The specific techniques used to encode the digital bits into analog signals are called modulation protocols. The various modulation protocols define the exact methods of encoding and the data transfer speed. In fact, you cannot have a modem without modulation protocols. A modem typically supports more than one modulation protocols.
The raw speed (the speed without data compression) of a modem is determined by the modulation protocols. High-speed modems are modems that feature modulation protocols at 9600 bps or higher. A 2400-bps modem with data compression that can theoretically yield a 9600-bps throughput is not a high-speed modem.
"CCITT" is a French acronym for the International Telegraph and Telephone Consultative Committee. CCITT, a United Nations agency, is an international telecommunications standards committee that makes recommendations on a broad range of subjects concerning data communications.
Bell 103 (300 bps U.S. Standard) Bell 212A (1200 bps U.S. Standard) CCITT V.22 (1200 bps standard outside the U.S.) CCITT V.22bis (2400 bps International Standard)Some 2400-bps modems also support the following protocols:
CCITT V.21 (300 bps standard outside the U.S.) CCITT V.23 (1200/75 and 75/1200 bps, used in Europe)In the past, most 2400-bps modems did not support any error correction or data compression protocols. Recently, however, many modem manufacturers have introduced 2400-bps modems with extra features like data compression, error correction and fax capability.
Unlike 2400-bps modems where a single modulation protocol (V.22bis) is supported by all modem makers, there are several proprietary modulation protocols used by modems from different manufacturers. Two notable examples are the U.S.Robotics HST and the Telebit PEP.
In the past three months, U.S. Robotics, Telebit and Motorola Codex have introduced new modems using proprietary protocols that yield better throughput than V.32bis modems.
The original Courier HST modem ran at 9600 bps. U.S. Robotics later (in 1989) improved the speed of the Courier HST to 14400 bps. In March 1992, U.S. Robotics further enhanced HST to 16800 bps (which yields an effective throughput around 2000 cps with compressed files). Don't be surprised to see a U.S. Robotics modem with a 19200-bps HST mode in the near future.
Although U.S. Robotics remains committed to the HST modems, there are now three different high-speed Courier modems available: the Courier HST (which only supports the HST protocol), the Courier V.32bis (which only supports V.32bis) and the Courier HST Dual Standard (which supports both the HST and the V.32bis protocols).
The TrailBlazer Plus owes its success partly to its builtin support for the UUCP g-protocol, thus allowing efficient and flawless UUCP session. PEP also performs well even with noisy telephone lines. The actual throughput is around 14400 bps. The TrailBlazer Plus has an installed base of more than 120,000 units.
Telebit also introduced a cheaper (and slower) PEP modem, the T1000, in 1988.
Like U.S. Robotics, Telebit recently announced Turbo PEP which has an effective data tranfer throughput of 2300 cps (with compressed files). The Telebit WorldBlazer is a dual-mode modem which supports both V.32bis and Turbo PEP.
Two modems can establish a connection only when they share a common modulation protocol. To connect at high speed, two modems have to support the same high- speed modulation protocol. Therefore, a modem with a proprietary modulation protocol can only establish a high-speed connection with another modem from the same manufacturer. A U.S. Robotics HST modem can only establish a high- speed connection (at 9600 or 14400 bps) with another HST or an USR Dual Standard modem. A Courier HST modem cannot establish a high-speed connection with a Courier V.32bis modem. They can only connect at 2400 bps. (All high- speed modems in the market support the CCITT V.22bis modulation protocol).
On the other hand, two V.32 modems can talk to each other at 9600 bps. They do not have to be from the same manufacturer. Two V.32bis modems can talk to each other at 14400 bps. A V.32 modem can talk to a V.32bis modem at 9600 bps.
An ISDN line carries three digital channels: two "B" channels that carry various kinds of data at 64,000 bps and a "D" channel at 16,000 bps that can carry control signals or serve as a third data channel. A single ISDN channel can transfer uncompressed data bidirectionally at 64,000 bps. Combine that with a data compression scheme and you will be able to transfer data at hundreds of kilobits per second.
Eventually, ISDN will provide widely available, low-cost digital communications for voice and data communication. Until ISDN is firmly in place, high-speed modems will be with us for a while.