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A Layman's Explanation of High Speed Modems
Stan Simmons
Business Forms Estimating Systems, Inc.
April 4, 1991
High speed modems have 4800 bps or faster data transfer rates. Until
recently most high speed modems used proprietary modulation schemes. In
other words, one manufacturers' modem would not connect to another
manufacturers' modem. Standards exist now that allow the interconnection
of different manufacturers modems.
MODULATION STANDARDS
In the mid 1980's the International Consultative Committee on Telephone
and Telegraph (CCITT in French) established the V.32 standard. The V.32
standard describes how modems should "talk" to each other using two-way
signaling at 4,800 and 9,600 bps over dial-up telephone lines.
Unfortunately the V.32 standard did not provide a method for error
control. Since V.32 signaling is more sensitive to noise and echoes on
the telephone line than lower-speed protocols are, you need an error
control scheme to retain accuracy.
In early 1991, the CCITT issued the V.32bis standard. The V.32bis
standard adds 7,200, 12,000, and 14,400 bps transfer rates and a faster
renegotiation protocol to the V.32 standard.
It is important to understand that the V.32 standard primarily describes
the electrical signaling scheme used over the telephone wire. Other
standards, such as MNP, V.42, and V.42bis, describe actions taking place
above the level of electrical signaling. So you can have modems using
different combinations of signaling and error correction protocols.
ERROR CONTROL STANDARDS
Microcom developed its own standard for asynchronous data error control,
Microcom Network Protocol (MNP). MNP Class 4 is the most commonly used
version of this family of error control. The error checking operates
independently of the signaling scheme used by the modem.
In 1988, the CCITT issued a hardware-implemented asynchronous error
correction standard called V.42, which describes two error correction
schemes. The primary protocol is Link Access Procedure for Modems (LAPM).
The secondary protocol is functionally equivalent to MNP Class 4. The
LAPM method offers slightly better error recovery and reliability than
MNP Class 4.
While the V.42 and MNP Class 4 protocols help maintain reliability, they
do little to improve throughput. Both protocols convert asynchronous data
characters to a synchronous data stream, making it a bit oriented
protocol instead of character oriented. Most asynchronous characters
consist of one start bit, eight data bits, and one stop bit, for a total
of ten bits per character. V.42 removes the start and stop "framing"
bits, which results in a 20% increase in efficiency. However, in order
for the protocol to work, V.42 adds about 12% in overhead back into the
transmission. The resulting 8% cushion helps maintain full transfer speed
during periods of moderate error correction activity (usually caused by
noisy telephone lines.)
For all practical purposes, the result of the V.42 link is an error free
transmission. Using the 16 bit redundancy check, it will detect every
error that is 16 bits or smaller, with 100% probability. As a result, the
chances of an error occurring are so small that you can, in practice,
ignore them.
DATA COMPRESSION STANDARDS
The next step in increasing throughput involves data compression. Data
compression can be used to provide a modem with an effective data
throughput rate that is higher than the modem's bps transmission speed.
The amount of this increase in throughput will depend largely on the type
of data being transferred.
Microcom introduced the MNP Class 5 data compression protocol. Software
supporting the MNP Class 5 protocol offers the ability to compress files
to half their original size during transmission, thus providing a 100%
increase in speed. However, 80-85% increases in speed are more typical.
MNP Class 5 requires concurrent error correction using MNP Class 4.
In late 1989, the CCITT issued the V.42bis standard, describing how to
implement data compression in hardware. V.42bis uses the Lempel-Ziv
compression algorithm and offers a 35% greater data compression than MNP
Class 5. For 9600 bps modems this means a potential throughput of 38,400
bps. For most file transfers, however, a throughput of 19,200 bps on non-
compressed files can be expected.
The V.42bis standard adapts to the data flow more quickly than MNP Class
5, turning data compression on and off as required. This gives it an
advantage over MNP Class 5 when transmitting previously compressed files,
since the MNP Class 5 compression algorithm can cause compressed files to
expand, reducing throughput. V.42bis simply passes pre-compressed data
through without trying to compress it. V.42bis compression software only
works with hardware that uses the V.42 error correction protocol.
DATA TRANSFER
When using a file transfer protocol to send and receive data, the type of
protocol used will have a big effect on the speed gain due to
compression. In general, a protocol that uses long data blocks (the
longer the better) will transfer files quicker. To take full advantage of
MNP or V.42 error correction, you should select the software's no-error-
correction option.
To make use of the data compression, the modems need to be driven at full
capacity. In other words, the data needs to be present at enough volume
(file transfers and batch operations) and speed to get maximum
compression benefits. For a V.32bis connection with V.42bis compression
the serial ports should be set for 38,400 bps.
In order for data compression to take place, both the answer and
originate modems at each end of the telephone line must have compression
and error correction enabled. If one unit does not have data compression
enabled, only error correction takes place.
Overall, on-the-fly compression with V.42bis on a V.32bis connection is
the most desirable and economical mode of operation for most
applications.
COMPUSERVE
At the present time all of CompuServe's 9600 bps modems are US Robotics
Dual Standard modems. These modems support the V.32 modulation standard,
the V.42 error correction protocol, and V.42bis & MNP level 5 compression
protocols. The US Robotics Dual Standard is also upgradeable to V.32bis
modulation.
CompuServe sets the modems to V.32 mode, and leaves both the MNP level 5
and V.42bis data compression enabled. But, even when using compression,
nothing is gained during normal operation because the ports are locked at
9600 bps. If an error occurs during transmission the re-transmitted
frames will be compressed. The result is that the throughput will remain
close to the maximum port speed even with some phone line noise or other
interference.
CompuServe's Host-Micro Interface (HMI), used by the CompuServe
Information Manager (CIM) and other CompuServe software products, uses B+
protocol full-time as the transport layer, and results in a measure of
data compression due to "bit packing" of transmitted data into a smaller
number of bits, using a technique similar to V.42. As with V.42, it
primarily acts to maintain throughput at a high level by offsetting the
protocol overhead, rather than increasing throughput significantly beyond
that achieved at the same baud rate without compression or error
detection.
CIM and other HMI products enjoy continuous error detection and
correction as a function of the B+ protocol transport layer, and this
error correction, being integral to the HMI, cannot be disabled. As a
result, the use of other error correction protocols such as MNP-4 or V.42
"in series" with the software's own error correction may be, in many
cases, redundant and unnecessary, and can actually slow down data
transfer and/or interfere with flow control. For this reason, it is
sometimes suggested that hardware error correction not be activated when
using HMI products.
CompuServe does not, at this time, permit data transfer rates at the port
above 9600 baud. The reasons for this have mainly to do with the
"backbone" of CompuServe's network, which handles the overall data
traffic for many users simultaneously, and the need to manage the
expansion of local nodes and the backbone itself in tandem.
CompuServe's dial-up data network currently includes approximately 20,000
"1200 bps equivalents," each representing the load on the network
presented by a port operating at 1200 baud. Logically, a port operating
at 9600 baud represents 8 "1200bEs" in terms of the demands placed upon
the network. Currently, 9600 baud ports represent approximately 3% of the
total number of ports, but account for as much as 15% of network load.
During the current fiscal year, CompuServe plans to expand the number of
9600 baud ports in the network by a full 200 percent. Overall, the impact
on the CompuServe network "backbone" will be an increase in total data
traffic by as much as 50 percent. Such an increase requires effective
planning and more than a little control over how, when and where the
expansion is performed.
If CompuServe was to allow the use of data compression to increase the
effective data rate at the port, and hence demand on the network, by a
factor of as much as 4:1, the effective increase in network load as a
result of expansion of the 9600 baud ports could easily jump to 200
percent of the current load. Needless to say, that's not something that
can be done with a "flip of a switch." The network "backbone" must be
expanded in tandem with the addition of 9600 baud ports; there's much
more involved than simply hooking up a 9600 baud modem at the port end.
Hardware and software must be replaced, enhanced and reconfigured, and
new facilities brought on-line on the host end to deal effectively with
the increased amount of data coming into the computer centers.
While the primary benefits of data compression are not available to
CompuServe users now, they will be available in the not-distant future.
For now, CompuServe's primary concern is to make sure that the expansion
of the 9600 baud service does not negatively impact other users of the
network, while providing maximum benefit from the expansion and the
availability of basic 9600 baud service.
ELECTRONIC BULLETIN BOARD SYSTEMS
Electronic Bulletin Board System's (BBS's) have been around for a number
of years now, and their numbers are growing on an almost daily basis. All
major cities boast several BBS's, and many smaller cities have at least
one BBS. Most BBS's limit each user's time on-line, so it is in your best
interest to transfer as much data as possible while you are on-line.
High speed modems can allow as much as 16 times the data transfer per
unit of time over standard 2400 bps modems. US Robotics, and several
other modem manufacturers, often provide high speed modems to established
BBS system operators (SysOps) at or below cost. These promotions are
usually in the best interest of the manufacturer. The SysOps become
familiar with the product and recommend it to the users.
If you are connecting to a BBS over long distance a high speed modem the
savings on your phone bill can greatly offset the initial cost of high
speed data transfer. The best way to save on telephone line charges is to
use scripts or front end programs to automate your BBS activities. Many
BBS's use a similar format for message and upload/download areas. As
BBS's become more standardized perhaps better front end programs for the
various BBS's will be written.
DISCLAIMER
The information in this document is correct to the best of my knowledge.
I make no warranty as to accuracy of the information, nor do I accept any
responsibility for the use or misuse of it. This document may be freely
copied and distributed in any form, as long as it is presented unaltered,
in its entirety and not for profit. Copyright (c) 1991, Stan Simmons.