Telecommunications channels
Telecommunications channels are the different media used to carry information between different locations. These include traditional cables and wires known as guided media, and wires and more recent innovations such as satellite and micro-wave which are unguided media. We will now examine the benefits and applications of these different media. When doing this, the main factors that need to be considered are the physical characteristics, data transmission method, performance and cost.
Characteristics of guided media
The main types of cabling used in LANs are based on copper cabling. Data are transmitted along this by applying at one end a voltage, which is received at the other. A positive voltage represents a binary one and a negative voltage represents a binary zero. There are three main types of cabling used in networks:
Telecommunications channels are the different media used to carry information between different locations. These include traditional cables and wires known as guided media, and wires and more recent innovations such as satellite and micro-wave which are unguided media. We will now examine the benefits and applications of these different media. When doing this, the main factors that need to be considered are the physical characteristics, data transmission method, performance and cost.
Characteristics of guided media
The main types of cabling used in LANs are based on copper cabling. Data are transmitted along this by applying at one end a voltage, which is received at the other. A positive voltage represents a binary one and a negative voltage represents a binary zero. There are three main types of cabling used in networks:
- Twisted-pair cabling consists of twisted copper wire covered by an insulator. The two wires form a circuit over which data can be transmitted. The twisting is intended to reduce interference. Shielding using braided metal may also be used to reduce external interference. A cable may have more than one pair, such as category 5 cable generally known as CAT5. CAT5 consists of four twisted pairs inside an outer insulating cover. The cable is used for many applications such as computer networking, telephony, low-voltage power distribution and home automation applications such as movement detectors and transmission of high-definition television. CAT5 can support network speeds up to 1 Gbps
- Co-axial cable consists of a single solid copper core surrounded by an insulator and a braided metal shield. ‘Co-ax’ can be used to connect devices over longer distances than twisted-pair. It is possibly best known as the means used to connect an antenna to a television set. Co-axial can be used for transmission speeds up to 500 Mbps. There are different grades of co-ax available, with a standard named CT100 suitable for transmission of digital TV around the home. CT125 offers even better performance.
- Fibre-optic cable consists of thousands of fibres of pure silicon dioxide. Packets are transmitted along fibre-optic cables using light or photons emitted from a light-emitting diode at one end of the cable; detection is by a photo-sensitive cell at the other end. Fibreoptic cables give very high transmission rates since the cable has very low resistance. This is well known as a method by which cable TV is delivered to homes. Fibre optic transmission rates can be as much as 100 times greater than those of co-axial. They are also much smaller and lighter than co-axial or twisted-pair cabling. Other advantages are that they have a much lower data error rate than other media and are harder physically to ‘tap’ into and thus offer greater security.
Characteristics of unguided or wireless media
For wide-area network cables are still commonly used, but they are being superseded by the use of unguided media. This method uses signals transmitted through air and space from a transmitter to a receiver. It tends to be faster than wired methods. Wireless transmissions can be used for different business applications at different scales:
- Wireless infra-red transmission can be used for sending data between a portable PC or personal digital assistant (PDA) and a desktop computer. Laser printers can receive documents for printing via wireless infra-red transmission from desktop or laptop computers that do not need to be connected to the printer.
- Wireless transmission can also be used locally to form a wireless LAN. Here a microwave or narrowband radio transmitter and receiver may be used to connect different buildings. Wireless LANs are often used across college campuses. They have the benefit that the cost of laying cabling is not incurred. This makes them particularly suitable where it is not clear whether a link is needed in the long term.
- Microwave transmission can be used to beam information through the atmosphere. The maximum distance that can separate microwave transmitters is 45 km, since the signal follows a straight line and the curvature of the earth limits transmission distance. This can make microwave an expensive method of transmitting data, but the cost can be reduced if it is combined with satellite methods.
- Satellite transmission operates at two orbit levels, high orbit at 22,300 miles in a geostationary orbit and at a lower orbit. Messages are sent from a transceiver at one location on the earth’s surface and are bounced off the satellite to another transceiver. Because of the distances involved, this can give a time delay of up to a quarter of a second, which is evident in interviews conducted by satellite. A range of frequencies can be used. Satellite applications include television, telephone and data transmission.
- Both guided and unguided media use a number of transmission schemes such as OFDM in order to improve their efficiency. Orthogonal frequency-division multiplexing (OFDM) is used in applications such as digital television and audio broadcasting, wireless networking and broadband internet access.
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