Building your PC:
Network Card 1 — modems
Introduction
Network cards are computer peripherals that allow one machine to communicate directly with another. This could be through the medium of telephone wires, twisted-pair ethernet cables or radio-frequency signals. Communications can be either between local machines (a Local Area Network or LAN) or remotely with servers and distant computers (a Wide Area Network or WAN) or any combination of these. A home network is an example of a LAN and communication with your ISP is an example of a WAN.
Page Map |
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| Modem Cards | Acceleration of Modem Connections |
| History | Broadband Modem |
| Telephone Modems: The Future | Cable Modem |
| Winmodem | Other Modems |
Modem Cards:
A modem is a device that modulates a carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. The name modem itself being a portmanteau work constructed from its function as a modulator and demodulator. The most familiar form of modem being the computer peripheral that converts the digital 1s and 0s of of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone System (POTS), and once received on the other side, converts those sounds back into 1s and 0s. Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or "bps".
History:
Interestingly modems were first introduced in the 1950s to connect terminals located at various airbases, radar sites and command-and-control centers to the SAGE director centers scattered around the US and Canada. IBM was the primary contractor for both the computers and the modems used in the SAGE system. During the 1950s in the US AT&T maintained a monopoly on the use of its phone lines, only allowing AT&T-supplied devices to be connected to its network. In response to the growing number of computer users they they introduced both a wideband service and a low-speed modem running at 200 baud in 1958. In 1960 the mane Data-Phone was introduced by them (this replaced the device's earlier name of 'digital subset'. The 202 Data-Phone was a half-duplex asynchronous service that was marketed extensively in late 1960. In 1962 the 201A and 201B Data-Phones were introduced. They were synchronous modems using two-bit-per-baud phase shift keying (PSK). The 201A operated half-duplex at 2000 bps over normal phone lines, while the 201B provided full-duplex 2400 bps service on four-wire leased lines, the send and receive channels running on their own set of two wires each. As a result of AT&T's monopoly over what could electronically be connected to their phone lines a market for modems that could be mechanically coupled to the telephone grew. This led to devices that could be connected to the handset, known as acoustically-coupled modems. Particularly common models from the 1970s being the Novation CAT and the Anderson Jacobson.
In December 1972 Vadic introduced the VA3400 (four years after AT&T's monopoly was broken). This device provided full duplex operation at 1200 bps over the dial network and included an asynchronous interface so that accept data was clocked by start and stop bits rather than a modem-supplied clock, which allowed a dial connection of 1200 baud to be maintained to remote computers.
The next major advance in modems came in 1981 when Hayes Communications released their Smartmodem. Though essentially the same as earlier AT&T modems the Hayes device was attached to a small controller that let the computer send commands to it to operate the phone line. The command set included instructions for picking up and hanging up the phone, dialing numbers, and answering calls. Even today the basic Hayes command set remains the basis for computer control of most modern modems. This advance did away with the need for a telephone and hence an acoustic coupler to connect it to the telephone. The Smartmodem was the first device that could be connected directly to the telephone line.
Well into the mid 1980s modems remained limited to 300 and 1200 bps however, by the late 1980s modems were being pushed to rates up to 19,200 bps, the maximum the phone lines could handle. However, operations at these speeds were pushing the limits of telephone lines and were generally very error prone. This led to the introduction of error correction systems built into the modems, made most famous with Microcom's MNP systems. The addition of compression along with error checking enabled the effective data transfer rate to be pushed beyond that which could be supported by the telephone lines alone. Another common feature of these high-speed modems was the concept of fallback, allowing them to talk to less-capable modems. During the call initiation the modem would play a series of signals into the line and wait for the remote modem to "answer" them. They would start at high speeds and progressively get slower and slower until they heard an answer. Thus two USR modems would be able to connect at 9600 bps, but when another user with a 2400 bps modem called in, the USR would "fall back" to the common 2400 bps speed.
Starting in the late 1980s modems were produced that could counteract a 'feature' of telephone systems. Normally the phone system sends a small amount of the outgoing signal, called sidetone, back to the earphone, in order to give the user some feedback that their voice is indeed being sent. However this same signal can confuse the modem which cannot determine whether the signal is being received locally or remotely. The way around this problem was 'echo cancellation'. By using the sidetone's well-known timing, a slight delay, it was possible for the modem to tell if the received signal was from itself or the remote modem. This led to the v.32 standard and the first modems that could both send and receive signals concurrently at 'full speed'. In 1991 Rockwell released a new modem chipset that supported v.32 and MNP as well as the newer v.32bis and the higher-compression v.42bis standards and included 9600 bps fax capability.
During the early 1990s, as modem speeds increased further there was a move from specialized modem chipsets to commodity 'flexible' parts that could be re-configured if needed. Rapid commoditization of modems drove almost all the original vendors out of the market and prices collapsed.
The next advance came with the rapid introduction of digital telephone systems during the 1990s. Digital lines are based on a standard using 16-bits of data for every voice sample, sampling 4000 times a second, for a total data rate of 64 kbps. However, many systems use in-band signalling for command data, inserting one bit of command data per byte of signal, and thereby reducing the real throughput to 56k. With some suitable probing, it is not difficult to discover whether the line is digital all the way, and in that case, use as much of the bandwidth as possible. This led to an effort to build a new 56 kbps standard, originally known as V.pcm (PCM referring to the pulse code modulation used in digital telephony), and later as v.90. The standardization effort started around 1996. Today almost every modem sold supports the v90 standard.
Telephone Modems: The Future
A standard modem consists of two functioning parts: an analog section for generating the signals and operating the telephone and a digital section for set-up and control. In reality both components are integrated into a single chip. Though, in terms of the modem's operation the division remains in effect. This division is reflected in the two modes that a modem can be in either: data mode where information is sent to or from the computer over the telephone lines; and a command mode n which the modem listens to the data from the computer for commands, and carries them out. One drawback of this mode of operation is that it's not really possible for the modem to know whether a string it has been passed is a command or a fragment of data. Very strange things can occur when these two strings become confused and the only way to sort the problem out is to re-initialize the modem. Almost all modern modems also act as a fax. Software running on the host computer can convert any image into fax-format, which can then be sent using the modem. For many users today the fax component of a modem is the only element they actually use.
Winmodem
At this point I should at least mention the Winmodem. This is a stripped-down modem for Windows-based machines where a number of the tasks traditionally handled by hardware are dealt with by software instead. In most cases the computer's built-in sound hardware is used to generate the tones normally handled by the analog portion of the modem. The main downside of a Winmodem s that the software generating the modem tones is not that simple, and the performance of the computer as a whole often suffers when it is being used. This can be a real problem if you're gaming on-line and using the modem to communicate with other gamers. Despite the advent of Broadband (see below), for the moment at least modems remain the most popular means of accessing the internet (though this situation probably won't last much longer) and the traditional 56k modem is now rapidly losing ground to broadband and/or cable modems.
Broadband Modem
Broadband, a technology more accurately described as Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional modem can provide. The distinguishing feature of ADSL is that it is asymmetric this means that data flow in one direction is greater than in the other. Thus the download speed is significantly higher than the upload speed. At the moment download speeds can reach 8Mbps whilst upload speeds are typically 512kbps. The next generation of ADSL, ADSL2+ will offer speeds up to 25 Mbps for spans of up to 1.5km from the exchange. ADSL usually uses the frequence range of 25.875–138kHz for upstream communication and 138ᄄkHz for downstream communication. As a result ADSL lines usually need filters to separate the normal telephone line from the frequency bands generally reserved for ADSL communication thus reducing cross-talk.
The computer is connected to an ADSL line via an ADSL (sometimes known as Broadband) modem. This functions to convert the digital signal from the computer into a high-frequency analog signal that can be transmitted down the telephone line. However, they are far more advanced than traditional telephone modems in that they are capable of modulating/demodulating hundreds of channels simultaneously. Many ADSL modems also incorporate the functions of ethernet routers so that they can be used as the basis of a fast home network.
Acceleration of Modem Connections
Recently companies have arisen which pre-cache internet content which can accelerate modem data transfer rates for website access to the equivalent of 512kbps. Though it will not speed-up email connections or other data uploads it is potentially useful if you use the internet as an information resource but do not use it frequently enough to warrant the cost of ADSL or other broadband technologies. Once such company is OnSpeed, which can be accessed here:
Cable Modem
A cable modem is a specialized form of broadband modem designed to modulate a data signal over cable television infrastructure. Cable modems are primarily used to deliver broadband Internet access, taking advantage of unused bandwidth on a cable television network. Cable-based internet access is generally more reliable than ADSL, however it does suffer the drawback that users in a given neighbourhood share the available bandwidth provided by a single coaxial cable line. Therefore, connection speed can vary depending on how many people are using the service at the same time. (This is also true of DSL, however there are usually more connections between the users and telephone exchanges than found in cable systems so the drop-off in speed with multiple users is smaller for DSL). Many cable internet providers are reluctant to offer cable modem access without tying it to some kind of television subscription package.
Other Modems
New types of modems, utilizing additional delivery technologies are now beginning to appear on the market. One of the main ones is a Satellite modem, where the modem itself is used to establish data transfers using a communications satellite as a relay. These all use the Digital Video Broadcasting standard as a basis and there are two main types: one-way satmodems download data via the satellite but use a telephone or cable system as the uplink path. Two-way satmodems use a satellite-based uplink channel as well so they can be used as stand-alone products. Satellite modems do not currently offer the data throughput of ADSL and cable modems but they do offer broadband internet communications for those for whom other forms of broadband communication are not available.
The latest development in modem technology is the power-line modem. Here a modem is used to convert a binary data stream from the computer into a sequence of signals with predefined characteristics (frequencies, levels) that can be overlaid onto the cables of the standard domestic power grid. The data are then communicated using the power line infrastructure. Technically this is challenging as power lines are generally noisy and losses can be great but the first products based on this technology are now coming to market.
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