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Micro House PC Hardware Library Volume II: Network Interface Cards And Modems Micro House PC Hardware Library Volume II: Network Interface Cards And Modems
by Micro House International, Inc. and Scott Mueller
Que, Macmillan Computer Publishing
ISBN: 078971664x   Pub Date: 06/17/98
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This section examines these and other proprietary modem protocols.

HST
The HST is a 14,400 bps and 9,600 bps modified half-duplex proprietary modulation protocol used by U.S. Robotics. Although common in BBSes, the HST is now all but extinct, due to V.32 modems having become more competitive in price. HST modems run at 9,600 bps or 14,400 bps in one direction, and 300 or 450 bps in the other direction. This is an ideal protocol for interactive sessions. Because echo-cancellation circuitry is not required, costs are lower.
U.S. Robotics also marketed modems that used the standard protocols as well as their proprietary standard. These dual standard modems incorporated both V.32bis and HST protocols, giving you the best of the standard and proprietary worlds and enabling you to connect to virtually any other system at the maximum communications rate. They were at one time among the best modems available; I used and recommended them for many years.
DIS
The DIS is a 9,600 bps proprietary modulation protocol by CompuCom, which uses dynamic impedance stabilization (DIS), with claimed superiority in noise rejection over V.32. Implementation appears to be very inexpensive, but like HST, only one company makes modems with the DIS standard. Because of the lower costs of V.32 and V.32bis, this proprietary standard will likely disappear.
MNP
MNP offers end-to-end error correction, meaning that the modems are capable of detecting transmission errors and requesting retransmission of corrupted data. Some levels of MNP also provide data compression.
As MNP evolved, different classes of the standard were defined, describing the extent to which a given MNP implementation supports the protocol. Most current implementations support Classes 1 through 5. Higher classes usually are unique to modems manufactured by Microcom, Inc. because they are proprietary.
MNP generally is used for its error-correction capabilities, but MNP Classes 4 and 5 also provide performance increases, with Class 5 offering real-time data compression. The lower classes of MNP usually are not important to you as a modem user, but they are included in the following list for the sake of completeness:
  MNP Class 1 (block mode) uses asynchronous, byte-oriented, half-duplex (one-way) transmission. This method provides about 70 percent efficiency and error correction only, so it’s rarely used today.
  MNP Class 2 (stream mode) uses asynchronous, byte-oriented, full-duplex (two-way) transmission. This class also provides error correction only. Because of protocol overhead (the time it takes to establish the protocol and operate it), throughput at Class 2 is only about 84 percent of that for a connection without MNP, delivering about 202 cps (characters per second) at 2,400 bps (240 cps is the theoretical maximum). Class 2 is used rarely today.
  MNP Class 3 incorporates Class 2 and is more efficient. It uses a synchronous, bit-oriented, full-duplex method. The improved procedure yields throughput about 108 percent of that of a modem without MNP, delivering about 254 cps at 2,400 bps.
  MNP Class 4 is a performance-enhancement class that uses Adaptive Packet Assembly and Optimized Data Phase techniques. Class 4 improves throughput and performance by about 5 percent, although actual increases depend on the type of call and connection, and can be as high as 25 to 50 percent.
  MNP Class 5 is a data-compression protocol that uses a real-time adaptive algorithm. It can increase throughput up to 50 percent, but the actual performance of Class 5 depends on the type of data being sent. Raw text files allow the highest in case, although program files cannot be compressed as much and the increase is smaller. On precompressed data (files already compressed with ARC, PKZIP, and so on), MNP 5 decreases performance, and therefore is often disabled on BBS systems.
V-Series
The Hayes V-series is a proprietary error-correction protocol by Hayes that was used in some of its modems. Since the advent of lower-cost V.32 and V.32bis modems (even from Hayes), the V-series has all but become extinct. These modems used a modified V.29 protocol, which is sometimes called a ping-pong protocol because it has one high-speed channel and one low-speed channel that alternate back and forth.
CSP
The CSP (CompuCom Speed Protocol) is an error-correction and data-compression protocol available on CompuCom DIS modems.

FAXModem Standards

Facsimile technology is a science unto itself, although it has many similarities to data communications. These similarities have led to the combination of data and faxes into the same modem. You now can purchase a single board that will send and receive both data and faxes. All of the major modem manufacturers have models that support this capability.


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