two-stage process for detection and negotiation of LAP-M error control.  
  V.42 also incorporates support for MNP levels 1-4.  
    
  V.42bis - a complementary extension of V.42 which defines a specific data  
  compression scheme for use in conjunction with error control. [BTLZ -  
  (British Telecom Lempel-Ziv) the compression technology used in V.42bis].  
    
  + V.44  - ITU-T data compression scheme for use in conjunction with error  
  + control (uses LZJH compression algorithm).  
    
  | V.90 - ITU-T recommendation for asymmetric data signalling rates of up  
  | tp 56Kbps in the direction of a digitally connected server to a capable  
  | client, and up to 33.6Kbps in the direction of the client to the server.  
    
  + V.92 - ITU-T recommendation for asymmetric data signalling rates of up  
  + to 56Kbps in the direction of a digitally connected server to a capable  
  + client, and up to 48Kbps in the direction of the client to the server.  
    
  V.110 - Support of data terminal equipments with V-Series type interfaces  
  by an integrated services digital network  
    
  V.120 - Support by an ISDN of data terminal equipment with V-Series type  
  interfaces with provision for statistical multiplexing  
    
  V.FC - V."Fast Class" is the name given to a proprietary modem technology  
  jointly developed by Rockwell and Hayes that is based on the basic features  
  of the proposed V.34 recommendation during the March-April 1993 time frame.  
  While under development by ITU-T, V.34 was given a number of "working"  
  names, the most common of which was "V.FAST". V.FC is not an ITU-T  
  recommendation, despite the "V." prefix. V.FC is incompatible with V.34,  
  but many modem vendors may offer products which support both protocols.  
    
  + V.PCM - the "working" name of the V.90 protocol while it was under  
  + development by ITU-T SG16.  
    
  V.Mach - A proposed 43.2Kbps duplex modulation scheme, designed to operate  
  over POTS.  
    
  X.25 - An ITU-T narrow-band (under 56Kbps) packet switching standard. A  
  forerunner to frame relay and ATM.  
    
  XMODEM - Xmodem is a file transfer protocol.  It was the first file  
  transfer protocol to come into widespread use in the microcomputer world.  
  It was developed by Ward Christensen in the mid-1970's to transfer files  
  between machines running the CP/M operating system. Until the late 1980's,  
  because of its simplicity and public domain status, Xmodem remained the  
  most widely used microcomputer file transfer protocol.  
    
  The file to be transmitted is divided into 128-byte blocks.  Each block is  
  sent in sequence, together with a special starting character, an  
  identifying number, and a checksum.  The checksum is a number created by  
  adding all of the bytes in the block together.  
    
  1K-Xmodem is an extension to Xmodem which increases the block size from 128  
  to 1,024 bytes, thereby increasing transmission speeds.  
    
  YMODEM - As a file transfer protocol, the Ymodem protocol was first  
  released as part of Chuck Forsberg's program YAM  ("Yet Another Modem").  
  Ymodem is actually an extension to Xmodem, to overcome some of the  
  limitations of Xmodem.  
    
  Ymodem allows information pertaining to the transmitted file to be sent  
  along with the data, such as filename and length.  Ymodem also increases  
  the size of the Xmodem block from 128 to 1,024 bytes and adds the  
  capability to transmit "batches" of files.  In fact, Ymodem is sometimes  
  referred to as Xmodem-1K, and vice versa.  
    
  Ymodem-G is a streaming protocol built around the philosophy that no news  
  is good news.  It sends the entire file before waiting for an  
  acknowledgment. If the receiving end detects an error in mid-stream, it  
  aborts the transfer.  YmodemG is not an error correcting protocol.  
    
  ZMODEM - As a file transfer protocol, the Zmodem protocol was commissioned  
  by Telenet and placed in the public domain.  Like Ymodem, it was also  
  designed by Chuck Forsberg and was developed as an extension to Xmodem, to  
  overcome some of the limitations of Xmodem.  
    
  Zmodem implements many of the features of the Kermit protocol, as well as  
  the capability to "step down" to the Xmodem or Ymodem protocols on demand.  
    
  Among the key features of Zmodem are a 32-bit CRC offering a degree of  
  error detection 100,000 times greater than Xmodem CRC, a server facility,  
  batch transfers, and fast error recovery.  
    
  Zmodem was also specifically engineered to avoid sending certain sequences,  
  such as ESCape-carriage return-ESCape, that the Telenet network uses to  
  control the connection.  
    
  DATA TRANSFER:  
    
      Microsoft/IBM/Digital Research-DOS is a single threaded, non-  
  re-entrant, non-preemptive, interrupt driven system. Relative to serial  
  communications, when a byte of data is accumulated in the UART, it signals  
  the CPU via an IRQ to process the byte. IRQs are hierarchical in nature  
  With the most essential system functions being assigned the highest  
  priority IRQs. DOS does not place a particularly high priority on the  
  serial port IRQs, and data transfer errors can occur with high data flow  
  rates, or when interrupt servicing is retarded.  
    
     The transmission of each byte (or character) requires that 10 actual  
  bits be moved; a start bit, the eight data bits, and a stop bit. The  
  theoretical maximum throughput in CPS for a given DCE is thus derived by  
  the formula DCE/10=CPS. Error correction and data compression complicate  
  this somewhat. Error-correcting modems do not transmit the framing bits,  
  and are thus able to deliver 25% more data throughput versus modems which  
  do not implement error correction. For an error-correcting link the  
  equation becomes {DCE/10} + {DCE/10}x.25=CPS (note that the numbers derived  
  by these equations represent maximums, and that insertion of protocol  
    
  --- MPost/2 v2.0a  
   * Origin: Marsh BBS (c) Dawson Creek BC Canada (1:17/23)