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PRELIMINARY
XRT79L71
58
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
Performance Monitoring Bits (Parity)
The DS3 Frame includes numerous bits that are used to support performance monitoring of a DS3 signal, as it
is transmitted from one DS3 terminal to another over coaxial cable or optical fiber, typically when mapped into
a SONET/SDH signal. Depending upon the framing format chosen, each DS3 frame contains at least one type
of parity bit and may contain two types of parity bits. P-bits are available in both the M13/M23 and C-bit Parity
Framing formats. However, the C-bit Parity Framing format also includes three additional CP or Path Parity
bits. Each of these types of parity bits are described in some detail below.
P-Bits (Applies to M13/M23 and C-bit Parity Framing Formats)
Each DS3 M-frame consists of two (2) P-bits. These two P-bits carry the parity information of the previous DS3
frame for performance monitoring. These two P-bits must be identical, within a given DS3 frame. As a
Transmitting DS3 Terminal assembles a DS3 frame, prior to transmitting this DS3 signal to the remote terminal
equipment, it computes the EVEN parity over all 4704 payload bits within a given DS3 frame. The Transmitting
DS3 Terminal then inserts the resulting parity information into the two P-bit fields within the very next outbound
DS3 frame. These two P-bits are set to "1" if the payload data within the previous DS3 frame consists of an
odd number of "1s". Conversely, the two P-bits are set to "0" if the payload data within the previous DS3 frame
consists of an even number of "1s".
As a Receiving DS3 Terminal receives a given incoming DS3 frame, it locally computes its value for the P-bits.
Afterwards, this Receiving DS3 Terminal compares its locally-computed P-bits with the value within the P-bit
positions within the very next incoming DS3 frame. If these two P-bit values match, then the Receiving DS3
Terminal presumes that the first of these two incoming DS3 frames was received in an error-free manner. If
these two P-bit values DO NOT match, then the Receiving DS3 Terminal presumes that the first of these two
DS3 frames was received in an erred manner.
For information on how the Receive DS3/E3 Framer block handles P-bits within the DS3 data-stream that it
CP-Bits (Applies only to the C-bit Parity Framing Format)
Each DS3 M-frame consists of three (3) CP-bits. Just like the P-bits these three CP-bits carry the information
of the previous DS3 frame for performance monitoring. These three CP-bits must be identical, within a given
DS3 frame. As a Transmitting DS3 Terminal assembles a DS3 frame, prior to transmitting this DS3 signal to
the remote terminal equipment, it computes the EVEN parity over all 4704 payload bits within a given DS3
frame. The Transmitting DS3 Terminal then inserts the resulting parity information into the three CP-bit fields
within the very next outbound DS3 frame. These three CP-bits have a role that is very similar to that of P-bits.
However, in some DS3 applications, there is a difference between P and CP-bits that should be noted.
P-bits are used to support error detection of a DS3 data-stream as it travels from one terminal equipment to
an adjacent piece of terminal equipment (e.g., a single DS3 link between two terminals).
CP-bits, which are often times referred to as DS3 Path Parity bits, are used to support error detection within
a DS3 data-stream, as it travels from the Source Terminal equipment where this particular DS3 data-stream
originated, to the Sink Terminal equipment where this particular DS3 data-stream is terminated.
As a
consequence, once these CP-bits are computed and inserted into the outbound DS3 data-stream at the
Source Terminal Equipment, these CP-bits cannot be altered by any Mid-Network equipment until this DS3
data-stream is terminated at the Sink Terminal Equipment. In contrast, the values of P-bits within this same
DS3 data-stream can be re-computed and potentially changed by any Mid-Network equipment.
How CP-bits are Processed throughout the Network
The following section describes how CP-bits are processed at three locations within the network.
The Source Terminal Equipment
The Mid-Network Terminal Equipment
The Sink Terminal Equipment
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