Communication links

A communication link requires access devices at each end of some medium. For example, the medium might be a telephone line with a DSL modem at each end, a wireless radio link with WiFi radios at each end, or a CAT 5 cable with Ethernet interfaces at both ends.

Length and speed are two key characteristics of a communication link. For example, a typical home CAT5 cable link has a speed of 100 Mb/s and can be 100 meters long. A typical home WiFi link will be up to 54 Mb/s and can be up to a few hundred feet. (Speed drops as distance increases).

Speed and distance covered improve as technology advances. For example, the initial Ethernet links used thick, clumsy cables and had a speed of 10 Mb/s. Today, we use flexible CAT5 cable and LAN speeds are typically 100 Mb/s or 1 Gb/s in our homes and offices, and an Ethernet running over optical fiber can reach across a city. The WiFi links in my house have a maximum speed of 11 Mb/s, because my equipment is old.

Note that a link can often be upgraded by changing the medium access device, without changing the medium. For example, the first link from my house to a network (before the Internet) ran at 100 bits per second. Today, I have a roughly 1 Mb/s link to the Internet over the same 50 year old copper wires belonging to the phone company. (The phone company name changed, but not the wires).

This speed increase is due to better equipment (access devices) at the ends of the link. The telephone company and I now have DSL modems at the ends of the link. This has great practical significance. It enables us to upgrade the speed of LANs, undersea cables and inter-city backbone links at relatively small cost.

Local telephone companies have a valuable asset since they own wires connecting nearly all residences and businesses to their central offices. CATV companies typically have connections to homes, but not businesses.

During any new construction, one should consider installing communication media even if there is no immediate application.

Some media and common applications are:

MediumApplications
Telephone company copper wireTelephone central office to customer premises -- DSL and telephone
Coaxial copper cableCATV head end to customer premises -- TV, Internet, telephony
CAT 5 copper wireLAN
Plastic optical fiberhome and auto WAN
Glass optical fiberLAN and WAN backbones
Terrestrial radioLAN, PAN, MAN, cellular telephone
Satellite radioTelevision, rural Internet, developing nations

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