Before you start
Objectives: learn the specifics of Coaxial network cable.
Prerequisites: no prerequisites
Key terms: cable, coaxial, conductor, connector, network, type, resistance
Coaxial cables are usually implemented with a bus networking topology. It is not typically used in ring or star topologies. It’s called coaxial because it uses two conductors within the single cable. Both of those conductors share a common access. That’s why we call it coaxial, or common access.
Image 205.1 – Coaxial Cable
Coaxial network cabling is made up of four different components. In the center is an inner conductor. Inner conductor is usually made out of copper and it’s used to actually conduct the data signals. Surrounding that inner conductor is an insulator, which is usually made out of PVC plastic. It is there to keep the signal separated so that the ground doesn’t touch the inner conductor. Outside this PVC insulator is another, secondary mesh conductor. It’s usually made out of aluminum or sometimes out of copper that’s been coated with tin as well. Now, this mesh is also a conductor but the data doesn’t flow on that conductor. It is just used for grounding so that is why we have PVC insulator in between the mesh conductor and inner conductor. This whole assembly is encased in a secondary PVC sheath.
It had a good degree of resistance to electromagnetic interference. If the cabling runs, for example, next to a high voltage electrical line, or maybe runs through the ceiling across a florescent light, these types of situations can result in a lot of electromagnetic interference. Electromagnetic noise can be picked up by networking cable. Coaxial network cable resisted that pretty well. It was also really tough, so it resisted physical damage.
It was moderately expensive to implement and install. It’s less flexible and harder to install. The thing that took coaxial network cabling out of favor was the fact that newer networking standards didn’t support coaxial cable.
Not all coaxial network cables are the same. There’s different types or different grades of cables. Each different type, or grade, has different electrical properties. It’s important that we use the correct type of coaxial cable for the given implementation we’re using.
The first type of coaxial cable, and the one that’s important for networking, is RG-58. It was used for older Ethernet networks and it was very widely implemented. The central conductor of RG-58 cable used a conductor made of copper that had a coating of tin around it so it had a silvery appearance. The central conductor had 50 ohms of resistance.
The RG59 is used for cable TV. RG-59 uses a copper plated steel central conductor and is designed to have 75 ohms of resistance. Because of that we can not use this type of cable for computer networks. We can’t use RG-59 in the place of RG-58.
It is usually used for satellite TV systems. RG6 is very similar to RG-59. The difference is in the central conductor. RG-6 uses a solid copper central conductor, and is also designed for 75 ohm resistance rating.
The first connector is a BNC connector. BNC can stand for Bayonet Neill-Concelman connector, or British Naval Connector. It’s used to connect older Ethernet networks using coaxial cabling.
Image 205.2 – BNC Connector
The second connector is F-type connector. Generally, F-type is used for cable TV and satellite TV connections. It’s a twist on connector and unlike the BNC connector, which is molded on, the F-type connector is screwed onto the end and it makes the appropriate contacts. The one place in networking where we use an F-type connector is if we are connecting a cable modem to a cable TV service. Cable TV providers can provide Internet access through their broadband cable TV lines.
Image 205.3 – F-type Connector
Coaxial cable has two conductors, but only one is used for data transfer. It has good resistance to EMI. Types of coaxial cables are RG-58, RG-59, and RG-6. RG-58 is used in computer networks.