Data Communications Notes

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Transmission Media

• Transmission medium – Physical path between transmitter and receiver – May be guided (wired) or unguided (wireless) – Communication achieved by using em waves • Characteristics and quality of data transmission – Dependent on characteristics of medium and signal – Guided medium ∗ Medium is more important in setting transmission parameters – Unguided medium ∗ Bandwidth of the signal produced by transmitting antenna is important in setting transmission parameters ∗ Signal directionality · Lower frequency signals are omnidirectional · Higher frequency signals can be focused in a directional beam • Design of data transmission system – Concerned with data rate and distance – Bandwidth ∗ Higher bandwidth implies higher data rate – Transmission impairments ∗ Attenuation ∗ Twisted pair has more attenuation than coaxial cable which in turn is not as good as optical fiber – Interference ∗ Can be minimized by proper shielding in guided media – Number of receivers ∗ In a shared link, each attachment introduces attenuation and distortion on the line

Guided transmission media • Transmission capacity (bandwidth and data rate) depends on distance and type of network (point-to-point or multipoint) • Twisted pair – Least expensive and most widely used – Physical description ∗ Two insulated copper wires arranged in regular spiral pattern ∗ Number of pairs are bundled together in a cable ∗ Twisting decreases the crosstalk interference between adjacent pairs in the cable, by using different twist length for neighboring pairs – Applications ∗ Most common transmission media for both digital and analog signals

Transmission Media


∗ Less expensive compared to coaxial cable or optical fiber ∗ Limited in terms of data rate and distance ∗ Telephone network · Individual units (residence lines) to local exchange (end office) · Subscriber loops · Supports voice traffic using analog signaling · May handle digital data at modest rates using modems ∗ Communications within buildings · Connection to digital data switch or digital pbx within a building · Allows data rate of 64 kbps – Transmission characteristics ∗ Requires amplifiers every 5-6 km for analog signals ∗ Requires repeaters every 2-3 km for digital signals ∗ Attenuation is a strong function of frequency · Higher frequency implies higher attenuation ∗ Susceptible to interference and noise ∗ Improvement possibilities · Shielding with metallic braids or sheathing reduces interference · Twisting reduces low frequency interference · Different twist length in adjacent pairs reduces crosstalk – Unshielded and shielded twisted pairs ∗ Unshielded twisted pair (utp) · Ordinary telephone wire · Subject to external electromagnetic interference ∗ Shielded twisted pair (stp) · Shielded with a metallic braid or sheath · Reduces interference · Better performance at higher data rates · More expensive and difficult to work compared to utp – Category 3 and Category 5 utp ∗ Most common is the 100-ohm voice grade twisted pair ∗ Most useful for lan applications ∗ Category 3 utp · Transmission characteristics specified up to 16 mhz · Voice grade cable in most office buildings · May have data rates up to 16 Mbps over limited distances · Typical twist length 7.5 to 10 cm ∗ Category 4 utp · Transmission characteristics specified up to 20 mhz ∗ Category 5 utp · Transmission characteristics specified up to 100 mhz · Data grade cable in newer buildings · May have data rates up to 100 Mbps over limited distances · Much more tightly twisted, with typical twist length 0.6 to 0.85 cm, for better performance • Coaxial cable

Transmission Media


– Physical description ∗ Consists of two conductors with construction that allows it to operate over a wider range of frequencies compared to twisted pair ∗ Hollow outer cylindrical conductor surrounding a single inner wire conductor ∗ Inner conductor held in place by regularly spaced insulating rings or solid dielectrical material ∗ Outer conductor covered...
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