Cable Modems Essay, Research Paper
In recent years the Internet has become extremely popular. As web pages become more complex and software downloads become larger the demand for faster speed modems is becoming a major issue in the computer market. With the advent of the cable modem the speed of “surfing” the Internet is drastically increased. In comparison to the 56k modems, cable modems are up to 100 times faster, at a rate of 500Kbs per second. Also, there is no log-on time because cable modems have a static connection to the Internet. A cable modem is a device that allows high-speed data access using cable TV networks. These cable networks are designed in a tree and branch architecture. The head of the network receives signals from a satellite. The signal is then sent throughout the network to the cable subscribers. Because the signal dissipates over long lengths of cable, many amplifiers are needed to increase the strength. Recently fiber optics have been employed to help cable networks. Fiber lines require fewer amplifiers and can transmit a higher bandwidth. This results in faster up and downstream transmissions. Cable networks were originally designed to transmit data in one direction only. The intent of the networks was to send data from the head unit to the individual subscribers. This data comes in 6MHz frequencies that range from 42MHz to 750MHz. Each channel requires one 6MHz frequency to transmit. This type of data transfer is known as downstream transmission. New applications now necessitate upstream data transfer, data sent from the subscriber to the head unit. Features that the new cable modems offer consist of operations such as two-way video conferencing and two-way audio applications such as telephony. Also, uploading other information over the Internet, such as software, requires 2 way transmissions. In order to accomplish this signals uploaded are transmitted within 5 MHz to 40 MHz. This allows for 4 different channels. The problem with the frequencies used in upstream transmissions is that there are many interference problems. Cable modems run a system known as LAN, local area network. The LANs that cable networks use are different from traditional LANs. In LANs the Ethernet uses Carrier Sense Multiple Access with Collision Detection, CSMA/CD. This type of LAN has to detect when a media is not in use in order to transmit data. Stations detect collisions by comparing the transmission they sent against the signals it receives. If a collision is detected, each sending station waits a random time and tries again. Even though this system is less efficient and reliable then a Triple Division Multiple Access, the system works well as long as the media is not heavily used in a way that causes numerous collisions. The cable network’s topology limits CSMA/CD’s efficiency in detecting collisions and avoiding them. Because cable networks are such large LANs the propagation delay is much bigger. This is because it takes longer for a computer’s transmitting signal to reach another computer. This may cause a computer to think it is safe to start transmitting when another transmission is on its way to a collision with the second computers transmission. The folded topology also hinders the effectiveness of Carrier Sense Multiple Access with Collision Detection. All transmissions are sent from the network stations on the upstream channel to the head end and back out the downstream channel. The network stations send upstream and listen downstream. They do not listen on the upstream channel or send on the downstream channel. Therefore, signals must travel all the way to the head end and back before another station notices that the bus is in use. This makes a physical separation of a few hundred yards into a long signal propagation time.
The long signal propagation delays of the residential cable plant also make CSMA/CD collision detection less efficient. Ethernet collision detection relies on a station listening to its own transmission long enough to make sure that it will not be corrupted by a collision. This time is twice the maximum propagation delay. This guaranteed listening time is accomplished by setting a minimum packet length that takes at least twice the maximum propagation delay to clock out of the Ethernet interface. This limitation can be significant when propagation delays are long, as they are in the residential cable plant.To the user, a cable LAN looks like a digital channel that simply transmits the 1’s and 0’s of computer data. Because long wires prolong, delay, and distort electrical signals with noise, the actual signal transmission is far more complex than it appears. First, sequences of bits are coded, or transformed into configurations of voltage levels that allow both efficient use of the channel and recovery of bit transitions even when sending and receiving clocks are not synchronized. Depending on the coding algorithm used, error detection and correction information may also be incorporated into the bit stream. Then the coded bits are modulated onto an analog waveform, enabling frequency-shifted transmission. When the signal reaches its destination, the receiver demodulates the analog waveform and decodes the bit stream, thus recovering the user’s original sequence of 1’s and 0’s. Decoding and demodulation are the functions performed by high-speed modems such as 56 K modems. These topics have been researched in digital transmission over the local telephone plant. The choice of coding and modulation schemes affects how successfully data can be transmitted over the cable plant in the presence of noise. Although many schemes are possible, the problem is that senders and receivers must use matching schemes. In the cable LAN environment, this problem translates into a requirement for the head end modem to share at least one coding and modulation scheme with each subscriber.The speed of a cable LAN is described by the bit rate of the modems used to send data over it. As this technology improves, cable LAN speeds may change. As of now, cable modems range in speed from 500 K to 10 M. This is about 17 to 340 times the bit rate of the 28.8 K telephone modem. This speed represents the peak rate at which a subscriber can send and receive data. The effective average bandwidth seen by each subscriber depends on how busy the LAN is. Therefore, a cable LAN will appear to provide a variable bandwidth connection to the Internet.Still, the high peak data rates of cable LANs can make them a very effective Internet access network. For example, a 28.8 K modem user who wants to view a simple graphical image that is about 40 Kbytes using the Internet would take more than 10 seconds for the image to download. While a cable LAN user could view the image in under a second. The contrast in speeds is even more evident when large files of a Mg or more are downloaded. Using a 28.8 K modem one can hardly enjoy streaming video or listen to the latest music WAV files the Internet has to offer. With all the graphical interfaces the new web pages have to offer surfing the Internet with the traditional telephone modems is more of a hassle then it is fun and enjoyable. http://CableDatacomNews.comhttp://catv.org/paragonpdx.com/http://www.godset.dk/cablemodem/14.htmhttp://www.discover.com/archive/index.html