
PRELIMINARY
XRT79L71
96
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
Setting these bit-fields to "0" configures the Transmit Section of the XRT79L71 to accept the corresponding
overhead bit(s) via the Transmit Payload Data Input Interface - Serial Input and to insert it/them into the
corresponding overhead bits, within the outbound DS3 data-stream. Conversely, setting these bit-fields to "1"
configures the Transmit Section of the XRT79L71 to (1) ignore value(s) of the corresponding overhead bits
within the data stream (that is being accepted by the Transmit Payload Data Input Interface block) and (2) to
allow the Transmit DS3/E3 Framer block to internally generate the corresponding overhead bit(s).
Why take advantage of this feature?
suppose that the user wishes to pass a Channelized DS3 data-stream that is of the M23 framing format
through the Transmit Section of the XRT79L71. If this DS3 signal is of the channelized type (e.g., it is carrying
either 28 DS1 or 21 E1 signals) and is of the M23 framing format, then the C-bits within this DS3 data-stream
contains important bit-stuffing information that was generated by the Source or M13 Multiplexer terminal where
the seven (7) DS2 signals were multiplexed together thereby creating this DS3 signal. If the value of these C-
bits are lost or overwritten by the time that this particular DS3 signal reaches the M13 De-Multiplexer terminal,
then the M13 De-Multiplexer terminal will not be able to de-multiplex out the seven DS2 signals from this DS3
signals. As a consequence, the 28 DS1 or 21 E1 signals that are being carried within this DS3 signal will be
lost. Therefore, whenever a user is handling a channelized DS3 signal that is of the M23 framing format, the
user must make sure that that value of the C-bits within this DS3 data-stream are not altered.
Problem with the Transmit DS3 Framer Block
As our channelized, M23 framing format DS3 signal is routed through the Transmit Section of the XRT79L71, it
will first pass through the Transmit Payload Data Input Interface block. Afterwards, this signal will now pass
through the Transmit DS3 Framer block.
According to Table 16, as this DS3 signal passes through the Transmit DS3 Framer block, then all of the
following operations will occur.
The Framing (F and M) bits will be re-inserted into this outbound DS3 data-stream.
P-bits will be computed and will be re-inserted into this outbound DS3 data-stream.
The X-bits will be set to the appropriate value depending upon whether the corresponding block is declaring
the LOS, OOF/LOF or AIS defects, or not.
The C-bits will be set to the value "0".
The last item in the above list (e.g., The C-bits will be set to the value "0") creates a problem for our
Channelized, M23 Framing format DS3 signal.
What can be done about this?
Fortunately, there is a way to preserve the value of the C-bits, as this Egress Direction DS3 signal passes
through the Transmit DS3 Framer block.
In short, the user can preserve the value of these C-bits by
configuring the Transmit DS3 Framer block to accept and externally insert certain overhead bits that it receives
from the Transmit Payload Interface block into its outbound DS3 data-stream.
More specifically, this requirement can easily be achieved by taking advantage of this ability to externally insert
the overhead bits into the outbound DS3 data-stream.
In this specific case, the user can configure the
Transmit DS3 Framer block to simply externally accept the C bits within the DS3 data-stream that it receives,
Transmit DS3 F-Bit Mask # 4 Register (Address = 0x1139)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
UDL Bit # 3
(C23)
UDL Bit # 2
(C22)
UDL Bit # 1
(C21)
X Bit # 2
FEAC Bit
(C13)
NA Bit (C12)
AIC Bit
(C11)
X Bit # 1
R/W
X