The Physical Layer: Data Transmission Basics. Encode data as energy at the data (information) source and transmit the encoded energy using transmitter hardware: Possible Energy Forms: Electrical, light, radio, sound. The transmitted energy is decoded at destination back into data.
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The amount of time needed for information to propagate from source to destination through the channel. It is the distance divided by the signal propagation speed (usually the speed of light). Depends on:
= signal propagation speed / data transmission rate
Example: Channel data-rate is 10Mbps. One bit is transmitted in 10-7
seconds. Since signals propagate in a medium at about 200,000 km/s, ie 2*108
m/s, bit-length is: 10-7 * 2 * 108 = 20 meters.
Mature technology, rugged and inexpensive;
Maximum transmission speed is limited: Low noise immunity
Relatively short maximum transmission distances.
Suitable for LANs.
Twisted pair plastic-coated copper wires
Coaxial cable cross-section
Compared to copper wiring: Higher speed ( > 1Gbps), more resistant to electro-magnetic interference, spans longer distances making it suitable for subnet channels, more expensive; less rugged. Requires only a single fiber.
Maximum data rate = 2 H log2 V bits/sec
where V is the number of discrete levels.
Maximum data rate = 2 H log 2 2 = 2 H
Maximum number of bits/sec = H log2 (1 + S/N)
30 dB = 10 log10 S/N
Signal to noise ratio = S/N = 10 30/10 = 103 = 1000
Maximum transmission rate = 3000 log2 (1 + 1000) » 30000 bps
serial communication and the RS-232C interface.
Analog signal: The local loop
Time division multiplexing is used to combine multiple
T1, T2 etc. lines onto single higher-bandwidth lines.
Data Multiplexing in SONET
Circuit switching Message switching Packet switching
Additional rate: 622.08 Mbps.
A single ATM cell