COMMUNICATIONS and NETWORKING CLASS 6 COMP 421 /CMPET 401 COMMUNICATIONS and NETWORKING CLASS 6

Encoding Techniques Digital data, digital signal Easy encoding / Less Complex Less Expensive Analog data, digital signal Can transmit data over Digital Network Digital data, analog signal Modems / Fiber / Unguided Media Analog data, analog signal Cheap & Easy Baseband Transmission / FDM

Analog Data Choices easy conversion process Analog data, analog signal Analog data, digital signal permits use of modern digital switching and transmission equipment

Digital Data Choices Digital data, analog signal equipment is less expensive than digital to analog conversion some transmission media (e.g. optical fiber or unguided media) can only send analog signals

Transmission Choices Analog transmission Digital transmission Only transmits analog signals, without regard for data content Attenuation overcome with amplifiers Digital transmission Transmits analog or digital signals Uses repeaters rather than amplifiers

Advantages of Digital Transmission The signal is exact Signals can be checked for errors Noise/interference are easily filtered out A variety of services can be offered over one line Higher bandwidth is possible with data compression

Encoding schemes Analog data, Analog signal Analog data, Digital signal voice analog analog digital Telephone CODEC Digital data, Analog signal Analog transmission means a transmitting analog signals without regard to their content; the signal may represent analog data (e.g. voice) or digital data (e.g. binary data). Digital transmission is concerned with a transmitting binary signal Digital data, Digital signal digital Modem analog digital Digital transmitter digital

Encoding and Modulation x(t) g(t) x(t) g(t) Encoder Decoder digital or analog digital t s(f) m(t) s(t) Modulator Demodulator m(t) For digital signal, a data source g(t), which may be either digital or analog, is encoded into a digital signal x(t). Analog tranmission uses a continuous constant-frequency signal known as the carrier signal. The frequency of the carrier signal is choosen to be compatible with the transmission medium. Data is transmitted using a carrier signal by modulation, which is the process of encoding source data onto a carrier signal with frequency fc digital or analog analog f fc fc

Why encoding? Three factors determine successfulness of receiving a signal: S/N (Signal to Noise Ratio) Data rate Bandwidth With other factors held constant, the following statements are true. An increase in data rate increases bit error rate An increase S/N decreases bit error rate An increase in bandwidth allows an increase in data rate [Stalling, p98]

Encoding Schemes' Evaluation Factors Signal spectrum Clocking Error detection Signal interference & noise immunity Cost and complexity Five factors are used to evaluate the various encoding scheme: Signal spetrum A lack of high-frequency components means that less bandwidth is required for transmission. No dc component is desirable. Clocking Suitable encoding provide some synchronization mechanism to determine the beginning and end of each bit position. Error detection Some error detection can be built into the encoding scheme Signal interference & noise immunity Some encoding scheme has superior performance in the presence of noise Cost and complexity Higher signaling rate to achieve a greater data rate results expensive devices.

Digital Data, Digital Signal / Characteristics Uses discrete, discontinuous, voltage pulses Each pulse is a signal element Binary data is encoded into signal elements

Terms (1) Unipolar Polar Data rate Duration or length of a bit All signal elements have same sign Polar One logic state represented by positive voltage the other by negative voltage Data rate Rate of data transmission in bits per second Duration or length of a bit Time taken for transmitter to emit the bit

Terms (2) Modulation rate Mark and Space Rate at which the signal level changes Measured in baud = signal elements per second Mark and Space Binary 1 and Binary 0 respectively

Interpreting Signals Need to know Timing of bits - when they start and end Signal levels Factors affecting successful interpretation of signals: Signal to noise ratio Data rate Bandwidth

Comparison of Encoding Schemes (1) Signal Spectrum Lack of high frequencies reduces required bandwidth Lack of dc component allows ac coupling via transformer, providing isolation It is important to concentrate power in the middle of the bandwidth

Comparison of Encoding Schemes (2) Clocking issues Synchronizing transmitter and receiver is essential External clock is one way used for synchronization Synchronizing mechanism based on signal is also used & preferred (over using an external clock)

Spectral density 0.5 1 1.5 1.5 B8ZS,HDB3 NRZ-L, NRZI 1 AMI, Pseudoternary Mean square voltage per unit bandwidth 0.5 Manchester, Differential Manchester NRZ make efficient use of bandwidth. most of the frequency in NRZ and NRZI signals are between dc and half the bit rate. Manchester& Different Manchesterh has the bulk of the energy between one-half and one times the bit rate. Thus the bandwidth is reasonably narrow and contain no dc component. AMI make use of bandwidth less than the bandwidth of NRZ [Stallings, p102] 0.5 1 1.5 -0.5 Normalized frequency (f/r)

Comparison of Encoding Schemes (3) Error detection Can be built into signal encoding Signal interference and noise immunity Some codes are better than others Cost and complexity Higher signal rate (& thus data rate) lead to higher costs Some codes require signal rate greater than data rate

Encoding Schemes Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Bipolar -AMI (Alternate Mark Inversion) Pseudoternary Manchester Differential Manchester B8ZS HDB3

Digital Data, Digital Signal 1 1 1 1 1 NRZ NRZI Bipolar -AMI Pseudoternary Definition of Digital Signal Encoding Formats Nonreturn-to-Zero-Level (NRZ-L) 0 = hign level 1 = low leve l Nonreturn to Zero Inverted (NRZI) 0 = no transition at beginning of interval (one bit time) 1 = transition at beginning of interval Bipolar-AMI 0 = no line signal 1 = positive or negative level, alternating for successive ones Pseudoternary 0 = positive or negative level, alternating for successive zeroes 1 = no line signal Manchester 0 = transition from high to low in middle of interval 1 = transition from high to low in middle of interval Differential Manchester Always a transitiion in middle of interval 0 = no transition at beginning of interval B8ZS Same as bipolar AMI, except that anystring of eight zeros is replaced by a string with two code violations HDB3 Same as bipolar AMI, except that any string of four zeros is replaced by a string with one code violation [Stallings, p99,100] Manchester Differential Manchester

Nonreturn to Zero-Level (NRZ-L) Two different voltages: 0 - Low Level 1 - High Level Voltage constant during bit interval Most often, negative voltage for one value and positive for the other

Nonreturn to Zero Inverted Nonreturn to zero inverted on ones Constant voltage pulse for duration of bit Data encoded as presence or absence of signal transition at beginning of bit time Transition (low to high or high to low) denotes a binary 1 No transition denotes binary 0 An example of differential encoding (Data represented by changes rather than levels)

NRZ

NRZ pros and cons Pros Cons Used for magnetic recording Easy to engineer Makes good use of bandwidth Cons dc component Lack of synchronization capability Used for magnetic recording Not often used for signal transmission

Bipolar-AMI Zero represented by no line signal One represented by positive or negative pulse One pulses alternate in polarity No loss of sync if a long string of ones happens (zeros still a problem) No net dc component  Can use a transformer for isolating transmission line Lower bandwidth Easy error detection

Pseudoternary One represented by absence of line signal Zero represented by alternating positive and negative No advantage or disadvantage over bipolar-AMI

Bipolar-AMI and Pseudoternary

Trade Off for Multilevel Binary Not as efficient as NRZ With multi-level binary coding, the line signal may take on one of 3 levels, but each signal element, which could represent log23 = 1.58 bits of information, bears only one bit of information Receiver must distinguish between three levels : (+A, -A, 0) Requires approx. 3dB more signal power for same probability of bit error

Biphase Manchester Used by IEEE 802.3 (Ethernet) Transition in middle of each bit period Transition serves as clock and data One is represented by a transition from low to high Zero is represented by a transition from high to low Used by IEEE 802.3 (Ethernet)

Differential Manchester Always a transition in the middle of the interval for clocking Transition at start of a bit period represents zero No transition at start of a bit period represents one Note: this is a differential encoding scheme used by IEEE 802.5 (Token Ring)

Biphase Pros and Cons Con At least one transition per bit time and possibly two Maximum modulation rate is twice NRZ Requires more bandwidth Pros Synchronization on mid bit transition (self clocking) No dc component Error detection Absence of expected transition points to error in transmission

Modulation Rate The modulation Rate is at which signal elements are generated In General the Modulation Rate D = R/b where R=Data Rate=bits/sec b=number of bits per signal element Data Rate (bit Rate 1/Tb) where Tb is bit duration For Manchester Encoding maximum Rate is: 2/Tb

Scrambling Techniques Used to reduce signaling rate relative to the data rate by replacing sequences that would produce constant voltage for a priod of time with a filling sequence that accomplishes the following goals: Must produce enough transitions to maintain synchronization Must be recognized by receiver and replaced with original data sequence is same length as original sequence

Scrambling Techniques No dc component No long sequences of zero level line signal No reduction in data rate Error detection capability As an example, fax machines use the modified Huffman code to accomplish this.

B8ZS Same as Bipolar AMI with 8 Zeros Substitution B8ZS: Abbreviation for bipolar with eight-zero substitution Same as Bipolar AMI with 8 Zeros Substitution Based on Bipolar-AMI If octet of all zeros and last voltage pulse preceding was positive, encode as 000+-0-+ If octet of all zeros and last voltage pulse preceding was negative, encode as 000-+0+- Causes two violations of AMI code This is unlikely to occur as a result of noise Receiver detects and interprets the sequence as octet of all zeros

B8ZS A one is sent on a T1 by sending a pulse, as opposed to not sending a pulse. The alternating mark rule means that if the last pulse sent was of a positive going polarity, the next pulse sent must be negative going. If a T1 device receives two pulses in a row and they are of the same polarity a bipolar violation (BPV) has occurred. In B8ZS a specific combination of valid pulses and bipolar violations is used to represent a string of eight zeroes, whenever the user data contains eight zeroes in a row

B8ZS Since a T1 uses a single pair of wires in each direction and the only signals on those wires are the pulses which represent data; the only way to recover clock and retain synchronization on a T1 is by detecting the rate at which pulses are being received. All of the equipment in a T1 circuit must operate at the same rate because all of the equipment must sense the T1 at the correct time in order to determine if a pulse (1) or no pulse (0) has been received at each bit time. Since only ones are sent as pulses and zeroes are represented by doing nothing, if too many zeroes are sent at a time there will be no pulses on the T1 at all and the clock circuitry in all of the hardware will rapidly fall out of synchronization. Thus the design of AMI requires that a certain ONES DENSITY be maintained, that a certain minimum of the bits over a certain period of time be guaranteed to be a ONE (pulse). This is why AMI circuits require DENSITY enforcement

B8ZS Briefly stated; on average one bit in eight must be a one and no more than (varies according to specific standard) so many zeroes may be sent in a row. In order to be able to satisfy the ones density requirement on an AMI T1 one bit out of every eight is taken away from the user, not available for voice or data traffic, and that 1 bit in 8 is always sent as a one. Once this has been done the requirement for ones density is satisfied and the user is free to send any data pattern in the remaining bandwidth.

B8ZS The rate of a T1 is 1.544 megabits per second. 8K is used for framing leaving 1.536MBPS. The 1.536 is usually divided into 24 timeslots (DS0s) or "channels" each being inherently 64KBPS. By taking the 1 bit in 8 that is reserved to satisfy ones density the user is left with 56K per timeslot.

AMI AMI = Alternate Mark Inversion. This is the original method of formatting T1 data streams. In AMI a zero is always sent by doing nothing, at the time when a pulse might otherwise be sent, a pulse is not sent to represent a zero. A one is sent on an AMI T1 by sending a pulse, as opposed to not sending a pulse. The alternating mark rule means that if the last pulse sent was of a positive going polarity, the next pulse sent must be negative going. If an AMI T1 device receives two pulses in a row and they are of the same polarity a bipolar violation (BPV) has occurred. Thus AMI has a rudimentary error checking capability with a 50% probability of detecting altered, inserted or lost bits end to end.

ESF Extended Super Frame A DS level and framing specification for synchronous digital streams over circuits in the North America. A DS1 "frame" is composed of 24 eight-bit bytes plus one framing bit (193 bits). 8000 bytes per second come from each source, and thus 8000 frames per second are transported by the DS1 signal. The result is 193*8000 = 1,544,000 bits per second. In the original standard, the framing bits continuously repeated the sequence 110111001000, and such a 12-frame unit is called a super-frame. In voice telephony, errors are acceptable (early standards allowed as much as one frame in six to be missing entirely), so the least significant bit in two of the 24 streams was used for signaling between network equipments. This is called robbed bit signaling

ESF To promote error-free transmission, an alternative called the extended super-frame (ESF) of 24 frames was developed. In this standard, six of the 24 framing bits provide a six bit cyclic redundancy check(CRC-6), and six provide the actual framing. The other 12 form a virtual circuit of 4000 bits per second for use by the transmission equipment, for call progress signals such as busy, idle and ringing. DS1 signals using ESF equipment are nearly error-free, because the CRC detects errors and allows automatic re-routing of connections.

HDB3 High Density Bipolar 3 Zeros Based on bipolar-AMI String of four zeros replaced with one or two pulses Note: The following is the explanation for the HDB3 code example on the next slide (see rules in Table 5.4, page 142): Assuming that an odd number of 1's have occurred since the last substitution, since the polarity of the preceding pulse is "-", then the first 4 zeros are replaced by "000-". For the next 4 zeros, since there have been no Bipolar pulses since the 1st substitution, then they are replaced by"+00+" since the preceding pulse is a "-". For the 3rd case where 4 zeros happen, 2 (even) Bipolar pulses have happened since the last substitution and the polarity of the preceding pulse is "+", so "-00-" is substituted for the zeros.

B8ZS and HDB3 See Table 5.4 for HDB3 Substitution Rules (Assume odd number of 1s since last substitution) See Table 5.4 for HDB3 Substitution Rules

Digital Data, Analog Signal Transmitting digital data through PSTN (Public telephone system) 300Hz to 3400Hz bandwidth modem (modulator-demodulator) is used to convert digital data to analog signal and vice versa Three basic modulation techniques are used: Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PSK)

Modulation Techniques

Amplitude Shift Keying Values represented by different amplitudes of carrier Usually, one amplitude is zero i.e. presence and absence of carrier is used Susceptible to sudden gain changes Inefficient Up to 1200bps on voice grade lines Used over optical fiber

ASK Vd(t) Vc(t) VASK(t) Signal power frequency spectrum Frequency In ASK, the amplitude of a single-frequency known as the carrier frequency is switched between two levels at a rate determined by the bit rate of the transmitted binary data signal. Bandpass filter is used to limit the band of frequencies based on Nyquist’s theorem. [Halsall, p.59] Signal power frequency spectrum Frequency fc-3f0 fc-f0 fc fc+f0 fc+3f0

Frequency Shift Keying Values represented by different frequencies (near carrier) Less susceptible to error than ASK Up to 1200bps on voice grade lines High frequency radio (3-30 MHz) Higher frequency on LANs using co-ax

FSK Data signal vd(t) Carrier 1 v1(t) v2(t) Carrier 2 FSK(t) Signal In FSK, two fixed amplitude carrier signals are used, one for a binary 0 and the other for a binary 1. The different between the two carriers is known as the frequency shift. The modulation operation is equivalent to summing together the outputs of two separate ASK modulators. FSK is the modulation method that was used in all early low bit rate modems. [Halsall p.62] Signal power frequency spectrum Frequency f1 f2

FSK in modem (on Voice Grade Line) 400 980 (1070) 1850 (2225) 1180 (1270) 1650 (2025) 3400 Amplitude Frequency(Hz) PSTN bandwidth frequency spectrum

Phase Shift Keying Phase of carrier signal is shifted to represent data Differential PSK Phase shifted relative to previous transmission rather than some reference signal

PSK bit rate = signaling rate Data Signal vc(t) Carrier vc(t) Phase coherent vPSK(t) Differential v’PSK(t) In PSK, the phase of the carrier signal is shift to represent data. Two type of PSK are used. Phase coherent PSK : Used two fixed carrier signals to represent a binary 0 and 1 with a 180° phase different, and is known asst used two fixed carrier signals to represent a binary 0. the disanvantage of this scheme us that a reference carrirer signal is required at the receiver against with the phase of the received signal is compared, this required more complex demodulation circuit. Differential PSK : A phase shift of 90° relative to the current signal indicates a binary 0 is the next bit while a phase shift 270° indicates a binary 0.[Halsall, p.65] bit rate = signaling rate 180=0 0=1 Differential example: for every logic 1, 180 degree phase shift phase diagram

Quadrature PSK More efficient use by each signal element representing more than one bit e.g. shifts of /2 (90o) Each element represents two bits Can use 8 phase angles and have more than one amplitude 9600bps modems use 12 angles , four of which have two amplitudes

Multilevel Modulation Method 00 01 10 11 +90°=01 +180°=10 0°=00 +270°=11 0° +90° +180° +270° 4-PSK phase diagram More sophisticated modulation methods are used which involve either multiple signal levels or a mix of the basic scheme, particularly amplitude and phase. More bit rate can be achieved if signaling element represent more than one bit. QPSK ( Quadrature PSk or 4-PSK) : Used four different phase change( 0°, 90°, 180°, 270°) to enables each phase change to convey 2 bits (bitrate=2*signaling rate). QAM (Quadrature Amplitude Modulation or 16-QAM) : Phase and amplitude changes, 16 levels persignal element and hence 4 bit symbols (bitrate=4*signaling rate). [Halsall, p.67] bit rate = n x signaling rate

Performance of Digital to Analog Modulation Schemes Bandwidth ASK and PSK bandwidth directly related to bit rate FSK bandwidth related to data rate for lower frequencies Requires more analog bandwidth than ASK In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK

Analog Data, Digital Signal Digitization Conversion of analog data into digital data Digital data can then be transmitted using NRZ-L or using other codes Digital data can then be converted to analog signal Analog to digital conversion done using a CODEC Pulse code modulation Delta modulation

Analog data, Digital signal Two principle techniques used PCM (Pulse Code Modulation) DM (Delta Modulation) Sampling clock PAM signal PCM signal Digitization is a process of convering analog data into digital data. The digital signal is converted back into analog signal at the receiver. The device used for converting analog data into digital form, and recovering the original analog data is known as CODEC (Coder-Decoder). Sampling Circuit Quantizer and compander Analog voice signal Digitized voice signal

Pulse Code Modulation(PCM) (1) If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal (Proof - Stallings appendix 4A) Voice data limited to below 4000Hz Require 8000 sample per second Analog samples (Pulse Amplitude Modulation, PAM) Each sample assigned digital value

Pulse Code Modulation(PCM) (2) 4 bit system gives 16 levels Quantized Quantizing error or noise Approximations mean it is impossible to recover original exactly 8 bit sample gives 256 levels Quality comparable with analog transmission 8000 samples per second of 8 bits each gives 64kbps

Pulse Code Modulation(PCM) (3) The process starts with an analog signal, which is sampled by PAM sample. the resulting pulse are quantized to produced PCM pulses and then encoded to produce bit stream. At the receiver end, the process is reversed to reproduce the analog signal.

PCM 011100011011001100 Sampling signal based on Nyquist theorem Original signal 3.9 4.2 3.4 3.2 2.8 PAM pulse 1.2 PCM is based on the sampling theorem. The original signal is assumed to be bandlimited with a bandwidth of B. Signal is sampled at a rate 2B. Samples signal are represents as narrow pulse whose amplitude is proportional to the value of the original signal and is known as PAM (Pulse Amplitude Modulation). The amplitude of each PAM pulse is approximated by an n-bit interger,. In the sample above, n=3. Thus 8=23 levels are used for approximating the PAM pulses. PCM pulse with quantized error 3 4 3 3 4 1 011 100 011 011 001 100 PCM output 011100011011001100

Nonlinear Encoding Quantization levels are not necessarily equally spaced. The problem with equal spacing is that the mean absolute error for each sample is the same, regardless the signal level. Lower amplitude values are relatively more distorted. Nonlinear encoding reduces overall signal distortion Can also be done by companding

Without nonlinear encoding With nonlinear encoding Quantizing level 15 15 14 Strong signal 14 13 13 12 12 11 11 10 10 9 8 Weak signal 9 8 7 7 6 6 5 Quantization levels are not necessary equally spaced. The problem with equal spacing is that the mean absolute error for each sample is the same, regardless the signsl level. Lower amplitude values are relatively more distorted. [Stallings, p.118] 4 5 4 3 3 2 2 1 1 Without nonlinear encoding With nonlinear encoding

Nonlinear Encoding Prior to the input signal being sampled and converted by ADC into a digital form, it is passed through a circuit known as a compressor. Similarly, at the destination, the reverse operation is perform on the output of the DAC by a circuit known as expander.

Delta Modulation Analog input is approximated by a staircase function Move up or down one level () at each sample interval Binary behavior Function moves up or down at each sample interval

Delta Modulation - example

Delta Modulation - Performance Good voice reproduction PCM - 128 levels (7 bit) Voice bandwidth 4khz Should be 8000 x 7 = 56kbps for PCM Data compression can improve on this e.g. Interframe coding techniques for video

Analog Data, Analog Signals Why modulate analog signals? Higher frequency can give more efficient transmission Permits frequency division multiplexing Types of modulation Amplitude Frequency Phase

Multilevel Modulation Method Quadrature Amplitude Modulation (QAM) Combines differential phase and amplitude shifts to achieve 16 distinct states, thereby allowing 4 bits to be represented by a single signal More sophisticated modulation methods are used which involve either multiple signal levels or a mix of the basic scheme, particularly amplitude and phase. QPSK ( Quadrature PSk or 4-PSK) : Four different phase changes (0°, 90°, 180°, 270°) to enables each phase change to convey 2 bits (bitrate=2*signaling rate). QAM (Quadrature Amplitude Modulation or 16-QAM) : Phase and amplitude changes, 16 levels per signal element and hence 4 bit symbols (bitrate=4*signaling rate). 16-QAM phase diagram

V.34 Modulation V.34 Also known as V.FAST. It will allow modems to operate at 28Kb/s. Uses multidimensional trellis coding and line probing equalization, power control and framing. Adaptive Pre-Emphasis or Precoding is a new form of adaptive equalization that modifies the transmitted signal as well as the receiver. Trellis Coding in more complex forms (64-state 4D, 32-state 4D, etc.) make more efficient use of constellation space. Non-linear encoding wraps the constellation space to bring the inner points closer and increase the distance between the outer points. Shell Mapping forms circular constellations which are optimum shape. Shaping distributes consolation points nearer the center, which is less sensitive to noise. Adaptive Power Control changes the levels to produce the best performance over impaired channels. This capability may also improve performance over analog cellular services. Scaling maintains the best transmit power levels when different modulation technologies are employed. Framing encodes bits over multiple symbols. This increases the systems ability to support different combinations of symbol and data rates and makes it possible to integrate a secondary channel.   V.FC V.FAST Class developed by Rockwell International. It is based on the V.34 proposed design, but it is an interim solution. It does not support the V.8 handshaking mechanism for full V.34 compatibility (it will require a software modification) V.8 negotiation using a modulated calling tone and answer tone transfers information about two modem’s functional capabilities in 5 seconds or less.

The 56K Modem The V.90 modulation uses PAM. Each symbol is a different voltage level. 128 symbols multiplied by 8000 symbols per second, gives a 56,000 bits per second downstream rate. If the environment is noisy, less voltage levels are used. For example, if 64 are in use, then the speed will be 48,000 bits per second in a 56Kbps connection, the server is a digital modem. The PAM modulation requires at least 45dB SNR. The minimum RX level a receiver can pull in is 34db below TX. For upstream transmission, the information is transmitted in the old way, analog, using QAM, A2D, through the PSTN, D2A and analog again. The upstream rate is limited to 31.2Kbps

END Class