Traffic Analysis Overview

8/3/2019 Traffic Analysis Overview

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Dave Corbett

Traffic Analysis Overview

Networks, whether voice or data, are designed around many different variables. Twoof the most important factors that you need to consider in network design are serviceand cost. Service is essential for maintaining customer satisfaction. Cost is always afactor in maintaining profitability. One way that you can factor in some of the serviceand cost elements in network design is to optimize circuit utilization.

This document describes the different techniques you can use to engineer andproperly size traffic-sensitive voice networks. It discusses several different trafficmodels and explain how to use traffic probability tables to help you engineer robustand efficient voice networks.

Traffic Theory Basics

Network designers need a way to properly size network capacity, especially asnetworks grow. Traffic theory enables network designers to make assumptions abouttheir networks based on past experience.

Traffic is defined as either the amount of data or the number of messages over acircuit during a given period of time. Traffic also includes the relationship betweencall attempts on traffic-sensitive equipment and the speed with which the calls arecompleted. Traffic analysis enables you to determine the amount of bandwidth youneed in your circuits for data and for voice calls. Traffic engineering addresses

service issues by enabling you to define a grade of service or blocking factor. Aproperly engineered network has low blocking and high circuit utilization, whichmeans that service is increased and your costs are reduced.

There are many different factors that you need to take into account when analyzingtraffic. The most important factors are described in the following sections:

• Traffic Load Measurement

• Grade of Service

• Traffic Types

• Sampling Methods

Of course, other factors might affect the results of traffic analysis calculations, butthese are the main ones. You can make assumptions about the other factors.

Traffic Load Measurement

In traffic theory, you measure traffic load. Traffic load is the ratio of call arrivals in aspecified period of time to the average amount of time taken to service each call

during that period. These measurement units are based on Average Hold Time

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(AHT). AHT is the total time of all calls in a specified period divided by the number ofcalls in that period, as shown in the following example:

(3976 total call seconds)/(23 calls) = 172.87 sec per call = AHT of 172.87 seconds

The two main measurement units used today to measure traffic load are erlangs andcentum call seconds (CCS).

One erlang is 3600 seconds of calls on the same circuit, or enough traffic load tokeep one circuit busy for 1 hour. Traffic in erlangs is the product of the number ofcalls times AHT divided by 3600, as shown in the following example:

(23 calls * 172.87 AHT)/3600 = 1.104 erlangs

One CCS is 100 seconds of calls on the same circuit. Voice switches generallymeasure the amount of traffic in CCS.

Traffic in CCS is the product of the number of calls times the AHT divided by 100, asshown in the following example:

(23 calls * 172.87 AHT)/100 = 39.76 CCS

Which unit you use depends highly on the equipment you use and what unit ofmeasurement they record in. Many switches use CCS because it is easier to workwith increments of 100 rather than 3600. Both units are recognized standards in thefield. The following is how the two relate: 1 erlang = 36 CCS.

Although you can take the total call seconds in an hour and divide that amount by

3600 seconds to determine the traffic in erlangs, you can also use averages ofvarious time periods. These averages allow you to use more sample periods anddetermine the proper traffic.

Echo Cancellation

Echo is the refl ection of sound that arrives to the listener a period of time after

the direct sound is heard. A certain amount of echo is experienced on most voice

calls and up to a certain point is tolerated. When analog signals are converted to

digital signals and then compressed using codecs, echo is often amplifi ed to the

point where it severely degrades the quality of the call. DSP resources are used

to assist in the elimination of echo when converting from one voice signal into

another. Echo cancellation is performed by default.

Effects of Network Elements on Echo

The following network elements in a VoIP network can have an effect on echo:loudness, hybrid transformers, telephones, routers, and quality of service (QoS).These elements are discussed in the following sections:



• Effect of Loudness on Echo

• Effect of Hybrid Transformers on Echo

• Effect of Telephones on Echo

• Effect of Routers on Echo

• Effect of QoS on Echo)

Effect of Loudness on Echo

As mentioned previously in this document, the louder an echo is, the more annoyingit is. To better understand the part that loudness contributes to echo, it is appropriate

to examine a few characteristics of sound and its transmission.

Effect of Hybrid Transformers on Echo

Echo sources are points of signal leakage between analog transmit and receivepaths. Hybrid transformers are often prime culprits for this signal leakage.

The typical analog telephone terminal is a 2-wire device: A single pair of conductors

is used to carry both the Tx and Rx signals. For analog trunk connections, known as4-wire transmission, two pairs of conductors carry separate Tx and Rx signals. Digitaltrunks (T1/E1) can be considered virtual 4-wire links because they also carryseparate Tx and Rx signals. A hybrid is a transformer that is used to interface 4-wirelinks to 2-wire links. Figure 6 shows a hybrid transformer in an analog tail circuit.

Basics of Echo Canceler Operation

The role of the echo canceler is to remove the echo portion of the signal coming outof the tail circuit and headed into the WAN. The echo canceler removes the echo bylearning the electrical characteristics of the tail circuit and forming its own model ofthe tail circuit in its memory. Using this model, the echo canceler creates an"estimated echo" signal based on the current and past Rx signal (Bob's voice). Bob'svoice is run through this functional model to come up with an estimate of what Bob'secho signal would sound like. This estimated "Bob echo" is then subtracted from theactual Tx signal coming out of the tail circuit. The time that is required (after the userstarts talking) to adjust the level of attenuation needed to the original signal is calledthe convergence time. Mathematically, the subtraction of the estimated "Bob echo" isperformed as follows:

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Tx signal sent from the gateway back to Bob

= Tx signal - "estimated Bob's echo"

= (Alice's voice + Bob's echo) - "estimated Bob's echo"

= Alice's voice + (Bob's echo - estimated Bob's echo)

= Alice's voice (if the estimation is accurate)

The quality of the estimation is continuously improved by monitoring the estimationerror.

TELR Calculation Example

Following are examples of how TELR is calculated for three scenarios of a telephoneconversation.

• Single talk operation (training)

TELR = talker phone SLR + WAN attenuation + tail circuit attenuation + tail circuitERL + tail circuit attenuation + WAN attenuation + talker phone RLR

TELR = 8 dB + 0 + 0 + 12 dB + 0 + 9 dB - 6 dB = 23 dB

• Single talk operation (after convergence)

TELR = talker phone SLR + WAN attenuation + ACOM (ERLE + ERL) + WANattenuation + talker phone RLR

TELR = 8 dB + 0 + 55 dB (CP:1 8 dB, NLP: 25 dB, local loop ERL: 12 dB) + 9 - 6

= 66 dB

• Double Talk Operation

TELR = talker phone SLR + WAN attenuation + ACOM (ERLE + ERL) + WANattenuation + talker phone RLR

TELR = 8 dB + 0 + 30 dB (CP:18 dB, local loop ERL: 12 dB) + 9 - 6 = 41 dB

Sources:

http://www.cisco.com/en/US/docs/ios/solutions_docs/voip_solutions/EA_ISD.

html#wp1050306

And other internet.






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Traffic Analysis Overview