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COMT 220

Managing Telecommunications

Systems

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Managing Telecommunications

• Configuration Management• Security Management• Accounting Management• Alarm Management• Performance Management

– This includes Traffic Engineering, or Network Design

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Traffic Engineering

• Understand the Applications a Systems is to Support

• Examine the Historical Amount of Resource Demand

• Determine the Amount of Resources Needed

• When a User Request for a Resource is not met, call it Blockage

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Examples of Resources

• CO Trunks for Local Calls• Trunks to an IXC for Long Distance

Calls• Call Setup, Tone Generators, etc.

in the PBX• Operators (“Agents”) in a Call

Center

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Why is “Blockage” unavoidable?

• Providing facilities for every user at all times is too expensive, both for companies and for carriers.

• Blockage will occur whenever users must share facilities.

• Reducing Blockage raises costs.• Managing means balancing service

and cost

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How is Blockage Measured

• Observe facilities during some period of time (usually an hour)

• Count the number of calls attempted, and the number of calls that failed. Express the result as a percentage.

• P.05 means 5 calls out of 100 failed.• P.12 means 12 calls out of 100 failed.

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What is Traffic?

• Traffic measured the amount of information that flows over the network.

• For Voice Calls, we define the Average Holding Time as the information amount.

• Holding Time = Conversation Time + Operating Time

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Traffic Measurements

• Peg Count– Number of calls per hour.

• CCS (100 Call Seconds)– Sum of holding times per hour, in 100

second increments.

• Erlangs– Sum of holding times per hour, in 1

hour increments.

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Examples

• During one hour, we find 100 calls with average holding time of 3 minutes.– Peg Count is 100– Sum of holding times is 300 minutes

= 18,000 seconds = 180 CCS– Sum of holding times is 300 minutes

= 5 hours = 5 Erlangs

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COMT 220

“Traffic Engineering”, orHow Many Trunks Are Needed

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The Concept of Probability

• Rolling 1 dice and getting a “1”– The “Event” has a 1 in 6 chance

(“probability”)– p = 1/6 = 0.16666 = 16.666%

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A Choice of Outcomes

• Roll a “1” or a “2” on a single dice throw– 1/6 chance for “1”– 1/6 chance for “2”– 2/6 chance overall (1/6 + 1/6) to get “1” or “2”

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Two Simultaneous Events• Roll a “1” on both dice simultaneously

– 1/6 chance that dice A has a “1”

– Out of those 1/6 rolls where A has a “1”:• 1/6 chance that dice B has a “1”

– 1/36 chance that both have a “1” (1/6 * 1/6)

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Probability Concepts in Telephony

• Check on one telephone user, once per minute.

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Continued…

• Record the number of minutes during one hour when you find the user on the phone.

• Say 10 minutes out of 1 hour• The probability of the user being

on the phone at any given time is 10/60 = 1/6.

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Two Users

• Two users, each with a probability of 1/6 of using the phone.

• Probability that both are using the phone is:

• 1/6 * 1/6 = 1/36

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How about Noone Using the Phone?

• Each user has a 5/6 probability of not using the phone.

• Probability of both users off the phone is:

• 5/6 * 5/6 = 25/36

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One Phone in Use?

• User A is on the phone (1/6), and User B is not (5/6)

• Probability is 1/6 * 5/6 = 5/36• User A off and User B on is also 5/36

Prob.• Overall probability of one call is 10/36.

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Put Everything Together

• No Phone in Use: 25/36 (69%)

• One Phone in Use: 10/36 (28%)

• Two Phones in Use: 1/36 (3%)

• Total Probability 36/36 (as it should be)

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Examples With More Users

• Your Assignment (Practice):– Three Users– 1/6 Probability for each to be on the

phone– What are the chances of no, 1, 2, or 3

calls?

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Traffic Engineering Variables

No. of Circuits

Grade of Service Offered Traffic

Offered Traffic = Carried Traffic + Blocked Traffic

Carried Traffic = (1 - E(c,a) ) * Offered Traffic

Blocked Traffic = E(c,a) * Offered Traffic

[E(c,a) represents the blockage with “c” circuits and “a” Erlangs offered]

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Caller Choices

ServerQueueFirst Attempt

Call Connected

Failed Calls Abandon

Failed Calls Retry

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Recall Behavior

• Retry Attempts Add to System Congestion• Traffic Engineering Corrections Must Be Made,

Usually via Iterative Methods• Retry Systems are Similar to Queuing Systems

and Can Overload Like Queueing Systems

ServerFirst Attempt

Call Connected

Some Failed Calls AbandonSome Failed Calls Retry

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Queueing

ServerQueueFirst Attempt Call Connected

Call Enter Queue if All Servers Are Busy

• Average Queue Delay Replaces Blocking as the Performance Criterion

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Inbound Facilities, ACDs

• Inbound Call Center– Blocking Trunk Group on the Access Side– One or More Queues

ACD

IVR

AG1

AG2

ACD Agent Splits

Queues

BlockingTrunkGroup

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Inbound Facilities, ACDs cont…

• Queues and Access Trunks Interact– Queue Length Part of Call Holding Time– Access Group Blocking “Protects” Queues

• Traffic Engineering– Erlang B and Erlang C; Need to Iterate– Event Simulation

ACD

IVR

AG1

AG2

ACD Agent Splits

Queues

BlockingTrunkGroup