User's Manual

5.8GHz PDH User’s Manual
15
A3. IDU: n*E1
Display the number of E1 equipments. If it displays 8*E1, it means that the
equipment has eight
E1 interface.
A4. Code: AMI/HDB3
Display the local end’s IDU dispatch model number. After pressing the OK button,
use the Right/Left button to choose the model number. At present, you may select
either of the two numbers: AMI or HDB3.
AMI
：
AMI (Alternate Mark Inversion) is a synchronous clock encoding technique which uses
bipolar pulses to represent logical 1 value. A logical 0 is represented by no symbol
and a logical 1 by pulses of alternating polarity.
Example of AMI encoding
The pattern of bits " 1 0 0 0 0 1 1 0 " encodes to "+ 0 0 0 0 - +"
HDB3：
HDB3 (High Density Bipolar Order 3 Encoding) is based on Alternate Mark Inversion
(AMI), but extends this by inserting violation codes whenever there is a run of 4 or
more 0's. This and similar (more complex) codes have replaced AMI in modern
distribution networks. The purpose of this is to prevent long runs of 0's in the data
stream, sometimes called a "run length limited" code. Encode any pattern of more
than four bits as B00V, where B is a balancing pulse. The value of B is assigned
as + or - , so as to make alternate "V"s of opposite polarity. The receiver removes
all Violation pulses, but in addition a violation preceded by two zeros and a pulse
is treated as the "BOOV" pattern and both the violation and balancing pulse are
removed from the received bit stream. This restores the original bit stream.