Part 3a: Cables and Network Communication Systems

Transmission Media
Transmission media refers to the many types of cables and other mediums that carry the signal from the sender to the receiver. Copper based cable is the most common form of medium, and is used for virtually all links except long distance. A cable medium often introduces unwanted changes to the signal, which limits the speed (how fast we can send information) and frequency range of the signals that can be transmitted.

Open Wire
Seldom used nowadays, but was used extensively in rural areas to provide 12 speech channels over a single wire atop telephone poles. Open wire cable has been replaced by micro-wave systems or newer technologies. Being exposed to the weather, dust particles and salt spray were deposited on the insulators that secured the cable to the telegraph pole. When storms and wet weather arrived, the dust turned to mud, and provided a path of least resistance for the signal, thus the signal was diverted down the pole to ground and significantly reduced in strength. Lightning strikes were also a hazard and special ceramic arrestors were used to protect equipment attached to open wire lines.

Twisted Pair Cable
Before long, the open overhead wires servicing users from the telephone centers were replaced by multi-strand cable , which was buried underground and protected by many sheaths of polyurethane plastic. This prevented water seeping into the cable and affecting the signal. It also helped overcome some of the limitations that open wire systems had. The major problem now is people digging them up! These multi-strand cables used a pair of wires for each user, and there were about 500 pairs or more per cable. Each wire was twisted around each other wire in order to try and reduce unwanted noise (hence the term twisted pair).

Cable was laid by the telephone company along each street, from the telephone center, to the customers house. Thus each customer had a physical set of wires which ran from their telephone set all the way into the telephone exchange. The use of multi-strand underground cable, still in widespread use today, made the delivery of low-cost telephone services to the general public.

Twisted pair cable is the most common form of cable today, used to connect telephone subscribers to exchanges (switching centers) and wire buildings. Two insulated wires are twisted around each other, and combined with others into a cable. In general, each twisted pair supports a single voice channel. Twisted pair is also used to interconnect PC's on a Local Area Network (LAN).

Twisted pair used in Local Area Networks has several ratings. Category 3 has a speed rating of 10 million bits per second (the speed of Ethernet), whereas category 5 has a speed rating of 100 million bits per second. CIT is cabled using Category 5 UTP cable. UTP (unshielded twisted pair) is cable which has no ground shield. Cables are often provided with a ground shield which helps to reduce signal interference from external sources, thus making the signal travelling down the cable less prone to alteration. Twisted pair cable is provided in two forms, UTP and STP (shielded twisted pair).

Unshielded twisted pair cable is the predominant cable used today. Two conductors are coated with a plastic sheath then twisted around each other. These pairs are then twisted around other pairs to make a multi-pair cable. The twisting of the wires around each other helps to reduce unwanted signals being induced into the wires. It is used for telephone wiring inside buildings, as telephone cables which link customer houses and buildings to telephone switching exchanges, and for implementing local area networks.

Coaxial Cable Systems
There became a need to inter-connect telephone centers in different towns together. The use of open wire or multi-strand cable for this was unsuitable, as the capacity of each cable was too low (number of simultaneous speech conversations per cable) in order to make it economically viable. Hence the introduction of coaxial cable. Till recently, coaxial cable was extensively used to support toll traffic and long distance links. Today, it is being replaced by micro-wave, satellite or fiber-optic links. Coaxial cable is a two wire conductor with a larger bandwidth than twisted pair cable. It is used in television, radio, and Ethernet LANs. In voice communication systems, each coaxial cable supports about 60 speech channels.

It has a single core, with an outer conductor which acts as a shield. The signal is transmitted on the inner core. The inner core and the outer shield are separated by an insulator, either plastic or mica. The cable is enclosed in polyurethane to protect it and give it some strength.

It is important not to bend the cable too tightly, as this damages the insulator which separates the inner core from the shield. The transmission qualities of coax are affected by the properties of the type of insulator used. Coaxial cable could carry up to 1200 speech circuits per cable. Coaxial cable operates better than open wire, as it is buried in the ground and not subject to the elements. The systems used to implement multi-speech channels on coaxial cable are called broad-band systems.

Thin Ethernet is cheap to install and is rated at 10 million bits per second. Each end of the cable is terminated using a 50ohm terminator. Failure to terminate each end of the cable, or a break in the cable, causes the network to fail.

Coaxial cable is used extensively in networking and data communications. A center conductor is separated from an outer conductor by an insulator medium. The cable cannot be crushed or bent sharply, as this damages the insulation between the conductors and thus alters the electrical characteristics of the cable. When used for local area networking, it links PC's together. The networking protocol commonly used with coaxial cable is ETHERNET, which describes how data is formatted and transmitted along a shared cable system.

As open wire gave way to better systems, so coaxial cable has given way to others. The problem with coaxial cable is the number of speech channels available per cable. With the high demands for data communications between computers, increased telephone circuits between cities and countries, not to mention television channels, other mediums have taken over from coaxial cable. Coaxial cable became popular in the 1980's as a method of interconnecting computers, specifically Local Area Networks (LAN's), because it was cheap and easy to install. In addition, some cable TV systems use coaxial cable to supply programming content to subscribers. In New Zealand during 1995, Kapiti Coast Television introduced a cable TV subscription service on the Kapiti Coast which used coaxial cable to supply TV programming content to paying customers.

Micro-wave systems
As the demand for more and more speech circuits grew by customers wanting to make long distance calls, the telephone companies had to expand the capacity to meet this demand. One such system which was used was Microwave, which does not use cable as a transmission medium, rather it uses the air. Using very high frequency signals, microwave support thousands of telephone channels and several television channels on the one circuit. Microwave is a radio system which uses very high frequencies to send and receive data. Because of the high frequencies involved, stations are located about 30 kilometers apart and in line of sight (visible to each other). Microwave systems have sufficient bandwidth capacity to support a large number of voice channels and one or two TV channels.

Transmitters and receivers must be located within sight of each other, about 30kilometers apart. Microwave does not bend round corners or jump over hills.

Dishes and towers were expensive to construct and with the distance limitations, meant it was expensive to go very long distances.

Nowadays, many companies use microwave systems to interconnect buildings at high speed digital links of 2 million bits per second or greater. Sometimes, this is a cheaper solution than linking buildings using fiber optic cable, specially in inner cities where cabling is a problem, or across rivers etc where terrain prohibits the use of existing physical cabling methods. This allows companies to link their networks in different buildings together into one common network, allowing the sharing and accessing of information. Today, microwave systems are used in a number of areas, such as linking local area networks together between campus buildings, sending radio signals from a radio station to its transmitter site, and the sending of video or audio signals from an outside sports event back to a TV broadcasting studio.

Satellite systems
Ground stations with large dishes communicate with a communications satellite in geo-stationary orbit around the earth. Each channel is managed by a transponder, which can support thousands of speech channels and about 4 TV channels simultaneously. The cost of satellite links is still very expensive (about $4M per transponder). It is primarily used for intercontinental links.

Satellite systems are comprised of ground based transmitter and receiver dishes, with an orbital satellite circuit (called a transponder). Signals are transmitted to the orbiting satellite, which relays it back to another ground station. The footprint coverage of a single satellite system is very large, covering thousands of square kilometers (enough for entire countries such as New Zealand). For example, in New Zealand in 1997, the SKY television network announced plans and introduced a pay per view satellite service for New Zealander subscribers. Satellite is used to carry television channels and telephone conversions between countries.

These use an orbiting satellite to communicate with ground stations. They support tens of thousands of speech channels and tens of television channels.

The cost is very high per circuit, and signals are received using a special dish. Satellite TV is an example of such a system.