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Hardware Building Blocks and Encoding COM211 Communications and Networks CDA College Pelekanou Olga Email: [email protected] www.cdacollege.ac.cy/site/info-com-technology-ll/ Performance Bandwidth: number of bits per time unit. (a) 1 (b) 1 We can talk about bandwidth at the physical level, but we can also talk about logical processto-process bandwidth. Latency: time taken for a message to travel from one end of the network to the other. Latency Latency Propagatio n Transmit Queue Propagatio n Distance / Speed of light Transmit Size / Bandwidth 2.0 108 m / s in a fiber Speed of light 2.3 108 m / s in a cable 3.0 108 m / s in a vaccum Delay and Bandwidth Delay Bandw idth This product is analogous to the volume of a pipe or the number of bits it holds. It corresponds to how many bits the sender must transmit before the first bit arrives at the receiver. Delay may be thought of as one-way latency or round-trip time (RTT) depending on the context. Throughput Throughput Transfer size / Transfer t ime (effective end-to-end throughput) Transfer t ime RTT (1/Bandwidt h) Transfer size We often think of throughput as measured performance. Implementation inefficiencies may cause the achievable bit rate to be less than the bandwidth for which the networks was designed. Throughput and Transfer Time Example: A user fetches a 1-MB file across a 1-G pbs network with a round-trip time of 100 ms. Compute the transfer time. 1 TransferTime RTT ( 9 ) 106 8 100ms 8ms 108ms 10 EffectiveThroughput 1Mbps / 108ms 74.1Mbps Shannon’s Theorem Real communication have some measure of noise. This theorem tells us the limits to a channel’s capacity (in bits per second) in the presence of noise. Shannon’s theorem uses the notion of signal-to-noise ratio (S/N), which is usually expressed in decibels (dB): dB 10 log 10 (S / N ) In a typical analog system, e.g. analog telephone system, dB = 30, which gives 30 = 10 * log10(S/N) S/N = 1000 for a typical analog system, including plain old telephone systems Shannon’s Theorem Shannon’s Theorem: C B log 2 (1 (S / N )) C: achievable channel rate (bps) B: channel bandwidth For POTS, bandwidth is 3000 Hz (upper limit of 3300 Hz and lower limit of 300 Hz), S/N = 1000 C 3000 log 2 (1 1000) 30Kbps Question: How come you get more than this with your modem? Information: Frequency audible to human ears: 20-20KHz Jitter Interpacket gap 4 Packet source 3 2 1 4 Network 3 2 1 Packet sink Jitter is a variation (somewhat random) of the latency from packet to packet. Jitter is most often observed when packets traverse multiple hops from source to destination. Building Blocks Networks nodes/ End Devices Links Dedicated cables Leased lines Last-mile links Wireless Network Node/End Device Memory: getting larger and larger as we can see the last years, but never enough! Processor: Moore’s law still holds for speed On a typical networked application, one must keep in mind the computation to communication ratio. Links and Signals Links: Twisted pair, coax, optical fiber, the ether; half-duplex or full-duplex. Signals: Waveforms that travel on some medium T (period) 1 f T (frequency) Frequency & Wavelength Wavelength: the distance between a pair of adjacent Maxima or minima of a wave, denoted as λ. λ *f = c, c is the speed of light in a given medium. Example: take c = 300 M meters/second, f = 100 M Hz, its wavelength λ = 3 meters Spectrum f(Hz) 10 0 2 10 10 4 6 10 Radio 4 10 5 10 6 10 7 10 8 10 10 10 Microw ave 8 10 9 10 10 12 14 10 Infrared 10 10 10 11 10 TV 18 10 20 22 10 X ray 12 10 Coax FM 10 UV Satellite AM 16 Terrestrial microw ave 13 10 10 24 Gamma ray 14 10 15 10 Fiber optics 16 10 Encoding – NRZ (Non Return to Zero) Signals that maintain constant voltage levels with no signal transitions (non return to a zero voltage level) during a bit interval. 4B/5B Encoding Insert extra bits into the stream to break up long sequences of 0s and 1s. Doesn’t allow more than one leading 0 and no more than two trailing 0s. 4 bits f 5 bits 4B/5B Encoding 4-bit Data Symbol 5-bit Code 0000 11110 0001 01001 0010 10100 0011 10101 0100 01010 0101 01011 0110 01110 0111 01111 1000 10010 1001 10011 11111 = idle line 1010 10110 00000 = dead line 1011 10111 1100 11010 1101 11011 1110 11100 1111 11101 24 16 25 32 16 codes are “left over” and some can be used for purposes other than encoding data. For instance: 00100 = halt 7 codes violate the “one leading 0, two trailing 0s rule”.