XR72L52 Datasheet, PDF (418/480 Pages) Exar Corporation – TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L52
REV. 1.0.1
xr
TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
FIGURE 177. ILLUSTRATION OF AMI LINE CODE
Data 1 0 1 1 0 0 0 1 0 1 1 1 1 0 1 1 0 1 1 0 0 1 1 0 0 0 0 1
Line Signal
RxPOS
RxNEG
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.
6.3.1.2.2 HDB3 Decoding
The Transmit E3 LIU Interface block and the associated LIU embed and combine the data and clocking infor-
mation into the line signal that is transmitted to the remote terminal equipment. The remote terminal equip-
ment has the task of recovering this data and timing information from the incoming E3 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 E3 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 E3 data, thereby causing the clock and data recovery process of the re-
ceiver to fail. Therefore, some approach is needed to insure that such a long string of consecutive zeros can
never happen. One such technique is HDB3 (or High Density Bipolar -3) encoding.
In general the HDB3 line code behaves just like AMI with the exception of the case when a long string of con-
secutive zeros occurs on the line. Any 4 consecutive zeros will be replaced with either a "000V" or a "B00V"
where "B" refers to a Bipolar pulse (e.g., a pulse with a polarity that is compliant with 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 "000V" or a "B00V" is made to insure that an odd number of Bipo-
lar (B) pulses exist between any two Bipolar Violation (V) pulses. The Receive E3 LIU Interface block, when
operating with the HDB3 Line Code is responsible for decoding the HD-encoded data back into a unipolar (bi-
nary-format). For instance, if the Receive E3 LIU Interface block detects a "000V" or a "B00V" pattern in the in-
coming pattern, the Receive E3 LIU Interface block will replace it with four (4) consecutive zeros. Figure 178
presents a timing diagram that illustrates examples of HDB3 decoding.
FIGURE 178. ILLUSTRATION OF TWO EXAMPLES OF HDB3 DECODING
Line Signal
RxPOS
0 00V
B0 0V
RxNEG
Data
101100000111
10 1 1 0
11 0 0 1 1 0 0 0 0 1
6.3.1.2.3 Line Code Violations
402