Transmission medium
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Transcript of Transmission medium
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:
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Wireless transmission can be divided into:– Radio waves– Microwaves– Infrared waves
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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