Traffic Engineering

1
Traffic Engineering

• CST 331/431Roger G. Clery
• 2005

2
Network Design Principles

• Voice Traffic Engineering
• POTS
• Cell Phones
• Trunked Radio Services
• Can also be used for packet traffic

3
Design is a Composite of 4 Characteristics/Factor
s

• Service Level
• Load
• Behavior of Users in placing and holding calls
• Capacity of Network Trunks Ports

4
Calculations are based on Busy Hour Traffic

• Traffic and Blockage are LOWER the rest of the
  day.
• The Busy Hour need not be a clock hour.

5
Design is a Composite of the following
Characteristics/Factors

• The network owners grade of service objective
  (Probability of Blockage)
• Anticipated load measured in
• Call Attempts
• Call Holding Time
• Behavior of Users in placing and holding calls
• Capacity of Network Trunks Ports

6
Design is a Composite of the following
Characteristics/Factors

• The network owners grade of service objective
  (Probability of Blockage)Service is never
  perfect!10 P 0.10 is not so good0.25 P
  0.0025 is very good

7
Design is a Composite of the following
Characteristics/Factors

• The network owners grade of service objective
  (Probability of Blockage)
• Anticipated load measured in
• Call Attempts
• Call Holding Time
• Erlangs
• How much traffic is in this system?

8
Design is a Composite of the following
Characteristics/Factors

• The network owners grade of service objective
  (Probability of Blockage)
• Anticipated load measured in
• Call Attempts
• Call Holding Time
• Erlangs
• Behavior of Users in placing and holding
  callsWhat happens when a call is blocked

9
Design is a Composite of the following
Characteristics/Factors

• The network owners grade of service objective
  (Probability of Blockage)
• Anticipated load measured in
• Call Attempts
• Call Holding Time
• Erlangs
• Behavior of Users in placing and holding calls
• Capacity of Network Trunks PortsHow many?

10
Three Types of Blockage

• BCC Blocked Calls Cleared Calls leave the
  system either permanently or for some extended
  time

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Three Types of Blockage

• BCC Blocked Calls Cleared Calls leave the
  system either permanently or for some extended
  time
• BCD Blocked Calls Delayed Calls are Queued
  until capacity to serve is available This call
  is important to us, please stay on the line and
  your call will be answered sometime.

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Three Types of Blockage

• BCC Blocked Calls Cleared Calls leave the
  system either permanently or for some extended
  time
• BCD Blocked Calls Delayed Calls are Queued
  until capacity to serve is available This call
  is important to us, please stay on the line and
  your call will be answered sometime.
• BCH Blocked Calls Held users redial immediately
  until they get through, talk time is reduces by
  the time spent dialing.

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How to measure traffic

• Sum of all calls over some time period
• Usually One Hour 60 Minutes 3600 Seconds
• Add up all talk time
• If in minutes divide by 60
• If in seconds divide by 3600

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Examples of Traffic
Erlangs
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Traffic Measurement One Erlang

• 4 people each talk 15 minutes during a one hour
  time period.
• 10 people each talk 6 minutes during a one hour
  time period.
• 6 people each talk 10 minutes during a one hour
  time period.
• 30 people each talk 120 seconds during a one hour
  time period.

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Traffic Measurement One Erlang

• 4 people each talk 15 minutes during a one hour
  time period. 4 x 15 60 60/60 1
• 10 people each talk 6 minutes during a one hour
  time period. 10 x 6 60 60/60 1
• 6 people each talk 10 minutes during a one hour
  time period. 6 x 10 60 60/60 1
• 30 people each talk 120 seconds during a one hour
  time period. 30 x 120 3600 3600 /
  3600 1 ERLANG

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Estimate traffic load

• 4 sales people talk 30 minutes and 3 clerks talk
  10 minutes
• The average call is 7 minutes long and at peak
  time 10 calls arrive an hour
• 72,000 seconds of traffic are generated in an 8
  hour day peak is 20 of total

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Estimate traffic load

• You talk for two hours straight. (trick
  question)Traffic can never be more than 1
  Erlang in an hour per user, line or
  deviceTraffic is measured in a one hour period
  the BUSY HOUR

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You could use a formula/function but Looking it
up in a table is much more efficient
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Selection of traffic table

• Calls are queued BCD use Erlang C
• Calls are high priority Final route Use
  Poisson
• All others use Erlang B

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Using the Tables

• You need to know 2 of 3 variables
• Servers- trunks- agents
• Grade of Service/Blocking or probability of
  queueing
• Traffic in Erlangs

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Using the Tables

• Need to know 2 of 3 variables
• Servers- trunks- agents
• Grade of service or probability of queueing
• Traffic in Erlangs
• Close is OK Use as a guide
• For Erlang C table you need to know the average
  length of a call

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Practice Practice Practice

• Installing a PBX 3 Erlangs of traffic at peak
  hour, grade of service required is P.05 how many
  trunks required?
• New office will have 16 sales agents average 3
  calls per hour each and average call length is 6
  minutes how many trunks required?

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Erlang B Table
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Erlang B Table Find P 0.05
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Erlang B Table Find 3 Erlangs

• Find P 0.05 Column, then look down to find 3, 3
  isnt there but 2.96 is close enough
• Last, go left it the same row to find Trunks. 6
  Trunks ! easy!

27
Part 2 16 agents, 3 calls, 6 minutes

• 16 x 3 x 6 234 minutes
• 234/60 3.9 Erlangs
• What is the Blockage? P value?
• High 0.10
• Low 0.01
• Super Low 0.001

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Solution to part 2

• You dont want to miss any sales so low blockage
  is required.P 0.01 would require 10 trunksP
  0.001 would require 12 trunks
• Extrapolating.. How about 11 trunks

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Practice Practice Practice

• 10 Erlangs are handled by 16 trunks what is the
  grade of service?
• 24 trunks can carry ______ Erlangs at P.01 grade
• 3.1 Erlangs of traffic are traveling to the 4th
  planet of Alpha Centari using sub-space channels.
  Grade of service is to be P.01, how many
  sub-space channels are required?

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Always use the B Table

• Except when very critical service is required
• Fire, Police, Hospital Emergency
• Final trunk routes in phone company
• Then use
• Poisson

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Web based B Calculator
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