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Layering allows the parts of a protocol to be designed and tested without a combinatorial explosion of cases, keeping each design relatively simple. Layering also permits familiar protocols to be adapted to unusual circumstances. For example, the mail protocol above can be adapted to send messages to aircraft. Just change the V.42 modem protocol to the INMARS LAPD data protocol used by the international marine radio satellites.
In telephony, failure rates of 1x10-4 bit per bit are faulty (they interfere with telephone conversations), while 1x10-5 bit per bit or more should be dealt with by routine maintenance (they can be heard).
Communication systems correct errors by selectively resending bad parts of a message. For example, in TCP (the internet's Transmission Control Protocol), messages are divided into packets, each of which has a checksum. When a checksum is bad, the packet is discarded. When a packet is lost, the receiver acknowledges all of the packets up to, but not including the failed packet. Eventually, the sender sees that too much time has elapsed without an acknowledgement, so it resends all of the packets that have not been acknowledged. At the same time, the sender backs off its rate of sending, in case the packet loss was caused by saturation of the path between sender and receiver. (Note: this is an over-simplification: see TCP for more detail)
In general, the performance of TCP is severely degraded in conditions of high packet loss (more than 0.1%), due to the need to resend packets repeatedly. For this reason, TCP/IP connections are typically either run on highly reliable fiber networks, or over a lower-level protocol with added error-detection and correction features (such as modem links with ARQ). These connections typically have uncorrected bit error rates of 1x10-9 to 1x10-12, ensuring high TCP/IP performance.
In packet switched networks, the equivalent functions are performed using router update messages to detect loss of connectivity.
wikipedia.org dumped 2003-03-17 with terodump