Computer Network Communications

Computer Network Communications

Assessment

The following report will look at, and describe transmission media used in network communications. Some of the more common media will be discussed and explained in detail.We will begin the report with a look at BandwidthIn basic terms Bandwidth is the range of transmission frequencies a network uses, so the greater the bandwidth, the more information that can be transferred over the network at any one time.

A simple way to understand bandwidth is to think of highways and cars. Bandwidth is the number of lanes on the highway. If you are the only car on a highway, you can travel very quickly. If you are stuck in the middle of rush hour, you may travel very slowly since all of the lanes are being used up.In a wire based communication channel, bandwidth is often limited by the wires themselves. The make up of the wires can cause degradation in high frequency transmission, so the signal may only be able to travel a few centimetres so there is a need for amplifiers and repeaters although these can produce problems of their own. In the past there have been efforts to minimise bandwidth signals Low bandwidth signals are less susceptible to noise interference

Low bandwidth signals allow for a greater number of communication exchanges to take place within a specified band of frequencies.

This is no longer the case because in digital and fibre optic systems, there is more and more demand for increased data speeds so less need for bandwidth conversionsBaud RateIn common usage the baud rate of a modem is how many bits it can send or receive per second. Technically, baud is the number of times per second that the carrier signal shifts value - for example a 1200 bit-per-second modem actually runs at 300 baud, but it moves 4 bits per baud (4 x 300= 1200 bits per second)This is NOT how many bits-per-second the modem can transmit because there may be more than two states to the signal.

A baud being 1 electronic state change per second The number of line changes per second

The Baud rate is 4 (4 changes per second) Data Rate

Data rate is the amount of data in digital form, that is moved from one place to another in a given time. The data transfer rate can be viewed as the speed of travel of a given amount of data from one place to another. In general, the greater the bandwidth (remember in the first paragraph, the highways and cars scenario) of a given path, the higher the data transfer rate.

Data transfer is usually measured in bits per second. An example being, a typical low-speed connection to the Internet may be 33.6 kilobits per second. On Ethernet local area networks, data transfer can be as fast as 10 megabits per second. Network switches are planned that will transfer data in the terabit range. In earlier telecommunication systems, data transfer was sometimes measured in characters or blocks (of a certain size) per second. AttenuationReduction of signal strength during transmission. Attenuation is the opposite of amplification, and is normal when a signal is sent from one point to another

To put it in simple terms attenuation is a reduction in strength of a signal. It can happen with any type of signal, digital or analogue. If the signal attenuates too much, it becomes unintelligible, which is why most networks require repeaters at regular intervals. The extent is usually expressed in more common terms being decibels In conventional and fibreoptic cables attenuation is determined as the number of decibels per foot, 1000 feet, kilometer, or mile. If the attenuation is less per unit distance the cable will be more efficient. In some cases it will be necessary to transmit signals, using cable over a long distance. In these cases repeaters of in certain cases repeaters may be inserted along the length of the cable. Repeaters, as we know from networking takes in the signal, boosts it to full strength and sends it back out therefore overcoming the attenuation. This operation increases the maximum attainable range the cable is able to communicate NoiseIs a combination of electrical and electromagnetic energy that degrades signal and data quality. Noise can occur in both digital and analogue systems and affects all types of communication, including files, text, programs, images, audio and telemetry.

In circuits such as internet connection through phone lines, external noise can be picked up from appliances in the vicinity, even as far as outer space. This noise however is normally of little or no consequence. During thunderstorms, or places where a lot of electrical appliances are in use, external noise can affect communications. The data transfer rate in an internet hookup can be slowed down because the system has to adjust its speed to match the line conditions.

A wireless system rather than a hard wired system noise is a more significant problem. At a low frequency such as 300kHz, atmosphere and electrical noise are more severe than at a higher frequency like 300MHz. Internal noise, which is generated inside wireless receivers, is less dependant on frequency. The less external noise there is at high frequency, the more significant the internal noise becomes.

The most traditional method to deal wit noise has been to minimize the signal bandwidth, the less space a signal occupies the less noise is passed through. But of course by reducing bandwidth this will reduce the maximum speed of the data being delivered. Digital Signal Processing using fibre optics, which is far less susceptible to noise has been recently developed to minimize the affects of noise.Optical fibreFiber optic cabling consists of a center glass core surrounded by several layers of protective materials (See fig. 5). It transmits light rather than electronic signals, eliminating the problem of electrical interference. This makes it ideal for certain environments that contain a large amount of electrical interference. It has also made it the standard for connecting networks between buildings, due to its immunity to the effects of moisture and lighting.

Fiber optic cable has the ability to transmit signals over much longer distances than coaxial and twisted pair. It also has the capability to carry information at vastly greater speeds. This capacity broadens communication possibilities to include services such as video conferencing and interactive services. The cost of fiber optic cabling is comparable to copper cabling; however, it is more difficult to install and modify. 10BaseF refers to the specifications for fiber optic cable carrying Ethernet signals.

MicrowavesMicrowaves are electromagnetic waves with wavelengths longer than those of infrared light, but relatively short for radio waves.they are used to provide links where it is either impractical or it may be too expensive to lay a physical media. Linking isolated communities, crossing deserts, swamps.etc. buildings as well as weather conditions can disturb the microwave beam as it travels through the eartrhs atmosphere. Microwave communications may be used reliably over distances which exceed 50km,this will depend on the height of the tower. Two towers for example could be placed 80km apart.Microwaves are used excessively in broadcasting transmissions eg news bulletins, film sequences, because microwaves pass easily through the earth's atmosphere with a lot less interference than longer wavelengths. There is also a lot more bandwidth in the microwave spectrum than in the rest of the radio spectrum. Microwaves are used in television news to transmit a signal from a remote location to a television station from a specially equipped van. SatelliteA precisely aligned microwave beam onto which the data is modulated, is transmitted from the ground to the satellite. It is then relayed to a destination which has been pre determined using an electronic circuit which is known as a transponder. One satellite will have a number of these transponders, each of them covering a particular band of frequencies (satellite channels have extremely high bandwidth of around 500MHz. allowing the satellite to provide a vast amount of high bit ratechannels using a technique known as multiplexing. The multiplexer divides the total size of the channel into a lot of sub channels.Technicians strategicaly place communication satellites to rotate at the same rate as the earth therefore appearing to be stationary (geostationary orbit) relative to a fixed point on the ground. The orbit is chosen so that it has a direct line of site to the transmitting and receiving stations.