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From: JNavas@NavasGrp.com (John Navas)
Newsgroups: comp.dcom.modems
Subject: The Navas 28800 Modem FAQ -- 1/5
Date: Sat, 25 Nov 1995 21:36:49 GMT
Organization: The Navas Group, Dublin, CA, USA
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THE NAVAS 28800 MODEM FAQ [MODEM PICTURE]
(Answers to Frequently Asked Questions)
BREAKING NEWS: {NEW!
Microsoft releases Unimodem V driver for Windows 95[1]
Hilgraeve offers free upgrade for Windows 95 HyperTerminal[2]
Penril introduces 33600 modem[3]
Motorola introduces Premier 33.6[4]
33600 upgrade for USR Sportster 28800[5]
Hayes comes up with another reorganization plan[6]
USRobotics changes Sportster S-registers[7]
USRobotics challenges Diamond in bidding for Hayes[8]
AT&T bails out of consumer modems[9]
Diamond Multimedia acquires Supra[10]
FIFO problem still exists in SMC "multi-I/O" chips[11]
ZyXEL[12] releases long-awaited V.34 modem (ISDN-upgradable
Elite 2864[13])
Motorola Lifestyle and Power Class modems can damage your serial
port.[14]
Copyright 1995 John Navas, All Rights Reserved. Updated:
Saturday, November 25, 1995.
Permission is granted to copy for private non-commercial use
only.
Please note: The author has no present connection with any modem
company (other than as a customer), and does not endorse the
products of any company. This information was compiled by the
author and is provided as a public service. Neither the author nor
any organization mentioned herein are responsible for any errors
or omissions, or for any consequential problems that might result.
USE AT YOUR OWN RISK.
Please note: The author does not have the time to give
individual technical support, so please do not email requests for
assistance. Instead, post them on the Usenet newsgroup
comp.dcom.modems[15]. Thank you.
Posted as http://web.aimnet.com/~jnavas/modem/faq.html. Email
comments and suggestions to JNavas@NavasGrp.com[16]
Button[17] [The Navas Group home page[18]]
___________________________________
Why don't I get 28800 bps speed on my connections?[19]
What's the difference between V.FC and V.34? What's next?[20]
What are split/asymmetric speeds?[21]
What is Selective Reject (SREJ)?[22]
Do I need a 16550 UART? What is a UART?[23]
Where can I get a 16550 UART?[24]
Why am I getting CRC errors (overruns)?[25]
What's wrong with my dialup SLIP/PPP connection?[26]
Any Trumpet Winsock tips?[27]
Why 1024 bytes for the Windows COM buffer?[28]
What is "retraining"?[29]
What are "fall-back" and "fall-forward"?[30]
How do I set a speed greater than 19200 bps in Windows?[31]
What about third-party comm drivers for Windows?[32]
How can I share a modem between data and WinFax?[33]
How can I display the status of an internal modem under
Windows?[34]
Why does Windows lock up when I access my modem?[35]
Why can't I get back on-line after I escape to command
mode?[36]
What are the best (data/fax) comm programs for Windows?[37]
Why do I sometimes have problems connecting?[38]
Why do I sometimes get abruptly disconnected?[39]
Why do connections sometimes seem to run slower and slower?[40]
Does USR Adaptive Answer work with Procomm Plus for
Windows?[41]
Any other USRobotics tips?[42]
Any other Windows 3.x tips?[43]
What are "voice" modems?[44]
What is ISDN?[45]
What are SDSL, ADSL, and HDSL?[46] {Updated!
Any Windows 95 tips? What is TAPI? What is Plug and Play?[47]
{Updated!
Any other sources of related information?[48]
How can I connect my modem to a digital phone system?[49]
What modem initialization string should I use?[50]
Where can I get information on my Rockwell-based modem?[51]
Which 28800 modem should I buy?[52]
Where can I get a good deal on a modem?[53]
Which modem companies have a full Internet presence?
Archtek Telecom[54]
Boca Research[55]
Global Village[56]
Hayes (and PPI)[57]
Intertex[58] {Updated!
Megahertz[59]
Microcom[60]
Motorola ISG[61] {Updated!
Multi-Tech[62]
Penril[63]
Supra[64] {Updated!
Telebit[65]
USRobotics[66] {Updated!
Zoom[67]
ZyXEL[68] {Updated!
___________________________________
Why don't I get 28800 bps speed on my connections?
If you are unable to get connections at 28800 with your
V.FC/V.34 modem, here are the probable causes and what you may be
able to do about them.
If you consistently connect at 26400 or above, consider yourself
fortunate -- it's simply not possible to go faster than that on
many phone circuits. (This is not false modem advertising -- 28800
modems are designed to wring as much speed out of the actual
real-world connection as possible, and 28800 speed is only
possible on a near-perfect connection.)
Even if you consistently connect at lower speeds (e.g., 24000 or
even 21600), there may be little you can do, but you can at least
try the following:
Make sure that your serial port is locked at 38400 or higher
(57600 recommended). This is usually set within your comm
application, not the Windows Control Panel (see "How do I set a
speed greater than 19200 bps in Windows?[69]").
Try connecting to known good 28800 numbers (e.g., Hayes at
800-"US-HAYES; Multi-Tech at 800/392-2432; USR at
708/982-5092). This will at least tell you whether the problem
is at your end or the other end of the connection. (The USR
number is particularly useful, because you can get an on-line
reading of connection quality from a USR BBS command.)
Watch out for dialin numbers that are being forwarded to a
distant location. It's a "dirty little secret" that many BBS
(bulletin board systems) and ISP (Internet service providers)
use Call Forwarding to extend their local calling areas. (Where
location A to location C is a toll call, and an intermediate
location B is local to both A and C, Call Forwarding from B to
C effectively makes A to C a local call.) Although this can
work fairly well at lower speeds (e.g., 14400), the extra
connection hop(s) can degrade the signal enough to limit higher
speeds (even as compared to a direct long distance call).
If possible, test for premises problems by disconnecting all
your premises wiring (and equipment) from the incoming telco
terminating block, and hooking your modem directly to it. If
your connections are better, you have a premises problem that
you may be able to isolate and fix. Premises problems (faulty
wiring and/or equipment like cheap phones and fax machines) are
a frequent cause of 28800 connection problems.
If that doesn't help, listen carefully to the quality of your
voice connections. Note that you must dial a known quiet
number, since many otherwise good phone lines exhibit excessive
noise until you actually connect. (Dialing a single digit is
not enough.) After you connect, if you hear more than very
faint hiss and/or hum, then you probably have a line problem.
While a quiet line is important, there are other line problems
that can reduce your speed: bandwidth (frequency response),
distortion, etc. It is difficult to test for these problems
without proper test equipment, but it's still a good idea to
listen carefully for audible problems, particularly if you can
find a number that will send you test tones.
You may be able to get your phone company to improve the
quality of your line. Since phone companies are often reluctant
or even unwilling to work on data problems, it may help to
report that you are also having fax problems. Or you can try
asking for a data or fax "specialist." Ideally you want the
service technician to bring the right kind of test equipment, a
sophisticated line or transmission test set, not just the
normal basic tester. It may also help to ask for a BERT (bit
error rate tester) or "data test set."
Sometimes switching to a different cable pair from the CO
(central office) will help. In extreme cases the author has
resorted to ordering a new line, making sure that it is good
when installed, and then canceling the old line.
You may be told that you need a special "data" line, more
properly called a "conditioned" circuit, which is considerably
more expensive than a standard "voice-grade" circuit. Don't
waste your money. All you need is a good quality "voice-grade"
circuit.
A final note: Add-on noise filters will not help -- they are the
modem equivalent of snake oil. Your 28800 modem already has all
the filtering it can use. An add-on filter will do nothing at
best, and it may well make things worse.
Button[70] [Return to top[71]]
___________________________________
What's the difference between V.FC and V.34? What's next?
V.FC A proprietary specification for speeds up to
28800 bps, based on an early draft of the
V.34 standard, that was rushed to market by
modem chipset leader Rockwell
International[72]. It works reasonably well,
but lacks the sophistication and robustness
of V.34; for example, V.FC does not support
split speeds[73]. Many (but not all) V.FC
modems are upgradable to V.34, but the modem
typically has to be returned to the
manufacturer for a "datapump" change.
V.FC-only modems will not connect above 14400
bps to V.34 modems that lack V.FC support.
With the advent of V.34, V.FC is rapidly
fading as modems are upgraded. In the opinion
of the author there is now little or no
reason to get a modem that has V.FC but not
V.34, although until V.FC fades completely
there will still be a significant advantage
to a modem that supports both V.34 and
V.FC.
V.34[74] A true international standard for speeds up
to 28800 bps that is more sophisticated and
robust than V.FC. Some (but not all) V.34
implementations support split speeds[75].
Extensions to V.34 for additional speeds up
to 33600 (on extraordinarily good
connections) have been drafted and are
expected to be approved in 1996, although
early products are already starting to
appear from some companies (e.g.,
USRobotics[76]). V.34 is rapidly replacing
V.FC in the marketplace. V.34-only modems
will not connect above 14400 bps to V.FC-only
modems. In the opinion of the author V.34 is
clearly the modem standard of choice. It may
well prove to be the last widely-supported
analog modem standard.
28800-33600 fax Fax is currently limited to 14400 bps.
Efforts are underway to define a 28800-33600
bps fax specification, which would probably
be widely adopted. However, general
availability is probably a few years away.
Voice See "What are "voice" modems?[77]"
Plug and Play[78] See "What is Plug and Play[79]"
ISDN[80] See "What is ISDN?[81]"
SDSL/ADSL/HDSL See "What are SDSL, ADSL, and HDSL?[82]"
Universal Serial Bus A better way to connect a modem to your
computer than the conventional UART or
parallel port. (See "What is a UART?[83]")
Expected to become a standard feature of many
chipsets and motherboards (e.g., those
manufactured by Intel) and operating systems
(e.g., Windows 95), although USB is not yet
available. (See "Universal Serial Bus Home
Page[84]")
See "MODEMS: THE MAKE-IT-HAPPEN MACHINES FOR THE ULTIMATELY WIRED
OF THE 90'S[85]"
Button[86] [Return to top[87]]
___________________________________
What are split/asymmetric speeds?
In the past, most standard modems were only capable of
transmitting and receiving at the same speed. (The exceptions were
proprietary modems such as the USR HST that used greatly different
transmit and receive speeds.) Since most connections were made at
the maximum speed, there was little reason to support different
transmit and receive speeds.
With the advent of speeds up to 28800 bps that is no longer
true. It's now quite common to have a connection where at least
one (and often both) speeds must be limited to less than 28800
bps, and it's not uncommon to find that a connection will support
faster speed in one direction than the other. (One reason is that
send and receive channels are separated for transmission between
telephone switching offices.)
Having been designed to optimize performance over a wide variety
of conditions, V.34 includes an optional specification for
asymmetric (differing or split) transmit and receive speeds. For
example, a connection might support a transmit speed of 28800 bps
but a receive speed of only 26400 bps; without split speed, the
speed in both directions would have to be limited to 26400 bps.
For this reason split speed capability is a worthwhile and
desirable feature.
Not all V.34-compliant modems support split speeds. For example,
at the time of this writing most "glue 'n go" Rockwell V.34
clones, do not support split speeds. On the other hand, others,
including all USR V.34 modems (Sportster 28800/33600 as well as
the Courier V.34), Supra, and Motorola, do support split speeds.
Button[88] [Return to top[89]]
___________________________________
What is Selective Reject (SREJ)?
Selective Reject (SREJ) is an optional, advanced LAPM (V.42)
error correction capability that allows the receiving modem to
request retransmission of a given block while continuing to
receive later blocks; i.e., to receive the retransmission late
(out of order). Without it, the entire transmission has to start
over at the retransmitted block, which can result in later blocks
being resent unnecessarily. Hence, Selective Reject can improve
throughput where there is a significant error rate, particularly
over links with long delays (e.g., satellite links).
Not all modems support Selective Reject, which is typically
found only in high-end products.
Button[90] [Return to top[91]]
___________________________________
Do I need a 16550 UART? What is a UART?
A UART (an acronym for Universal Asynchronous
Receiver-Transmitter) is simply an interface chip. Your computer
needs one to "talk" over a serial connection to serial devices
such as serial printers and modems. Each serial device needs a
UART chip to "talk" to your computer, even modems that are
installed inside your computer.
In computer terms the UART is very old technology. We continue
to use them because we own so much software that depends on there
being a specific kind of UART. Efforts to replace the UART and its
particular serial architecture are ongoing (e.g., Universal Serial
Bus[92]), but it's doubtful that the UART will be replaced anytime
soon.
There are many types of UART chips, but only two main types that
are used in IBM-compatible personal computers:
8250/16450. Major weaknesses of the 8250 and 16450 are a lack
of built-in flow control, and buffers that can only hold a
single character. This means that the UART must be promptly
serviced by the system processor each time a character is
received, or that character will be wiped out by the following
character, a condition called "overrun." (See "Why am I getting
CRC errors (overruns) when downloading?[93]")
8250/8250A. The 8250 is the UART used in the original IBM PC.
"Urban legends" notwithstanding, the 8250 and the improved
8250A are capable of speeds up to 115,200 bps.
16450. Slightly improved over the 8250, the main advantage of
the 16450 is that it works better on computers with higher
internal speeds. From a functional standpoint, however, it is
essentially indistinguishable from the 8250, and suffers from
the same weaknesses.
16550. A substantial advance over the 8250/16450, the 16550 has
both a fully 8250-compatible mode and a newer mode that
provides 16-character FIFO (an acronym for First In, First Out)
buffers. When the old mode is enabled, it works just like an
8250; when the newer mode is enabled, the FIFO buffers greatly
extend the time available for responding to incoming
characters. The receive FIFO buffer has a variable threshold
that can be set to values of 1, 4, 8, or 14 characters, with
higher numbers giving less time to respond before the
16-character FIFO buffer is full; on the other hand, higher
numbers can give slightly better performance, by allowing the
system to process incoming characters in larger "chunks." A
good rule of thumb is a threshold of 8 characters. (See "What
about third-party comm drivers for Windows?[94]") Most (but not
all) internal modems come with a 16550 UART.
16550. Certain versions of the original 16550 were buggy. It is
best to avoid them.
16550A. Improved version of the 16550. Later types, the 16550AF
and 16550AFN, are essentially indistinguishable, but even less
likely to be buggy. These are the UARTs of choice. Look for the
National Semiconductor[95] brand.
16552. Two improved 16550 UARTs in a single package.
Clones. More recent designs typically package UART
functionality (often two of them) in a single "multi-I/O" chip.
Many of them work very well; however, some of them are buggy.
(See "Why does Windows lock up when I try to access my
modem?[96]")
Some internal modems emulate UARTs with on-board logic rather
than using actual UART chips; these products can provide more
effective buffering than a conventional UART. More advanced UARTs
(e.g., 32-character FIFO buffers, built-in flow control) are
becoming available, but they are generally not needed. Parallel
modem interfaces and the Hayes ESP card are alternatives to UARTs,
but they require special drivers and provide no significant
advantage over the 16550A in the opinion of the author.
In general, a single-task operating system like MS-DOS can get
by with an 8250/16450 UART with serial port speeds of 19200-57600
bps (depending on the speed of the system processor) and
well-written software. This is usually adequate for even a 28800
bps modem. However, a multitasking operating system like Windows,
OS/2, or UNIX, will usually need a 16550 UART. (Nevertheless, if
you are stuck with an 8250/16450, you can probably still use a
28800 bps modem if you are careful.)
You can tell what kind of UART chip you have by running
Microsoft's MSD[97] (which comes with Windows), being sure to run
it outside of Windows. (The reason is that Windows can hide the
true state of the UART from MSD.)
The Universal Serial Bus (USB) is a better way to connect a
modem to your computer than the conventional UART or parallel
port. USB is expected to become a standard feature of many
chipsets and motherboards (e.g., those manufactured by Intel) and
operating systems (e.g., Windows 95), although it is not yet
available. (See "Universal Serial Bus Home Page[98]")
Button[99] [Return to top[100]]
___________________________________
Where can I get a 16550 UART?
If your 8250 or 16450 UART is a single chip mounted in a
socket, it is easy to replace it with a 16550 UART chip. At the
time of this writing WH Networks[101] was selling real 16550
UART chips for US$10 each. (Replacement of "multi-I/O" chips is
usually not practical.)
If you only need serial and parallel ports, Quickpath makes a
good, albeit pricey, "multi-I/O" card called the "4 Serial 2
Parallel" that is available at retail (e.g., $100 at Central
Computer in Santa Clara, CA, 408/248-5888), featuring:
Four serial ports with high IRQ support that use real 16552
chips.
Two parallel ports
Quickpath also makes a more complete, albeit pricey,
"multi-I/O" card called the "Port-Folio 550E" that is
available at retail (e.g., $120 at NCA Computer Products in
Sunnyvale, CA, 408/739-9010)
Pros:
Four serial ports with high IRQ support that use real 16552
chips
Two parallel ports
Four floppy support
IDE and game port
Con: Floppy support is only 1.44 Mb, not 2.88 Mb (inexcusable
in a product at this price in the opinion of the author)
Another good high-end "multi-I/O" card is the GSI Model 32,
also available at retail (e.g., $120 at NCA Computer Products
in Sunnyvale, CA, 408/739-9010).
Pros:
Two serial ports with (limited) high IRQ support that use a
real 16552 chip
One bi-directional parallel port
Supports four floppies, up to 2.88 Mb (4 Mb unformatted), with
drive remapping
Enhanced IDE support for two drives (including large drives)
On-board Flash BIOS
Con: BIOS, IDE, and floppy support cannot be disabled
A source of low-cost I/O cards with 16550-type UARTs is:
Byte Runner Technologies
406 Monitor Lane
Knoxville, TN 37922
voice: 800/274-7897 or 615/966-3667
fax: 615/675-3458
The "T-9004 Plus" ($32 plus a modest shipping charge)
features:
Two 16550-compatible serial ports (COM1-4) with high IRQ
support
One EPP/ECP bi-directional parallel port (IRQ 7 or 5)
Supports two floppies, up to 2.88 Mb (4 Mb unformatted)
Legacy header for (separate) PCI IDE controller
But see warning below
SIIG (Fremont, CA) makes I/O cards that are available at retail
(e.g., $70 at Fry's Electronics in Fremont, CA, 510/770-3797).
The "IDE Professional" (Model 2404) features:
Two 16550-compatible serial ports (COM1-4) with high IRQ
support
One EPP/ECP bi-directional parallel port (IRQ 7 or 5)
Supports two floppies, up to 2.88 Mb (4 Mb unformatted)
IDE controller (hard disk or CD-ROM, primary or secondary
channel)
Game port
But see warning below
Pros of the Byte Runner and SIIG cards:
High IRQ support makes it a lot easier to have a serial mouse
and more than one modem.
EPP/ECP parallel ports can be a lot faster than slow, old-style
parallel ports.
2.88 Mb floppy support can make your "floppy tape" (e.g.,
HP/CMS Jumbo 250/350) twice as fast as 1.44 Mb.
Con of the Byte Runner and SIIG cards:
Both use the SMC[102] FDC37C666GT "multi-I/O" chip, which can
cause lockups under Windows 3.1. (Warning: see "Why does
Windows lock up when I try to access my modem?[103]")
Button[104] [Return to top[105]]
___________________________________
<<cont>>
