XRT74L73 Datasheet, PDF (338/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
PRELIMINARY
FIGURE 138. 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 trans-
formers, is that this line code has no dc component, thereby
eliminating dc distortion in the line.
5.3.1.2.2 HDB3 Decoding
The Transmit E3 LIU Interface block and the associ-
ated 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 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) technol-
ogy for clock recovery is that it relies on transitions in
the line signal, in order to maintain lock with the in-
coming E3 data-stream. Therefore, these clock re-
covery 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 receiver to
fail. Therefore, some approach is needed to insure
that such a long string of consecutive zeros can nev-
er 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
consecutive zeros occurs on the line. Any 4 consec-
utive 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 al-
ternating polarity scheme of AMI.) The decision be-
tween inserting an "000V" or a "B00V" is made to in-
sure that an odd number of Bipolar (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 (binary-for-
mat). 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 139 presents a timing diagram that illustrates
examples of HDB3 decoding.
FIGURE 139. 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
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