B3T Line Coding

International BRI circuits, commonly called ISDN-2 or ISDN Basic Access (IBA), use a different type of line coding that is called 4 Binary 3 Ternary (4B3T). This coding scheme is a bit more complicated than most and is also referred to as Modified Monitoring State Block Code 43 (MMS43). The basic premise is to take four binary digits and, using a lookup table, convert them to be used in a strict three voltage state environment of negative, zero, and positive voltage. This function creates an approximately 25 percent reduction in baud rate as transmitted on the BRI circuit. Refer to Table 8-8 for a listing of the binary conversion.

Table 8-8 Lookup Table for 4B3T Line Coding

Binary Digits

Bit #

Voltage

Bit #

Voltage

Bit #

Voltage

Bit #

Voltage

0001

1

0-+

2

0-+

3

0-+

4

0-+

0111

1

-0+

2

-0+

3

-0+

4

-0+

0100

1

-+0

2

-+0

3

-+0

4

-+0

0010

1

+-0

2

+-0

3

+-0

4

+-0

1001

1

+0-

2

+0-

3

+0-

4

+0-

1110

1

0+-

2

0+-

3

0+-

4

0+-

1011

2

+-+

3

+-+

4

+-+

1

---

0011

2

00+

3

00+

4

00+

2

-- 0

1101

2

0+0

3

0+0

4

0+0

2

-0-

1000

2

+00

3

+00

4

+00

2

0--

0110

2

-++

3

-++

2

-- +

3

-- +

1010

2

++-

3

++-

2

+--

3

+--

1111

3

++0

1

00-

2

00-

3

00-

0000

3

+0+

1

0-0

2

0-0

3

0-0

0101

4

0++

1

-00

2

-00

3

-00

1100

4

+++

1

-+-

2

-+-

3

-+-

Binary representation of 4B3T is shown in Figure 8-23.

Figure 8-23 4B3TLine Coding

8-Bit Word: 01110100

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