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New Member

VoIP over VSAT (good bandwidth but high delay)

Hi All

This interesting scenario requiring the help of all VoIP gurus. I am trying to setup a VoIP call over a VSAT link. The link has ample bandwidth but terrible delays.

We've had a number of enquires about the viability of voice over VSAT using traditional VoIP technologies, and have been trying to get this working as a proof of concept for the last week or so. A sample scenario - the customers are situated in remote/rural areas where typically there is no cellular reception and/or telco infrastructure is not readily available. They are willing to compromise quality for service availability. The VSAT links they have provide 2MB upstream and 384kbps downstream rates and is capable of transporting IP. Bandwidth is not an issue but the roundtrip delay is 800-1000ms on average. Not good I know.

I am using a 3640 and VG200 router with FXS interfaces and analogue phones. Default static routes with basic dial peer configurations are used and G729 codes as a start. We have full IP connectivity between the 2 routers but the call setup over the voip call leg fails. No surprise here although I was kinda hoping to at least get a call thorugh with maybe no voice or bad quality. I captured various debug traces and through comparing succesful call setup debugs, suspect that the H323 call setup timers might need tuning (if at all possible).

I've read though the Cisco press article about the first VoIP call from space, using Cisco's Softphone, but could not find any technical info on what settings had to be tweaked. I'm busy completing the Call Manager setup and plan to test the softphone as well.

Has anyone had any past experience with VoIP over VSAT? Any ideas, help or info would be greatly appreciated. I have various debugs available should you be interested in them.

Thanks

3 REPLIES
Cisco Employee

Re: VoIP over VSAT (good bandwidth but high delay)

Well, you ask a good questions here.

Please remember that the call from space was from the space shuttle. The shuttle only flies at about 280 miles altitude and this was a one way stream. A geosychronous satellite ip 22,300 miles altitude and the call requires a round trip. That been said, I think you can see that your delay is much, much greater than what the shuttle folks had.

it probably is h323 timing out that is causing some of your problems.

Here is a link for setting the h225 timers:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121t/121t2/dtcfgtim.htm

Give this a try and see if it works for you.

Silver

Re: VoIP over VSAT (good bandwidth but high delay)

We have had some success in getting voip to work across satellite links into remote sites in the past. As the long as the users have an understanding that it will be almost like using a 2 way radio then it should be OK. We did not have to change any of the H225 timers so there could be something more basic stopping it from working. Can you post the configs from both routers here so we can take a look ?

New Member

Re: VoIP over VSAT (good bandwidth but high delay)

Thanks for your interest and responses. After upgrading the IOS code on one of the 3640 routers to the problem was solved. One router was running 12.2.3 Enterprise software and the other 12.0.5XK1 IP Plus software. I have not looked in detail for the cause but can confirm that it's definately IOS related (downgrading as a test re-introduced the problem). Call setup now completes without having to tweak any H225 parameters and the quality is very good once you get used to the delay. We've succesfully tested various codecs and they all worked well with G711 being most preferred by the users.

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