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Design Question - VoIP WAN Router Sizing

I would appreciate thougts on WAN router sizing for a VoIP infrastructure. Following is what I believe is pertinent information.

1) hub and spoke topology. DS3's at hub site. Multiple T1's at spoke sites.

2) spoke sites may have anywhere from 10 to 200 telephones, and 20 to several hundred data ports.

3) the requirement is 4 FXO for local PSTN access / failover plus one port to connect to local paging systems

4) centralized Call Manager. Conference calling is essential, so a Conference Server has been specified.

5) the WAN router at the spoke sites will provide various QoS functions, local DSP resources, SRST, etc

6) I have no information about call volume or busy hour usage. the customer in question has only said that during defined busy hours, they estimate that "75% of the phones are in use"

7) as always, the customer is pushing back on cost. this is a 1500+ phone proposal, plus LAN switches, so the overall cost is up in the millions.

As I see my options, 3745's appropriately configured, can serve. Smaller sites can use a combo of a 1760V and a 2651V to meet the requirement.

My question for this forum - given that I have no real information about call volume, at what number of phones would people think it prudent to up the router from the lower end pair ( 1760+2651 ) to the 3745?

a) at a site with 2xT1 and PPP multilink - 20 phones? 50 phones?

b) at a site using 3xT1 and PPP multilink - 30 phones? 60 phones?

Multilink is preferable. customer is not interested in dedicating a T1 to voice only.

my concern is that if I use the lower end pair, and it proves insufficient, I'm on the hook for a free upgrade ( it's that kind of customer ) whereas obviously, the 3745 is more expensive, and subject ot rejection because of cost.

Any general guidelines, any general thoughts are welcome.


Re: Design Question - VoIP WAN Router Sizing

The best guide to decide on the design would be:

Voice Design and Implementation Guide

Hope this helps.

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