The older speeds of 300, 1200, and 2400 baud were well-defined. However, as modem manufacturers began to devise methods for gaining higher speeds, each modem manufacturer started to use a proprietary method or algorithm, incompatible with modems from other manufacturers.
When studying telecommunications standards, it is important to understand the differences between half duplex and full duplex transmissions. In a half duplex (HDX) transmission, a data packet is sent by one system and received by the other. Another data packet CANNOT be sent until the receiving system sends an acknowledgement back to the sender.
In a full duplex (FDX) transmission, both the sending and receiving systems communicate with each other simultaneously; in other words, both modems can send and receive data at the same time. This means a modem can be receiving a data packet while acknowledging the receipt of another.
Today, the United Nations Consultative Committee for International Telephony and Telegraphy (CCITT) defines standards for most high-speed communications.
Even modern high-speed modems are much slower than other methods of computer communication. A high-speed modem typically operates at 14,400 baud, while an Ethernet LAN connection operates at 10,000,000 bps. In order to boost data throughput, high-speed modems typically offer one or more data compression algorithms. These algorithms can boost the throughput of a high-speed modem to speeds above 54,000 bps.
Note that these algorithms are sensitive to the data being transmitted. If the data has already been compressed (for example, with the compress command), the data compression methods of high-speed modems will offer little or no benefit, and might even reduce data throughput.
When using a modem with data compression technology, the speed of the DTE/DCE connection between the computer and the modem should be greater than the nominal data rate of the connection between modems. For example, with a V.32bis modem with V.42bis data compression, the data rate of the modem (the speed at which the modem communicates across telephone lines) is 14,400 baud. When the V.42bis compression is active, actual data throughput can reach 57,600 bps. To accommodate the greater throughput offered by data compression, the speed of the DTE/DCE between the computer and the modem should be set to 57,600 bps (38.4Kbps).
Some modems implementing data compressions and modem modulation schemes may yield a higher data throughput than some systems and asynchronous adapters can accommodate.
Today, the CCITT defines standards for high-speed communications including data compression algorithms. CCITT standards are usually named V.nn, where nn is a number.
Following is a list of some common communications standards defined by the CCITT:
Another de-facto standard is the Microcom Networking Protocol (MNP) which was originally developed by Microcom, Inc. Available in versions (called classes) 1-9, MNP is a high-performance, high-speed protocol that was available before the advent of the CCITT standards. With MNP, errors in the transmitted data packets are detected by the remote modem causing it to request a retransmission of the data packet in error. The capability to recognize and quickly correct data errors makes MNP one of today's most common protocols.