The efficiency improvement factor is the ratio of total bytes to be sent pre-compression to the bytes actually sent. So if the router had 50,000 bytes to send, and the compression reduced that to 25,0000 bytes actually sent, then the ratio would be 2.
The display shows bytes saved and bytes sent, so you need to add the 2 together then divide by bytes saved to get the efficiency improvement factor. In my example bytes saved would be 25,000 and bytes sent would be 25,000.
RTP header compression is on a link by link basis. You must configure it on both ends of each link. The efficiency improvement factor on each link should not be affected by the number of routers in the path between 2 endpoints.
I have frame relay links running rtp header compression with G729 codec calls. The efficiency improvement factor shows 2.71 on all links. Each uncompressed packet has an IP header of 20 bytes, an UDP/RTP header of 20 bytes,and 20 bytes payload for a total 60 bytes. Compression reduces the header to 4-5 bytes, so the 60 becomes 24 for a calculated improvement factor of 2.5. Including the frame header drops it down to to 2.38. Not sure why the router says it does a bit better, but the calculation comes pretty close.
