Cabletron Systems CSX1200, CSX1000 manual Compression and CCP

CONFIGURING OTHER ADVANCED OPTIONS

Compression Options

effect is to increase effective interconnect bandwidth by decreasing transmission time. If negotiation for compression fails, data is transmitted uncompressed.

The compression algorithm implemented is STAC-LZS. This algorithm is used in all of STAC’s data compression products. This software version is fully compatible with STAC’s data compression compressor chips including the multi-tasking features. STAC-LZS data compression is performed by replacing redundant strings in a data stream with shorter tokens. The STAC-LZS uses a compression history, or sliding window, as opposed to a structured dictionary. This allows greater flexibility and a greater number of possible string comparisons during compression process. The compression history automatically discards old information as new information is processed. Both the device and system must perform compression using the STAC-LZS data compression algorithm. The peer and remote compression algorithms must be synchronized, this is accomplished by negotiating compression at channel connect time. Once this has been accomplished compressed data can be transmitted. If a transmission problem should ever occur the problem is detected and compression re-synchronized by the execution of a pre-defined protocol.

COMPRESSION AND CCP

The Compression Control Protocol (CCP) is one of a suite of protocols which operate under the umbrella of the IETF’s Point-to-Point Protocol (PPP) suite. CCP implementation permits compression and decompression on PPP links.

During call establishment, an appropriately configured system will attempt to negotiate compression 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 options used by CCP include:

•STAC-LZS compression algorithm

•one history

•sequence number check mode or extended mode

During CCP negotiations, the system will always propose the use of Sequence Number check mode first for inbound traffic. The peer has the option to accept or reject this proposal. If the peer rejects the 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 the Point-to-Point links. Such compressed data packets will be encapsulated as described in the CCP specification. Received data packets not so encapsulated will be considered to be uncompressed data and will be forwarded on in the order they were received. Transmitted packets whose compressed size increases to the point of exceeding the link’s Maximum Receive Unit (MRU) will be sent uncompressed.

When using Sequence Number check mode and a non-zero number of histories, the STAC-LZS algorithm requires that incoming data packets be decompressed in the order they were compressed. The sequence numbers are used to assure proper ordering and that no packets have been lost. Should a packet loss be detected, the system will send a CCP Reset-Request packet as described in the CCP specification to the peer and will discard any accumulated history and queued receive packets. The peer will be expected to also discard its outbound history and respond with a CCP Reset-Acknowledgment. At this point, both sides will have been resynchronized and compressed data transfers can continue.

Workgroup Remote Access Switch 347