Modems Revealed - Again
By Grant Kwai
Copyright (c) 1992 Apple Users' Group, Sydney
Republished from Applecations, a publication of the Apple Users' Group, Sydney, Australia.
Here are a few extensions to last month's article.
Bits Per Second and Baud
These two terms are generally interchanged in 'normal' conversation. When we talk about the speed of a modem we normally use the term BAUD, as in: "I have a 2400 baud modem". Technically this is incorrect. Bits Per Second (BPS) and Baud are not the same. [Guilty as charged from last month's article - Sub-Ed.]
BPS is the total number of data bits per second transmitted (e.g. 2400). The baud rate, on the other hand, refers more to how the transmission occurs in that it is the actual rate of signal transmissions per second. For example: The modulating technique employed by most 2400 bps modems is to transmit four data bits per baud, at 600 baud (2400 bps = 4 bits x 600 baud).
Per-plex-ing
Here is a reminder about the meaning of the terms simplex, half-duplex and full-duplex.
Simplex - One way only communication. Very good examples of Simplex communication are the ordinary television or radio. They both just receive transmissions.
Half-Duplex - Two way communication but only one way at a time. Conversation between two people is an example. One person listens while the other speaks, and then they swap roles.
Full-Duplex - Two way communication at the same time. For an example of full-duplex communication, imagine a conversation where two people listen and speak at the same time.
More Speed Protocols
Bell 103 - A 300 bps protocol using full duplex. This is mainly used in the U.S.A. The rest of the world abides by the V21 CCITT standard.
Bell 212A - This is the Bell 1200 bps protocol. It operates using full duplex, and again is mainly used only in the U.S.A.
HST - Stands for High Speed Technology. This is a propriety method of U.S. Robotics (USR) an American company. It operates slightly differently to most other protocols and unlike V32, is not full duplex. It transmits data at a theoretical rate of 14,400 bps in one direction and either 300 bps or 450 bps in the other. This can alternate, depending on which end has the larger data packet to send. 14,400 bps is achieved by using TCQAM to encode at 7 bits per baud at 2400 baud. One bit is used as parity.
As you can see, it doesn't offer high speed transmissions in both directions. A HST modem can only connect to another HST for high speed transfers. They do, however, follow the standards for 300 bps, 1200 bps and 2400 bps. USR now make modems which offer both HST and V32/V32bis.
PEP - An acronym for Packetized Ensemble Protocol. A propriety method of Telebit. PEP uses a method called Dynamic Adaptive Quadrature Amplitude modulation, DAQAM for short. PEP is used in the Trailblazer series of modems (from Netcomm). A PEP modem will only communicate with another PEP modem at high speeds. PEP uses an adaptive duplex.
That is, if data is coming only from one end, it will act as if it is a half duplex modem, offering maximum send rate. When data must be transferred from both ends, PEP will act to insure maximum data transmission rates for both ends at a reduced speed, hence behaving more like full duplex (For example, full duplex may be half the maximum speed). Communication speeds for PEP can be as high as 20,600 bps.
Some less commonly known protocols have also been used over the years. Some of these are:
V17 - This was a half duplex implementation of V33, a 14,000 bps protocol. It was mainly used in Group III faximilies (fax machines)
V27ter - An asynchronous 4800/2400 bps protocol. It was also used in Group III faximilies.
V29 - A 9600 bps standard which operates using half duplex. Some older modems (made in the U.S.A. generally and before V32 became standard) offer this protocol. It is however, normally seen in Group III faximilies operating at 9600/7200 bps.
V33 - This is a version of V32 without echo cancellation. It is synchronous and operates at 14,400 bps and uses QAM and TCM.
V35 - A now obsolete 19,200 bps high speed standard originally used on leased lines.
More MNP Protocols
Microcom Inc have also developed Microcom Network Protocols (MNP) other than those explained last month.
MNP level 1 is rarely used today. It uses asynchronous, half duplex transmission. It, however, only gave approximated 70% efficiency.
MNP level 7 provided increased data compression than MNP level 5 when used in conjunction with MNP 4. A 3:1 compression ratio is quoted. However, since this is still second-rate compared to V42bis, it is not widely used.
MNP level 9 was later introduced. This offered 4:1 compression ratio, however by this time V42bis had a predominant market following. Hence, MNP level 9 has never really taken off.
MNP level 10 is the latest protocol developed by Microcom. It is a fallback protocol which is more intelligent than its predecessors. It has the ability to slow down transmissions during bad line conditions. However, it also has the ability to increase speed again if the line quality should improve.

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