Welcome! to the Modems section

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

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

Contents

• Introduction
• How do 56k modems work?
• Requirements for 56k high speed modems
  • A telephone connection with adequate bandwidth
  • A single digital-to-analog conversion
  • The ISP must have digital links
  • local exchange must be digital
  • Modem compatible to Network
  • Install latest firmware version?
  • Have sufficient computer processing capacity
  • Have correct UART and software
  • No premises problems
  • Have a good quality line
  • Have a good quality modem
• Speeds aint Speeds Connect vs Carrier
• Questions and Answers
• Bottlenecks on the Internet
• Do you really need a faster modem?
• Recommended reading for more valuable information
• Acknowledgments
• About the author
• And finally
• Definitions
• Appendix A - Comparison of 2 connection's frequency response
• Appendix B - V.90 ain't V.90 : The Good Oil

Introduction

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, K56flex^TM, x2^TM 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

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:

1 A telephone connection with adequate bandwidth. A "standard" telephone connection has a bandwidth of about 3100 Hz, from 300 Hz to 3400 Hz. In practice many connections, but not all, have a much wider bandwidth and high speed modems including V.34 (33,600 bit/s) take advantage of this.

2 There must be only a single digital-to-analog conversion between your ISP and your modem. This is another way of saying that a computer at the ISP must directly control the analog output from the codec in your local exchange.

Nearly every item I've read about 56k modems says "There must be only one Analog-to-Digital conversion between your ISP and your modem". This confused me for a long time until I realised that they should have said "There must be only one Digital-to-Analog conversion between your ISP and your modem".

Since writing the above I have tracked down the ITU press release of 6 February 1998 on the 56k modem standard which says "V.90 modems are designed for connections which are digital at one end and have only one digital-to-analog conversion".

3 Your ISP must have digital links direct to the digital network. The ISP must have one or more high speed digital links connected to a digital exchange which is part of the digital network. At the exchange the digital link bypasses the individual line cards with the codecs, and connects directly with the digital switch. Incoming PCM codes representing the output from your modem are fed into a digital signal processor (dsp) at your ISP, and the data stream recovered.

4 Your local exchange must be digital and a part of the same digital network. The link between your ISP and the codec at your local exchange must be completely digital. Any intermediate conversion from the digital signal to analog and back to digital will prevent the use of 56k modems.

We are very fortunate in Australia. It has been reported that Telstra is well on the way to completing the changeover to its Future Mode of Operation (FMO). The network should be almost 100% digital by the end of 1998 with small pockets here and there still to be finished.

In the meantime you can find out whether or not your exchange is digital from your local Telstra Shop. The staff at the Telstra Shop told me you can also find out by calling the information number on your telephone bill.

5 Your modem must have been designed to operate on the Australian digital telephone network. Australian digital telephony uses A-law companding when converting between analog and digital signals while the American system uses both mu-law and some A-law companding. The two companding laws produce different analog outputs for the same PCM code. The two are not compatible. In Australia the 56k modems must use A-law companding, or be capable of being set to use A-law by an initialisation string. The wrong setting could reduce the maximum download speed.

modems designed and built for use on the Australian telephone system should default to the A-law setting. Modems bought directly from overseas may default to the mu-law

In addition, the Australian dial tone, ring tone, engaged signal and other tones are often different from those used in other countries. Unless the modem is designed to recognise these signals it will not operate properly.

6 Should your modem have the latest version of the firmware installed? "Firmware" is the code stored in the modem memory that controls its operation. Installing the latest version may cure many of your problems. Modem manufacturers are continually revising the firmware to improve its performance. Like all software, modifying the code to fix one problem may cause new problems. Check first to see if you really need the new version. If you do the latest version can usually be downloaded from the manufacturers web site.

Most 56k modems store the code in flash ROM so they can be upgraded easily. Read the handbook for your modem for details or contact the firm you bought the modem from for information on how to upgrade the modem.

WARNING. You must use the upgrade intended for your modem. Do not under any circumstances use an upgrade intended for another model or make of modem. Follow the manufacturer's instructions exactly. The Internet newsgroups are full of horror stories of people who have used the wrong code and "fried" their modems.

7 Your computer must have sufficient processing capacity to handle the high speed incoming data. If you have a HSP (Host System Processing) or "win-modem" your computer needs even more processing power. These modems keep down cost by transferring functions such as data compression and decompression to the computer's processor instead of handling them within the modem. Some computer manufacturers are pre-installing either of these types to keep the cost of a system as low as possible in a very competitive market.

There are two potential problems with these two types of modem, (a) upgrading the modem or operating system may upset the operation of the modem, and (b) some application programs are not fully "Windows compatible", they hog the CPU and data is lost. In particular, there have been complaints from games enthusiasts that the modems are running slow when playing a game.

8 Your UART and software must be capable of handling high speed data. Your computer should be fitted with at least a 16550 UART. If you have a recent model computer you should have no problems. If you have an older model computer and operating system you may need to buy a new serial card and the software to make use of its internal buffer.

9 Problems that can occur in your own premises. What other equipment is connected to the same line - other phones, fax machine, answering machine, cordless phone, extension bell, automatic dialler, etc? Go around and unplug the lot of them and see if it improves the speed and/or reliability. If it does you will need to disconnect them each time before logging on. Don't rely on other people not plugging a phone back in and dialling. It usually happens when there's only a tiny bit to go on that huge program you have been trying to download for weeks. This is not an acceptable reason for murder. Hide the cords instead.

A better approach is to install a modem socket or automatic privacy switch. These should be the first connection on the line as it comes from the exchange. A modem socket allows the modem to disconnect all other equipment from the line. Some modems only disconnect one side of the line leaving the other equipment as a large capacitance to ground. This can still cause problems. A privacy switch will do the same job, but make sure it breaks both sides of the line.

Make sure there are no loose terminals, plugs or sockets anywhere in the phone line or connection to the modem.

Neatness can be a health hazard: Tying all the interconnecting cables and power cables into a single neat bundle running along the back of the desk can allow noise spikes on the mains to enter your modem cable and cause errors. Stay with a bird's nest, or keep cables well separated.

10 You must have a good quality line between your local exchange and your modem. The quality of the connection between your modem and the exchange is critical. It is one of the two key items which finally will determine the maximum speed at which your modem can operate, or whether it will work at all so I'll look at it in some detail.

The telephone network was designed to carry speech, and it does that very well. But speech is very forgiving. You can carry on a conversation even if there is a substantial drop in volume, or if the line is noisy. Just as you can across the room or face to face at a noisy party. The human brain tends to filter out unwanted information. modems on the other hand are very demanding. They need a quiet line and a steady signal.

The Australian telephone network has been built, expanded and refined over the last hundred years or so. More modern equipment has been installed when it was no longer economic to maintain the existing equipment, or when new facilities were to be offered. Coaxial cable or radio links replaced the open wire lines linking capital cities, and this in turn has been replaced with fibre optic cable. Manual exchanges and telephonists were replaced by automatic step by step exchanges. They were gradually replaced by crossbar exchanges, which in turn are being replaced by digital exchanges. Analogue circuits between exchanges have mostly been replaced by digital equipment. As mentioned above the network is now almost 100% digital.

Despite this, there is a very significant part of the network that has not been upgraded and is still analog, the cable network between your premises and your local exchange.

Think of the traffic analogy again. You have a clear 60 km/h road from the ISP to your local exchange. If you have a clear road from the exchange to your house you can continue to drive at 60 km/h all the way to your front gate. Not everyone has this luxury. The local council may have installed road humps or other measures that slow your speed. There may be 10 cm of water over the road or there may be children or animals running across the road. There may be so many potholes that you have to drive slowly to avoid them. In the worst case the council won't have enough money to remake the road for some years.

The condition of the wires in the cable can act like pot holes to slow you down. Unless the cables have deteriorated to the extent that speech is no longer possible there is little chance that the telephone company will spend money to upgrade. When they do, the cure may be worse than the disease.

There is still a lot of old to very old construction in the cables between the exchange and your home or business. Early cables were made from copper wire which had been insulated with a wrapping of paper. Two insulated wires were twisted together to form a pair and the required number of pairs were bundled to form a cable. This was wrapped in a layer of kraft paper, then a lead sheath extruded over it for protection. Modern cables use plastic insulation of the wires and a plastic sheath.

A cable's life was not a happy lot. Rats and termites chew holes in the sheath. Tree roots crush the cable. Thieves used to strip the lead sheath and sell it for scrap. Workmen used to damage the cables with their picks and shovels. These days they use a backhoe to do a thorough job. Once the sheath is damaged water gets in. Wet paper is a poor insulator, current flows between the wires and considerable heat is generated. I've seen cable where the paper insulation has been charred for several meters.

Low insulation resistance: When a small amount of moisture gets into a cable current can flow between the conductors. This usually shows up as a hissing or crackling noise. It can also happen in the older types of telephone wiring used indoors.

Crosstalk: There is always a small amount of coupling between adjacent pairs in a cable. Sometimes this is high enough so that you can hear other conversations.

Split pairs: It's sometimes hard to decide which two wires belong together as a pair. If one wire in each of two pairs is swapped in a joint both pairs will be unbalanced and subject to increased cross-talk and interference.

Multiples: Around the end of World War 2 there were relatively few private telephones. When new cables were installed it was common practice to run a cable down each side of the street with sufficient capacity to allow one phone for each house. The street cables were connected to a cable pillar. Large size cable was in short supply and expensive. If it was predicted that there would be no more than one phone to every four houses in the next five years or so a smaller capacity cable was run between the exchange and the cable pillar. Each pair of wires in the exchange cable was connected to a pair from each street cable. When a telephone was connected to a pair in one street, the pairs in the other streets were not disconnected.

It is likely that some of this construction is still in existence. Although it is fine for voice it can pick up noise, and the multiple pairs can affect the impedance of the line.

Dry joints: In older construction the joints between two lengths of cable were made by stripping the insulation off, twisting the two bare wires together and slipping an insulating sleeve over the joint. Any small amount of oxides that formed in the joint were burnt off by the ring voltage and the current flowing in the line during a call. Sometimes this can produce a hissing or cracking noise in the telephone. It's annoying during speech but will cause errors and reduce the speed of a modem connection or stop it working altogether. Dry joints can also cause distortion of the analog waveform causing errors.

If you move to a new address: You may very well find out that your existing modem works better at the new address, or the same, or worse or not at all. A different make or model of modem may get you working again or the line may be so bad that you are stuck with a low speed connection. If high speed is absolutely vital it might be worth making a number of calls from the new house to your ISP(s) before you buy or rent.

In some cases, the high speed connection will not work at all, even though your exchange is digital. Just because your mate over the road can connect reliably at 50,000 bit/s doesn't mean that you can too. He may be connected to a different piece of cable.

Loading Coils: You will see these referred to frequently in American magazines. They are small inductance coils that are connected in series with the telephone line at 1830 m intervals to reduce the loss of high frequencies. However, they limit the highest frequency which can be transmitted to about 3400 Hz. This may prevent the use of the higher modem speeds. There are very few loading coils left in use in Australia, and are only likely to be found on very long lines between the exchange and customer.

Pair gain equipment: - Integrated and non-integrated RIMs: Cable pairs are an expensive and scarce resource. Upgrading an existing cable, or adding a new cable back to the exchange is not a trivial operation. Telstra and its predecessors have tried a variety of ways of providing the additional pairs quickly. This includes using a party line with two or more subscribers sharing a single pair, multiplexers of various types and pair gain systems. Currently Telstra is using RIMs - Remote Integrated Multiplexers.

Each RIM handles up to 480 lines and connects to the exchange via a fibre optic cable. There are two different types of RIMs in use, the integrated and the non-integrated. Both types of RIM convert each customer's lines to a standard 64 kbit/s PCM channel and multiplexes these into a single high speed digital stream.

The fibre optic cable between the exchange and an integrated RIM connects directly to the digital switch bypassing the line cards. The integrated RIM effectively moves the exchange closer to the customer. It should allow the use of 56k modems, but this will depend on the quality of the digital-to-analog converter and filter in each channel. You may also need to choose a modem that has been optimised for short lines if you are close to the RIM.

The operation of a non-integrated type of RIM is quite different. Instead of connecting directly to the digital switch the fibre optic cable connects to a terminal, effectively a second RIM, at the exchange. The analog side of the terminal has one pair of wires for each pair connected to the remote unit. As far as the exchange equipment is concerned these pairs are treated as if they were copper pairs directly to the customers. The use of a non-integrated RIM violates the requirement that there be a single digital-to-analog conversion, and that the D/A converter be directly controlled by the equipment at the ISP. As a result, a non-integrated RIM prevents the use of 56k modems.

The older pair gain systems, multiplexers, etc. also demultiplex back to pairs connected to the MDF. In all cases there is no continuous copper pair between the subscriber and the MDF. Most of these systems will also preclude the use of 56k modems.

I have been told that there are about one million lines already connected to RIMs that are part of Telstra's Future Mode of Operation (FMO) and the number of lines connected is growing. There is an unknown number of lines connected through other pair gain systems.

There are reports in Internet newsgroups that modems that have previously operated at speeds of 50,000 bit/s on copper pair will only connect at 28,800 bit/s when the service is transferred to some types of RIM. These modem users are extremely unhappy and very vocal. It is all very well for Telstra to say they are only required to provide a 2400 bit/s connection. The reality is that the customers have lost the high speed access they had become used to. They are angry and upset.

Concentrators: These are really a tiny exchange moved out close to a group of customers. They are designed on the assumption that only a few customers will be using their phones at the one time. Therefore, there many less lines are need back to the parent exchange. This approach fails when many of the customers make lengthy calls.

11 Quality of the design and construction of the modem. This is the second key item which will finally determine the maximum speed at which your modem can operate, or whether it will work at all on each dial up connection. It all depends on the quality of the design and construction, and how well it matches the characteristics of the telephone line.

The modem has a number of important functions quite apart from converting the digital stream from your modem into the line signals and converting the signals incoming from the line into data for your computer. These include:

• Loop the telephone line to tell the exchange it is connected.
• Disconnect all equipment beyond the modem.
• Separate the two directions of transmission. It is essential that the least possible amount of the transmitted energy be allowed to get back into the receive side where it would interfere with the received signal.
• Measure the frequency response of the line and provide compensation in both directions.
• Continuously monitor the line quality, the error rate and the number of retransmissions.
• In conjunction with the equipment at the ISP adjust the line speed as needed to get the best overall throughput.
• Carry out error correction on the received data.
• Carry out compression and decompression of data as required.
• Keep information on a wide range of parameters which can be used to generate a report for the user on demand.

Because of the very wide range in voltage of the input signal from the line the distortion must be kept at a very low level, generally less than 0.2%. Distortion can be caused by semiconductors or by saturating or overloading the input transformer. This is a particular problem with the miniature PCMCIA card modems intended for use with laptops. At low frequencies the PCMCIA transformers can cause distortion of up to 1.0%

It appears that some models or makes of modem work very well on short or medium length lines, but not on long lines. Others work well on long lines but not on short lines. This is one of the reasons you may have to try several different modems before you find the best one for your telephone line.

There is no doubt that manufacturers could produce modems that will perform reliably over a wider range of line conditions if they were prepared to spent the time and the money to develop and build them. However, this is not likely to be an economical proposition with so many people looking for a $100 modem to go with their $10,000 computer. Yet it is your modem that controls the initial testing of the connection, determines how well it can match the line conditions and sets the speed and other operating parameters.

DIL signal - Digital Impairment Learning. During the hand shaking sequence your modem asks the ISP to send a DIL signal. Your modem uses this signal to try and locate any extra A/D and D/A conversions, robbed bit signalling, conversion between A-law and mu-law and any other problems in the digital part of the connection.

Just how well your modem handles a particular digital impairment will depend on the design of the DIL signal, and how your modem analyses the DIL probe. Since K56flex and x2 use different methods the V.90 specification has left this part up to the manufacturers. This will probably cause a significant difference in performance on difficult circuits.

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:

Q1. I've bought a new 56k modem and it connects at less than 56,000 bits/sec
This is normal. As long as you are connected to a reasonable quality telephone line your maximum connection speed will generally be between 42,000 and 52,000 bit/s from the ISP to you, and 33,600 bit/s from you to the ISP. The difference is deliberate, and is part of the V.90 specification. For more details see the section "How do 56k modems work?"

Q2. If the maximum connection speed cannot be faster than about 52,000 bit/s why are they called 56k modems?
It's generally agreed that this was a bad marketing mistake as it gives the impression that the modems are capable of connecting at 56,000 bits/sec.

Q3. I've bought a new 56k modem and it's slower than my old 33,600 bit/s modem.
This is one of the most common complaints in the newsgroups. Your new modem may be a poor match to the telephone line. The set up may not be correct. You may be trying to force it to run at too high a speed on a noisy line so that there are many more errors and retransmissions.

Q4. My new 56k modem won't connect at all from my house, but connects at 48,000 bits/sec from my friend's house.
Both 56k and V.34 modems are very sensitive to the quality of the telephone connection. It is very likely that you have a very poor quality line from your exchange, there is serious interference or there are too many other pieces of telephone equipment connected to the line as well as your modem.

Q5. I am thinking of upgrading to a 56k modem. I currently have a 33.6 kbit/s. Will there be a noticeable difference? I was told that the current telephone lines can only transfer data at a 33.6 kbit/s rate.
This is partly true. 56k modems still send data from you to your ISP at a maximum of 33,600 bit/s. As shown earlier, 56k modems take advantage of the digital telephone network to send data from your ISP to you at speeds approaching 56,000 bit/s. If you have a poor quality telephone line you will not get a high speed connection.

Q6. What sort of modem should I buy?
The one that will do what you want it to in all the places you want to use it. That may sound trite. However, there are big differences in the characteristics of cable pairs and the ability of modems to cope with these differences. You may have to try several makes and models at each location until you find one that best suits your needs. Even then you may have to accept lower performance at some sites. It's a bit like buying shoes, you keep trying until you find one you are comfortable with.

If you are regularly getting connection speeds between 28,800 and 33,600 bit/s none of the 56k modems will double the speed nor halve the download time. If you will be satisfied with a modest increase in speed of say 30% to 50% (42,000 to 52,000 bit/s) then go ahead and try one or more brands of modem until you find one that works reliably on you present telephone line. Remember that if you move house the new modem may not connect at greater than 33,600 bit/s at the new location.

Make sure you know which standard (K56flex, x2 or V.90) is used by the ISPs you intend to connect. They are not interchangeable.

If you are buying a new computer as a package you may be better off buying the modem separately from one of the stores that specialises in modems. There are so many things that can go wrong with modem communications that you can usually save substantial time and money by dealing with a specialist who can help you get the best performance from your modem and computer.

In either case: Get a written guarantee that the full purchase price will be refunded if the modem does not work satisfactorily on your phone line. This cannot be stressed too strongly.

Alternative warning:
DO NOT BUY A 56K MODEM WITHOUT A WATERTIGHT MONEY BACK GUARANTEE IF IT FAILS TO CONNECT REGULARLY AT HIGH SPEED.

If you cannot get the modem working at a reasonable speed within a few days don't wait for a miracle. They don't happen to computer equipment. Take it back in as new condition complete with all the handbooks, cables, packing, etc and get your money back.

If you are buying a current K56flex or x2 modem get a written guarantee that it uses "Flash ROM" to store its control program so it can be upgraded to the V.90 standard, and that the retailer can arrange to have this done for you if you wish. Expect to pay a fee for the upgrade, whether you do it yourself or have it carried out on your behalf. It is, after all, an upgrade to a more expensive model.

Try and buy a modem which has Flash ROM with a large enough capacity to hold the firmware for both V.90 and either K56flex or x2. This will allow you to swap between the two. Anecdotal evidence suggest there are times and conditions where one is better than the other.

It is also important to be able to upgrade the control program in the modem. Manufacturers are continually changing the control programs in their modems to improve the performance. The upgraded software is often available for down loading from their web sites.

Do not buy a V.90 modem, or upgrade your present modem to V.90 until all the ISPs you connect to offer V.90 connections. A V.90 modem may not connect as a K56flex or x2 modem if your modem and the ISP modems do not use the same technology. Otherwise it will drop back to a 33,600 bit/s or lower connection. This is a further reason for buying a modem with enough flash RAM to store the firmware for both V.90 and either K56flex or x2.

Try and avoid the HSP and win-modems. Apart from the extra load they place on the CPU they need special software. New versions of software may not be readily available if you change your operating system.

Q7. What else should I look out for when buying a a high speed modem.
There are considerable differences in the performance of different makes of modems. It seems that if you have a first rate line almost any modem will do. If you have a poor quality line from your exchange you need one of the modems that has been designed to cope with this.

Be wary of reports in computer magazines. From what I have read, most of the tests have been conducted in somewhat artificial circumstances.

Q8. Will my high speed modem work from a hotel room. Can I connect a high speed modem to my phone at work? The phone is an extension from the company's PABX.
CAUTION: I have recently read of cases where connecting a modem to a digital PABX extension has killed the modem. I have no other details at present. To be safe, RTFM - Read The Flaming Manual, or failing that get an assurance from the modem and the PABX manufacturers.

If the hotel or work uses a digital PABX with direct digital links to the local exchange, and the numbers are part of the local exchange number range then you may get high speed connection. In other cases you should be able to connect as a V.34 modem but not necessarily at 33,600 bit/s. If the PABX uses another form of digital coding such as ADPCM (Adaptive PCM) or Delta coding you are likely to get very poor connections.

Q9. My friends all have 56k modems, yet when we dial each other we can only connect at 33,600 bit/s or less. Why can't we connect at higher speeds.
56k modems are designed to receive at speeds up to 56,000 bit/s and to send at speeds up to 33,600 bit/s. Even if you connect the modems back-to-back with a short length of cable you cannot do better than 33,600 bit/s. You may get even less or very poor performance if the modems are expecting at least a kilometre of cable between them.

Q10. My modem carries an "Austel or ACA Approved" sticker, but it does not perform very well.
The regulatory authorities do not test or guarantee how well the equipment meets the claims made for it. Their sole interest is to make sure that the equipment does not put any voltage, current or signal onto the telephone line that will injure staff, damage telephone equipment or cause interference to other users.

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:

Curt's High Speed Modem Page: http://www.net-boy.com
Loads quickly. Huge amount of valuable information. Regularly updated.

John Navas: http://www.aimnet.com/~jnavas/modem/faq.html
John Navas's 28000 - 56k Frequently asked questions.

Geoffrey Welsh Home Page: http://home.inforamp.net/~crs0794/
Another excellent source for information.

General 56k site: http://www.56k.com
Valuable information and help with K56flex, x2 and V.90, including problems when upgrading to V.90

56k=v.Unreliable: http://808hi.com/56k
Detailed information on the downside of 56k, Problems and solutions.

Who are we going to sue: http://www.infoworld.com/cgi-bin/displayNew.pl?/metcalfe/980413bm.htm
Dissatisfied users of American telephone networks.

General V.90 site: http://www.v90.com
Information and help on V.90

IBM Help Library: http://help.ibm.net/helplib/
Series of helpful papers including one on line noise and its effects.

Compaq Computer Corporation Communication Products Division: http://www.compaq.com/support/techpubs/whitepapers/prt005a0798.html
Detailed information on problems caused by lines and exchange equipment.

Australia: aus.comms
Whole world but mainly USA: comp.dcom.modems

Acknowledgments.

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".

Definitions

56k - short for K56flex, x2 and V.90

Analog Telephone Network - Speech, data and fax signals are carried from the sender to the receiver as an analog signal. The old network was designed for a maximum loss of 30 dB between any two phones. In simple terms, only 0.1% of the signal power leaving the first phone arrived at the second. The rest was dissipated in the lines connecting the two.

A/D - Analog to Digital - An analog signal such as speech or music is sampled at regular intervals and encoded into a series of digital codes which can be stored in a computer, on a compact disk or sent over a digital network.

Analog Transmission - Information flows as a continuous signal. Human speech is an example. Analog signals grow weaker the further they travel and pick up noise and interference. They can be amplified, but the noise and interference is amplified with them.

Bandwidth - One of the most misused and misunderstood terms in digital transmission. Bandwidth is defined as "the difference between the upper and lower frequencies in a band" (Macquarie Dictionary - first edition). The bandwidth of a telephone circuit is generally accepted as about 3,100 Hz although it will often exceed this in practice. Modem designers have gradually pushed the digital capacity of this limited bandwidth from 300 bits/sec to 33,600 bits/sec. In other word, bandwidth is irrelevant when talking about the rate at which digital data is transmitted.

Nobody talks about the bandwidth of a freeway. They talk about its capacity and measure this in vehicles per hour. You should distrust everyone who talks or writes about the "bandwidth" of a digital connection or the "Information Super Highway" when they really mean it's capacity to carry data. If they do not understand the difference between bandwidth and capacity the rest of their information is likely to be suspect.

Codec - short for encoder/decoder - A combination of an encoder and a decoder operating in opposite directions of transmission in the same equipment. It takes an analog signal and codes it into a digital signal suitable for transmission over the digital network. At the far end, another codec decodes the digital stream into a replica of the original analog signal. Band limiting filters which follow this codec remove unwanted components from the analog signal and are the primary restriction on useable bandwidth.

Digital Pads - Basically a digital volume control. They consist of a series of look up tables of PCM codes stored in memory, one table for each step of gain or loss required. Each incoming PCM code is used to address the specified table for the required change in gain and the equivalent PCM code read out. Equivalent to the resistor networks or "pads" used in analog systems to control the gain.

Digital Pads are an Australian invention, but fortunately have not been used in this country in the past. The position is changing, and digital pads are used in several of the newer telephone exchanges. Because the pads change the PCM codes their use is likely to reduce the range of codes available, and hence the maximum data speed.

Digital Telephone Network - The telephone system uses Pulse Code Modulation (PCM). A codec at your local exchange samples your voice, modem or fax signal 8,000 times a second and the loudness is converted to an eight bit code. These codes are sent through the telephone network and converted back to a replica of the original signal by another codec at the distant exchange.

In a fully digital network all exchanges and links between exchanges are digital. Therefore the codec at your local exchange is connected to the codec at the local exchange of the number you are calling by a digital path, giving the best possible connection between the two subscribers.
D/A - Digital to Analog : The decoding of the digital codes to the equivalent analog signal. The sound card in your computer uses a D/A converter.

Digital Transmission - Information flows as a series of pulses. Morse code is an early form of digital transmission. Digital signals also grow weaker the further they travel and also pick up noise and interference. However, the original signals can be regenerated at intervals to recover the original digital signal free of noise and interference.

ISP - Internet Service Provider such as Melbourne PC User Group.

MDF - The Main Distributing Frame - The cable pairs from the customers are connected to terminals on one side of the MDF. The pairs from the exchange equipment are connected to terminals on the other side of the MDF. Now any telephone number can be connected to any cable pair by running a  jumper  - a pair or wires - between the terminals for the number and the terminals for the desired cable pair.

Mixed Analog and Digital Networks - If the telephone circuit isn't completely digital the original signal will be converted to and from PCM several times. Each conversion introduces small errors or distortion which is not noticeable in speech. Lower speed modems can cope with the distortion, but it upsets the operation of higher speed modems. More than one conversion stops 56k modems cold.

Modem - short for modulator/demodulator - A combination of a modulator and a demodulator operating in opposite directions of transmission in the same equipment. The transmit half of your modem modulates a carrier signal with the digital data from the computer putting it into a form suitable for sending over an analog network. At the far end the receive half of another modem demodulates the incoming signal to recover the digital data.

Robbed Bit Signalling (also called bit stealing) - This is used in some American telephone networks to pass control information from one exchange to another. They use the least significant bit of a PCM codes in each sixth frame. The robbed codes cannot be used for data and must be detected and ignored.

Hopefully we will never have to contend with robbed bit signalling. However, once other telephone companies set up competing networks they may not follow the existing Australian practice.

UART - Universal Asynchronous Transmitter Receiver - This is the integrated circuit that converts parallel data to and from serial data to interface the computer to the modem.

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 |
|Level+---------------------------------------------------+Atten|
|    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 |
|Level+---------------------------------------------------+Atten|
|    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.

1. In spite everything you have seen in the press and magazines, the current V.90 standard is not the final version.

2. The client modem (your modem) controls the testing carried out during the setting up of a connection. The ISP equipment (the server) provides the test signals as and when requested by your modem.

3. The server must be able to provide all the various test signals that the client modem needs to decide how to get the best possible connection.

4. Each individual or group of modem manufacturer(s) has their own ideas about the best test signals to use.

1. You cannot buy a V.90 modem, nor upgrade an existing K56flex or x2 modem to V.90 in the certain knowledge that it will give a stable high speed connection to any ISP you choose.

2. The firmware in your modem is not referred to as beta code, although in many cases it seems like it. Modem manufacturers continue to make revised firmware available to fix known problems or provide better performance. Read the detailed information supplied with the new firmware before installing it.

3.DO NOT install the new code unless it is certain that it will improve performance. The newsgroups are full of messages such as "I flashed my modem to the new version and the performance is much worse. How do I put it back to what it was before?"

4. The manufacturers of the server equipment are also continually looking at ways to improve the performance of the equipment, and to make it compatible with a wider range of modems. Each new version of the beta code they release will improve the performance for some users, have little effect for other users, and may reduce the performance for others. We have seen this with the Club's V.90 line recently, and I understand that we have reverted to the previous version of the firmware.

1. If V.90 works for you, use it and enjoy it, but have some compassion for those not so fortunate.

2. If you are having problems with V.90 but you can still connect at a lower speed, try and resolve them by


(a) Searching the web pages listed in the Recommended Reading section.

(b) Using the search engine http://www.dejanews.com/ which archives all the material which appears in the newsgroups. Search this area before you submit questions to the newsgroups. If yours is a common problem there is a high probability that the answer is already there.

(c) Asking for help in comp.dcomp.modems or aus.coms giving exact details of your problem together with the make and model of your modem and the current version of the firmware installed. Your modem handbook will tell you how to extract this information from your modem.

If all else fails, just use the modem at whatever speed it chooses, and hope that a different version of firmware for either your modem or the ISP server will give you better results.

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
e-mail
Alan's Site

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
All writings on this page are Copyright Alan Fowler ©1998, 1999, 2000, 2001
Writings used on this site with the express permission of the author
All trademarks respected as the property of their respective owners