Bandwidth requirements

Unanswered Question
Jan 6th, 2009

I`ve been asked to look into Cluster over the WAN for a particular site though I don` recommend it .

From Cisco SRND Set up

ICCS- call traffic

We have 2x server for around say 300 -500 user in two buildings - A & B. Cisco states that for below 10k BCHA then you need 1.544Mbs for ICCS or based upon a formula if you have more then 10KBCHA -

Total BHCA/10000 *(1+0.006*Delay )

Database replication

A T1 * per server in each remote location i.e Site 1 Publisher, Site 2 Subscriber and a TFTP/MOH so Site 2 requires T1*2 (Sub and TFTP) to the central site

Question 1

Obviously 10k BHCA is quiet a large through put what if you only have a small number of phones 300-500 which will provide less then the 10K. Is there a different formula for less then 10K for ICSS, is the T1 still a min. requirement

Question 2

If you configure CUCM so that each site is designed - admittedly not good practice, but to only have the phones to register to the "sites" CUCM server so to restrict call traffic between the two sites again is the T1 a minium again. obvious if you use EM then this effect this set up .

I`m being asked by support these questions purely based upon a SI who has suggested splitting the two server Cluster across two sites which only has a 10 Mbs link between them which is being used by data as well and that the back up link is only 2Mbs, My recommendation is to have the two CUCM servers in Site A and have the necessary SRST router in place in Site 2 and have the phones register to Site A as 1st and 2nd choice with 3rd choice the SRST router. But I would like to understand the requirements a bit more - is there any movement the sizes mentioned above, Cisco SRND

Based on the SRND we would require a min 2x T1 or 3Mbs

ICCS - 1.544Mbs

Databas 1.544Mbs based upon a single remote server


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Chris Deren Tue, 01/06/2009 - 09:00

1. Correct, a T1 is the minimum to spit CM servers.

2. There is no call traffic going through CM servers, only signaling so why are you worrying about using the remote server for redundancy? 300 phones will not generate that much signaling bandwidth.

So, how many servers do you have? if you are planning on putting only one CM server at a remote location you need 1 T1 not 2, (assuming latency is below 40 ms).

As to which design is better it depends on the requirements, if the remote site is critical and needs all the features of CM and SRST will not do the job splitting CM across WAN may be a better solution (assuming sufficient bandwidth). Another alternative is to configure CME as SRST and provide more functionality than simple SRST. There is no extra cost for it.



iptuser55 Tue, 01/06/2009 - 09:48

I`m referencing the attachment- Is this not correct then ? Based on the attachment we are lookng at 3Mbs? for a remote site to have a Sub on their site- 1.544 for replciation and 1.544 for ICCS Again am I wrong in this ? The additional 1.544Mbs is based upon the attachment, admitedly there will not be a lot of call traffic from say 300 handset even if we terminate the E1`s on Site A so all I/C, O/G are across the WAN link but it seems you need the figure they suggest?

Chris Deren Tue, 01/06/2009 - 09:59

I guess you are right, initially this was the case for only deployments where the latency is above 40 ms (up to 80), I guess now Cisco just standardized and made a generic statement. Interesting.


iptuser55 Tue, 01/06/2009 - 10:28

I know, the dB replication figure I`m happy with though it should also be dependent on the size/ number of devices in the Cluster so small cluster small link ???? it would be good if the ICCS could be reduced down to less then 10k figure as they use a formual for more then 10k BHCA so why not less then


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