Gprs Architecture

GPRSSYS
GPRS Architecture: Interfaces
and Protocols
Training Document





6-64442
Issue 4.0
©Nokia Oyj

1 (55)

GPRS Architecture: Interfaces and Protocols

The information in this document is subject to change without notice and describes only the
product defined in the introduction of this documentation. This document is intended for the
use of Nokia's customers only for the purposes of the agreement under which the document is
submitted, and no part of it may be reproduced or transmitted in any form or means without
the prior written permission of Nokia. The document has been prepared to be used by
professional and properly trained personnel, and the customer assumes full responsibility
when using it. Nokia welcomes customer comments as part of the process of continuous
development and improvement of the documentation.
The information or statements given in this document concerning the suitability, capacity, or
performance of the mentioned hardware or software products cannot be considered binding
but shall be defined in the agreement made between Nokia and the customer. However,
Nokia has made all reasonable efforts to ensure that the instructions contained in the
document are adequate and free of material errors and omissions. Nokia will, if necessary,
explain issues which may not be covered by the document.
Nokia's liability for any errors in the document is limited to the documentary correction of
errors. NOKIA WILL NOT BE RESPONSIBLE IN ANY EVENT FOR ERRORS IN THIS
DOCUMENT OR FOR ANY DAMAGES, INCIDENTAL OR CONSEQUENTIAL (INCLUDING
MONETARY LOSSES), that might arise from the use of this document or the information in it.
This document and the product it describes are considered protected by copyright according
to the applicable laws.
NOKIA logo is a registered trademark of Nokia Oyj.
Other product names mentioned in this document may be trademarks of their respective
companies, and they are mentioned for identification purposes only.
Copyright ©Nokia Oyj 2004. All rights reserved.

2 (55) ©Nokia Oyj

6-64442
Issue 4.0

Contents

Contents
1 Module objectives ..................................................................................4
2 Introduction ............................................................................................5
3 Network elements...................................................................................7
3.1 Packet Control Unit (PCU) .......................................................................8
3.2 Channel Codec Unit (CCU)......................................................................8
3.3 Serving GPRS Support Node (SGSN) .....................................................8
3.4 Gateway GPRS Support Node (GGSN)...................................................9
3.5 GPRS MS...............................................................................................10
3.6 Domain Name Servers...........................................................................12
3.7 Firewalls.................................................................................................12
3.8 Border Gateway.....................................................................................13
3.9 Charging Gateway..................................................................................13
4 GPRS interfaces ...................................................................................14
5 Transfer of packets between GSNs ....................................................16
6 Nokia GPRS solution ...........................................................................19
6.1 Nokia GPRS functionality.......................................................................19
6.2 Nokia Base Station Subsystem (BSS) ...................................................22
6.3 Nokia Core Network Subsystem (CNS) .................................................25
6.4 Nokia Network Management System (NMS)..........................................32
7 Key points .............................................................................................35
7.1 GPRS architecture: key points...............................................................35
7.2 Nokia GPRS solution: key points ...........................................................36
8 Review questions .................................................................................38
9 Appendix – GPRS transmission plane protocols..............................40




6-64442
Issue 4.0
©Nokia Oyj

3 (55)


GPRS Architecture: Interfaces and Protocols

1 Module objectives
At the end of the module, the participant will be able to:
• Name the GPRS specific network elements and their most important
functions
• Name and explain five important open interfaces in the GPRS network
• Explain the principle of the GPRS Tunnelling Protocol (GTP)
without using any references.
4 (55) ©Nokia Oyj

6-64442
Issue 4.0

Introduction

2 Introduction
GPRS provides mobile users access to value-added WAP services and different
external packet switched networks. These networks can be, for example, the
Internet or corporate intranets. The GSM-BSS provides the radio interface, and
the GPRS core network handles mobility and access to external packet networks
and services. This is shown in Figure 1.
External
Packet
Networks
Value-Added
Services
(WAP)
Radio Resource
& Radio Link
Management
NSS
Switching/routing,
mobility & connection
management
GPRS (ps)
Core Network
GSM (cs)
Core Network
BSS

Figure 1. GPRS access to packet switched networks
The GPRS network acts in parallel with the GSM network, providing packet
switched connections to the external networks. The requirements of a GPRS
network are the following:
• The GPRS network must use as much of the existing GSM infrastructure
with the smallest number of modifications to it.
• Since a GPRS user may be on more than one data session, GPRS should
be able to support one or more packet switched connections.
• To support the budgets of various GPRS users, it must be able to support
different Quality of Service (QoS) subscriptions of the user.
• The GPRS network architecture has to be compatible with future 3rd and
4th generation mobile communication systems.
• It should be able to support both point-to-point and point-to-multipoint
data connections.
• It should provide secure access to external networks.
6-64442
Issue 4.0
©Nokia Oyj

5 (55)


GPRS Architecture: Interfaces and Protocols

A GPRS network must provide all of the functionality of a GSM network for
packet switched networks and more. The GPRS is expected to perform the
functions of a traditional mobile communication network and a traditional
packet switched computer network. These functions are itemised below:
• Capability to separate circuit switched and packet switched traffic from
mobile station (MS)
• Radio resource management, that is, allocation of radio resources to
GPRS subscribers across the air interface
• Interfaces to Internet, intranets, Public Data Networks (PDN), and other
Public Land Mobile Networks (PLMN)
• Authenticate subscriber requests for packet switched resources
• Encrypt data transmitted on the air interface for security purposes
• Data compression for data transmitted over the air interface
• Interact with databases (HLR/VLR) containing subscriber information
such as IMSI, security data, and subscription information
• Mobility management as in GSM
• Location management as in GSM
• Handover as a GPRS subscriber moves within a coverage area
• Power control to minimise the transmitted power by the user
• Network management that facilitates GPRS network management
• Generation and collection of network performance statistics
• Generation and collection of charging or billing information
• Signalling links between the GPRS network elements
• Routing of packets to appropriate destination
• Protocol conversion between networks that may use different protocols
• Buffering of data at GPRS nodes
• Allocation of static or dynamic address for packets originating from MS
• Protection of the GPRS network from security threats
• Capability to monitor target subscriber by law enforcement agencies
• Translation between logical names and IP addresses using Domain Name
System (DNS)
• Facilitation of roaming subscribers so that they can connect to home
networks
• Delivery of SMS messages through the GPRS network
• Redundancy mechanisms if one or more network elements were to fail
• Translation between private and public addresses using NAT and NAPT
• Detection of faulty or stolen GPRS handsets
6 (55) ©Nokia Oyj

6-64442
Issue 4.0

Network elements

3 Network elements
Figure 2 shows the architecture of a GPRS network. The GPRS system brings
some new network elements to an existing GSM network. These elements are:
• Packet Control Unit (PCU)
• Serving GPRS Support Node (SGSN): the MSC of the GPRS network
• Gateway GPRS Support Node (GGSN): gateway to external networks
• Border Gateway (BG): a gateway to other PLMN
• Intra-PLMN backbone: an IP based network inter-connecting all the
GPRS elements
• Charging Gateway (CG)
• Legal Interception Gateway (LIG)
• Domain Name System (DNS)
• Firewalls: used wherever a connection to an external network is required.
Not all of the network elements are compulsory for every GPRS network.
BSC
BTS
BTS
TRAU
BSC
BTS
BTS
TRAU
BSS
BSS
NSS
MSC/VLR GMSC
HLR EIR
AC
PSTN/
ISDN
MS
GPRS
MS
corp.
network
WAP
PDN
SGSN
IP-
backbone
PCU
PCU
CCU
CCU
CCU
CCU
CG
Billing
Centre
BG
Inter-PLMN
Network
LIG
LEA
DNS
GGSN
F
W

Figure 2. GPRS architecture
6-64442
Issue 4.0
©Nokia Oyj

7 (55)


GPRS Architecture: Interfaces and Protocols

3.1 Packet Control Unit (PCU)
The PCU separates the circuit switched and packet switched traffic from the
user and sends them to the GSM and GPRS networks respectively. It also
performs most of the radio resource management functions of the GPRS
network. The PCU can be either located in the BTS, BSC, or some other point
between the MS and the MSC. There will be at least one PCU that serves a cell
in which GPRS services will be available. Frame Relay technology is being
used at present to interconnect the PCU to the GPRS core.
PCU
Decides dynamically, which
resources are allocated to cs and
ps usage, based on
• load situation
• priority, and
• operator set rules
BSC
cs Radio Resource
Management
PCU
ps Radio Resource
Management
cs resources ps resources

Figure 3. PCU – its position within the BSS
3.2 Channel Codec Unit (CCU)
The CCU is realised in the BTS to perform the Channel Coding (including the
coding scheme algorithms), power control and timing advance procedures.
3.3 Serving GPRS Support Node (SGSN)
The SGSN is the most important element of the GPRS network. The SGSN of
the GPRS network is equivalent to the MSC of the GSM network. There must at
least one SGSN in a GPRS network. There is a coverage area associated with a
SGSN. As the network expands and the number of subscribers increases, there
may be more than one SGSN in a network. The SGSN has the following
functions:
8 (55) ©Nokia Oyj

6-64442
Issue 4.0

Network elements

• Protocol conversion (for example IP to FR)
• Ciphering of GPRS data between the MS and SGSN
• Data compression is used to minimise the size of transmitted data units
• Authentication of GPRS users
• Mobility management as the subscriber moves from one area to another,
and possibly one SGSN to another
• Routing of data to the relevant GGSN when a connection to an external
network is required
• Interaction with the NSS (that is, MSC/VLR, HLR, EIR) via the SS7
network in order to retrieve subscription information
• Collection of charging data pertaining to the use of GPRS users
• Traffic statistics collections for network management purposes.
3.4 Gateway GPRS Support Node (GGSN)
The GGSN is the gateway to external networks. Every connection to a fixed
external data network has to go through a GGSN. The GGSN acts as the anchor
point in a GPRS data connection even when the subscriber moves to another
SGSN during roaming. The GGSN may accept connection request from SGSN
that is in another PLMN. Hence, the concept of coverage area does not apply to
GGSN. There are usually two or more GGSNs in a network for redundancy
purposes, and they back up each other up in case of failure. The functions of a
GGSN are given below:
• Routing mobile-destined packets coming from external networks to the
relevant SGSN
• Routing packets originating from a mobile to the correct external network
• Interfaces to external IP networks and deals with security issues
• Collects charging data and traffic statistics
• Allocates dynamic or static IP addresses to mobiles either by itself or
with the help of a DHCP or a RADIUS server
• Involved in the establishment of tunnels with the SGSN and with other
external networks and VPN.
From the external network's point of view, the GGSN is simply a router to an IP
sub-network. This is shown below. When the GGSN receives data addressed to
a specific user in the mobile network, it first checks if the address is active. If it
is, the GGSN forwards the data to the SGSN serving the mobile. If the address
is inactive, the data is discarded. The GGSN also routes mobile originated
packets to the correct external network.
6-64442
Issue 4.0
©Nokia Oyj

9 (55)


GPRS Architecture: Interfaces and Protocols

Host
155.222.33.55
Corporate subnetwork
131.44.15.xxx
GPRS subnetwork
155.222.33.xxx
Host
131.44.15.3
Router
Router
LAN
Internet

Figure 4. GPRS network as seen by another data network
3.5 GPRS MS
Different GPRS MS classes were introduced to cope with the different needs of
future subscribers. The mobiles differ in their capabilities.
class
A
simultaneous
•attach
•activation
•monitor
no simultaneous
traffic
simultaneous
•attach
•activation
•monitor
•invocation
•traffic
of GSM and GPRS
pure GPRS or
alternative use of
GSM and GPRS only
class
B
class
C

Figure 5. GPRS network as seen by another data network
10 (55) ©Nokia Oyj

6-64442
Issue 4.0

Network elements


Three GPRS MS classes were defined:
• Class A:
With a class A mobile GSM circuit switched services and GSM GPRS
services can be simultaneously activated. A subscriber can get data
from an active GPRS link while simultaneously making a phone call. A
class A mobile allows also a simultaneous attach, activation and
monitor of the classical GSM and GPRS services.
• Class B:
A class B mobile allows a simultaneous attach, activation and monitor
of the circuit switched GSM and GPRS services. It does not allow a
simultaneous transmission of user data on GSM and GPRS. For
instance, a subscriber has established a GPRS data connection and
receives data packets. A mobile terminating GSM circuit switched call
is indicated. The subscriber accepts the call. While he is making the
voice call, the GPRS virtual connection is “held or busy”, but no packet
data transfer is possible. Having terminated the voice call, packet data
can again be transmitted via the still existing GPRS virtual connection.
• Class C:
A class C mobile is either a pure GPRS MS or it supports both GSM
circuit switched services and GPRS. If it supports both then it can be
used only in one of the two modes. If a subscriber switches his mobile
into GPRS mode, he can originate or terminate GPRS calls, but he can
no longer originate or terminate GSM circuit switched calls.
In GPRS and HSCSD, increased data rates can be achieved by channel
bundling. Channel bundling is the allocation of several timeslots to a MS. In
other words, the mobile stations have a multislot capability. In the specification
05.02, the individual GSM multislot MS classes are specified.

Maximum number of
slots
Minimum number of
slots
Type Multislot
class
Rx Tx Sum Tta Ttb Tra Trb
1 1 1 2 3 2 4 2 1
2 2 1 3 3 2 3 1 1
3 2 2 3 3 2 3 1 1
4 3 1 4 3 1 3 1 1
5 2 2 4 3 1 3 1 1
6 3 2 4 3 1 3 1 1
7 3 3 4 3 1 3 1 1
8 4 1 5 3 1 2 1 1
9 3 2 5 3 1 2 1 1
10 4 2 5 3 1 2 1 1
11 4 3 5 3 1 2 1 1
12 4 4 5 2 1 2 1 1
13 3 3 NA NA a) 3 a) 2
6-64442
Issue 4.0
©Nokia Oyj

11 (55)


GPRS Architecture: Interfaces and Protocols

14 4 4 NA NA a) 3 a) 2
15 5 5 NA NA a) 3 a) 2
16 6 6 NA NA a) 2 a) 2
17 7 7 NA NA a) 1 a) 2
18 8 8 NA NA 0 0 0 2
19 6 2 NA 3 b) 2 c) 1
19 6 3 NA 3 b) 2 c) 1
21 6 4 NA 3 b) 2 c) 1
22 6 4 NA 2 b) 2 c) 1
23 6 6 NA 2 b) 2 c) 1
24 8 2 NA 3 b) 2 c) 1
25 8 3 NA 3 b) 2 c) 1
26 8 4 NA 3 b) 2 c) 1
27 8 4 NA 2 b) 2 c) 1
28 8 6 NA 2 b) 2 c) 1
29 8 8 NA 2 b) 2 c) 1
a) =1 with frequency hopping
=0 without frequency hopping.
b) =1 with frequency hopping or change from Rx to Tx.
=0 without frequency hopping and no change from Rx to Tx.
c) =1 with frequency hopping or change from Tx to Rx.
=0 without frequency hopping and no change from Tx to Rx.
3.6 Domain Name Servers
These devices convert IP names into IP addresses, for example,
server.nokia.com to 133.44.15.5. There is a primary DNS server and a
secondary DNS server. Details of DNS were described in Introduction to
TCP/IP module, and information is also found in the IP CORE Course.
In the specifications, the DNS functionality is included in the SGSN. However,
the main vendors have chosen to separate the DNS functions from the SGSN.
3.7 Firewalls
A firewall protects an IP network against external attack (for example, hackers
from the mobile users or from the Internet). In the case of GPRS, the firewall
might be configured to reject all packets that are not part of a GPRS subscriber-
initiated connection. The firewall can also include NAT (Network Address
Translation), see theIntroduction to TCP/IP module.
12 (55) ©Nokia Oyj

6-64442
Issue 4.0

Network elements

In the specifications for GPRS, the firewalls are not included. It is however
included here due to the fact that operators usually need to implement firewalls
in their GPRS network (for security reasons).

3.8 Border Gateway
The Border Gateway (BG) is a router that can provide a direct GPRS tunnel
between different operators' GPRS networks. This is referred to as an inter-
PLMN data network. It is more secure to transfer data between two operators'
PLMN networks through a direct connection rather than via the public Internet.
The Border Gateway will commence operation once the GPRS roaming
agreements between various operators have been signed. It will essentially
allow a roaming subscriber to connect to company intranet through the Home
GGSN via the visiting PLMN network.
3.9 Charging Gateway
GPRS users have to be charged for the use of the network. In a GSM network,
charging is based on the destination, duration, and time of call. However, GPRS
offers connectionless service to users, so it not possible to charge subscribers on
the connection duration. Charging has to be based on the volume, destination,
QoS, and other parameters of a connectionless data transfer. These GPRS
charging data are generated by all the SGSNs and GGSNs in the network. This
data is referred to as Charging Data Records or CDRs. One data session may
generate a number of CDRs, so these need to be collected and processed. The
Charging Gateway (CG) collects all of these records, sorts them, processes it,
and passes it on to the Billing System. Here the GPRS subscriber is billed for
the data transaction. All CDRs contain unique subscriber and connection
identifiers to distinguish it. A protocol called GTP' (pronounced GTP prime) is
used for the transfer of data records between GSNs and the Charging Gateway.

6-64442
Issue 4.0
©Nokia Oyj

13 (55)


GPRS Architecture: Interfaces and Protocols

4 GPRS interfaces
The GPRS system introduces new interfaces to the GSM network. Figure 6
illustrates the logical architecture with the interfaces and reference points of the
combined GSM/GPRS network.
HLR
Air (Um)
Gb
Gs
Gr
Gf
Gn
Gp
Inter-PLMN
GPRS
backbone
Gp
External
packet
network
Gi
SGSN
EIR
BSC
MSC/
VLR
SMS-
GMSC
Gd
GGSN
BG
Signalling and data
Signalling
Ga
Ga
CG
SGSN
Gn
Gc

Figure 6. GPRS interfaces
Connections from the GPRS system to the NSS part of the GSM network are
implemented through the SS7 network. The GPRS element interfacing with the
NSS is SGSN. The important interfaces to the NSS are the SGSN-HLR (Gr),
SGSN-EIR (Gf), and SGSN-MSC/VLR (Gs). The other interfaces are
implemented through the intra-PLMN backbone network (Gn), the inter-PLMN
backbone network (Gp), or the external networks (Gi).
The interfaces used by the GPRS system are described below:
• Um between an MS and the GPRS fixed network part. The Um is the
access interface the MS uses to access the GPRS network. The radio
interface to the BTS is the same interface used by the existing GSM
network with some GPRS specific changes.
• Gb between a SGSN and a BSS. The Gb interface carries the GPRS
traffic and signalling between the GSM radio network (BSS) and the
GPRS network. Frame Relay based network services is used for this
interface.
14 (55) ©Nokia Oyj

6-64442
Issue 4.0

GPRS interfaces

• Gn between two GSNs within the same PLMN. The Gn provides a data
and signalling interface in the Intra-PLMN backbone. The GPRS
Tunnelling Protocol (GTP) is used in the Gn (and in the Gp) interface
over the IP based backbone network.
• Gp between two GSNs in various PLMNs. The Gp interface provides the
same functionality as the Gn interface, but it also provides, together with
the BG and the Firewall, all the functions needed for inter-PLMN
networking, that is, security, routing, etc.
• Gr between an SGSN and the HLR. The Gr gives the SGSN access to
subscriber information in the HLR. The HLR can be located in a different
PLMN than the SGSN (MAP).
• Ga between the GSNs and the CG inside the same PLMN. The Ga
provides a data and signalling interface. This interface is used for sending
the charging data records generated by GSNs to the CG. The protocol
used is GTP', an enhanced version of GTP.
• Gs between a SGSN and a MSC. The SGSN can send location data to the
MSC or receive paging requests from the MSC via this optional interface.
The Gs interface will greatly improve the effectiveness of the radio and
network resources in the combined GSM/GPRS network. This interface
uses BSSAP+protocol.
• Gd between the SMS-GMSC and an SGSN, and between SMS-IWMSC
and an SGSN. The Gd interface is available for more efficient use of the
SMS services (MAP).
• Gf between an SGSN and the EIR. The Gf gives the SGSN access to
GPRS user equipment information. The EIR maintains three different
lists of mobile equipment: black list for stolen mobiles, grey list for
mobiles under observation and white list for other mobiles (MAP).
• Gc between the GGSN and the HLR. The GGSN may request the
location of an MS via this optional interface. The interface can be used if
the GGSN needs to forward packets to an MS that is not active.

There are two different reference points in the GPRS network. The Gi is GPRS
specific, but the R is common with the circuit switched GSM network:
• Gi between a GGSN and an external network. The GPRS network is
connected to an external data networks via this interface. The GPRS
system will support a variety of data networks. Because of that, the Gi is
not a standard interface, but merely a reference point.
• R between terminal equipment and mobile termination. This reference
point connects terminal equipment to mobile termination, thus allowing,
for example, a laptop-PC to transmit data over the GSM-phone. The
physical R interface follows, for example, the ITU-T V.24/V.28 or the
PCMCIA PC-Card standards.
6-64442
Issue 4.0
©Nokia Oyj

15 (55)


GPRS Architecture: Interfaces and Protocols

5 Transfer of packets between GSNs
User data packets are sent over the GPRS backbone in 'containers'. When a
packet coming from an external packet network arrives at the GGSN, it is
inserted in a container and sent to the SGSN. The stream of containers inside
the GPRS backbone network is totally transparent to the user: To the user, it
seems like he/she is connected directly via a router (the GGSN) to external
networks. In data communications, this type of virtual stream of containers is
called a tunnel. We say that the GSNs are performing tunnelling of user packets,
see Figure 7.
U
ser
packet
U
ser
packet
U
ser
packet
U
ser
p
acket
SGSN
GGSN
The stream of containers
forming a tunnel.
U
s
e
r
p
a
c
k
e
t

Figure 7. User packets over the GPRS backbone in ‘containers’
The protocol that performs the tunnelling in GPRS is called GPRS Tunnelling
Protocol (GTP). We can say that we transport GTP packets between the SGSN
and the GGSN.
Over the GPRS backbone, IP packets are used to carry the GTP packets. The
GTP packets then contain the actual user packets. This is shown in Figure 8.
The user packet, for example, a TCP/IP packet that carries some part of an
e-mail, is carried inside a GTP packet. The GTP packet is carried over the
GPRS backbone using IP and TCP or UDP (in the example, UDP).
The GTP packet headers, including the tunnel ID (TID), will tell the receiving
GSN who the user is. The tunnel ID includes the user IMSI (and another user
specific number). The TID is a label that tells the SGSN and the GGSN, whose
packets are inside the container.
16 (55) ©Nokia Oyj

6-64442
Issue 4.0

Transfer of packets between GSNs

User packet
Tunnel ID:
IMSI…
THE GTP PACKET
IP (+TCP/UDP)
Who is the user?
To which GSN?
GSN IP-
address
E.g. a TCP/IP packet
carrying e-mail

Figure 8. GTP container
From the point of view of the user and the external network, the GTP packets
that contain the user packets could be transferred between the GSNs using any
technology, for example, ATM, X.25, or Frame Relay. The chosen technology
for the GPRS backbone is IP.
All the network elements (the GSNs, the charging gateway, etc.) connected to
the GPRS backbone must have an IP address. IP addresses used in the backbone
are invisible to the MS and to the external networks. They are what we call
private IP addresses. That is, the user packets are carried in the GPRS core
between the SGSN and the GGSN using the private IP addresses of the
GPRSbackbone.
This concept of tunnelling and hiding backbone addresses ('private') to the user
level is illustrated in the following figures. Figure 9 shows a close-up of the user
and backbone IP address levels. Figure 10 shows the GTP tunnel related to the
user payload, and the relationship between the protocol stacks in the Gi and Gn
interfaces.
GGSN
GTP
IP IP
IP
backbone data using private IP addresses
SGSN MS
IP
GTP
Tunnel
user data using 'public' IP addresses

Figure 9. Transfer of packets between the GGSN and the MS
6-64442
Issue 4.0
©Nokia Oyj

17 (55)


GPRS Architecture: Interfaces and Protocols

BTS BSC
SGSN
GGSN
Internet
GPRS
Core
Network
SS7
HLR
MSC/
VLR
L1
L2
IP
GTP
USER
PAYLOAD
UDP
TCP/UDP
IP
APP
L1
L2
Tunnelled
payload
Server

Figure 10. GTP tunnelling and user payload

Note
For additional information on the GPRS transmission protocols, see the
Appendix – GPRS transmission plane protocols.
18 (55) ©Nokia Oyj

6-64442
Issue 4.0

Nokia GPRS solution

6 Nokia GPRS solution
6.1 Nokia GPRS functionality
This section describes the capabilities of the Nokia GPRS solution.
6.1.1 SGSN to MSC/VLR interface (Gs)
The Nokia solution includes the SGSN to MSC/VLR interface (Gs). For class-A
and class-B mobile terminals, an association is made in the VLR and SGSN to
indicate that they refer to the same physical mobile. This will give the following
functions:
• Class-A and B GPRS mobiles can receive paging for circuit switched
calls via GPRS channels. The GPRS is then suspended and the mobile
moves to circuit switched mode to start the call.
• Combined routing area and location area attaches and updates.
• Combined detaches from GPRS and circuit switched services.
6.1.2 Cell reselection
There are no handovers as such in GPRS. Instead, there is cell reselection,
which is made autonomously by the mobile. The parameters used by the mobile
for cell reselection are sent from the network and can be different for each cell.
There is an option in the GPRS specifications to enable network controlled cell
reselection. This requires GPRS terminals to send measurement results to the
BSS.
6.1.3 SMS through GPRS
Nokia GPRS supports the sending of SMSs via the GPRS network. This is more
efficient for GPRS mobiles using SMS since it frees up the GSM signalling
channels for other purposes. SMS through GPRS is achieved by the Gd
interface (SGSN to Gateway MSC). For each subscriber, a parameter in the
HLR indicates if the SMS will be delivered via GPRS or via GSM channels.
6-64442
Issue 4.0
©Nokia Oyj

19 (55)


GPRS Architecture: Interfaces and Protocols

6.1.4 Charging
Charging information is collected by the SGSN and GGSN, and is then
delivered to the Nokia Charging Gateway, where it is processed and forwarded
to the billing system.
6.1.5 Roaming
A Border Gateway (see Figure 11) enables users to use a secure GPRS-
tunnelled connection to their home network when roaming (via an inter-operator
backbone network), rather than connecting via the public Internet.
Inter PLMN
backbone
network
GPRS
Backbone
IP Network
Border
Gateway
GPRS
backbone
IP network
GGSN GGSN Internet
Operator A
(Home network)
Operator B
Roaming user
Border
Gateway
Secure GPRS Tunnelled Connection

Figure 11. Roaming via Border Gateway and Inter-PLMN network'
6.1.6 Quality of Service
In each cell, the total circuit switched capacity is available for use by GPRS
dynamically. Circuit switched calls always have priority over GPRS traffic, so
implementing GPRS will not reduce the quality of service (QoS) given to
speech- and circuit switched data subscribers.
If a guaranteed minimum quality of service for GPRS users is required, it is
possible to reserve a number of timeslots per cell that can only be used for
GPRS traffic.
6.1.7 Access to Internet
Direct Internet Connection (transparent access) access is the simplest way to
connect to the Internet, because all services are provided by the GGSN itself, or
20 (55) ©Nokia Oyj

6-64442
Issue 4.0

Nokia GPRS solution

to be more exact, the access point (AP), which is associated with the physical
interface to the Internet. This is illustrated in Figure 12.
GGSN
GPRS
backbone
IP Network
MS BSC BTS SGSN
ISP
Backbone
AP
Ethernet

Figure 12. Transparent access - Direct Internet connection
The address and netmask of the AP will be dictated by the network
configuration of the operator's ISP backbone. The Internet ISP backbone sees
the AP as a router, and the mobile stations (MSs) as nodes in the AP's subnet.
6.1.8 Corporate access solutions
A key difference for an operator will be the ability to provide secure
connections to corporate networks (and hence capture the corporate's complete
mobile telecommunications business). The Nokia GPRS solution includes
features that allow offering of secure and reliable corporate access solutions.
Multiple access points
One Nokia GGSN can connect to many corporate intranets. For each GPRS
subscriber, the access point (AP) used (that is, which external network to
connect the user to), is defined in the subscriber data in the HLR.
DHCP and RADIUS server access
The Nokia solution supports both static IP address allocation (defined in the
HLR) and dynamic IP address allocation, either from:
• RADIUS/DHCP servers within the operators network, external ISP or
corporate network, or from
• The GGSN's internal address pool.
The RADIUS and DHCP clients included in the Nokia GGSN can retrieve IP
addresses from corresponding servers located, for example, in corporate
networks. This also allows the subscriber to be authenticated against the
corporate intranet server (non-transparent access). Small corporate intranets
may not have a RADIUS server. In this case, the GGSN internal pool can be
used to allocate IP addresses.
6-64442
Issue 4.0
©Nokia Oyj

21 (55)


GPRS Architecture: Interfaces and Protocols

Secure connections: Non-transparent access
A dedicated and Virtual Private Network (VPN) connection or a non-
transparent access to an intranet covers both corporate nets and independent
ISPs. In this case, part of the service is provided outside the GPRS operator’s
network. The AP must provide secure access to that external part, and co-
operate with its infrastructure (for example authentication, address allocation,
and accounting).
GGSN
GPRS
backbone
IP Network
Intranet
Internet
Service
Infra servers
- RADIUS
- DNS
MS BSC BTS SGSN
ISP
Backbone
AP
AP
GTP

Figure 13. Non-transparent access - Dedicated and VPN connections
Whether the AP is connected to a VPN device or an intranet, it is regarded as a
router and the MSs are part of its subnet. When the GPRS and intranet operators
sign an interworking agreement, they should also define RADIUS addresses and
(if necessary) DHCP server addresses. In addition, netmask and the default
route can be exchanged when the agreement is made.
6.2 Nokia Base Station Subsystem (BSS)
The functions performed by the Nokia BSS elements to support GPRS include:
• Communication with GPRS MS using CS 1-4 and MCS 1-9 (EGPRS)
• Separation of circuit switched (CS) and packet switched (PS) traffic and
sending of the PS traffic to SGSN
• Support of GPRS protocols such as RLC, MAC, BSSGP, and FR
22 (55) ©Nokia Oyj

6-64442
Issue 4.0

Nokia GPRS solution

• Allocation of channels and radio blocks to MS using the USF flag
• Multiplexing and demultiplexing of data transmission on a TS
• Controlled by the NMS (network management system).

Most operators will have an existing GSM network that may have to be
upgraded for GPRS. These changes are discussed in this section. The features
available in the Nokia GSM/EDGE BSS10 are:
• Adaptive Multirate Codec (AMR), which can significantly improve
speech quality and provide more capacity even in low C/I conditions.
• Enhanced Data Rates for Global Evolution (EDGE), which should
increase the air interface throughput by a factor of 3. New EDGE-capable
TRX will be introduced for the UltraSite and MetroSite.
• Tri Band - Common BCCH for GSM 900 and GSM 1800 TRX in the
same cell
• GSM - WCDMA networking for inter-system handover and hence
seamless services to end-users
• MS location services, which will enable location-based services to be
offered.
• Automated radio network tuning with NetAct' OSS 3.1.
6.2.1 BTS
All Nokia 2nd-generation, Talk-family, Nokia PrimeSite and Nokia MetroSite
BTSs support GPRS coding schemes CS1 and CS2 without any hardware
changes.
6.2.2 BSC
Implementing GPRS has no effect on the capacity of the Nokia BSC to handle
circuit switched calls. To support GPRS, new plug-in units are required in the
BSC. These units are stated below:
• One Packet Control Unit (PCU unit) must be installed into each BCSU
signalling unit of the BSC (one slot is vacant).
• One Switching Unit (SW64) must be installed into each Group Switch
(GSW), since there are new internal PCM links with GPRS, and their
numbering is fixed.
• If needed, new ET cartridges (ET5C) could also be installed. In a fully
configured BSC, these might already be installed.
6-64442
Issue 4.0
©Nokia Oyj

23 (55)


GPRS Architecture: Interfaces and Protocols


Figure 14. New plug-in units in BSC for GPRS
Nokia PCU unit
The PCU unit is a new plug-in unit of the BCSU. It takes care of switching
GPRS traffic between Abis interface (TRAU frames) and Gb interfaces (FR)
using the Group Switch (GSW). The PCU also supports the new protocols
(RLC, MAC, FR) required for GPRS operation and handles GPRS channel
allocation and radio resource management functions.
The following specifications apply to the PCU:
• BSC contains eight active and one redundant PCU in a n+1 configuration.
• One PCU can handle 256 GPRS channels, that is, TCHs of 32 TRXs.
• One PCU can be connected to a maximum of 64 BTSs and/or 64 cells.
• The data processing capacity of one PCU is 2 Mbit/s.
• A BSC must be fully (hardware) equipped, that is, each BCSU must have
one PCU, but the number of active PCUs is determined by software
licence.
24 (55) ©Nokia Oyj

6-64442
Issue 4.0

Nokia GPRS solution

• Each PCU has a separate frame relay interface to the SGSN (Gb
interface). This can be configured in 64 kbit/s steps, from two to 31 PCM
timeslots.
6.3 Nokia Core Network Subsystem (CNS)
6.3.1 MSC/VLR
The MSC/VLR is not involved in GPRS data transfer, but supports signalling
for class-A and class-B mobiles as well as SMS delivery.
To allow support for terminals attached to both GSM and GPRS services (A and
B type), Nokia has implemented the Gs interface between the MSC/VLR and
the SGSN. This interface supports paging and combined LA and RA update
procedures for class-A and B mobiles.
6.3.2 HLR and EIR
The Nokia-combined Home Location Register (HLRi), Authentication Centre
(AC), and Equipment Identity Registry (EIR) is based on the DX200 switching
platform.
As for circuit switched services, subscriber information for GPRS is stored in
the Home Location Register (HLR). The HLR supports procedures such as
GPRS attach/detach and authentication. Nokia has implemented interfaces
between the HLR and SGSN (Gr) and the Equipment Identity Register (EIR)
and SGSN (Gf). The interface is implemented using MAP v.3.
For SMS support, one new parameter per subscriber has been added in the HLR
to indicate whether short messages should be delivered via the MSC or the
SGSN.
6.3.3 Nokia Serving GPRS Support Nodes
The Serving GPRS Support Node (SGSN) of a GPRS network is equivalent to
the MSC of a GSM network. It combines the functions of a digital switching
platform with the functions necessary for interfacing between the GSM Base
Station Subsystem (BSS) and IP backbone networks.
6-64442
Issue 4.0
©Nokia Oyj

25 (55)


GPRS Architecture: Interfaces and Protocols


Figure 15. The Nokia SGSN
The SGSN is connected to one or several BSCs (Base Station Controllers) by
the Gb interface. The number of BSCs connected to a SGSN depends on the
amount of data traffic expected. One SGSN can support BSCs working under
several different MSCs. The SGSN can be physically located at the MSC or
BSC site. There must at least one SGSN in a GPRS network.
The SGSN is connected to the GPRS network via the GPRS backbone. The
GPRS backbone is a private network that provides IP connectivity between the
GPRS network elements in order to carry signalling, traffic and charging data.
This network is not directly accessible from the public Internet.
The primary function of the Nokia SGSN is to convert the IP network protocol
to the protocols used in the BSS and the mobile terminal. It then passes the data
to the relevant GGSN when a connection to an external data network is
required. Additional functions performed by the Nokia SGSN include mobile
terminal authentication and mobility management, as well as user data
compression to and from the terminal. Finally, it handles signalling interfaces
with the MSC/VLR and HLR, collects charging and statistical information, and
provides flexible network management interfaces.
The Nokia SGSN consists of a number of functional units, each with its own
processor and back-up facility carrying out a number of tasks. These functional
units have independent tasks, but communicate when and as necessary using a
common message bus. It is not the intention of this section to give a complete
explanation of all the units in this section, but only a simple overview of them
to give an understanding.

The SGSN consists
26 (55) ©Nokia Oyj

6-64442
Issue 4.0

Nokia GPRS solution

• Packet Processing Unit (PAPU)
The main purpose of the PAPU is to process user data and protocol
conversion between the BSS and the GPRS backbone network and vice
versa. It handles the GPRS mobility management (GMM) procedures
such as GPRS attach, location management, and GPRS detach. It is
responsible for session management (SM) including PDP context
activation, modification, and deactivation, as well as SMS delivery. It
takes care of ciphering and compression between the MS and SGSN.
Up to 16 +1 PAPUs can be found in a Nokia SGSN
• Signalling and Mobility Management Unit (SMMU)
The main purpose of the SMMU is to support subscriber mobility
management using SS7-based interfaces Gr, Gd, Gs, and Gf. It has a
temporary storage for GPRS subscriber data called visiting GPRS
subscriber database, similar to the VLR in a MSC. If Gs interface is
implemented, the SMMU handles all the combined GPRS/GSM
mobility management for those GPRS and IMSI attached subscribers
that have class A or B capabilities. It supports the SMS delivering
procedure forwarding the short messages either to SMS-GMSC through
Gd interface or to the MS after checking related subscriber information
on the visiting GPRS subscriber database. Up to 4 +1 SMMU can be
within a Nokia SGSN.
• Operation and Maintenance Unit IOMU)
The OMU acts as an interface between the user and the exchange and
takes automatic recovery measures, as needed, based on its collected
fault data. The tasks of the Operation and Maintenance Unit include
traffic control functions, maintenance, system configuration
administration, and system management.
• Marker and Charging Unit (MCHU)
The Charging Unit within the MCHUcollects and stores charging
information, generates call detail records (CDRs) and transfers them to
Charging Gateway (CG) through the Ga interface. If CG is not present
in the GPRS network, the CDRs are saved and transferred off-line to
Billing and Customer Care System (BCCS). The Marker within the
MCHU controls and supervises the group switch (GSW), hunts free
circuits, and is responsible for establishing and releasing all
connections.
• a Group Switch (GSW) used for semi-permanent connections between
the SGSN and the BSS, and the SGSN and the NSS
• units to connect transmission systems to the GSW (Exchange
Terminal, ET), and
• a high-speed Message Bus (MB) for interconnecting computer units
6-64442
Issue 4.0
©Nokia Oyj

27 (55)


GPRS Architecture: Interfaces and Protocols

Exchange
Terminal
(ET)
Gr,Gs, Gd
(to NSS)
Group
Switch
(GSW)
Clock and
Sync. Unit
(CLS)
Exchange
Terminal
(ET)
Group
Switch
(GSWB)
Clock and
Sync. Unit
(CLS)
Packet
Processing
Unit
(PAPU)
Signalling &
Mobility
Man. Unit
(SMMU)
Operation &
Maintain.
Unit
(OMU)
Marker &
Charging
Unit
(MCHU)
WDU
N+1 N+1
2N 2N
Gn
(to GGSN)
Charging
Gb
(to BSC)
WDU

Figure 16. Logical architecture of the Nokia SGSN
The main units supporting GPRS are PAPU and SMMU. Each of the functional
units is described in the following chapters. The SMMU and PAPU have N+1
redundancy, whereas the MCHU, OMU, MB, GSW and CLS have 2N
redundancy.
6.3.4 Nokia Gateway GPRS Support Nodes
The Gateway GPRS Support Node (GGSN) provides the interconnection
between the GPRS network and external packet data networks such as the
Internet. The main functions of the GGSN are:
• Interfacing the GPRS backbone to external data networks
• GTP tunnelling to the SGSN
• Gathering of charging and statistics information
• Network management interfaces
• Dynamic IP address allocation to mobile station (MS)
• Protecting the GPRS backbone from external attacks.
Charging in GGSN
Various charging information is provided, for example, data volume uplink or
downlink, PDP context active time, tariff change, and access point name. This
relates to the network or services used, static or dynamic IP address usage, PDP
IP address, and cause for record close.
28 (55) ©Nokia Oyj

6-64442
Issue 4.0

Nokia GPRS solution

GGSN interworking services
The allocation of IP addresses can be carried out by one of the following:
• GGSN allocates the address from an internal pool.
• Internal DHCP client can be used to access a DHCP server in the intranet
or ISP network.
• Internal RADIUS client can be used to access a RADIUS server in the
intranet or ISP network.
The exact functions provided by interconnecting networks depend on which of
the following three connection options is required (see Figure 17):
• Direct Internet connection to an existing ISP infrastructure owned by the
operator
• Dedicated connection to an intranet outside the operator's network.
• Indirect connection to an intranet employing a Virtual Private Network
(VPN). The virtual connections run between the VPN software in the
GGSN and the VPN software in the accessed intranet.

Figure 17. Various connections between GPRS and external networks
6-64442
Issue 4.0
©Nokia Oyj

29 (55)


GPRS Architecture: Interfaces and Protocols

6.3.5 Nokia Charging Gateway (CG)
CG and CDRs
In the GPRS standardisation, ETSI gives the following two possibilities to
implement the CG functionality:
• Stand-alone
• Integrated in GSNs (SGSN and GGSN).
In the Nokia GPRS solution, the CG is a stand-alone network element, as shown
in Figure 18. Why is that?
• Better service for the operator: A separate mediation device is not
needed. If CG is in GSNs, a mediation device is needed because GSNs
are not able to send Charging Detailed Records (CDRs) to the Customer
Care and Billing System (CCB). If GSNs send CDRs to the CCB, there
are as many access points to the CCB as there are GSNs. This means
more workload.
• Each CG has one access point to the CCB.
• An ETSI-specified real-time transfer protocol, GTP' (enhanced GPRS
Tunnel Protocol), is used to transfer CDRs to the CCB.
When a subscriber switches the terminal on, the PDP context is activated and
the GGSN starts to send CDRs to the CG. The GGSN informs SGSN to which
CG unit it sends CDRs.
SGSN
Operator
IP backbone
GGSN
Border Gateway
Inter operator
IP network
Internet
Billing System
Charging gateway GTP'
GTP'

Figure 18. Implementation of the Nokia CG in the GPRS backbone
GGSN has only one type of CDRs, that is the G-CDR,
The SGSN sends four CDR types:
30 (55) ©Nokia Oyj

6-64442
Issue 4.0

Nokia GPRS solution

1. S-CDR – The content is like G-CDR plus some other fields.
2. M-CDR is for mobility management.
3&4. SMS-CDR
The GPRS Charging Gateway receives CDRs from GGSNs and SGSNs using a
real-time transfer protocol (GTP’). The CG forms an intermediate storage for
the CDRs. Therefore, no long-term storing capabilities are required at GSNs.
In normal conditions all CDRs from the same PDP context are transferred to the
same CG. If one CG is down, a redundancy procedure built in the GTP' protocol
allows the transfer of CDRs from the GSNs towards other CGs.
CG functionality
The Nokia Charging Gateway will consolidate and pre-process the CDRs before
passing them to the billing system. Standard billing system interfaces FTAM,
FTP over TCP/IP and NFS are implemented in CG. The CG implementation
enables the auditing and tracing of every phase of CDR processing. The CG
validates and consolidates CDRs, and produces them in a format suitable for the
Customer Care and Billing System (CCBS).
Nokia CG benefits
CG offers the following major benefits:
• Reliable storage capability for CDRs, eliminating the need for costly and
failure risk storage capabilities at the GSNs
• Fast transfer of CDRs from the GSNs and CG hot billing internal
architecture are future-proof for hot billing and prepaid, enabling credit
control and fraud detection in an early phase.
• Automated validation makes it easier to detect errors.
• Reduction of the CCB processing load by pre-processing and
consolidating
• The audit trail function enables tracing of CDR-processing phases and
CDR contents, for example, in case of customer complaints.
An overview of the CG functions and management is shown in Figure 19.
6-64442
Issue 4.0
©Nokia Oyj

31 (55)


GPRS Architecture: Interfaces and Protocols

SGSN
GGSN
Billing
System
CG
•Receiving CDRs from GSNs
•Real-time transfer protocol
•Fixed CDR format (Rel 1)
•Intermediate CDR storage
•CDR consolidation
•Pre-processing
•Handling erroneous CDRs
•Interfaces towards Billing
System / CCB
•CDR transfer towards BS
•Configuration management
•CDR transfer settings
•backup/restore
•tariff changes
•Fault management
•current alarms
•alarm history
•Performance management
•KPIs
•Time management
•synchronising with
other NEs
•Security management
•password settings
•Management methods
•SNMP
Charging Gateway Management

Figure 19. CG functions
6.4 Nokia Network Management System (NMS)
The Nokia solution to support the operation of a network is known as the Nokia
NetAct. The structure of the solution is not built around the technology but the
functions and processes that an operator must perform to ensure the operation
of the business.
Nokia has launched the Nokia NetAct Framework in order to support the
transition from 2G to 3G. It also extends the multivendor integration capability
of the Network Management System (NMS).
NetAct provides a full-scale management capability for both packet data and
traditional voice traffic, independent of technology. Thanks to this, it is possible
to deploy new technologies with the same system that manages the current
infrastructure.
The following figure illustrates the different functions that the NetAct supports.
All the functions are brought together in a common framework and are
connected to the physical network elements through a UMA (Unified Mediation
and Adaptation) object, which allows NetAct to talk to Nokia elements and 3rd
party systems.
32 (55) ©Nokia Oyj

6-64442
Issue 4.0

Nokia GPRS solution

Workflow Manager
Workflow Manager
Reporter
Reporter
Administrator
Administrator
3rd Party Tools
3rd Party Tools
Rating &
Charging
Rating &
Charging
Planner
Planner
Configurator &
Provisioning
Configurator &
Provisioning
Monitor
Monitor
Service Quality
Manager
Service Quality
Manager
Unified Mediation
and Adaptation
Unified Mediation
and Adaptation
Network
Network
Common network
typology
Common WEB GUI

Figure 20. Nokia NetAct Framework
The Nokia network elements (e.g. RNC, AXC, BTS) themselves provide the
necessary functions on commissioning, setting up, or troubleshooting the
individual equipment. Nokia NMS − sitting on top of the managed network
elements − provides tools for making large-scale modifications at the network
level.
Business
Management
Systems
Service
Management
Systems
Network
Management
Systems
Element
Management
Systems
Funct ions of Service management systems:
• take care of subcriber data
• provision services and subscribers
• collect and rate, bill offered services
• create, promote and monitor services
Network management system (NMS):
• collect information from the underlying networks
and pre/post-process the raw data
• analyse and distribute information
• optimise network capacity and quality
Element management systems (EM):
• EMs are part of the NE (RNC, BTS, AXC,etc.)
functionality
• monitor the functioning of the equipment
• collect raw data (performance indicators)
• local GUI provided for site engineers
• mediate towards the NMS system
MIS
Planning
System
BTS
BTS
AXC
RNC
NE x
NE y
CCB
NE =network element, CCB =customer care and
billing, MIS =management information system
NMS

Figure 21. How the TMN is visualised in the Nokia GSM/UMTS solution
6-64442
Issue 4.0
©Nokia Oyj

33 (55)


GPRS Architecture: Interfaces and Protocols

6.4.1 IP configuration management
In the Nokia NMS solution, the technology differences between IP and other
subnetworks have been scaled down, so shared management methods can be
used. This results in major savings, both in time and O&M resources. IP core
network planning can be integrated into the operator's general network planning
process. The tasks include capacity planning and IP routing set-up in order to
ensure proper functionality of this transport network. In the element
management layer, access to the actual element can be supported by web-based
applications, so the needed configurations of the IP elements are always visible
to O&M personnel from any O&M screen. Special emphasis has been placed on
security management applications.
Internet / Intranet
Management Data
IP Core Development IP Core Development
•IP Core Planning
•IP Core Implementation
•IP Core Analysis
Nokia NMS for UMTS Nokia NMS for UMTS
•Shared Applications
•Common Resources
•Shared Processes
Element Management

Figure 22. Management of the IP backbone in the Nokia NetAct
34 (55) ©Nokia Oyj

6-64442
Issue 4.0

Key points

7 Key points
7.1 GPRS architecture: key points
• A GPRS network is expected to perform the functions of GSM network
and data network.
• The new elements of the GPRS network are the PCU, SGSN, GGSN,
CG, BG, DNS, and Firewalls.

• The functions of the Gateway GPRS Support Node (GGSN)are the
following:
− Routing mobile-destined packets from external networks to
relevant SGSN
− Routing packets originating from an MS to the correct external
network
− Interfacing to external IP networks
− Collecting charging data and traffic statistics
− Allocating dynamic IP addresses to mobiles either by itself or with
the help of a DHCP or a RADIUS server.

• The functions of the Serving GPRS Support Node (SGSN) are the
following:
− Converting protocols used in IP backbone to protocols used in the
BSS and MS
− Handling of authentication and mobility management
− Routing data to relevant GGSN when connecting to an external
network
− Collecting charging data and traffic statistics
− Handling of ciphering and data compression.

• The interfaces in the GPRS network are the following:
− Gb SGSN to BSS
− Gn between GSNs (GTP)
− Gr between SGSN and HLR (MAP)
− Gs SGSN to MSC (BSSAP+)
6-64442
Issue 4.0
©Nokia Oyj

35 (55)


GPRS Architecture: Interfaces and Protocols

− Gi GGSN to external data networks
− Gf SGSN and the EIR (MAP)
− Gd between SGSN and the GMSC (SMSC)
− Ga between GSNs and CG.
• Tunnelling is the process by which user packets are transported
encapsulated in containers and transported through a network.
• The tunnelling protocol in GPRS is called the GPRS Tunnelling Protocol
(GTP) over the GPRS backbone. The backbone is an IP network.
• Tunnelling is used when:
a. the packets with private IP addresses have to transmitted through a
public network
b. packets of one protocols have to be sent through a network that
does not understand it
c. for security reasons.
7.2 Nokia GPRS solution: key points
• The components of the Nokia GPRS solution are: the Nokia SGSN, the
Nokia GGSN, the Nokia LIG, Nokia Charging Gateway (CG), Nokia
BSC, the Nokia NMS for GPRS and the Nokia BTSs.
• The features available in the Nokia GSM/EDGE BSS10 are adaptive
multirate codec (AMR), Enhanced Data Rates for Global Evolution
(EDGE), Tri Band - Common BCCH, GSM-WCDMA networking, MS
location services, Automated radio network tuning.
• The capacity of the Nokia SGSN:
− Maximum attached subscriber capacity 120 000 (SG1)/ 240 000
(SG2)
− Up to 300 000 short messages in the busy hour
− Mean switching capacity of 48 Mbit/s (SG1) and 100 Mbit/sec
(SG2)
− Capacity can be configured in steps of 25% of maximum capacity.
• The Nokia CG is a stand-alone element, which collects CDRs from
GSNs. The CG validates and consolidated CDRs, and produces them in a
format suitable for the Customer Care and Billing System (CCBS).
• The new features in T12 are Capacity Indication Tool, Capacity
Estimation Tool, NMS backup, Service Access Control, Multivendor
Integration ( Corba Integration Kit, ASCII Alarm Forwarding feature),
Radio Network Management (Automatic Picocell Planning, Channel
36 (55) ©Nokia Oyj

6-64442
Issue 4.0

Key points

Finder), GPRS backbone management (Configuration, Firewall and VPN
Management, Backbone Name and Address Management), GPRS
Performance Management (ASCII Interface for GPRS Measurement
Data, Basic Report Set) and Routing Area IP manager.

6-64442
Issue 4.0
©Nokia Oyj

37 (55)


GPRS Architecture: Interfaces and Protocols

8 Review questions
1. Draw the GPRS architecture including the main GPRS network elements.













2. What are the functions of SGSN?

3. What are the functions of GGSN?

4. Which interfaces exist in GPRS (draw into sketch in Question 1)?

5. Which protocol is used on each of the interfaces?

6. What is GTP?

7. Why is GTP used?

8. What modifications need to be performed to Nokia BTS?

38 (55) ©Nokia Oyj

6-64442
Issue 4.0

Review questions


9. What are the important specifications for the Nokia PCU solution?




6-64442
Issue 4.0
©Nokia Oyj

39 (55)


GPRS Architecture: Interfaces and Protocols

9 Appendix – GPRS transmission plane
protocols
1. Overview of protocols used in GPRS
A GPRS network introduces many new protocols designed to convey user data
in a reliable and secure way. Information is passed between the existing GSM
network and the GPRS network by employing protocols on two separate planes:
• Transmission plane protocols are used for the transmission of user data
and control functions.
• Signalling plane protocols are used to convey signalling information
that controls and supports the transmission plane functions.
The transmission plane protocols convey user data in the form of IP datagrams
from the mobile station to external networks, such as the Internet or corporate
data networks.
The signalling plane contains many protocols that are already employed in
existing GSM network elements.
2 GPRS transmission plane protocols
When looking at the transmission protocols, we can assume, that a connection
between the MS and the external PDN has been established. The network
elements GGSN, SGSN, PCU/BSC, BTS/CCU are now responsible for the user
data transfer. The user data transfer can be decomposed into several steps:
• The user data has to be transmitted from the external PDN to the
GGSN; and it has to be transmitted from the GGSN to the external
PDN. The user data transfer is organised via the interface Gi.
• The subscriber’s packets (PDU) have to be exchanged between the
GGSN, which is interfacing the external PDN, and the SGSN, which is
interfacing the BSS and locally serving the GPRS Ms. The user data
transfer between the two network entities is organised based on the Gn-
interface specification.
• User data has to be transmitted in a well defined way between the
SGSN and the MS. The transmission is organised on a logical level
peer-to-peer between the SGSN and GPRS MS. This is done with the
protocols SNDCP and LLC (see figure below).
• The SNDCP and LLC protocol are used to specify the user data
exchange between the GPRS MS and the SGSN on a logical, peer-to-
peer level. Lower layer protocols define, how user data is exchanged
40 (55) ©Nokia Oyj

6-64442
Issue 4.0

Appendix – GPRS transmission plane protocols

between the BSS network elements and SGSN and GPRS MS.
The protocols BSSAP and NS are responsible for user data transmission
between SGSN and PCU, while
• the protocols RLC, MAC, and GSM RF (Radio Frequency Layer and
Physical Link Layer) organise the user data transport between PCU +
CCU and the GPRS MS. (see figure below).
(Note: the subject is easier to understand for a beginner, if he assumes the PCU
and CCU to be realised at the BSC site.)
MAC
GSM
RF
RLC
LLC
SND
CP
IP/
X.25
MAC
GSM
RF
RLC
NS
L1bis
BSS
GP
Relay
NS
L1bis
BSS
GP
LLC
SND
CP
L1
L2 L2
IP IP
UDP /
TCP
UDP /
TCP
GTP GTP
Relay
IP/
X.25
MS BSS SGSN
GGSN Um Gb Gn Gi
L1
BSSGP: BSS GPRS Protocol LLC: Logical Link Control SNDCP: SubNetwork Dependent
NS: Network Service MAC: Medium Access Control Convergence Protocol
GTP: GPRS Tunnelling Protocol RLC: Radio Link Control TCP: Transmission Control Protocol
IP: Internet Protocol UDP: User DatagramProtocol
L2‘
IP/
X.25
L1‘
Relay
L2‘
IP/
X.25
L1‘
L2‘
IP/
X.25
L1‘
Relay
Application/ Higher level protocols
Rou-
ter

Figure 23. Transmission plane protocols

2.1 Transmission protocols in the Gn interface
The Gn interface forms the GPRS backbone network. The protocol layers L1,
L2, IP, and TCP/UDP form a transmission network solution, based on standard
Internet protocols or standard layer 1 and 2 link protocols. Only the highest
layer protocol, the GPRS Tunnelling Protocol (GTP) is GPRS specific.
6-64442
Issue 4.0
©Nokia Oyj

41 (55)


GPRS Architecture: Interfaces and Protocols

2.1.1. Transport
The L1 and the L2 protocols are vendor dependent OSI layer 1 and 2 protocols
that carry the IP datagrams for the GPRS backbone network between the SGSN
and the GGSN.
TheInternet Protocol (IP) datagram in the Gn interface is internally used in
the GPRS backbone network. The GPRS backbone (core) network and the
GPRS subscribers use different IP addresses. This makes the GPRS backbone
IP network invisible to the subscribers and vice versa. The GPRS backbone
network carries the subscriber IP or X.25 traffic in a secure GPRS tunnel.
All data from the mobile subscribers or external networks is tunnelled in the
GPRS backbone.
TCP or UDP are used to carry the GPRS Tunnelling Protocol (GTP) PDUs
across the GPRS backbone network. TCP is used for user X.25 data and UDP is
used for user IP data and signalling in the Gn interface.
GGSN
SGSN
PDN
(z.B. X.25, IP) Gi
IP/X.25
L2
Link Layer
IP
Internet
Protocol
UDP
User
Datagram
Protocol
TCP
Transmission
Control
Protocol
GTP
GPRS
Tunnelling
Protocol
L1
Physical
Layer
L2‘
Link Layer
L1‘
Physical
Layer
IP / X.25
Relay
L2‘
Link Layer
L1‘
Physical
Layer
IP / X.25
L2
Link Layer
IP
Internet
Protocol
UDP
User
Datagram
Protocol
TCP
Transmission
Control
Protocol
GTP
L1
Physical
Layer
Gn
•outside the
scope of the
Rec.
•depends on
agreement
•PDU en-/de-
capsulation
•Tunnelling protocol
between GSNs
• Signalling
between GSNs
•TCP: reliable
•UDP: unreliable
but fast
•UDP =minimum
solution in NSS
•IPv4 or IPv6
•path selection
(next hop)
•datagram format
adjustment
•outside the scope
of the Rec.
•operator
dependent

Figure 24. . GPRS Tunnelling Protocol principle
2.1.2 GPRS Tunnelling Protocol (GTP)
The GPRS Tunnelling Protocol (GTP) allows multi-protocol packets to be
tunnelled through the GPRS backbone between GPRS Support Nodes (GSNs).
This is illustrated above.
The GTP can have proprietary extensions to allow proprietary features. The
relay function in the SGSN relays the user PDP (Packet Data Protocol) PDUs
(IP or X.25) between the Gb and the Gn interfaces.
42 (55) ©Nokia Oyj

6-64442
Issue 4.0

Appendix – GPRS transmission plane protocols

GTP is defined both for the Gn interface, that is, the interface between GSNs
within the same PLMN, and the Gp interface between GSNs in different
PLMNs.
The UDP/IP and TCP/IP are examples of paths that may be used to multiplex
GTP tunnels. The choice of path is dependent on whether the user data to be
tunnelled requires a reliable link or not. Two modes of operation of the GTP
layer are therefore supported for information transfer between the GGSN and
SGSN.
• unacknowledged (UDP/IP)
• acknowledged (TCP/IP).
A UDP/IP path is used when the user data is based on connectionless protocols,
such as IP. A TCP/IP path is used when the user data is based on connection-
oriented protocols, such as X.25.
The GTP layer supports both modes simultaneously.
In the transmission plane, the tunnel created by the signalling plane is used to
carry user data packets between network elements connected to the GPRS
backbone network, such as the SGSNs and GGSNs. No other systems need to
be aware of GTP, for example, the MSs are connected to a SGSN without being
aware of GTP.
UDP
Header
UDP
Header
GTP
Header
GTP
Header
user IP Data
GPRS
Backbone IP
Header
GPRS
Backbone IP
Header
User Data Payload (T-PDU)
(user IP Data)
User Data Payload (T-PDU)
(user IP Data)
GTP
Header
GTP
Header
User Data Payload (T-PDU)
(user IP Datagram)
User Data Payload (T-PDU)
(user IP Datagram)
UDP
Header
UDP
Header
GTP
Header
GTP
Header
User Data Payload (T-PDU)
(user IP Datagram)
User Data Payload (T-PDU)
(user IP Datagram)
UDP Layer
GTP Layer
Backbone
IP Layer

Figure 25. The GTP protocol header being added to user data
A GTP tunnel is defined by two associated PDP contexts in different GSN
nodes and is identified by a Tunnel ID (TID). A GTP tunnel is necessary for
forwarding packets between an external packet data network and an MS. The
Tunnel ID identifies the MM and PDP contexts (MM Context ID and a NSAPI).
The NSAPI (Network Service Access Point Identifier) is a fixed value between
0 and 15 that identifies a certain PDP context. It identifies a PDP context
belonging to a specific MM context ID.
6-64442
Issue 4.0
©Nokia Oyj

43 (55)


GPRS Architecture: Interfaces and Protocols


The GTP header
The GTP header contains 16 octets and is used for all GTP messages.
The information contained in the GTP header includes the following:
• The type of GTP message (signalling messages =1-52, but when used for
data transmission the GTP message type =255).
• The length of the GTP message (G-PDU) in octets.
• A Sequence Number to provide a transaction identity for signalling
messages and a growing sequence number for tunnelled T-PDUs.
(A T-PDU is an IP datagram from an MS or a network node in an
external packet data network. The T-PDU is the payload that is tunnelled
in the GTP tunnel).
• A flag to indicate whether an LLC frame number is included or not.
• An LLC frame number that is used for the Inter SGSN Routing Update
procedure to co-ordinate the data transmission on the link layer between
the MS and the SGSN.
• A TID (Tunnel Identifier) that points out MM and PDP contexts.
The content of the GTP header differs depending on whether the header is used
for signalling messages or user data (T-PDUs).

Tunnel ID (TID) format
The Tunnel Identifier (TID) consists of the following:
• Mobile Country Code (MCC)
• Mobile Network Code (MNC)
• Mobile Subscriber Identification Number (MSIN)
• Network Service Access Point Identifier (NSAPI)
These represent the MM and PDP contexts.
2.2. Higher layer peer-to-peer transmission between SGSN and MS
The protocols are responsible for a peer-to-peer user data transmission between
MS and SGSN:
• Subnetwork Dependent Convergence Protocol Layer (SNDCP)
• Logical Link Control Layer (LLC).
44 (55) ©Nokia Oyj

6-64442
Issue 4.0

Appendix – GPRS transmission plane protocols

SGSN
SNDCP
SubNetwork
Dependent
Convergence
Protocol
FR
Frame Relay
L1bis
Physical
Layer
BSSGP
BSS
GPRS
Protocol
LLC
Logical Link
Control
MS
SNDCP
RLC
Radio Link
Control
MAC
Medium Access
Control
GSM RF
IP / X.25
LLC
Um Gb
•compression
•multiplexing/de-multiplexing
•segmentation & re-assembly
•logical connection
•acknowledge/
unacknowledged peer-to-peer
operation
•ciphering
•SAPs to higher layer
(SNDCP, GMM, SMS)
MAC
GSM
RF
RLC
FR
L1bis
BSS
GP
Relay

Figure 26. Higher Layer peer-to-peer user data transport
2.2.1 Logical Link Control (LLC)
The Logical Link Control (LLC) layer offers a secure and reliable logical link
between the MS and the SGSN for upper layer protocols. It is independent of
lower layers.
The LLC layer is responsible to transmit
• signalling and control information,
• SMS, and
• Subnetwork Dependent Convergence Protocol (SNDCP) packets.
SNDCP provides a mapping and compression function between the
network layer (IP or X.25 packets) and the lower layers. It also
performs segmentation, reassembly, and multiplexing.
Two modes of operation of the LLC layer are defined for information transfer:
unacknowledged and acknowledged. The LLC layer can support both modes
simultaneously.
In acknowledged mode, the receipt of LLC-PDUs is confirmed. The LLC layer
retransmits LLC-PDUs if confirmation has not been received within a certain
timeout period.
In unacknowledged mode, there is no confirmation required for LLU-PDUs.
6-64442
Issue 4.0
©Nokia Oyj

45 (55)


GPRS Architecture: Interfaces and Protocols

Signalling and SMS is transferred in unacknowledged mode.
In unacknowledged mode, the LLC layer offers he following two options:
• Transport of "protected" information means that if errors occur within
the LLC information field, the frame will be discarded.
• Transport of "unprotected" information means that if errors occur
within the LLC information field, the frame will not be discarded.
The LLC layer supports several different QoS delay classes with different
transfer delay characteristics.
The network layer protocols for signalling, SMS, and user data are multiplexed
to the lower layers in the following way :
• SAPI is the Service Access Point Identifier, which is used to identify the
points where the LLC provides a service to a higher layer. SAPIs have
different priorities.
• TLLI is the Temporary Logical Link Identity, which unambiguously
identifies the logical link between the MS and SGSN. TLLI is used for
addressing at the LLC layer.

Figure 27. Multiplexing of network protocols
LLC provides the services necessary to maintain a ciphered data link between
an MS and an SGSN.
46 (55) ©Nokia Oyj

6-64442
Issue 4.0

Appendix – GPRS transmission plane protocols

The LLC connection is maintained as the MS moves between cells served by
the same SGSN. When the MS moves to a cell being served by a different
SGSN, the existing connection is released and a new logical connection is
established with the new SGSN.
LLC is independent of the underlying radio interface protocols. In order to
allow LLC to operate with a variety of different radio interface protocols, and to
ensure optimum performance, it may be necessary to adjust, for example, the
maximum LLC PDU length and the LLC protocol timer values. Such
adjustments can be made through negotiation between the MS and the SGSN.
The maximum length of an LLC PDU shall not be greater than 1600 octets
minus the BSSGP protocol control information.
A logical communication pipe is established between the GGSN and the MS
through a SGSN. The LLC protocol link is established between the MS and the
SGSN upon GPRS attach. The GPRS Tunnelling Protocol (GTP) establishes a
tunnel between the SGSN and the GGSN at PDP context activation. In the LLC
header, the NSAPI (Network layer Service Access Point Identifier) identifies
which application inside the MS the packet belongs to.
2.2.2 SNDCP (Subnetwork Dependent Convergence Protocol)
Network layer protocols are intended to be capable of operating over a wide
variety of subnetworks and data links. GPRS supports several network layer
protocols providing protocol transparency for the users of the service.
To enable the introduction of new network layer protocols to be transferred over
GPRS without any changes to GPRS, all functions related to the transfer of
Network layer Protocol Data Units (N-PDUs) are carried out in a transparent
way by the GPRS network. This is one of the requirements of SNDCP.
Another requirement of the SNDCP is to provide functions that help to improve
channel efficiency. This is achieved by means of compression techniques.
The set of protocol entities above the SNDCP consists of commonly used
network protocols. They all use the same SNDCP entity, which then performs
multiplexing of data coming from different sources to be transferred using the
service provided by the LLC layer. The Network Service Access Point Identifier
(NSAPI) is an index to the PDP context of the PDP that is using the services
provided by the SNDCP (see Figure 4). One subscriber may have several PDP
contexts and NSAPIs.
Each active NSAPI uses the services provided by the Service Access Point
Identifier (SAPI) in the LLC layer. More than one NSAPIs may be associated
with the same SAPI.
6-64442
Issue 4.0
©Nokia Oyj

47 (55)


GPRS Architecture: Interfaces and Protocols


Figure 28. SNDCP used to multiplex different protocols
2.3 Transmission protocols in the Gb interface (BSS layers)
SGSN
SNDCP
SubNetwork
Dependent
Convergence
Protocol
FR
Frame Relay
L1bis
Physical
Layer
BSSGP
BSS
GPRS
Protocol
LLC
Logical Link
Control
BSS (PCU)
BSSGP
BSS
GPRS
Protocol
FR
Frame Relay
L1bis
Physical
Layer
Gb
•LLC frame transmission
(transparent)
•routing information
•QoS information
•no error correction
unreliable BSSGP
PDU transport
E1/T1 (PCM30/PCM24)

Figure 29. BSSAP, NS, and L1bis
48 (55) ©Nokia Oyj

6-64442
Issue 4.0

Appendix – GPRS transmission plane protocols

The Gb interface allows many users to be multiplexed over the same physical
link using Frame Relay (FR). Bandwidth is allocated to a user upon activity
(when data is sent or received) and is reallocated immediately thereafter. This
is, in contrast to the A interface, where a single user has the exclusive use of a
dedicated physical resource throughout the lifetime of a call irrespective of
activity.
GPRS signalling and user data are sent in the same transmission plane and
therefore no separate dedicated physical resources are required to be allocated
for signalling purposes.
Data rates over the Frame Relay Gb interface may vary for each user without
restriction, from zero data to the maximum possible line rate (for example
1984 kbit/s, which is the maximum available bit rate of a 2Mbit/s (E1) link).
2.3.1 Physical Layer Protocol
Several physical layer configurations and protocols are possible at the Gb
interface and the physical resources are allocated by Operation & Maintenance
(O&M) procedures. Normally a G703/704 2Mbit/s connection is provided.
2.3.2 Network Services layer
The Gb interface Network Services layer is based on Frame Relay. Frame
Relay virtual circuits are established between the SGSN and BSS. LLC PDUs
from many users are statistically multiplexed onto these virtual circuits. These
virtual circuits may traverse a network of Frame Relay switching nodes, or may
just be provided on a point to point link between the PCU and the SGSN (if the
PCU and SGSN are co-located). Frame Relay is used for signalling and data
transmission over the Gb interface.
The following characteristics apply for the Frame Relay connection:
• The maximum Frame Relay information field size is 1600 octets.
• The Frame Relay address length is two octets.
• Frame Relay Permanent Virtual Circuits (PVC) are used.
• The Frame Relay layer offers detection of errors, but no recovery from
errors.
• One or more Frame Relay PVCs are used between an SGSN and a BSS to
transport BSSGP PDUs.
6-64442
Issue 4.0
©Nokia Oyj

49 (55)


GPRS Architecture: Interfaces and Protocols

2.3.3 Base Station System GPRS Protocol (BSSGP)
The Base Station System GPRS Protocol (BSSGP) transfers control and
signalling information and user data between a BSS and the SGSN over the Gb
interface.
The primary function of BSSGP is to provide Quality of Service (QoS), and
routing information that is required to transmit user data between a BSS and an
SGSN.
A secondary function is to enable two physically distinct nodes, the SGSN and
PCU, to operate node management control functions.
There is a one-to-one relationship between the BSSGP protocol in the SGSN
and in the BSS/PCU. If one SGSN handles multiple BSSs/PCUs, the SGSN has
to have one BSSGP protocol device for each BSS/PCU.
The main functions for the BSSGP protocol are to:
• provide a connectionless link between the SGSN and the BSS
• transfer data in an unconfirmed way between the SGSN and the BSS
• provide for bi-directional control of the data flow between the SGSN and
the BSS
• handle paging requests from the SGSN to the BSS
• give support for deleting old messages in the BSS, for example when an
MS changes BSSs
• support multiple layer 2 links between the SGSN and the BSS.
2.4 Transmission protocols in the Um interface (BSS protocols)
BSS (PCU, CCU) MS
SNDCP
RLC
Radi o Link
Control
MAC
Medi um Access
Control
GSM RF
phy. Li nk & RF
IP / X.25
LLC
Um
RLC
Radio Link
Control
MAC
Medi um Access
Control
GSM RF
phy. Li nk & RF
•LLC segmentation/ re-assembly
•acknowledged/ unacknowledged
mode
•Backward Error Correction BEC
•Access signalling procedures
•physical channel bundling
•sub-multiplexing
•physical channel organisation
•channel coding
•GSMK

Figure 30. RLC, MAC, and physical layers
50 (55) ©Nokia Oyj

6-64442
Issue 4.0

Appendix – GPRS transmission plane protocols

2.4.1 Physical layer
The physical layer can be divided into the Radio Frequency (RF) layer and the
Physical Link layer.
The Radio Frequency (RF) is the normal GSM physical radio layer. Among
other things the RF layer specifies:
• the carrier frequency characteristics and GSM radio channel structures
• the radio modulation scheme used for the data
• the radio transmitter and receiver characteristics as well as performance
requirements.
The GSM RF physical layer is used for GPRS with the possibility for future
modifications.
The Physical Link layer supports multiple MSs sharing a single physical
channel and provides communication between the MSs and the network.
Network controlled handovers are not used in the GPRS service. Instead,
routing area updates and cell updates are used.
The Physical Link layer is responsible for:
• Forward Error Correction (FEC) coding, allowing the detection and
correction of transmitted code words and the indication of incorrectable
code words
• the interleaving of one RLC Radio Block over four bursts in consecutive
TDMA frames.
2.4.2 Medium Access Control (MAC)
The Medium Access Control (MAC) protocol handles the channel allocation
and the multiplexing, that is, the use of physical layer functions.
The GPRS MAC function is responsible for:
• Providing efficient multiplexing of data and control signalling on both the
uplink and downlink. This process is controlled by the network. On the
downlink, multiplexing is controlled by a scheduling mechanism. On the
uplink, multiplexing is controlled by medium allocation to individual
users (for example, in response to a service request).
• Mobile originated channel access, contention resolution between
channel access attempts, including collision detection and recovery.
• Mobile terminated channel access, scheduling of access attempts,
including queuing of packet accesses.
• Priority handling.
6-64442
Issue 4.0
©Nokia Oyj

51 (55)


GPRS Architecture: Interfaces and Protocols

2.4.3 The Radio Link Control (RLC)
The Radio Link Control (RLC) protocol offers a reliable radio link to the upper
layers. Two modes of operation of the RLC layer are defined for information
transfer: unacknowledged and acknowledged. The RLC layer can support both
modes simultaneously.
The RLC function is responsible for:
• Providing transfer of Logical Link Control layer PDUs (LLC-PDU)
between the LLC layer and the MAC function.
• Segmentation and reassembly of LLC-PDUs into RLC Data Blocks. See
Figure 2.
• Backward Error Correction (BEC) procedures enabling the selective
retransmission of uncorrectable code words. This process is generally
known as Automatic Request for Retransmission (ARQ).

Note
The Block Check Sequence for error detection is provided by the Physical Link
layer.
Information Field
FH FCS
Information
Field
BH BCS
Information
Field
BH BCS
Information
Field
BH BCS
Normal TDMA
Burst
RLC Block
LLC Frame
LLC
Layer
Normal TDMA
Burst
Normal TDMA
Burst
Normal TDMA
Burst
RLC/MAC
Layer
Physical
Layer
FH =Frame Header
FCS =Frame Check Sequence
BH =Block Header
BCS =Block Check Sequence (When SDCCH coding is used, BCS corresponds to the Fire code)

Figure 31. Segmentation of LLC-PDUs into RLC data blocks
52 (55) ©Nokia Oyj

6-64442
Issue 4.0

Appendix – GPRS transmission plane protocols

3. Signalling plane
In the signalling plane, the GTP specifies a tunnel control and management
protocol which allows the SGSN to provide GPRS network access for an MS.
The signalling plane also handles path management and location management.
Signalling is used to create, modify and delete tunnels. The underlying
protocols TCP/UDP, IP, L2, L1 are used as transport solution for GTP
signalling transport.
Between the MS and the SGSN, GPRS Mobility Management and Session
Management Information are exchanged. The protocols below MM in the BSS,
SGSN and MS are used for signalling transport.
SGSN
GMM/SM
GPRS Mobility
Management
and
Session
Management
NS
Network
Service
L1bis
Physical
Layer
BSSGP
BSS
GPRS
Protocol
LLC
Logical Link
Control
MS
GMM/SM
GPRS Mobility
Management
and
Session
Management
RLC
Radio Link
Control
MAC
Medium Access
Control
GSM RF
IP / X.25
LLC
Logical Link
Control
Um Gb
•GPRS attach / detach
•security
•routing area update,
location update
•PDP context activation,
modification & deactivation
•logical connection
•acknowledge/ unacknowledged
peer-to-peer operation
•ciphering
•SAPs to higher layer (SNDCP,
GMM, SMS)
MAC
GSM
RF
RLC
NS
L1bis
BSS
GP
Relay

Figure 32. Signalling plane over BSS
6-64442
Issue 4.0
©Nokia Oyj

53 (55)


GPRS Architecture: Interfaces and Protocols

Signalling via the interfaces Gc, Gf, Gd, and Gr is based on the modified mobile
specific SS7 protocols MAP and BSSAP.
L1
SCCP
Signalling Conne-
ction Control Part
TCAP
Transaction
Capabilities
Application Part
MAP
Mobile
Application
Part
MTP L2
MTP L3
L1
SCCP
TCAP
MAP
Mobile
Application
Part
MTP L2
MTP L3
L1
SCCP
MTP L2
MTP L3
L1
SCCP
MTP L2
MTP L3
BSSAP+
BSSAP+
BSS
Application
Part +
SGSN
Gr,f,d
SGSN MSC/VLR Gs
HLR
Gc
GGSN
GPRS-
specific
MAP
extension
Subset of
BSSAP
function-
alities

Figure 33. Signalling to registers and SMSC

54 (55) ©Nokia Oyj

6-64442
Issue 4.0

Appendix – GPRS transmission plane protocols

References
Nokia DX200 SGSN Product Description
Nokia GPRS Charging Gateway Product Description
Nokia GGSN Product Description
Nokia GPRS Solution Description
Nokia GPRS System Description
3GPP Specification 23.060

3GPP Specification 23.064
3GPP Specification 24.008
3GPP Specification 24.011
3GPP Specification 24.064
3GPP Specification 24.065
3GPP Specification 27.060
3GPP Specification 27.070
3GPP Specification 28.014
3GPP Specification 28.016
3GPP Specification 28.018
3GPP Specification 29.002
3GPP Specification 29.016
3GPP Specification 29.018
3GPP Specification 29.060

6-64442
Issue 4.0
©Nokia Oyj

55 (55)