Coaxial Cable

Coaxial cabling consists of a single central copper conductor core surrounded by a foam filler layer. Both these layers are surrounded by a braided metal shield and then by another outer covering sheath that is usually made of plastic.

Layer Functions

The central solid copper core is the signal carrying layer. The inner foam layer; which is fire resistant, was also intended to provide insulation between the center conductor and the braided metal shield which acts to reduce (even block) any outside EMI from fluorescent lights, motors, and other computers as well as helping to make coaxial cable tear resistant. The final layer; usually composed of some waterproof material such as plastic, is there for overall protection and to provide resistance to environmental damage such as that which water might cause.

Distance – Overall coaxial cable is significantly more resistant to signal interference and can span greater single segment cable runs than twisted pair cable (STP & UTP). This means that the distance in between end nodes can be significantly longer than for standard CAT cable.

Inflexible – However; coaxial cable is notoriously difficult to install due to its inflexibility (thick coaxial cable in particular). This inflexibility is contributed to by: the outer braided metal layer, the thick foam layer and the overall thickness in diameter of the inner conducting (signal carrying) layer, all of which combine to make coaxial cable; as a package, far thicker and more unwieldy than UTP and STP.

Two Types of Coaxial Cable

Two types of coaxial cable have been used as transmission media in computer networking; thick coaxial cable and thin coaxial cable. The thick and thin refer to the comparative diameters of one type of coaxial cable against the other (that is to say one is thicker and stiffer relative to the other).

Thick Coaxial Cable or Thicknet was the first variant to be used with Ethernet networks and has an extra thick protective plastic cover to assist in resistance (tolerating) to damp/humid environment tolerance by keeping moisture away from the center conductor. It is this extra layer that makes thick coaxial cable thicker than thin coaxial cable and hence the name.

10BASE5 – When carrying Ethernet signals the term 10BASE5 is used to describe the specifications to which the cable must be compliant. The 10 refers to the cable’s maximum data throughput of up to 10 Megabits/sec while the BASE tells us that this cable is intended for use with baseband signaling as opposed to broadband signaling and the 5 refers to the maximum segment length being 461.5 meters (approximately 1600 feet or 500 yards).

Note that baseband signaling means signals and/or systems whose range of frequencies is measured from zero to a maximum bandwidth or highest signal frequency. Generally the entire spectrum range is used with every transmission and so only one signal can be propagated across the medium at a time.

10BASE5 network segments could support up to 100 nodes and was the better choice for running longer lengths in a linear bus network compared to 10BASE2 (thin coaxial cable).

Specifications – 10BASE5 cables are rather stiff and measured approximately 9.5 mm (0.375 inches) in diameter with an impedance of 50 ohms.

The cable and had to be one linear run with 50 ohm resistive terminators installed at both ends and transceivers installed every 2.5 meters. The transceivers are necessary to ensure that reflections due to the presence of multiple taps are not in phase thereby helping to reduce the amount of noise and signal degradation. Suitable attachment locations were marked on the cable. It was also permitted to connect transceivers using N connectors at the end of a cable segment. T-Connectors are not allowed.

Transceivers are connected to nodes via an interface that was known as the Attachment Unit Interface (AUI) which is a 15-pin, double row D-Sub type of connector that used clips rather than screws for cable resistance (to stop the cable from being accidently dislodged).

Thin Coaxial Cable or Thinnet was referred to as 10BASE2 when carrying Ethernet signals. As with Thicknet (10BASE5) the 10 means that the cable is rated at a maximum data throughput rate of 10 Megabits/sec, the BASE tells us that baseband signaling was used rather than broadband and the 2 refers to a maximum manufacturer specified segment length of 200 yards (185 meters). When wiring a 10BASE2 network, special care has to be taken to ensure that cables are properly connected to all T-connectors and appropriate terminators are installed.

Applications – Thin coaxial cable was popular in school networks (especially linear bus topologies) and while more flexible than thick coaxial cable still presented problems as it is very cumbersome and difficult to work with compared to both STP and UTP. The characteristics of 10BASE2 and thin coaxial cable meant that it was ideal for small networks of 2-4 machines. However; a failure at any point, in the network cabling tends to prevent all communications which made 10BASE2 difficult and time consuming to administer, manage, maintain and troubleshoot.

Limited Implementation – We never saw a wholesale rollout of mainstream thin coaxial cable and 10BASE2 network implementations primarily due to competition from the cheaper CAT 3 cable and the arrival of CAT 5 cable which are both considerably cheaper and easier to work with (install).

Coaxial Cable Connectors

The Bayone-Neill-Concelman (BNC) or bayonet connector is the most common type of connector to be found in association with coaxial cabling. Thin coaxial cable networks also used other types of connectors and adapters including: T-connectors, barrel connectors and terminators.

As is usually the case with wired transmission media the connector represents the physically weakest and most easily disrupted point in the network cabling infrastructure. It is therefore highly advisable to use coaxial BNC connectors that are crimped onto the cable rather than screwed onto the cable as this will produce consistently stronger and more reliable connectivity.

Vampire Taps

One of the major advantages of 10BASE5 cable was that it allowed for new connections to be made while all existing connections remained live (still in use). The device used to do this is called a vampire tap and with practice the cable installer is able to clamp the vampire tap onto the existing cable by forcing a spike inside the tap to pierce through the outer shielding layers to make contact with the solid copper core that performed the transmission and reception of the signal.

When a special crimping tool was used; other spikes, which bit into the outer conductor, were deployed at the same time as the copper core was being spiked. The crimping tool is readily available from hardware stores, electronics stores and electrical supplies stores. However; it was not long before we began to see vampire taps built into the transceiver because this allowed the use of more flexible multi-wire cables.

Terminators

All varieties of coaxial cable need to be terminated if there is no device attached to both ends of a segment.

Repeaters

Special devices called repeaters are used in scenarios where there is a greater distance between machines than the manufacturer’s cable specifications cite as being the maximum distance that this particular copper cable could be run if you hope to get any worthwhile service. In order to increase the maximum distance between nodes beyond that stipulated in the manufacturer’s specifications devices known as repeaters can be used.

About Author

Leave A Reply