modems-faq

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U.S. Robotics modem with a 19200-bps HST mode in the near future.

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).

II.2.d. Telebit PEP and Turbo PEP

Telebit introduced the TrailBlazer in 1985 that employed a proprietary 
modulation protocol called PEP (Packetized Ensemble Protocol). 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).

The TrailBlazer Plus owes its success partly to its builtin 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.

Like U.S. Robotics, Telebit recently announced Turbo PEP which has an 
effective data tranfer throughput of 2300 cps (with compressed files). The 
Telebit WorldBlazer is a dual-mode modem which supports both V.32bis and Turbo 
PEP.

II.2.e. 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.

II.2.f. CompuCom CSP

While every modem manufacturer was 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 (CompuCom 
Speed Protocol). The SpeedModem Champ was the only modem with a proprietary 
protocol that costs less than a generic V.32 modem. (The internal SpeedModem 
Champ was priced at $169. An external version was $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 MNP2-MNP4 error control and 
MNP5 data compression. (Unfortunately, CompuCom went out of business 
recently.)

II.2.g. Other proprietary modulation protocols

There are modems using other proprietary modulation protocols. For example, 
Motorola Codex (a subsidiary of Motorola Inc.) just announced the 326XFast 
Series of modems. Although you may see those modems referred to as "V.fast", 
they use a proprietary protocol. (It could take another 18 months for CCITT to 
finalize the V.fast standard.) Motorola Codex is promising a free ROM upgrade 
when the V.fast standard is established.

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.

II.3.   Things to come

A V.32bis modem may seem fast comparing to a standard 2400 bps modem. But  
there are applications that will require even faster modems to be really 
usable.

II.3.a. V.fast

CCITT is working on a new modem standard, dubbed V.fast. If all goes well, the 
next modem standard can materialize before the end of 1993, 1994, 1994. A V.fast modem is 
expected to reach a raw speed of 19,200-24,000 bps over standard dial-up 
telephone lines. (By the way, V.fast will probably be known as "V.34" when 
it's finalized.)

II.3.b. 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.


III.    Error Control Protocols

Besides high-speed modulation protocols, all current models of high-speed 
modems also support error control and data compression protocols.

III.1.  V.42 and MNP4

There are two standards for error control (error-correcting, error correction) 
protocols: MNP 4 and V.42. The Microcom Networking Protocol, MNP, is developed 
by Microcom. MNP2 to MNP4 are error correction protocols. MNP5 is a data 
compression protocol. V.42 is established by CCITT. V.42 actually incorporates 
two error control schemes. V.42 uses LAP-M (Link Access Procedure for Modems) 
as the primary scheme and includes MNP4 as the alternate scheme. Therefore, a 
V.42 modem will be able to establish an error-controlled connection with a 
modem that only supports MNP 4.

A modem that uses a proprietary modulation protocol may also use a non-
standard error control protocol. For example, Hayes V-series Smartmodem 9600 
supports an error control protocol called LAP-B. CompuCom's SpeedModem Champ 
also uses a non-standard error control protocol.

III.2.  V.42 & MNP4 can provide errorfree connections

Modems without error control protocols, such as most 2400-bps Hayes-compatible 
modems, cannot provide error-free  data communications. The noise and other 
phone line anomalies are beyond the capabilities of any standard modem to 
deliver error- free data.

V.42 (and MNP 2-4) copes with the phone line impairments by filtering out the 
line noise and automatically retransmitting corrupted data. If you have used a 
standard Hayes-compatible modem, you probably notice some garbled characters 
(like "@8d_\nw`[ce") show up on your screen from time to time. When two modems 
establish an error-controlled connection, they are said to have a reliable 
link and are capable of filtering out those garbled characters caused by the 
line noise. Notice that the line noise is still there, it just does not show 
up on your screen or the screen on the remote system.

The filtering process used by V.42 (and MNP 2-4) is similar to the error 
correction scheme used by file transfer protocols (such as Xmodem). The two 
modems use a sophisticated algorithm to make sure that the data received match 
with the data sent. If there is a discrepancy, the data is resent.

What is the difference between error control protocols (such as V.42) and file 
transfer protocols (such as Xmodem)?

For one thing, file transfer protocols provide error detection and correction 
only during file transfers. File transfer protocols do not provide any error 
control when you are reading e-mail messages or chatting with other people 
online. In other words, an error control protocol is "on" all the time during 
your online session and file transfer protocols are "on" only some of the 
times, namely when you are sending or receiving files.

Even though an error control protocol is "on" all the time, we still need file 
transfer protocols when two modems establish a reliable link. A modem works 
with bit streams, timing and tones. It does not understand what a file is. 
When you download or upload a file, your communications software needs to take 
care of the details related to the file: the filename, file size, etc. This is 
handled by the file transfer protocol which does more than error-checking.

Some file transfer protocols, most notably Ymodem-g and Imodem, are developed 
to handle file transfer without performing any error-checking. The idea of 
using a protocol like Ymodem-g is to eliminate the redundancy - thus improve 
the transfer speed. Ymodem-g and Imodem should only be used with modems that 
provide built-in error control protocols. These file transfer protocols do not 
provide any error-detection or recovery capability. If a problem occurs during 
the file transfer, the transfer session will be aborted.

Protocols like Ymodem-g or Imodem depend on the modems to provide assurance 
for the integrity of data being transferred. However, you should know that a 
reliable link between two modems does not provide absolute guarantee for the 
data integrity during file transfer. When you call a remote computer, there 
are really three links involved in the process. Besides the link between the 
two modems, there are still one link between your computer and your modem and 
another link between the remote modem and the remote computer. When two modems 
make a reliable connection using V.42 or MNP 4, only the data integrity 
between the two modems is  ensured. It is still possible for errors to occur 
at either end between the serial port and the modem (in the cable) or in the 
computer itself. (Fortunately, such errors are rare.)

For extra protection, you may still want to use a file transfer protocol  such 
as Zmodem which also performs error checking even if you have a reliable link 
with the remote system. There is a common misconception that Ymodem-g is much 
faster than other file transfer protocols. Although Ymodem-g is significantly 
faster than Ymodem, it offers little over Zmodem. Zmodem has proven to be 
extremely efficient. (See benchmark below)

        Filename        Ymodem          Ymodem-g        Zmodem
        --------------------------------------------------------
        the-wave.txt    1527 cps        3261 cps        3296 cps
        dayrpt.arc       761 cps        1042 cps        1025 cps
        dayrpt.wks      1244 cps        2314 cps        2337 cps
        sunset.arc       745 cps         987 cps         965 cps
        sunset.pic      1297 cps        2594 cps        2588 cps
        text109k.arc     814 cps        1089 cps        1064 cps
        text109k.txt    1351 cps        2812 cps        2885 cps

Note:   The seven test files used throughout this article are available on the 
Hayes BBS (800-874-2937). It is an excellent source for information about 
Hayes products. The BBS also provides a database for thousands of BBS in the 
U.S. Best of all, it is free.

Unless noted otherwise, the results are obtained by using the following:

Computer: Mac SE with Mobius Two Page Display with 68030 accelerator 
Modem: ATI 9600etc/e (the modem is set as V.32 with V.42bis enabled) 
Operating System: System 7.0 
Communication Software: ZTerm (Comm port set to 38400 bps) 
File Transfer Protocol: Zmodem

All results are reported by ZTerm. (I use several communication programs on 
both IBM PC and Mac. All of them show the average throughput while file 
transfer is in progress, but ZTerm actually produces a report after the 
transfer is completed).

III.3.  V.42 & MNP4 can improve throughput

The other benefit of V.42 (or MNP4) is that it can improve throughput. Before 
sending the data to a remote system, a modem with V.42 (or MNP 4) assembles 
the data into packets and during that process it is able to reduce the size of 
the data by stripping out the start and stop bits.

A character typically takes up 1 start bit, 8 data bits and 1 stop bit for a 
total of 10 bits. When two modems establish a reliable link using V.42 or MNP 
4, the sending modem strips the start and stop bits (which subtracts 20% of 
the data) and sends the data to the other end. The receiving modem then 
reinserts the start and stop bits and pass the data to the remote computer.

Therefore, even without compressing the data you can expect to see as much as 
1150 characters per second on a 9600 bps connection. (Although the modem 
subtracts 20% of the data, the speed increase is less than 20% due to the 
overhead incurred by the error control protocol.) Here are the test results 
obtained by downlaoding the same file (1) without any error control protocol, 
(2) with MNP4, and (3) with V.42. No data compression protocol is used.

        Filename        No EC           MNP4            V.42
        --------------------------------------------------------
        the-wave.txt    935 cps         1151 cps        1128 cps
        dayrpt.arc      863 cps         1023 cps        1002 cps
        dayrpt.wks      898 cps         1071 cps        1052 cps
        sunset.arc      838 cps          971 cps         953 cps
        sunset.pic      903 cps         1080 cps        1065 cps
        text109k.arc    908 cps         1085 cps        1064 cps
        text109k.txt    937 cps         1150 cps        1127 cps


III.4.  Are MNP4 and V.42 useful?

Absolutely. Anyone that has ever used a standard modem can appreciate the 
benefit of an error-free connection. And the increase in data throughput, 
though modest, is nothing to sneeze at.


IV.     Data Compression Protocols

Besides error control protocols, all current high-speed modems also support 
data compression protocols. That means the sending modem will compress the 
data on-the-fly and the receiving modem will decompress the data to its 
original form.

IV.1.   MNP5 and V.42bis

There are two standards for data compression protocols, MNP5 and CCITT 
V.42bis. Some modems also use proprietary data compression protocols.

A modem cannot support data compression without utilizing an error control 
protocol, although it is possible to have a modem that only supports an error 
control protocol but not any data compression protocol. A MNP5 modem requires 
MNP 4 error control protocol and a V.42bis modem requires V.42 error control 
protocol.

Also note that although V.42 include MNP4, V.42bis does not include MNP5. 
However, virtually all high-speed modems that support CCITT V.42bis also 
incorporate MNP5.

The maximum compression ratio that a MNP5 modem can achieve is 2:1. That is to 
say, a 9600 bps MNP5 modem can transfer data up to 19200 bps. The maximum