Line Coding
Line Coding Basics:
Transmission of serial data over any distance, be it a twisted pair, fiber optic
link, coaxial cable, etc., requires maintenance of the data as it is transmitted through
repeaters, echo chancellors and other electronically equipment. The data integrity
must be maintained through data reconstruction, with proper timing, and
retransmitted. Line codes were created to facilitate this maintenance.
In selecting a particular line-coding scheme some considerations must be
made, as not all line codes adequately provide the all-important synchronization
between transmitter and receiver. Other considerations for line code selection are
noise and interference levels, error detection and error checking, implementation
requirements, and the available bandwidth.
Question: How do we evaluate which one is better?
Answer: We have to come up with a standard to compare the techniques.
Comparison & Evaluation Standards:
1. Signal Spectrum.
+ Lack of high frequency components → Small BW → Multiplexing
+ Lack of D.C. component → A.C. coupling via transformer → electrical
isolation → low interference
+ Good spectrum shape → Concentrated power to the center of the
transmitted signal spectrum → Less signal
distortion → Less ISI.
2. Clocking
Synchronization mechanism inherent to the encoding strategy.
3. Error detection
Some error detection capability built into the physical signaling encoding scheme.
4. Signal interference & Noise immunity
Superior performance in the presence of noise → Differential Encoding.
5. Cost and complexity.
Now I lest some of the line codes with there proprieties and application:
• Nonreturn to zero (NRZ)
0 = low level, 1 = high level. And unipolar NRZ: 0V and “high” volts (e.g.
TTL 0V/5V). Bipolar NRZ: positive and negative.
It has manly two types:
•
Nonreturn to Zero level (NRZ-L)
Commonly used code to generate or interpret digital data by digital devices.
•
Nonreturn to Zero, invert on (the rising edges of) One (NRZ-I)
More reliable to detect transition in the presence of noise. (i.e. non-differential
always compares to the threshold.) NRZ techniques are used for digital magnetic
recording. This line code is shown in Fig1.
Problem with NRZ-L & NRZ-I
1-Possesses D.C. components.
2-Lack of synchronization capabilities.
Figure 1 the NRZ line code.
• Multilevel Binary.
Bipolar-AMI (Alternate Mark Inversion) & Pseudoternary:
No less of synchronization (Bipolar –AMI: 111… Pseudoternary: 000..,) No net
d.c. component. Less BW compared to NRZ. Error detection for isolated errors →
From inherent pulse-alternation property.
Problem with Bipolar-AMI & Pseudoternary:
1) Binary data in multilevels – Less efficient in information encoding. Needs
3dB more power compared to NRZ → Fixed S/N : Higher BER.
2) Possible Loss of synchronization → SCRAMBLING. (Bipolar-AMI :
consecutive 0s → constant voltage zero level. Pseudoternary : consecutive 1s → constant
voltage zero level.)
Bipolar-AMI used in Primary ISDN interface (with 1.544Mbps). And
Pseudoternary used in Baric ISDN interface (with 192 kbps).
Figure 2 the Multilevel Binary line code.
• Biphase.
Have two types:
•
Manchester code – Transition in the middle of the bit period.
Transition in middle of bit period
Mid-bit transition serves as clock and data
1 = low-to-high transition
0 = high-to-low transition
•
Differential Manchester Code.
Mid-bit transition is clock
Data is encoded at the beginning of the bit period
0 = transition at beginning of bit period
1 = no transition at beginning
Modulation Rate ~ near twice of NRZ → Requires much lager BW! (For both).
Advantages:
1) Synchronization → Self-clocking codes. = Biphase codes.
2) No D.C. component → Always a balanced 0 & 1 levels usage.
3) Error detection → Noise would have to invert both signal levels sequentially to
create an undetected error pattern.
Manchester Code used for IEEE 802.3 standard for baseband coaxial cable
and twisted pair CSMA/CD bus LANs. And the Differential Manchester Code used
for IEEE 802.5 Tocken Ring LAN, using shielded twisted pairs.
Figure 3 the Biphase line code.
• Bipolar with 8-Zeros Substitution (B8ZS)
Modification to Bipolar-AMI to eliminate string of 0 bits:
–Replace any octet of all 0 (00000000) with:
000+-0-+
if previous non-zero signal was +
000-+0+-
if previous non-zero signal was -
–This causes 2 code violations, so receiver knows it is a substitution byte, not
a transmission error.
Advanteges:
•Good clock recovery
•Most of the transmitted energy is in middle of the spectrum; no D.C. component
•B8ZS is used with pulse code modulation (PCM) on T1 lines (1.544 Mbps); B3ZS
and PCM are used on T3 lines.
It is Commonly used in North America.
• High Density Bipolar-3 Zeros (HDB3)
Modification to Bipolar-AMI to eliminate zero strings:
–Replace any 4 zero bits (0000) with:
odd
even
000+ -00-
if previous non-zero signal was +
000- +00+
if previous non-zero signal was -
–Alternate (odd/even occurrence) between the two
–Each replacement causes one code violation
•Good clock recovery; most of energy is in middle of the spectrum; no D.C.
component; not as robust as B8ZS
HDB3 is used on E-series public carrier lines (E1 is 2.048Mbps), Commonly
used in Europe and Japan
Figure 4 B8ZS and HDB3 line codes.
Appendix A: table of must common line codes and it’s Algorithms.
Appendix B: the power spectral density for some line codes.