How Routers Use Compression

Dictionary based algorithms use two modes of operation: continuous mode, and packet mode.

Continuous mode monitors a data stream of characters to create and maintain the dictionary, the data stream can consist of multiple network protocols such as: IP, IPX, etc. In continuous mode it is necessary that the dictionaries at the compression, and decompression side be kept synchronised all the time, by means of a reliable data link such as: PPP, X25, LAPB, etc.

By means of synchronisation we can make sure that both dictionaries are kept alike, in fact it is no possible for the decompressor dictionary to decode a frame that is corrupted, as it would cause data to be decompressed inaccurately, and the two dictionaries to diverge.

Packet mode operation is similar to continuous mode operation, it also needs to monitor data to create and keep its dictionary in order. However packet mode is limited to only one network packet, which means that the dictionary only has to be synchronised within that network packet boundaries.