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XRT72L52
196
TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. 1.0.3
NOTE: One of the reasons that the AMI Line Code has been chosen for driving copper medium, isolated via transformers,
is that this line code has no dc component, thereby eliminating dc distortion in the line.
4.3.1.2.2
B3ZS Decoding
The Transmit DS3 LIU Interface block and the associated LIU embed and combine the data and clocking
information into the line signal that is transmitted to the remote terminal equipment. The remote terminal
equipment has the task of recovering this data and timing information from the incoming DS3 data stream.
Most clock and data recovery schemes rely on the use of Phase-Locked-Loop technology.
One of the
problems of using Phase-Locked-Loop (PLL) technology for clock recovery is that it relies on transitions in the
line signal, in order to maintain lock with the incoming DS3 data-stream. Therefore, these clock recovery
scheme, are vulnerable to the occurrence of a long stream of consecutive zeros (e.g., no transitions in the
line). This scenario can cause the PLL to lose lock with the incoming DS3 data, thereby causing the clock and
data recovery process of the receiver to fail. Therefore, some approach is needed to insure that such a long
string of consecutive zeros can never happen. One such technique is B3ZS (or Bipolar 3 Zero Substitution)
encoding.
In general the B3ZS line code behaves just like AMI with the exception of the case when a long string of
consecutive zeros occurs on the line. Any 3 consecutive zeros will be replaced with either a 00V or a B0V
where B refers to a Bipolar pulse (e.g., a pulse with a polarity that is compliant with the alternating polarity
scheme of the AMI coding rule). And V refers to a Bipolar Violation pulse (e.g., a pulse with a polarity that
violates the alternating polarity scheme of AMI.) The decision between inserting an 00V or a B0V is made to
insure that an odd number of Bipolar (B) pulses exist between any two Bipolar Violation (V) pulses. The
Receive DS3 Framer, when operating with the B3ZS Line Code is responsible for decoding the B3ZS-encoded
data back into a unipolar (binary-format). For instance, if the Receive DS3 Framer detects a 00V or a B0V
pattern in the incoming pattern, the Receive DS3 Framer will replace it with three consecutive zeros.
Figure 64 presents a timing diagram that illustrates examples of B3ZS decoding.
4.3.1.2.3
Line Code Violations
FIGURE 63. AMI LINE CODE
FIGURE 64. ILLUSTRATION OF TWO EXAMPLES OF B3ZS DECODING
Data
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
RxPOS
RxNEG
Line Signal
Data
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
RxPOS
RxNEG
0 0
V
Line Signal
B
0 V