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56K Modems – an overview How fast can a modem go?
The speed at which a modem transmits and receives data is expressed in terms of bits per second, or with modern high-speed modems, thousands of bits per second, abbreviated to Kbps. Modem speeds have increased over the years from a starting point of around 300bps to today’s 33.6Kbps and even 56K devices. When we start looking at why there is a limit to the speed at which a modem can run, it is important to understand a little about what it is that a modem actually does and what the telephone network does. Information inside your computer is in digital format: all of the data is stored as a series of zeroes and ones. Normal telephone lines are analogue; they were designed to transmit voice data as a series of constantly changing electrical peaks and troughs of different frequencies and varying loudness to represent intelligible speech. The term ‘modem’ actually stands for MOdulator/DEModulator, this being the process by which the digital signals from a computer are transformed into analogue signals, by a technique known as modulation, for transmission over telephone lines that were originally designed for the transmission of voice rather than digital data. A modem, however, has to make sure that the data it sends can be received correctly, despite the existence of external influences, such as noise on the line. Finally, at the receiving end, the analogue signal has to be converted back to the original digital data by the technique known as demodulation. It is this conversion process of modulation and demodulation at each end of the telephone connection, coupled with the limited bandwidth (the amount of information that can be carried at any one time) and the inherent noise of an analogue telephone line that imposes a limit on the maximum speed at which a modem can reliably transfer data. In the late seventies, William Shannon of Bell Laboratories came up with Shannon's Law, which set a theoretical limit for modem speeds over analogue circuits at about 34Kbps. Current 33.6Kbps V.34 modems push analogue telephone lines to the limit, and represent the ‘state of the art’ for conventional modems. In future, increases in dial-up bandwidth will depend on all-digital circuits (see ‘Into the future’ below). At one time it was thought that 33.6Kbps would be as fast as you could get - to get higher speeds, you'd need modems using totally different technologies that just wouldn't work over standard phone lines. The advent of 56K modems has changed that. You can now have a faster connection using your existing telephone line and new modems that work just like your old modem, but faster, although there are some provisos. Today's telephone network is increasingly digital. In particular, the telephone connection between the telephone company and the Internet Service Provider (ISP) is often digital rather than analogue. Digital lines still have some inherent noise, and are still subject to performance limits, but they have less noise and a higher potential speed than the analogue variety. In fact, one of the few remaining types of analogue circuits is the local analogue loop that connects the normal household telephone to the telephone exchange. The actual telephone network that connects exchanges together is digital in nature, but is designed to connect these analogue local loops together. The network itself, therefore, has analogue-to-digital converters, situated at the exchange, to take traffic from the home telephone or modem and convert it for transmission across the digital network, and digital-to-analogue converters at the remote exchange to reverse the process for transmission to the remote user’s telephone or modem. Let’s look at what happens when two home users ‘A’ and ‘B’ connect their computers together using dial-up modems. The telephone network will first convert the analogue signal transmitted from user A's modem to a digital signal which will then be transmitted digitally from the telephone exchange in user A's area to the telephone exchange in the user B's area. At user B’s telephone exchange, the digital signal will be converted back to an analogue signal before being sent via analogue phone line to user B's modem. The digital-to-analogue conversion of signals received at user B’s exchange causes no problem to transmission. However, the telephone network's analogue-to-digital converter at user A’s exchange introduces noise problems (called quantization noise) as a result of which the maximum speed is therefore determined, according to Shannon's Law, at around 34Kbps. Where an Internet Service Provider has a digital connection to the telephone company, 56K technology takes advantage of the fact that the network has no analogue-to-digital conversions in the end-user's downloading (downstream) path, which effectively eliminates the quantization noise problems introduced by the telephone network. Several companies have created techniques that take advantage of the digital portion of the telephone network to achieve higher speeds than were possible with a purely analogue pathway. These new techniques treat the telephone system as a digital network that has just one analogue link between the user and the telephone exchange. The basis of 56K technology is different to that used in existing analogue modems. In particular, it depends completely on the existence of a half-digital connection to achieve its higher speeds, and can therefore only give its speed advantage over 33.6Kbps modems in certain situations. The most common of these is when you connect to the Internet via an Internet Service Provider (ISP) who provides 56K modem support, such as Infotrade. The reason 56K modems are able to break Shannon's barrier is that, strictly speaking, they aren't modems at all. They depend on digital connections from the telecom company, through the Internet Service Providers, and onto the Internet. In this digital world, Shannon's limitations no longer apply. This necessity for a half-digital connection does impose some fundamental requirements. Your Internet service provider (ISP) must have digital phone lines to the public switched telephone network (PSTN) and there cannot be more than one analogue-to-digital conversion in your local phone loop. Furthermore, there are two major 56K technologies that are non-interoperable: you have to buy a 56K modem that uses the same 56K technology used in your Internet Service Provider's modem. See ‘Not one standard but two’, below. If any of these restrictions apply to you, you can't use 56K modems to their full advantage, and your connection will be limited to V.34 speeds (a maximum of 33.6Kbps). A 56K connection is asymmetric: it's faster for downloading than for uploading. Downloading means receiving data from another computer (for example, viewing a Web page or receiving email). Uploading means sending data to another computer (for example, sending email or sending keystrokes and mouse clicks to a Web page). The download speed can be as much as 56Kbps, but the upload speed will be only 28.8Kbps or 33.6Kbps. Why the limit? When downloading over a 56K connection, the downstream data from the exchange to your modem is digitally encoded instead of modulated, and is changed from a digital signal to an analogue signal at the modem. Digital-to-analogue conversions are perfectly accurate, so downloading at speeds up to 56Kbps is possible. When uploading, data is sent in a modulated form and has to be converted from an analogue signal to a digital signal. Analogue-to-digital conversions are imperfect, so noise is introduced into the signal, resulting in a lower upload speed.
However, this lower upload speed isn't significant for most users. Most Internet surfers spend far more time downloading than uploading, and most of their uploading consists of little more than key strokes and mouse clicks. Many other high-speed technologies, e.g. ADSL and cable modems, (see ‘Into the future’ below) also have asymmetric rates. V.34 modems (28.8Kbps and 33.6Kbps) support asymmetric connections to compensate for poor line quality in one direction, but in this case the upload and download rates are usually very close to one another. The asymmetric nature of 56K technology does mean that it is not a panacea. In particular, it makes 56K impractical for any application where the amount of data flowing in each direction is roughly equal, for example in the case of video-conferencing. For those applications, ISDN is superior. Nevertheless, if surfing the Web is what you do most, it’s the download speed from the Internet to you that you’re going to benefit from. With standard modems you can normally connect any modem to another of the same speed with no problem because they follow international standard protocols, specified by an organisation known as the International Telecommunications Union (ITU). A protocol is simply the set of rules that the transmitting and receiving modems both abide by so that they can understand each other. For example, V.34 is the name of the standard that defines the protocol for 33.6Kbps modems to talk to each other. The first and most important thing you need to know about 56K modems is that there are two 56K protocols, and that these protocols are incompatible. These proprietary 56K protocols are known as x2™ from U.S. Robotics (now merged with 3Com), and K56flex™ from Rockwell and Lucent. Modem manufacturers who produce 56K modems will use one or the other of these two protocols. If you buy a modem that uses K56flex protocol, you won't be able to call a modem using the x2 protocol and get the full speed. You will, however, still be able to connect at 28.8 or 33.6Kbps, so in that sense you haven’t really lost anything compared with a 33.6Kbps modem. An ITU standard for 56K isn't likely until sometime during the second half of 1998 at the earliest. Which one of these will become the standard? In all probability it will be neither, but instead it will most likely be a combination of elements of both the current x2 and K56flex technologies. Until there's a standard, which one should you choose: K56flex or x2? One answer is to choose the one your Internet Service Provider is using or is going to use (or choose an ISP who provides support for both). Whatever you do, you should check that the modem you are going to buy will be upgradable - preferably by a free software program - to the final ITU-T 56K standard when it is established. That way your investment is protected for the future. If you were one of the first people to buy a 28.8Kbps modem, you may have discovered that most of the places that you called still used 14.4Kbps modems, so you could only connect at 14.4K. It's always been this way with modems: they can only connect at the highest speed supported by either modem, or by the quality of the telephone connection. Many people who buy high-speed analogue modems are sometimes surprised that they don't necessarily connect at the full 33.6Kbps (or 28.8Kbps) rated speed, and connections at 26400 or 24000 are common. Let’s look at why this is. The rated speed for a modem is the maximum at which the modem is capable of performing, given an optimum telephone connection. However, the modem is only one part of the communications link and it must be remembered, as discussed earlier, that it is the inherent noise of an analogue telephone line that imposes a limit on the maximum speed at which a modem can reliably transfer data. Modems will automatically select the highest speed at which this reliability can be maintained, which will frequently be less than its rated maximum. Because the part of the connection between your computer and the telephone company is analogue, 56K modems will still suffer from line noise. The upside is that with 56K connections, the link between the telephone company and the Internet service provider is digital, which has much less noise. Even if you can't get a 28.8Kbps or 33.6Kbps connect on your existing analogue modem, you may be able to get a higher connect speed with 56K. The technology used in 56K modems has two possible drawbacks. Firstly, for 56K to work, you cannot have more than one analogue-to-digital conversion between your home or office and the Internet service provider (ISP). If there's more than one, you can't use 56K, and your connection will be limited to V.34 speeds (a maximum of 33.6Kbps). If your office is on a PBX, you may also have problems getting speeds higher than 33.6 or 28.8K. Secondly, there are two major 56K technologies, K56flex™ and x2™, and these are mutually exclusive, so you have to buy a 56K modem that uses the same 56K technology used in your Internet Service Provider's modem, or choose an Internet Service Provider who offers both, such as Infotrade. Because of Federal Communications Commission (FCC) limitations, x2 is limited to 53Kbps in the U.S. To reach the speed of 56Kbps, an x2 modem would need to increase the power sent down the telephone line beyond a limit set by the FCC years ago. Note that because of its different technological approach, K56flex is not affected in this way. This isn't a major issue, as most people probably wouldn't connect at the full 56Kbps anyway, due to the telephone line constraints that prevent many people from connecting at 28.8 or 33.6Kbps (see ‘Modem connection speeds’). In any event, the difference between 53Kbps and 56Kbps is hardly of great significance. The whole concept of 56K technology depends on the ISP having digital phone lines connecting their modems to the telephone company. The digital lines are much clearer than analogue phone lines, so higher connection speeds are possible. And another big advantage of 56K is that although the ISP has to have a special digital connection, you can continue to use the local analogue phone line you already have. Other technologies already exist that are faster than 56Kbps. The most popular is the Integrated Services Digital Network (ISDN) which is a completely digital circuit. The downside to ISDN is the cost. Digital leased lines are also available, called T1, T2, T3 or T4 - these are, however, very expensive. Entering the late stages of development is another technology, Asymmetric Digital Subscriber Line (ADSL). Whilst ISDN is limited to 64kbps, ADSL uses speeds of 640kbps to upload, and speeds in excess of 6Mbps (mega-bits per second) to download. Also entering the market are "Cable" (cable phone/cable TV) companies and even power companies. Norweb, for example, recently announced technology that allows data to be transferred over electricity power lines into the home at speeds of over 1Mbps (megabit per second).
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