ProfitPress Mega CDROM2 …eeware (MSDOS)(1992)(Eng)/ProfitPress-MegaCDROM2.B6I/MAGAZINE/MISC/MNMAR19.ZIP/BUZZP1

Text File | 1991-02-18 | 9.5 KB | 240 lines

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[The world of computers is riddled with buzz words -- technical jargon for the
various parts of computers, their functions, and applications. In telecommu-
nications it's the same thing. Terms like Baud, Bits, Parity, MNP, Half Duplex,
and Full Duplex can make a TBBS system designer's life seem more complex than
it really is. The problem is, these buzz words are attached to many of the
components and concepts that a TBBS system designer must grasp to make the most
of online system implementation and even to explain a system's operation to its
users.









Fortunately, most telecommunications terminology isn't hard to understand --
once it's been explained by someone who knows what the terms mean and can speak
English clearly enough to break them down in understandable language. Alan
Applegate is just such a person and we at eSoft are lucky enough to have him on
our technical support staff.

In the following special three-part series, Alan will tackle many of the common
telecommunications buzz words you'll encounter as a TBBS system designer and
bring them a lot closer to home with straightforward, plain-English definitions
and step-by-step explanations... Ed.]














Modem Standards --------------- No doubt you've wondered at one time or another
about modem standards. There are currently several active standards, and they
involve more than just the modem's actual operating speed.  Without these
standards, modems from one manufacturer most likely couldn't "talk" to modems
made by another manufacturer.  Consequently, at least a basic understanding of
modem standards is also necessary if you want to make the right choices when
selecting modems for use on your TBBS system.

Generally speaking, 300, 1200 and 2400 bps modems each use a different standard
that is adhered to by all modems and modem makers. (It should be noted that
standards for 300 and 1200 bps are different in the United States than they are
in Europe.)

Standards for 9600 bps transmission have been established for some time, but
the technology to implement those standards was, until recently, expensive. To
get around the high cost of using the existing standard, modem manufacturers
have created several of their own proprietary high-speed modem standards. This
is why so many high-speed modems will only "talk" to another high-speed modem
of the same brand.







Data transmission speeds, however, are not the only type of modem standard.
Actually, modem standards are grouped into four distinct areas or "layers."
These are shown in the illustration below:

Modulation ---------- Modulation is the starting (or bottom) layer for all
modems  ("modem" means MOdulator - DEModulator). Each layer builds upon the
next.

Modulation refers to the signaling method that is used by the modem. Two modems
must use the same modulation method in order to understand each other. Each
data rate uses a different modulation method, and sometimes there is more than
one method for a particular rate. An example of this is the Bell 212A and V.22
modulation standards (described below); they both specify 1200 bps modulation,
but they work differently, and are not directly compatible.











Negotiation ----------- Negotiation refers to the manner in which two modems
establish which modulation method will be used during a connection. Modems
"listen" to the tones sent by a remote modem to determine what modulation
method will be used. Since different modulation methods often use different
answer tones, these can be used by the calling modem to determine which method
to use. Negotiation standards have been created to make the process easier.
These standards dictate the sequence of events that will occur when a modem
answers the phone, eliminating the guesswork associated with the "listen to the
tones" method. Negotiation is part of many modem standards.

Error Correction ---------------- Error correction refers to an ability that
some modems have to identify errors during a transmission, and to automatically
re-send data that appears to have been damaged in transit. If error correction
is used, both modems must adhere to the same error correction standard to make
it work. Fortunately, there are error correction standards which are followed
by most modem manufacturers.






Data Compression ---------------- Data compression refers to a built-in ability
in some modems to compress the data they're sending, automatically "squeezing"
data to a smaller size as it is sent. This, of course, saves time and can
result in considerable money saved by long-distance modem users. Depending on
the type of files that are sent, data can be compressed by as much as 50% of
its original size, effectively doubling the speed of the modem.

In this scenario, a 2400 bps modem with data compression is capable of sending
some files as quickly as a 4800 bps modem WITHOUT data compression. Not all
types of data can be compressed by 50%, but gains can nearly always be real-
ized.

We'll take a look at each of the various data compression standards later in
this series, but first let's examine those modem standards that are associated
directly with the transmission speed of the modem.







Standards for 300 and 1200 Bps ------------------------------ Most 300 bps
modems follow the standard created initially by AT&T, called Bell 103, and are
common in the United States. Most modems manufactured for use outside the
United States support the CCITT V.21 standard instead, and are not compatible
with Bell 103 modems. Some modems can be set to follow either standard.

AT&T also created the Bell 212A standard for 1200 bps modems.  It's become the
common standard in the United States. Most modems manufactured for use outside
the United States support the CCITT V.22 standard instead, and are not compati-
ble with the Bell 212A modems. Some modems can be set to follow either stan-
dard.  Most modems manufactured since 1985 are capable of differentiating
between the two standards, and can effectively handle either one.

2400 Bps Standards ------------------ The international standard for 2400 bps
communications is CCITT V.22bis. This is used by modems manufactured for use
both inside and outside the United States. Most 2400 bps modems include
automatic detection of the data rate fall back, if a data rate lower than 2400
bps is detected at the other end of the connection.






9600 Bps Modems -- Are There Standards? ---------------------------------------
Contrary to what might be believed, standards for high speed data transmission
have been in place for some time. Acknowledged standards came in two forms -- a
half duplex standard, commonly used in fax machines and called V.29, and a full
duplex standard called V.32 (we'll take a look at half and full duplex later in
the series). The technology required to implement the V.32 standard remained
prohibitively expensive for many years. This forced most modem manufacturers to
create their own less- expensive proprietary transmission methods.

U.S. Robotics, for example, created the Courier HST, ("High Speed Technology").
This design is not full duplex, meaning that it does not support high speed
transmission in BOTH directions.  Current HST modems send data at 14,400 bps in
one direction, and 450 bps in the other direction. The high speed channel
changes direction depending on which side of the transmission has the most data
to send. HST modems can only talk at high speed with other HST modems, although
they also adhere to existing standards for 300, 1200 and 2400 bps operation.

Telebit, another modem manufacturer, created PEP ("Packetized Ensemble Proto-
col"), which is used in their Trailblazer modem series. Like the HST, PEP
modems will only connect at high speed with other PEP modems.




Hayes also developed their own technology for high speed transmission, in the
absence of an inexpensive standard. Like the others, Hayes high speed modems
only talk high speed to other Hayes modems.

Fortunately, the cost of V.32 high speed transmission technology has come down
drastically in recent years, and is displacing other high speed proprietary
protocols in popularity. This means that, finally, high speed modems are
starting to communicate with a common standard. U.S. Robotics' new Courier HST
Dual Standard is one example of a new high speed modem utilizing both U.S.
Robotics' own HST transmission standard and the V.32 high speed standard. The
new Hayes V-series Ultra Smartmodem 9600 is another "multiple-standard" high
speed modem that utilizes the V.32 standard.

Next month we'll discuss the CCITT and the international telecommunications
standards that are set by this prestigious committee. We'll even de-mystify the
whole family of MNP standards. Also we'll examine the data compression stan-
dards.  What works, what doesn't and what is realistic to expect from data
compression in a modem? MNP vs. V.42bis -- don't miss it!

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