This article may require cleanup to meet Wikipedia's quality standards. Please improve this article if you can. (April 2009)

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

It uses three states:

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

This means we have 16 (2⁴) input combinations to represent, using 27 output combinations. 000 is not used to avoid long periods without a transition. To achieve an overall zero DC bias, 6 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 3 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, −+−−+−−−+++.

[edit] Encoding table

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

Input	Accumulated DC offset
	1	2	3	4
0000	+ 0 +   (+2)	0 − 0   (−1)
0001	0 − +   (+0)
0010	+ − 0   (+0)
0011	0 0 +   (+1)			− − 0   (−2)
0100	− + 0   (+0)
0101	0 + +   (+2)	− 0 0   (−1)
0110	− + +   (+1)		− − +   (−1)
0111	− 0 +   (+0)
1000	+ 0 0   (+1)			0 − −   (−2)
1001	+ − +   (+1)			− − −   (−3)
1010	+ + −   (+1)		+ − −   (−1)
1011	+ 0 −   (+0)
1100	+ + +   (+3)	− + −   (−1)
1101	0 + 0   (+1)			− 0 −   (−2)
1110	0 + −   (+0)
1111	+ + 0   (+2)	0 0 −   (−1)

[edit] Decoding table

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
0 0 0	N/A
+ 0 +	0000
0 − 0	0000
0 − +	0001
+ − 0	0010
0 0 +	0011
− − 0	0011
− + 0	0100
0 + +	0101
− 0 0	0101
− + +	0110
− − +	0110
− 0 +	0111
+ 0 0	1000
0 − −	1000
+ − +	1001
− − −	1001
+ + −	1010
+ − −	1010
+ 0 −	1011
+ + +	1100
− + −	1100
0 + 0	1101
− 0 −	1101
0 + −	1110
+ + 0	1111
0 0 −	1111

[edit] See also

Other line codes that have 3 states:

• hybrid ternary code
• bipolar encoding
• MLT-3 encoding
• B3ZS

v • d • e Pulse-code modulation (PCM) digital encoding Main articles Unipolar encoding · Bipolar encoding · On-off keying Basic line codes Return to zero (RZ) · Non-return-to-zero, level (NRZ/NRZ-L) · Non-return-to-zero, inverted (NRZ-I) · Non-Return-to-Zero, space (NRZ-S) · Biphase (Bi-φ) · Manchester · Differential Manchester Extended line codes Conditioned Diphase · 4B3T · 4B5B · 2B1Q · Alternate Mark Inversion · Modified AMI code · Coded mark inversion · MLT-3 encoding · Hybrid ternary code · 6b/8b encoding · 8b/10b encoding · 64b/66b encoding · Eight-to-fourteen modulation · Delay/Miller encoding · TC-PAM Optical line codes Carrier-Suppressed Return-to-Zero · Alternate-Phase Return-to-Zero See also: Quantization · Sampling · Category:Line codes

This article related to telecommunications is a stub. You can help Wikipedia by expanding it. v • d • e