Considerations on migrating from TDM PRI's to SIP Trunking?

Question

Hello,
We are planning a migration from traditional TDM PRI's to SIP trunking for inbound toll-free and outbound LD telephone traffic. Currently, we have 7 x PRI's connected to a 3945 router feeding our CUCM 8.6 system. This works fine. We are planning on making use of a 100 Mb data circuit to bring in SIP service from our carrier. This would terminate on our 3945 voice gateway. We would have roughly the same number of SIP concurrent call paths as we have PRI channels (162).
I know that the 3945 makes heavy use of DSP resources to handle the PRI traffic. Are DSP resources needed for SIP traffic as well?
Will the SIP usage (162 SIP concurrent call paths) cause similar router utilization as the PRI's (router CPU, memory, etc.)?
What are other things we should look out for in this migration?
Thank you!
Brian

Answer 1

1. Just to add to the excellent tip provided by George (+5), Here is the capacity matrix for the 3900 gateways. Your 3945 can support 950 concurrent calls. So in terms of capacity, you are well taken care of.
 Number of IP-to-IP Calls per Platform
Platform
Maximum Number of Simultaneous Calls (Flow-Through)
Cisco 3945E
2500
Cisco 3925E
2100
Cisco 3945
950
Cisco 3925
800
Cisco 2951
500
Cisco 2921
400
Cisco 2911
200
Cisco 2901
100
Cisco ASR 1004; and Cisco ASR 1006 Router Processor 2 (RP2)
5000; 16000*
Cisco ASR 1002, ASR 1004, and ASR 1006 RP1
1750
Cisco AS5350XM and AS5400XM
600
Cisco 3845
500
Cisco 3825
400
Cisco 2851
225
Cisco 2821
200
Cisco 2811
110
Cisco 2801
55
2. You should definitely make provisions for dsps. You may need DSPs for MTP, xcoding, etc. Especially with SIP, MTP may be needed for DTMF mistmatch, supplementary services etc
3. One of the most important thing to consider is the codec you will use for your calls. Your users are used to PSTN (TDM). Using G711 on your sip calls is not even the same as the traditional PSTN. The quality will be noticeable. Using G729 is going to be distinct. I have seen where users complain of this during a deployment. Using G729 will be a rude shock and they may not like it.
4. You also need to consider your analogue options etc FAX. What fax protocol does your ITSP support etc...We have been sued before by a customer because their fax machines didnt work. The ITSP said it supported T.38 while in reality it didnt.
5. In terms of memory utilization and CPU, I will think that it should be less for IP-IP call. This is because in an IP-TDM call, your router is constantly encoding via your dsp the IP payload (codecs) to TDM  for transimissiont ot he PSTN. This wont be happening any more.