4B3T

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4B3T, which stands for 4 (four) Binary 3 (three) Ternary, is a line encoding scheme used for ISDN PRI interface. 4B3T represents four binary bits using three pulses.

Description[edit]

It uses three states:

  • + (positive pulse),
  • 0 (no pulse), and
  • − (negative pulse).

This means we have 24 = 16 input combinations to represent, using 33 = 27 output combinations. 000 is not used to avoid long periods without a transition. 4B3T uses a paired disparity code to achieve an overall zero DC bias: six triplets are used which have no DC component (0+−, 0−+, +0−, −0+, +−0, −+0), and the remaining 20 are grouped into 10 pairs with differing disparity (e.g. ++− and −−+). When transmitting, the DC bias is tracked and a combination chosen that has a DC component of the opposite sign to the running total.

This mapping from 4 bits to three ternary states is given in a table known as Modified Monitoring State 43 (MMS43). A competing encoding technique, used for the ISDN basic rate interface where 4B3T is not used, is 2B1Q.

The sync sequence used is the 11-symbol Barker code, +++−−−+−−+− or its reverse, −+−−+−−−+++.

Encoding table[edit]

Each 4-bit input group is encoded as a 3-symbol group (transmitted left to right) from the following table. Encoding requires keeping track of the accumulated DC offset, the number of + pulses minus the number of − pulses in all preceding groups. The starting value is arbitrary; here we use the values 1 through 4, although −1.5, −0.5, +0.5 and +1.5 is another possibility.

MMS 43 coding table[1]
Input Accumulated DC offset
Hex Binary 1 2 3 4
0 0000 + 0 +   (+2) 0−0 (−1)
1 0001 0 − +   (+0)
2 0010 + − 0   (+0)
3 0011 0 0 +   (+1) − − 0   (−2)
4 0100 − + 0   (+0)
5 0101 0 + +   (+2) − 0 0   (−1)
6 0110 − + +   (+1) − − +   (−1)
7 0111 − 0 +   (+0)
8 1000 + 0 0   (+1) 0 − −   (−2)
9 1001 + − +   (+1) − − −   (−3)
A 1010 + + −   (+1) + − −   (−1)
B 1011 + 0 −   (+0)
C 1100 + + +   (+3) − + −   (−1)
D 1101 0 + 0   (+1) − 0 −   (−2)
E 1110 0 + −   (+0)
F 1111 + + 0   (+2) 0 0 −   (−1)

This code forces a transition after at most five consecutive identical bits, or four consecutive zero bits.

Decoding table[edit]

Decoding is simpler, as the decoder does not need to keep track of the encoder state, although doing so allows greater error detection. The 000 triplet is not a legal encoded sequence, but is typically decoded as binary 0000.

Ternary Binary Hex Ternary Binary Hex Ternary Binary Hex
0 0 0 N/A N/A − 0 0 0101 5 + − − 1010 A
+ 0 + 0000 0 − + + 0110 6 + 0 − 1011 B
0 − 0 0000 0 − − + 0110 6 + + + 1100 C
0 − + 0001 1 − 0 + 0111 7 − + − 1100 C
+ − 0 0010 2 + 0 0 1000 8 0 + 0 1101 D
0 0 + 0011 3 0 − − 1000 8 − 0 − 1101 D
− − 0 0011 3 + − + 1001 9 0 + − 1110 E
− + 0 0100 4 − − − 1001 9 + + 0 1111 F
0 + + 0101 5 + + − 1010 A 0 0 − 1111 F

See also[edit]

Other line codes that have 3 states:

References[edit]

  1. ^ "Wired Communications T-SMINTO 4B3T Second Gen. Modular ISDN NT (Ordinary)" (PDF) (Data sheet). Version 1.1. Infineon. November 2001. PEF 80902.