XRT74L74
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
REV. P1.1.1
PRELIMINARY
296
condition), then it will inform the Remote Terminal
Equipment of this fact by commanding the Local
Transmit E3 Framer block to set the “A” bit-field, with-
in the next outbound E3 frame, to “1”. The Local
Transmit E3 Framer block will continue to set the “A”
bit-field (within the subsequent outbound E3 frames)
to “1” until the Receive E3 Framer block no longer ex-
periences problems in receiving the E3 frame data. If
the Remote Terminal Equipment receives a certain
number of consecutive E3 frames, with the “A” bit-
field set to “1”, then the Remote Terminal Equipment
will interpret this signaling as an indication of a Far-
End Receive Failure (e.g., a problem with the Local
Terminal Equipment).
Conversely, if the Receive E3 Framer block (at a Lo-
cal Terminal Equipment) is not experiencing any
problems receiving E3 frame data from a Remote
Terminal Equipment, then it will also inform the Re-
mote Terminal Equipment of this fact by commanding
the Local Transmit E3 Framer block to set the “A” bit-
field within an outbound E3 frame (which is destined
for the Remote Terminal) to “0”. The Remote Termi-
nal Equipment will interpret this form of signaling as
an indication of a normal operation.
A detailed discussion into the practical use of the A
bit-field is presented in Section 4.2.2.
6.1.1.2
The “N” Bit
The “N” bit is typically used to transport PMDL (Path
Maintenance Data Link) information, from one termi-
nal to the next. However, the “N” bit-field can also be
used to transport a proprietary data link, if configured
according.
A detailed discussion into the practical use of the N-
bit field is presented in Section 4.2.2.
6.2
T
HE
T
RANSMIT
S
ECTION
OF
THE
XRT74L74 (E3,
ITU-T G.751 M
ODE
O
PERATION
)
When the XRT74L74 has been configured to operate
in the E3, ITU-T G.751 Mode, the Transmit Section of
the XRT74L74 consists of the following functional
blocks.
Transmit Payload Data Input Interface block
Transmit Overhead Data Input Interface block
Transmit E3 Framer block
Transmit HDLC Controller block
Transmit LIU Interface block
Figure 105 presents a simple illustration of the
Transmit Section of the XRT74L74 Framer IC.
Each of these functional blocks will be discussed in
detail in this document.
6.2.1
face Block
The Transmit Payload Data Input Inter-
F
IGURE
105. T
HE
XRT74L74 T
RANSMIT
S
ECTION
WHEN
IT
HAS
BEEN
CONFIGURED
TO
OPERATE
IN
THE
E3 M
ODE
Payload Data
Interface Block
Transmit
Payload Data
Interface Block
Framer Block
Transmit DS3/E3
Framer Block
Block
Transmit LIU
Interface
Block
Transmit E3
Controller/Buffer
Transmit E3
Controller/Buffer
TxPOS
TxNEG
TxLineClk
TxOHFrame
TxOHEnable
TxOH
TxOHClk
TxOHIns
Interface Block
Transmit
Overhead nput
Interface Block
TxOHInd
TxSer
TxNib[3:0]
TxInClk
TxNibClk
TxFrame
From Microprocessor
Interface Block