Chapter 4 Transmission Media Overview Guided – wire (twisted pair, coaxial cable, optical fiber) Unguided – wireless (broadcast radio, terrestrial microwave, satellite) Characteristics & quality of data transmission is determined by medium & signal For guided medium, the medium itself is more important in determining the transmission limitations For unguided medium, the bandwidth produced by the antenna is more important Key concerns are data rate and distance in the design of trans. sys.

Design Factors Bandwidth: Transmission impairments: e.g. Attenuation Higher bandwidth (HZ) gives higher data rate (bps) Transmission impairments: e.g. Attenuation Limits the transmission distance Twisted pair -> coaxial cable -> optical fiber Interference: from competing signals in overlapping frequency bands can distort or wipe out a signal. Number of receivers: In guided media Point-to-point link or share link with multiple attachment: each attachment introduce some attenuation & distortion on the line, limiting distance and/or data rate

Electromagnetic Spectrum for Telecom

Guided Transmission Media Twisted Pair Coaxial cable Optical fiber

Twisted Pair & Applications Most common medium: least expensive Telephone network Between house and local exchange (subscriber loop) Within buildings To private branch exchange (PBX) For local area networks (LAN) 10Mbps or 100Mbps

Twisted Pair - Transmission Characteristics Pros and Cons Cheap; Easy to work with Low data rate; Short distance <- bigger attenuation Analog Signal: Amplifiers every 5km to 6km Digital Transmission: repeater every 2km or 3km Limited distance Limited bandwidth (1MHz) Limited data rate (100Mbp) Susceptible to interference and noise

Unshielded and Shielded TP Unshielded Twisted Pair (UTP) Ordinary telephone wire Cheapest Easiest to install Suffers from external EM interference Shielded Twisted Pair (STP) Metal braid or sheathing that reduces interference More expensive Harder to handle (thick, heavy)

UTP Categories Cat 3 Cat 4 Cat 5 up to 16MHz Voice-grade cable found in most offices Twist length of 7.5 cm to 10 cm Cat 4 up to 20 MHz Cat 5 up to 100MHz Commonly pre-installed in new office buildings Twist length 0.6 cm to 0.85 cm

Coaxial Cable

Applications & Transmission Characteristics Television distribution : Cable TV Long distance telephone transmission Can carry 10,000 voice calls simultaneously (via FDM) Being replaced by fiber optic Short distance links between computer devices Transmission Characteristics: Analog Amplifiers every few km; Closer if higher frequency Up to 500MHz Digital: Repeater every 1km; Closer for higher data rates

Optical Fiber - Transmission Characteristics Optical Fiber: transmit a signal-encoded beam of light by means of total internal reflection Act as waveguide for freq. of 1014 to 1015 Hz Portions of infrared and visible spectrum Three Transmission Modes: Step-index multimode; Graded-index multimode; Single-mode Two Type of Light Sources: Light Emitting Diode (LED) Cheaper; Wider operating temp range; Last longer Injection Laser Diode (ILD): More efficient; Greater data rate WDM: multiple beams of light at different freq. are transmitted on the same fiber. This is a form of FDM.

Optical Fiber Transmission Modes

Optical Fiber - Properties & Applications Greater capacity: in the unit of Gbps, Tbps Smaller size & lighter weight Lower Attenuation EM Isolation: not affected by external EM interference Greater Repeater Spacing: every hundreds of km Applications: Long-haul trunks ; Metropolitan trunks; LANs Rural exchange trunks Subscriber loops

Wireless Transmission Unguided media Transmission & reception via antenna Directional Focused beam Careful alignment required Omnidirectional Signal spreads in all directions Can be received by many antennae

3 Frequency Ranges 30MHz to 1GHz: 2GHz to 40GHz: Microwave frequency Suitable to Omnidirectional app. Refer to as broadcast radio range 2GHz to 40GHz: Microwave frequency Highly directional beams are possible Suitable to point- to-point transmission Can also be used for satellite comm. 3 x 1011 to 2 x 1014 Infrared portion of the spectrum Local apps. Within confined areas, e.g. a single room

Terrestrial Microwave Physical Description: the most common type of antenna is the parabolic “dish” Focuses a narrow beam: to achieve line-of-sight trans. to the receiving antenna Apps.: Long haul telecommunications Higher frequencies give higher data rates common freq : 2 – 40 GHZ

Satellite Microwave Satellite is a microwave relay station Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency Requires geo-stationary orbit Height of 35,784km Applications: Television Long distance telephone Private business networks

Broadcast Radio Omnidirectional Freq: 30 MHZ to 1GHZ FM radio UHF and VHF television Trans. is limited to the line of sight Suffers from multipath interference

Infrared Transceivers modulate noncoherent infrared light Two parties must be within line of sight (or reflection) Blocked by walls e.g. TV remote control