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TrafficEngineeringPrinciples forVoice Network
Design
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Steve Quan
squan@cisco.com
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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
• 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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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
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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
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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
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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
Lost CallsLost Calls
HeldHeld
Lost CallsLost Calls
DelayedDelayed
Finite SourcesFinite Sources
Lost CallsLost Calls
ClearedCleared
Lost CallsLost Calls
HeldHeld
Lost CallsLost Calls
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
• Constant or exponential holding time
• 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
• Infinite sources
• Constant or exponential holding time
• 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
• Infinite sources
• 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
• Infinite sources
• 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?
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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
Table Extracted from T. Frankel’s “ABC of the Telephone”
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Agenda
• Review of Circuit Switch Fundamentals
• Basic Voice Traffic Engineering
•• How Costs Impact Trunk GroupsHow Costs Impact Trunk Groups
• Voice Traffic Impact on a Packet BasedData Network
• How Voice Quality Is Impacted by aPacket Based Data 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
• Basic Voice Traffic Engineering
• How Costs Impact Trunk Groups
•• Voice Traffic Impact on a Packet BasedVoice Traffic Impact on a Packet BasedData NetworkData Network
• How Voice Quality Is Impacted by aPacket Based Data 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
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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
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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
• Basic Voice Traffic Engineering
• How Costs Impact Trunk Groups
• Voice Traffic Impact on a PacketBased Data Network
•• 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
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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
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Impact on Delay Budget—
Multiple Conversions
Delay #2
Delay #1Site A
Site B
Site C
Private LineNetwork
FixedFixedDelayDelay
VariableVariableDelayDelay
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
DejitterDejitter BufferBuffer 5050 msecmsec
Tandem SwitchTandem Switch — —
Delay #1 TotalDelay #1 Total 110110 msecmsec
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Impact on Delay Budget—
Second Link
FixedFixedDelayDelay
VariableVariableDelayDelay
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
DejitterDejitter BufferBuffer 5050 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
Table Extracted from T. Frankel’s “ABC of the Telephone”
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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.
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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
Table Extracted from T. Frankel’s “ABC of the Telephone”
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