Chapter 3. Data Transmission Concepts and Terminology Data and Signal Transmission Impairments

Transmission Terminology Guided and unguided media Simplex, Half-duplex, Full-duplex tx Time-Domain representation Frequency-Domain representation

Time Domain Representation

Frequency Domain Representation

Data Rate vs. Bandwidth Case A (transmission system bandwidth = 4MHz) (4/p)[sin(2pft) + (1/3)sin(2p(3f)t)]  101010… (Treat the waveform as a bit string of 1s & 0s) Bandwidth 4MHz = 3f – f = 2f =  f = 2MHz Data rate = 2f bps = 4 Mbps Case B (transmission system bandwidth = 4MHz) (4/p)[sin(2pft) + (1/3)sin(2p(3f)t)+(1/5)sin(2p(5f)t)] Bandwidth 4MHz = 5f – f = 4f  f = 1MHz Data rate = 2f bps = 2 Mbps

Data Rate vs. Bandwidth (cont) T= 1/f B

Data and Signals Analog data Digital data a function of time and occupy a limited frequency spectrum can be represented by a electromagnetic signal occupying the same spectrum Digital data can be represented by digital signals, with a different voltage level for each of the two binary digits

Data and Signals (cont) Analog signal Digital signal Two alternatives: (1) signal occupies the same spectrum as the analog data;(2) analog data are encoded to occupy a different portion of spectrum. Analog data are encoded using a codec to produce a digital bit stream. Analog data Digital data are encoded using a modem to produce analog signal. Two alternatives: (1) signal consists of two voltage levels; (2) digital data are encoded to produce a digital signal with desired properties. Digital data

Treatment of Signals Analog transmission Digital transmission Is propagated through amplifiers; same treatment whether signal is used to represent analog data or digital data. Assumes that the analog signal represents digital data. Signal is propagated through repeaters. Analog signal Not used Digital signal represents a stream of 1s and 0s, which may represent digital data or may be an encoding of analog data. Signal is propagated through repeaters. Digital signal

Analog Signal

Digital Signal Data

Transmission Impairments Attenuation distortion (strength of a signal) Guided media: logarithmic Unguided media: complex function of distance and of the makeup of atmosphere. Increasing function of frequency Delay distortion (peculiar to guided media) The velocity of propagation of a signal through a guided medium varies with frequency Inter-symbol interference Some of the signal components of one bit position will spill over into other bit positions

Transmission Impairments Noise Thermal noise (white noise) Due to thermal agitation of electrons Function of temperature Uniformly distributed across the frequency spectrum Cannot be eliminated, place an upper bound on comm. Inter-modulation noise Different frequencies share the same tx medium Crosstalk Electrical coupling Impulse noise

Channel Capacity Nyquist Bandwidth Shannon Capacity Formula Case of a channel that is noise free Given a bandwidth of B, the highest signal rate that can be carried is 2B If the signals to be transmitted are binary (2 voltage levels), then the data rate is 2B bps Shannon Capacity Formula C = B log2 (1+SNR) Theoretical maximum that can be achieved Consider white noise only