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Why Don't High Speed Modems Work At Their "Rated" Speed?

Alan M. Fowler FIEAust CPEng

There was a little girl,
Who had a little curl,
Right down the middle of her forehead,
When she was good,
She was very, very good,
But when she was bad she was horrid.

B. R. T. .Machetta "Home life of Longfellow"

Which sums up the performance of any modem designed to operate at speeds higher than 14,400 bit/s.



You can buy cars capable of sustained speeds of 200 km/h but you cannot expect to use them regularly at that speed.

You can buy data modems claimed to be capable of speeds of 33,600 bits/sec or 56 kbits/sec but you cannot expect them to operate at those speeds on every telephone connection you dial.

Why not? The answer is the same in both cases - the infra-structure, both roads and the telephone network, is not designed to operate at those speeds.

There are many people who have bought one of these high speed modems and are now confused and angry because they are not getting the expected performance. There will be many more people upset before this sorry tale comes to an end.

There seems to be very little readily available practical information on high speed modems and what is required to get the best out of them. This report is based on information gathered from a large number of sources to help understand the problems and requirements for using these modems. It is intended to help those who are having problems at present and those who are thinking of buying a new modem.

How do 56k modems work?

The three standards, K56flexTM, x2TM and V.90 are very similar, and the differences between them are only in the detail. For convenience the three will be referred to simply as 56k modems. Previous high speed modems operate at the same maximum speed for both up- and down loading. 56k modems upload (from you to your ISP) at a maximum of 28,800 to 33,600 bit/s, depending on the manufacturer and model. They download (from ISP to you) at speeds up to a theoretical maximum of 56,000 bit/s. In practice the download speed will be in the range of approximately 40,000 to 52,000 bit/s

56k modems are designed to take advantage of the new digital telephone networks. These use Pulse Code Modulation (PCM) to convert your voice, fax or modem signal into a digital stream at your local exchange. The amplitude of the analog signal is measured 8000 times per second. Each measurement produces a PCM code in the form of an eight bit byte to represent the amplitude.

Instead of 256 uniform steps the Australian PCM codec uses seven segments with the steps in each segment being twice the size of those in the previous segment. The result is approximately a logarithmic response. The eight bit code represents one sign bit, three exponent and four magnitude bits. These codes are sent through the telephone network to the distant exchange at 64,000 bit/s. A codec on the line card at that exchange converts the PCM codes back to a replica of the original signal and sends it to the telephone, fax or modem you called. Information from the far end is sent to you the same way on a parallel digital stream.

Instead of sending data to you as a modulated tone, the ISP sends a series of PCM codes that produce a modulated voltage waveform as a series of steps at the output of the codec. The size of each step is a precisely controlled voltage. The ratio of the maximum output to the minimum step is about 2000 to 1 (or 66 dB). The smallest steps are easily masked by noise and cannot be used. During the initial hand shaking the two modems work out which PCM codes will produce voltage steps which will be recognised reliably. These codes are then stored in a table usually referred to as the constellation.

In an ideal world your modem would be able to recognise all 256 possible steps that can be produced by an eight bit code so your ISP would simply send actual data instead of coding it and the connection would operate at 64 kbit/s. Real worlds are never ideal. Remember that the telephone network was designed for speech. The telephone has to faithfully reproduce the full range of loudness from a lover's whispers to the shouting of an irate, aggressive customer.

If 128 steps (7 bits) can be recognised reliably then the maximum speed will be 7 x 8000 = 56,000 bit/s. Similarly if only 64 steps (6 bits) can be recognised the maximum speed will be 6 x 8000 = 48,000 bit/s, 40,000 bit/s for 32 steps and 32,000 bit/s for 16 steps. If somewhere between 32 and 64 steps can be recognised reliably then the maximum speed will between 32 kbit/s and 48 kbit/s. K56flex modems work at speeds of 32, 34, 36 , ...... 48, 50, 52, 54, 56 kbit/s. V.90 modems work at speeds from 32 to 56 kbits/s in increments of 1333.3 bit/s. x2 speeds are also multiples of 1333.3 bit/s, although x2 does not include these V.90 rates: 34.6 or 36 kbit/s.

A special purpose computer at your Internet Service Provider (ISP) breaks up the outgoing digital stream into groups of eight or less bits. It selects the relevant eight bit PCM codes from the constellation and sends them to the codec at your local digital exchange. The codec converts the PCM codes to the step waveform which is sent to your modem along your telephone line. Your 56k modem translates each step into the equivalent group of bits and assembles these into a digital stream to send to your computer.

There is a further complication. Telecommunications Authorities around the world set a limit on the maximum power, or loudness of the signal, that may be transmitted along a telephone line. This means that not all of the largest steps can be used limiting the maximum downstream data rate to about 53 or 54 kbit/s. This is a carryover from the days of all analog transmission when a louder modem signal could overload amplifiers on trunk calls, and cause interference to other users. That will become irrelevant when the telephone network is fully digital, and the authorities may agree to relax the power limit. The increase in signal level may make it possible to go closer to the magic figure of 56,000 bits/sec. However, the main beneficiaries are likely to be those users whose maximum speed has been limited by noise on their local loop.

The above is a very simplified description of the operation of all 56k modems. Actual operation is much more complex, and there are considerable differences between the details of the operations of K56flex, x2 and V.90 modems.

Requirements for 56k high speed modems:

Speeds ain't Speeds, Connect vs Carrier.

There seems to be a lot of confusion about the reported speeds of modems. This is due partly to a misunderstanding of which speed is being reported, and how the speed may vary during an online session.

First of all, there is the speed of the connection between the computer serial port and the modem. The UART in the serial port adds an extra bit to the start and end of each byte being sent to the modem to tell the receiving UART where each byte begins and ends. The port speed must be set to at least 25% higher than the line speed you expect to use. For a 28,800 of 33,600 bit/s the port speed should be 38,400. or 57,600 bit/s and for a 56k modem it should be set to 115,200 bit/s. If you are using compression the port speed needs to be set higher to cope with the increased load.

For those using Windows 95 make sure you have the correct .INF file, it is properly installed and that the settings will give the correct CONNECT message. Getting this wrong is one of the most common causes of confusion.

High speed modems select an initial line speed during the handshake sequence and usually report this as the CARRIER speed. For 33,600 bit/s modems the transmit speed may be different from the receive speed. Some modems choose and report a safe operating speed during the handshake sequence, then decide on an actual working speed once they have handled live data. The modems at each end may decide between themselves to change either or both speeds during a session. If line noise increases causing errors the speeds may drop, if the line improves they may increase.

As an experiment I checked the speeds reported by my Dynalink 33,600 bit/s modem at the end of each session for about two weeks. With an initial carrier speed of 31200 the maximum speed was up to 33,600 bit/s and the last speed (NOT lowest speed) before logoff was as low as 12,000 bit/s. The modem does not keep a record of the lowest speed, so it could have been less than 12,000 bit/s during the session.

56k modems behave in the same way and continually adjust the speed up or down to match the line conditions and the amount of noise and interference present.

At best, the CONNECT speed reminds you of the port speed setting, and the CARRIER speed tells you that you have a live connection to the other end. At worst, it causes no end of worry because you think the modem is not running fast enough. Don't worry. It's running as fast as it can on the particular connection. If it is much less than the modems top speed you probably have a poor telephone connection which you may or may not be able to fix.

Questions and answers.

Questions about the use of high speed modems including V.34, K56flex, x2, and the new V.90 standard modems keep cropping up in the melbpc.general, melbpc.connectivity and aus.comms newsgroups.

The following answers have been kept as brief as possible. For much more detailed information have a look at the web pages listed in "Further Information".

Basically the questions can be reduced to:

Bottlenecks on the Internet.

The Internet is not an unlimited resource. The Internet does not have unlimited capacity. Digital capacity costs real money, and since governments have sold, or are selling, the communication networks the new owners want fast returns for their investment. Governments were prepared to get their money back in the long term. As more and more people access the Internet it will grind to a halt until it looks like the South Eastern Freeway - better known as "The Longest Parking lot in the World". The Information Super Highway is about to become the Information Super Tollway.

The servers storing the information do not have unlimited capacity. The "free" information everyone wants is stored on computers that somebody has had to pay for. They are not going to increase the capacity and/or speed of those computers unless they can recover their costs. Either they start charging as some do already, or they include paid advertisements with the information you want effectively slowing down access.

Poorly designed Web Pages. These are another source of frustration and delay as they seem to take forever to load. Many of them seem to have been designed without any thought for the reader. I suggest you measure how long each takes to load then e-mail and tell them. Ask if they would consider making provision for a text-only version as well.

Downloading software. MelbPC members should have a look and see whether the wanted software is available on the BBS. In my experience downloads are far quicker from the BBS than the Internet.

Do you really need a faster modem?

Finally, before you decide to buy, have a look at the REAL speeds you are getting now. What really matters is the average speed or throughput - the total number of characters you get in a fixed period. Watch the receive and transmit LEDs if you have an external modem. You usually get bursts of download mixed up with periods of inactivity. A one second burst at 48,000 bit/s followed by 9 seconds of inactivity is an average throughput of 4800 bit/s.

Have a look at the speeds if your program shows this. I often find that a download will start at 2 K bytes/sec and progressively decrease. I've had it go as low as 6 bytes/sec i.e. about 50 bits/sec on a particularly busy server.

A faster modem will not be any help if there are bottle-necks in the Internet.

I have seen many calls recently for 100 Mbit/s service to all users. Now what can you do with such a high speed. For a start you can completely fill your new 6.1 Gbyte hard disk in less than ten minutes .....

There is little reason to buy a very fast car if all your driving is in stop and go motoring on badly clogged roads.

What does the future hold?

The big unknown is what will happen to the telephone network once Telstra is fully privatised. The most likely outcome is that control of the company will end up in foreign hands despite legislation designed to stop this happening. Major decisions will be made in board rooms on the other side of the world. Unprofitable parts of the network will be sold. Untimed local call areas will shrink until they only cover your local exchange, in line with "world's best practice".

Recommended reading for more valuable information.

There is a huge amount of very valuable material in the following references. If you hunt through it, and read it carefully, you will find the answers to almost all of your problems. If this doesn't help then post a question to one of the newsgroups, starting with the Australian newsgroups.

When writing to a newsgroup give a short subject heading which explains your problem, give sufficient details so that others can recognise your equipment and help you. Don't swamp the readers with too much information. There is a nice balance. Look and see how others have framed their questions, and the value of the replies received. Subject headings like "Help" will probably be ignored.

Remember that much of the material in the following references is related to the American telephone network, and may not apply to Australia. We should be very thankful that we have such a high quality network.

The information in the following web pages is being updated continually:

News groups for modem problems, help and discussions.


There are far too many people who have helped to list them personally. To all those who provided information and discussed problems with me, to the authors of the very detailed web pages and to those who take part in the newsgroups I give my heartfelt thanks.

About the author.

Alan Fowler joined the Post-Master General's Department in 1945 as a Technician-in-training and retired as Principal Engineer, Telecommunication Science and Technology Branch of the Research Laboratories in 1992. In between he worked in telephone equipment, ABC studios and transmitters including Radio Australia and the Transmission and Switching Branches of the Research Laboratories. His first real contact with modems and data transmission was in setting up the 2400 bit/s data links across Australia for the first Moon Landing. He is a Fellow of the Institution of Engineers, Australia and an Emeritus Member of the Electrical College of that Institution.


And finally.

Remember the first rule of computing when buying new hardware: "Beware of Geeks bearing gifts".






Appendix A.

Comparison of the frequency response of two connections.

| -18 |  X X X X X X X X X X X X X x x x x x       |  1 |
| -20 |  X X X X X X X X X X X X X X X X X X X X x    |  3 |
| -22 |  X X X X X X X X X X X X X X X X X X X X X    |  5 |
| -24 | X X X X X X X X X X X X X X X X X X X X X X X   |  7 |
| -26 | X X X X X X X X X X X X X X X X X X X X X X X   |  9 |
| -28 | X X X X X X X X X X X X X X X X X X X X X X X x  | 11 |
| -30 | X X X X X X X X X X X X X X X X X X X X X X X X  | 13 |
| -32 | X X X X X X X X X X X X X X X X X X X X X X X X  | 15 |
| -34 | X X X X X X X X X X X X X X X X X X X X X X X X  | 17 |
| -36 | X X X X X X X X X X X X X X X X X X X X X X X X  | 19 |
| -38 | X X X X X X X X X X X X X X X X X X X X X X X X x | 21 |
|    0 0 0 0 0 0 1 1 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3    |
|    1 3 4 6 7 9 0 2 3 5 6 8 9 1 2 4 5 7 8 0 1 3 4 6 7    |
|    5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5    |
|    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0    |
            Frequency Hz.

Good Quality Connection.
Should allow a V.90 modem to run at high speed if there are no other impairments.

| -16 |  x X X X X x x                  |  1 |
| -18 |  X X X X X X X X x x x              |  3 |
| -20 |  X X X X X X X X X X X X x x           |  5 |
| -22 |  X X X X X X X X X X X X X X X X x x       |  7 |
| -24 |  X X X X X X X X X X X X X X X X X X X x     |  9 |
| -26 |  X X X X X X X X X X X X X X X X X X X X X    | 11 |
| -28 |  X X X X X X X X X X X X X X X X X X X X X    | 13 |
| -30 | X X X X X X X X X X X X X X X X X X X X X X X   | 15 |
| -32 | X X X X X X X X X X X X X X X X X X X X X X X   | 17 |
| -34 | X X X X X X X X X X X X X X X X X X X X X X X   | 19 |
| -36 | X X X X X X X X X X X X X X X X X X X X X X X   | 21 |
| -38 | X X X X X X X X X X X X X X X X X X X X X X X x  | 23 |
| -40 | X X X X X X X X X X X X X X X X X X X X X X X X  | 25 |
| -42 | X X X X X X X X X X X X X X X X X X X X X X X X  | 27 |
| -44 | X X X X X X X X X X X X X X X X X X X X X X X X  | 29 |
| -46 | X X X X X X X X X X X X X X X X X X X X X X X X  | 31 |
|    0 0 0 0 0 0 1 1 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3    |
|    1 3 4 6 7 9 0 2 3 5 6 8 9 1 2 4 5 7 8 0 1 3 4 6 7    |
|    5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5    |
|    0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0    |
            Frequency Hz.

Poor Quality Line.
Maximum line speed less than 19,200 bit/s. The difference in signal strengths at 150 and 300 Hz is greater than the 10 dB permitted. The signal strength measured at 3750 Hz is below the -50 dB limit.



Appendix B.

V.90 ain't V.90 - The Good Oil

Or to put it more bluntly, not all modems are created equal.

There are four important points to keep in mind when discussing V.90 modems and any problems with their use.

As a Result:

And Finally:

Collective wisdom:

The answer to your problem is almost certainly out there somewhere. Conversely, you may very well have the exact piece of information that other users need to solve their problem.

So read the newsgroups for a little while to get a feel for the way they operate. Take an active part in any discussion where you can make a useful contribution. It's a two way street and you will be surprised just how much you can learn. I started doing this about a year ago, and have learnt a tremendous amount in that time.

Copyright 1998 Alan M. Fowler FIEAust CPEng
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All of the views expressed on this page are Alan Fowler's and are not intended to represent those of any other person or commercial entity. All advice given is drawn from personal experience and/or the experience of others and is genuinely intended to assist. Please take responsibility for your actions - I cannot and will not be held responsible for any damage caused to your hardware or software by actions taken which are inspired by writings on this or other pages. All efforts have been taken to ensure the veracity of what is written - if you know of any errors or omissions then please let me know & I will amend/add it at the earliest opportunity.

Updated 27 June 2000
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