Networks: Data Encoding 1 Physical Layer – Part 2 Data Encoding Techniques

Analog and Digital Transmissions:

Networks: Data Encoding 2 Analog and Digital Transmissions Figure 2-23.The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs.

Data Encoding Techniques:

Networks: Data Encoding 3 Data Encoding Techniques Digital Data, Analog Signals [ modem ] Digital Data, Digital Signals [ wired LAN ] Analog Data, Digital Signals [ codec ] Frequency Division Multiplexing (FDM) Wave Division Multiplexing (WDM) [ fiber ] Time Division Multiplexing (TDM) Pulse Code Modulation (PCM) [ T1 ] Delta Modulation

Digital Data, Analog Signals [Example – modem]:

Networks: Data Encoding 4 Digital Data, Analog Signals [Example – modem] Basis for analog signaling is a continuous, constant-frequency signal known as the carrier frequency. Digital data is encoded by modulating one of the three characteristics of the carrier: amplitude , frequency , or phase or some combination of these.

PowerPoint Presentation:

Networks: Data Encoding 5 A binary signal Frequency modulation Amplitude modulation Phase modulation Figure 2-24.

Modems:

Networks: Data Encoding 6 Modems All advanced modems use a combination of modulation techniques to transmit multiple bits per baud . Multiple amplitude and multiple phase shifts are combined to transmit several bits per symbol. QPSK (Quadrature Phase Shift Keying) uses multiple phase shifts per symbol. Modems actually use Quadrature Amplitude Modulation (QAM). These concepts are explained using constellation points where a point determines a specific amplitude and phase.

Digital Data, Digital Signals [the technique used in a number of LANs]:

Networks: Data Encoding 8 Digital Data, Digital Signals [the technique used in a number of LANs] Digital signal – is a sequence of discrete, discontinuous voltage pulses. Bit duration :: the time it takes for the transmitter to emit the bit. Issues Bit timing Recovery from signal Noise immunity

NRZ ( Non-Return-to-Zero) Codes:

Networks: Data Encoding 9 NRZ ( N on- R eturn-to- Z ero) Codes Uses two different voltage levels (one positive and one negative) as the signal elements for the two binary digits. NRZ-L ( N on- R eturn-to- Z ero- L evel) The voltage is constant during the bit interval. NRZ-L is us ed for short distances between terminal and modem or terminal and computer. negative voltage 0 positive voltage

NRZ ( Non-Return-to-Zero) Codes:

Networks: Data Encoding 10 NRZ ( N on- R eturn-to- Z ero) Codes NRZ-I ( N on- R eturn-to- Z ero- I nvert on ones) The voltage is constant during the bit interval. NRZI is a differential encoding (i.e., the signal is decoded by comparing the polarity of adjacent signal elements.) 1 existence of a signal transition at the beginning of the bit time (either a low-to-high or a high-to-low transition) 0 no signal transition at the beginning of the bit time

Bi –Phase Codes:

Networks: Data Encoding 11 Bi –Phase Codes Bi- phase codes – require at least one transition per bit time and may have as many as two transitions. the maximum modulation rate is twice that of NRZ greater transmission bandwidth is required. Advantages: Synchronization – with a predictable transition per bit time the receiver can “synch” on the transition [self-clocking] . No d.c. component Error detection – the absence of an expected transition can used to detect errors.

Manchester encoding :

Networks: Data Encoding 12 Manchester encoding There is always a mid-bit transition {which is used as a clocking mechanism}. The direction of the mid-bit transition represents the digital data. Consequently, there may be a second transition at the beginning of the bit interval. Used in 802.3 baseband coaxial cable and CSMA/CD twisted pair. 1 low-to-high transition 0 high-to-low transition Textbooks disagree on this definition!!

Differential Manchester encoding :

Networks: Data Encoding 13 Differential Manchester encoding mid-bit transition is ONLY for clocking. Differential Manchester is both differential and bi-phase. Note – the coding is the opposite convention from NRZI. Used in 802.5 (token ring) with twisted pair. * Modulation rate for Manchester and Differential Manchester is twice the data rate inefficient encoding for long-distance applications. 1 absence of transition at the beginning of the bit interval 0 presence of transition at the beginning of the bit interval

Bi-Polar Encoding:

Networks: Data Encoding 14 Bi-Polar Encoding Has the same issues as NRZI for a long string of 0’s. A systemic problem with polar is the polarity can be backwards. 1 alternating +1/2 , -1/2 voltage 0 0 voltage

Analog Data, Digital Signals [Example – PCM (Pulse Code Modulation)]:

Networks: Data Encoding 16 Analog Data, Digital Signals [Example – PCM (Pulse Code Modulation)] The most common technique for using digital signals to encode analog data is PCM. Example: To transfer analog voice signals off a local loop to digital end office within the phone system, one uses a codec . Because voice data limited to frequencies below 4000 HZ, a codec makes 8000 samples/sec. (i.e., 125 microsec/sample).

Networks: Data Encoding 19 Figure 2-31. (a) The original bandwidths. (b) The bandwidths raised in frequency. (c) The multiplexed channel. Frequency-division Multiplexing

Networks: Data Encoding 22 Time-division Multiplexing

PowerPoint Presentation:

Networks: Data Encoding 23 Statistical Multiplexing - Concentrator

Pulse Code Modulation (PCM):

Networks: Data Encoding 24 Pulse Code Modulation (PCM) Analog signal is sampled. Converted to discrete-time continuous-amplitude signal (Pulse Amplitude Modulation) Pulses are quantized and assigned a digital value . A 7-bit sample allows 128 quantizing levels.

Pulse Code Modulation (PCM):

Networks: Data Encoding 25 Pulse Code Modulation (PCM) PCM uses non-linear encoding, i.e., amplitude spacing of levels is non-linear. There is a greater number of quantizing steps for low amplitude. This reduces overall signal distortion. This introduces quantizing error (or noise). PCM pulses are then encoded into a digital bit stream. 8000 samples/sec x 7 bits/sample = 56 Kbps for a single voice channel.

PowerPoint Presentation:

Networks: Data Encoding 26

PowerPoint Presentation:

Networks: Data Encoding 27 PCM Nonlinear Quantization Levels

Networks: Data Encoding 29 The T1 carrier (1.544 Mbps). TDM Figure 2-33. T1 Carrier (1.544Mbps)

Delta Modulation (DM):

Networks: Data Encoding 30 Delta Modulation (DM) The basic idea in delta modulation is to approximate the derivative of analog signal rather than its amplitude. The analog data is approximated by a staircase function that moves up or down by one quantization level at each sampling time. output of DM is a single bit. PCM preferred because of better SNR characteristics.