Network Traffic Load
Content
COMMUNICATION SWITCHING TECHNIQUES
Contents
Network Traffic Load and Parameters
Grade of Service and Blocking Probability
Modeling Switching Systems
Loss Systems
Delay Systems
Overview
The telecommunication system has to service the voice traffic and data traffic.
The traffic is defined as the occupancy of the server. The basic purpose of the traffic
engineering is to determine the conditions under which adequate service is provided to
subscribers while making economical use of the resources providing the service. The
functions performed by the telecommunication network depends on the applications it
handles. Some major functions are switching, routing, flow control, security, failure
monitoring, traffic monitoring, accountability internetworking and network management.
To perform the above functions, a telephone network is composed of variety of
common equipment such as digit receivers, call processors, interstage switching links
and interoffice links etc. Thus traffic engineering provides the basis for analysis and
design of telecommunication networks or model. It provides means to determine the
quantum of common equipment required to provide a particular level of service for a
1
given traffic pattern and volume. The developed model is capable to provide best
accessibility and greater utilization of their lines and trunks. Also the design is to provide
cost effectiveness of various sizes and configuration of networks.
The traffic engineering also determines the ability of a telecom network to carry a
given traffic at a particular loss probability. Traffic theory and queuing theory are used to
estimate the probability of the occurrence of call blocking. Earlier traffic analysis based
purely on analytical approach that involved advanced mathematical concepts and
complicated operations research techniques. Present day approaches combine the
advent of powerful and affordable software tools that aim to implement traffic
engineering concepts and automate network engineering tasks.
In this unit, the traffic design requirements, various probability distributions, loss
systems and delay systems are described.
4.1 Network Traffic Load and Parameters
In a telephone network, the traffic load on a typical working day during the 24
hours is shown in figure. Obviously, there is little use of the network during 0 and 6
hours when most of population is asleep. There is a large peak around mid-forenoon
and mid-afternoon signifying busy office activities. The afternoon peak is, however
slightly smaller. The load is low during the lunch hour period i.e. 12.00-14.00 hours. The
period 17.00-18.00 hours is characterized by low traffic signifying that the people are on
the move from offices to their residences. The peak of domestic calls occurs after 18.00
hours when persons reach home and reduced tariff applies. In many countries including
India the period during which the reduced tariff applies has been changed to begin later
than 18.00 hours and one may expect the domestic call patterns also to change
accordingly. During holidays and festival days the traffic pattern is different from the
shown in figure. Generally there is a peak of calls around 10.00 hours just before people
leave their homes on outings and another peak occurs in the evening.
In a day, the 60 minute interval in which the traffic is the highest is called the
busy hour (BH). In figure the 1-hour period between 11.00 and 12.00 hours is the busy
hour. The busy hour may vary from exchange to exchange depending on the location
and the community interest of the subscribers. The busy hour may also show seasonal,
2
weekly and in some places even daily variations. In addition to these variations, there
are also unpredictable peaks caused by stock market or money activity, weather,
natural disaster, international events, sporting events etc. To take into account such
fluctuations while designing switching networks, three types of busy hours are defined
by CCITT in its recommendations:
1. Busy Hour: Continuous 1 hour period lying wholly in the time interval
concerned for which the traffic volume or the number of call attempts is
greatest.
2. Peak Busy Hour: The busy hour each day it usually varies from day to day or
over a number of days.
3. Time Consistent Busy Hour: The 1 hour period starting at the same time
each day for which the average traffic volume or the number of call attempts
is greatest over the days under considerations.
For ease of records the busy hour is taken to commence on the hour or half hour
only.
Not all call attempts materialize into actual conversations for a variety of reasons
such as called line busy, no answer from the called line and blocking in the trunk groups
or the switching centers. A call attempt is said to be successful or completed if the
3
called party answers. Call Completion Rate (CCR) is defined as the ratio of the number
of successful calls to the number of call attempts. The number of call attempts in the
busy hour is called Busy Hour Call Attempts (BHCA), which is an important parameter
in deciding the processing capacity of a common control or a stored program control
system of an exchange. The CCR parameter is used in dimensioning the network
capacity. Networks are usually designed to provide an overall CCR of over 0.70. A CCR
value of 0.75 is considered excellent and attempts to further improve the value is
generally not cost effective. A related parameter that is often used in traffic engineering
calculations is the busy hour calling rate which is defined as the average number of
calls originated by a subscriber during the busy hour.
The busy hour calling rate is useful in sizing the exchange to handle the peak
traffic. In a rural exchange, the busy hour calling rate may be as low as 0.2, whereas in
a business city it may be as high as three or more. Another useful information is to know
how much of the day’s total traffic is carried during the busy hour. This is measured in
terms of day-to-day busy hour traffic ratio which is the ratio of busy hour calling rate to
the average calling rate for the day. Typically this ratio may be over 20 for a city
business area and around six or seven for a rural area.
The traffic load on a given network may be on the local switching unit, inter office
trunk lines or other common sub systems. For analytical treatment all the common
subsystems of a telecommunication network are termed as servers or link or trunk. The
traffic on the network may then be measured in terms of the occupancy of the servers in
the network. Such a measure is called the traffic intensity which is defined as
Ao = Period for which server is occupied / total period of observation.
Generally the period of observation is taken as one hour. Ao is obviously
dimensionless. It is called erlang (E) to honour the Danish telephone engineer
A.K.Erlang, who did pioneering work in traffic engineering. A server is said to have 1
erlang of traffic if it is occupied for the entire period of observation. Traffic intensity may
also be specified over a number of servers.
Erlang measure indicates the average number of servers occupied and is useful
in deriving the average number of calls put through during the period of observation.
4
Contents
Network Traffic Load and Parameters
Grade of Service and Blocking Probability
Modeling Switching Systems
Loss Systems
Delay Systems
Overview
The telecommunication system has to service the voice traffic and data traffic.
The traffic is defined as the occupancy of the server. The basic purpose of the traffic
engineering is to determine the conditions under which adequate service is provided to
subscribers while making economical use of the resources providing the service. The
functions performed by the telecommunication network depends on the applications it
handles. Some major functions are switching, routing, flow control, security, failure
monitoring, traffic monitoring, accountability internetworking and network management.
To perform the above functions, a telephone network is composed of variety of
common equipment such as digit receivers, call processors, interstage switching links
and interoffice links etc. Thus traffic engineering provides the basis for analysis and
design of telecommunication networks or model. It provides means to determine the
quantum of common equipment required to provide a particular level of service for a
1
given traffic pattern and volume. The developed model is capable to provide best
accessibility and greater utilization of their lines and trunks. Also the design is to provide
cost effectiveness of various sizes and configuration of networks.
The traffic engineering also determines the ability of a telecom network to carry a
given traffic at a particular loss probability. Traffic theory and queuing theory are used to
estimate the probability of the occurrence of call blocking. Earlier traffic analysis based
purely on analytical approach that involved advanced mathematical concepts and
complicated operations research techniques. Present day approaches combine the
advent of powerful and affordable software tools that aim to implement traffic
engineering concepts and automate network engineering tasks.
In this unit, the traffic design requirements, various probability distributions, loss
systems and delay systems are described.
4.1 Network Traffic Load and Parameters
In a telephone network, the traffic load on a typical working day during the 24
hours is shown in figure. Obviously, there is little use of the network during 0 and 6
hours when most of population is asleep. There is a large peak around mid-forenoon
and mid-afternoon signifying busy office activities. The afternoon peak is, however
slightly smaller. The load is low during the lunch hour period i.e. 12.00-14.00 hours. The
period 17.00-18.00 hours is characterized by low traffic signifying that the people are on
the move from offices to their residences. The peak of domestic calls occurs after 18.00
hours when persons reach home and reduced tariff applies. In many countries including
India the period during which the reduced tariff applies has been changed to begin later
than 18.00 hours and one may expect the domestic call patterns also to change
accordingly. During holidays and festival days the traffic pattern is different from the
shown in figure. Generally there is a peak of calls around 10.00 hours just before people
leave their homes on outings and another peak occurs in the evening.
In a day, the 60 minute interval in which the traffic is the highest is called the
busy hour (BH). In figure the 1-hour period between 11.00 and 12.00 hours is the busy
hour. The busy hour may vary from exchange to exchange depending on the location
and the community interest of the subscribers. The busy hour may also show seasonal,
2
weekly and in some places even daily variations. In addition to these variations, there
are also unpredictable peaks caused by stock market or money activity, weather,
natural disaster, international events, sporting events etc. To take into account such
fluctuations while designing switching networks, three types of busy hours are defined
by CCITT in its recommendations:
1. Busy Hour: Continuous 1 hour period lying wholly in the time interval
concerned for which the traffic volume or the number of call attempts is
greatest.
2. Peak Busy Hour: The busy hour each day it usually varies from day to day or
over a number of days.
3. Time Consistent Busy Hour: The 1 hour period starting at the same time
each day for which the average traffic volume or the number of call attempts
is greatest over the days under considerations.
For ease of records the busy hour is taken to commence on the hour or half hour
only.
Not all call attempts materialize into actual conversations for a variety of reasons
such as called line busy, no answer from the called line and blocking in the trunk groups
or the switching centers. A call attempt is said to be successful or completed if the
3
called party answers. Call Completion Rate (CCR) is defined as the ratio of the number
of successful calls to the number of call attempts. The number of call attempts in the
busy hour is called Busy Hour Call Attempts (BHCA), which is an important parameter
in deciding the processing capacity of a common control or a stored program control
system of an exchange. The CCR parameter is used in dimensioning the network
capacity. Networks are usually designed to provide an overall CCR of over 0.70. A CCR
value of 0.75 is considered excellent and attempts to further improve the value is
generally not cost effective. A related parameter that is often used in traffic engineering
calculations is the busy hour calling rate which is defined as the average number of
calls originated by a subscriber during the busy hour.
The busy hour calling rate is useful in sizing the exchange to handle the peak
traffic. In a rural exchange, the busy hour calling rate may be as low as 0.2, whereas in
a business city it may be as high as three or more. Another useful information is to know
how much of the day’s total traffic is carried during the busy hour. This is measured in
terms of day-to-day busy hour traffic ratio which is the ratio of busy hour calling rate to
the average calling rate for the day. Typically this ratio may be over 20 for a city
business area and around six or seven for a rural area.
The traffic load on a given network may be on the local switching unit, inter office
trunk lines or other common sub systems. For analytical treatment all the common
subsystems of a telecommunication network are termed as servers or link or trunk. The
traffic on the network may then be measured in terms of the occupancy of the servers in
the network. Such a measure is called the traffic intensity which is defined as
Ao = Period for which server is occupied / total period of observation.
Generally the period of observation is taken as one hour. Ao is obviously
dimensionless. It is called erlang (E) to honour the Danish telephone engineer
A.K.Erlang, who did pioneering work in traffic engineering. A server is said to have 1
erlang of traffic if it is occupied for the entire period of observation. Traffic intensity may
also be specified over a number of servers.
Erlang measure indicates the average number of servers occupied and is useful
in deriving the average number of calls put through during the period of observation.
4
