5

A method for insuring minimum logic 1 density by substituting

bipolar code in place of strings of 0’s is called BNZS or Bipolar

with N Zero Substitution. B6ZS is used commonly in North

American T2 (6.3212MHz) carriers. For every string of 6

zeros, bipolar code is substituted according to the following

rule:

• If the immediate preceding pulse is of (-) polarity, then

code each group of 6 zeros as 0+- 0+-, and if the

immediate preceding pulse is of (+) polarity, code each

group of 6 zeros as 0+- 0-+.

One can see the consecutive logic 1 pulses of the same

polarity violate the AMI coding scheme.

e.g.,

B8ZS is used commonly in North American T1 (1.544MHz)

and T1C (3.152MHz) carriers. For every string of 8 zeros,

bipolar code is substituted according to the following rules:

1. If the immediate preceding pulse is of (-) polarity, then

code each group of 8 zeros as 000-+ 0+-.

2. If the immediate preceding pulse is of (+) polarity then

code each group of 8 zeros as 000+-0-+.

e.g.,

The BNZS coding schemes, in addition to eliminating DC

wander, minimize timing jitter and allow a line error

monitoring capability.

Another coding scheme is HDB3, high density bipolar 3, used

primarily in Europe for 2.048MHz and 8.448MHz carriers. This

code is similar to BNZS in that it substitutes bipolar code for 4

consecutive zeros according to the following rule:

1. If the polarity of the immediate preceding pulse is (-) and

there have been an odd (even) number of logic 1 pulses

since the last substitution, each group of 4 consecutive

zeros is coded as 000-(+00+).

2. If the polarity of the immediate preceding pulse is (+) then

the substitution is 000+(-00-) for odd (even) number of

logic 1 pulses since the last substitution.

e.g.,

The 3 in HDB3 refers to the coding format that precludes

strings of zeros greater than 3. Note that violations are

produced only in the fourth bit location of the substitution

code and that successive substitutions produce alternate

polarity violations.

01

00

0

0

00

0

0

0

11 1

1

00

-

+

+

-

00

+

-

-

+

V

V

V

V

B6ZS (+)

B6ZS (

PCM CODE

6

V = VIOLATION

11

1

1

0

0

00

00

00

0

0

0

0

0

0

00

0

0

-

-

++

V

V

V

V = VIOLATION

PCM CODE

B8ZS (

B8ZS (+)

8

PCM CODE

01

0

0

0

0

0

00 00

0

0

0

0

0

0

11 1

1

00

0

0

0

-

+

--

+

+

V = VIOLATION

HDB3 (-)

HDB3 (+)

V

V

V

V

44

HC-5560