cisco press - principios, diseño e ingenieria de trafico redes de voz

8/8/2019 cisco press - principios, diseño e ingenieria de trafico redes de voz

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TrafficEngineeringPrinciples forVoice Network

Design

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Steve Quan

[email protected]



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Understand How Voice Networks AreEngineered Today

Presentation Goal:

Voice Traffic Engineering

With the intent that this information willbetter prepare the communications

professional to integrate voice and data

over a single network



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Agenda•• PBX FundamentalsPBX Fundamentals

• Basic Voice Traffic Engineering

• How Costs Impact Trunk Groups

• Voice Traffic Impact on a PacketBased Data Network

• How Voice Quality Is Impacted by aPacket Based Data Network



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Overview/Review of PBX

Traditional Telephone ServiceTraditional Telephone Service

C.O.

# of Telephones=

# of Lines

PBX Voice ServicePBX Voice Service

GrowthGrowth

CostCostFeaturesFeaturesControlControl

10’s100’s

C.O.

# of Telephones>

# of Lines (trunks)

Switch SwitchPBX



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PBX Overview/Review

Digits Are Dialed

InternalInternal

CallCall

ExternalExternal

CallCall

ManipulateDigits

TrunkTrunk

Available ?Available ?

Seize TrunkSeize Trunk

OutpulseOutpulse DigitsDigits

PBX

Lines Trunks

Telephone Goes “off-hook”Telephone Goes “off-hook”



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PBX Networking

• Examples of servicesfor long distance calls

10’s100’s

10’s100’s

Switch PBX

C.O. C.O. TorontoVancouver

C.O. Trunks

Tie Trunks

604–666–5678 416–345–2424

C.O. Trunks

Tie Trunks

PSTN

FX

416–526–1234

SwitchPBX



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The New Voice Networking

10’s100’s

10’s100’s

Switch PBX

PSTN

C.O. C.O. TorontoVancouver

C.O Trunks

Tie Trunks

604–666–5678 416–345–2424

C.O Trunks

Tie Trunks

DataData

DeviceDeviceDataData

DeviceDeviceData Network

FX

416–526–1234

SwitchPBX



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Agenda• PBX Fundamentals

•• Basic Voice Traffic EngineeringBasic Voice Traffic Engineering






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Basic Voice Traffic Engineering:

A Four-Step Process• Step 1: Obtain traffic data

• Step 2: Profile traffic

Determine the busy hour

• Step 3: Determine number of physicaltrunks required too meet traffic

• Step 4: Determine the least-costcombination of trunks

Iterative comparison



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Step 1: Obtain Traffic Data• Conceptually simple; difficult in practice

• Sources of traffic information

Carrier bills

Shows only chargeable calls

Carrier design studies or traffic reports

Traffic reports from PBX

CDR (Call Detail Report)

Reports specific to manufacturer

Third party software and hardware availablefor analysis



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Sample CDR Reporttran_date duratio ckt dialing_no dialed_no charge extensionfacility_na pbx

------------------- -------- ---- ------------- ------------- ------- --------- ------------- -----

08/01/97 - 00:05:00 2 30 61445 1181352196009 0.68 0 IDDD_SJ1 SJ1

08/01/97 - 00:07:00 2 1 71820 1181352196009 0.68 0 IDDD_SJ1 SJ1

08/01/97 - 00:31:00 1 30 77456 1181352196028 0.34 0 IDDD_SJ1 SJ1

08/29/97 - 23:35:00 1 30 77458 1181352196028 0.34 0 IDDD_SJ1 SJ1

08/30/97 - 04:29:00 2 6 66151 1181352196028 0.68 0 IDDD_SJ1 SJ1

08/30/97 - 20:50:36 2 30 61035 1181352196009 1.02 0 IDDD_SJ1 SJ1

-- --------- ----- ------------

Query Sum ary

-- --------- ----- ------------

Total Calls: 595Total Minutes: 2382.3

Total Cost: 900.17



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Step 2: Profiling the Traffic—

Group into Categories• Inbound vs. outbound

• Call distance

Local calls

Intrastate long distance

Interstate long distance

International

• Type of call



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Defining the Traffic

A = C x TA = C x T

A, the Traffic Flow Is the Product of:A, the Traffic Flow Is the Product of:

C the Number of Calls Originated DuringC the Number of Calls Originated Duringa Period of 1 Hour and T, the Averagea Period of 1 Hour and T, the Average

Holding Time of the CallHolding Time of the Call



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For Example

PBXPBX11

200 Calls of an Average Duration of Two Minutes AreGenerated During a Period of One Hour. The Traffic

Flow Is Equal to 400 Call-Minutes per Hour



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Converting to a

Common Measurement

• Converting call-minutes to call-hours,divide by 60

• In our example: 400/60 = 6.67 call-hours

• Typically we use erlangs or thecontinuos use of a circuit for one hour

• Another common measurement is CCS

(Centum Call Seconds)1 erlang = 36 CCS



Traffic Variation

Traffic

Traffic

8AM 6PM

M T W T F

Traffic

Jan Dec

Traffic Varies by Hour, Day, Month and Year

Traffic

98 99 00 01

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Busy Hour

• Busy hour =

Total traffic in a month x % in busy day x %in busy hour

• BH is always used to determinethe required number of trunks



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Adjusting the Holding Time

• Holding time = total time trunk in use

Dialing + Call Setup + Ringing +Conversation + Release

• Factors affecting holding time

Call processing may or may not be included

Telephone bills only have conversation time

Call timing data “in” varies by PBX manufacturerOther sources of trunk use: Ring-no-answer,busy signal, etc.



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Adjusting the Holding Time (Cont.)

• If additional holding time is not included,the result is too few trunks

Add 10% to 16% to length of all calls

• Call billingIf billing is in 6 second increments

No adjustment is required

1-minute increments, on average, is 30 seconds too long

Using this information for trunking will result in toomany trunks

Subtract 30 seconds from each call



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Example

A Bill Shows 404 Calls Totaled 1834 Minutes.A Bill Shows 404 Calls Totaled 1834 Minutes.Billing Is in 1 Minute Increments.Billing Is in 1 Minute Increments.

What Is the Adjusted Traffic?What Is the Adjusted Traffic?

404 x (0.5) = 202404 x (0.5) = 202

1834 - 202 = 1632 “Real Traffic”1834 - 202 = 1632 “Real Traffic”



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Traffic Probability

• Traffic source characteristics

• How lost calls are handled

• How the switch handlestrunk allocation



Number of Originating Sources

• Infinite—Probability ofcall arrival is constantand does not dependupon the state of

occupancy• Finite—The number of

sources is relativelysmall and the arrivalrate is proportional tothe number of sources

not already engaged ina call

23

Poisson Distribution with10 Trunks and a P of .01

Infinite 4.13

Number of Sources Traffic Capacity (erlangs)

100 4.26

75 4.3550 4.51

25 4.84

20 5.08

15 5.64

13 6.03

11 6.95

10 10

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Random Traffic

.02

.04

.06

.08

.010

.012

5 10 15 20 25 30 35

P(x)

Mean, u = 15.02Variance = 15.67VMR = 1.04

x



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Smooth Traffic

5 10 15 20 25 30 35

Mean, u = 14.97Variance = 10.82VMR = .72

.02

.04

.06

.08

.010

.012

P(x)

x



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x

Rough or Peaked Traffic

.02

.04

.06

.08

.010

.012

5 10 15 20 25 30 35

P(x)

Mean, u = 15.21Variance = 20.82VMR = 1.37



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Handling Lost Calls

Trunk

C = Carried TrafficC = Carried TrafficO = OfferedF = First Attempt

B = Blocking Factor

Calls Cleared

Calls Held

Calls Delayed

Lost Calls Cleared (LCC)—Give up on a Busy Signal

Lost Calls Held (LCH)—Redial on a Busy Signal

Lost Calls Delayed (LCD)—Sent SomewhereElse When Busy



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Switch Trunk Availability

Inputs Trunk

Inputs Trunk

Inputs Trunk

Inputs Trunk

Limited AvailabilityLimited Availability Full AvailabilityFull Availability



Step 3: Trunk Group Provisioning

• Theory of large trunk groups:

The larger the trunk group, the more efficient they are

The benefit of adding another trunk gets smalleras the trunk group gets larger

29

TrafficTraffic TrafficTrafficTrunksTrunks ErlangsErlangs AvgAvg/Trunk/Trunk

IncreaseIncrease IncreaseIncrease

22 0.150.15 0.080.08

44 0.870.87 0.220.22 580%580% 580%580%

88 3.153.15 0.390.39 2100%2100% 362%362%

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Defining Grade of Service

• Grade of Service

% of calls blocked, % of calls delayed

Average delay of all calls

Average delay of delayed calls

• Grade of Service is basedon the busiest hourbusiest hour

BH is when the most traffic is offeredBH varies between days, weeks,and months



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Determining Grade of Service

• GoS dependent on strategicobjectives

• Acceptable grade of service uniqueto each organization

• Too poor can be moremore expensiveexpensive

than too good



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Traffic Tables

• Eliminates the use of equations

Input: Given a volume of traffic and a GoS

Output: Number of trunks required

• Pick the model that best fitsyour application

Number of sources: Finite and infinite

Traffic characteristics: Random, smooth or roughHow blocked calls are handled

Switch availability



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Traffic Table Flow ChartTraffic TheoryTraffic Theory

Infinite SourcesInfinite Sources

Lost CallsLost Calls

ClearedCleared


HeldHeld


DelayedDelayed

Finite SourcesFinite Sources


ClearedCleared


HeldHeld


DelayedDelayed

Erlang B Poisson

Delay TheoryDelay Theory Engset

Binomial

Delay

See Next Slide

SmoothSmooth PeakedPeaked

Neg. Binomial

ExtendedErlang B

Table Modified from T. Franlel’s “ABC of the Telephone”



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Traffic Table Flow Chart (Cont.)Delay TheoryDelay Theory

ExponentialExponentialHolding TimeHolding Time

First inFirst inFirst outFirst out

RandomRandom

Last inLast in

First outFirst out

Erlang C

ConstantConstantHolding TimeHolding Time

First inFirst inFirst outFirst out

RandomRandom

Last inLast in

First outFirst out

Crommelin-Pollaczek



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Erlang B

• Infinite sources

• Lost calls cleared

• Constant or exponential holding time

• Random traffic

• Application

Outbound trunks with overflow, i.e. alternateroutes are used

• Used throughout the worldas the standard



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Poisson or Molina• Infinite sources

• Lost calls held


• Random traffic

• Application

Outbound trunks with no overflow and queue,trunk group of last resort

• Overstates trunk requirements

Not heavily used outside of the U.S.



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Extended Erlang B

• % of lost calls return



• Random traffic

• Application

Inbound trunks, e.g. 1–800

Outbound trunks with no overflow and queue

• Most Accurate, but hard to identifyreturned calls



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Erlang C

• Lost calls delayed


• Exponential holding time• Random traffic

• Calls served in arrival order

• ApplicationACD agents, trunks with queuingbut no overflow



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Engset

• Lost calls cleared

• Finite sources

•Constant or exponentialholding time

• Equal traffic distribution (smooth)

• Application

Small PBX or Key System with tie linesto a central PBX and using overflowto the PSTN



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Design Example #1

• 352 hours of first-attempt traffic

in the month• 22 business days/month

• 10% call processing

• 15% of traffic in busy hour

• Chance of blocking = 1%

How Many Trunks to Handle this Traffic?



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Design Example #1

First, What Is the Busy Hour?First, What Is the Busy Hour?



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Design Example #1

First, What Is the Busy Hour?First, What Is the Busy Hour?

BH = 352 hrBH = 352 hr ÷÷ 22 d/m x 0.15% busy hour x 1.10 call processing22 d/m x 0.15% busy hour x 1.10 call processing

= 2.64 E= 2.64 E



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Design Example #1


• Overflow excess traffic

• No queuing

Second, What Table Would You Use?Second, What Table Would You Use?



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ErlangErlang BB

Design Example #1

Second, What Table Would You Use?Second, What Table Would You Use?



Finally, Calculate the Number of Trunks Required?NN PP

.003.003 .005.005 .01.01 .02.02 .03.03 .05.05

11 .003.003 .005.005 .011.011 .021.021 .031.031 .053.053

22 .081.081 .106.106 .153.153 .224.224 .282.282 .382.382

33 .289.289 .349.349 .456.456 .603.603 .716.716 .9.9

44 .602.602 .702.702 .87.87 1.0931.093 1.2591.259 1.5251.52555 .995.995 1.1321.132 1.3611.361 1.6581.658 1.8761.876 2.2192.219

66 1.4471.447 1.6221.622 1.9091.909 2.2762.276 2.5432.543 2.9612.961

77 1.9471.947 2.1582.158 2.5012.501 2.9362.936 3.253.25 3.7383.738

88 2.4842.484 2.732.73 3.1283.128 3.6273.627 3.9873.987 4.5434.543

99 3.0533.053 3.3333.333 3.7833.783 4.3454.345 4.7484.748 5.3715.371

1010 3.6483.648 3.9613.961 4.4624.462 5.0845.084 5.535.53 6.2166.216

1111 4.2674.267 4.6114.611 5.165.16 5.8425.842 6.3286.328 7.0777.077

1212

4.9044.904

5.2795.279

5.8765.876

6.6156.615

7.1417.141

7.957.95

1313 5.5595.559 5.9645.964 6.6086.608 7.4027.402 7.9677.967 8.8358.835

1414 6.2296.229 6.6646.664 7.3527.352 8.2018.201 8.8048.804 9.739.73

1515 6.9136.913 7.3767.376 8.1088.108 9.019.01 9.659.65 10.6310.63

Table Extracted from T. Frankel’s “ABC of the Telephone”

Design Example #1

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NN PP

Finally, Calculate the Number of Trunks Required?

.003.003 .005.005 .01.01 .02.02 .03.03 .05.05

11 .003.003 .005.005 .011.011 .021.021 .031.031 .053.053

22 .081.081 .106.106 .153.153 .224.224 .282.282 .382.382

33 .289.289 .349.349 .456.456 .603.603 .716.716 .9.9

44 .602.602 .702.702 .87.87 1.0931.093 1.2591.259 1.5251.52555 .995.995 1.1321.132 1.3611.361 1.6581.658 1.8761.876 2.2192.219

66 1.4471.447 1.6221.622 1.9091.909 2.2762.276 2.5432.543 2.9612.961

77 1.9471.947 2.1582.158 2.5012.501 2.9362.936 3.253.25 3.7383.738

88 2.4842.484 2.732.73 3.128 3.6273.627 3.9873.987 4.5434.543

99 3.0533.053 3.3333.333 3.7833.783 4.3454.345 4.7484.748 5.3715.371

1010 3.6483.648 3.9613.961 4.4624.462 5.0845.084 5.535.53 6.2166.216

1111 4.2674.267 4.6114.611 5.165.16 5.8425.842 6.3286.328 7.0777.077

1212

4.9044.904

5.2795.279

5.8765.876

6.6156.615

7.1417.141

7.957.95

1313 5.5595.559 5.9645.964 6.6086.608 7.4027.402 7.9677.967 8.8358.835

1414 6.2296.229 6.6646.664 7.3527.352 8.2018.201 8.8048.804 9.739.73

1515 6.9136.913 7.3767.376 8.1088.108 9.019.01 9.659.65 10.6310.63

Design Example #1

46


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Agenda

• Review of Circuit Switch Fundamentals


•• How Costs Impact Trunk GroupsHow Costs Impact Trunk Groups

• Voice Traffic Impact on a Packet BasedData Network




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Step 4: Least-Cost Trunking

• Calculate the number of trunks required tocarry the busy hour traffic

• Account for traffic overflowing to alternate

paths, i.e. lost calls cleared

• Primary and alternate routes (overflow) aredictated by economics, e.g. carrier pricing

• Packet based networks can serve as alternate

routes, given we understand the costassociated with its use



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Cost of Service

• Cost per minuteoften-quoted parameter

Unique to each organization

Varies with each organization

• In reality, costs should include:

transmission, equipment,administration and maintenance



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Cost-Optimization Rule

• Use average usage figures instead of

busy hour calculations• Use the least costly circuits until

the cost per hour becomes moreexpensive than the next best route



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Back to Our Example

• 2.64 Erlangs in the busy hour requireseight trunks for a P of .01

• 352 hours of first-attempt traffic in the month

• 22 business days per month

• 10% call processing

• Average hourly usage is equal to:

352 hours/month / 22 days/month / 8 hours/day* 1.10 processing = 2.22.2 ErlangsErlangs per hourper hour



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Example

• XYZ carrier has offered our company threechoices for carrying voice traffic betweenits head office, and its sales office

1.DDD $25 per hour

2.Savings Plan A $60 fixed charge plus $18 per hour

3.Tie trunk $500

Which Option Is Best?Which Option Is Best?

Example (Cont.)



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Graph of Cost vs. Usage

Comparison of XYZ Pricing

$-

$100

$200

$300

$400

$500

$600

$700

$800

$900

$1,000

0 5 10 15 20 25 30 35 40

Hours Per Month

DDD

Plan A

Tie Trunk

… Depends on Traffic Volume



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Economic Cross over Points

• 8.57 hours between DDD and Plan A

• 24.4 hours between Plan A and Tie Trunks



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Number Offerred Carried Cumulative Grade

Trunks Hours per Trunk Carried of Service

1 2.2 0.688 0.688 0.688

2 1.513 0.565 1.253 0.4313 0.947 0.419 1.672 0.24

4 0.528 0.271 1.943 0.117

5 0.257 0.149 2.093 0.049

6 0.107 0.069 2.161 0.018

7 0.039 0.027 2.188 0.005

8 0.012 0.009 2.197 0.002

9 0.003 0.003 2.199 0

ERL-B Shows Traffic on Each Trunk based on 2.2 Erlangs

Traffic Distribution Based on

Average Usage



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DDDDDD

Plan APlan A

DDDDDD

Tie TrunkTie Trunk

Tie TrunkTie Trunk

Tie TrunkTie Trunk

Tie TrunkTie Trunk

Tie TrunkTie Trunk

Average Trunk Usage on a

Monthly BasisBased on 2.2 Erlangs

0.3% Dropped

68.8%

56.5%

41.9%

27.1%

14.9%

6.9%

2.7%

0.9%

121

99.4

73.7

47.7

26.2

12.1

4.71.6

Based on 176 Erlangs per Trk per Mo.



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Agenda

• Review of Circuit Switch Fundamentals



•• Voice Traffic Impact on a Packet BasedVoice Traffic Impact on a Packet BasedData NetworkData Network




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Voice Traffic Means More

Overall TrafficPBX

? ?

??

PBX

PBX

PBX

PBX

?PBX

PBX

PBX

• We have reviewed the methodology forcalculating the number of voice trunks

• Next we need to see how this impacts thebandwidth required in the data network



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Erlang to Packet Conversion

• 1 Erlang = 60 minutes = 64Kbps * 3600 seconds/ 8bits/byte = 28.8Mbytes

• ATM

1 Erlang = 655K cells (AAL1)

= 600K cells (AAL5)

• Frame Relay

1 Erlang = 1.44M frames (20 byte frames) or 400 fps

• IP

1 Erlang = 450K packets (64 byte packets) or 125 pps



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Packet Overhead

• ATM

AAL1–6 bytes for every 47 bytes of payload

AAL5–5 bytes for every 48 bytes of payload

• Frame Relay4–6 bytes overhead, payload variable to 4096 bytes

• IP/H.323

(Frame Ovhd) + (IP) + (UDP) + (RTP)

Varies + 24 + 8 + (12–72)

RTP Header Compression



Voice Compression

Encoding/Encoding/CompressionCompression

ResultResultBit RateBit Rate

G.711 PCMG.711 PCMA-Law/A-Law/u u -Law-Law

6464 kbpskbps (DS0)(DS0)

G.726 ADPCMG.726 ADPCM 16, 24, 32, 4016, 24, 32, 40 kbpskbps

G.727 E-ADPCMG.727 E-ADPCM

G.729 CA-CELPG.729 CA-CELP 88 kbpskbps

G.728 LD-CELPG.728 LD-CELP 1616 kbpskbps

G.723.1G.723.1 6.3/5.36.3/5.3 kbpskbpsVariableVariable

16, 24, 32, 4016, 24, 32, 40 kbpskbps

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G.729 Voice Compression

• Will reduce the bandwidth requiredto 1/8 th, 1 Erlang = 3.6Mbytes

• G.729 coder will output voice frames

in 10 msec segments. 1–10msecframe = 10 bytes

• Bandwidth may or may not fit perfectlyinto a packet. For example, a 48 byteATM cell will have at least 8 bytesof padding



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Don’t Forget the

Signaling Overhead

• Out of Band—Separate 64K channelfor T1 or E1

• Inband—Robbed signaling does notadd anything to the overhead

• Any miscellaneous call

setup information



Traffic Distribution—How Do Voice

Trunks Equate to Bandwidth?Using Our Earlier ExampleUsing Our Earlier Example

64

2.64 Erlangs during BH

72.5%62.3%49.7%35.7%22.5%

12.2%5.7%

2.3%1.0%Dropped

2.2 Erlangs during AH

68.8%56.5%41.9%27.1%14.9%

6.9%2.7%

0.9%0.3%Dropped

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Traffic Distribution—How Do Voice

Trunks Equate to Bandwidth?• Still use 8 trunks to give us the required grade of service

• At any point in time we could be using 8 * 64Kbps (PCM)or 512Kbps of Bandwidth

• During the busy hour we will be using 2.64 trunks x64Kbps or 169Kbps or 33%

• During the average hour we will be using 2.2 trunks x64Kbps or 141Kbps or 27.5%

• Over a period of a month we will be using 352 Erlangs out

of a possible 1408(8 trks *8 hrs/day *22 days) or 25%



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Data TrunkData Trunk

• Do we take the most busy trunks and move them on tothe data network, or do we take the least busy trunks?

• The decision will be based on desired voice quality andcosts, but will have an impact on the traffic loaddelivered to the data network

Other Trunk Selection Criteria



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Network Topology

• Subscriber lines vs. trunks

• Star vs. mesh

PBX to Tandem PBX

PBX to PBX, network acts as Tandem switch

• Voice server functions

Voice Mail, ACD, fax server



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Traditional Tandem PBX Network

• The trunks are moved over on to the data network

• Traffic patterns are easily discernible

PBX

PBX

PBX PBX

PBX

PBXPrivate Network

TandemTandemPBXPBX

TandemTandemPBXPBX

TandemTandemPBXPBX



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PBX

PBX

PBX PBX

PBX

PBXPrivate Network

Network Acts as Tandem PBX

• Traffic is switched directly, no in and out of a tandem switch;therefore, backbone traffic is reduced

• Still must run PBX trunks to something

VirtualVirtual

TandemTandemPBXPBX



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Voice Servers—Voice Mail Etc.

• Factor in connections to ACDs,voice mail, fax servers andgateways

PBX

PBX

PBX PBX

PBX

PBXPrivate Network

TandemTandemPBXPBX

TandemTandemPBXPBX

TandemTandemPBXPBX

SRVRSRVR



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Advantages of Using the Network

to Perform the Switching

Single larger trunk group reduces PBXand WAN switch hardware

Simplified dial plan

Full Mesh dialing

Reduced WAN bandwidth

Reduced call setup delay

Eliminates multiple compression cycles



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Agenda

• PBX Fundamentals




•• How Voice Quality Is Impacted by aHow Voice Quality Is Impacted by aPacket Based Data NetworkPacket Based Data Network



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Voice Quality Factors

• Line quality

• Coding and compression

• Delay/delay variation

• Multiple conversions:

Analog to digital

Compression—Recompression



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Compression MethodCompression Method MOSMOS ScoreScoreDelayDelay

((msecmsec))

PCM (G.711)PCM (G.711) 4.44.4 0.750.75

32K ADPCM (G.726)32K ADPCM (G.726) 4.24.2 11

16K LD-CELP (G.728)16K LD-CELP (G.728)

8K CS-ACELP (G.729)8K CS-ACELP (G.729) 4.24.2 1010

8K CS-ACELP (G.729a)8K CS-ACELP (G.729a) 1010

Voice Quality Guidelines

3–53–54.24.2

4.24.2



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Circuit Switching

• PBX trunks are sized based on Voice Trafficengineering principles

• Dedicated time slots for trunks resultin wasted bandwidth

• Delay and delay variation areminor concerns

TDM

CC

AA

BB

Wasted Bandwidth

TDM

PBX PBX



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Voice Delay Guidelines

One Way DelayOne Way Delay

((msecmsec))DescriptionDescription

0–1500–150 Acceptable for Most User ApplicationsAcceptable for Most User Applications

150–400150–400 Acceptable Provided ThatAcceptable Provided ThatAdministrations Are AwareAdministrations Are Aware

of the Transmission Time Impactof the Transmission Time Impact

on the Transmission Qualityon the Transmission Quality

of User Applicationsof User Applications

400+400+ Unacceptable for General NetworkUnacceptable for General Network

Planning Purposes; However, It IsPlanning Purposes; However, It Is

Recognized That in Some ExceptionalRecognized That in Some ExceptionalCases This Limit Will Be ExceededCases This Limit Will Be Exceeded

ITU’s G.114 Recommendation



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Delay in Perspective

Cumulative Transmission Path Delay

Time (msec)

0 100 200 300 400

Delay Target

500 600 700 800

Satellite QualitySatellite Quality

Fax Relay, BroadcastFax Relay, BroadcastHigh QualityHigh Quality

CB ZoneCB Zone



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Fixed Delay Components

• Propagation—Six microseconds per kilometer

• Serialization

• ProcessingCoding/compression/decompression/decoding

Packetization

Processing Delay

Propagation Delay

Serialization Delay—Buffer to Serial Link



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Variable Delay Components

• Queuing delay

• Dejitter buffers

• Variable packet sizes

DejitterBuffer

QueuingDelay

QueuingDelay

QueuingDelay



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Back to Our Example

• We have eight trunks

• We are going to use CS-ACELP that

uses 8 Kbps per voice channel

• Our uplink is 64 Kbps

• Voice is using a high priority queue

and no other traffic is being used



Calculate Delay Budget

PropagationDelay—32 ms

CoderDelay25 ms

Serialization Delay

3 ms

Dejitter Buffer50 ms

QueuingDelay6 ms

LosAngeles Munich

(Private Line Network)

TotalTotal 110110 msecmsec

DejitterDejitter BufferBuffer 5050 msecmsec

3232 msecmsec

Network Delay (e.g., Public Frame RelayNetwork Delay (e.g., Public Frame Relay SvcSvc))

Serialization Delay 64Serialization Delay 64 kbpskbps TrunkTrunk 33 msecmsec

2121 msecmsecMax Queuing Delay 64Max Queuing Delay 64 kbpskbps TrunkTrunk

55 msecmsec

PacketizationPacketization Delay—Included inDelay—Included in CoderCoder DelayDelay

CoderCoder Delay G.729 (5Delay G.729 (5 msecmsec Look Ahead)Look Ahead)

Propagation Delay (Private Lines)Propagation Delay (Private Lines)

FixedFixedDelayDelay

VariableVariableDelayDelay

CoderCoder Delay G.729 (10Delay G.729 (10 msecmsec per Frame)per Frame) 2020 msecmsec

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Variable Delay Calculation

• We have eight trunks, so in the worst case we willhave to wait for seven voice calls prior to ours

• To put one voice frame out on a 64Kbps linktakes 3msec

• 1 byte over a 64Kbps link takes 125 microseconds.We have a 20 byte frame relay frame with 4 bytes ofoverhead. 125 * 24 = 3000 usecs or 3 msec

• Does not factor in waiting for a possible data

packet or the impact of variable sized frames• Assumes voice prioritization of frames



Impact on Delay Budget—

Multiple Conversions

Delay #2

Delay #1Site A

Site B

Site C

Private LineNetwork



DELAY #1DELAY #1

CoderCoder Delay G.729Delay G.729 2525 msecmsec

PacketizationPacketization DelayDelay

(Included in(Included in CoderCoder Delay)Delay)

Max Queuing Delay 64Max Queuing Delay 64 kbpskbps TrunkTrunk 21msec21msec

Serialization Delay 64Serialization Delay 64 kbpskbps TrunkTrunk 33 msecmsec

Propagation Delay (Private Lines)Propagation Delay (Private Lines) 3232 msecmsec


Tandem SwitchTandem Switch — —

Delay #1 TotalDelay #1 Total 110110 msecmsec

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Impact on Delay Budget—

Second Link



DELAY #1 TotalDELAY #1 Total 110110 msecmsec

DELAY #2DELAY #2

CoderCoder Delay G.729Delay G.729 2525 msecmsec

PacketizationPacketization DelayDelay

(Included in(Included in CoderCoder Delay)Delay)

Max Queuing Delay 2Max Queuing Delay 2 MbpsMbps TrunkTrunk .7.7 msecmsec

Serialization Delay 2Serialization Delay 2 MbpsMbps TrunkTrunk 0.10.1 msecmsec

Propagation Delay (Private Lines)Propagation Delay (Private Lines) 55 msecmsec


Delay #2 TotalDelay #2 Total 8080 msecmsec

Total DelayTotal Delay 190190 msecmsec

86

Delay #2

Delay #1Site A

Site B

Site C

Private LineNetwork

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Summary

• Voice traffic engineering principlesstill apply

• Use packet-based voice trunks oneconomics and quality issues

• Biggest impact on voice will be dueto a data network’s delayand delay variation



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Thank You



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Supplementary

Examples



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Design Example #2

How many tie lines are required to support a remote officekey system if the following conditions exist:

There are 20 SubscribersAverage five calls per hour per userAverage duration of 100 seconds per callAnd 5% of the traffic will overflow to PSTN?

1

20

PBX

?

PSTN5%



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Using Engset

Six Trunks Satisfy the Requirements

Design Example #2

PP

NN 0.0010.001 0.0020.002 0.0030.003 0.0050.005 0.010.01 0.020.02 0.030.03 0.050.05

L = 20L = 20

55 0.880.88 1.031.03 1.131.13 1.281.28 1.531.53 1.841.84 2.072.07 2.432.4366 1.341.34 1.541.54 1.681.68 1.871.87 2.172.17 2.562.56 2.832.83 3.26

77 1.881.88 2.132.13 2.292.29 2.522.52 2.882.88 3.333.33 3.653.65 4.144.14

88 2.492.49 2.772.77 2.962.96 3.233.23 3.653.65 4.164.16 4.514.51 5.065.0699 3.153.15 3.483.48 3.73.7 3.993.99 4.464.46 5.025.02 5.425.42 6.026.02




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Design Example #3:

Mixed-Model• Call center design

Uses 1–800 number to sales, service, etc.

Some call centers provide functionto several companies

Every minute can equate to $10,000s

• Real-time issues

Peaked traffic

Agent breaks, sickness, etc.



Call Center Design Elements

• Call centers use a device, called an ACDto queue and distribute calls to agents

May be part of a PBX or a separate device

• Holding time of a trunk

RingACD

AnswersAgent

AnswersAgent

Hangs-upAgentReady

Queue Delay Agent Handle Time

Time Trunk Is OccupiedTime Trunk Is Occupied

Time Agent Is OccupiedTime Agent Is Occupied

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Call Center Traffic Models

• Erlang C is used to design numberof agent positions

Queuing

No overflow

• Extended Erlang B for trunks

High re-trial rate

No overflow



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Design Example #3

ACD? Trunks

Lines

How many lines are required to support a call center that has:

• 230 calls per hour with an average delay not longer than 30 sec

• No more than 20 percent of the calls delayed over one minute?

• Average hold time is 120 seconds

• Queuing is first in first out



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Design Example #3

• A = C * T

• A = 230 * 120 = 7.67Erlangs

3600

• D1 = 30 sec = .25Holding Time

120 sec

A = Traffic flowC = Number of calls originated in 1 hourT = Average holding timer of one call

D1 = Average Delay on all calls as a percentage of the holding timeD2 = Average Delay on calls delayed, as a percentage of the holding timet = Time of no greater than delay as a percentage of the holding timeP(t) = Probability of delay greater than t

• t = 60 sec = .5 HoldingTime

120 sec

• P(t) (for t = .5) must belessthan 20%



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Using Erlang C

1. Found the Entry for Erlangs = 7.7

2. Found the Entry for D1 < .25

3. Looking at t = 0.5 find P(t) < 0.20

4. 10 Trunks will meet my requirements

P(t) for t=P(t) for t=

AA NN P(0)P(0) D1D1 D2D2 0.10.1 0.20.2 0.30.3 0.40.4 0.50.5 0.750.75 11

7.7 88 0.8890.889 2.942.94 3.333.33 0.8560.856 0.8310.831 0.8060.806 0.7820.782 0.7590.759 0.7040.704 0.6530.653

99 0.5640.564 0.4340.434 0.7690.769 0.4960.496 0.4350.435 0.3820.382 0.3360.336 0.2950.295 0.2130.213 0.1540.154

1010 0.3460.346 0.15 0.4350.435 0.2740.274 0.2180.218 0.1730.173 0.1380.138 0.109 0.0620.062 0.0350.0351111 0.2020.202 0.0610.061 0.3030.303 0.1450.145 0.1040.104 0.0750.075 0.0540.054 0.0390.039 0.0170.017 0.0070.007

Design Example #3


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