Compression Techniques (Linktionary term)Compression provides a way to improve throughput on wide area links such as LAN interconnections with modems, bridges, and routers. However, compression must be done in real time, which imposes restrictions on the compression algorithm. One of those restrictions is a requirement for lossless compression. No bits may be lost. Picture two remote LANs connected by a pair of modems, bridges, or routers. Compression is done by the link devices or by computers at each end of the link.

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As data streams into the compressor at one end of the link, that data must be compressed on the fly and transmitted with little or no delay (depending on what is acceptable). The receiver must decompress the data at the same rate to prevent delays. The data that is flowing into the link devices is in frame or packet form.

In contrast, compressing and saving a file to a disk on a desktop computer seems like a simple task given the relative amount of time that the compression can take place. Modem manufacturers have used many of the data- compression techniques mentioned earlier, but a variation of the Lempel- Ziv technique has become popular with the acceptance of the ITU V. Yahoo Group And Files Downloader Crackle. V. 4. 2 is able to monitor data and determine whether compression improves data throughput. For example, files that are already compressed or encrypted do not benefit from further compression.

Note: Encrypted data, by definition, should not have repeating patterns as that may expose the data to attack. Compressing encrypted data actually causes it to expand since there are no patterns that benefit compression. Likewise, there is usually little reason to compress data that has already been compressed since expansion may occur. In some cases, using software- based compression is preferred. For example, Microsoft recommends turning off hardware compression on the modem and using its own software compression for RAS (Remote Access Server) dial- up connections, It claims its RAS data compression can be two to four times as efficient as modem compression.

There are other modem compression protocols, such as the MNP (Microcom Networking Protocol) class 5 and class 7 series, but the V. A protocol called CCP (Compression Control Protocol) provides a way for two linked systems to negotiate the type of data compression over a PPP (Point- to- Point Protocol) connection. Point- to- point leased lines and ISDN connections provide wide area connectivity for many organizations.

Compression is the best way to optimize the use of these lines. Two methods may be used to compress data across links: Stream mode    In this mode, data is continually compressed as it crosses the link. However, the symbol dictionaries, which continually adapt to the stream, must be kept synchronized between the sender and receiver. Thus, a reliable data protocol must be used to ensure synchronization. Packet mode    In this mode, each packet is handled individually as opposed to being included in a stream.

Compression is applied to data either before or after it is encapsulated in a packet. The header is left unchanged so packets can be switched or routed through a mesh network. Dictionary synchronization is only required within the boundary of each packet.

Note that stream mode is designed for point- to- point links because the whole data stream is compressed, including packet headers. Since the headers are compressed, the packets cannot go through routers unless the routers decompress the packets, and this adds delays and other problems. Packet mode is best when transporting packets across mesh, hop- to- hop networks.

An option available in packet mode is whether to compress data before or after putting it in the packet. If you compress the data before putting in the packet, there are fewer packets to send. If you compress after putting it in the packet, you retain the same number of packets. The former is the preferred method on networks that charge by the packet. If encryption is required, data should be compressed and then encrypted before placing it in the packet due to the problem of trying to encrypt compressed data as mentioned in the previous note. The packet mode method can be used over public networks such as the Internet. If the sending and receiving router also perform encryption on the packet data, the packet transmission forms a VPN (virtual private network) across the Internet.

RFC 1. 14. 4 (Compressing TCP/IP Headers for Low- Speed Serial Links. February 1. 99. 0). TCP/IP packets before they cross the link. These headers are relatively large, so compression can improve throughput. Header compression is common for PPP connections.

For example, when configuring a Windows dial- up configuration to use PPP, you can choose IP header compression. In some configurations, it is called Van Jacobson or VJ header compression after the author of the aforementioned RFC. Note that some ISPs may not support IP header compression (if you are configuring a PPP Internet connection). Other significant compression protocols or standards work is outlined below. MPPC (Microsoft Point- to- Point Compression)    MPPC was developed by Microsoft in 1. PPP links. MPPC is defined in RFC 2.

Microsoft Point- To- Point Compression Protocol, March 1. FRF. 9    A specification defined by the Frame Relay Forum to compress data over frame relay networks. Compression is handled across switched links, so data is compressed and the header is left uncompressed.

The Frame Relay Forum is at http: //www. IETF Secure Shell (secsh) Working Group    The goal of this group is to update and standardize the SSH protocol, which provides secure remote logins and data transfers over TCP/IP connections. The protocol encrypts, authenticates, and compresses data. The working group's Web page is at http: //www. The following Internet RFCs provide information about compression and its use in TCP/IP networks: RFC 1.

ZLIB Compressed Data Format Specification version 3. RFC 1. 95. 1 (DEFLATE Compressed Data Format Specification version 1. RFC 1. 95. 2 (GZIP file format specification version 4. RFC 2. 39. 3 (IP Payload Compression Protocol)RFC 2. IP Payload Compression Using DEFLATE)RFC 2.

IP Payload Compression Using LZS)RFC 2. IP Header Compression)RFC 2. Compressing IP/UDP/RTP Headers for Low- Speed Serial Links)Copyright (c) 2.

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