topic Re: Fragmenting packets over Ethernet to improve voice quality in Voice over IP

Fragmenting packets over Ethernet to improve voice quality

rocampo — Thu, 27 Feb 2003 09:55:15 GMT

Setup:

1750--Ethernet---Satellite Receiver---128K Sat Link---Sat. Recvr--Ether---1750

Question:

Since the Satellite Link is only 128K there will be a problem of Serialization Delay. Is there a way to fragment packets and Interleave over ethernet ? The same way it is done over Frame Relay or MLPPP.

In this setup how does one lower down "Serialization Delay" ?

Re: Fragmenting packets over Ethernet to improve voice quality

rbristow — Mon, 03 Mar 2003 17:58:16 GMT

rocampo,

With a Satellite link, most the latency will be travel time and not serialization delay. I would expect VoIP quality to be an issue, since the users will have to tolerate long delays and have to wait to make sure the person on the other end has actually finished speaking before starting to speak. VoIP packets are generally small enough that they are already at or near Ethernet minimum packet size of 64 KBytes, so they are not fragmented on Ethernet. However, you may want to look into the QOS / COS capabilities of your LAN switches. But the real issue for you will be the large amount of latency across the satellite link. Be sure to use QOS on the satellite link, to send along VoIP packets on a preferred basis over other less time sensitive traffic. And you may want to see if you can use RTP header compression on the Satellite link to shrink the size of VoIP frames.

Regards,

Rob Bristow

AT&T Solutions

CCIE #3335

Re: Fragmenting packets over Ethernet to improve voice quality

rocampo — Thu, 06 Mar 2003 02:37:46 GMT

We have working satellite setup with the router connected to the Satellite receiver via Serial with 512ms roundtrip delay. We dont have problems with this setup because we can fragment packets using MLPPP using the serial interface.

Serialization delay is a factor to consider in the problem that I raised because the link would be used for both Voice and Data. A 1500 byte data packet would need 93ms to be placed on the wire, add to that latency and other delay factors and this would exceed the recommended 200ms one way delay for VOIP.

Re: Fragmenting packets over Ethernet to improve voice quality

mazhar71 — Fri, 14 Mar 2003 10:05:36 GMT

Hi,

You can get what you want using a feature called PPP over Ethernet Server & PPP over Ethernet Client. Using this feature you can open PPP session on ethernet media. So if your satallite connection is transparent this is suitable for you.

Regards

Mazhar

CCIE 6683 R&S,C&S