XRT74L73 Datasheet, PDF (257/488 Pages) Exar Corporation – 3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

PRELIMINARY
XRT74L73
3 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.0.1
FIGURE 94. A SIMPLE ILLUSTRATION OF THE RECEIVE OVERHEAD OUTPUT INTERFACE BLOCK
RxOHFrame
RxOH
RxOHClk
RxOHEnable
RReecceeiviveeOOvveerrhheeaadd
OOuutptpuuttIInnteterrfafaccee
BBlolocckk
From Receive
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 XRT74L73 has been de-
signed to handle and process both the payload type
and overhead type bits for each DS3 frame.
The Receive Payload Data Output Interface block,
within the Receive Section of the XRT74L73, has
been designed to handle the payload bits. Likewise,
the Receive Overhead Data Output Interface block
has been designed to handle and process the over-
head bits.
The Receive Overhead Data Output Interface block
unconditionally outputs the contents of all overhead
bits within the incoming DS3 data stream. The
XRT74L73 does not offer the user a means to shut off
this transmission of data. However, the Receive
Overhead Output Interface block does provide the
user with the appropriate output signals for external
Data Link Layer equipment to sample and process
these overhead bits, via the following two methods.
• Method 1- Using the RxOHClk clock signal.
• Method 2 - Using the RxClk and RxOHEnable out-
put signals.
Each of these methods are described below.
4.3.4.1 Method 1 - Using the RxOHClk Clock
signal
The Receive Overhead Data Output Interface block
consists of four (4) signals. Of these four signals, the
following three signals are to be used when sampling
the DS3 overhead bits via Method 1.
• RxOH
• RxOHClk
• RxOHFrame
Each of these signals are listed and described below
in Table 57 .
258