Digital Line Encoding Converting standard logic level to a form more suitable to telephone line transmission. Six factors must be considered when selecting a line encoding format; 1.transmission voltage & DC component 2.Duty cycle 3.Bandwidth consideration 4.Clock and framing bit recovery 5.Error detection 6.Ease of detection and decoding

Why Digital Signaling? Low cost digital circuits The flexibility of the digital approach (because digital data from digital sources may be merged with digitized data derived from analog sources to provide general purpose communication system)

Digital Modulation Using Digital Signals to Transmit Digital Data –Bits must be changed to digital signal for transmission –Unipolar encoding Positive or negative pulse used for zero or one –Polar encoding Uses two voltage levels (+ and - ) for zero or one –Bipolar encoding +, -, and zero voltage levels are used

Non-Return to Zero-Level (NRZ-L) Two different voltages for 0 and 1 bits. Voltage constant during bit interval. –no transition, no return to zero voltage More often, negative voltage for one value and positive for the other.

Non-Return to Zero Inverted (NRZ-I) 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

Multilevel Binary( 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 (zeros still a problem) No net dc component 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

Manchester 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. 1  low-to-high transition 0  high-to-low transition Consequently, there may be a second transition at the beginning of the bit interval. Used in baseband coaxial cable and CSMA/CD twisted pair.

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

Bipolar With 8 Zeros Substitution (B8ZS) Based on bipolar-AMI If octet of all zeros and last voltage pulse preceding was positive encode as If octet of all zeros and last voltage pulse preceding was negative encode as Causes two violations of AMI code Unlikely to occur as a result of noise Receiver detects and interprets as octet of all zeros

B8ZS and HDB3

High Density Bipolar 3 Zeros (HDB3) Also based on bipolar-AMI String of four zeros replaced with one or two pulses