Traffic Engineering
Traffic Engineering provides the basis for the analysis and
design of telecommunication networks.
We have already calculated blocking probabilities due to the
unavailability of switching paths.
But it is not only the switching elements but also many other
common shared subsystems (digit receivers, interstage
switching links, call processors and trunks between
exchanges) in a telecommunication network that contribute
to the blocking of a subscriber call.
Besides, the load or the traffic pattern on the network varies
during the day with heavy traffic at certain times and low
traffic at other times.
Traffic engineering enables one to determine the ability of a
telecommunication network to carry a given traffic at a
particular loss probability.
It provides a means to determine the quantum of common
equipments required to provide a particular level of service
Network traffic load and Parameters
Obviously, there is little use of the
network during the dead of night
when most of the population is
asleep.
There is a large peak around
mid-forenoon and mid-afternoon
signifying busy office activities.
The afternoon peak is, however,
slightly small.
The load is low during the
lunch-hour period (12.00-14.00 lunch-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.
During holidays and festival days
the traffic pattern is different from that shown in fig.
Network traffic load and Parameters
In a day, the 60-minute interval in which the traffic is the
highest is called the busy hour (BH).
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, weekly and in some
places even daily variations.
In addition to these variations, there are also unpredictable
peaks caused by stock market 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 E.600:
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.
Peak Busy Hour: The busy hour in each day; it usually varies from day to day, or over a number of days.
Network traffic load and Parameters
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 called party answers.
Call Completion Rate (CCR) is defined as the ratio of the number of
successful calls to the number of call attempts. It 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.
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.
A related parameter used in TE is the busy hour calling rate (BHCR) which is defined as the average number of calls originated by a
subscriber during the busy hour.
Example: An exchange serves 2000 subscribers. If the average BHCA is 10000 & the CCR is 60%, calculate the BHCR.
Solution: Average busy hour calls=BHCAxCCR=6000 calls
BHCR= average busy hour calls/ total no. of subscribers=3
Network traffic load and Parameters
The BHCR is useful in sizing the exchange to handle the peak traffic.
In a rural exchange, the BHCR 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-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. For analytical treatment in our course, all the common subsystems of
a telecommunication network are collectively termed as
servers.
The traffic on the network may then be measured in terms of theoccupancy of the servers in the network. Such a measure is called the
traffic intensity
which is defined as:A0=period for which a server is occupied/total period of observation
Generally, the period of observation is taken as one hour. A0 is
obviously dimensionless. It is called
erlang(E)
to honor the Danish telephone engineer A.K. Erlang, who did pioneering work in TE. 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
Network traffic load and Parameters
Example: In a group of 10 servers, each is occupied for 30
minutes in an observation interval of two hours. Calculate
the traffic carried by the group.
Solution: Traffic carried per server
=occupied duration/total duration=30/120=0.25 E
Total traffic carried by the group=10x0.25=2.5E, this
actually indicates the average no. of servers occupied.
Example: A group of 20 servers carry a traffic of 10 erlangs.
If the average duration of a call is three minutes, calculate
the number of calls put through by a single server and the
group as a whole in a one-hour period.
Solution: Traffic per server=10/20=0.5E, i.e., a server is
busy for 30 minutes in one hour.
Number of calls put through by one server=30/3=10 calls
Total number of calls put through by the group=10x20=200
calls.
Network traffic load and Parameters
Traffic intensity is also measured in centum call second (CCS) (valid only in telephone circuits) which represents a call-time product. One CCS may mean one call for 100 seconds duration or 100 calls for one second
duration each or any other combination.
Sometimes, call seconds (CS) and call minutes (CM) are also used.
Note that, 1E= 36 CCS=3600 CS=60CM
Example: A subscriber makes three phone calls of three minutes, four minutes and two minutes duration in a one-hour period. Calculate the subscriber traffic in erlangs, CCS & CM.
Solution: Subscriber traffic in erlangs=busy period/total period=(3+4+2)/60=0.15E
Traffic in CCS= (3+4+2)x60/100=5.4 CCS Traffic in CM=(3+4+2)=9 CM
Two important parameters are required to estimate the traffic intensity or the network load:
Average call arrival rate, C
Average holding time per call, th
So, the load offered to the network, A= Cxth
C & th must be expressed in like time units. For example, if C is in number of Calls per minute, th must be in minutes per call.
Network traffic load and Parameters
Example: Over a 20- minute observation interval, 40 subscribers initiate calls. Total duration of the calls is 4800 seconds. Calculate the load offered to the network by the subscribers and the average subscriber traffic.
Solution: Mean arrival rate, C=40/20= 2 calls/minute Mean holding time, th= 4800/(40x60)= 2 minutes/call Therefore, offered load=2x2=4E
Average subscriber traffic=4/40=0.1E
It is possible that the load generated by the subscribers sometimes
exceeds the network capacity. There are two ways in which this overload traffic may be handled: the overload traffic may be rejected without being serviced or held in a queue until the network facilities become available. In the first case, the calls are lost and in the second case the calls are
delayed. Correspondingly, two types of systems, called loss systems and delay systems are encountered.
Conventional automatic telephone exchanges behave like loss systems whereas operator assisted manual exchanges can be considered as delay systems.
In data networks, circuit-switched networks behave as loss systems
whereas store-and-forward (S&F) message or packet networks behave as delay systems. But, in a S&F network if the queue buffers become full, then further requests have to be rejected.
The basic performance parameters for a loss system are the grade of service & the blocking probability, and for a delay system, the service
Network traffic load and Parameters
Average delays, or probability of delay exceeding a certain limit, or variance of delays may be important under different circumstances.
The traffic models used for studying loss systems are known as blocking or
congestion models and the ones used for studying delay systems are called queuing models.
In loss systems, the overload traffic is rejected and hence is not carried by the network. The amount of traffic rejected by the network is an index of the quality of the service offered by the network. This is termed grade of service (GOS) and is defined as the ratio of lost traffic to offered traffic. Accordingly, GOS=(A-A0)/A where, A= offered traffic, A0= carried traffic, A-A0=lost traffic.
The smaller the value of GOS, the better is the service. The recommended value for GOS is 0.002.
Usually, every common subsystem in a network has an associated GOS
value. The GOS of the full network is determined by the highest GOS value of the subsystems in a simplistic sense.
Since the volume of traffic grows as the time passes by, the GOS value of a network deteriorates with time. In order to maintain the value within reasonable limits, initially the network is designed to have a much smaller GOS value than the recommended one so that the GOS value continues to be within limits as the network traffic grows.
Network traffic load and Parameters
The blocking probability PB is defined as the probability that all the servers in a system are busy. At the first instance, it may appear that the blocking probability is the same measure as the GOS which is generally not true. For example, in a system with equal number of servers and subscribers,
the GOS is zero as there is always a server available to a subscriber. On the other hand, there is a definite probability that all the servers are busy at a given instant and hence the blocking probability is nonzero.
The fundamental difference is that the GOS is a measure from the
subscriber point of view whereas the blocking probability is a measure from the network or switching system point of view.
In order to distinguish between these two terms clearly, GOS is called call congestion or loss probability and the blocking probability is called time congestion.
In the case of delay systems, GOS as defined above is not meaningful. The probability that a call experiences delay, termed delay probability, is a
useful measure.
If the offered load far exceeds the network capacity, then the queue
lengths become very large and the system is unstable as they would never be able to clear the offered load. An easy way of bringing the system back to stable region of operation is to make it behave like a loss system until the queued up traffic is cleared to an acceptable limit. This technique of maintaining the stable operation is called flow control.
In recent times, a more general term called QOS is being used which includes other factors like quality of speech, error-free transmission
