All modems in a network must be using the same error-correction protocols for it to work. Fortunately, most modems use the same protocols. Most use the V.42 error correction protocol. With it, the modem can detect damaged data streams, and the data will be resent. The V.42bis protocol is just like V.42, but incorporates data compression.
Data compression allows modems to use higher bps's.
A 14.4 modem, with data compression, can boast transmission rates of 57,600 bps. A 28.8 modem will boast transmissions of 115,200 bps. It takes advantage of a digital phone network, allowing transmission rates of up to 128 Kbps. ISDN requires an ISP that is capable of it, plus an initial setup fee and a monthly charge. You must also buy a special ISDN modem.