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AMI LINE CODE HDB3:
gautam

Posts: 27

Joined: 08 Oct 2010 16:36

01 Sep 2012 17:57

AMI LINE CODE HDB3:

AMI LINE CODE HDB3:

T1 and E1 career uses AMI codes to maintain system synchronization. Modified AMI codes are Alternate Mark Inversion (AMI) line codes in which bipolar violations may be deliberately inserted to maintain system synchronization

The clock rate of an incoming T-carrier signal is extracted from its bipolar line code. To prevent loss of synchronization when a long string of zeros is present in the payload, deliberate bipolar violations are inserted into the line code, to create a sufficient number of transitions to maintain synchronization;

"B" denotes a balancing mark with the opposite polarity to that of the preceding mark, while "V" denotes a bipolar violation mark, which has the same polarity as the preceding mark.

HDB3 - Used in all levels of the European E-carrier system, the high density bipolar of order 3 (HDB3) code replaces any instance of 4 consecutive 0 bits with one of the patterns "000V" or "B00V". The choice is made to ensure that consecutive violations are of differing polarity, i.e. separated by an odd number of normal + or - marks.

Every time there are 4 consecutive zeros in the code they will be replaced by either 000?, 000+, +00+ or ?00?.

To determine which pattern to use, you have to count the number of pluses (+) and the number of minuses (?) since the last violation bit V that you used and subtract one from the other. If the result is an odd number then 000? or 000+ is used. If the result is an even number then +00+ or ?00? is used.

To determine which polarity to use you must look at the pulse preceding the four zeros. If 000V form must be used then V simply copies the polarity of last pulse, if B00V form must be used then B and V chosen will have the opposite polarity of the last pulse

HDB3 encoding rules -

Transmitted bit HDB3 Encoded bit

0 0

1 AMI

0000 000V ( 3 ZEROS and a violation)

0000 0000 B00V B00V



Example 1 - The pattern of bits

1 0 0 0 0 1 1 0

Encoded in HDB3 is

+ 0 0 0 V ? + 0

Example 2 - The pattern of bits

1 0 1 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 "

Encoded in HDB3 is " + 0 ? 0 0 0 V 0 + ? B 0 0 V ? + B 0 0 V 0 0 " which is:

+ 0 ? 0 0 0 ? 0 + ? + 0 0 + ? + ? 0 0 ? 0 0

Example 3 - The pattern of bits

" 1 0 1 0 0 0 0 1 0 0 0 0 1 1 0 0 0 0 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 0 1 0 0 0 0"

Encoded in HDB3 is "+ 0 ? 0 0 0 V + 0 0 0 V ? + B 0 0 V + ? + 0 0 0 V ? + ? + B 0 0 V + 0 ? B 0 0 V " which is:

+ 0 ? 0 0 0 ? + 0 0 0 + ? + ? 0 0 ? + ? + 0 0 0 + ? + ? + ? 0 0 ? + 0 ? + 0 0 +