Workgroup Remote Access Switch 347C ONFIGURING O THER ADVANCED O PTIONSCompression Optionseffect is to increase effective interconnect bandwidth by decreasing transmission time. Ifnegotiation for compression fails, data is transmitted uncompressed.The compression algorithm implemented is STAC-LZS. This algorithm is used in all of STAC’s datacompression products. This software version is fully compatible with STAC’s data compressioncompressor chips including the multi-tasking features. STAC-LZS data compression is performedby replacing redundant strings in a data stream with shorter tokens. The STAC-LZS uses acompression history, or sliding window, as opposed to a structured dictionary. This allows greaterflexibility and a greater number of possible string comparisons during compression process. Thecompression history automatically discards old information as new information is processed. Boththe device and system must perform compression using the STAC-LZS data compressionalgorithm. The peer and remote compression algorithms must be synchronized, this isaccomplished by negotiating compression at channel connect time. Once this has beenaccomplished compressed data can be transmitted. If a transmission problem should ever occur theproblem is detected and compression re-synchronized by the execution of a pre-defined protocol.C OMPRESSION AND CCPThe Compression Control Protocol (CCP) is one of a suite of protocols which operate under theumbrella of the IETF’s Point-to-Point Protocol (PPP) suite. CCP implementation permitscompression and decompression on PPP links.During call establishment, an appropriately configured system will attempt to negotiatecompression using CCP and STAC-LZS. The system will support either of two STAC-LZS modes,sequence numbers or extended mode. This negotiation will take place on all calls. Specific optionsused by CCP include:• STAC-LZS compression algorithm• one history• sequence number check mode or extended modeDuring CCP negotiations, the system will always propose the use of Sequence Number check modefirst for inbound traffic. The peer has the option to accept or reject this proposal. If the peer rejectsthe proposal and counter-proposes STAC-LZS Extended mode, it will be accepted by the system.For outbound traffic, the system will accept either Sequence Number or Extended Mode.Once compression has been negotiated, transfers of compressed data can take place across thePoint-to-Point links. Such compressed data packets will be encapsulated as described in the CCPspecification. Received data packets not so encapsulated will be considered to be uncompresseddata and will be forwarded on in the order they were received. Transmitted packets whosecompressed size increases to the point of exceeding the link’s Maximum Receive Unit (MRU) willbe sent uncompressed.When using Sequence Number check mode and a non-zero number of histories, the STAC-LZSalgorithm requires that incoming data packets be decompressed in the order they werecompressed. The sequence numbers are used to assure proper ordering and that no packets havebeen lost. Should a packet loss be detected, the system will send a CCP Reset-Request packet asdescribed in the CCP specification to the peer and will discard any accumulated history andqueued receive packets. The peer will be expected to also discard its outbound history and respondwith a CCP Reset-Acknowledgment. At this point, both sides will have been resynchronized andcompressed data transfers can continue.