Chapter 5, Stallings

Data and Digital Signals Chapter 5, Stallings COMP312 - Data and Digital Signals – p.1/37 Lecture Outline • Today: Digital Signals ◦ Digital Data on Digital Signals ◦ Analogue Data on Digital Signals • Tomorrow: Analogue Signals ◦ Analogue Data on Analogue Signals ◦ Digital Data on Analogue Signals COMP312 - Data and Digital Signals – p.2/37 OSI Protocol Model 7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Link 1. Physical Working Here COMP312 - Data and Digital Signals – p.3/37 What is a Digital Signal? • “A digital signal is a sequence of discrete, discontinuous voltage pulses” • Digital signals are desirable as we can use repeaters rather than amplifiers to transmit over longer distances. • That is, they do not suffer from additive noise like an analogue signal may. COMP312 - Data and Digital Signals – p.4/37 ◦ Fibre Optics ◦ ADSL (Asymmetric Digital Subscriber Line) ◦ Wireless Ethernet ◦ CDMA ◦ GSM ◦ Plain old telephone service ◦ Ethernet over UTP • Answer: only Ethernet over UTP Media examples • Question: Which of the following media use digital signals? COMP312 - Data and Digital Signals – p.5/37 ◦ ADSL (Asymmetric Digital Subscriber Line) ◦ Wireless Ethernet ◦ CDMA ◦ GSM ◦ Plain old telephone service ◦ Ethernet over UTP • Answer: only Ethernet over UTP Media examples • Question: Which of the following media use digital signals? ◦ Fibre Optics COMP312 - Data and Digital Signals – p.5/37 ◦ Wireless Ethernet ◦ CDMA ◦ GSM ◦ Plain old telephone service ◦ Ethernet over UTP • Answer: only Ethernet over UTP Media examples • Question: Which of the following media use digital signals? ◦ Fibre Optics ◦ ADSL (Asymmetric Digital Subscriber Line) COMP312 - Data and Digital Signals – p.5/37 ◦ CDMA ◦ GSM ◦ Plain old telephone service ◦ Ethernet over UTP • Answer: only Ethernet over UTP Media examples • Question: Which of the following media use digital signals? ◦ Fibre Optics ◦ ADSL (Asymmetric Digital Subscriber Line) ◦ Wireless Ethernet COMP312 - Data and Digital Signals – p.5/37 ◦ GSM ◦ Plain old telephone service ◦ Ethernet over UTP • Answer: only Ethernet over UTP Media examples • Question: Which of the following media use digital signals? ◦ Fibre Optics ◦ ADSL (Asymmetric Digital Subscriber Line) ◦ Wireless Ethernet ◦ CDMA COMP312 - Data and Digital Signals – p.5/37 ◦ Plain old telephone service ◦ Ethernet over UTP • Answer: only Ethernet over UTP Media examples • Question: Which of the following media use digital signals? ◦ Fibre Optics ◦ ADSL (Asymmetric Digital Subscriber Line) ◦ Wireless Ethernet ◦ CDMA ◦ GSM COMP312 - Data and Digital Signals – p.5/37 ◦ Ethernet over UTP • Answer: only Ethernet over UTP Media examples • Question: Which of the following media use digital signals? ◦ Fibre Optics ◦ ADSL (Asymmetric Digital Subscriber Line) ◦ Wireless Ethernet ◦ CDMA ◦ GSM ◦ Plain old telephone service COMP312 - Data and Digital Signals – p.5/37 • Answer: only Ethernet over UTP Media examples • Question: Which of the following media use digital signals? ◦ Fibre Optics ◦ ADSL (Asymmetric Digital Subscriber Line) ◦ Wireless Ethernet ◦ CDMA ◦ GSM ◦ Plain old telephone service ◦ Ethernet over UTP COMP312 - Data and Digital Signals – p.5/37 Media examples • Question: Which of the following media use digital signals? ◦ Fibre Optics ◦ ADSL (Asymmetric Digital Subscriber Line) ◦ Wireless Ethernet ◦ CDMA ◦ GSM ◦ Plain old telephone service ◦ Ethernet over UTP • Answer: only Ethernet over UTP COMP312 - Data and Digital Signals – p.5/37 Digital Signals Data Digital Signal Data Encoder Decoder COMP312 - Data and Digital Signals – p.6/37 Digital Data on Digital Signals • Data is already in digital form; goal is to have high performance digital encoding schemes • Logic to encode and decode signals not terribly complicated COMP312 - Data and Digital Signals – p.7/37 Digital Encoding Scheme Performance Evaluated in terms of • Spectrum. High frequencies may be attenuated, DC results in power transfer. • Clocking. Receiver needs to maintain synchronisation with transmitter. • Error Detection. Can any errors be detected without additional techniques? • Noise immunity. Will spikes cause errors in the signal? • Cost/complexity. How difficult are the receiver and transmitter to build? COMP312 - Data and Digital Signals – p.8/37 Digital Encoding Schemes • Non-Return to Zero Schemes • Multilevel Schemes • Biphase Schemes • Coding Schemes COMP312 - Data and Digital Signals – p.9/37 Non-Return to Zero • 0: low voltage • 1: high voltage COMP312 - Data and Digital Signals – p.10/37 Non-Return to Zero Level • More commonly used in practice. • 0: high voltage • 1: low voltage COMP312 - Data and Digital Signals – p.11/37 Non-Return to Zero Level • Voltage levels are constant for each bit period • Simplest schemes to engineer • Bandwidth efficient • Poor noise immunity • Synchronisation problems with long strings of 1’s or 0’s COMP312 - Data and Digital Signals – p.12/37 Non-Return to Zero Invert on Ones • Differential encoding. ◦ 0: no transition at beginning of interval ◦ 1: transition at beginning of interval • Gives greater noise immunity as transitions are easier to detect than levels. • No inherent polarity. • Used in USB with bit-stuffing. COMP312 - Data and Digital Signals – p.13/37 Multilevel Schemes • Have some redundancy - can detect some errors • Are very bandwidth efficient • Provide synchronisation on "marks" but not on non-marks. • Worse noise immunity due to multiple levels - higher bit error rates. • More expensive than NRZ codes. COMP312 - Data and Digital Signals – p.14/37 Bipolar Alternate Mark Inversion Mark is ‘1’ bit. COMP312 - Data and Digital Signals – p.15/37 Pseudoternary Mark is ‘0’ bit. COMP312 - Data and Digital Signals – p.16/37 Symbol rate • Previous schemes have had a 1:1 mapping of bit to symbol emitted • Not so good at keeping clock synchronisation • What if we increased the ratio of symbols per bit? • Baud: unit of symbol rate COMP312 - Data and Digital Signals – p.17/37 Biphase Schemes • Always at least one transition per bit period • Double the bandwidth requirements • No DC component • Very good synchronisation • Greater noise immunity COMP312 - Data and Digital Signals – p.18/37 Manchester • Used in 10Mbps Ethernet over UTP. • 0: high to low in middle of interval • 1: low to high in middle of interval COMP312 - Data and Digital Signals – p.19/37 Differential Manchester • Used in token ring LANs • 0: transition at beginning of interval • 1: no transition at beginning of interval COMP312 - Data and Digital Signals – p.20/37 Scrambling • Although biphase codes solve many problems, their bandwidth requirements are undesirable on long distance connections. • Scrambling removes long strings of constant line levels with transitions. • Removes potential DC components • Provides synchronisation • Can add error detection capability • May reduce required line rate COMP312 - Data and Digital Signals – p.21/37 Scrambling Schemes • NRZ Codes are scrambled by using a mathematical transformation to produce a random looking bit stream with many transitions. The receiver reverses the transformation to produce the original data stream. • Multilevel schemes can be scrambled by replacing long sequences of non-marks with defined patterns using polarity violations. • Example is B8ZS based on Bipolar-AMI with replacement of strings of eight zeroes • Following a positive mark use 000+-0-+ • Following a negative mark use 000-+0+- COMP312 - Data and Digital Signals – p.22/37 B8ZS COMP312 - Data and Digital Signals – p.23/37 100Mbps Ethernet coding: 4B5B • 10Mbps Ethernet uses Manchester encoding: ◦ Requires twice as much bandwidth as the original signal ◦ That is, 10Mbps Ethernet requires 20Mhz clock • 100Mbps Ethernet uses 4B5B: ◦ Encodes a set of 4 bits using a 5 bit code defined in dictionary ... ◦ ... designed to provide clock synchronisation. ◦ Comes at a cost of overhead; 5 signal elements are required to encode 4 bits. ◦ That is, 100Mbps Ethernet requires 125Mhz clock COMP312 - Data and Digital Signals – p.24/37 4B5B 0000 11110 11111 idle 0001 01001 11000 delimiter 0010 10100 10001 delimiter 0011 10101 01101 delimiter 0100 01010 00111 delimiter 0101 01011 00100 transmit error 0110 01110 other invalid 0111 01111 1000 10010 1001 10011 1010 10110 1011 10111 1100 11010 1101 11011 1110 11100 1111 11101 COMP312 - Data and Digital Signals – p.25/37 • Answer: NRZI; • 100Mbps Ethernet over fibre uses NRZI; 100Mbps Ethernet over UTP uses MLT-3 4B5B coding • Question: What coding scheme was used on the previous slide? COMP312 - Data and Digital Signals – p.26/37 • 100Mbps Ethernet over fibre uses NRZI; 100Mbps Ethernet over UTP uses MLT-3 4B5B coding • Question: What coding scheme was used on the previous slide? • Answer: NRZI; COMP312 - Data and Digital Signals – p.26/37 4B5B coding • Question: What coding scheme was used on the previous slide? • Answer: NRZI; • 100Mbps Ethernet over fibre uses NRZI; 100Mbps Ethernet over UTP uses MLT-3 COMP312 - Data and Digital Signals – p.26/37 MLT-3 MLT−3 1 0 0 1 1 1 1 0 0 0 0 1 0 1 1 1 0 1 1 0 1 1 1 0 COMP312 - Data and Digital Signals – p.27/37 8B10B • 4B5B is not DC-balanced • 8B10B is DC-balanced and is preferred for higher symbol rates. ◦ 8 bits are encoded into 10 symbols ◦ used in 1Gbps Ethernet, SATA, Firewire, PCI Express, DVI. ◦ still require 25% more bandwidth than bit rate COMP312 - Data and Digital Signals – p.28/37 64B66B • 64-bit to 66-bit line code used in 10Gbps Ethernet • 2 bit preamble followed by 64-bits of data (scrambled) • If preamble is ‘01’, following bits are entirely

Load more