Teletraffic Engineering

Teletraffic EngineeringDepartment of Electrical Engineering

Traffic TheoryCovers specific types of random processes intelecommunications Average connection duration Average number of users Busy time Service time Call arrival2Traffic EngineeringRequired in telecommunications network planning to ensure that network costs are minimized without compromising the quality of service (QoS) delivered to the user of the network. It is based on probability theory and can be used to analyze mobile radio networks as well as other telecommunications networks.Mobile radio networks have traffic issues that do not arise in the fixed line PSTN. A mobile handset, moving in a cell, receives a signal with varying strength. This signal strength is subject to: slow fading, fast fading interference from other signals, 3Traffic EngineeringTraffic engineering balances the following factors based on given amount of traffic Grade of Service (GOS) Resources (e.g. trunk channels) Two types of systems implemented to provide voice communications Blocking Voice or data is blocked (by a busy signal) if network resource (e.g trunk channel) is not available. GOS = Blocking probability Delay System Voice or data is queued until network resource is available GOS = Queuing Probability and average time in queue4Traffic Engineering Holding Time - the length of time that a resource is being held(e.g the duration of a phone call)Traffic volume - for an interval is the sum of all the traffic holding times for that intervalTraffic intensity = traffic volume / time interval which is a measure of demand

5Traffic Engineering Erlangs - describe traffic intensity in terms of the number of hours of resource time required per hour of elapsed timeCCS( Centum Call Seconds) - measures the exact same traffic intensity as the Erlangs but expresses it as the number of 100 second holding times required per hour. Traffic registers sample stations every 100 seconds per hour to check for busies. Since there are 36 sets of hundred seconds in an hour36 CCS = 1 Erlangs6Traffic Measurement UnitesErlangs:Traffic intensity (named after of a Danish mathematician) is the average number of calls simultaneously in progress over a certain time. It is a dimensionless unit. Erlang one hour of continuous use of one channel = 1 Erlang 1 Erlang = 1 hour (60 minutes) of traffic In data communications, an 1 E = 64 kbps of data In telephone, 1 Erlang = 60 mins = 1 x 3600 call seconds % of Occupancy7Erlangs - ExampleFor example, if a group of user made 30 calls in onehour, and each call had an average call duration of 5minutes, then the number of Erlangs this represents isworked out as follows:

Minutes of traffic in the hour = number of calls x durationMinutes of traffic in the hour = 30 x 5Minutes of traffic in the hour = 150Hours of traffic in the hour = 150 / 60Hours of traffic in the hour = 2.5Traffic figure = 2.5 Erlangs8Call holding time is the length of time during which a traffic source engages a traffic path or channel. 1 3 minutes typical, >10 minutes infrequent for voice. Call Holding Time

H = average holding time, 3 minutes.Negative exponentialBusy HourBusy hour is that continuous 60 minutes time span of the day during which the highest usage occurs.

Busy HourNote: may not occur at the same time every dayweekly variationweek day /weekend variationseasonal variation

Mathematical formulas assume the busy hour traffic intensity is the average of an infinite number of busy hours.

Busy HourADPH (Average Daily Peak Hour) one determines the busiest hour separately for each day (different time for different days), and then averages over e.g. 10 days the resolution of the start time of the busy hour may be either a full hour (ADPH-F) or a quarter of an hour (ADPH-Q)TCBH (Time Consistent Busy Hour) a period of one hour, the same for each day, which gives the greatest average traffic over e.g. 10 daysFDMH (Fixed Daily Measurement Hour) a predetermined, fixed measurement hour (e.g. 9.30-10.30); the measured traffic is averaged over e.g. 10 daysaFDMH aTCBH aADPH

Traffic Density / Intensity Traffic Density is defined as the number of simultaneous calls at a given moment.Traffic Intensity,A represents the average traffic density (occupancy) during any one hour period.

Occupancy is any use of a traffic resource regardless of whether or not a connection (call) is completed.Occupancy is the probability of finding the trunk busy is equal to the proportion of time for which the trunk is busyLoss and Delay SystemsA Loss System is one in which a call attempt is rejected when there is no idle resource to serve the call. (BCC Blocked Call Cleared)Blocked callsA Delay System is one in which call attempts are held in a waiting queue until resource are available to serve the calls.Delayed calls

Offered, Carried and Blocked TrafficOffered traffic is the traffic intensity that would occur if all traffic submitted to a group of circuits could be processed.Carried traffic is the traffic intensity actually handled by the group.Blocked traffic is that portion of traffic that cannot be processed by the group of circuits (I.e. offered traffic minus carried traffic).Blocked traffic may be rejected, retried or offered to another group of circuits (overflow).System UtilizationLet A be the offered trafficC be the carried traffic, B is blocking probabilityLost traffic (A- C) = B*AProbability of blockingSystem utilization:

Traffic IntensityIs a measure of the average occupancy of a resource during a specified period of time, normally a busy hour.The traffic intensity offered by each user is:A = H ErlangswhereH is the average holding time of a call is the average number of call requested/hourIf there are U users and an unspecified number of channels.The total offered traffic intensity is:AT = UA ErlangsBusy hours traffic: Calls/busy hours *Mean call hold time17Traffic Intensity - contd.In a trunks system of C channels and equally Distributed traffic among the channels, the traffic intensity per channel is:AC = UA/C Erlangs/channelsThe traffic volumeis a measure of the total work done by a resource orfacility, normally over 24 hours VT = A * T Erlangs-Hours18Traffic IntensityTraffic Flow = (no. of calls)*(mean call holding time)

Example :If 100 calls are generated in 1 hour of 3 minutes average duration we have 3*100 = 300 call minutes or 300/60 = 5 call hours.ErlangsThe international dimensionless unit of telephone traffic is called the Erlang after A. K. Erlang (1878 1929) a Danish scientist.Defined as one circuit occupied for one hour.1 Erlang = 1 Callhour / hour

Busy hour trafficErlangs = (Calls/busy hour)*(mean call holding time)

(careful with units, all times in hours)1 Erl = 36 CCSExample IA call established at 1am between a mobile and MSC. Assuming a continuous connection and data transfer rate at 30 kbit/s, determine the traffic intensity if the call is terminated at 1.50am.Solution:Traffic intensity = (1 call)*(50 mins)*(1 hour/60 min) = 0.833 ErNote, traffic intensity has nothing to do with the data rate, only the holding time is taken into account.Note: If the traffic intensity > 1 Erlang: The incoming call rate exceeds the outgoing calls, thus resulting in queuing delay which will grow without bound (if the traffic intensity stays the same). If the traffic intensity is < 1 Erlang, then the network can handle more average traffic.21Example 2Consider a PSTN which receives 240 calls/hr. Each call lasts an average of 5 minutes. What is the outgoing traffic intensity to the public network.Solution:A = *H = 240 calls/hr and H = 5 minutesA = (240 calls /hr) x (5 min/call) = 1200 min/hr

Erlang cannot have any unit soA= 1200 min/hr * (1 hour/60 minutes) = 20 Erlangs

So 20 hours of circuit talk time is required for every hour of elapsed time. An average of T1 voice circuits busy at any time is 20. (Or 20 hours of continuous use of 20 channels.)22Example 3Call established at 2 am between a central computer and a data terminal. Assuming a continuous connection and data transferred at 34 kbit/s. What is the traffic if the call is terminated at 2-45am?Solution:Traffic = (1 call)*(45 min)*(1 hour /60 min) = 0.75 Erlangs.

Its nothing to do with the data rate of communication, only the call holding time.A group of 20 subscribers generate 50 calls with an average holding time of 3 minutes, what is the average traffic per subscriber?Example 4Solution:Traffic = (50 calls)*(3min)*(1 hour/60 min) = 2.5 Erlangs Average traffic per subscriber = 2.5 / 20 = 0.125 Erlangs per subscriber.

Individual (residential) calling rates are quite low and may be expressed in milli-Erlangs, i.e. 0.125 Erlangs = 125 milli-Erlangs.

There are 100 subscribers with the following telephone traffic profile: 20 make 1 call/hour for 6 minutes; 20 make 3 calls/hour for half a minute; 60 make 1 call/hour for 1 minute. Example 5Solution:The traffic they generate is:20x1x (6/60) = 2 E20x3x(0.5/60) = 0.5 E60x1x(1/60) = 1 E

a total traffic of 3.5 E.

On average, each subscriber generates 3.5/100 = 35 mE.In practice on average telephone subscribers generate between 25 to 35 mE during the busiest hour

Grade of Service, GoSGrade of Service is a measure of the probability that a percentage of the offered traffic will be blocked or delayed. the ability to interconnect users the rapidity with which that connection is madeCommonly expressed as the fraction of calls or demand thatfails to receive immediate service (blocked calls)is forced to wait longer than a given time (delayed calls) For example, if GOS = 0.05, one call in 20 will be blocked during the busiest hour because of insufficient

Grade of Service, B

B = Lost traffic Offered trafficOrB = Lost callOffered call

Grade of Service (GoS)Grade of Service (GoS)A measure of the performance of a telephone systemGOS is a measure of the ability of a user to access a trunked system during the busiest hourAlso an indication of the user not being able to secure a channel during the busiest hourTelephone networks are designed with specified GOS, usually for the busiest hour. If a subscriber is able to make a call during the busiest hour, he will be able to make a call at any other time

Grade Of Service, B is used to observe and measure how many calls are offered, carried and lost in the system.

The lower this number, the higher the GOS.B = Number of calls lost Number of calls offered B = Traffic lost Traffic offered Offered TrafficThe offered traffic: Volume of traffic offered to a switch that are all processed is defined as:

Offered traffic = carried traffic + overflow

The carried traffic: The actual traffic carried by a switch.Overflow (blocked) traffic: Portion of the traffic not processed.Busy Hour Call Attempts (BHCA) Used to evaluate and plan capacity for telephone networks Is the number of telephone calls made at the peak hour The higher the BHCA, the higher the stress on the network processors. Not to be confused with Busy Hour Call Completion (BHCC), which truly measures the throughput capacity of the network.30For example, if GOS = 0.05, one call in 20 will be blocked during the busiest hour because of insufficient capacity

For GOS = 0.02. This means that two users of the circuit group out of a hundred will encounter a call refusal during the busy hour.

Example 6 1200 calls are offered to a channel and 6 calls are lost. Duration of a call is 3 minutes. Find:Offered traffic, ACarried trafficLost trafficGOS, BCongestion time Solution:A = CH/T = (1200 x 3) / 60 = 60 ECarried traffic = [(1200-6) x 3] / 60 = 59.7 ELost traffic = (6 x 3) / 60 = 0.3 EB= Lost traffic / Offered traffic = 6/1200 = 0.005 Congestion time= B x 1 hours (second)= 0.005 x 60 x 60= 18 seconds

On average during the busy hour, a company makes 120 outgoing calls of average duration 2 minutes. it receives 200 incoming calls of average duration 3 minutes. Find;A. The outgoing trafficB. The incoming trafficC. The total trafficExample 7A. The outgoing traffic is 120 x 2/60 = 4EB. The incoming traffic is 200 x 3/60 = 10EC. Total Traffic is 4E + 10E = 14ESolution:Chart1333335716701108766506886927451382211533

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