Introduction to Communication Networks Spring Unit 3 Physical Layer

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1 Introduction to Communication Networks Spring 2007 Unit 3 Physical Layer EECS 122 SPRING 2007

2 Acknowledgements For this unit we have extensively used slides from the book by WIliam Stallings (our supplementary texbook). Some figures/tables have been taken from a book by B. Forouzan, Data Communications and Networking, Mc Graw Hill, 2004 Some Figures have been taken form the book by F. Halsall Computer Networking and the Internet, Fifth edition, Pearsons education limited, 2005 Some figures have been used form the earlier issues of the EECS 122 tought by Prof Jean Walrand. 2of 50

3 Fiber, radio... Something in common? In both cases only specific frequency bands are used, different form the natural spectrum of the signal. Think of voice... The useful signal has to be shifted where desired... We will call this pass-band transmission We do this, using modulation... 3of 50

4 Pass-band Transmission (Analog Data and Signals!) We shift the frequency spectrum elsewhere... The different kinds of modulation can be combined 4of 50

5 Why to Modulate Analog on Analog? Take Amplitude Modulation AM: If the modulated signal is a sinusoid with frequency f c, and data are within [0, B] Hz then the resulting spectrum is from f c to (f c +B) or from (f c -B) to f c One possible justification: smaller differences in attenuation... 5of 50

6 Digital Data / Digital Signal Encoding Digital Data: Sequence of bits to transmit Physical signal: Physical value e.g. voltage, changing in discrete time epochs. Encoding (channel coding): Mapping of bit sequences to signals. This can be done in many ways... Example: Unipolar (unbalanced) signaling Polar (balanced) signaling 6of 50

7 Digital Signal Encoding (2) Signaling period - T Number of signal levels - M Within the signaling period K changes of the signal level are possible, the number of changes per second = baud rate! There are M K signal shapes possible. Usually only some N < M K signal shapes are permitted. Achievable bit rate R=(lg 2 N)/T Bit rate is not necessarily equal to baud rate 7of 50

8 Digital Signal Encoding (3) Criteria for selection Signal spectrum : lack of high-frequency component lack of dc component (transformers!) transmitted power concentrated in the middle of bandwidth Clocking reconstruction at the receiver long periods of constant values avoided Some error detection features Polarity of the signal negligible (only the change counts) Noise immunity 8of 50

9 Digital Signal Encoding (4a) NRZ-L(Nonreturn-to-Zero-Level) 0 = high level 1 = low level Or exactly the other way round NRZI (Nonreturn-to-Zero-Inverted) polarity not important!!! 0 = no transition at beginning of interval 1 = transition at beginning of interval Bipolar AMI Assures signal without DC component 0 = no line signal 1 = positive or negative level, alternating Note: 3 level of signal, bit rate=baud rate Thus: the possible bit rate could be log 2 (3)= 1.58, but really only one bit /signaling time is send... 9of 50

10 The Discussed Codes.. 10 of 50

11 Digital Signal Encoding (4b) Pseudoternary 0 = positive or negative level, alternating zeros 1 = no line signal Manchester: Solves the problem of long constant values 0 = transition high > low in middle of interval 1 = transition low > high in middle of interval Or another way around. Note: Bit rate lower than baud rate!! Differential Manchester: as above, plus unimportant polarity! Always a transition in middle of interval 0 = transition at beginning of interval 1 = no transition at beginning of interval 11 of 50

12 NRZ vs. Manchester of 50

13 Spectral Efficiency of Codes 13 of 50

14 4-bit/5-bit code (used in 100Mb/Ethernet) [Walrand] Another approach to improve this efficiency comes later! 14 of 50

15 Intersymbol Interference The Received Signals for the Message Sequence 1 0 1, Showing the Effect of Intersymbol Interference with Rectangular Pulses (The effect of signals outside the sequence is not shown) 15 of 50

16 The Ways to Limit the Intersymbol Interference Why signal attenuation VARIABLE as function of frequency Cutting off higher frequencies Solutions: Amplifying different frequencies by different amounts Proper shape of the signals Raised cosine Proper sampling times at the receiver Bit synchronization And splitting the band (will be discussed later!) 16 of 50

17 Shaping of the Signal: Raised Cosine 2A cos(2παt) s( t) = 2 T 1 (4αt ) t sin(2α T t 2α T ) 17 of 50

18 Lack of Intersymbol Interference for the Raised Cosine Signal Note : Some intersymbol interference does not necessarily destroy the transmission! a) Message sequence b) Signal component for first 1 c) Signal component for 0 d) Signal component for second 1 e) Composite received signal 18 of 50

19 Comment: Reality. This does NOT mean that such shape of signals is really frequently used... But there are ALWAYS requirements as for the signal shape. See example from the CCITT V.10 recommendation (low bit rate transmission over twisted pairs...) 19 of 50

20 Proper Sampling Times... Clock at the sender and clock at the receiver are independent, there is always some DRIFT. Increasing the clock accuracy is not keeping pace with the increase of the speed of transmission.., If the receiver will sample the signal in wrong points in time, the errors in decoding will increase. Bit synchronization: assuring the proper epochs of sampling the individual bits 20 of 50

21 Errors Due to Wrong Bit Synchronization Example: Bipolar AMI 21 of 50

22 Asynchronous Transmission Asynchronous transmission: Symbols e.g. 8 bit coded - may start at any time, the gap between the symbols is arbitrarily long... Good for data with large gaps (keyboard) Simple Cheap Overhead of 2 or 3 bits per character (~20%) 22 of 50

23 Asynchronous Transmission general [Halsall] Scheme of the transmitter Serial transmission of a symbol (two stop bits) 23 of 50

24 Asynchronous Transmission sender [Halsall] Detection of new character start is triggered by signal change At least one signal change per character 24 of 50

25 Asynchronous Transmission details [Halsall] Operation of the receiver with clock rate equal to x baud rates; Clock rate equals respectively 1*, 4*, 16* the baud rate... Increasing x makes the synchronization more precise. This transmission mode works only with pretty low bit rates, and short symbols (a few bits/symbol) 25 of 50

26 Influence of different clock rates [Halsall] 26 of 50

27 Synchronous Transmission: Synchronous transmission: once transmission of bit stream has been started, the stream is continuously sent... Note: if there is NOTHING to send, just fillers should be transmitted... Initial synchronization to the sender is achieved using a training sequence with known bit sequence, e.g.: ( ). Update of the synchronization within the stream! Not less frequently than every XX bits... How to achieve? 27 of 50

28 Asynchronous vs. Synchronous transmission [Forouzan] 28 of 50

29 Synchronous: Explicit Clock Signal??? [Halsall] NO!: Sending directly the clock from the sender to the receiver works on a board but NOT in Telecommunications! 29 of 50

30 Self Synchronizing Codes [Halsall] Self synchronizing codes (e.g. Manchester) assure a transition of the signal level each bit (Manchester in the middle of each bit). 30 of 50

31 DPLL: Digital Phase-Locked-Loop (1) [Halsall] Introduce an initial synchronization pattern Be sure about the maximum number of bits with constant signal. Introduce a mechanism for adjusting the sampling epochs after observing the signal change 31 of 50

32 DPLL: Digital Phase-Locked-Loop (2) [Halsall] 32 of 50

33 Multilevel Schemes Note: The bit rate is twice (or 3 times!) the baud rate of 50

34 Which Bandwidth is needed? Nyquist result: Using M signal levels, In absence of noise; we could get at most : C [Bps] = 2B log 2 M where B is the bandwidth in Hz. Great idea: just increase the number of signal levels? Not really... There is noise... Shannon result: In presence of noise, theoretical upper bound is: C [Bps] = B log 2 (1 + SNR) for error free transmission without limits on delay! 34 of 50

35 The reality... Unfortunately limits are usually not achievable... And we DO have time limits... As result: there are errors in transmission. We do ususally look at the measure expressed as BIT ERROR RATE the ratio of bits in error... We plot it usually as a function of : Eb/No the ratio of (signal energy / bit) to (noise power density/ Hz) 35 of 50

36 Bit Error Rate for Multilevel PSK (example) Probability of error (BER) 36 of 50

37 The usage of spectrum Rememeber the spectrum efficiency of codes? The so far discussed digital transmission works with low pass channels of 50

38 Digital goes also on analog 38 of 50

39 Mixing the amplitude and phase modulation [Forouzan] 39 of 50

40 More complex in modems... [Forouzan] 40 of 50

41 Let us return to the Intersymbol Interference Increasing bit-rat causes shorter and shorter bits...(using more and more bandwidth) - the ISI becomes deadly. Solution: Multi-tone modulation split the bit stream into sub-streams Split the bandwidth into several sub-channels Assign each sub-stream of bits to a single sub-channel In a given time epoch each sub-channel will have in general different attenuation. The modulation will be different form sub-channel to sub-channel (and change in time!) THE technology for multiple new, high speed communication systems in which bandwidth IS an issue (therefore NOT in fiber optics ) 41 of 50

42 Example - DMT used in DSL home access [Forouzan] 42 of 50

43 Orthogonal Frequency Division Modulation [brodersen] OFDM an advanced form of multi-tone modulation! N carriers Symbol: 2 periods of f 0 B Data coded in frequency domain f Symbol: 4 periods of f 0 Symbol: 8 periods of f 0 + Transformation to time domain: each frequency is a sine wave in time, all added up. Transmit f Channel frequency response Receive time f Decode each frequency bin separately Time-domain signal B Frequency-domain signal 43 of 50

44 Features of Communication Channels Direct No intermediate devices Point-to-point Direct Channel, only 2 devices share link Multi-point More than two devices share the link Parallel link vs, Serial Link Simplex vs. Half Duplex vs. Duplex Properties of the Communication Channels 44 of 50

45 Serial vs. Parallel Transmission [Forouzan] 45 of 50

46 Simplex vs. Half Duplex vs. Full Duplex Simplex data flow Data One direction, e.g. Television Data Half-Duplex data flow delay Either direction, but only one way at a time, e.g. police radio Data Full-Duplex data flow Both directions at the same time e.g. telephone Data Data no delay 46 of 50

47 Properties of the Communication Channel Throughput Bits can be inserted in the channel with some speed. If bits arrive with higher speed than the channel can adopt than they have to be stored and delayed before transmission!! Delay Bits arrive at the receiver with a delay relative to their transmission. The minimal delay is defined by the signal propagation, further delay might occure if the channel is not a direct one. Errors in transmission Bits might be corrupted, but also lost.. less frequently duplicated.. Ideal channel unlimited throughput, no errors, no delay 47 of 50

48 Data for real media... [walrand] The propagation speed of guided media is usually assumed around 2/3 of the light speed 5 μs/km The bit rates of optical transmission approach 160 Gbits/s for a single carrier wavelength ( colors of light ), tens of carrier wavelengths can be supported by a single fiber 48 of 50

49 Different cases... [Walrand] 49 of 50

50 Bits are stored in the transmission! transmission speed time to send a Packet (1024 bytes) Bytes (packets) on media between New York and California (delay 30 ms) 64 kbit/s 128ms 240 (0,23) 2 Mbit/s 4,1ms (7,3) 10 Mbit/s 820 μs (36,6) 1 Gbit/s 8,2 μs (3662) 10 Gbits/s 0,82 μs (36620) Information on media = transmission speed x delay delay = distance / 0.66 * speed of light 50 of 50

51 Throughput, Propagation [Forouzan], modified With excessive data arrival intensity (as compared to the throughput!) a backlog of bits might start building up! This will be discussed later QUEUEING! 51 of 50

52 Data and Signals - Repetition 52 of 50

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