3Com MSR 50 Series Configuration Manual page 377

Configuring PPP Link Efficiency Mechanism 377
(including IP, UDP, and RTP) headers or TCP headers. The following describes how
compression operates by taking RTP header compression as an example.
The real-time transport protocol (RTP) is virtually a UDP protocol using fixed port
number and format. The protocol includes a 40-byte header and a data section.
There is a concern that the 40-byte header which is composed of a 20-byte IP
header, an 8-byte UDP header and a 12-byte RTP header, is too large when
compared with the 20 bytes to 160 bytes typical payloads of RTP. To reduce
unnecessary bandwidth consumption, you can use IPHC to compress headers.
After compression, the 40-byte header can be reduced to 2 to 5 bytes. If the
payload is 40 bytes, the compression ratio will be (40+40) / (40+5), about 1.78,
which is very efficient. The process of IPHC is illustrated in the following figure.
Figure 94 IP header compression
Incoming
packets
RTP header compression
Output
queue
Queue
Traffic
classifying
Non-RTP traffic
Stac LZS compression
Stac LZS compression is a link-layer data compression standard developed by Stac
Electronics. Stac LZS is a Lempel-Ziv-based algorithm that compresses only packet
payloads. It replaces a continuous data flow with binary code that can
accommodate to the change of data. While allowing for more flexibility, this
requires more CPU resources.
VJ TCP header compression
VJ TCP header compression was defined in RFC 1144 for use on low-speed links.
Each TCP/IP packet transmitted over a TCP connection contains a typical 40-byte
TCP/IP header containing an IP header and a TCP header that are 20-byte long
each. The information in some fields of these headers, however, is unchanged
through the lifetime of the connection and needs sending only once, while the
information in some other fields changes but regularly and within a definite range.
Based on this idea, VJ TCP header compression may compress a 40-byte TCP/IP
header to 3 to 5 bytes. It can significantly improve the transmission speed of some
applications, such as FTP, on a low-speed serial link like PPP.
Link Fragmentation and Interleaving
On the low speed serial link, real-time interactive communication (such as Telnet
and VoIP) is performed, and block and delay may occur when large packets are
transmitted. For example, if a voice packet arrives when large packets are being
scheduled and waiting for being transmitted, it has to wait until all the large
packets have been transmitted. As for the real-time applications, large packets can