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end-to-end traffic analysis example

Unanswered Question
Apr 11th, 2006
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Hi!

I was reading the voip traffic analysis example. See http://www.cisco.com/univercd/cc/td/doc/cisintwk/intsolns/voipsol/ta_isd.htm#xtocid35

I cannot understand why to calculate the bandwidth needed on the link between the U.K. and the U.S., 96DSOs is used in the calculation instead of 48DSOs.

-> Full Rate: 96 DS0s * 26.4 kbps = 2.534 Mbps


Same goes for the bandwidth needed on the link between the U.K. and China. Shouldn't we use 96DSOs here instead of 72 DSOs?

-> Full Rate: 72 DS0s * 26.4 kbps = 1.9 Mbps


TIA for any enlightenment!

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jbouwkamp Mon, 04/17/2006 - 13:47
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The 96 and 72 DSO statements are addressing the number of PSTN DSOs needed at each location. Since the scenario is not using voice over IP for connections to the PSTN you will need one DS0 per call. The calucations for bandwidth required between sites is to handle all calls coming in all DSOs.

So while it has been determined that 96 DSO connections to the PSTN are required for the US location, you will need 2.534Mbs of bandwidth between sites to support all PSTN calls forwarding across the WAN. Same thing for China, since China will have 72 DSO connections to the PSTN you will need 1.9Mbs of WAN bandwidth between sites in case all calls are forwarded across the WAN at the same time.

So while 96 DSOs are required to telco, you could get away with WAN links that support 2.534Mbs (2 T1s) but this does not address other considerations like actually having some data traverse the link or Cisco's best practice of not exceeding 33% of a link for voice.


Hope that helps...

gohsekchye Mon, 04/17/2006 - 21:21
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Thanks jbouwkamp, but I am still puzzled by your explaination.


Just to check my understanding, here are some of the text extracted from the example:


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


U.K. = (36,000 min per day) * 17% = (6,120 min per busy hour)/60 = 102 BHT

U.S. = (12,882.4 min per day) * 17% = (2,190 min per busy hour)/60 = 36.5 BHT

China = (28,235.3 min per day) * 17% = (4,800 min per busy hour)/60 = 80 BHT

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

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

China, at 80 erlangs of BHT with P.01 GoS, the Erlang B table shows you that you can use one of two numbers. At P.01 blocking you 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.


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Based on my understanding of this paragraph, the numbers of PSTN DSOs required at each site are as follows:

UK: 120 DS-0s

US: 48 DS-0s

China: 96 or 120 DS-0s



My confusion is why the example uses 96 DS-0s to calculate bandwidth needed on the link between US and UK as follows:

Full Rate: 96 DS0s * 26.4 kbps = 2.534 Mbps


What is the relationship of the DS-0s required at each site and the amount bandwidth needed BETWEEN the sites?


Thanks for any enlightenment.

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