Data Transmission Common media concepts

Data Transmission and Media

Guided media: Twisted pair: Applications: LAN, Telephones Twist decreases crosstalk Tighter the twists, the better the cable Bundled pairs vary twist rate Most common medium Telephones, 10 Base-T, 100 Base-T, ISDN Inexpensive, flexible, easy to work Useful for shorter applications Useful to 100 Mbps (1 GHz for CAT7)

Data Transmission and Media Twisted pair: Analog or digital: amplifiers and repeaters necessary Very susceptible to interference: limited bandwidth, data rate, distance Shielding (STP), variable twist lengths reduce interference but increase costs Telephone wire and LAN cable are unshielded (UTP).

Data Transmission and Media Near-end crosstalk (NEXT): Signal coupling from one pair of connectors or conductors to another (in dB: the higher the better) Summary: UTP and STP are easy to use, inexpensive, flexible and very strong. Length and bandwidth are limited.

Data Transmission and Media Guided media: Coaxial cable Also 2 conductors, but one is an insulated hollow conductor that acts as shield. Inner conductor is suspended in solid dielectric or spaced rings Shield and other physical characteristics allow for higher frequencies, longer distances, less interference, less crosstalk Expensive compared to fiber, microwave Fading fast

Data Transmission and Media

Coaxial cable: Used for analog and digital signals: LAN’s, long distance telephone, television distribution Also require amplifiers and repeaters, but over longer distances and higher frequencies (analog to 400MHz)

Data Transmission and Media Guided cable: optical fiber Thin, flexible medium made from glass (fused silica) or plastic capable of conducting ultra-high frequency signals (light) Ultra-pure fiber: best performance-highest cost Multicomponent glass: moderate/moderate Plastic: low/least Cylindrical with 3 concentric sections: the core, the cladding, the jacket

Data Transmission and Media Optical fiber: Core and cladding have different refractive characteristics Jacket absorbs light Used in long-distance telecommunications, military applications, LAN’s Several advantages over coaxial and UTP/STP

Data Transmission and Media Advantages to fiber: Capacity: data rates over 100’s of Gbps Longer distances, fewer repeaters: greater than 900 miles Small size, light weight Lower attenuation Electromagnetic isolation: not affected at all by external electromagnetic fields (interference, impulse noise, crosstalk)

Data Transmission and Media

Optical fiber: Long trunks: 900 miles, high capacity (20K to 60K voice channels) Metropolitan trunks: 8 miles, 100K voice channels, no repeaters, connect telephone exchanges to each other and to backbone Rural exchange trunks: miles, link towns and different telco’s (5K voice channels) Subscriber loop trunks: central exchange to subscriber LAN’s: 100 Mbps, price is attractive

Data Transmission and Media Types of light technologies: LED’s: (light emitting diode) Advantages: inexpensive, wide temperature range, longer life. Most common Disadvantages: less efficient, lower data rate ILD’s: (injection laser diode) Advantages: more efficient, higher data rate Disadvantages: expensive, shorter life, narrower temperature range Detectors Photo Diodes

Data Transmission and Media infrared Optical fiber is most efficient at 3 infrared wavelengths: 850nm: used for short distances 850 nm LED: most common-limited to 100Mbps and few kilometers 1300nm: LED or laser-longer distance, higher rates 1500nm: laser, highest rates and distance

Data Transmission and Media Fiber/satellite will replace coax and terrestrial microwave Price/performance Fiber dominance has 3 effects: Prices are dropping Services are expanding Fiber is crowding out even satellite

Mediums Fiber Optics use total internal reflection This occurs in a transparent medium whose index of refraction is higher that surrounding medium