SmartSwitch Router User Reference Manual 319Chapter 22: WAN Configuration GuideAverage Packet SizeIn most cases, the larger the packet size, the better the potential compression ratio. This isdue to the overhead involved with compression, as well as the compression algorithm.For example a link which always deals with minimum size packets may not perform aswell as a link whose average packet size is much larger.Nature of the DataIn general, data that is already compressed cannot be compressed any further. In fact,packets that are already compressed will grow even larger. For example, if you have a linkdevoted to streaming MPEG videos, you should not enable compression as the MPEGvideo data is already compressed.Link IntegrityLinks with high packet loss or links that are extremely over-subscribed may not performas well with compression enabled. If this is the situation on your network, you should notenable compression histories (this applies only to PPP compressions; in Frame Relaycompression, histories are always used).Compression histories take advantage of data redundancy between packets. In anenvironment with high packet loss or over-subscribed links, there are many gaps in thepacket stream resulting in very poor use of the compression mechanism. Compressionhistories work best with highly-correlated packet streams. Thus, a link with fewer flowswill generally perform better than a link with many flows when compression histories areutilized.The “no history” (max-histories = 0) option causes packets to be compressed on a packet-by-packet basis, thus packet loss is not a problem. Also, the number of flows is not anissue with this option as there is no history of previous packets.Latency RequirementsThe use of compression may affect a packet’s latency. Since the compressed packet issmaller, less time is needed to transmit it. On the other hand, each packet must undergo acompression/decompression process. Since the compression ratio will vary, the amount oflatency will also vary.Example ConfigurationsThe following command line displays an example for Frame Relay:frame-relay set payload-compress ports se.3.1.300
