Sahithyan's S3 — Data Communication and Networking

Digital to Digital Encoding

Multilevel Binary

Encoding more than 1 bit per 1 symbol. Achieved through having more than 2 voltage levels. “Binary” is not implying 2 levels; it’s used as it is used in digital communication. Requires more signal power (approx. $3\,\text{dB}$ , or $2\text{x}$ ).

Scrambling

The process of replacing the constant voltage levels with a filling sequence. To prevent the receiver from detecting the signal incorrectly.

Filling sequence

A bit sequence to replace a sequence of constant voltage level. Must produce enough transitions to sync. Must be recognized by the receiver, and replaced with original. Has same length as original.

NRZ-L

Short for Non-Return to Zero-Level. Two different voltage levels: 1 and 0. Either can be higher. Either can be positive or negative. Usually one is positive and the other is negative. Voltage is constant during bit interval.

NRZ-I

Short for Non-Return to Zero-Inverted. A differential encoding scheme. Constant voltage during bit interval, similar to NRZ-L. Transition at start of a bit denotes a binary 1. No transition denotes a binary 0.

Bipolar-AMI

Short for Alternate Mark Inversion. Most common bipolar encoding scheme. 0 is represented by 0. 1 is represented by non-zero level, with alternating polarity.

Pros:

No DC component
No loss of sync due to long sequences of 1s
Uses less bandwidth compared to Manchester
Can detect errors (bipolar violation)

Cons:

Long sequence of 0s can cause loss of sync
More complex compared to NRZ.
Limited error detection (not all errors can be detected)

Pseudoternary

Opposite of AMI. No advantage or disadvantage compared to AMI.

Manchester

Aka. Phase encoding, or PE. A biphase encoding scheme. Each bit is represented by a transition at the middle part of the bit period.

Low to high: 1
High to low: 0

2 types of transitions:

mid-bit transition
Always present. Carries data and works as a clock signal.
start of the bit transition
Only present when consecutive bits are the same. Works as a timing cue.

Used by IEEE 802.3 which defines the physical and data-link layer’s media access control of ethernet.

Differential Manchester

Aka. Differential Phase encoding, or DPE. A biphase encoding scheme and a differential encoding scheme. Midbit transition is used for clocking. Non-midbit transitions denote the data. Transition at the start of a bit is 0. No transition means 1.

Used by IEEE 802.5 which is used to build local area networks.

B8ZS

Bipolar with 8-zero substitution. Based on Bipolar-AMI. Used to prevent long sequence of zeros in bipolar-AMI signals. Replaces 8 consecutive zeros with a special sequence that intentionally includes 2 bipolar violations.

If the octet is full of zeros and:

Last voltage pulse preceding was +ve then encode as 000+-0-+
Last voltage pulse preceding was -ve then encode as 000-+0+-

The intentional violations are placed to make sure the replacement is detected correctly. They don’t mess up error detection because the specific 2 violations couple doesn’t occur because of noise.

HDB3

High Density Bipolar 3-level encoding. Similar to B8ZS. Replaces 4 consecutive zeros with patterns containing a non-zero pulse, to maintain synchronization. The exact pattern depends on the number of pulses since the last substitution, to keep it DC-balanced.

The substitution pattern includes:

a violation pulse (V)
(optional) a balancing pulse (B)

Maintains clock synchronization. Avoids long run of 0s. Signal will be DC-balanced.

When 4 consecutive 0s are detected:

Count positive (+) and negative (−) pulses since last violation (V). If the difference (imbalance) is:
- Odd: use 000V
- Even: use B00V
Check the polarity of the pulse before 0000:
- For 000V: V = same polarity as last pulse
- For B00V: B and V = opposite polarity of last pulse

Imbalance of non-zero pulses	Substitution pattern	Meaning
Even	000V	Adds a V same polarity as last pulse
Odd	B00V	Adds a B, V of opposite polarity