In my last post, I discussed SigComp and how it relates to the wonderful world of IMS. SigComp though is only part of the IMS compression story. There is more. Much more.
Just in case you forgot – messages are big. We would all like to shrink them so that they use less of operators’ precious bandwidth. We tackled the big message problem by compressing the content using SigComp.
But there is one “minor” issue I left out last time – the issue of IP. IP is a nice enough protocol, and is required for IMS (you remember the IP Multimedia Subsystem…).
SIP messages are sent over TCP or UDP, which in turn are sent over IP. RTP packets (you know, that media we want to see or use) are sent over UDP, which again means it is over IP.
Last time, we dealt with the SIP message issue. But what about the IP, UDP, TCP and RTP?
All these have their own headers that add lots of overhead to the messages themselves. We’re talking about 40 bytes for an IPv4 packet sent over RTP (UDP and IP included). And if we have to send 50 of those packets every second just to keep our audio running, we have some pretty heavy packets to deal with!
The solution to this problem is ROHC (Robust Header Compression). It is defined in various RFCs, with different modes of operation. I won’t delve into the technical details, but would like to point out one very interesting thing: it’s in the operating system.
Since ROHC is used to tweak the size of IP, UDP and TCP headers and compress them to be 1 or 3 bytes, it requires support from the operating system itself. By operating system, I mean your “average” IP stack that you get for free with it.
What all this means to us, is very simple. To implement IMS – and on a client no less – requires not only application implementation but also an operating system with support for all the architecture’s special needs.
Technorati Tags:
IMS,
SIP,
SigComp,
3GPP,
Tsahi Levent-Levi,
IMTC
No comments:
Post a Comment