of the monopoly formerly held by AT&T in the United States. RETRAIN - A retrain is where the two modems suspend operations and renegotiate the best possible connection all over again. RPI - (R)ockwell (P)rotocol (I)nterface. RPI is a technology that allows error correction and data compression (ECC) technologies to be performed in the PC host computer, rather than in the modem hardware, as has been done traditionally. Technically speaking, RPI is an asynchronous HDLC (High Level Data Link Control) link that operates between a PC and a modem, allowing data compression and error correction to take place on the host PC. This allows RPI to operate using standard PC COMM ports, or standard serial cards. The V.42bis bit stream presented to the remote modem from an RPI modem looks exactly like that which would come out of a traditional hardware implementation of V.42bis, fully synchronous and full duplex. MNP levels 2,3,4,5 are supported as well as V.42 LAP-M, and V.42bis. To get ECC with RPI, as one would get using a hardware implementation of ECC, the PC comm application needs either a driver or a software package that supports RPI; the modem itself needs firmware that supports RPI. RTS - (R)equest (T)o (S)end - flow control parameter which indicates a device is ready to transmit data. SHANNON/HARTLEY THEOREM - further refines Nyquist Theory by taking into account the signal-to-noise ratio of the communications channel in addition to the bandwidth. It is expressed as follows: Maximum bits Available per second = bandwidth x LOGbase2 ( 1 + Signal-to-noise ratio ) SYMBOL RATE - The proper term for what most mistakenly refer to as baud rate. A "symbol" is a waveform transmitted by the modem, which contains a certain number of encoded bits of data to be moved across the link. In simple terms, the Symbol can be viewed as an analog representation of a digital number. Consider a stream of data bits: 1 0 0 1 1 1 0 1 0 0 Now take them two at a time : 10 01 11 01 00 As can be seen there are 4 possible situations, 00, 01, 10, and 11 which one could call A, B, C, & D respectively. The symbol for 00 would be A and for 01 it would be B, and for 10, C and for 11, D (these symbols could be anything agreed to). Thus if you detected a C at your receiver you would know that the transmitter was actually sending you 10 etc. etc. The actual symbols are not alpha characters, of course. They could be, for example, the signal constellation points in the non-trellis coded telephone line signal. Consider this simple constellation: A + ^ | B+-------+C | v + D The points at O, 90, 180 and 270 could be the four (4) distinct Symbols. They would correspond to the A, B, C, D in the example above. We would have just 4 symbol types to detect. The Symbol would change on every other data bit and thus the Symbol Rate would be half the Data Rate. For 4800 BPS the Symbol Rate would be 2400 Symbols per Second. Now consider 9600 BPS. If we wanted the Symbol Rate to still be 2400, as this sets the required bandwidth needed on the telephone channel to carry the modem signal, we must reduce the 9600 rate to 2400. 9600 ---- = 4 2400 This means we must process 4 bits at a time. Four (4) bits can be in 16 combinations as 0000, 0001, 0010 etc. Thus we must have 16 symbol types or points in the un-coded line signal constellation. For Example: + + + + + + + + + + + + + + + + This should be viewed as an expansion of the first constellation example above where the inner 4 points are "like" the 4 points in the above example but rotated 45 degrees. Each point (symbol) would now represent 4 bits as 0000, 0001, 0010, 0011, 0100 etc, etc. Thus, the Symbol Rate is one fourth the data rate. Now for 12000 BPS and a Symbol Rate of 2400, we see we must process 5 bits at a time, and for 14400 BPS and a Symbol Rate of 2400 be must process 6 bits at a time. This latter example would give us an un-coded constellation of 64 points. The noise levels in the channel determine how many bits are encoded in each symbol; lower noise levels allow a greater number of bits per symbol. The bandwidth of the channel limits how many of these symbols may be sent each second. SYNCHRONOUS - Describes a form of data transmission in which blocks of data are sent at strictly timed intervals. Because timing is uniform, no Start or Stop bits are required. TCM - (T)rellis (C)oded (M)odulation. TCM is an optional error correction method included in the V.32 standard. TCM allows modems to check for transmission errors with a redundancy bit, which results in fewer errors on noisy lines. Modems can tolerate twice the noise in the channel than otherwise, using only QAM. Trellis Encoding works with other error correction methods such as V.42. Unlike other methods, TCM is true error correction, in that for small errors, it does not force a retransmit, but actually fixes them. Instead of using a simple 16-point constellation, for example, you force the modem to a higher-density 32-point constellation. The 32-point --- MPost/2 v2.0a * Origin: Marsh BBS (c) Dawson Creek BC Canada (1:17/23) SEEN-BY: 3/0 633/267 640/954 712/0 313 550 620 848 @PATH: 17/23 140/1 261/38 712/848 633/267