various physical medium

Classify the various physical medium and highlight their merits and demerits.

Network links are implemented on media that is either guided (wired) or unguided (wireless).

GUIDED MEDIA

The guided media is broadly classified into Twisted-pair, Coaxial and Fiber-optic cable

Twisted-Pair Cable

The least-expensive and most commonly-used transmission medium is twisted-pair. The copper wires are twisted together to reduce the electrical interference.

 

Twisted-pair cables are either shielded (STP) or unshielded (UTP). The EIA has classified UTP cable into various categories. Some are:

Category	Data Rate in Mbps	Usage
CAT-1	< 0.1	Telephone
CAT-4	20	Token Ring
CAT-5	100	LAN

UTP cable is commonly used for LANs such as 10Base-T and 100Base-T. The most common UTP connector is RJ45.

UTP cable suffers from attenuation and need repeaters for long distance transmission (every 100m in case of CAT-5).

Twisted-pair cables are also used in telephone lines to provide voice and data channels. DSL lines also use the high-bandwidth capability of UTP cables.

Coaxial Cable

 

It has a central core conductor enclosed in an insulating sheath, which in turn is encased in an outer conductor of metal foil.

Coaxial cable carries signals of higher frequency ranges than those in twisted-pair cable To connect coaxial cable to devices, coaxial connectors such as BNC connector, BNC T

connector and BNC terminator are used.

Coaxial cables are categorized by their radio government (RG) ratings. Some are:

Category	Usage	Bandwidth	Distance
RG-59	Cable TV
RG-58	Thin Ethernet	10–100 Mbps	200 m
RG-11	Thick Ethernet	10–100 Mbps	500 m

The attenuation is much higher in coaxial cables than in twisted-pair cable. The signal weakens rapidly and requires the frequent use of repeaters.

Coaxial cable was widely used in analog telephone networks and in cable TV networks, but was later replaced by optical fiber.

Coaxial cable is used to provide connection to user premises as it requires narrow bandwidth and is cost-effective.

Fiber-Optic Cable

A fiber-optic cable transmits signals in the form of light and use property of reflection.

 

The outer jacket is made of PVC/Teflon. Inside the jacket are Kevlar strands to strengthen the cable. Below the Kevlar is another plastic coating to cushion the fiber. 

Fiber-optic supports two modes: multimode and single mode.
	Cable	Bandwidth	Distance
	Multimode fiber	100 Mbps	2 km
	Single-mode fiber	100–2400 Mbps	40 km

The attenuation is much lesser and few (10 times less) repeaters are required. Supports higher bandwidth and longer transmission distance.

Fiber-optic cables are immune to interference and corrosive-resistant.

Installation and maintenance require expertise and fiber-optic cables are more expensive. Two fiber-optic cables are required for duplex communication (light is unidirectional)

Cable TV companies use optical fiber to provide the backbone structure. LANs such as 100Base-FX network and 1000Base-X also use fiber-optic cable.

UNGUIDED MEDIA

Unguided media transport signals through free space.

It can be broadly classified as Radio waves, Micro waves and Infrared. Signals can travel through ground, sky or line-of-sight propagation.

The spectrum pertaining to radio waves and microwaves is divided into bands, regulated by government authorities. It ranges from very low to extremely high frequency.

 

Radio Waves

The frequency range is 3 kHz – 1 GHz.

Radio waves are omni-directional and hence antennas need not be aligned.

The waves transmitted by one antenna are susceptible to interference by another.

Radio waves that propagate in the sky mode, can travel long distances and is used for long-distance broadcasting such as AM radio.

Radio waves of low and medium frequencies can penetrate walls and used in FM radio, maritime radio, cordless phone, paging, etc.

Microwaves

Microwaves have frequencies between 1 and 300 GHz.

Microwaves are unidirectional. Hence sending and receiving antennas should be aligned. A pair of antennas can be aligned without interfering with another aligned pair.

Microwave propagation is line-of-sight.

Very high-frequency microwaves cannot penetrate walls.

They are used in cellular phone networks, satellite networks, and wireless LANs.

Infrared

Infrared waves frequencies ranges from 300 GHz to 400 THz.

It can be used for short-range communication only within a closed space. Infrared waves have high frequencies and cannot penetrate walls.

IrDA has established standards for communication between devices such as keyboards, mouse, PCs, and printers.

Infrared signals transmit through line of sight with very high data rate of transmission.

Last mile links

If two nodes to be connected are on opposite sides of the country/town, then only option is to lease a dedicated link from the telephone company which is expensive. The less expensive options available are last-mile links that span from the home to a service provider. Some common services and their bandwidth are:

Service	Bandwidth
POTS	28.8–56 Kbps
ISDN	64–128 Kbps
xDSL	16 Kbps–55.2 Mbps
CATV	20–40 Mbps