X.75
X.75 is an ISDN transmission standard from ITU-T similar to X.25 . It is a network protocol for the transmission of data packets between the user's ISDN terminal and the exchange . On the B channel are HDLC frames ( frames ) sent, the numbers confirmed and possibly to multiple processes routed must be. In contrast to the older ARQ protocols , X.75 is significantly more efficient because it belongs to the group of Go-Back-N protocols. With X.75, data transfer rates of up to 64,000 bit / s are possible, with channel bundling (simultaneous use of two B channels) twice as much. X.75 is the preferred protocol in Europe that BBS or FidoNet users use. In now rare cases it is also the transmission standard with which ISDN subscribers surf the Internet without DSL . Since around 1997, however, " PPP over ISDN " according to RFC 1618 has been established for this purpose . In North America, V.120 is more often used for BBS access instead of X.75 , as X.75 is set to the data rate 64,000 bit / s, while in North America the ISDN networks historically ignored the MSB and therefore only data rates up to 56,000 bit / s reach. In contrast, mobile ( GSM ) devices in particular also use the V.110 protocol on both continents . X.75 is still used in proprietary environments, e.g. B. used for sending data from MDE devices. However, with the increasing spread of Embedded Linux or Windows CE , X.75 is being displaced and replaced by the TCP / IP capable PPP. X.75 does not specify from the start which window size and number of frames must be used. In BBS environments, 7 frames of 2048 bytes each are usually used. This means that - together with the ZedZap , Z-Modem , Janus , Hydra or BiModem protocols - data rates can be achieved at which the net - is almost the same as the gross throughput .
10/8 arithmetic trick?
In connection with the data transmission speed of X.75 terminal adapters , one often comes across information such as 8000 bytes / s. In the case of asynchronous transmissions with the data frame 8n1, the data rate is usually calculated because each byte is framed by a start and a stop bit. At 64,000 bit / s, according to this calculation, the transfer rate should only be 6400 byte / s. However, since the terminal adapter converts the asynchronous protocol of the EIA-232 into the synchronous X.75, the start and stop bits are omitted and are replaced by opening flags 01111110 High-Level Data Link Control # block structure at the beginning and end of the 2k frames. If the terminal adapter is controlled with more than 64,000 bit / s (115,200 bit / s is usual), it is possible to achieve a transfer rate of almost 8000 byte / s. In practice, however, this speed depends on the data content, because certain bit combinations are bypassed by the X.75 protocol by bit stuffing.
