EE 489 Telecommunication Systems Engineering University of Alberta

EE 489 Telecommunication Systems Engineering University of Alberta Dept. of Electrical and Computer Engineering Introduction to Traffic Theory Wayne Grover TRLabs and University of Alberta

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering A note on sources of this material • The following material on traffic theory / traffic engineering was initially developed as printed handwritten notes from 1998 to 2001 by W. Grover for EE 589. • In 2002 John Doucette set these materials into the present powerpoint format for use in EE 589. • The ppt versions of the original notes, with updating and some revisions by W. Grover, 2007, are made available courtesy J. Doucette for use in EE 489. • Related Reading in Bellamy 3 rd Edition: – Chapter 12, pp. 519 -567. Material prepared by W. Grover (1998 -2002) 2

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Traffic Engineering • One billion+ terminals in voice network alone – Plus data, video, fax, finance, etc. • Imagine all users want service simultaneously • In practice, low overall utilization – Random duration at random times • Balance cost and practicality with acceptably low chance of network blocking. • We use traffic engineering to “dimension” the network, i. e. mainly to decide on how many transmission paths (trunks) are needed between node and the sizes of switches or routers needed. Material prepared by W. Grover (1998 -2002) 3

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Characterization of Circuit-Switched Traffic • Calling Rate ( ) – also called Arrival Rate – Average number of calls or “connections” initiated per unit time (units. “attempts per hour”) – Each arrival independent of other calls – Random in time If receive calls from a terminal in time T: If receive calls from m terminals in time T: Group calling rate Per terminal calling rate Material prepared by W. Grover (1998 -2002) 4

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Characterization of Telephone Traffic (2) • Calling rate assumption: – Number of calls in time T is Poisson distributed: • Time between calls is negative exponentially distributed: Material prepared by W. Grover (1998 -2002) 5

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Characterization of Telephone Traffic (3) • Holding Time (h) – Mean length of time a call lasts – Probability of lasting time t or more is exponential in nature: – Real sampled voice data fits very closely to the negative exponential form above – As non-voice “calls” begin to dominate, more and more calls have a constant holding time characteristic • Departure Rate ( ): Material prepared by W. Grover (1998 -2002) 6

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Real Holding Time Sample Data Material prepared by W. Grover (1998 -2002) 7

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Exponential Form of Holding Time • Memory-less property – Call “forgets” that it has already survived to time T 1 • Proof: Recall: Material prepared by W. Grover (1998 -2002) 8

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Exponential Form of Holding Time • Understanding Memorylessness in Call holding times It means that: – whether a connection has already existed for 1 minute or one hour… – the probabiity that it will last another minute (or any other unit time)… – is the same. • Counterintuitive (? ) but very accurate actually. • Can understand it (or any memoryless process) as being analogous to repeated coin tossing Material prepared by W. Grover (1998 -2002) 9

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Traffic Volume (V) = # calls in time period T h = mean holding time V = volume of calls in time period T • Units can be “ccs”: ccs – Hundred call seconds “c” “s ” – 1 ccs is volume of traffic equal to: – one circuit busy for 100 seconds, or – two circuits busy for 50 seconds, or – 100 circuits busy for one second, etc. Material prepared by W. Grover (1998 -2002) 10

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Traffic Intensity (A) • This is the rate of “traffic flow”. flow = # calls in time period T h = mean holding time Recall: T = time period of observations Recall: = calling rate = departure rate V = call volume • Units: – “ccs/hour”, /hour or – dimensionless (if h and T are in the same units) “Erlang” Erlang unit Material prepared by W. Grover (1998 -2002) 11

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering The “Erlang” • Dimensionless unit of traffic intensity. Characterizes the intensity of any stream of demands for circuit-switched (or connectionoriented data) connections • Named after Danish mathematician A. K. Erlang (1878 -1929) • Usually denoted by symbol E. • 1 Erlang is equivalent to the traffic intensity that keeps: – one circuit busy 100% of the time, or – two circuits busy 50% of the time, or – four circuits busy 25% of the time, etc. • e. g. , 26 Erlangs is equivalent to traffic intensity that keeps : – 26 circuits busy 100% of the time, or – 52 circuits busy 50% of the time, or – 104 circuits busy 25% of the time, etc. Material prepared by W. Grover (1998 -2002) 12

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Erlang (2) • How does the Erlang unit correspond to ccs? ccs • Percentage of time a terminal is busy is equivalent to the traffic generated by that terminal in Erlangs, or • Average number of circuits in a group busy at any time • Typical usages: – residence phone -> 0. 02 E – business phone -> 0. 15 E – interoffice trunk -> 0. 70 E Material prepared by W. Grover (1998 -2002) 13

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Traffic Offered, Carried, and Lost • Offered Traffic (TO ) equivalent to Traffic Intensity (A) – Takes into account all attempted calls, whether blocked or not, and uses their expected holding times • Also Carried Traffic (TC ) and Lost Traffic (TL ) • Consider a group of 150 terminals, each with 10% utilization (or in other words, 0. 1 E per source) and dedicated service: service 1 150 each terminal has an outgoing trunk (i. e. terminal: trunk ratio = 1: 1) 1 150 TO = A = 150 x 0. 10 E = 15. 0 E TC = 150 x 0. 10 E = 15. 0 E TL = 0 E Material prepared by W. Grover (1998 -2002) 14

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Traffic Offered, Carried, and Lost (2) • A = TO = TC + TL Traffic Intensity Offered Traffic Carried Traffic Lost Traffic • TL = TO x Prob. Blocking (or congestion) = P(B) x TO = P(B) x A • Circuit Utilization ( ) - also called Circuit Efficiency – proportion of time a circuit is busy, or – average proportion of time each circuit in a group is busy Material prepared by W. Grover (1998 -2002) 15

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Example #1 • Traffic Engineered solution for the 150 terminals at 0. 1 E. . . Material prepared by W. Grover (1998 -2002) 16

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Grade of Service (gos) • In general, the term used for some traffic design objective • Indicative of customer satisfaction • In systems where blocked calls are cleared, usually use: • Typical gos objectives: – in busy hour, range from 0. 2% to 5% for local calls, however – generally no more that 1% – long distance calls often slightly higher • In systems with queuing, gos often defined as the probability of delay exceeding a specific length of time Material prepared by W. Grover (1998 -2002) 17

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Grade of Service Related Terms • Busy Hour – One hour period during which traffic volume or call attempts is the highest overall during any given time period • Peak (or Daily) Busy Hour – Busy hour for each day, usually varies from day to day • Busy Season – 3 months (not consecutive) with highest average daily busy hour • High Day Busy Hour (HDBH) – One hour period during busy season with the highest load Material prepared by W. Grover (1998 -2002) 18

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Hourly Traffic Variations Material prepared by W. Grover (1998 -2002) 19

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Daily Traffic Variations Material prepared by W. Grover (1998 -2002) 20

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Seasonal Traffic Variations Material prepared by W. Grover (1998 -2002) 21

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Seasonal Traffic Variations (2) Material prepared by W. Grover (1998 -2002) 22

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Grade of Service Related Terms (2) • Average Busy Season Busy Hour (ABSBH) – One hour period with highest average daily busy hour during the busy season – For example, assume days shown below make up the busy season: ABSBH Highest Note: Red indicates daily busy hour Material prepared by W. Grover (1998 -2002) 23

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Grade of Service Related Terms (3) • Ten High Day Busy Hour (10 HDBH) – One hour period with highest average load for the 10 highest day loads for that hour – For example: 10 HDBH Highest Note: Red indicates 10 highest hourly loads for each hour Material prepared by W. Grover (1998 -2002) 24

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Grade of Service Related Terms (4) • Examples of grade-of-service type specification statements: – – – 1. 5% of calls in ABSBH have dial tone delay more than 3 seconds blocking on trunk groups < 3% blocking through switch matrix < 0. 1% probability of packet delay > x msec less than 5% probability of dropped connection in progress < 1% per minute etc. • Note implications of designing to “busy hour” g. o. s. objectives: – simplifies design and forecasting problems – busy hour may change (unpredictably!) – the resulting network is “peak engineered” - same as the power network …may be greatly underutilized at off busy-hour times • Q. What could you do with this? Material prepared by W. Grover (1998 -2002) 25

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Typical Call Attempts Breakdown • Calls Completed - 70. 7% • Called Party No Answer - 12. 7% • Called Party Busy - 10. 1% • Call Abandoned - 2. 6% • Dialing Error - 1. 6% • Number Changed or Disconnected - 0. 4% • Network Blockage or Failure - 1. 9% Material prepared by W. Grover (1998 -2002) 26

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Traffic Theoretic Models for Blocked Calls • Blocked Calls Cleared (BCC) BCC – Blocked calls leave system and do not return – Good approximation for calls in 1 st choice trunk group with overflow available. • Blocked Calls Held (BCH) BCH – Blocked calls remain in the system for the amount of time it would have normally stayed for – If a server frees up, the call picks up in the middle and continues – Not a good model of real world behaviour (mathematical approximation only) – Tries to approximate call reattempt efforts • Blocked Calls Wait (BCW) BCW – Blocked calls enter a queue until a server is available – When a server becomes available, the call’s holding time begins Material prepared by W. Grover (1998 -2002) 27

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Blocked Calls Cleared (BCC) 2 sources 10 minutes Source #1 Offered Traffic 1 3 Source #2 Offered Traffic 2 4 1 st call arrives and is served Only one server 2 nd call arrives but server already busy Traffic Carried Total Traffic Offered: TO = 0. 4 E + 0. 3 E TO = 0. 7 E 1 2 3 4 Total Traffic Carried: TC = 0. 5 E 2 nd call is cleared 3 rd call arrives and is served 4 th call arrives and is served Material prepared by W. Grover (1998 -2002) 28

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Blocked Calls Held (BCH) 2 sources 10 minutes Source #1 Offered Traffic 1 3 Source #2 Offered Traffic 2 4 Total Traffic Offered: TO = 0. 4 E + 0. 3 E TO = 0. 7 E 1 st call arrives and is served Only one server 2 nd call arrives but server busy Traffic Carried 1 2 2 3 4 2 nd call is held until server free 2 nd call is served Total Traffic Carried: TC = 0. 6 E 3 rd call arrives and is served 4 th call arrives and is served Material prepared by W. Grover (1998 -2002) 29

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Blocked Calls Wait (BCW) 2 sources 10 minutes Source #1 Offered Traffic 1 3 Source #2 Offered Traffic 2 4 Total Traffic Offered: TO = 0. 4 E + 0. 3 E TO = 0. 7 E 1 st call arrives and is served Only one server 2 nd call arrives but server busy 2 nd call waits until server free Traffic Carried 1 2 2 3 4 Total Traffic Carried: TC = 0. 7 E 2 nd call served 3 rd call arrives, waits, and is served 4 th call arrives, waits, and is served Material prepared by W. Grover (1998 -2002) 30

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Blocking Probabilities • System must be in a Steady State – Also called state of statistical equilibrium – Arrival Rate of new calls equals Departure Rate of disconnecting calls – Why? • If calls arrive faster that they depart? • If calls depart faster than they arrive? Material prepared by W. Grover (1998 -2002) 31

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Binomial Distribution Model • Assumptions: – m sources – A Erlangs of offered traffic • per source: TO = A/m • probability that a specific source is busy: P(B) = A/m • Can use Binomial Distribution to give the probability that a certain number (k) of those m sources is busy: Material prepared by W. Grover (1998 -2002) 32

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Binomial Distribution Model (2) • What does it mean if we only have N servers (N<m)? – We can have at most N busy sources at a time – What about the probability of blocking? • All N servers must be busy before we have blocking Remember: Material prepared by W. Grover (1998 -2002) 33

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Binomial Distribution Model (3) • What does it mean if k>N? – Impossible to have more sources busy than servers to serve them – Doesn’t accurately represent reality • In reality, P(k>N) = 0 – In this model, we still assign P(k>N) = A/m – Acts as good model of real behaviour • Some people call back, some don’t • Which type of blocking model is the Binomial Distribution? – Blocked Calls Held (BCH) Material prepared by W. Grover (1998 -2002) 34

EE 489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Time Congestions vs. Call Congestion • Time Congestion – Proportion of time a system is congested (all servers busy) – Probability of blocking from point of view of servers • Call Congestion – Probability that an arriving call is blocked – Probability of blocking from point of view of calls • Why/How are they different? Time Congestion: Call Congestion: Probability that there are more sources wanting service than there are servers. Probability that all servers are busy. Material prepared by W. Grover (1998 -2002) 35