access when no GPRS
channels are allocated.
Besides, shared CCCHs are the only way to allow Class B mobile stations attached to GPRS
to listen to their Circuit-Switched Paging chan nel on CCCH.
PDCH handling
PDCHs are the physical channels dedicated to GPRS packet data and signaling traffic. They
are organized in a multiframe structure carried by a timeslot (see Figure 2).
52 multiframe number
B0

B1

B2

x

B3

B4

B5

x

B6

B0-B11 = RLC blocks composed by 4 burst

B7

B8

x

B9

B10

B11

x

x = idle frames

Figure 2: PDCH multiframe structure

The radio blocks from B0 to B11 are allocated to PCCCHs and PTCHs according to the following rules:
1.
2.

3.

The blocks are put in a logical order according to the following list of blocks:
B0 B6 B3 B9 B1 B7 B4 B10 B2 B8 B5 B11
The BCCH indicates the PDCH containing the PBCCH. The PBCCH is allocated downlink
to the first block of the list. The next 1 to 3 blocks of the list can be allocated to additional
PBCCHs (the total PBCCH block number reported by BS_PBCCH_B LKS parameter,
broadcast in the first PBCCH).
Additional PDCHs containing PCCCHs are indicated in the PBCCH: On these PDCHs, the
first BS_PBCCH_BLKS blocks of the list are used for PDTCH or PACCH in the downlink.

BR5.5 - SF10420 – V2.1

7/33

4.

On any PDCH with PCCCHs the next BS_PAG_BLKS_RES (broadcast in PBCCH, from 0
to (12 - BS_PBCCH_BLKS)) blocks of the list are used downlink for PAGCH, PNCH,
PDTCH or PACCH.

5.

The remaining blocks of the ordered list can be used to carry PPCH, PAGCH, PNCH,
PDTCH or PACCH.
On the uplink of a PDCH containing PCCCHs, blocks can be used as PRACH, PDTCH or
PACCH (PRACH are identified by the Uplink State Flag USF=FREE).

6.
7.

Optionally, the first BS_PRACH_BLKS (broadcast in PBCCH) blocks of the list are used
only for RACHs.

8.
9.

On PDCHs not containing PCCCHs, all bloc ks can be used as PDTCH or PACCH.
In any case, the current usage of a block is indicated by the message type.

Master / Slave Concept and Capacity on Demand Concept
The flexible and dynamic allocation / de -allocation of radio resources allows for efficient radio
resource sharing between circuit-switched and packet -switched services. Therefore two basic
concepts are used within GPRS:
•

Master / Slave concept
This concept uses a master channel, which is a more or less statically allocated channel
containing the GPRS CCCH, and one or more dynamically allocated slave channels carry ing the user data.

•

Capacity on Demand concept

With this concept, the network dynamically allocates capacity from the common pool of all
radio resources depending on the number of GPRS mobile stations (MS), their data
amounts, multislot capabilities and requested Quality of Service (QoS).
An optimized radio resource management mechanism also allows the use of resources due to
the gaps between two consecutive circuit-switched connections.
Timeslot combining
Timeslot combining allows the use of applications which need more throughput than that
achieved by using one timeslot only, and enables the operator to speed up simultaneous data
transmission for several users. A maximum of 7 timeslots can be combined using a single
BCCH carrier or as well a maximum of 8 timeslots using other carriers than BCCH.
Timeslot combining supports all MS multislot classes from 1 up to 29.
Channel Coding CS1 and CS2 on PDTCH
The introduction of GPRS into the networks in GSM Phase 2+ requires a modification of current channel coding. Four channel coding schemes (CS1 to CS4) are specified.
For the first GPRS release, the CS1 and CS2 coding schemes are implemented. CS1 and
CS2 differ in the number of transmitted dat a bits.
(Please note that CS3 could add only 10 % performance both for throughput and spectrum ef ficiency, while CS4 works in specific radio environments only.)
CS1 implements the basic coding for the RLC / MAC data and control blocks. The maximum
net data throughput performed by CS1 is about 8 Kbps under good radio conditions and
changes slowly as function of the C/I ratio. CS2 provides a higher data throughput (a maximum net data throughput of 12 Kbps) in good radio environments, the changes are more dependent on the C/I ratio.
The initial coding scheme for downlink is based on a default value, which is anchored in the
data base of the BSC. This initial value can be handled by O&M commands, and the default
value is prescribed by the operator. The initial value in the database works per cell class.
Support of 11 data bit packet random access burst on PDCH
In the current GSM, the burst carrying the random access uplink message contains 8 information bits. Evaluations have shown that this limitation is a bottleneck.

BR5.5 - SF10420 – V2.1

8/33

To avoid this bottleneck, an 11 information bit random access uplink message has been defined. The new 11 bit random access request message allows more frequent one phase access instead of two phase access.
In the one phase access procedure, an immediate channel assignment message assigning a
suitable channel allocation follows directly after the random request message, whereas in a
two phase access procedure an additional signaling step is necessary to find a suitable cha nnel allocation.
The 11 information bit random access request message speeds up the call set -up and ther efore decreases the signaling load, which makes the GPRS call duration quite short in comparison to circuit-switched connections and, enables more frequent call set -ups.
Power Control
Power control is important for spectrum efficiency as well as for minimizing power consum ption in the mobile stations. In order to minimize the impact on the existing frequency plans
when introducing GPRS, the amount of interference power generated is kept at a minimum.
The present GSM radio interface supports uplink and downlink power control, based on received signal level and received signal quality measurements during continuous two -way connections. This mechanism is not applicable to the unbalanced bursty nature of data communications. Therefore, new power control mechanisms are introduced as standardized by ETSI,
which fulfill the GPRS requirements. In BR5.5, MS open loop power control will be supported.
The algorithm is based on parameters configured by the customer. The specified algorithm
(GSM 5.08, Annex B) is implemented at the MS side as follows:
PMS = Γ0 – ΓCH - α*(C+48)
• PMS is the output power at the MS side.
•

Γ0 equals 39 dbm for GSM 900 and 36 dbm for DCS 1800.

•

ΓCH is an operator-dependent parameter set on the PTPPKF object. Its value is calculated
to reach a target value for received uplink signals at the BTS.

•
•

α is a constant system parameter broadcast on PBCCH or on BCCH.
C is the downlink signal level received at the MS side.

Quality of Service (Best Effort)
The GSM standards define a Quality of Service (QoS) for data transmissions over the network
for GPRS. The QoS is divided into four Delay Classes, the predictive classes Class1 to
Class3 and the non-predictive Class4, the so-called "Best Effort" class. BR5.5 supports
Class4 (Best Effort), thus providing an optimum of spectrum efficiency and being best suited
for Web access.
Operation and Maintenance Functionality
The Operation and Maintenance (O&M) functionality as provided for the SBS system is enhanced to cover the management of the BSS part of the GPRS network. This means that all
general management principles applicable to the GSM circuit-switched connections are also
valid for the GPRS packet -switched connections. The O&M functions for GPRS are available
from the Local Maintenance Terminal (LMT) as well as from the OMC-B. From there, the existing O interface is used for management purposes.
With the introduction of GPRS in the network, the operator is able to:
•

verify the correct dimensioning of the GPRS network (short - and long-term planning)

•

examine the GPRS network planning parameters

•
•

fine-tune the GPRS network configuration parameters
identify the QoS and performance of the GPRS network

Performance Measurements
In order to check the GPRS network performance, new performance measurements have
been introduced. This set of measurements provides information on the following functional-

BR5.5 - SF10420 – V2.1

9/33

ities and can be used to verify and optimize the GPRS configuration parameters in the network:
•
•

GPRS radio access
Radio resource usage

•
•

GPRS radio resource reassignment
Dynamic allocation and de-allocation of GPRS radio resources

•
•

Packet queuing
Used coding schemes

Possible Applications
One way the user can benefit from GPRS is by the packet nature of GPRS, which makes a
GPRS connection similar in many ways to a local area network (LAN) connection. Just as with a
LAN connection, once a GPRS mobile station registers with the network, it is ready to send and
receive packets.
Example:
•

A user with a lapt op computer could be working on a document without even thinking
about being connected, and then automatically receive new E -mail. The user could decide
to continue working on a document, then half an hour later read the E-mail message and
reply to it. All this time the user has had a network connection and not once had to dial in
(as s/he must today with circuit-switched connections). Furthermore, GPRS allows for simultaneous voice and data communication, so the user can still receive incoming calls or
make outgoing calls while in the midst of a data session.
Since there is almost no delay before sending data, GPRS is ideally suited for applications such
as extended communications sessions, E-mail communications, database queries, dispatch, and
stock updates to name just a few.
In addition, the high throughput of GPRS will overcome many obstacles in the use of graphical
Web-based applications in multimedia. For example, mobile users will have easy access to
graphically intensive Web-based map applications to get directions while traveling.
GPRS meets the needs of most data applications in a wide range. The following point -to-point
applications will be possible:
•
•

Mobile Internet / Intranet access with corresponding applications
Traffic guide and information systems

•

General information services (e.g. stock exchange, tourist information)

•

Entertainment

•
•

Mobile Office
Field sales / service

•

Group call-based services (e.g. stock information)

•

Wireless access to databases

•
•

Mobile Internet access
Electronic commerce

•

Point of sale

•

Electronic banking

•

Electronic cash

•
•

Messaging
Fleet management

•

Security / supervisory systems

•

Telemetry

•
•

Reservation systems (e.g. hotel, theater, and flights)
Highway charging systems

BR5.5 - SF10420 – V2.1

10/33

Impacts on BSS
GPRS is based on the existing GSM network infra structure, i.e. it introduces an overlaying architecture on the existing one with the definition of new entities and new interfaces.
In the BSS, the Packet Control Unit (PCU) located in the BSC is a new network component for
the BSS. In addition, the Channel Codec Unit (CCU) is a new BTS extension, which is easily installed via software download.
In detail, GPRS has the following impacts on the BSS:
BSC
The GPRS network structure as standardized in SMG requires a new interface in the BSC towards the Serving GPRS Support Node (SGSN) / Gateway GPRS Support Node (GGSN), the
so-called Gb interface. This is caused by the fact that with the GPRS functionality packet oriented data transfer and new protocols have to be handled in the otherwise circuit-switched
BSS.
In the BSS, this new interface is implemented by so-called Peripheral Packet Control Unit
(PPCU) cards, which are plugged into the BSC rack as additional units. The PCU is scalable
in steps of 64 channels per PPCU and can handle a maximum of up to 128 GPRS channels
per BSC.
TRAU
In the BSS, no change of the TRAUs, neither in hardware nor in software, is necessary for the
use of GPRS.
BTS
In the BSS, GPRS requires no hardware upgrade at all for the BTSs: The GPRS -relevant
CCU components can be introduced by simple software download.
Therefore, upgrade of a complete network for GPRS capability does not require any service
staff at BTS sites, which constitutes an enormous advantage in terms of time, cost and manpower.
Implementation
GPRS includes certain BSS modifications:
•
•

linking of the BSS to the new GPRS Support Nodes (GSN) via the Gb interface, that is the
installation of a PCU
transmission of the packet data through the BSS

•

the new channel coding schemes, i.e. the implementation of the CCU

•

combining of physical channels to achieve high transmission rates via the Um radio interface

BSC
Due to the packet -oriented data transfer as well as the appropriate protocols that are now also
handled in the BSS, the GPRS network structure has been provided with a new interface to
the SGSN / GGSN. In the SBS, this interface is implemented with the PCU, i.e. PCU cards are
inserted into the BSC rack.
BTSE
No hardware modification is required for the BTSE. This way, GPRS is supplemented with the
CCU by simple software download.
TRAU
The TRAU requires neither hardware nor software changes.
LMT
The terminal assigned to the SBS for the local operation and maintenance of the SBS network
elements gets O&M functions added to manage the new HW and SW elements.
Entity

Implementat ion Impact

MS

Must support GPRS by providing the respective protocols and functions

TRAU

No impact

BR5.5 - SF10420 – V2.1

11/33

BSC

Must be extended by adding the PCU, which handles the GPRS protocol stack
and functions

BTS

Must be extended for GPRS by software download

OMC

Must support GPRS

LMT

Must support GPRS

Table 1: Implementation impacts on network entities

Network Compatibility
One of the major aspects defining the GPRS standard was to minimize possible effects on the ex isting GSM network infrastructure.
With BR5.5, the effects could be reduced to the extension of only two network elements:
•

PCU for the BSS to reflect the new GPRS Gb interface onto the Abis interface, and the

•

CCU in the BTS.

Performance
Through the use of GPRS, the existing GSM network will be enhanced by packet data services.
These services rely on new network elements that will be particularly suited for specific packet
switching needs.
Since these new network elements are additionally assigned to the GSM network they will reli eve
the existing circuit-switched GSM data services. This will not only affect the traffic connections but
also the signaling channels.

Functional Split between BTS (CCU) and BSC (PCU)
The management of GPRS radio channels and the protocol stack conversion between the Gb
and Abis interfaces is carried out by the PCU. The PCU is implemented in the BSC and is interfaced to several CCUs located in the BTSs, as outlined in Figure 3:
GPRS functions implemented in the BTS (CCU) are:
•
•
•

Channel Coding, including Forward Error Correction (FEC) and interleaving
Radio Channel Measurement, including received quality level, received signal level, and
timing advance measurement information
Mapping of GPRS data and signaling on the Abis interface toward the BSC
BTS
CCU

Abis

Gb

CCU
BTS
CCU
CCU

BSC
P
C
U

SGSN

BTS
CCU
CCU

Figure 3: CCU and PCU location
GPRS functions implemented in the BSC (PCU) are:
•
•

Mapping of GPRS data and signaling on the Abis interface towards the BTS
MAC and RLC layer handling

BR5.5 - SF10420 – V2.1

12/33

•

Packet Data Unit (PDU) assembly and segmentation

•

PDCH RLC Automatic Request (ARQ) functions (i.e. based on MS request ACK / NACK),
including buffering and re-transmission of RLC blocks

•

Scheduling functions for PDCH data transfer

•

BSS GPRS Protocol (BSSG P) support

•

Layer 1 (Frame Relay) protocol support on the Gb interface toward SGSN via dedicated
link or embedded via Asub interface
Setup and release of GPRS resources on the Abis (Um) interface upon request

•

PCU MAC layer functions
•

PDCH multiframe management

Multiframe synchronism (via bit in the PCU frame)
• Data and control signaling multiplexing based on a scheduling mechanism and on users'
demand
The USF sent in the PCU frame allows one MS to transmit in the next uplink radio block. On
PCCCHs 7 MSs can be multiplexed, the eighth USF value means that the uplink radio block is
free to be used as a RACH burst. On PDCHs not carrying PCCCHs up to 8 MSs can be multiplexed.
•

Timing Advance (TA) management

When the BTS receives a Packet Channel Request message in a (P)RACH, it computes the
TA and sends both the result and the request to the BSC in a PCU frame or in an LAPD mes sage. The relevant Packet Resource (Immediate) Assignment will notify the MS of the proper
TA.
After this initial TA estimation, the TA will be updated continuously by the BTS. The BTS is
able to control the TA of the MSs without the intervention of the BSC.
• Broadcast information on PBCCH
If PBCCH exists, GPRS cell parameters will be broadcast on it.
• Power Control
MS Power Control uplink is implemented.
• Coding Scheme selection
The Coding Scheme applied in a cell is an O&M condition. CS-1 will always be used for
PACCH, PBCCH, PAGCH, PPCH, and PNCH.
The information about the Coding Scheme to be used is reported in the PCU frame.
•

Dynamic GPRS channel allocation

Active MSs are associated dynamically with one Temporary Frame Identifier (TFI) and one
USF upon resource assignment. Downlink packets are accepted by an MS if the MS TFI and
the packet TFI match. The USF contained in downlink packets id entifies the MS allowed to
transmit in the relevant uplink packet.
PCU RLC layer functions
• Segmentation and re-assembly of LLC-PDUs into RLC data blocks
•

Backwards Error Corrections (BEC) procedures to allow selective re-transmission of uncorrectable errore d frames (Automatic Retransmission Request, ARQ)

•

Packet acknowledge / not acknowledge (ACK / NACK)

The transfer of RLC data blocks can take place both in a reliable Acknowledged Mode and in
a faster Unacknowledged Mode. When the Acknowledged Mode is used, temporary / final
Packet ACK / NACK messages are transmitted on the PACCH to the remote peer to report the
status of the reception process. Transmitted RLC blocks are numbered through a Block Sequence Number (BSN).
• Temporary Frame Identifier (TFI) management
In the PACKET DOWNLINK ASSIGNMENT message, the PCU assigns a TFI to an MS, the
assigned TFI identifying the Temporary Block Flow (TBF) on a direction. The same TFI value
may be used concurrently for TBFs in opposite directions. Upon reception of a final PACKET

BR5.5 - SF10420 – V2.1

13/33

ACKNOWLEDGMENT message from the MS (last data block received successfully), the TFI
may be used for other users.
PCU LLC layer functions
The PCU has the responsibility of relaying the LLC layer between the RLC and the BSSGP. A
buffering function is able to compensate LLC-PDU frame peaks.
In LLC, for every cell queues are allocated for 4 QoS priority levels and one queue for signaling (in BR5.5, no QoS classes are supported, the transmission is of "Best Effort" type).
PCU BSSGP layer functions
• Bi-directional data flow control
One LLC-PDU is mapped in one UL-UNITDATA PDU and vice versa one DL-UNITDATA PDU
is mapped in one LLC-PDU.
•

Downlink queues are managed via flow control procedures.

• Paging request handling
At this level, paging requests issued by the SGSN through a PAGING PS PDU are managed
in the PCU.
•
•

•
•

Flushing of old data queues (e.g. when an MS changes the BSS)
In case a link with an MS is interrupted (e.g. because the MS goes Out of Coverage), the
reception of a FLUSH-LL PDU from an SGSN shall f lush all LLC-PDUs stored in the PCU.
Queued BSSGP signaling (e.g. Pages) is not affected.
Multiple level 2 link management
Use of RLC/MAC level information to build BSSGP PDUs and invoke RLC / MAC operations using BSSGP information.

PCU Frame Relay (Network Service) functions
• Permanent Virtual Connections (PVC) management
A PVC is identified via a Bearer Channel Identifier (corresponding to the physical link, e.g. a
64 Kbps timeslot in a 2Mbps PCM link) and a Data Link Control Identifier (DLCI, addressing
f ield in the header of an FR frame).
• Load sharing management
Network Service-Service Data Units (NS-SDU) are distributed over NSVC on the Gb interface,
in order to distribute traffic load and reorganize the traffic after of a failure.
• Frame Relay support
Protocols and Interfaces
In the BSS, GPRS data are carried from the BTS to the SGSN over the Abis and Gb interfaces
according to the stack protocol described in Figure 4.
Um interface
The PDCH is the physical radio interface within GPRS. It differs in channel coding, multi-frame
structure and MS multiplexing mechanisms from a circuit-switched traffic channel. On the Um
interface, the common channels Random Access Channel (RACH), Paging Channel (PCH),
and Access Grant Channel (AGCH) can be shared for channel requests, paging and assignment commands between GPRS and circuit-switched connection services. GPRS traffic is
transferred over timeslots taken from the common TCH pool.
Abis interface
On the Abis interface, GPRS data and RLC / MAC-associated signaling are mainly transferred
via 16 Kbps channels in frames of a fixed length of 320 bits, i. e. in so-called PCU frames (an
extension of the existing TRAU frames).
RLC / MAC signaling sent over shared control channels is logically multi plexed in the LAPDs
between the BTSs and the BSCs. Inband signaling and GPRS traffic are encapsulated in PCU
frames by the BTS.
Gb interface
The Gb interface connects the BSS, i.e. the PCU, to the GSN via Frame Relay protocol (FR),
allowing the exchange of signaling information and user data, also in a multi-vendor environment. In contrast to the A interface, where a user is provided with a certain physical resource

BR5.5 - SF10420 – V2.1

14/33

for the duration of the entire connection, a resource on Gb is only assigned when active (wh ile
data are being sent or received).
The Gb interface is implemented as either a dedicated PCM link towards the SGSN or as an
embedded bundle of timeslots of the Asub interface, which are transparently routed via the
TRAU to the SGSN.
Figure 4 shows the protocol stack that is used for data transmission in the GPRS network.
Application

Um

Gb

Abis

IP/X.25/
CLNP
SNDCF

SNDCF

LLC

LLC

LLC Relay

RLC

GTP

RLC

MAC

MAC
new TRAU
frame

GSM RF

GSM RF

new TRAU
frame

BSSGP

BSSGP

L2'

Frame
Relay

Frame
Relay

L1'

e

MS

BSC/PCU

BTS

SGSN

Figure 4: GPRS protocol stack

Data packet

Data packet

Network Layer

Data packet (encrypted & compressed)
SNDCP Layer

LLC frame

Hdr

Info Field FCS Hdr

Info Field FCS

H d r Info Field FCS

LLC Layer

RLC frame

Hdr

Info Field FCS Hdr

Info Field FCS

H d r Info Field FCS

RLC/MAC
Layer

Burst

Burst

Burst

Burst

Burst

Burst

Burst

Physical Layer

Figure 5: Data Flow between several protocol layers on the MS and SGSN.
The layers carry out the following functions:
GSM RF
The GSM RF is the physical radio channel used to transfer packet data.
MAC
The Medium Access Control provides the access to the physical radio resources. It is respo nsible for the ph ysical allocation of a PDTCH.
RLC
The RLC layer provides a reliable link over the air interface that fits the block structure of the
physical channel. Therefore it segments and re-assembles the LLC frames. Additionally, it
performs sub -multiplexing to support more than one mobile station by one physical channel,
and channel combining to provide up to 8 physical channels to one mobile station.
LLC
The Logical Link Control (LLC) layer provides a logical connection between the mobile station
and the SGSN even if no physical connection is established. The physical connection is set up
by the RLC / MAC layer when there are data to be transmitted.
BSSGP
The BSSGP is used to transfer the LLC frames together with related information between the
SGSN and the PCU. Such information includes QoS and routing information.
SNDCF
The Sub-Network Dependent Convergence Function (SNDCF) performs the following tasks:

BR5.5 - SF10420 – V2.1

15/33

•

Encryption

•

Compression

•

Segmentation / re -assembling

• Multiplexing / de -multiplexing of signaling information and data packets.
The encryption function is used to support data privacy, whereas the compression and segmentation functions are performed to limit the amount of data transferred by the LLC layer.
Higher layers
The higher layers are not within the scope of GPRS because these layers are independent of
the underlying network.

Functional Description
GPRS provides the mobile subscribers with means for services like point -to-point data transfer. In
detail, these are:
• High system availability because of an extensive and improved system maintenance concept and maintenance functions
•
•

Support of the standardized Gb interface
Support of subscriber mobility including Routing Area update and cell update

•

Support of the paging function to find subscribers at unknown locatio ns

•

Support of GSM security functions (authentication) for protection against misuse and fraud

•

Support of the acknowledged and unacknowledged logical link control operation mode,
which allows the usage of multi-purpose applications
For the operator, GPRS offers the following functionality:
•

High system availability because of an extensive and improved system maintenance concept and maintenance functions:

-

The consistency of data within the system is checked by audit programs which
will be running periodically or on demand. By detecting errors, the applications will be requested to correct the data and audit symptom data will be collected.
Collecting of symptom data in the case of error detection in software applica-

tions.
-

•
-

Escalation to higher recovery levels in the case of frequent software errors.
Different recovery levels are defined. A recovery can also be requested manually
by the operating personnel. Depending on the recovery level, data are initialized and OS
resources are released.
The start -up info service holds information about the software state of the processors within the system.
The operator is able to manage
the network resources and network changes,

the data specific for the GPRS network nodes,
the parameters of the different protocols that are used for GPRS.
The list below gives an overview of the data that are managed by the configuration manag ement:
-

Management of connections to other entities (e.g. from SGSN to the PCUs)

-

Management of internal connections between components

-

Management of office and project data
Own entity functions and own entity address

•
•

Standardized Gb interface according to GSM
Support of GSM security functions to prevent misuse and fraud

•

Support of two LLC modes (acknowledged and unacknowledged), hence the adaptati on
to different applications.

BR5.5 - SF10420 – V2.1

16/33

•

Support of a reliable transmission via the logical link.

•
•

Support of the simultaneous use of up to 4 different independent data links.
Support of the transport of the L3 signaling data packets.

Implementation
PCU and CCU
The GPRS features within the BSS are implemented in two functional units:
•

The PCU located in the BSC provides resource allocation and protocol conversion between the BTS and the SGSN. The PCU acts (just like the BSC) as a statistical multiplexer and router (see Figure 6). It receives RLC packets from the Abis channel related to
more than one mobile station and packs them into LLC frames. These LLC frames are
then routed to the SGSN and vice versa together with other LLC frames coming from
other Abis channels.

The PCU is one unit composed of two cards named Peripheral Packet Control Unit (PPCU).
While the first card provides service, the second one is in cold standby.
Each BSC can include two PCU units. In this case, the traffic is divided statically to both units
by means of configuration settings.
• The CCU performs channel coding functions and channel measurement functions.
PCU
The PCU is a functional unit within the BSC that provides resource allocation and protocol
conversion between the BTS and the SGSN. Looking at the protocol stack, the PCU is er sponsible for:
•
•

Channel Access Control functions, e.g. access requests and grants
PDCH scheduling functions for uplink and downlink data transfer

•
•

Radio Channel Management functions, e.g. power control, congestion control, broadcast
control information, etc.
PDCH RLC ARQ functions, including buffering and re -transmission of RLC blocks

•
•

LLC layer PDU segmentation into RLC blocks for downlink transmission
RLC layer PDU re-assembly into LLC blocks for uplink transmission

•

BSSGP protocol provides PCU – SGSN communication in terms of BVCI (BSSGP Virtual
Connection Identifier)

•

Network Service functions provide PCU – SGSN communication in terms of Virtual Channel (Network Service Virtual Channel NSVC)

CCU
The functions inside the CCU are:
•

Channel coding functions, including FEC and interleaving

•

Radio channel measurement functions, including received quality level, received signal
level and information related to timing advance

• Continuous Timing Advance
PCU frames are transferred across the Abis interface every 20 ms (fixed length of 320 bits).

PCU Frame
PCU Frame
PCU Frame
PCU Frame

Frame Relay

PCU

LAPD

Abis

TDPC

Gb
BSC

BR5.5 - SF10420 – V2.1

SGSN

17/33

Figure 6: PCU as multiplexer and router
PCU and PPCU Internal Structure
In order to introduce the GPRS service in the SBS, a new unit has been designed to support
packet data interworking between the Gb and the Abis interfaces. The evaluation of the feasibility
study phase has revealed that the amount of messages exchanged between the Gb and the Abis
interfaces needs a dedicated processing resource in order to avoid capability losses in normal
GSM traffic. The new PPCU unit will be inserted in the BSC rack instead of PPLDs, as can be
seen in Figure 7.
The internal physical connections are represented in Figure 8. The BSC can be configured with a
maximum of two PCUs (each one redounded).
PCU-0
• PPCU-0 replaces PPLD -15 (not used on the BSC) in the frame. PPCU -1 substitutes
PPLD-12.
PPLD-14, PPLD-13, and PPLD-11 are removed from the frame.
PCU-1
• PPCU-0 substitutes PPLD-8 in the frame, and PPCU-1 replaces PPLD-9.
PPLD-10 and PPLD-7 are removed from the frame.
The new layout of the BSC module can be seen in Figure 8. The capacity of each PCU is selected via O&M commands in terms of bandwidth reserved on the Abis and Gb interfaces. The
minimum bandwidth allowed, the sum of the Abis and Gb interfaces, is 32 * 64 Kbps which
means that 4 PPLDs are removed from the system and replaced with two PPCU (0/1) cards.

.

15 14 1 3 12 11 1 0 9

4 3 2 1 8 7 6

5 4

3

1

FUSE & ALARM
PANEL

.
.

.

2 1 0 1

1

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

0

1

5 4 3 2 1 0
8

6

.
.

1

5 4 3

1

FUSE & ALARM
PANEL

.
.
.
.
.
.
.
.
.

0

0 1 0 0

1 0 2 1 0 1

1

DK 40
IXLT
UBEX
SN16
TDPC
MEMT

.

MPCC
MPCC

0 0 0 0 0 0 0 1 1 1 1 1 1 1 1

.

MEMT
TDPC
SN16
UBEX
IXLT
DK 40

DK 40
IXLT
UBEX
SN16
TDPC
MEMT

.0

1 0
MPCC
MPCC

.

0 1 0 0

MEMT
TDPC
SN16
UBEX
IXLT
DK 40

.

0

.

0

PWRS

.

.

8 7 6

PLLH
PPLD
PPLD
PPLD
PPCC
PPCC
LICDS
LICD
LICD
PLLH
PWRS

PWRS

PLLH
PPLD
PPLD
PPLD
PPCC
PPCC
LICDS
LICD
LICD
PLLH

.

PWRS

.

.

.
.

EPWR

5

.

.

EPWR

0

.

PPLD
PPLD
PPLD
PPLD
Removed
PPCU 1
LICDS
LICD
LICD
LICD
LICD

.

EPWR

.

EPWR

.

PPLD
PPLD
PPLD
PPLD
PPLD
PPLD
LICDS
LICD
LICD
LICD
LICD

.

.
.

PPCU 1
Removed
Removed
PPCU 0
Removed
Removed
PPCU 0

PPLD
PPLD
PPLD
PPLD
PPLD
PPLD
PPLD

8 7 6

.

.
.

LICD
LICD
LICD

LICD
LICD
LICD

.
.

0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1

.
.
.
.

Figure 7: The BSC module without GPRS (left) and ready for GPRS (right)

BR5.5 - SF10420 – V2.1

18/33

PPCC 0

PPCC 1

PPLD 0

PPLD 1

PPLD 3

PPLD 4

PPLD 5

PPLD 6

PPLD 7

PPLD 8

PPLD 9

PPLD 10

PPLD 11

PPLD 12

PPLD 13

PPLD 14

PPCC 0

PPCC 1

PPLD 0

PPLD 1

PPLD 3

PPLD 4

PPLD 5

PPLD 6

LICD 0

PPLD 2

SN
LICD 8

LICD 0

PPLD 2

SN
LICD 8

PPLD 7

PPLD 8

PPLD 9

PPLD 10

PPLD 11

PPLD 12

PPLD 13

PPLD 14

PCU 0

PPCC 0

PPCC 1

PPLD 0

PPLD 1

PPLD 2

PPLD 3

PPLD 4

PPLD 5

PPLD 6

LICD 0

SN
LICD 8

PPLD 7

PPLD 8

PPLD 9

PPLD 10

PCU 1

PPLD 11

PPLD 12

PPLD 13

PPLD 14

PCU 0

Figure 8: Internal BSC physical connections

Table 2 shows the capacity reduction in terms of LAPD channels with GPRS introduction, both for
the SN16 and SN64 network cards.
Card

No. of PCU

No. of LAPD channel

SN 16

0

112

or

1

80

SN 64

2

48

Table 2: Number of LAPD channels
The maximum number of LAPD channels is obtained by retaining the PPLD-11 frame.
The amount of data traffic handled by the PCU depends on the number of Kbps reserved for the
PCU unit on the internal connection between SN16 (or SN64) and the PCU.
Access
On the air interface, the cell structure organization remains the same as in the actual implement ation. An additional identifier is introduced to group the cells supporting GPRS service in the Loc ation Area (LA). This inf ormation is named Routing Area (RA) which can be less than or equal to
the LA. One LA can include more than one RAs.
The mobile stations with access to GPRS service receive information on the service in the Sy stem Information (SI) messages on the BCCH chan nel. SIs 3 and 4 are modified in order to insert
parameters for GPRS. The new SI 13 is sent on the air interface carrying all parameters for
GPRS network access.
In GPRS service, there is no handover: When the mobile station leaves one cell, it starts a cell re selection procedure. The RR links are disconnected and a new access operation is started. The
integrity and sequence of data is handled in the RLC layer.

BR5.5 - SF10420 – V2.1

19/33

Channel Configuration
The packet data logical channels are mapped onto the physical channels that are dedicated to
packet data. The physical channel dedicated to packet data traffic is called PDCH.
For GPRS, three types of channels have to be considered:
• Packet Data Traffic Channel (PDTCH)
•

Packet Broadcast Control Channel (PBCCH)

• Packet Common Control Channel (PCCCH)
All channels described above are allocated on the BCCH TRX. This ensures that radio planning
allows maximum cell power.
PDCH channels are "normal" TCH channels allocated, on demand, to a GPRS service. Up to 8
mobile stations can be multiplexed on one PDTCH channel. If a service requires more bandwidth,
it is possible to allocate to it up to 7 PDTCHs. This way, these channels are allocated to the same
mobile station and they have the same ARFCN, MAIO, HSN, and TSC.
PBCCH and PCCCH are semi-permanent channels configured with O&M commands.
The PDTCH allocation is handled by LV3 Radio, i.e. the master in the radio resource allocation.
For each PDTCH, a network connection must be set up in order to connect one 16 Kbps on the
Abis interface with one 16 Kbps PDT on the PCU unit. By request of the PCU, the TDPC releases
the PDTCH channel.
The PBCCH and PCCCH have a semi-permanent connection between one Abis 16 Kbps and
one PDT. The radio channels allocated to the PCCCH and PBCCH can no longer be used for
normal speech traffic.
The BCCH information is also replicated in the PBCCH – if it is active in the cell – to allow circuitswitched operation, even if the mobile station is monitoring the PBCCH only. In other words, the
PBCCH works like a second BCCH for mobile stations supporting GPRS.
The PCCCH works like the CCCH in normal operation and as PBCCH - if it is active in the cell - it
also carries the information about circuit-switched operation.
Network Service Control
The Sub-Network Service ent ity provides communications service to Network Service Control
peer entities. The peer-to-peer communication across the Gb interface between remote Network
Service Control entities is performed over Network Service Virtual Connections (NSVC). The
Network Service Control takes care of the end-t o-end NSVCs’ communication between the PCU
and the SGSN.
The Network Service Control entity is responsible for the following functions:
• NSDU transmission
The Network Service Data Units (NSDUs) are transmitted on the NSVCs. The NSDUs are encapsulated in the Network Service Control PDUs, which in turn are encapsulated in the Sub Network Service PDUs. On each NSVC, data are transferred in order.
•

Load sharing

The load sharing function distributes the NSDU traffic among the available (i.e. unblocked)
NSVCs.
• NSVC management
A blocking procedure is used by an NS entity to inform an NS peer entity when an NSVC becomes unavailable for NS user traffic. An unblocking procedure is used for the reverse operation. A reset procedure is used between peer NS entities in order to set an NSVC to a determined state, after events resulting in possibly inconsistent states of the NSVC on both sides of
the Gb interface. A test procedure is used to check whether an NSVC is operating properly between peer NS entities.
When the Sub-Network Service entity detects that an NSVC becomes unavailable (e.g. DLCI failure detection) or when the NSVC becomes available again (e.g. DLCI failure recovery), the Network Service Control entity is informed.
Quality of Service

BR5.5 - SF10420 – V2.1

20/33

GSM 03.60 currently specifies five different attributes within the QoS profile associated with each
PDP context. These attributes are:
•
•

Precedence class
Delay class

•
•

Reliability class
Peak throughput class

• Mean throughput class
During the QoS profile negotiation between the mobile station and the network, the mobile station
can request a value for each QoS attribute; the network always attempts to provide adequate resources to support the negotiated QoS profile.
For an uplink data transfer, the QoS profile is communicated by the mobile station as priority information in the PACKET_CHANNEL_REQUEST message. For a downlink data transfer, the BSSGP
provides the means to transfer the full QoS profile together with each downlink LLC PDU from the
SGSN to the BSS, which is the controller of the media utilization on the radio interface. In the lat ter case, the following QoS parameters are included in each LLC-PDU transferred to the BSS:
•

Precedence class

•

Peak throughput

• LLC-PDU lifetime
Taking into account the available radio resources and the multislot capabilities of the mobile station, the PCU decides if and how the requested QoS may be satisfied. This means that the core
algorithm of the PCU would try to satisfy the requested QoS by acting on many factors , for example changing the coding scheme on the air interface (CS2 has more transfer capacity than CS1),
allocating more radio resources (capacity on demand), reshuffling subscribers in the available
PDCHs according to the mobile station multislot capabilities, delay of the subscriber according to
the subscriber priority, etc.
In the first phase of GPRS, the QoS supported is the so-called “Best Effort”. This means that the
PCU main scheduler queues the mobile stations’ requests without considering the QoS attributes.
PCCCH and PBCCH Channel Allocation
To introduce GPRS in the GSM network, new channel types have been specified. The PBCCH
works like a secondary BCCH in the supporting GPRS cell. The PCCCH works like a secondary
CCCH in the supporting GPRS cell. Both the PBCCH and PCCCH are recognized only by mobile
stations supporting GPRS.
These two channel types are allocated on the BCCH carrier, and no hopping is admitted for these
channels in the SBS.
To create these channels means to create a semi -permanent connection between the Abis 16
Kbps channels and one 16 Kbps PDT in one PCU.
These channel types can be handled by adding a new parameter to the CHAN creation command
in order to specify the combination used for the GPRS channel.
The PBCCH or PCCCH creat ion can be allowed only if the PTPPKF object is created. The PDT is
selected automatically by the system.
Since, from the BTS point of view, these channels are managed in a completely different way
than the other common control channels, the BTS is notified of the creation of these channels.

BR5.5 - SF10420 – V2.1

21/33

Create Channel
PBCCH/PCCCH

Update Data Base
of MPCC / TDPC
Allocation
Possible

No
Request Network
Connection
between A bis /PDT

Yes
Send Data Base
to PCU
PTPPKF
Created

No

Send Data Base
to BTS

Yes
Choose PDT on
serving PCU

PDT
Available

CREATE
NACK

Request to send
System Information
to TL3RD

No

CREATE
NACK

Yes

Figure 9: Logical flow chart of a CREATE PBCCH / BCCCH command
The framing of these channels is managed by the PCU directly. The mobile stations are notified
via a SI message on the BCCH channel that these channels are active in a cell.
Special SI called Packet System Information (PSI) is sent on the PBCCH and PCCCH. When
mobile stations that support GPRS mode are in a cell supporting GPRS service and where these
kinds of channels are allocated, they listen to the information broadcast on these channels instead of to the one broadcast on the BCCH.
Figure 9 represents the logical operational flow to be performed for the allocation of the PCCCH /
PBCCH.
These channels are created in the "Locked" state, that means that no power can be sent on them
until the UNLOCK command is performed. The PCU is informed of the availability of the channels
because it needs to know when they can be used.
When the channel is working, the channel is transmitting at the power of the BCCH TRX. Locking
the PBCCH or PCCCH means to switch off the power at the BTS side and stop using it for paging
and access grant on the PCU side.
Framing of channel and SI transmission is continued. The GPRS service in the cell can be provided, even if no PBCCHs or PCCCHs are configured or available.
PDCH Channel Allocation Strategy
PDCHs are normal channels dynamically allocated for GPRS service. The allocation of a PDCH
channel is performed on the BCCH TRX carrier or a different one. The resource allocation / de allocation is driven by the PPCU software; the channel chosen and the channel activation / release to the BTS is a TL3RD task. The rules for the PDTCH allocation in the multislot configuration are as follows:
•

same frequency hopping law

•

same training sequence code

•
•

same MAIO
adjacent timeslot number

•

a maximum of 8 timeslots allocated per mobile station

BR5.5 - SF10420 – V2.1

22/33

The first three rules must be observed during configuration, which means that all TCHs on the
BCCH carrier must abide by the same hopping law and the same training sequence code. The
fourth law is observed dynamically for the timeslot selection. If a request for a new PDCH comes
from the PCU, the TL3RD tries to allocate a new PDCH adjacent to the previous one. In the case
of no free timeslots adjacent to the busy one, the adjacent timeslots are in a stable call state, and
there is at least one free channel in the cell, a forced intracell handover is initiated in order to
move the call camped in the adjacent timeslot to another one (see Figure 10) using the same rule
as the HSCSD. It must be taken into account that HSCSD calls cannot be forced. Because the intracell handover procedure doesn't need more than 0.4 sec., no packet queuing notification is
sent to the PCU software in order to keep the channel allocation in the standby state.
In the case of channel locking (PBCCH, PCCCH, and PDTCH) the PDT and the corresponding
radio channels are released to the idle state.
BCCH
TS 0

Busy 1 Busy 2 PDT 0 Busy 3 Busy 4 Free 0 Busy 5
PBCCH timeslots 0 - 7

BCCH
TS 0

Busy 1 Busy 2 PDT 0 Busy 3 Busy 4 Free 0 Busy 5

Intracell HO tried & successful for busy 3 stable state call
Other
Channels

Busy 6 Free 1 Busy 7 Free 2 Busy 8 Busy 9 Free 3 Busy 0

BCCH
TS 0

Busy 1 Busy 2 PDT 0 Free 4 Busy 4 Free 0 Busy 5

Other
Channels

Busy 6 Busy 3 Busy 7 Free 2 Busy 8 Busy 9 Free 3 Busy 0

BCCH
TS 0

Busy 1 Busy 2 PDT 0 PDT 1 Busy 4 Free 0 Busy 5

Busyx: TS allocated to some call
Free x: TS not allocated
PDT x: TS used by GPRS-MS

Figure 10: PDTCH multislot strategy allocation when a new PDTCH is requested for
the same MS and no adjacent PDTs adjacent to the one in use are free
A maximum of 8 PDCHs can be allocated for each cell; this parameter can be set by an O&M
command for each PTPPKF (object PTPPKF, parameter GMANPAL).
The PCU tries to schedule the MS with it’s maximum TS usage depending on MS multislot class.
If there is no PDTCH allocated in one cell and the MS needs to establish a TBF, the PCU must allocate the maximum number of TSs supported by the MS, if there is at least one TS free on the
cell the TBF is accepted; otherwise the TBF is rejected. If another MS needs to setup a TBF and
the number of TSs supp orted are the same or less than those used by the previous MS, the new
mobile is allocated to the already allocated TS. If the MS requires more TSs then additional re quests are sent to the TDPC in order to satisfy the maximum MS capacity and the MS is shared
between the already existing TS and the new time slot.
In the case of an incoming CS (Circuit Switch) call (Normal Assignment or External Incoming HO,
single slot) the following algorithm applies to the cell having no free channel:
•

If the incoming CS call finds the cell congested, the first thing attempted is to preempt one
vulnerable CS call.

•

If preemption cannot be started for whatever reason (feature not enabled, the incoming
CS call has a PCI set to 0, ...) a directed retry is started.
If not even the directed retry can be started (because the feature is not enabled or the feature is enabled but the BTS is sending a condition for an Intercell HO message without a
cell list) GPRS preemption is attempted. The GPRS TS having the highest TS number
(except some very special cases that only occur when the incoming CS call is half rate

•

BR5.5 - SF10420 – V2.1

23/33

and GPRS TSs are mixed FULL_RATE_ONLY or FULL_AND_HALF) is selected to be
preempted.
• If the GPRS preemption cannot be started because no GPRS TS are available (or because of very special cases due to Half Rate CS), the queuing procedure is started.
The following algorithm applies to the case of an incoming circuit switch call (Normal Assignment
or External Incoming HO, HSCSD 1+1) to a cell having no free channel:
•
•

If the incoming CS call finds the cell congested, the first thing attempted is to preempt one
vulnerable CS call.
If preemption cannot be started for whatever reason (feature not enabled, incoming CS
call having the PCI set to 0, ...) then a directed retry is started.

•

If not even the directed retry can be started (because the feature is not enabled) GPRS
preemption is attempted. The GPRS TS to be preempted is selected choosing the one
with the highest TS number of GPRS assigned TSs (except some very special cases that
occurs when the incoming CS call is Half Rate and GPRS TSs are mixed
FULL_RATE_ONLY or FULL_AND_HALF).

•

If not even the directed retry can be started (feature enabled but the BTS sending condition is for an intercell HO without a cell) it is assigned to a normal phase 2 data call (if
necessary the GPRS preemption is started for this call).

•

If the GPRS preemption cannot be started because no GPRS TS are available (or because of very special cases due to Half Rate CS), the queuing procedure is started.
The following behavior applies to normal assignments or external incoming HOs; no GPRS preemption is executed in the case of CS internal intercell/intracell HOs because the BTS, when the
HO is not managed, repeats the HO indication (same rule used in the existing CS preemption).
System Information Management
SIs are regularly broadcast by the network on the BCCH and the busy TCH. SI broadcasts can be
grouped in 6 classes – as described in GSM 04.07 – based on the information they contain. On
the basis of this information, the mobile station is able to decide whether and how it may gain ac cess to the network via the current cell.
With the introduction of GPRS, the SI 3 and 4 rest octets are modified, while SI 13 is added. In
addition, if the PBCCH is allocated to the cell, new System Information called Packet System Information (PSI), required by GPRS, is broadcast on this new logical channel.
All mobile stations camping in a GPRS -supporting service cell in which PCCH is allocated, listen
to the PSI instead of to the traditional SIs. A GPRS mobile station reading the PCCH can receive
non-GPRS paging incoming from the network, because on this GPRS broadcast channel traditional SI is also sent to guarantee this event.
When a mobile station moves to a new cell, it switches to the BCCH. The mobile station listens to
the SIs 3, 4, 7 or 8. If the cell does not support GPRS, the mobile station cannot perform packet
access to the network. If the cell supports GPRS, the mobile station reads SI 13. A mobile station
that h as read SI 13 but without first having read SIs 3, 4, 7 or 8 may assume that the current cell
supports GPRS service.
System Information 3 and 4
These messages are broadcast on the BCCH. They contain new additional information about
supporting GPRS service in the cell.
System Information 13
This message is broadcast on the BCCH only if GPRS is supported in the cell. It indicates:
•

•

If the PBCCH is active in the cell:
If the PBCCH is not configured, the mobile station in idle packet mode reads SI 13 according to GSM 05.08. In case the PBCCH is active in the cell, additional SI related to GPRS is
sent on the PBCCH.
If SI 1 is necessary for packet access in the cell:

Here, the mobile station is not allowed to initiate packet access until it has obtained SI 1.
The availability of SI 13 is sent on the BCCH using the SIs 3, 4 and, if sent, 7 or 8.

BR5.5 - SF10420 – V2.1

24/33

Packet System Information 1
PSI 1 is sent by the network on the PBCCH or PACCH giving information for cell selection, for
control of the PRACH, for the description of the control channel(s) and optional global power
control parameters. Special requirements for the transmission of this message apply on the
PBCCH (GSM 05.02).
Packet System Information 2
PSI 2 is sent by the network on the PBCCH or PACCH giving information on reference fre quency lists, mobile station allocations and PCCCH channel descriptions applicable for packet
access in the cell. A consistent set of these messages is required to completely decode the information.
Packet System Information 3
PSI 3 is sent by the network on the PBCCH or PACCH giving information on the BCCH alloc ation in the adjacent cells and cell selection parameters for serving cells and non-serving cells.
Special requirements for the transmission of this message apply on the PBCCH (GSM 05.02).
Packet System Information 3 bis
This optional message is sent by the network on the PBCCH and PACCH giving information
on the BCCH allocation in the adjacent cells and cell selection parameters for non -serving
cells. If the entire information does not fit into one PSI 3 bis message, the PSI 3 bis message
can be repeated. Special requirements for the transmission of this message apply on the
PBCCH (GSM 05.02).
Packet System Information 4
PSI 4 is optionally sent by the network on the PBCCH and PACCH giving information for directing the mobile station to make measurements on a list of serving cell PDCHs, during the
idle frame of those PDCHs. Special requirements for the transmission of this message apply
on the PBCCH (GSM 05.02).
Packet System Information 5
This optional message is sent by the network on the PBCCH or PACCH giving information for
measurement reporting and network -controlled cell reselection. If the entire information does
not fit into one message, the remaining information will be sent in the PSI 5 bis message. The
message is sent on PBCCH only if so indicated in PSI 1.
Packet System Information 5 bis
This optional message is sent by the network on the PBCCH or PACCH giving information for
measurement reporting and network -controlled c ell reselection. If the entire information does
not fit into one message, the remaining information will be sent in the PSI 5 bis message. The
message is sent on PBCCH only if so indicated in PSI 5.
Packet System Information 6
PSI 6 is sent by the network on the PBCCH giving information for scheduling of the PBCCH
messages.
In GSM networks, SI formatting is carried out by the TDPC, which sends them to the BTS to
be broadcast on the cells after having updated all SIs. In GPRS / GSM networks, PSI, with the
PCCCH active in the BSS area, must also be broadcast, again supported by the TDPC.
For this reason, the TDPC reads all parameters needed by SI 1 to SI 13 and by PSI 1 to PSI 6
from its own database. After having formatted all / only -modified PSI, the TDPC sends them to
the PCU which broadcasts the PSI on the air.
Paging Management
If in packet idle mode – for mobile stations supporting GPRS – the PCCCH is active in the serving cell,
the mobile station listens to the PBCCH and to the corresponding paging sub -channels. If the PCCCH
is not active in the considered cell, the mobile station listens to the BCCH and to the corresponding
paging sub-channels.
Paging sub-channels are, in any case, monitored according to the paging groups determined for the
mobile station in packet idle mode (defined in GSM 05.02) and its current DRX mode (defined in GSM

BR5.5 - SF10420 – V2.1

25/33

04.60 and GSM 05.02). Paging for GPRS is performed in the Routing Area (RA) instead of in the LA
as in standard GSM (A RA is defined for a GPRS cell and represents a cluster of cells; it is contained
in an LA). Moreover, if a mobile station is in transfer mode, it can be paged in the cell where it is
camping.
GPRS-Paging Using Paging Sub-channel on the CCCH
This type of paging is used to send paging information to mobile stations in idle mode and in
packet idle mode if the PCCCH is not active in the cell. Three types of paging messages can
be broadcast on this channel:
•

PAGING REQUEST TYPE 1

•
•

PAGING REQUEST TYPE 2
PAGING REQUEST TYPE 3

as described in GSM 04.07. Paging request messages can include more than one mobile station, as well as priority levels related to the mobile station identification.
For GPRS, just as for the well-known standard GSM service, a paging channel in combination
with DRX can also be organized. If a mobile station chooses to use the DRX mode (as indicated in Classmark), it indicates to the network to which additional paging groups it listens,
which allows acceptable access delay and/or acceptable battery consumption and/or the QoS
needed by the application.
A mobile station using DRX is only required to monitor the PCH blocks belonging to its paging
group in the same way as in GSM 05.02.
A mobile station not using DRX is required to monitor every PCH block on the same CCCH as
for DRX.
Paging reorganization is supported in the same way as for circuit-switched GSM.
The internal network message flow is as follows:
•

•
•

•

The SGSN, knowing how to use the DRX, sends a paging message to all PCUs supporting the proper RA. This message includes the information whethe r or not the DRX is used
and, through the SPLIT_PG_CYCLE parameter, if the enhanced DRX mechanism is
used.
The PCU forwards the PACKET PAGING REQUEST message combined with the er quested paging parameters over the internal interface to the BSC.
The BSC calculates the proper paging group and forwards, per LAPD connection, the
PACKET PAGING REQUEST messages to the paging queues inside the BTS. Additionally, the BSC evaluates all needed DRX parameters which have to be broadcast on the
BCCH.
The BTS queues all PACKET PAGING REQUEST messages and sends them, sorted by
first-in first-out, on the PCHs in the CCCH multiframe.

GPRS-Paging Using Paging Sub-channel on the PCCCH
This type of paging is used to send paging information to mobile stations in packet idle mode if
the PCCCH is active in the cell. The initiation procedure and paging request are specified in
GSM 04.60.
A mobile station using the DRX is required to monitor the PPCH. A mobile station not using
the DRX is required to monitor every PPCH block on the same PCCCH as for the DRX.
Paging reorganization may be supported in the same way as for circuit-switched GSM.
The internal network message flow is as follows:
•

•

The SGSN, knowing how to use the DRX, sends a paging message to all PCUs located in
the proper RA. This message includes the information whether or not the DRX is used
and, through the SPLIT_PG_CYCLE parameter, if the enhanced DRX mechanism is
used.
The PCU calculates the proper paging group and adds all PACKET PAGING REQUEST
messages on its paging gro up queues. Additionally, the PCU evaluates all needed DRX
parameters which have to be broadcast on the PBCCH.

BR5.5 - SF10420 – V2.1

26/33

•

The PCU includes the PACKET PAGING REQUEST messages into the RLC / MAC
blocks and schedules the messages into the PDCH multiframes which contain h
te
PCCCH. The RLC / MAC blocks are transferred via TRAU frames to the BTS, which
transmits the PACKET REQUEST message immediately.

GPRS-Paging Using Paging Sub-channel on the PACCH
This type of paging is used to send paging information to mobile stations in packet transfer
mode, if the PCCCH is active in the cell. The initiation procedure and paging request are
specified in GSM 04.60.
GPRS Mobile Classes and Crossed -paging Types
Three classes of GPRS mobile stations are defined:
Class A
Simultaneous and independent execution for both circuit-switched and GPRS operation is
possible. Therefore, the class A mobile station uses two independent receivers / transmitters. (The class A mobile station is a typical high-end MS.)
Class B
Simultaneous execution of circuit-switched and GPRS operation is possible, but the quality of service of a GPRS operation may decrease in the case of a pending or established
circuit-switched connection. The minimum requirement is sequential support of the services, whereas the mobile station must be able to monitor the CCCH. (The class B mobile
station is a typical all-purpose MS.)
Class C
Alternate use of circuit-switched and GPRS operation is possible. The mobile station supports either GPRS operation only or both circuit -switched and GPRS operation. In the latter case, only one service at a time is available by default or manual pre-selection. That
means, a class C mobile station is either a GPRS or a non -GPRS mobile station. (The
class C mobile station with exclusive GPRS capability is a typical low-cost MS for GPRS
supporting a particular application.)
Therefore, the introduction of GPRS in the GSM networks introduces a new concept in the
management of paging message transmission, because a class A or B mobile station can
be paged during data packet transfer for a traditional speech call or vice versa.
In the case of a class A mobile station, no problems will arise because the two calls can
be managed independently. In the case of a class C mobile station, no problems are expected because it supports only one mode at a time.
In the case of a class B mobile station, it is necessary to perform different actions than the
ones carried out till now.
GPRS-Paging to Class B Mobile Stations During Speech Calls
If, in a BSS, a paging message arrives from an SGSN node for a class B mobile station busy
in a circuit-switched (originating / terminated) call, it is necessary that the PCU asks the TDPC
to perform paging to the addressed mobile station. Since the mobile station’s position in the
RA is known, the TDPC scans the list of the cells belonging to the addressed RA and sends a
paging message on the PCH channel to each of them.
The addressed mobile station reads the paging message and can decide whether to
• stop the conversation and start normal procedures for this case,
•

ignore to read the paging and continue the current speech call, or

•

put on hold the current conversation, switch to GPRS as long as data are transferred and,
afterwards, switch back to non -GPRS mode to retrieve the first call.

Paging a Class B Mobile Station During Data Transfer Mode
If, in a BSS, a paging message arrives from an SGSN node for a class B mobile station busy
in a circuit-switched (originating / terminated) call, it is necessary that the PCU asks the TDPC
to perform paging to the addressed mobile station. Since the mobile station’s position in the

BR5.5 - SF10420 – V2.1

27/33

RA is known, the TDPC scans the list of the cells belonging to the addressed RA and sends a
paging message on the PCH channel to each of them.
The addressed mobile station reads the paging message and can decide whether to
•

stop the conversation and start normal procedures for this case,

•
•

ignore to read the paging and continue the current speech call, or
put on hold the current conversation, switch to GPRS as long as data a re transferred and,
afterwards, switch back to non -GPRS mode to retrieve the first call.

Paging a Class B Mobile Station in a GPRS -supporting Cell in Standby State
If, in a BSS, a paging arrives from the MSC for a class B mobile station while it camps in a cell
supporting GPRS and is in standby state – so its LA is known –, the TDPC needs to scan the
list of the cells belonging to the addressed LA. Then, for each cell belonging to the addressed
area and in which a PCCCH is not allocated, the TDPC sends a paging message on the PCH,
while for each cell in the addressed area and in which a PCCCH is allocated, it asks the PCU
to perform a paging. The PCU then broadcasts a paging message on the PPCH for each of
the indicated cells. The addressed mobile station reads on the paging channel, listens to the
paging message sent, and can switch back to non -GPRS mode to begin normal procedures to
obtain a circuit-switched connection for the time required by the conversation and, finally,
switches back to GPRS if the cell where it is located again supports this service.
Uplink Access to the Network by Mobile Stations
Access to the GPRS network uses a slotted-aloha protocol and is performed by sending a traditional 8 bit access burst type (GSM 05.01). In line with the ETSI specifications, a new enhanced
access burst type with 11 information bits can be sent by the mobile station to attempt access to
the GPRS network. It depends on the network whether this one or the other message is used: the
capability of the network to er ceive messages of 8 or 11 bit length is broadcast by the
ACCESS_BURST_ TYPE SI parameter that indicates the permitted access length; the 8 bit length is
called one phase access method and the 11 bit length two phase access method.
•

In the one phase access, the PACKET C HANNEL REQUEST is answered by the network with
the PACKET I MMEDIATE ASSIGNMENT reserving the resources on PDCH(s) for uplink transfer
of a number of radio blocks. This reservation is carried out according to the information on
the requested res ources that is comprised in the PACKET CHANNEL REQUEST. On the
RACH, there is only one cause value available for denoting GPRS and the network can
only assign uplink resources on 1 or 2 PDCH(s) or two different priorities. One phase access on PCCCH and CCCH is used in the case of paging response, cell update, and MM
procedure, as well as in all cases where the MS need not send more information than the
MS class and priority.

•

In the two phase access, the PACKET CHANNEL REQUEST is answered with the PACKET
I MMEDIATE ASSIGNMENT which reserves the uplink resources for transmitting the PACKET
RESOURCE R EQUEST. The PACKET R ESOURCE REQUEST message carries the complete description of the requested resources for the uplink transfer. This message is already sent
on the assigned PDCH(s) in the PACKET ASSOCIATED C ONTROL C HANNEL (PACCH). This
channel is a transparent link between the mobile station and the PCU. If the number of
PDCH(s) in the cell needs to be increased, the PCU requests additional channels from the
BSC. Afterwards, the PCU responds on the PACCH with the PACKET R ESOURCE
ASSIGNMENT reserving resources for the uplink transfer. Two phase access on PCCCH
and CCCH is used in the case of data transfer in unacknowledged mode as well as in all
cases described for one phase access when additional information needs to be carried in
the access phase.
The mobile station is always able to override the one phase access by sending the PACKET
RESOURCE REQUEST on the assigned resource to initiate the two phase access.
The PACKET I MMEDIATE ASSIGNMENT message includes the Timing Advance (TA) and Power Control (PC) information.
If there is no response to the PACKET C HANNEL R EQUEST within a pre-defined time period, the mobile station initiates a retry after a random back-off algorithm as defined in the existing GSM.

BR5.5 - SF10420 – V2.1

28/33

The mobile station request may also be rejected via a PACKET ACCESS REJECT message on the
same PCCCH on which the channel requests were received.
Currently, more PACKET C HANNEL REQUESTS can be received than can be served within a certain
time limit. To handle this, the PCU sends a PACKET QUEUING NOTIFICATION to the mobile station.
The message is transferred as an internal message from the PCU to the BSC and per LAPD to
the BTS, and is then sent on the AGCH to the mobile station. The notification includes information
that the PACKET C HANNEL REQUEST message has been received correctly and a PACKET
IMMEDIATE ASSIGNMENT may be sent later. The PACKET QUEUING NOTIFICATION can be concatenated with the PACKET I MMEDIATE ASSIGNMENT to another mobile station in the same downlink radio block. If the TA information becomes inaccurate for a mobile station, the PACKET POLLING can
be used to estimate the new TA before issuing the PACKET I MMEDIATE ASSIGNMENT. Therefore, the
PCU decides to send a message to the BSC and per LAPD to the BTS, and then sends the
PACKET POLLING on the PCH. The mobile station responds with a PACKET POLLING R ESPONSE
message, which is encoded in an access burst.
When the mobile station initiates the transmission of a C HANNEL R EQUEST message, it ignores the
PAGING REQUEST messages. A mobile station belonging to class A or B continues to monitor its
paging subchannel on the CCCH – if it is a GPRS mobile station class B, then it can decide
whether or not to abort the ac cess procedure.
Uplink Access on the CCCH
A mobile station initiates a packet transfer by sending a PACKET CHANNEL R EQUEST on the RACH
(8 information bits). The BTS decodes the burst and sends the contents combined with meas urement information per LAPD link to the BSC. The TDPC, as the manager of the radio re sources, activates a PDTCH in the cell, if not active, and sends its identity to the PCU (including
the cell ID). In case the GPRS resource is not enough, the PCU will ask for an additional PDTCH.
After successful channel activation at the BTS, the BSC switches a connection (via SN -16) between the PCU and the PDTCH(s) and delivers the PDTCH ID(s) to the PCU, which reserve the
TFI and one or more USFs. The established PDCH(s), the TFI and the USF(s) are then included
in the PACKET UPLINK ASSIGNMENT to the mobile station via LAPD on the AGCH.
Uplink Access on the PCCCH
A mobile station initiates a packet transfer by sending a PACKET CHANNEL REQUEST on the
PRACH. The PRACH can be more or less dynamically a llocated on block basis (four consecutive
bursts) by the PCU. Therefore, the PCU needs to inform the receiver of the BTS that uplink capacity is provided on the uplink for sending PACKET C HANNEL R EQUESTS.
On the PRACH, more efficient new 11 information bit access bursts can be used. Therefore, the
BTS needs an O&M command with the information which kinds of access burst have to be decoded on all PDCH(s) in the cell. The mobile station gets the information on the network capability for receiving the new access burst type by a specific broadcast message. The extra three bits
enhance the information contents to make one phase access more probable. The BTS decodes
the burst and sends the contents together with the measurement information per TRAU frame to
the PCU.
If additional PDCH(s) need to be allocated in the cell, the PCU requests channel(s) from the BSC,
which manages the common pool of radio resources between GPRS and circuit-switched connections. Therefore a priority and preemption mechanism is necessary. A circuit-switched call is
given priority over a GPRS call. Whenever a new GPRS call is set up and no timeslot is available
on the BCCH, an intracell handover is made for the circuit-switched call. If no space is available,
a directed retry is carried out. If this does not work either, the existing GPRS calls are downgraded (e.g. 2 timeslots on one timeslot). If this fails, too, the call is rejected.
For a new circuit-switched call, an intracell handover is first carried out on existing circuitswitched calls. If this does not work, a cell selection/re -selection is carried out for the existing
GPRS calls. Then, the other GPRS calls are downgraded as described above. As last option, a
GPRS call may be disconnected (circuit-switched prioritized over GPRS).
Afterwards, the PCU assigns a PDCH(s) to the mobile station. This assignment is an RLC block
which is transferred with the help of a TRAU frame from the PCU to the BTS and is then sent on
the PAGCH.

BR5.5 - SF10420 – V2.1

29/33

As described in the above section, two different access methods are possible. But since more
code points are available on the PRACH (11 bit AB is used) there is less need for the two phase
access.
If there is no response to the PACKET C HANNEL REQUEST within a pre-defined period, the mobile
station initiates a retry after a random back-off time defined by the GPRS specific back -off algorithm.
The mobile station request may also be rejected via a PACKET ACCESS REJECT message on the
same PCCCH on which the channel request was received.
More PACKET CHANNEL REQUESTS can be received than can be served within a certain time limit.
To handle this, the PCU sends an RLC block called PACKET Q UEUING NOTIFICATION via TRAU
frame to the BTS and then on the PAGCH to the mobile station. The notification includes the information that the PACKET CHANNEL R EQUEST message has been received correctly and a PACKET
IMMEDIATE ASSIGNMENT may be sent later. If the TA information becomes inaccurate for the mobile
station, an RLC block called PACKET POLLING is sent on the PPCH to the mobile stat ion. The mobile station responds with a PACKET POLLING R ESPONSE message, which is encoded in an access
burst.

BR5.5 - SF10420 – V2.1

30/33

Man Machine Interface
The operator administrates and maintains the new GPRS functionality via the new Radio Com mander. The individual network elements of the BSS will be accessed from the Radio Com mander (OMC -B) for management purposes via the existing O interface and the BSCs.
The O&M functions for GPRS will be available from the OMC -B as well as from the LMT.
The following new objects have been modeled and can be accessed via O&M commands:
• FRL (Frame Relay Link) object
This is a functional object representing the physical link connection on the Gb interface.
• NSVC (Network Service Virtual Connection) object
This is a functional object representing the end-to-end communication between the BSS
and the SGSN.
• PCMG (PCM line on Gb Interface) object
This is a functional object representing the direct physical connection between the BSC
and the SGSN.
•

PCU (Packet Control Unit) object
This is a functional object representing a packet control unit designed to implement GPRS
in SBS.

•

PPCU (Peripheral Packet Control Unit) object
This is an equipment object representing the new card designed to implement the PCU
unit. The PPCU objects are provided in redundant configuration (active / cold standby) for
each PCU
• PTPPKF (Point-to-point Packet Function) object.
This is a functional object representing the presence of GPRS in a specific cell. The state
of this object allows / denies the service in the cell. The PTPPKF object number corresponds to the BTS number, thus, e.g. PTPPKF-0 is the virtual circuit connected to BTS-0.
The Radio Commander (Operation and Maintenance Center for the BSS OMC -B) provides O&M
functions to manage GPRS features:
•
•

Gb and Um interfac e configurations
PCU Fault and Maintenance functions, and

• Performance Measurements.
These O&M functions for GPRS will be available from the OMC -B (via the existing O interface
and the BSCs) as well as from the LMT.
With the introduction of GPRS, the netwo rk provider is able to:
•

Verify the correct dimensioning of the GPRS network (for short - and long-term planning)

•

Check the parameters of the GPRS network planning

•
•

Provide fine-tuning of the GPRS network configuration parameters
Determine the QoS and performance of the GPRS network

Objects
The following new objects have been modeled and can be accessed via O&M commands:
• FRL (Frame Relay Link) object
This is a functional object representing the physical link connection on the Gb interface.
• NSVC (Network Servi ce Virtual Connection) object
This is a functional object representing the end-to-end communication between the BSS and
the SGSN.
•

PCMG (PCM line on the Gb interface) object

This is a functional object representing the direct physical connection between the BSC and
the SGSN. This line carries 32 timeslots of 64 Kbps that can handle a maximum of 31 FRLs.
• PCU (Packet Control Unit) object
This is a functional object representing a PCU designed to implement GPRS in the SBS.

BR5.5 - SF10420 – V2.1

31/33

Each PCU is able to handle a maximum of 64 PDTs on the Abis interface and a maximum of
16 FRLs on the Gb interface. The number of PDTs and the number of FRLs can be selected
when creating the PCU object.
One level of addressing is provided for the PCU: the PCU number (range: 0 to 1). The PCU
can be considered "Enabled" when one super-ordinate PPCU is providing service.
All attributes that are common for all subordinated objects (FRL, NSVC, PTPPKF) are inserted
on the PCU object in order to avoid duplicate data and keep it easier to configure GPRS on
the SBS. According to this idea, both PPCU cards are implicitly handled, from the configuration point of view, acting on the PCU functional object, i.e. the creation of the PCU implies the
creation of the pair of cards and the deletion of the PCU implies the deletion of both cards.
This way, it is possible to manage all the hierarchy acting on the single PCU object instead of
two cards.
• PPCU (Peripheral Packet Control Unit) object
This is an equipment object representing the new card designed to implement the PCU unit.
The PPCU objects are provided in redundant configuration (active / cold standby) for each
PCU. Each card is able to handle a maximum of 2 Mbps of data transmission split in two
parts: Abis and Gb interface. The throughput in both interfaces can be selected via configuration commands on the PCU objects.
Two levels of addressing are provided for the PPCU. The first is the PCU number (range: 0 to
1), the second one is the PPCU copy (range: 0 to 1). The PPCU can be considered " Enabled"
only after the software has been loaded correctly.
The PPCU object cannot be created or deleted directly by the operator, but it is implicitly
equipped during the creation of the PCU object and deleted with the PCU deletion.
• PTPPKF (Point-to-point Packet Function) object
This is a functional object representing the presence of GPRS in a specific cell. The state of
this object allows or refuses the service in the cell. The state of the PTPPKF object can be affected by a BTS state change, a specific command on the object, or by a state change on the
NSVC object.
Commands
Table 3 gives an overview of all commands allowed ("Yes") or not allowed ("No") for each new
GPRS-specific object introduced in the SBS system.
Configuration command
Object CREATE DELETE
SET
GET
FRL

Yes

Yes

NSVC

Yes

PCMG

Yes

PCU

State command
LOCK

UNLOCK SWITCH

GET

Yes

Yes

Yes

Yes

No

Yes

Yes

No

Yes

Yes

Yes

No

Yes

Yes

Yes

Yes

Yes

Yes

No

Yes

Yes

Yes

Yes

Yes

Yes

Yes

No

Yes

PPCU

No

No

No

Yes

Yes

Yes

Yes

Yes

PTPPKF

Yes

Yes

Yes

Yes

Yes

Yes

No

Yes

Table 3: O&M commands for GPRS specific objects

BR5.5 - SF10420 – V2.1

32/33

Interworking / Dependencies
With the features of the GPRS, there is interworking with:
• The hardware for GSN
• Point-to-point service (GPRS is the basis for point -to-point)
Restraint of O&M functions of the current release (BR5.5) with concern of the following features
that are available in complete O&M functions within the next release (BR6.0):
•
•

Packet transfer on non BCCH TRX
Automatic horizontal allocation

Prerequisites
The prerequisites required for BR5.5 have been limited to the following two items:
• PCU, added to the BSC,
• CCU, added to the BTS (by software download).
The following is required:
•

Hardware for PCU

•

BSC must support the Gb interface according the respective ETSI standard

Recommendations
Relevant GSM recommendations
GSM No. (ETSI TS)

02.60,
03.60, 03.64,
04.07, 04.60,
05.01, 05.02, 05.08

Remarks
BR5.5 features the following restrictions with respect to the standard:
• No support of BTS downlink power control at carriers used for GPRS channels
•

Restraint of O&M functions of packet transfer on non BCCH TRX

•

Restraint of O&M functions of automatic horizontal allocation

•
•

GPRS measurement results are reported "by file"
Abis transpo rt mechanism is a proprietary PCU frame format

•

In the PCU, no segmentation and re -assembling is foreseen and there is no flow control
mechanism for uplink queues.

•
•

The CS3 and CS4 coding schemes are not used
No upgrade from CS1 to CS2 or vice versa

•

"Best Effort" is the only QoS to be supported

•
•

Only point-to-point service possible (without connection according to IP V4)
MS controlled cell re-selection only (no network driven cell re-selection)

•

GPRS is supported in the “inner area” of an extended cell

Summary
The new GPRS enables customers to develop new business opportunities as evolutionary steps
towards UMTS. An investment in Siemens GPRS hardware today, is a step in the right direction
towards the third generation of mobile communications and new UMTS networks.

BR5.5 - SF10420 – V2.1

33/33