Modem Tutorial - Modulation Protocols

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Modulation Protocols

Modem stands for MOdulator/DEModulator. A modem converts digital signals generated by the computer into analog signals which can be transmitted over a telephone line and transforms incoming analog signals into their digital equivalents.

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 protocol.

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.

2400 bps Modems

A 2400 bps Hayes-compatible modem typically supports the following modulation protocols:

• Bell 103 (300 bps U.S. Standard)
• Bell 212A (1200 bps U.S. Standard)
• CCITT V.22 (1200 bps standard used outside the U.S.)
• CCITT V.22bis (2400 bps International Standard)

• CCITT V.21 (300 bps standard used outside the U.S.)
• CCITT V.23 (1200/75 and 75/1200 bps, used in Europe)

High-speed Modems

There are two standard modulation protocols for high-speed modems: V.32 and V.32bis. Both are standards established by the CCITT.

V.32

This is the standard for 9600 (and 4800) bps modems. CCITT V.32 is adopted by the CCITT in 1984. But the market has not taken off until recently. V.32 modems used to cost more than modems using proprietary modulation protocols (Hayes introduced the Smartmodem 9600, a V.32 modem, in 1988 with a $1999 price tag). But it is no longer true. At present, street prices for most V.32 modem are below $500. Every modem manufacturer is making V.32 modems now. Packet-switching networks like Sprintnet (Telenet) and CompuServe are also starting to support V.32 modems. Companies that make modems with proprietary modulation protocols are making modems with "dual standard." U.S. Robotics, Telebit, Hayes and CompuCom all have modems that support V.32 and their own proprietary protocols.

V.32bis

V.32bis, established in early 1991, is the CCITT standard for 14400 bps modems. A V.32bis modem also can fall back to 12000, 9600, 7200 and 4800 bps. V.32bis is downwardly compatible with V.32.

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.

U.S. Robotics HST (High Speed Technology)

Until the recent surge of V.32 modems, the U.S. Robotics HST was the de facto standard in the PC-based BBS community. U.S. Robotics introduced the Courier HST modem in 1986 and pioneered the market for high-speed modems in the IBM PC environment. The immense popularity of the HST modems was partly due to the generous discount program U.S. Robotics offered to the BBS Sysops (SYStem OPerators). Many modem manufacturers have implemented similar Sysop discount programs, but most BBS sysops remain loyal to the U.S. Robotics modems.

The original Courier HST modem ran at 9600 bps. U.S. Robotics later improved the speed of the Courier HST to 14400 bps.

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).

Telebit PEP (Packetized Ensemble Protocol)

Telebit introduced the TrailBlazer in 1985 that employed a proprietary modulation protocol called PEP. While the Courier HST is popular among BBS, Telebit modems dominate the UNIX UUCP and Usenet communities. (Usenet, UUCP and the Internet are discussed in Part II of "The Joy of Telecomputing").

The TrailBlazer Plus owes its success partly to its built-in 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.

Hayes Express 96

Hayes entered the high-speed modem arena in 1987 with the introduction of the V-series Smartmodem 9600. The modem used a proprietary modulation protocol called Express 96 (also known as Hayes "Ping Pong" protocol). The V-series modems have not been as successful as the U.S. Robotics or the Telebit modems.

CompuCom CSP (CompuCom Speed Protocol)

While every modem manufacturer is jumping on the V.32 bandwagon, CompuCom bucked the trend and came out with the SpeedModem Champ in early 1991. It's a 9600 bps modem with a proprietary modulation protocol called CSP. The SpeedModem Champ has one strong selling point. It is the only modem with a proprietary protocol that costs less than a generic V.32 modem. The internal SpeedModem Champ is priced at $169. An external version is available for $199. Hundreds of PC-based bulletin board systems have installed the SpeedModem Champ. The Champ also works as a Hayes-compatible 2400 bps modem with MNP 2-4 error control and MNP-5 data compression.

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.

V.fast

CCITT is working on a new modem standard, dubbed V.fast. If all goes well, the next modem standard can materialize before 1993. A V.fast modem is expected to reach a raw speed of 19,200-24,000 bps over standard dial-up telephone lines.

ISDN

In a couple of years we may not need modems at all. Integrated Services Digital Network (ISDN) has been coming for years. When will ISDN really become available for the rest of us? It depends on your local telephone company. It is estimated that by the end of 1994 about half the telephone connections in the U.S. will has access to it. With ISDN, you won't need a modem since no modulation or demodulation will be necessary. You will need an ISDN adapter instead.

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.

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