Transmission Media Physical layer Choosing the physical media Transportation of a raw bit stream. Choosing the physical media Bandwidth, delay, cost, … Two flavors Guided media e.g. Copper wire, fiber optics, … Unguided media e.g. Radio, lasers, ...

Magnetic media Physical transportation of magnetic or optical disk. e.g. Exabyte tape: 50x50x50 cm box can hold 1000 7 GB tapes. Fed Ex anywhere is US in 24 hrs. Effective bandwidth: 648 Gbps. 1000 tapes = $5000, reuse > 10 times, $200 shipping. $700 for 7000 GB, about 10 cents per GB. Large propagation delay.

Two-wire open lines Each wire is insulated from the other and are open to free space. Connected equipment < 50m apart, < 19.2 kbps. Signal encoded using voltage or current level relative to some ground reference. Used to connect data terminal equipment (DTE) to data communication equipment (DCE) (e.g. modem). Wires enclosed in single protected multicore cable, or flat ribbon cable.

Problems with two wire open lines Crosstalk Cross coupling of electrical signals between wires caused be capacitive coupling. Susceptible to noise due to open structure. Noise primarily from electromagnetic radiation from other electrical signal sources. May amplify signal on only one wire creating an additional difference signal. Solution: twist the two lines together.

Twisted pair Oldest and most common transmission medium. Connects most telephones to the telephone equipment. Usually many pairs are bundled together. Improved immunity to spurious noise. i.e. Noise picked up by both wires. Reduced crosstalk. Used for both analog and digital transmission. Bit rates depend on thickness and distance of wire. ~1Mbps, < 100m Shielded and unshielded twisted pair.

Problems with twisted pair Skin effect At high bit rates the current tends to flow on the outer surface of the wire. Uses less of the available cross section. Increases electrical resistance of wires for higher frequency signals increasing attenuation. More signal power is lost due to radiation effects. Solution: More sophisticated driver and receiver electronics. Another transmission medium.

Coaxial cable Signal and ground wire Solid center conductor running coaxially inside a solid (usually braided) outer circular conductor. Center conductor is shielded from external interference signals.

Properties of coaxial cable Better shielding allows for longer cables and higher transfer rates. 100 m cables 1 to 2 Gbps feasible (modulation used) 10 Mbps typical Used for long haul routes by Phone Co. Mostly replaced now by optical fiber. Still widely used for Cable TV and LANs. Baseband vs. broadband coax.

Fiber Optics Optical fiber cable carries the transmitted information in the form of a fluctuating beam of light in a glass fiber. Light has much wider bandwidth than electrical waves. Light waves are immune to EM interference and crosstalk. Good to use in electrically noisy environment. Achievable bandwidth > 50 Tbps. Current limitation: electrical/light interface (1 Gbps).

Components of an optical transmission system 1. Light source 2. Transmission medium 3. The detector Light means a 1 bit, no light means a 0 bit. Transmitter LED or injection laser diode. Detector (photodiode or photo transistor) generates an electrical pulse when light falls on it. Unidirectional data transmission system. Electrical signal to light signal and back again.

Trapping the light inside the fiber Light is reflected and refracted at the silica/air boundary. Amount of refraction depends on the silica and air indices of refraction. If the angle of incidence is above critical value then total internal reflection is achieved.

Modes of fiber Fiber consists of two parts: the glass core and glass cladding with a lower refractive index. Light propagates in 1 of 3 ways depending on the type and width of the core material. Multimode stepped index fiber Both core and cladding have different but uniform refractive index. Relies on total internal reflection; Wide pulse width. Multimode graded index fiber Core has variable refractive index (light bends as it moves away from core). Narrow pulse width resulting in higher bit rate. Singlemode fiber (> 100 Mbs) Width of core diameter equal to a single wavelenth.

Fiber cables Multimode: diameter of core is ~50 microns. About the same as a human hair. Single mode: diameter of core 8-10 microns. They can be connnected by connectors, or by splicing, or by fusion.

Fiber vs. copper Fiber (pros) Fiber (cons) Higher bandwidth, Lower attenuation, Immune to electromagnetic noise and corrosive chemicals, Thin and lightweight, Security (does not leak light, difficult to tap). Fiber (cons) Not many skilled “fiber engineers,” Inherently unidirectional, Fiber interfaces are expensive.

Wireless transmission Electromagnetic waves are produced by moving electrons and can propagate through free space. Frequency: oscillations per second of EM wave. Wavelength: distance between consecutive maxima. Speed of light: 3 x 108 m/s in a vacuum. EM waves can be broadcast/received via antenna attached to an electrical circuit.

Electromagnetic spectrum

Frequency bands and data rate Amount of info an EM wave can carry is related to its bandwidth. Can encode 3-8 bits per cycle with EM waves. Data rate deduced from width of frequency band. FCC: who gets to use what frequency band.

Radio Transmission Radio waves Easy to generate, travel long distances, and penetrate buildings easily. Omnidirectional. Low frequencies Pass through obstacles well, Quick power drop off (e.g. 1/r2 in air). High frequencies Travel in straight lines and bounce off obstacles. Absorbed by rain. Subject to electrical interference

Microwave transmission Microwave waves Travel in straight lines and thus can be narrowly focused. Easy to avoid interference with other microwaves. Parabolic antenna is used to concentrate the energy (improves SNR). More popular before fiber. Waves do not pass through buildings. Multiple towers used as repeaters.

Infrared Waves Short range communication. Directional. e.g. Remotes on VCR’s and TV’s. Directional. Do not pass through walls. Behaves more like visible light. Can be used for LANs indoors only. Can just use visible unguided light (lasers).