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Navas 28800-56K Modem FAQTM 

(Answers to Frequently Asked Questions)

[Modem Picture]

Cable modem/DSL users: see Navas Cable Modem/DSL Tuning GuideTM


Copyright 1999-2008 The Navas GroupSM, All Rights Reserved.
Permission is granted to copy for private non-commercial use only.
Send mirror and commercial license inquiries to John Navas.

Posted as <http://modemfaq.navasgroup.com/faq_b.htm>.

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What's a good download speed?

Here are some general, rough guidelines. Bear in mind that data download speed is affected by many things other than modem-to-modem connection speed, including: the load on the remote system; the speed of any network links (e.g., Internet congestion); the speed of the local computer-to-modem port (e.g., serial port speed); the load on the local system; quality of the communications software; and the transfer protocol (e.g., ZMODEM or FTP).
(e.g., .ZIP) SPEED (CPS)
(e.g., text) SPEED (CPS)
14400 1600 3200
21600 2400 4800
24000 2670 5330
26400 2930 5870
28800 3200 6400
31200 3470 6930
33600 3730 7470
36000 4000 8000
40000 4440 8890
44000 4890 9780
48000 5330 10670
52000 5780 11560
56000 6220 12440
64000 7110 14220


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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), 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:

  1. 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?")
  2. 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?") Most (but not all) internal modems come with a 16550 UART.
  3. 16650. Like a 16550, but with a 32-character FIFO instead of a 16-character FIFO.
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.

The standard serial port (UART) on a PC-compatible computer is limited to 115.2 Kbps, which is adequate for V.34 (28800/33600) modems but a bottleneck for 56K modems (see "What are 56K modems?") and ISDN (see "What is ISDN?"). Some serial ports feature clock multiplier (2X-4X) technology that lets them run at speeds higher than 115.2 Kbps without special drivers; e.g., with the port configured (through jumper or software) for 2X clock doubling, when the port is set to 115.2 Kbps it actually runs at 230.4 Kbps. (See "Where can I get a 16550 UART?") However, this will only work with modems that specifically support such higher serial port speeds.

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 (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 95/98), although it is not yet available. (See "Universal Serial Bus Home Page")
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What is "retraining"?

What are "fall-back" and "fall-forward"?

It's very important to properly distinguish between (a) retraining and (b) fall-back/forward. They are frequently confused but they are not the same thing. If you turn on the modem speaker with M2 you can easily hear the difference.


Retraining is like the initial training -- several seconds where the modems probe the line in order to configure themselves (equalization, nonlinear coding, pre-emphasis, pre-coding, shaping, mapping, etc.). Either modem can request a retrain, but the retrain will only occur if the other modem grants it. You can hear the distinctive training sounds if the modem speaker is on. During the several seconds of retraining no data can flow. Because of that we want the modems to only retrain when truly necessary, as when line conditions have changed significantly and permanently, something that is usually rare. Retraining may not necessarily result in a higher speed. Excessive retrains can therefore be a serious problem.


Fall-back/forward is simple speed shifting that is relatively quick, so we would like the modems to fall back when needed to keep the error rate low, and fall forward when possible to improve throughput. With the modem speaker on you will hear only a small beep or blip in the carrier hiss. If a noise burst causes a fall-back, we want the modems to fall-forward as soon as the burst is gone. What we don't want is for the modems to ping-pong back and forth between two speeds, because that would result in lower throughput than simply remaining at the lower speed. Again, fall-back/forward can be requested by either modem, but will only occur if the other modem permits it.
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What is Caller ID?

Caller ID is a technology that makes it possible for the called party to see the telephone number of the caller on a special device while the incoming call is ringing. It is also possible for a modem with Caller ID capability to obtain the telephone number data and report it to special software, which can use the data for purposes like security validation, event logging, and/or data access (e.g., of a caller's credit history). Caller ID is an optional service that is being offered by more and more local telephone companies now that the FCC has mandated nationwide deployment. (See "Caller ID to be available nationwide")

Caller ID has obvious applications in telephone mail-order, emergency services, dial-in communications, and for those that simply want to screen their calls. However, it can also invade the privacy of a caller, disclosing information that the caller would prefer not to disclose. (Your telephone number is a key that could be used to instantly access the vast amount of information about you that has undoubtedly been compiled in public and private databases without your knowledge or consent.) Caller ID can be blocked on a per-call or a per-line basis. Check with your local telephone company.
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What is Distinctive Ring?

How can I use a single phone for fax/data/voice?

There are basically four methods of sharing an incoming phone line (without resorting to custom programming):
  1. Install a "Call Router" device on your regular phone line. (The cost is in the range of $50-300, depending on features. Sources on the Internet include Black Box, Hello Direct, and Sparco Communications.) You plug your modem, fax machine, answering machine, and/or voice telephone into the device. The device answers incoming calls, and directs them to the appropriate connected unit. Some devices do this with a series of voice prompts that instruct the caller to press different phone buttons for different services. Others do it by listening to the incoming call, and making a selection (guess) based on what they "hear." Drawbacks to this method are that voice prompting can confuse calling modems, and that your computer would need a second modem to receive both fax and data calls.
  2. "Silent Answer." This is a fax modem feature in certain Diamond/Supra modems that allows a fax modem to share a voice line. The modem answers, but does not use the normal fax answer tones. Instead it listens for tones from a calling fax. If it hears such tones, it automatically picks up the call and goes into fax mode, possibly cutting off an answering machine. Otherwise, it does not disturb the call. The drawback to this method is that it does not support data calls.
  3. Obtain "Distinctive Ring" service, and use a modem and software that support it. This service, offered by some (but not all) local telephone companies, gives you more than one phone number for a single phone line. Each number has a "distinctive" ringing pattern that can be detected by attached equipment. An attached fax/modem and software that support Distinctive Ring can then tell whether a call is coming in on the fax, data, or voice number, and handle the call in the appropriate manner. (Distinctive Ring can also be used with certain kinds of Call Routers. See above.) Distinctive Ring is usually not free, but less expensive than multiple phone lines. The drawbacks to this method are the monthly charge, and the possibility that the service may not be available in your area.
  4. Use a modem and software that support incoming Call Discrimination (also called "Adaptive Answer" or "Call Select"). Certain fax/data modems can discriminate between fax and data calls; certain so-called "voice" modems can discriminate between fax, data, and voice calls. The modem answers the call, and uses a combination of listening and trying (tones) to determine (guess) the type of incoming call. A voice modem and appropriate software can also employ voice prompting like certain Call Routers (see above). The modem then notifies the software of the type of call. The software is expected to take the appropriate action. Some modems do a better job of Call Discrimination than others.
The personal preference of the author is Call Discrimination. The problem is that good software that supports Call Discrimination has been hard to find. Although Version 2.xx of Procomm Plus for Windows has such support, it does not work well with all modems (see "Does USR Adaptive Answer work with Procomm Plus for Windows?"), and the author does not otherwise recommend the program (see "What are the best [data/fax] comm programs for Windows?"). Microsoft TAPI promised to address this need, but TAPI has been slow in coming. (See "What is TAPI?") Symantec WinFax PRO 7.0, also available as part of the Symantec CommSuite 95, finally delivers on that promise. Here's how to make Call Discrimination work on the USR Sportster 28800 Vi in Windows 95/98 with fax and the Dial-Up Networking Server: Having done all this, the author's Windows 95/98 system, even with just the Symantec CommBar running, will answer calls on a Sportster 28800 Vi (internal), and connect them to either WinFax or the Windows 95/98 Dial-Up Networking Server as appropriate. It's also possible for the author to retrieve faxes remotely. (When connected to the Dial-Up Networking Server from a remote location, it's even possible for the author to access UNIX hosts running Samba that are connected to the Windows 95/98 PC over an Ethernet TCP/IP network.) All in all, it's an impressive display of technology. The one catch is that certain failure modes will cause WinFax to go out of fax auto-answer mode, even though the option is still checked; if and when this happens, open Setup | Receive and click OK.

If for data you need "mini-BBS" capability instead of networking, you can substitute the Host Mode in Symantec WinComm PRO 7.0, also included in the Symantec CommSuite 95, although this has not been tested by the author. The principal drawback is that WinComm PRO 7.0 currently suffers from bugs and other problems, including excessive CPU usage in Host Mode. Or you can try some other TAPI-complaint application (e.g., remote access software), but be warned that this is still new territory, and that you may have difficulty getting everything to work together properly.

Symantec TalkWorks, also included in the Symantec CommSuite 95, adds voice support, but has not been tested by the author.
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How can I connect my modem to a digital phone system?

Your modem won't work on the kind of digital phone that you typically find in businesses and hotel rooms, and it can be hard to find a standard phone jack when you need one. There are a number of devices on the market to solve the problem. Typically you have to disconnect the handset from a digital phone, plug the handset into the device, and then plug the device into the handset jack on the phone. Then you connect your modem to the device with a standard phone cord.

The author gives a qualified recommendation to the KonexxKonnector Model 111. The product can be powered by a 9v battery or from an AC power adapter, both of which are included. The biggest weakness of the Konnector Model 111 is that, like other similar products, you have to experiment with a non-intuitive 4-position switch.
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How can I improve game play over modems?

The objective is to reduce "latency," the time it takes for game play signals to travel from your computer to the remote computer and vice versa.

General advice:

If connecting over the Internet:

What you should not do is disable error control and data compression! Recommendations to do that are urban myths that are not supported by the facts. The intention is to reduce latency due to packetizing in the modem. Often the recommendation includes advice to lower the modem speed (e.g., to 9600 bps) in order to reduce the error rate over the modem connection. However, that all generally makes things worse, not better.

Here's what "ping" tells us about latency. Much like a sonar ping (popularized in submarine movies), a TCP/IP ping is a short data packet that is reflected back to you by the target system. The round-trip ping time therefore shows the cumulative latency at both ends of the connection in both directions. Smaller times are better:

Chart of TCP/IP Ping Times to Local POP

min (ms)
mode (ms)
max (ms)
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Any other sources of related information?

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Where can I get more help?

Usenet newsgroups, which are like giant electronic bulletin boards, with a different "board" for each topic, can be excellent resources -- you can often find answers to questions you didn't even know you had. If you don't find what you are looking for, you can post your own questions, and usually get answers within a day or two, sometimes within hours. Just remember that a good deal of nonsense also gets posted to Usenet.

NOTE: It is very important to "learn the ropes" before posting to newsgroups. Resources to help you do this include:

Search Usenet newsgroups

Browse Usenet newsgroups

Modem-related Usenet newsgroups include:
Unmoderated discussion of fax modem-related issues (not conventional fax machines).
Unmoderated discussion of ISDN. (See "What is ISDN?")
Unmoderated discussion of data modem-related issues.
Unmoderated discussion of "cable modem"-related issues. (See "What are 'cable modems?'")
Unmoderated discussion of Microsoft Windows communications applications
Unmoderated discussion of IBM OS/2 communications-related issues
Unmoderated discussion of Apple Macintosh communications-related issues
Unmoderated discussion of communications hardware (e.g., serial ports) for PC-compatible computers
Modem-related Microsoft newsgroups

(Similar to Usenet, but run by Microsoft)

Unmoderated discussion of communications over telephone lines (e.g., data/fax/voice modems, ISDN) under Windows 95
Unmoderated discussion of Dial-Up Networking (DUN) under Windows 95
Unmoderated discussion of Dial-Up Networking (DUN) under Windows 98
Unmoderated discussion of modem communications over telephone lines (data/fax/voice) under Windows 98
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