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

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