Specifications

Installation and Operation Manual Appendix C Operating Environment
FCD-E1LC Ver. 1.0 E1 Environment C-3
E1 Line Signal
The E1 line signal uses the High-Density Bipolar 3 (HDB3) code. The HDB3 coding
format is an improvement of the alternate mark inversion (AMI) code.
In the AMI format, “ones” are alternately transmitted as positive and negative
pulses, whereas “zeros” are transmitted as a zero voltage level. The AMI format
cannot transmit long strings of “zeros”, because such strings do not carry timing
information.
The HDB3 coding rules restrict the maximum length of a “zero” string to three
pulse intervals.
Longer strings are encoded at the transmit end to introduce non-zero pulses. To
allow the receiving end to detect the artificially introduced pulses and enable
their removal to restore the original data string, the encoding introduces
intentional bipolar violations in the data sequence. The receiving end detects
these violations and when they appear to be part of an encoded “zero” string – it
removes them.
Other bipolar violations may also be caused by transmission errors. Therefore,
any bipolar violations which cannot be interpreted as intentional coding violations
can be separately counted, to obtain information on the quality of the
transmission link.
Timeslot 0
Timeslot 0 of E1 frames is used for two main purposes:
• Delineation of frame boundaries. For this purpose, in every second frame,
timeslot 0 carries a fixed pattern, called frame alignment signal (FAS). Frames
carrying the FAS are defined as even frames, as they are assigned the
numbers 0, 2, 4, etc. when larger structures (multiframes) are used.
The receiving equipment searches for the fixed FAS pattern in the data
stream using a special algorithm, a process called frame synchronization.
Once this process is successfully completed, the equipment can identify each
bit in the received frames.
• Interchange of housekeeping information. In every frame without FAS (odd
frames), timeslot 0 carries housekeeping information. This information is
carried as follows:
 Bit 1 - this bit is called the international (I) bit. Its main use is for error
detection using the optional CRC-4 function (CRC-4 stands for Cyclic
Redundancy Check, using a fourth-degree polynomial). This function is
described below.
 Bit 2 is always set to 1, a fact used by the frame alignment algorithm.
 Bit 3 is used as a remote alarm indication (RAI), to notify the equipment
at the other end that the local equipment lost frame alignment, or does
not receive an input signal.
 The other bits, identified as S
a4
through S
a8
, are designated national bits ,
and are actually available to the users, provided agreement is reached as
to their use. RAD equipment with SNMP agents can use S
a
bits for