Transmission mediumTransmission medium We can say that transmission media belong to layer zero

Transmission mediumTransmission medium Transmission media can be divided into two broad categories:

– Guided media include twisted-pair cable, coaxial cable, and fiber-optic – Unguided media is usually air

Guided mediaGuided media Guided media, which are those that provide a conduit from one device

to another, include twisted-pair cable, coaxial cable, and fiber-optic cable.

A signal traveling along guided media is directed and contained by the physical limits of the medium

Twisted-pair and coaxial cable use metallic (copper) conductors that accept an transport signals in the form of electric current

Optical fiber is a glass cable that accepts and transports signals in the form of light

Twisted-Pair CableTwisted-Pair Cable Consists of

– two conductors (normally copper)– each with its own plastic insulation– twisted together

Twisted-Pair CableTwisted-Pair Cable One of the wires is used to carry signals to the receiver And the other is used as a ground reference Receiver uses the difference between the two level Interference (noise) and crosstalk may affect both wires and create

unwanted signals Receiver operates only on the difference between these unwanted signals

– If the two wires are affected by noise or crosstalk equally Receiver is immune (the difference is zero)

– If the two wires are parallel The effect of these unwanted signals is not the same in both wires (one close and

one farther)– By twisting the pairs, a balance is maintained

Twisted-Pair CableTwisted-Pair Cable The number of twists per unit of the length (e.g. inch) determines the

quality of the cable– More twists mean better quality

Unshielded versus ShieldedUnshielded versus Shielded Two common twisted-pair cable used in communications

– Unshielded twisted-pair (UTP)– Shielded twisted-pair (STP)

STP has a metal foil or braided-mesh covering– Preventing the penetration of noise or crosstalk– It is bulkier and more expensive

CategoriesCategoriesEIA: the Electronic Industries association classifies unshielded twisted-

pair cable into seven categories which determined by quality (1 lowest and 7 highest)

ConnectorsConnectorsMost common UTP connector is RJ45

– R J stands for Registered Jack

PerformancePerformanceOne way measure performance is to compare

– attenuation vs. frequency and distance.– A twisted-pair can pass a wide range of frequencies.– Figure 7.6 shows with increasing frequency, attenuation in decibels per kilometer

(dB/km), sharply increase with frequency above 100 kHz.– Gauge is a measure of the thickness of the wire.

ApplicationApplication Used in telephone Lines (UTP) DSL lines (UTP) Local area networks

Coaxial CableCoaxial Cable Carries signals of higher frequency ranges than twisted-pair cable Coax has a central core conductor of solid or stranded wire (usually

copper) Encased in an outer conductor of metal foil, braid, or a combination of

the two The outer metallic wrapping Outer conductor is also enclosed in an insulating sheath Whole cable is protected by a plastic cover

Coaxial Cable StandardsCoaxial Cable Standards Coaxial cables are categorized by their radio government (RG) ratings Each RG number denotes a unique set of physical specifications

– Wire gauge of the inner conductor– Thickness and type of the inner insulator– Construction of the shield– Size and type of the outer casing

Each RG ratings is adapted for a specialized function

Coaxial Cable ConnectorsCoaxial Cable Connectors Coaxial cable connector is BNC (Bayone-Neill-Concelman) Figure shows the BNC connector, the BNC T connector, and the BNC

terminator

ApplicationsApplications It was used in analog and digital telephone networks (replaced by

fiber-optic) Cable TV networks (replaced by fiber-optic) Traditional Ethernet Lans

Fiber-Optic CableFiber-Optic Cable It is made of glass or plastic and transmits signals in the form of light If a ray pf light traveling through one substance and enters another

(more or less dense), the ray changes directions

Optical fibers use reflection to guide light through a channel

Fiber-Optic Cable Fiber-Optic Cable Glass or plastic core is surrounded by a cladding of less dense glass or

plastic

Propagation ModesPropagation Modes Current technology supports two modes (multimode and single mode)

for propagating light along optical channels Multimode: multiple beams from a light source move through the core

in different paths Multimode can be implemented in two forms:

– Step-index– Graded-index

Propagation ModesPropagation Modes

Fiber SizesFiber Sizes By the ratio of the diameter of their core to the diameter of their

clading [micrometers]

Cable compostionCable compostion

Fiber-Optic ConnectionsFiber-Optic Connections

Fiber-optic use three different type of connectors1. Subscriber Channel (SC) connector

Used in cable TV and it uses a push/pull locking system

2. The Straight-Tip (ST) connector– Used for connecting cable to networking devices

3. MT-RJ in new connector with the same size as RJ45

Fiber-Optic ConnectionsFiber-Optic Connections

Applications

It used in backbone networks For cable TV with coaxial cable (a hybrid network)

Advantages and Disadvantages Advantages and Disadvantages of Opticalof Optical

Advantages over twisted-pair and coaxial– Higher bandwidth

Less signal attenuation – Signal with fiber-optic can run for 50 km with requiring regeneration– 5 km for coaxial or twisted-pair cable

Immunity to electromagnetic interference– Electromagnetic noise cannot affect fiber-optic cables

Resistance to corrosive materials– Glass is more resistant to corrosive materials than copper

Light weight More immune to tapping

– Fiber-optic cables definitely more immune to tapping than copper cables

Advantages and Disadvantages Advantages and Disadvantages of Opticalof Optical

Disadvantages– Installation/maintenance

Because it is new technology, need expertise– Unidirectional

If we need bidirectional we need two fibers– Cost

The cables and interfaces are expensive

UNGUIDED MEDIA: WIRELESSUNGUIDED MEDIA: WIRELESS

Unguided media: transport electromagnetic waves without using a physical conductor (wireless communication)

– Signal broadcast through air

UNGUIDED MEDIA: WIRELESSUNGUIDED MEDIA: WIRELESS

Unguided signals can travel from the source to destination in several ways:

– Ground propagation– Sky propagation– Line-of-sight propagation

UNGUIDED MEDIA: WIRELESSUNGUIDED MEDIA: WIRELESS

Ground propagation (below 2 MHz)

– Radio waves travel through the lowest portion of atmosphere– Waves emanate in all directions from the transmitting antenna– Distance depends on the amount of power in the signal

Sky propagation (2 - 30 MHz)

– Higher frequency radio waves radiate upward into ionosphere where they reflected back to earth

– This type of transmission allows for greater distance with lower power output

UNGUIDED MEDIA: WIRELESSUNGUIDED MEDIA: WIRELESS

Line-of-sight propagation (above 30 MHz)

– Very high-frequency signals are transmitted in straight lines directly from antenna to antenna

– Antennas must be directional– Facing each other– Either tall enough or close enough

UNGUIDED MEDIA: WIRELESSUNGUIDED MEDIA: WIRELESS

Electromagnetic spectrum defined as radio waves and microwaves is divided into eight ranges, called bands:

UNGUIDED MEDIA: WIRELESSUNGUIDED MEDIA: WIRELESS

Wireless transmission can be divided into:– Radio waves– Microwaves– Infrared waves

UNGUIDED MEDIA: WIRELESSUNGUIDED MEDIA: WIRELESS

Radio waves– Radio waves: waves range in frequencies between 3 KHz and 1 GHz – Microwaves: waves ranging in frequencies between 1 and 300 GHz – Radio waves are omnidirectional (propagated in all directions) – The sending and receiving antennas do not have to be aligned– Disadvantage

One antenna interferences another antenna when they using same frequency– Radio waves that propagate in the sky mode, can travel long distance [AM radio]– Radio waves [low and medium frequencies] can penetrate walls inside a building

Advantage: an AM radio can be received inside a building Disadvantage: we cannot isolate a communication to just inside or outside a building

Applications– Useful for multicasting: Radio, television. Cordless phones and paging

system

UNGUIDED MEDIA: WIRELESSUNGUIDED MEDIA: WIRELESS

Microwaves– Microwaves are unidirectional– Antenna need to be aligned– Advantage: pair of antenna can be aligned without interfering with

another pair– Microwave propagation is line-of-sight– For long distance communication

Very tall towers Repeater

– Very high-frequency microwaves cannot penetrate walls Disadvantage if receiver inside a building

UNGUIDED MEDIA: WIRELESSUNGUIDED MEDIA: WIRELESS

Applications– Due to unidirectional properties, microwaves useful when

unicasting (one-to-one) communication Cellular phones Satellite networks Wireles LANs

UNGUIDED MEDIA: WIRELESSUNGUIDED MEDIA: WIRELESS

Infrared– From 300 GHz to 400 THz (wavelengths from 1 mm to 770 nm)– Use for short-range communication– It has high frequency, cannot penetrate walls

Prevents interference between one system and another– Remote control not interfere with our neighbors

– Can not be used outside because sun’s rays contain infrared waves (interference)

– (IrDA) Infrared Data Association established standards for communicating between devices:

Keyboards, mice, PCs and printers