Transmission media

Guided mediaThose that provide a conduit from one device to

another.Types include:

◦ Twisted pair◦ Coaxial◦ Fiber optic

A signal travelling along these medium is directed and contained by the physical limits of the medium.

Twisted pair and coaxial cable use metallic conductors, transporting signals in electrical current

Optical fiber is a glass or plastic cable that accepts and transports signals in form of light.

Twisted pair Cable Types:

◦unshielded twisted pair(UTP)◦Shielded twisted pair(STP)

Coaxial Cable

Categories of coaxial cables

Category Impedance Use

RG-59 75 W Cable TV

RG-58 50 W Thin Ethernet

RG-11 50 W Thick Ethernet

OPTICAL FIBER

An optical fiber (or fibre) is a

glass or plastic fiber (pipe) that

carries light along its length.

Light is kept in the "core" of the

optical fiber by

total internal reflection.

Unguided Media: WirelessRadio Waves

Microwaves

Infrared

Band Range Propagation Application

VLF 3–30 KHz Ground Long-range radio navigation

LF 30–300 KHz GroundRadio beacons and

navigational locators

MF 300 KHz–3 MHz Sky AM radio

HF 3–30 MHz SkyCitizens band (CB),

ship/aircraft communication

VHF 30–300 MHzSky and

line-of-sightVHF TV, FM radio

UHF 300 MHz–3 GHz Line-of-sightUHF TV, cellular phones,

paging, satellite

SHF 3–30 GHz Line-of-sight Satellite communication

EHF 30–300 GHz Line-of-sight Long-range radio navigation

Radio waves are used for multicast communications, such as radio and television, and paging systems.

Note:

Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs.

Note:

Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation.

Note:

Switching How to connect multiple devices so as

one to one communication is possible?Solution: different topologiesDrawbacks of different topologiesSwitched network consists of series of

interlinked nodes called switches.Switches are hardware and/or software

devices capable of creating temporary connections between two or more devices linked to the switch

Types of switchingCircuit switchingPacket switchingMessage switching

Circuit SwitchingCreates a direct physical

connection between two devicesIt may have n inputs and m

inputsMay implement space division

switching or time division switching

A circuit switch

Packet SwitchingDrawbacks of circuit switch

◦Less suited for data communication ◦Data rate is slow◦System is inflexible

In packet switch, data are transmitted in discrete units of variable length blocks called packets

Types:◦Datagram approach◦Virtual circuit approach

Datagram approachIn this approach, Each packet is treated

independently, here each packet is referred to as datagram

Each message is divided into several datagrams, and each datagram may go different paths to reach their destination

This can cause datagrams to arrive at destination out of order. The transport layer takes the responsibility of reordering these out of order datagrams before they are given to destination port

Virtual circuit approachHere the relationship between

each packet is maintainedWhen data is sent, all packets of

the transmission travel one after the another along a dedicated route

Can be implemented in two formats:◦Switched virtual circuit◦Permanent virtual circuit

Similar to circuit switching, in SVC, a virtual circuit is created whenever it is required and this connection exists only for the duration of the transmission

Only one route is established in SVCIn the case of PVC, the same virtual

circuit is established between two users but on a continuous basis

Much like a dedicated link to dedicated users

Message switchingIt is best known otherwise as

STORE AND FORWARDHere a node receives messages,

stores it until an appropriate route is free and then it sends it.

Not much in use today, due to requirement of large storage capacity and delays.