XRT72L52
REV. 1.0.1
xr
TWO CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
190
4.3.1.2.1
AMI or Alternate Mark Inversion, means that consecutive one's pulses (or marks) will be of opposite polarity
with respect to each other. This line code involves the use of three different amplitude levels: +1, 0, and -1.
The +1 and -1 amplitude signals are used to represent one's (or mark) pulses and the "0" amplitude pulses (or
the absence of a pulse) are used to represent zeros (or space) pulses. The general rule for the AMI line code
is: if a given mark pulse is of positive polarity, then the very next mark pulse will be of negative polarity and vice
versa. This alternating-polarity relationship exists between two consecutive mark pulses, independent of the
number of zeros that exist between these two pulses.
Figure 64
presents an illustration of the AMI Line Code
as would appear at the RxPOS and RxNEG input pins of the Framer, as well as the corresponding output sig-
nal on the line.
AMI Decoding
N
OTE
:
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 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 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 vul-
nerable 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 pro-
cess of the receiver to fail. Therefore, some approach is needed to insure that such a long string of consecu-
tive 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 con-
secutive 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 al-
ternating 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 Fram-
er, 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 incom-
ing pattern, the Receive DS3 Framer will replace it with three consecutive zeros.
Figure 65
presents a timing
diagram that illustrates examples of B3ZS decoding.
F
IGURE
64. AMI L
INE
C
ODE
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
RxPOS
RxNEG
Line Signal