Review: The physical layer. Bandwidth/Capacity bit rate/baud rate How to determine the number of bits per symbol? Relation between the bandwidth and capacity: –Nyquist’s theorem: maximum baud rate for noiseless channel. max baud rate = 2 * Bandwidth –Shannon's theorem: maximum bit rate for noisy channel. C = Bandwidth * log_2 (1 + S/N) Data encoding: –digital data to digital signals –digital data to analog signals »amplitude modulation, frequency modulation and phase modulation.

Simplex/half-duplex/duplex communication Multiplexing: when it is needed? –Time Division Multiplexing, Frequency division Multiplexing, Code division multiplexing.

A physical layer protocol: RS-232-C. –Four components: mechanical interface electrical interface functional interface procedureal interface –Terminology: DTE (data terminal equipment): computer DCE (data circuit-terminating equipment): modem

mechanical interface: –25-pin connector – mm wide – top row 1-13 –bottom row , –cable 15m electrical interface: –1: < -3V – 0: > 4V –bit rate: up to 20kbps

functional interface: which circuit should connect to which pin (Fig 2-21). 1: protective ground 2: transmit 3: receive 4: request to send 5: clear to send 6: data set ready 7: common return 8: carrier detect 20: data terminal ready (1) (3) (4) (5) (6) (7) (8) (20) (2)

procedural interface: legal sequence of events. – Example: DTE requests to send, DCE clear to send.