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SCN FAQ (Frequently Asked Questions)
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A Universal Modem Initialization String?


Setting Maximum Modem Speed

in Modem Initialization Strings

To maximize download speed (and browser speed), it is necessary to slow down most modem connections. The maximum connect speed you can obtain will actually slow your data rate in most cases.

Port Speeds (DTE)
Reducing Maximum Modem Connect Speeds (DCE)
WHY?? would you want to reduce your maximum speed?
Setting Maximum Speed
Getting 56K Performance from a 33.6K Modem
One Way to Test for Optimum Speed



DTE Port Settings

This is repeated from A Universal Modem Initialization String?

Your computer looks at the modem and considers it to be "data set equipment". Your computer doesn't directly know the connect speed over the phone lines and only works with the DTE (data terminal equipment) speed or port speed. (The reason for the DTE terminology is that a computer is not required to make a modem connection; most "green terminals" are not computers.)

The DTE speed may be selected by your software, in which case you won't have to worry about it.

If you have an internal modem running at 14,400 baud or higher, you will get the best efficiency if you choose at least the next higher DTE setting, and preferrably at least double the modem speed. For example if you have a 14,400 modem, select a port speed such as 19000 or 38800. (Note that the port settings beyond 9600 generally don't match common modem speeds. Do not use 14400 as a port speed.)

If you have an external modem and do not have what is known as a 16550 UART, you may need to set the port speed to 9600 or slower. If you have a standard 2400 baud modem, you may have to set your port speed to match that of the modem.


DCE Speed Settings on the Modem

On programs which list 'optional' or 'additional' commands (e.g., 'extra settings' in Windows95), the 'AT' is normally left off.

Disable MNP5 (varies - see documentation)
Disable MNP5 compression. MNP5 is a data compression protocol which tends to slow down downloads of most large files, including image files and zip files. The reason is that these files are typically already compressed. (SCN's dialup lines have this protocol disabled.) So MNP5 really has little use in most consumer applications.
Max. Speed Settings (varies - see documentation)
WHY?? would you want to reduce your maximum speed? Okay...

This concerns the DCE speed of the modem talking to another modem over the phone line. DTE, or port speed is discussed above.

It is possible to increase data transfer rates by reducing connect speed (DCE)! If the maximum speed exceeds that speed at which your modem (actually your phone connection) can achieve a clean connection, then your modem will be sending NAKs to the host's modem. (The host is SCN, or an internet provider.)

It's easy to see with a straight download from the host, and looks a lot like the kind of delays commonly seen on the internet. If your terminal program (eg., Telix) displays a data transfer rate during download, then you will see stalls in the download, followed by a reduction in the average download speed. To a much lesser extent, you can observe this in Windows95 by double clicking the red/green 'lights' modem icon.

This mostly affects compressed files, like .zip, .qwk (also a zip file), and internet image files -- i.e., 95% of the files where speed counts. If you are on Lynx (without images) this whole issue is irrelevant!

Modem stalls are not internet delays. Internet delays are between the local host (SCN or your internet provider) and a remote host (www.moron.com). On Lynx, internet delays appear as
"HTTP request sent; waiting for response." or
"1200 bytes of 9980000 received (and holding...)"

Your modem setting will not help with internet delays.

Lynx text is unaffected because

So anything over 9600 baud on Lynx (text) is just speed for the sake of speed.


How to Determine Download Speeds




What's Going On

Higher speed protocols jam more data into the same signal. That works nicely within the modem, but we run into the limits of what a voice quality line can handle. The modem uses slight variations in signal phase and frequency to increase its DCE baud rate. The phone line distorts the signal, and these slight variations become garbled.

When a signal is garbled, the signal doesn't match its checksum, and the receiving modem sends a NAK which is a logical "say again" signal. The transmitting modem then must back up and repeat the data stream from the location of the error. This typically takes several seconds.

This is much like having a fast talking person saying something that you decide you want to understand. If you ask the fast talker to repeat the sentences every time they run words together, then things go a lot slower than if they slowed down and didn't run the words together.

So we really have reached a technological ceiling in the vacinity of 24 to 38.6 KBaud on our voice lines. The top-of-line 56K modem may be able to connect at 51K, but the best speed may be in the 28K range.

That's not all wasted performance. I had a car that could cruise at 140 MPH. For some reason I didn't drive it that fast, but it still performed nicely at 70.




Setting Maximum Symbol Rate

Hey, if this looks too technical, that's because it is. It may be much easier to skip to the speed data. The idea is efficiency, not re-engineering the *&%$ modem.

This is probably the same as setting maximum speed, but focus directly on what I believe is the controlling issue. If so, it may permit 56K connections at higher than the optimum speed for 33.6 connections, and yet throttle-back both types of connections.

I don't have the Rockwell settings, so please skip down to the speed data

Symbol rates are as follows (register bits are for USRobotics): 0 1 Disable 2400 symbol rate. 1 2 Disable 2743 symbol rate. 2 4 Disable 2800 symbol rate. 3 8 Disable 3000 symbol rate. 4 16 Disable 3200 symbol rate. - blocks 28.8 kbps speeds 5 32 Disable 3429 symbol rate. - blocks 33.6 kbps speed 6 64 Reserved 7 128 Disable shaping.

Advanced encoding techniques boost performance by improving V.34 signaling. Like all high-speed modem signaling techniques, V.34 transmits and receives analog symbols that have certain amplitude, frequency, and phase characteristics. Each symbol is capable of representing up to 9 bits, which yields a total of 512 (29) different symbol states. For 28.8 kbps speeds, 3,200 symbols per second are transmitted. For 33.6 kbps speed, the modem sends 3,429 symbols per second.

For USRobotics modems, this is either s register s33 or s54, so (in from a terminal program) type:

s33? and
s54?
(s33=? will give you a list of options.) and see which one produces results.
Then, to disable 3429 symbol rate, use s__=32
To disable both 3200 and 3429 symbol rates, use s__=48 (which is the sum of 16+32).


Setting Maximum Speed

Typical settings are:
AT+MS=11,1,2400,26400
- for Rockwell chipsets, using the format
+MS=[,[][,[][,[]]]].
Start with +MS=? to get your options.
Choose the speed.


&U1 &N13
- for USR (undocumented). Normally &N settings lock a speed, but the &U1 setting frees the modem to go to lower speeds as necessary. The USR speeds (see documentation) are:
&N0 (default) - select maximum possible speed.
&N1     300 bps
&N2    1200 bps
&N3    2400 bps
&N4    4800 bps
&N5    7200 bps
&N6    9600 bps
&N7    12,000 bps
&N8    14,400 bps
&N9    16,800 bps (V.FC/V.34 only)
&N10   19,200 bps (V.FC/V.34 only)
&N11   21,600 bps (V.FC/V.34 only)
&N12   24,000 bps (V.FC/V.34 only)
&N13   26,400 bps (V.FC/V.34 only)
&N14   28,800 bps (V.FC/V.34 only)


so the command for USR is &U1 &N13 (for a 26,400 maximum speed)
So to answer the obvious question Why would someone deliberately downgrade their modem? The reason is that you can increase your data transfer rate by doing so. If you note significant "stalls" when downloading, then your modem is sending NAKs (say again)

You can also write some of these options to your modem's non-volatile memory. If you do so, first type the 'atZ' command to start with the existing memory configuration and not your program's initialization string. A typical sequence would be something like:

atZ <enter>
atW2 T <enter>
at&W <enter>

That will allow your communications program to call up that configuration, but only if the program calls the "atZ" command instead of the "at&F" command.

It's actually preferrable to write such limitations into a dialing prefix because if you connect to a site with digital lines (a 56K site), you can up the max. speed by 2400 or 4800.


Phone Line Quality and Line Noise


Factors involved in phone line quality include: Obviously some of these things are beyond the user's control, short of obtaining a T-1 connection. Some things can be controlled, however. Most people won't need to do anything (except maybe the Slick 50 56-KFlex Oil Treatment). If you are getting more-or-less consistent connections, you are probably okay. If your connect speed is 2400 below the maximum connect speed, consider it an enhancement to stability. (If you start with a 2400 baud modem and connect at 2400 below that, it will be very stable.)

You may find that the signal levels are not high enough to generate a maximum speed connection. That's usually not a major disadvantage, provided that the signal is clean. My own connection was at a consistent low level but also consistently clean. I'll take that over fast and dirty any day, and besides, think how much harder it is for my dog to eavesdrop on my conversations!


Which Modem Shall I Get?

First, it's always more economical to buy less than the fastest modem available. Unless you need and expect to use the highest speed, you will get good connections with a slightly slower modem. As of this writing, we are seeing a bit of a limit on the physical capability of modems which means that it is unlikely that you will actually get much more than the 30K from your connections on voice telephone lines.


Getting 56K Performance from a 33K Modem

Nothing magic here. The maximum theoretical speed from a voice phone line is 33.6 . This is because of the limits of what a voice quality line can handle. In order to run 56K, it is necessary that the ISP use a digital connection to the telephone switchboard. This eliminates the standard analog connection on their side of the telco trunk, so they can transmit at 56K.

It turns out that many users are not seeing connections much over 30K. If the maximum "clean" connection turns out to be 33.6K then a 33.6K modem will give you optimum performance. Put another way, if you had a 56K modem, you may find that maximum throughput is obtained by downgrading it to 33.6 when talking to a 56K site. Either type of modem may have to be downgraded to a lower figure such as 26.4 to talk with a modem on a voice line.

This also means that you should select conditioned lines for dialing the ISP. If the only conditioned lines are 56K lines, then use them even if you are running 33.6 .

So is a report from a dialup test that the line is "56K" capable so much bovine scatology?

Yep.

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The Limits of a Voice Quality Line

The reason modems are unable to exceed 33.6 on a "voice line to voice line" connection (and 56K on a single voice line connection) is Shanon's Law. Simply put, the speed of data transfer is limited to the audio resolution of the phone line.

Shannon's Law (named after Claude Shannon):
C = B log_2 (1 + S/N),   where C is achievable channel capacity, B is the bandwidth (typical telephone line is 3300 hz - 300 Hz, or 3000 Hz), log_2 is the logarithm to base 2, and S/N is the signal to noise ratio. A typical signal to noise ratio is 30 decibels, which (skipping some of the math) means the 'S/N' term is 1000.

The 'typical' S/N ratio is for an end-to-end connection. 56 Kbps rates are possible where the central switching office has the right equipment to put the signal on a higher bandwidth line to the ISP. You're still stuck with Shannon's limit going upstream because of the bandwidth loss in digitizing the analog signal. But coming downstream (the ISP sending information to you) the digital signal can be made analog with less loss, and therefore higher bandwidth.

Some of the other high-speed connections (ISDN, ASDL) attain higher capacity by using conditioned lines (less noise), or balanced-pairs so they can drive the signal at a higher level without spewing RFI all over.

Thanks to jj for the information on Shannon's law.


One Way to Test for Optimum Speed

  1. Internet Downloads vs. Modem Downloads

    If you download a file, the file first gets transferred from the remote host (the website or ftp site) to the local host (your ISP). The file then gets transferred to your computer. At that time your browser lets you know you are downloading.

  2. Timing the Download

    If you time the download; or simply look for stalls during the download, you will see that the download progresses in blocks of perhaps 1KB at a time. Every once in a while you will get the stalls I mention above. It helps to have some sort of indication of bits transferred so you can watch more than the Windows "lights".

  3. Different File Types

    The results are most repeatable with compressed files. These include .zip files and compressed executables such as antivirus updates. Internet images (.jpg and .gif) are compressed, as are I believe most other media files. That means that most bandwidth is consumed by compressed files even when browsing.

  4. Compare Times

    The most accurate way to compare this is to compare time of transfer of the same file. A close second is comparing the times of two similar files, divided by kilobytes. Your results will be greatly affected by variations in your phone connection quality, but you should get a good idea after a few times.

    If you can observe stalls, that should give you a good idea of what to look for.

  5. Optimum Speed

    At the optimum speed, you should see almost no stalls. Typically, I would expect perhaps one stall for 15 minutes or longer of intensive downloads.

    I have a 33.6 modem and connect to SCN, which has analog lines. I set my max. connect speed at 28,800. But that's my phone line. These settings will probably not speed your computer. If you are running Windoze, see Windows Setup (W95/98).


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    Stan Protigal - Comments about this site: email me

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