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New Member

VoIP or Vo ATM


We are going to implememte voice over a data link. We'll contract an ATM E1 with two PVC of 512Kbps. In one of them we are going to send voice packets.

We are going to connect our PBX (Hicom 350 E) with a CIS_VWIC-1MFT-E1= 1-Port RJ-48 Multiflex Trunk - E1.

What you recomend us, Voice over IP or Voice over ATM.

We'll connect only to sites (Argentina-CHile)



New Member

Re: VoIP or Vo ATM

Well here are my two cents.

From what I'm reading you're going to use both.

VoIP end to end, with packet to cell conversion between your two ATM router cards.

Using a PVC for voice only is your best bet.

Just make sure that you're using the right QoS parameters before and when the packets/call conversions.

What kind of bit rate is the ATM ciruit provided to you.

If you have a option request VBR-rt (real time), as

variable bit rate (VBR) circuits have a high service class.

The most significant architectural advancements at the router-switched boundary involve the management of the traffic queues. It is these queues that ensure that higher-priority data is expedited ahead of lower-priority data.

New Member

Re: VoIP or Vo ATM

The circuit that they are providing me is 512Kbps real time and it will be use only for voice.

Do you have a configuration example for the routers??

Do you suchess to use voice compresion?? if yes which one???



New Member

Re: VoIP or Vo ATM

Each router config varies from the other.

You can find different configs @

As far as voice channel compressions are concerned, that'll all depend on how many voice calls are going to go through the pipe. Without compression, PCM G.711 you'll be able to run maybe 6 voice channels at a time.

64kb (PCM) + 20kb(IP header) + ATM cell header = 90+ kbps.

If you need more chanels:

I would suggest you use G.729 CELP compression @ 24 kbps and you'll get appx. 20 voice channels.

If you compress RTP you can bring it down to appx.12kbps.

You can get even more voice channels if you enable VAD silence supression, that'll bring the voice channel bandwidth down to 7.3kbps.

This way you would be able to run approx. 60 voice channels.

I like very much the quality of the CELP compression.

VAD is not the best idea to use, but you be a judge of it after you get to test it.

Good Luck

New Member

Re: VoIP or Vo ATM

How do I restrict the channel use.???

This can be done in the router or I have to block the channels in the PBX.


New Member

Re: VoIP or Vo ATM

Why would you want to restrict any channels?

Being that you have an E1 PRI card with 30 voice channels, I would suggest you use CELP (G.729)compression at 12kbps (with ip overhead), and not enable VAD.

This way you can have all your 30 channels up and plenty of bandwidth for packet to cell overheads.

In this case you would not use silence compression (VAD), and you would have to compress the IP RTP header.

On the other hand, if you don't need 30 channels running at the same time, than you can get 20 voice channels using CELP without IP RTP header compression. These voice calls with take up to 24 kbps of bw, and the router CPU will not be utilized as much.

On the PBX side you don't have to do anything with voice channels. The TDM to packet conversion will be done on the router, and on the router you have to specify what compression algorithm you want to use, VoIP and pots dial-peers as well as class-maps and policy-maps for QoS. The QoS (CoS in your case) has to be mapped to the ATM (outside/inside) ports, so you can get high level Quality of Service.

New Member

Re: VoIP or Vo ATM

I don't understand how the router manage the congestion so I though that if a block some channels I prevent those issues.

Is that true????



New Member

Re: VoIP or Vo ATM

Routers are very capable of handling 30 and if need be (depending on the router) much more.

You can run as much as 360 G.711 (no compression) voice channels on a 3660 and even more on a 7200 VXR


By you blocking channles on PBX side, you're not doing anything but limit yourself extra voice channels,