A communication channel is a physical link between two end nodes of a telecommunication or computer network system. In other words it is a transmission medium which takes data from one end to other end of the a link between nodes.
There are many wired and wireless transmission mediums available today.
Wired mediums: optical fiber, twisted pair cable, LAN cable, coaxial cable
Wireless mediums: radio waves, free space optics (FSO), millimeter wave (mmWave)
Optical fiber is a tiny flexible cable made of fiber glass. Light travels inside fiber glass medium. Today optical fiber is the backbone of internet.
Twisted Pair Cable
In a twisted-pair cabling system two conductors of a circuit are rolled around each other in a twisted fashion. Twisted pair cable in computer network are widely used today. A typical LAN cable has 4-twisted pairs with different colour codes.
Coaxial cable has the ability to carry high frequency signals. A main copper core is surrounded by dielectric material. The dielectric material is further surrounded by copper sheet. This whole setup is enclosed in a plastic sheath. Coaxial in computer networking is used for broadband internet. It is also used in cable TV network and connecting high frequency transmitters/receivers.
Local area network ( LAN ) cable also known as Patch cable used to connect switch or router with the computer. A typical LAN cable with RJ-45 connector is shown below