# Understanding line coding on 10/100 mbps ethernet

While learning about the way bits are sent over ethernet (twisted pair), I came across this table:
What I don't understand is:

1. Do all of them except 100Base-TX use only 1 pair for both RX and TX?
2. What is the difference between MLT-3 and PAM-3?
3. (In 100Base-T1) How is it possible that a group of 4 bits is represented as a group of 3 bits?
4. (100Base-TX) Isn't saying that MLT-3 is used imply that NRZ-I is used? Why the redundancy?
• Not an answer because it's incomplete and mostly guessing, but: (1) it would seem so; it looks like the 1 stands for 1 pair, and it's used in automotive applications. (3) that sure seems like a typo. Feb 18, 2023 at 20:19
• That's too many questions, and most info is already available if you put the terms you are looking for into Wikipedia, where your existing page already points. The Q1 is a bit ambiguous anyway; you already have a list which Ethernet standards use which amount of pairs, and which all of them you mean? Feb 18, 2023 at 20:20

## 1 Answer

1. No. From the table, different 10Base-T versions can use 1 or 2 pairs, and different 100Base-T versions can use 1, 2 or 4 pairs. Both 10Base-T and 100Base-TX that are extremely common use 2 pairs, 1 pair per direction.

2. Both MLT-3 and PAM-3 have three voltage levels. MLT-3 is a specific line code that uses 3 voltage levels in a specific way, which steps between all voltage levels in a sequence. If the bit is 1, the sequence advances, and for 0 bits, the sequence does Not advance. Basically for strings of 1, the output is a sine wave. PAM-3 just means you have 3 voltage levels, but not how they are used.

3. You are correct; it isn't possible to take in 4 bits and output 3 bits. But that's not what 4B3T does. The 4B3T takes in 4 bits but it does not output 3 bits. It outputs 3 ternary (PAM-3) symbols.

4. 100Base-TX does use 4B5B, NRZ-I and MLT-3. There is really no redundancy involved. 4B5B and NRZ-I were already used on 100Base-FX. When the 100Base-TX was designed, the fiber medium dependent sublayer just was replaced with copper medium dependent sublayer which needs further tricks to fit 125 MBPS over Cat5 bandwidth of 100 MHz, and thus the MLT-3 encoding was borrowed from copper version of FDDI.