Fidonet COMM Echo Primer   
                             Revision 1.3.6 12/1/2000   
      
                   | = Revised Entry        + = New Entry   
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   For newcomers to this, the FidoNet International echo COMM, there follows a   
   discussion of terms which will be encountered frequently in the messages   
   herein.  A firm grounding in these will add considerable to understanding   
   the messages in this echo.   
      
    +========+                                               +========+   
    |Computer|   DTE-              DCE-               DTE-   |Computer|   
    |   A    |   Rate   +--A--+    Rate     +--B--+   Rate   |   B    |   
    |        |~~~~~~~~~~|Modem|~~~~~~~~~~~~~|Modem|~~~~~~~~~~|        |   
    +========+          +=====+             +=====+          +========+   
      
   Pictured above is a brief sketch of a complete signal circuit, consisting   
   of two computers (A & B) interconnected thru their Modems.   
      
   DEFINITIONS:   
      
   56Kbps Modems [Pre-V.90] - Rockwell, USR, Lucent Technologies, and Motorola   
   marketed incompatible chipsets/modems that operated in a server/client   
   format at up to 56Kbps over standard telephone lines prior to the adoption   
   of ITU-T V.90. USR implemented a protocol dubbed X2, and the remainder   
   combined efforts to implement a protocol dubbed K56Flex (a combination of   
   Rockwell's K56Plus and Lucent's VFlex/2 protocols). The X2 and K56Flex   
   protocols do not interoperate.   
      
   ADSL (Asymmetric Digital Subscriber Line) -  a modem technology that   
   converts existing twisted-pair telephone lines into access paths for   
   multimedia and high speed data communications. ADSL transmits more than   
   6Mbps to a subscriber, and as much as 640 kbps more in both directions.   
      
   An ADSL circuit connects an ADSL modem on each end of a twisted-pair phone   
   line, creating three information channels; a high speed downstream channel,   
   a medium speed duplex channel, and a POTS (Plain Old Telephone Service)   
   channel. The POTS channel is split off from the digital modem by filters,   
   thus guaranteeing uninterrupted POTS, even if ADSL fails. The high speed   
   channel ranges from 1.5 to 6.1 Mbps, while duplex rates range from 16 to 640   
   kbps. Each channel can be sub-multiplexed to form multiple, lower rate   
   channels.   
      
   ARQ - (A)utomatic (R)epeat Re(Q)uest - a general term which describes   
   detection and retransmission of defective blocks of data. When appended to a   
   CONNECT string (eg. CONNECT 28800/ARQ) it indicates that the modems have   
   negotiated some manner of error control for the link.   
      
   ASCII - (A)merican (S)tandard (C)ode for (I)nformation (I)nterchange. A   
   standard for defining codes for information exchange between equipment   
   produced by different manufacturers.   
      
   ASYNCHRONOUS - Describes data transmission technique in which the length of   
   time between transmitted characters may vary. Because the time lapses   
   between transmitted characters may vary, a receiving modem must be signaled   
   as to when the data bits of a character begin and when they end. The   
   addition of Start and Stop bits serves this purpose.   
      
   ATM - An international ISDN high-speed, high-volume, packet-switching   
   transmission protocol standard. ATM uses short, uniform, 53-byte cells to   
   divide data into efficient, manageable packets for ultrafast switching   
   through a high-performance communications network. The 53-byte cells   
   contain 5-byte destination address headers and 48 data bytes. ATM is the   
   first packet-switched technology designed from the ground up to support   
   integrated voice, video, and data communication applications. It is   
   well-suited to high-speed WAN transmission bursts. ATM currently   
   accommodates transmission speeds from 64 Kbps to 622 Mbps. ATM may support   
   gigabit speeds in the future.   
      
   BANDWIDTH - The frequency range available for use by modems on an ordinary   
   two-wire dial-up telephone line. This corresponds to the frequency range   
   required to reproduce the human voice, or approximately 3500Hz   
   (200-3700hZ).   
      
   BAUD - Perhaps the most mis-used term in all of the discussions posted in   
   this forum. It actually refers to the unit of measure for the number of   
   discrete changes of state which occur in a communication channel per second   
   (ie. the number of times per second that carrier frequencies are   
   modulated). It is an old term from the days of Frequency Shift Keyed   
   modems. The name honors Jean Maurice Emile Baudot, who invented a bit   
   encoding scheme for characters (it is/was not the same as that presently   
   used for encoding ASCII characters however).   
      
   Relative to FSK modems, the use of Baud referred to the rate that you could   
   shift from one FSK Tone to another. The tones directly represented the ones   
   and zeros of data being transmitted. In the early days they were generally   
   referred to as the Mark Frequency and the Space Frequency. Accordingly,   
   with this direct correlation of tones to 1s and 0s, the Baud Rate was the   
   same as the Bit Rate. [Note: The FSK transmission schemes referenced above   
   are to bi-frequency implementations such as V.21 and the Bell 103 protocol.   
   Multi-frequency FSK schemes also exist, but they have not been widely   
   implemented over the PSTN].   
      
   As more complex ways of transmission were devised it was natural to try to   
   extrapolate this concise definition to define their operation. An early   
   extrapolation was to Phase Shift Keyed (PSK) modems such as the V.26 Series   
   of modems. This was unfortunate, but it did actually occur. The   
   extrapolation went like this:  The PSK modem  generated a signal with 4   
   possible phase states and thus 4 possible phase changes. The states were 0,   
   90, 180 and 270 degrees of the carrier. The possible changes were the same.   
      
      
   --- MPost/2 v2.0a   
    * Origin: Marsh BBS (c) Dawson Creek BC Canada (1:17/23)