
XRT74L74
4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER
PRELIMINARY
REV. P1.1.1
163
As far as the XRT74L74 DS3/E3 UNI is concerned,
whether an OAM cell is an F4 or F5 type OAM cell,
is rather unimportant. The Receive Cell Processor
circuitry has been designed to recognize both types
of OAM cells, based upon their header byte pattern.
However, whether an OAM cell is a “Segment type” or
an “End-to-End type” is more important in regards to
UNI IC operation. The manner in which the Receive
Cell Processor handles “Segment” and “End-to-End”
OAM cells is described below.
4.3.2.4.1
Segment type OAM cells are only intended for point-
to-point transmission. In other words, a segment type
OAM cell will be created at a source node, transmis-
sion across a single link, to a destination node; and
then terminated at the destination node. This Segment
OAM cell is not intended to be read or processed by
any other nodes within the ATM Network.
Segment Type OAM Cells
How the Receive Cell Processor handles Segment
Type OAM Cells
The Receive Cell Processor has been designed to
recognize incoming OAM cells, based upon their
header byte pattern. Further, the Receive Cell
Processor is also capable of reading the header byte
patterns, in order to determine if the OAM cell is a
“Segment” type or an End-to-End type OAM cell. If
the incoming OAM cell is a “Segment” type OAM cell,
then the Receive Cell Processor will not write this cell
to the RxFIFO, within the Receive UTOPIA Interface
block and will discard this cell. This act of discarding
the OAM cell terminates it and prevents it from propa-
gating to other nodes in the network.
Note: If the User Cell Filter is configured to pass cells with
header bytes pattern ranges that includes that of the “Seg-
ment”-type OAM Cell, then the User Cell Filter settings will
take precedence and allow the “Segment”-type OAM Cell to
be written to the RxFIFO, within the Receive UTOPIA Inter-
face Block.
Although the Receive Cell Processor will discard this
“Segment” OAM cell, the Receive Cell Processor can
be configured to have the contents of this cell written
into the Receive OAM Cell Buffer, where it can be
read out and processed by the local μP/μC.
If a “1” is written to bit 3 (OAM Check Bit) within the
“RxCP Configuration” register (Address = 4Ch), then
all OAM cells that are received by the Receive Cell
Processor will be written into the Receive OAM Cell
buffer (located at 161h through 1A1h, in the UNI chip
address space).
Once the Receive Cell Processor has written the
OAM cell into the “Receive OAM Cell” buffer, then the
Receive Cell Processor will alert the local μP/μC of
this fact, by generating the “Received OAM Cell”
interrupt. If a “0” is written to bit 3 of the “RxCP Con-
figuration” register, then the Receive Cell Processor
will not write the contents of the OAM cells that it re-
ceives, to the “Receive OAM Cell” buffer.
Figure 32 presents an illustration depicting how the
Receive Cell Processor handles incoming Segment-
type OAM cells, if a “1” has been written to bit 3 (OAM
Check Bit) of the “RxCP Configuration” register.
RxCP Configuration Register (Address = 4Ch)
B
IT
7
B
IT
6
B
IT
5
B
IT
4
B
IT
3
B
IT
2
B
IT
1
B
IT
0
RxLCD
RDPChk
Pattern
RDPChk
Pattern Enable
Idle Cell
Discard
OAM Check
Bit
De-Scramble
Enable
RxCoset
Enable
HEC Error
Ignore
RO
R/W
R/W
R/W
R/W
R/W
R/W
R/W
0
x
x
x
x
x
x
x