Course contents

Unit 4: Ethernet and CablingLesson 4.8 (8 of 11 in this unit)37 of 104 in the Network Fundamentals course

Coaxial Cable

Coaxial cable carried the first Ethernet networks and still brings cable TV and broadband into millions of homes. Here is how it is built, the types you will see, and why it fell out of the office but not out of the world.

Beginner · 10 min read · Before this: Copper cabling, Network connectors

Covers Coax construction, Impedance: 50 vs 75 ohm, RG-6, RG-59, RG-58, Thinnet and thicknet Ethernet and Cable broadband.

Technical definition

Coaxial cable has a copper centre conductor surrounded by an insulating layer, a metal shield and an outer jacket, all sharing the same axis. The shield blocks outside interference and keeps the signal in.

Inside a coaxial cable
Inside a coaxial cableA cross-section of coaxial cable, from the outside in: an outer jacket, a braided metal shield, an insulating layer called the dielectric, and a copper centre conductor. The shield and the centre share the same axis, which is where the name coaxial comes from. The shield blocks outside interference and also keeps the signal from leaking out.Outer jacketprotects from water and wearBraided shieldblocks outside noise, keeps signal inInsulator (dielectric)holds the centre at a fixed spacingCentre conductorcarries the signal

On small screens, swipe sideways or focus the diagram and use the arrow keys.

Coaxial means the shield and the centre conductor share one axis. The fixed spacing is what gives the cable its impedance, 75 ohms for TV and broadband, 50 ohms for radio and old Ethernet.

How a coaxial cable is built

From the middle outward there are four layers. A centre conductor carries the signal. An insulator, the dielectric, keeps the centre exactly in the middle. A braided or foil shield surrounds it as the return path and as a barrier against interference. A plastic jacket protects the whole cable. Because the shield and the centre share one axis, the electric field stays inside the cable, so it neither picks up much noise nor leaks much signal.

Impedance: 50 and 75 ohm

Every coaxial cable has a characteristic impedance set by its geometry. 75 ohm cable is used for TV, video and broadband. 50 ohm cable is used for radio equipment and was used for coaxial Ethernet. Mixing them causes reflections, so the cable, connectors and equipment should all match.

Common coaxial cable types
Common coaxial cable typesCable; Impedance; Typical use. RG-6: 75 ohm Cable TV, cable broadband, satellite dishes RG-59: 75 ohm Short video runs such as CCTV RG-58 (thinnet): 50 ohm 10BASE2 Ethernet, about 185 m per segment RG-8 / RG-213 (thicknet): 50 ohm 10BASE5 Ethernet, about 500 m per segmentCableImpedanceTypical useRG-675 ohmCable TV, cable broadband, satellitedishesRG-5975 ohmShort video runs such as CCTVRG-58 (thinnet)50 ohm10BASE2 Ethernet, about 185 m persegmentRG-8 / RG-213(thicknet)50 ohm10BASE5 Ethernet, about 500 m persegment

On small screens, swipe sideways or focus the diagram and use the arrow keys.

Impedance and use for the cables you will meet. Thinnet and thicknet are legacy.

Coaxial Ethernet, and why it ended

The first Ethernet standards, 10BASE5 and 10BASE2, put every device on one shared coaxial bus, with terminators at both ends to absorb the signal. Every device was in the same collision domain, so only one could send at a time (see Collision domains). A break or a missing terminator disturbed the whole segment. Twisted pair with a hub, then a switch, replaced it.

Where coax is used now

Cable broadband runs over coaxial cable using the DOCSIS standard: the cable modem shares the line with neighbours and turns the signal into Ethernet (see Modems). Coax is also used for TV antennas, satellite dishes, CCTV and radio equipment. The connectors are BNC or F-type (see Network connectors).

Test yourself

Predict · scenario 1

Which cable would you expect to carry cable broadband into a home?

Predict · scenario 2

Why did 10BASE2 networks fail whenever someone broke the cable?

Predict · scenario 3

What do the two ends of a coaxial Ethernet bus need?

FAQ

Is coaxial cable still used for networking?
Yes, mostly as the access line for cable broadband, where a cable modem turns the signal into Ethernet. Inside buildings, twisted pair and fibre replaced coaxial for LANs.
What is the difference between RG-6 and RG-59?
Both are 75 ohm cables. RG-6 has a thicker centre and better shielding, so it loses less signal over distance and suits cable TV and broadband. RG-59 is thinner and used for shorter video runs such as CCTV.
Why did Ethernet move away from coaxial?
Every device shared one cable, so a single break or loose terminator stopped everyone, and it was hard to find faults. Star topologies with twisted pair and switches isolated each device on its own link.