XRT7250 Datasheet, PDF (328/463 Pages) Exar Corporation – DS3/E3 FRAMER IC

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XRT7250
DS3/E3 FRAMER IC
REV. 1.1.1
FIGURE 144. A SIMPLE ILLUSTRATION OF THE RECEIVE OVERHEAD OUTPUT INTERFACE BLOCK
RxOHFrame
RxOH
RxOHClk
RxOHEnable
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BBlolocckk
From Receive
E3 Framer Block
The E3, ITU-T G.751 frame consists of 1536 bits. Of
these bytes, 1524 bits are payload bits and the re-
maining 12 bits are overhead bits. The XRT7250 has
been designed to handle and process both the pay-
load type and overhead type bits for each E3 frame.
Within the Receive Section of the XRT7250, the Re-
ceive Payload Data Output Interface block has been
designed to handle the payload bits. Likewise, the
Receive Overhead Data Output Interface block has
been designed to handle and process the overhead
bits.
The Receive Overhead Data Output Interface block
unconditionally outputs the contents of all overhead
bits. The XRT7250 does not offer the user a means
to shut off this transmission of data. However, the
Receive Overhead Output Interface block does pro-
vide 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.
5.3.4.1 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 E3 overhead bits via Method 1.
• RxOH
• RxOHClk
• RxOHFrame
Each of these signals are listed and described below
in Table 63.
Interfacing the Receive Overhead Data Output In-
terface block to the Terminal Equipment (Method
1)
Figure 145 illustrates how one should interface the
Receive Overhead Data Output Interface block to the
Terminal Equipment when using Method 1 to sample
and process the overhead bits from the inbound E3
data stream.
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