Deploying Cisco® Voice over IP Solutions

End-to-End Traffic Analysis Example 25

Figure 1-5 Topology with broken connection.

End-to-End Traffic Analysis Example

With the proper traffic tables, defining the number of circuits needed to handle calls

becomes fairly simple. By defining the number of calls on the TDM side, you can also

define the amount of bandwidth needed on the IP leg of the call. Unfortunately, putting

them together can be an issue.

End-to-End Traffic Analysis: Problem

As illustrated in Figure 1-6, you have offices in the U.S., China, and the U.K. Because your

main office is in the U.K., you will purchase leased lines from the U.K. to the U.S. and to

China. Most of your traffic goes from the U.K. to the U.S. or China, with a little traffic going

between China and the U.S. Your call detail records show:

• U.K. 36,000 minutes/day

• U.S. 12,882.4 minutes/day

• China 28,235.3 minutes/day

2 Erlangs

2 Erlangs

2 Erlangs

10 Erlangs

10 Erlangs

10 Erlangs

2 Erlangs

3 Erlangs

3 Erlangs

Physical link

Virtual link

A1

A2

A

B

B1

C

C1

26 Chapter 1: Understanding Traffic Analysis

Figure 1-6 End-to-end traffic analysis example topology.

In this network, you are making the following assumptions:

• Each node’s traffic has a random arrival pattern.

• Hold times are exponential.

• Blocked calls are cleared from the system.

• Infinite number of callers.

These assumptions tell you that you can use the Erlang B model for sizing your trunk

groups to the PSTN. You want to have a GoS of P.01 on each of your trunk groups.

End-to-End Traffic Analysis: Solution

Compute the traffic load for the PSTN links at each node:

U.K. = 36,000 mins per day × 17% = 6120 mins per busy hour / 60 = 102 BHT

U.S. = 12,882.4 mins per day × 17% = 2190 mins per busy hour / 60 = 36.5 BHT

China = 28,235.3 mins per day × 17% = 4800 mins per busy hour / 60 = 80 BHT

These numbers will effectively give you the number of circuits needed for your PSTN

connections in each of the nodes. Now that you have a usable traffic number, look in your

tables to find the closest number that matches.

For the U.K., a 102 BHT with P.01 GoS indicates the need for a total of 120 DS-0s to

support this load.

36,000 mins

per day

28,235.3 mins

per day

12,882.4 mins

per day

U.S. China

U.K.

End-to-End Traffic Analysis Example 27

U.S. traffic shows that for P.01 blocking with a traffic load of 36.108, you need 48 circuits.

Because your BHT is 36.5 Erlangs, you might experience a slightly higher rate of blocking

than P.01. By using the Erlang B formula, you can see that you will experience a blocking

rate of ~0.01139.

At 80 Erlangs of BHT with P.01 GoS, the Erlang B table (see Appendix A) shows you that

you can use one of two numbers. At P.01 blocking you can see that 80.303 Erlangs of traffic

requires 96 circuits. Because circuits are ordered in blocks of 24 or 30 when working with

digital carriers, you must choose either 4 T1s or 96 DS-0s, or 4 E1s or 120 DS-0s. Four E1s

is excessive for the amount of traffic you will be experiencing, but you know you will meet

your blocking numbers. This gives you the number of circuits you will need.

Now that you know how many PSTN circuits you need, you must determine how much

bandwidth you will have on your point-to-point circuits. Because the amount of traffic you

need on the IP leg is determined by the amount of traffic you have on the TDM leg, you can

directly relate DS-0s to the amount of bandwidth needed.

You must first choose a codec that you are going to use between PoPs. The G.729

codec is the most popular because it has high voice quality for the amount of compression

it provides.

A G.729 call uses the following bandwidth:

• 26.4 kbps per call full rate with headers

• 11.2 kbps per call with VAD

• 9.6 kbps per call with cRTP

• 6.3 kbps per call with VAD and cRTP

Table 1-5 lists the bandwidth needed on the link between the U.K. and the U.S.

Table 1-6 lists the bandwidth needed on the link between the UK and China.

Table 1-5 Bandwidth Requirements for U.K.–U.S. Link

Bandwidth

Consideration Full Rate VAD cRTP VAD/cRTP

Bandwidth

Required

96 DS0s

× 26.4

kbps = 2.534

Mbps

96 DS0s × 11.2

kbps = 1.075

Mbps

96 DS0s × 17.2

kbps = 1.651

Mbps

96 DS0s × 7.3

kbps = 700.8

kbps

Table 1-6 Bandwidth Requirements for U.K.–China Link

Bandwidth

Consideration Full Rate VAD cRTP VAD/cRTP

Bandwidth

Required

72 DS0s

× 26.4

kbps = 1.9 Mbps

72 DS0s × 11.2

kbps = 806.4

kbps

72 DS0s × 17.2

kbps = 1.238

kbps

72 DS0s × 7.3

kbps = 525.6

kbps