Coaxial Cable

Introduction

Coaxial cable: is an electrical cable consisting of a round conducting wire, surrounded by an insulating spacer, surrounded by a cylindrical conducting sheath, usually surrounded by a final insulating layer. It is used as a high-frequency transmission line to carry a high-frequency or broadband signal. Sometimes DC power (called bias) is added to the signal to supply the equipment at the other end, as in direct broadcast satellite receivers. Because the electromagnetic field carrying the signal exists (ideally) only in the space between the inner and outer conductors, it cannot interfere with or suffer interference from external electromagnetic fields.

Coaxial cables may be rigid or flexible. Rigid types have a solid sheath, while flexible types have a braided sheath, both usually of thin copper wire. The inner insulator, also called the dielectric, has a significant effect on the cable's properties, such as its characteristic impedance and its attenuation. The dielectric may be solid or perforated with air spaces. Connections to the ends of coaxial cables are usually made with RF connectors.

Radio-grade flexible coaxial cable.

A: outer plastic sheath

B: copper screen

C: inner dielectric insulator

D: copper core

There are two types of coaxial cables:

1. Thinnet

2. Thicknet

Thinnet

Also known as "Thin Ethernet" or Thinnet, 10BASE-2 is an IEEE standard for baseband Ethernet at 10MBps over thick coaxial cable. 10Base2 has a maximum distance of 185 meters. Thin Ethernet is five millimeters in diameter and used to connect machines up to 1,000 feet apart.

Thinnet (thin Ethernet) is an incarnation of the Ethernet standard in which coaxial cables are used in a LAN (local-area network) configuration to connect computers together. A Thinnet setup is capable of transmitting data at a rate of 10Mbps (megabits per second). It is also cheaper and easier to install than Thicknet.

The first variation on the original variety of Ethernet was simply to use a thinner coaxial cable and relax the constraints on how and where transceivers can connect. 10BASE-2 does this with coaxial cable that looks just like the cable used for receiving cable television or hooking up a television set to an antenna. The only difference in the cable itself is the impedance rating. A television cable is rated at 75 ohms and a 10BASE-2 cable is rated at 50 ohms. In a pinch, a small length of one can be substituted for the other.

The connectors used in 10BASE-2 are called BNC connectors for Berkeley Nucleonics Co.: they were originally used in nuclear physics. A transceiver is typically located on the same circuit board as the network interface and a male BNC connector protrudes from the device containing the network interface.

A thinnet network is an economical way to support a small department or workgroup. The cable used for this type of network is:

* Relatively inexpensive.

* Easy to install.

* Easy to configure.

External 10BASE-2 transceiver

A thinnet network can combine as many as five cable segments connected by four repeaters; but only three segments can have stations attached. Thus, two segments are untapped and are often referred to as "inter-repeater links." This is known as the 5-4-3 rule.

In the Figure down, there are five segments, four repeaters, and trunk segments 1, 2, and 5 are populated (have computers attached to them). Trunk segments 3 and 4 exist only to increase the total length of the network and to allow the computers on trunk segments 1 and 5 to be on the same network.

The following table summarizes 10Base2 specifications:

Category Notes

Maximum segment length 185 meters (607 feet).

Connection to network interface card BNC T connector.

Trunk segments and repeaters Five segments can be joined using four repeaters.

Computers per segment 30 computers per segment by specification.

Segments that can have computers Three of the five segments can be populated.

Maximum total network length 925 meters (3035 feet).