XRT72L56 Datasheet, PDF (290/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L56
SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.2
áç
PRELIMINARY
FIGURE 112. SIMPLE ILLUSTRATION OF THE TRANSMIT OVERHEAD DATA INPUT INTERFACE BLOCK
TxOHFrame
TxOHEnable
TxOH
TxOHClk
TxOHIns
TTrraannsmsmitit
OOvveerhrheeaadd
DDaatataInInppuut _t
InInteterfrafacceeBBlolocckk
To Transmit DS3 Framer Block
The E3, ITU-T G.751 Frame consists of 1536 bits. Of
these bits, 1524 are payload bits and the remaining
12 are overhead bits. The XRT72L56 has been de-
signed to handle and process both the payload type
and overhead type bits for each E3 frame. Within the
Transmit Section within the XRT72L56, the Transmit
Payload Data Input Interface has been designed to
handle the payload data. Likewise, the Transmit
Overhead Input Interface has been designed to han-
dle and process the overhead bits.
The Transmit Section of the XRT72L56 generates or
processes the various overhead bits within the E3
frame, in the following manner.
The Frame Alignment Signaling (FAS) Overhead
Bits
The FAS (Framing Alignment Signaling) bits are al-
ways internally generated by the Transmit Section of
the XRT72L56. Hence, the user cannot insert his/her
value for the FAS bits into the outbound E3 data
stream, via the Transmit Overhead Data Input Inter-
face.
The “A” (Alarm) Overhead bit
The “A” bit is used to transport the FERF (Far-End
Receive Failure) condition. This bit-field can be either
internally generated by the Transmit Section within
the XRT72L56, or can be externally generated and in-
serted into the outbound E3 data stream, via the
Transmit Overhead Data Input Interface. The Data
Link Related Overhead Bits
The “N” (National) Overhead bit
The E3 frame structure also contains the N bit which
can be used to transport a proprietary User Data Link
information and or Path Maintenance Data Link infor-
mation. The UDL (User Data Link) bits are only ac-
cessible via the Transmit Overhead Data Input Inter-
face. The Path Maintenance Data Link (PMDL) bits
can either be sourced from the Transmit LAPD Con-
troller/Buffer or via the Transmit Overhead Data Input
Interface.
Table 48 lists the Overhead Bits within the E3 frame.
In addition, this table also indicates whether or not
these overhead bits can be sourced by the Transmit
Overhead Data Input Interface.
271