9
x r
PRELIMINARY
E3 UNI FOR ATM
XRT7234
REV. P1.0.0
between the ATM Layer Processor and the UNI IC.
Figure 2. System Level Interfacing of the XR-T7234 E3 UNI (E3 data is transmitted over Optical Fiber).
The Transmit Cell Processor block will read in the ATM cell from the Tx FIFO. It will then (optionally) proceed to take
the first four octets of this cell and compute the HEC byte from these bytes. Afterwards the Transmit Cell Processor will insert
this HEC byte into the 5th octet position within the cell. The Transmit Cell Processor will also (optionally) scramble the
payload portion of the cell (bytes 6 through 53) in order to prevent the user data from mimicking framing or control
bits/bytes. Once the cell has gone through this process it will then be transferred to the Transmit E3 Framer.
If the Tx FIFO (within the Transmit Utopia Interface block) is depleted and has no (user) cells available, then the
Transmit Cell Processor will automatically generate Idle cells. These Idle cells will be processed in the exact same
manner as are the user cells, prior to transmission to the Transmit E3 Framer block. The Transmit Cell Processor gen-
erates these Idle Cells for “Cell-Rate” decoupling purposes. The Transmit Cell Processor also has provisions to allow
the user to generate an OAM cell via software control. Note: the OAM cells will be subjected to the same processing
(e.g., HEC Byte Calculation/Insertion and Cell Payload Scrambling) as are user and Idle cells.
The Transmit E3 Framer block will take these ATM cells (from the Transmit Cell Processor), and insert this data into
the payload portions of each outbound E3 frame. The Transmit E3 Framer will also generate overhead (OH) bytes
that support framing, performance monitoring (the EM byte), path maintenance data link as well as alarm and status
information originating from the “Near-End” Receiver section of the UNI. The purpose of these alarm and status
information bits is to alert the “Far-End” Terminal Equipment that the “Near-End” UNI Receiver has detected some
problems in receiving data from it. The Transmit E3 Framer will output this E3 data stream to an off-chip LIU (Line
Interface Unit) chip via the TxPOS, TxNEG, and TxLineClk output pins. The LIU chip will take on the responsibility of
driving the E3 data out on the E3 Transport Medium to the “Far-End” Terminal.
Likewise, whenever ATM cell data arrives to the UNI, over the E3 line, the Receive E3 Framer block will synchronize itself
to this incoming E3 Data Stream (containing ATM cells) via the RxPOS, RxNEG, and RxLineClk input pins, and pro-
ceed to “strip off” and process the OH bytes of the E3 frame. Once all of the OH bytes have been removed, the payload
portion of the received E3 Frame should consist of ATM cells.
ATM
Layer
Processor
Electrical
to
Optical
Converter
Microprocessor
Interface
ALE_AS
XR-T7234
TxData
[15:0]
To/From
Far End
E3 UNI
TxPOS
RxPOS
ATM Switch
Local “Housekeeping” Processor
D[15:0]
WRB_RW
RDS_DS
Rdy_Dtck
TxClav
RxClav
Transmit
Utopia
Interface
Block
Transmit
Cell
Processor
Transmit
E3
Framer
Receive
Cell
Processor
Receive
Utopia
Interface
Block
Receive
E3
Framer
Optical
to
Electrical
Converter
RxDa
[15:0]