Some about MODEM's

Some about Modems

see > RS-232 Pin - layout / NULL-Modem Cable

U.S. / Europe and International Data Transmission Standards

 Bell  103J    U.S. standard       300bps
 Bell  212A    U.S. standard       1200bps
 CCITT V.17    Int. fax standard   14400bps
 CCITT V.21    Int. standard       300bps
 CCITT V.22    Int. standard       1200bps
 CCITT V.22bis Int. standard       2400bps
 CCITT V.23    Int. standard       1200 and 75bps
 CCITT V.24    Serial interface standard (EIA/RS232-D)
 CCITT V.27ter Int. fax standard   4800bps
 CCITT V.29    Int. fax standard   9600bps
 CCITT V.32    Int. standard       9600 and 4800bps
 CCITT V.32bis Int. standard       14400/12000/9600/7200/4800bps
 CCITT V.34    Int. standard       28800/26400/24000/21600/19200 .. to 2400bps
 CCITT V.34+   Int. standard       same as V.34 plus X2 technology up to 56000bps
 CCITT V.42    Standard for detection and negotiation for LAPM
 CCITT V.42bis Extension of V.42 specifying the data compression protocol
 CCITT V.FAST  Int. standard       28800bps
 ITU   V.90    Int. standard       56000 kBit/s
       X2 and K56Flex Modem's are not compatible to V.90.
 Most of this Modems (X2 and K56Flex) can be upgraded for V.90 (Flash-code).
       V.110   ISDN Europe (old)   38400 kBit/s
       V.120   NDIS U.S.           57600 kBit/s
       X.75    ISDN Europe std.    64000 kBit/s
       T70     BTX <-> ISDN Europe 64000 kBit/s
      ISDN - Integrated Services Digital Network (Germany - Europe)
      NDIS - Network Device driver Interface Specification (USA)
      DSS1 - Digital Subscriber Signalling System No. 1 (aka Euro ISDN)
      1TR6 - ISDN-Protocol for D-channel / control for (up to 30) B-channels - Germany only
             (1TR6 is now obsolete -> use DSS1)

Error Control
There are two important sets of standards that define how transmission errors are to be detected and corrected. They are MNP (Microcom Networking Protocol) classes 2,3 and 4 and the CCITT V.42.
V.42 uses a protocol called LAP-M (Link Access Protocol for Modems). For compatibility, V.42 contains MNP 2-4.

Data Compression
To reduce the amount of data that must be transmitted, there are two important standards that describe automatic data compression. They are MNP class 5 and CCITT V.42bis.

V.42bis requires that both modems use V.42 error control with LAP-M. It is claimed that MNP 5 offers 2:1 compression and V.42bis offers 4:1 compression. If this were true, we would see the following effective speeds:

  2400bps  +  MNP 5   -->  4800bps
  2400bps  +  V.42bis -->  9600bps
  9600bps  +  MNP 5   -->  19200bps
  9600bps  +  V.42bis -->  38000bps
 14400bps  +  MNP 5   -->  28800bps
 14400bps  +  V.42bis -->  57600bps


The actual throughput is significantly less and depends on the type of data being transmitted.
(Because MNP/V.42 uses near the same algorithmus than compression programs, a packed file transmitted with MNP will have the same throughput, as a transmit w/o MNP, but when you send uncompressed ASCII-text or binary files then the transmission speed will be faster than w/o MNP or V.42)

AT Command's (ATtention cmd.)

The AT command-set is not standarized , always use the literature and software that comes with the modem. However, this description should assist, to understand the basic's of TELE-PROCESSING.


  Under no circumstances try to work / manipulate a MODEM during a THUNDER-STORM !

MODEM's are normally shipped, with a tailored software-package. During start testing a MODEM, always use the software that comes with the MODEM. The defaults included in such a modem program (protocol used for DIAL / RESET / COMMUNICATE) will work in near all situations.

When you enter AT command's, the input should be done, either in UPPER or LOWER case (do not mix).


MODEM Data Terminal Interface

The modem connector interfaces with the DTE via a 25-pin connector.
(for 9-pin DB-9 connector see > Serial DB-9 pin-layout).

Pin EIA-232C
Circuit
CCITT V.24
Circuit
Function
1 AA 101 Frame ground (FG)
2 BA 103 Transmitted data (TD)
3 BB 104 Received data (RD)
4 CA 105 Request to send (RTS)
5 CB 106 Clear to send (CTS)
6 CC 107 Data set ready (DSR)
7 AB 102 Signal ground (SG)
8 CF 109 Data carrier detect (CD)
9     +12 V dc voltage reference
10     -12 V dc voltage reference
11     Flow control
12 CI   Speed indicator (from DCE)
15 DB 114 Transmitter signal element timing (from DCE) (TC)
17 DD 115 Receiver signal element timing (from DCE) (RC)
20 CD 108/2 Data terminal ready (DTR)
21   140 Remote loopback command
22 CE 125 Ring indicator (RI)
23 CH   External speed control (from DTE)
24 DA 113 External transmit clock (ETC)
25 CN 141 Analog loopback command


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