Most computer-to-computer connections occur through a
serial port, a parallel port, or a network adapter. In this
chapter, you explore ways to connect your PC to other
computers. Such connections enable you to transfer and share
files, send electronic mail, access software on other
computers, and generally make two or more computers behave as
a team.
NOTE: For a much more detailed account of how
to upgrade and repair networks, refer to Que's Upgrading and
Repairing Networks, ISBN 0-7897-0181-2.
Using Communications Ports and
Devices
The basic communications ports in any PC system are the
serial and parallel ports. The serial ports are used primarily
for devices that must communicate bidirectionally with the
system. Such devices include modems, mice, scanners,
digitizers, and any other devices that "talk to" and receive
information from the PC.
Several companies also manufacture communications programs
that perform high-speed transfers between PC systems using
serial or parallel ports. Several products are currently on
the market that make nontraditional use of the parallel port.
You can purchase network adapters, floppy disk drives, CD-ROM
drives, or tape backup units that attach to the parallel port,
for example.
Serial Ports
The asynchronous serial interface is the primary
system-to-system communications port. Asynchronous
means that no synchronization or clocking signal is present,
so characters may be sent with any arbitrary time spacing.
Each character sent over a serial connection is framed by a
standard start and stop signal. A single 0 bit, called the
start bit, precedes each character to tell the
receiving system that the next 8 bits constitute a byte of
data. One or two stop bits follow the character to signal that
the character has been sent. At the receiving end of the
communication, characters are recognized by the start and stop
signals instead of by the timing of their arrival. The
asynchronous interface is character-oriented and has about a
20 percent overhead for the extra information needed to
identify each character.
Serial refers to data sent over a single wire, with
each bit lining up in a series as the bits are sent. This type
of communication is used over the phone system, because this
system provides one wire for data in each direction. Add-on
serial ports for the PC are available from many manufacturers.
You usually can find these ports on one of the multifunction
boards available or on a board with at least a parallel port.
Figure 11.1 shows the standard 9-pin AT-style serial port, and
Figure 11.2 shows the 25-pin version.
Serial ports may connect to a variety of devices such as
modems, plotters, printers, other computers, bar code readers,
scales, and device control circuits. Basically, anything that
needs a two-way connection to the PC uses the
industry-standard Reference Standard number 232 revision c
(RS-232c) serial port. This device enables data transfer
between otherwise incompatible devices. Tables 11.1, 11.2, and
11.3 show the pinouts of the 9-pin (AT-style), 25-pin, and
9-pin-to-25-pin serial connectors.
FIG.
11.1 AT-style 9-pin serialport
connector specifications.
FIG.
11.2 Standard 25-pin serialport
connector specifications.
Table 11.1 9-Pin (AT) Serial Port
Connector
Pin |
Signal |
Description |
I/O |
1 |
CD |
Carrier detect |
In |
2 |
RD |
Receive data |
In |
3 |
TD |
Transmit data |
Out |
4 |
DTR |
Data terminal ready |
Out |
5 |
SG |
Signal ground |
-- |
6 |
DSR |
Data set ready |
In |
7 |
RTS |
Request to send |
Out |
8 |
CTS |
Clear to send |
In |
9 |
RI |
Ring indicator |
In |
Table 11.2 25-Pin (PC, XT, and PS/2) Serial
Port Connector
Pin |
Signal |
Description |
I/O |
1 |
-- |
Chassis ground |
-- |
2 |
TD |
Transmit data |
Out |
3 |
RD |
Receive data |
In |
4 |
RTS |
Request to send |
Out |
5 |
CTS |
Clear to send |
In |
6 |
DSR |
Data set ready |
In |
7 |
SG |
Signal ground |
-- |
8 |
CD |
Carrier detect |
In |
9 |
-- |
+Transmit current loop return |
Out |
11 |
-- |
-Transmit current loop data |
Out |
18 |
-- |
+Receive current loop data |
In |
20 |
DTR |
Data terminal ready |
Out |
22 |
RI |
Ring indicator |
In |
25 |
-- |
-Receive current loop return |
In |
Table 11.3 9-Pin to 25-Pin Serial Cable Adapter
Connections
9-Pin |
25-Pin |
Signal |
Description |
1 |
8 |
CD |
Carrier detect |
2 |
3 |
RD |
Receive data |
3 |
2 |
TD |
Transmit data |
4 |
20 |
DTR |
Data terminal ready |
5 |
7 |
SG |
Signal ground |
6 |
6 |
DSR |
Data set ready |
7 |
4 |
RTS |
Request to send |
8 |
5 |
CTS |
Clear to send |
9 |
22 |
RI |
Ring indicator |
NOTE: Macintosh systems use a similar serial
interface defined as RS-422. Most external modems in use
today can interface with either RS-232 or RS-422, but it is
safest to make sure that the external modem you get for your
PC is designed for a PC, not a Macintosh.
The heart of any serial port is the Universal
Asynchronous Receiver/Transmitter (UART) chip. This chip
completely controls the process of breaking the native
parallel data within the PC into serial format, and later
converting serial data back into the parallel format.
There are several types of UART chips on the market. The
original PC and XT used the 8250 UART, which is still used in
many low-price serial cards on the market. In the PC/AT (or
other systems based on at least an 80286 processor), the 16450
UART is used. The only difference between these chips is their
suitability for high-speed communications. The 16450 is better
suited for high-speed communications than the 8250; otherwise,
both chips appear identical to most software.
The 16550 UART was the first serial chip used in the PS/2
line. This chip could function as the earlier 16450 and 8250
chips, but it also included a 16-byte buffer that aided in
faster communications. This is sometimes referred to as a
FIFO (first in/first out) buffer. Unfortunately, the
16550 also had a few bugs, particularly in the buffer area.
These bugs were corrected with the release of the 16550A UART,
which is used in all high- performance serial ports.
TIP: The 16550 UART chip is pin-for-pin
compatible with the 16450 UART. If your 16450 UART is
socketed, it is a cheap and easy way to improve serial
performance to install a 16550 UART chip in the socket.
Because the 16550A is a faster, more reliable chip than its
predecessors, it is best to look for serial ports that use it.
If you are in doubt about which chip you have in your system,
you can use the Microsoft MSD program (provided with Windows,
MS DOS 6.x, and Windows 95) to determine the type of UART you
have.
Another way to tell if you have a 16650 UART in Windows 95
is to right-click My Computer, and then click Properties. This
brings up the System Properties dialog box. Choose the Device
Manager tab, Ports, and then the communications port that you
want to check. Choose the Port Settings tab and then click the
Advanced button. This will bring up the Advanced Port Settings
box. If you have a 16650 UART, there will be a check mark in
the use FIFO Buffers option.
The original designer of these UARTs is National
Semiconductor (NS). Many other manufacturers are producing
clones of these UARTs, such that you probably don't have an
actual NS brand part in your system. Even so, the part you
have will be compatible with one of the NS parts, hopefully
the 16550. In other words, you should check to see that
whatever UART chip you do have does indeed feature the 16-byte
FIFO buffer as found in the NS 16550 part.
Some manufacturers have also begun making integrated chips
which take on the functions of multiple chips. Boca Research,
for instance, sells serial and parallel cards with little more
than one Integrated Circuit (IC) on them. Most of these
integrated chips function as a 16550 would; however, you
should make sure that they have 16550 compatibility before
purchasing them.
Table 11.4 lists the standard UART chips used in IBM and
compatible systems.
Table 11.4 UART Chips in PC or AT Systems
Chip |
Description |
8250 |
IBM used this original chip in the PC
serial port card. The chip has several bugs, none of
which are serious. The PC and XT ROM BIOS are written to
anticipate at least one of the bugs. This chip was
replaced by the 8250B. |
8250A |
Do not use the second version of the 8250
in any system. This upgraded chip fixes several bugs in
the 8250, including one in the interrupt enable
register, but because the PC and XT ROM BIOS expect the
bug, this chip does not work properly with those
systems. The 8250A should work in an AT system that does
not expect the bug, but does not work adequately at 9600
bps. |
8250B |
The last version of the 8250 fixes bugs
from the previous two versions. The interrupt enable bug
in the original 8250, expected by the PC and XT ROM BIOS
software, has been put back into this chip, making the
8250B the most desirable chip for any non-AT serial port
application. The 8250B chip may work in an AT under DOS,
but does not run properly at 9600 bps. |
16450 |
IBM selected the higher-speed version of
the 8250 for the AT. Because this chip has fixed the
interrupt enable bug mentioned earlier, the 16450 does
not operate properly in many PC or XT systems, because
they expect this bug to be present. OS/2 requires this
chip as a minimum, or the serial ports do not function
properly. It also adds a scratch-pad register as the
highest register. The 16450 is used primarily in AT
systems because of its increase in throughput over the
8250B. |
16550 |
This newer UART improves on the 16450.
This chip cannot be used in a FIFO buffering mode
because of problems with the design, but it does enable
a programmer to use multiple DMA channels and thus
increase throughput on an AT or higher class computer
system. I highly recommend replacing the 16550 UART with
the 16550A. |
16550A |
This chip is a faster 16450 with a
built-in 16-character Transmit and Receive FIFO buffer
that works. It also allows multiple DMA channel access.
You should install this chip in your AT system serial
port cards if you do any serious communications at 9600
bps or higher. If your communications program makes use
of the FIFO, which most do today, it can greatly
increase communications speed and eliminate lost
characters and data at the higher
speeds. |
If you want more information on various chips, check
out:
http://www.twinight.org/chipdir/chipdir.htm
High-Speed Serial Ports
Some modem manufacturers have gone a step further on
improving serial data transfer by introducing Enhanced
Serial Ports (ESP) or Super High Speed Serial
Ports. These ports enable a 28,800 bps modem to
communicate with the computer at data rates up to 921,600 bps.
The extra speed on these ports is generated by increasing the
buffer size. These ports are usually based on a 16550AF UART
or a 16550AF UART emulator with dual 1,024-byte buffers and
on-board data flow control, and can provide great benefit in
an environment where both your computer and the "receiving"
computer are equipped with these ports. Otherwise, just one of
the computers having an ESP doesn't yield any benefit.
As the need for additional serial devices continues to
increase, users are beginning to need more than the two com
ports that are standard in PCs. As a result, multi-port
serial cards were created. These cards generally have 2-32
ports on them. Often they also provide higher baud rates than
can be achieved on a standard serial port.
Most of the multiport serial cards on the market use
standard 16550 UARTs with a processor (typically an 80x86
based processor) and some memory. These cards can improve
performance slightly because the processor is dedicated to
handling serial information. However, it's not always the best
method for high-performance applications.
Some of the better multiport serial cards have broken the
model of the 16550 UART in favor of a single integrated
circuit. These cards have the advantage of higher sustainable
throughput without loss. One such card is the Rocketport by
Comtrol. It comes in ISA and PCI versions with up to 32 ports.
Each port is capable of 232K baud sustained.
Various manufacturers make versions of the 16550A; National
Semiconductor was the first. Its full part number for the
40-pin DIP is NS16550AN or NS16550AFN. Make sure that the part
you get is the 16550A, and not the older 16550. You can
contact Fry's Electronics or Jameco Electronics to obtain the
NS16550AN, for example.
Serial Port Configuration
Each time a character is received by a serial port, it has
to get the attention of the computer by raising an
Interrupt Request Line (IRQ). Eight-bit ISA bus systems
have eight of these lines, and systems with a 16-bit ISA bus
have 16 lines. The 8259 interrupt controller chip usually
handles these requests for attention. In a standard
configuration, COM1 uses IRQ4, and COM2 uses IRQ3.
When a serial port is installed in a system, it must be
configured to use specific I/O addresses (called
ports), and interrupts (called IRQs for
Interrupt ReQuest). The best plan is to follow the existing
standards for how these devices should be set up. For
configuring serial ports, you should use the addresses and
interrupts indicated in Table 11.5.
Table 11.5 Standard Serial I/O Port Addresses
and Interrupts
System |
COMx |
Port |
IRQ |
All |
COM1 |
3F8h |
IRQ4 |
All |
COM2 |
2F8h |
IRQ3 |
ISA bus |
COM3 |
3E8h |
IRQ4* |
ISA bus |
COM4 |
2E8h |
IRQ3* | *Note that
although many serial ports can be set up to share IRQ 3 and 4
with COM1 and COM2, it is not recommended. The best
recommendation is setting COM3 to IRQ 5. If ports above COM3
are required, it is recommended that you purchase a multiport
serial board.
You should ensure that if you are adding more than the
standard COM1 and COM2 serial ports, they use unique and
nonconflicting interrupts. If you purchase a serialport
adapter card and intend to use it to supply ports beyond the
standard COM1 and COM2, be sure that it can use interrupts
other than IRQ3 and IRQ4.
Another problem is that IBM never built BIOS support for
COM3 and COM4 into its original ISA bus systems. Therefore,
the DOS MODE command cannot work with serial ports
above COM2 because DOS gets its I/O information from the BIOS,
which finds out what is installed in your system and where
during the POST. The POST in these older systems checks only
for the first two installed ports. PS/2 systems have an
improved BIOS that checks for as many as eight serial ports,
although DOS is limited to handling only four of them.
To get around this problem, most communications software
and some serial peripherals (such as mice) support higher COM
ports by addressing them directly, rather than making DOS
function calls. The communications program Procomm, for
example, supports the additional ports even if your BIOS or
DOS does not. Of course, if your system or software does not
support these extra ports or you need to redirect data using
the MODE command, trouble arises.
Datastorm, the maker of Procomm, can be found at:
http://www.datastorm.com/
Windows 95 has added the support for up to 128 serial
ports. This allows for the use of multiport boards in the
system. Multiport boards give your system the ability
to collect or share data with multiple devices, while using
only one slot and one interrupt.
A couple of utilities enable you to append your COM port
information to the BIOS, making the ports DOS-accessible. A
program called Port Finder is one of the best, and is
available in the "general hardware" data library of the
PCHW forum on CompuServe.
Port Finder activates the extra ports by giving the BIOS
the addresses and providing utilities for swapping the
addresses among the different ports. Address swapping enables
programs that don't support COM3 and COM4 to access them.
Software that already directly addresses these additional
ports usually is unaffected.
CAUTION: Sharing interrupts between COM ports
or any devices can function some times and not others. It is
recommended that you never share interrupts. It will cause
you hours of frustration trying to track down drivers,
patches, and updates to allow this to work successfully--if
it's even possible in your system.
Modem Standards
Bell Labs and the CCITT have set standards for modem
protocols. CCITT is an acronym for a French term that
translates into English as the Consultative Committee on
International Telephone and Telegraph. The organization
was renamed the International Telecommunications Union (ITU)
in the early 1990s, but the protocols developed under the old
name are often referred to as such. Newly developed protocols
are referred to as ITU-T standards. A protocol is a
method by which two different entities agree to communicate.
Bell Labs no longer sets new standards for modems, although
several of its older standards are still used. Most modems
built in the last few years conform to the CCITT
standards.
The ITU is an international body of technical
experts responsible for developing data communications
standards for the world. The group falls under the
organizational umbrella of the United Nations, and its members
include representatives from major modem manufacturers, common
carriers (such as AT&T), and governmental bodies. The ITU
establishes communications standards and protocols in many
areas, so one modem often adheres to several different
standards, depending on its various features and capabilities.
Modem standards can be grouped into the following three areas:
Bell 103 |
CCITT V.29 |
Bell 212A |
CCITT V.32 |
CCITT V.21 |
CCITT V.32bis |
CCITT V.22bis |
CCITT
V.34 |
- Error-correction standards
- CCITT V.42
- Data-compression standards
- V.42bis
Other standards have been developed by different companies
(not Bell Labs or the ITU). These are sometimes called
proprietary standards, even though most of these
companies publish the full specifications of their protocols
so that other manufacturers can develop modems to work with
them. The following list shows some of the proprietary
standards that have become fairly popular:
56K Modems At the time of this writing, two
competing factions have developed for the development of
so-called 56K modems. U.S. Robotics, one of today's leaders
in modem technology, has developed a "standard" which they
call X2. Rockwell and others have proposed a 56KFlex
"standard."
It remains to be seen if one or the other of these
technologies will become "the standard" or if the ITU will
decide to develop a third option which is the standard.
Almost all modems today claim to be
Hayes-compatible, a phrase which has come to be as
meaningless as IBM-compatible when referring to PCs. It does
not refer to any communication protocol, but instead to the
commands required to operate the modem. Because almost every
modem uses the Hayes command set, this compatibility is a
given and should not really affect your purchasing decisions
about modems.
Not all modems that function at the same speed have the
same functionality. Many modem manufacturers produce modems
that have different feature sets at different price points.
The more expensive modem usually supports such features as
distinctive ring support and caller ID. When purchasing a
modem, be sure that it supports all the features that you
need.
The following are the Web sites for U.S. Robotics, Hayes,
Microcom, and Megahertz, respectively:
http://www.usrobotics.com/
http://www.hayes.com/
http://www.microcom.com/
http://www.megahertz.com/
The basic modem commands don't vary from modem manufacturer
to manufacturer as much as they did. Some modems, most notably
U.S. Robotics, allow you to query the command set by simply
sending AT$ to the modem.
A list of the basic commands can be found in Table 11.6 of
the sixth edition of this book, which is included on the
CD-ROM. However, the best sources of modem commands are the
manuals that came with the modem.
Baud Versus Bits Per Second (bps)
Baud rate and bit rate often are confused in discussions
about modems. Baud rate is the rate at which a signal
between two devices changes in one second. If a signal between
two modems can change frequency or phase at a rate of 300
times per second, for example, that device is said to
communicate at 300 baud.
Sometimes a single modulation change is used to carry a
single bit. In that case, 300 baud also equals 300 bits per
second (bps). If the modem could signal two bit values for
each signal change, the bps rate would be twice the baud rate,
or 600 bps at 300 baud. Most modems transmit several bits per
baud, so that the actual baud rate is much slower than the bps
rate. In fact, people usually use the term baud
incorrectly. We normally are not interested in the raw baud
rate, but in the bps rate, which is the true gauge of
communications speed.
Modulation Standards.
Modems start with modulation, which is the
electronic signaling method used by the modem (from modulator
to demodulator). Modems must use the same modulation method to
understand each other. Each data rate uses a different
modulation method, and sometimes more than one method exists
for a particular rate.
The three most popular modulation methods are:
- Frequency-shift keying (FSK). A form of frequency
modulation, otherwise known as FM. By causing and
monitoring frequency changes in a signal sent over the phone
line, two modems can send information.
- Phase-shift keying (PSK). A form of phase
modulation, in which the timing of the carrier signal wave
is altered and the frequency stays the same.
- Quadrature-amplitude modulation (QAM). A
modulation technique that combines phase changes with
signal-amplitude variations, resulting in a signal that can
carry more information than the other methods.
Bell 103
Bell 103 is a U.S. and Canadian 300 bps modulation
standard. It uses FSK modulation at 300 baud to transmit 1 bit
per baud. Most higher-speed modems will still communicate
using this protocol, even though it is obsolete.
Bell 212A
Bell 212A is the U.S. and Canadian 1200 bps modulation
standard. It uses differential phase-shift keying (DPSK) at
600 baud to transmit 2 bits per baud.
V.21
V.21 is an international data-transmission standard for 300
bps communications, similar to Bell 103. Because of some
differences in the frequencies used, Bell 103 modems are not
compatible with V.21 modems. This standard is used primarily
outside the United States.
V.22
V.22 is an international 1200 bps data-transmission
standard. This standard is similar to the Bell 212A standard,
but is incompatible in some areas, especially in answering a
call. This standard was used primarily outside the United
States.
V.22bis
V.22bis is a data-transmission standard for 2400 bps
communications. Bis is derived from the Latin meaning
second, indicating that this data transmission is an
improvement to or follows V.22. This data transmission is an
international standard for 2,400 bps and is used inside and
outside the United States. V.22bis uses QAM at 600 baud and
transmits 4 bits per baud to achieve 2,400 bps.
V.23
V.23 is a split data-transmission standard, operating at
1,200 bps in one direction and 75 bps in the reverse
direction. Therefore, the modem is only
pseudo-full-duplex, meaning that it can transmit data
in both directions simultaneously, but not at the maximum data
rate. This standard was developed to lower the cost of 1200
bps modem technology, which was expensive in the early 1980s.
This standard was used primarily in Europe.
V.29
V.29 is a data-transmission standard at 9,600 bps, which
defines a half duplex (one-way) modulation technique. This
standard generally is used in Group III facsimile (fax)
transmissions, and only rarely in modems. Because V.29 is a
half-duplex method, it is substantially easier to implement
this high-speed standard than to implement a high-speed
full-duplex standard. As a modem standard, V.29 has not been
fully defined, so V.29 modems of different brands seldom can
communicate with each other. This does not affect fax
machines, which have a fully defined standard.
V.32
V.32 is a full-duplex (two-way) data transmission standard
that runs at 9,600 bps. It is a full modem standard, and also
includes forward error-correcting and negotiation standards.
V.32 uses TCQAM (trellis-coded quadrature amplitude
modulation) at 2,400 baud to transmit 4 bits per baud,
resulting in the 9,600 bps transmission speed.
The trellis coding is a special forward error-correction
technique that creates an additional bit for each packet of 4.
This extra check bit is used to allow on-the-fly error
correction to take place at the other end. It also greatly
increases the resistance of V.32 2 to noise on the line.
In the past, V.32 has been expensive to implement because
the technology it requires is complex. Because a one-way, 9600
bps stream uses almost the entire bandwidth of the phone line,
V.32 modems implement echo cancellation, meaning that they
cancel out the overlapping signal that their own modems
transmit and just listen to the other modem's signal. This
procedure is complicated and was at one time costly. Advances
in lower-cost chipsets then made these modems inexpensive, and
they were the de facto 9,600 bps standard for some time.
V.32bis
V.32bis is a 14,400 bps extension to V.32. This protocol
uses TCQAM modulation at 2,400 baud to transmit 6 bits per
baud, for an effective rate of 14,400 bps. The trellis coding
makes the connection more reliable. This protocol is also a
full-duplex modulation protocol, with a fallback to V.32 if
the phone line is impaired. It is the communications standard
for dialup lines because of its excellent performance and
resistance to noise. I recommend the V.32bis-type modem.
V.32fast
V.32fast, or V.FC (Fast Class) as it is also called, was a
new standard being proposed to the CCITT. V.32fast is an
extension to V.32 and V.32bis, but offers a transmission speed
of 28,800 bps. It has been superseded by V.34.
V.34
V.34 is the latest in the world of modem standards. It has
superseded all the other 28.8Kbps standards, and is the
current state of the art in analog modem communications. It
has been proven as the most reliable standard of communication
at 28.8Kbps. A recent annex to the V.34 standard also defines
optional higher speeds of 31.2 and 33.6Kbps, which most of the
newer V.34 modems will be capable of. Many existing V.34
modems designed using sophisticated Digital Signal Processors
(DSPs) can be upgraded to support the new 33.6Kbps speeds by
merely installing a software upgrade in the modem. This is
accomplished by downloading the Modem ROM upgrade from the
manufacturer, and then running a program they supply to
"flash" the modem's ROM with the new code.
V.34 is the fastest communication now possible over an
analog serial connection. It is also the fastest that analog
communications are likely to get. Looming on the horizon is
the fact that the phone system eventually will be digital. All
further development on analog transmission schemes will end,
and new digital modems will be developed.
Error-Correction Protocols
Error correction refers to the capability of some
modems to identify errors during a transmission, and to
automatically resend data that appears to have been damaged in
transit. For error correction to work, both modems must adhere
to the same correction standard. Fortunately, most modem
manufacturers adhere to the same error-correction
standards.
MNP 1-4
This is a proprietary standard that was developed by
Microcom which provides basic error correction. The Microcom
networking Protocol (MNP) is covered in more detail in the
"Proprietary Standards" section.
V.42
V.42 is an error-correction protocol, with fallback to MNP
4, and version 4 is an error-correction protocol as well.
Because the V.42 standard includes MNP compatibility through
Class 4, all MNP 4-compatible modems can establish
error-controlled connections with V.42 modems. This standard
uses a protocol called LAPM (Link Access Procedure for
Modems). LAPM, like MNP, copes with phone-line impairments
by automatically retransmitting data corrupted during
transmission, assuring that only error-free data passes
between the modems. V.42 is considered to be better than MNP 4
because it offers about a 20 percent higher transfer rate due
to its more intelligent algorithms.
Data-Compression Standards
Data compression refers to a built-in capability in
some modems to compress the data they're sending, thus saving
time and money for long-distance modem users. Depending on the
type of files that are sent, data can be compressed to
one-fourth its original size, effectively quadrupling the
speed of the modem. For example, a 14,400 modem with
compression can yield transmission rates of up to 57,600 bps,
and a 28,800 modem can yield up to 115,200 bps.
MNP 5
Microcom continued the development of its MNP protocols to
include a compression protocol named MNP 5. This protocol is
discussed more fully in the section "Proprietary
Protocols."
V.42bis
V.42bis is a CCITT data-compression standard similar to MNP
Class 5 but providing about 35 percent better compression.
V.42bis is not actually compatible with MNP Class 5, but
nearly all V.42bis modems include the MNP 5 data-compression
capability as well.
This protocol can sometimes quadruple throughput, depending
on the compression technique used. This fact has led to some
mildly false advertising; for example, a 2400 bps V.42bis
modem might advertise "9600 bps throughput" by including
V.42bis as well, but this would be possible in only extremely
optimistic cases, such as in sending text files that are very
loosely packed. In the same manner, many 9600 bps V.42bis
makers now advertise "up to 38.4K bps throughput" by virtue of
the compression. Just make sure that you see the truth behind
such claims.
V.42bis is superior to MNP 5 because it analyzes the data
first, and then determines whether compression would be
useful. V.42bis only compresses data that needs compression.
Files found on bulletin board systems often are compressed
already (using PKZIP or a similar program). Further attempts
at compressing already compressed data can increase the size
of the file and slow things down. MNP 5 always attempts to
compress the data, which slows down throughput on previously
compressed files. V.42bis, however, compresses only data that
will benefit from the compression.
To negotiate a standard connection using V.42bis, V.42 also
must be present. Therefore, a modem with V.42bis data
compression is assumed to include V.42 error correction. These
two protocols, when combined, result in an error-free
connection that has the maximum data compression possible.
Proprietary Standards
In addition to the industry-standard protocols for
modulation, error correction, and data compression that
generally are set forth or approved by the ITU-T, several
protocols in these areas were invented by various companies
and included in their products without any official
endorsement by any standards body. Some of these protocols
have been quite popular at times and became pseudo-standards
of their own.
The most successful proprietary protocols are the MNP
(Microcom Networking Protocols) that were developed by
Microcom. These error-correction and data-compression
protocols are supported widely by other modem manufacturers as
well.
Another company that has been successful in establishing
proprietary protocols as limited standards is U.S. Robotics,
with its HST (high-speed technology) modulation protocols.
Because of an aggressive marketing campaign with BBS
operators, it captured a large portion of the market with its
products in the 1980s.
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 increase, 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.
Over the years, the CCITT has set international standards
for fax transmission. This has led to the grouping of faxes
into one of four groups. Each group (I through IV) uses
different technology and standards for transmitting and
receiving faxes. Groups I and II are relatively slow and
provide results that are unacceptable by today's standards.
Group III is the standard in use today by virtually all fax
machines, including those combined with modems. Whereas Groups
I through III are analog in nature (similar to modems), Group
IV is digital and designed for use with ISDN or other digital
networks. Because the telephone system has not been converted
to a fully digital network yet, there are very few Group IV
fax systems available.
Group III Fax
There are two general subdivisions within the Group III fax
standard--Class 1 and Class 2. Many times you will hear about
a FAXModem supporting Group III, Class 1 fax communications.
This simply indicates which protocols the board is able to
send and receive. If your FAXModem does this, it can
communicate with most of the other fax machines in the world.
In FAXModems, the Class 1 specification is implemented by an
additional group of modem commands that the modem translates
and acts upon. Earlier you learned about the V.29 modulation
standard. As stated in that section, this standard is used for
Group III fax transmissions.
Modem Recommendations
Today the cost of 33.6K-bps modems has dropped to between
$100 and $200, on average, including fax capabilities. I would
normally recommend that you purchase an internal modem if your
computer has space for it; however, I prefer external modems
myself, due to the additional troubleshooting capabilities
possible by watching the LEDs that indicate the modem's
status. Internal modems usually ship with a high-speed UART on
the modem card, thus eliminating the need to upgrade any
older, slower UARTs you may have in your PC. If you don't have
an internal slot for a modem, be sure that you have the
appropriate UART. Today, most modems come with multiple forms
of error correction or data compression.
Integrated Services Digital Network
(ISDN)
ISDN modems are the next step in telecommunications.
ISDN modems make the break from the old technology of analog
data transfer to the newer digital data transfer. Digital
technology allows you to send voice, data, images, and faxes
simultaneously over the same pair of wires at up to 128Kbps.
ISDN modems required an ISDN service for connection, which is
more readily available today. Prices for ISDN service vary
widely, depending on your location. U.S. prices--on
average--are approximately $130 to $150 for the initial
installation and $50 to $100 a month. There is usually a
connect-time charge as well that can range from 1 to 6 cents a
minute. These are all line charges paid to the telephone
company. You will also have to purchase an ISDN modem, and
there will also be an additional charge from your service
provider for Internet Access at ISDN speeds.
CAUTION: When purchasing an ISDN modem, you
will almost always want to purchase an internal version. An
ISDN modem with compression can easily exceed a serial
port's ability to reliably send and receive data. Consider
that even a moderate 2:1 compression ratio exceeds the
maximum rated speed of 232Kbps that most high-speed COM
ports support.
ISDN has become extremely popular in Europe, where leased
lines are often prohibitively expensive. ISDN modems have also
dropped considerably in price; what was once a $1,500 to
$2,000 device can now be
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