XRT72L50 Datasheet, PDF (164/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
4.2.2 The Transmit Overhead Data Input Interface
FIGURE 44. THE TRANSMIT OVERHEAD DATA INPUT INTERFACE BLOCK
TxOHFrame
TxOHEnable
TxOH
TxOHClk
Transmit
Overhead Data
Input Interface
Block
TxOHIns
áç
To Transmit DS3
Framer Block
The DS3 Frame consists of 4760 bits. Of these bits, 4704 bits are payload bits and the remaining 56 bits are
overhead bits. The XRT72L50 has been designed to handle and process both the payload type and overhead
type bits for each DS3 frame. Within the XRT72L50 Transmit Section, the Transmit Payload Data Input
Interface has been designed to handle the payload data. Likewise, the Transmit Overhead Data Input Interface
has been designed to handle and process the overhead bits.
The Transmit Section of the XRT72L50 generates or processes the various overhead bits within the DS3
frame, in the following manner.
The Frame Synchronization Overhead Bits (e.g., the F and M bits)
The F and M bits are always internally generated by the Transmit Section of the XRT72L50. These overhead
bits are used by the Remote Terminal Equipment for Frame Synchronization purposes. User values cannot be
inserted for the F and M bits into the outbound DS3 data stream via the Transmit Overhead Data Input
Interface. Any attempt to externally insert values for the F and M bits will be ignored by the Transmit Overhead
Data Input Interface “High” block.
The Performance Monitoring Overhead Bits (P and CP Bits)
The P-bits are always internally generated by the Transmit Section of the XRT72L50. The P bits are used by
the Remote Terminal Equipment to perform error-checking/detection of a DS3 data stream as it is transmitted
from one Terminal Equipment to adjacent Terminal Equipment (e.g., point-to-point checking). User values
cannot be inserted for the P-bits into the outbound DS3 data stream via the Transmit Overhead Data Input
Interface.
In contrast to P bits, CP bits are used to perform error-checking/detection of a DS3 data stream from the
Source Terminal Equipment to the Sink Terminal Equipment. In applications where a given DS3 data stream is
received via one port and is output via another port, it is necessary that the CP bit-values remain constant.
The only way to insure this is to (1) extract out the CP bit values via the Receiving Line Card and (2) insert
these CP-bit values into the outbound DS3 data stream via the Transmit Overhead Data Input Interface block.
Hence, the Transmit Overhead Data Input Interface block will permit the user to externally insert the CP bits
into the outbound DS3 data stream.
The Alarm and signaling related Overhead bits
Bits that are used to transport the alarm conditions can be either internally generated by the Transmit Section
within the XRT72L50 or externally generated and inserted into the outbound DS3 data stream via the Transmit
Overhead Data Input Interface or TxSER (Payload Data Pin). The DS3 frame overhead bits that fall into this
category are:
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