Forward performance monitoring cell generation in ATM OAM...

Details Forward performance monitoring cell generation in ATM OAM... Forward performance monitoring cell generation in ATM OAM...

2000-07-13

2004-02-24

Ngo, Ricky (Department: 2662)

Multiplex communications

Data flow congestion prevention or control

Flow control of data transmission through a network

C370S252000, C370S414000

Reexamination Certificate

active

06697332

ABSTRACT:

TECHNICAL FIELD
This invention relates to generation and insertion of forward performance monitoring (“Fwd PM”) cells during operation and maintenance (“OAM”) processing of cell traffic in asynchronous transfer mode (“ATM”) layer devices.
BACKGROUND
As shown in
FIG. 1
, an ATM layer device
10
(everything to the left of network cloud
12
) receives ATM cells from any one of a potentially large plurality of input sources
14
A,
14
B,
14
C,
14
D, etc. Each input source carries cells corresponding to a potentially large number of ATM connections. ATM layer device
10
processes the received cells and outputs them to network
12
(or to an ATM switch, not shown), by placing each cell in an appropriate one of a number of output queues
16
A,
16
B,
16
C,
16
D, etc. corresponding to input sources
14
A,
14
B,
14
C,
14
D, etc. As is well known, such processing involves carefully controlled admission of cells from the input sources by access controller
18
so as to avoid head-of-line blocking problems, facilitate proper management of the output buffers, and generally to provide a well-controlled traffic stream. The OAM performance monitoring (“OAM-PM”) functionality of ATM layer device
10
(as defined in ITU-T Recommendation I.610—“B-ISDN Operation and Maintenance Principles and Functions”, February 1999, hereafter “ITU-T I.610”) is represented schematically in
FIG. 1
by performance manager (“PM”) sub-block
20
.
More specifically,
FIG. 2
provides an enlarged depiction of ATM layer device
10
. For ease of illustration, only three ATM cell input sources
14
A
14
B,
14
C and their corresponding output queues
16
A,
16
B,
16
C are shown. Input source
14
A and its corresponding output queue
16
A is depicted as currently carrying cells corresponding to connections “
12
”, “
11
” and “
14
”; input source
14
B and its corresponding output queue
16
B is depicted as currently carrying cells corresponding to connections “
3
”, “
4
” and “
1
”; and, input source
14
C and its corresponding output queue
16
C is depicted as currently carrying cells corresponding to connection “
7
”. Access controller
18
admits cells from the input sources into processing queue
22
, and performs various operations on the cells as they pass through processing queue
22
, including identifying the connection each cell belongs to, identifying the PM session associated with that connection, and performing other ATM layer functions such as policing and fault management. PM
20
monitors the cell traffic stream passing through processing queue
22
to output queues
16
A,
16
B,
16
C, and counts the number of cells on each connection.
Periodically, PM
20
inserts a Forward Performance Monitoring (“Fwd PM”) cell into the cell stream within processing queue
22
, as prescribed by ITU-T I.610 and ITU-T Recommendation I.356—“B-ISDN ATM Layer Cell Transfer Performance”, October, 1996 (hereafter ITU-T I.356). More particularly, one Fwd PM cell is inserted for every approximately N cells on a connection, where N ranges from 128 to 32768 depending on the data rate of the connection. Because the Fwd PM cells must be inserted based on the number of cells transmitted on a connection, the operation of access controller
18
affects the number of PM cells to be transmitted. Furthermore, other ATM Layer functions (including, for example, cell rate policing as described in ATM Forum TM4.1—ATM Forum Traffic Management Specification Version 4.1, 1999) may affect the number of cells transmitted, thus affecting the number of Fwd PM cells that must be generated. Accordingly, it is convenient to insert the Fwd PM cells at the ATM Layer. However, this poses potential head-of-line blocking problems and/or output queue buffer overflow problems.
For example, output queue
16
A depicted in
FIG. 3
currently contains three cells (i.e. those labelled “
12
”, “
11
” and “
14
”). This leaves three empty slots (i.e. the three blank, leftmost slots in output queue
16
A) into which cells passing through processing queue
22
may be placed. Suppose now that PM
20
determines that it is necessary to insert a Fwd PM cell
24
corresponding to connection “
12
” into the cell stream within processing queue
22
. Fwd PM cell
24
is generated by PM
20
and contains, amongst other statistical indicia, a count of the total number of cells corresponding to connection “
12
” which PM
20
has detected passing through processing queue
22
since PM
20
last generated a Fwd PM cell corresponding to connection “
12
”. However, access controller
18
has already admitted into processing queue
22
three cells which are destined for output queue
16
A, namely the three cells labelled “
12
”. Thus, there is no room for insertion of Fwd PM cell
24
into processing queue
22
and thence into output queue
16
A without dropping one of the three connection “
12
” cells in processing queue
22
, which is unacceptable. (After generating and inserting Fwd PM cell
24
as aforesaid, PM
20
clears and restarts its count of the total number of cells corresponding to connection “
12
”.)
The prior art has evolved a variety of solutions to the aforementioned class of head-of-line blocking and/or output queue buffer overflow problems. One such solution, illustrated in
FIG. 4
, is to provide an access controller
18
A and PM
20
A adapted to “reserve” a cell slot within output queue
16
A into which a Fwd PM cell can be inserted. Specifically, PM
20
A signals access controller
18
A when PM
20
A determines that it is necessary to insert a Fwd PM cell
24
A corresponding, for example, to connection “
12
” into the cell stream within processing queue
22
. Access controller
18
A responds by reserving one of the empty slots in output queue
16
A for use by PM
20
A. This may necessitate momentarily halting admission of cells from input source
14
A into processing queue
22
until space becomes available in output queue
16
A. Access controller
18
A then signals PM
20
A, indicating grant of the reserved slot, and PM
20
A reacts by inserting Fwd PM cell
24
A into that slot. In practice however, complex timing and logic problems arise, making it difficult to properly implement such a “reservation” scheme, particularly at high frequencies.
Another prior art solution, illustrated in
FIG. 5
, is to provide an access controller
18
B and PM
20
B adapted for “loop-back” insertion of Fwd PM cells from PM
20
B to access controller
18
B. Specifically, when PM
20
B determines that it is necessary to insert a Fwd PM cell
24
B corresponding, for example, to connection “
12
” into the cell stream within processing queue
22
, PM
20
B generates Fwd PM cell
24
B and transmits cell
24
B to an input port of access controller
18
B, as indicated at
26
in FIG.
5
. Access controller
18
B eventually admits cell
24
B into processing queue
22
. By allowing access controller
18
B to handle insertion of cell
24
B into processing queue
22
, the loop-back technique overcomes the above-described problems of the prior art techniques illustrated in
FIGS. 3 and 4
. Specifically, access controller
18
B does not admit cell
24
B into processing queue
22
unless there is room in the appropriate output queue (in this case, output queue
16
A) for cell
24
B, thereby overcoming the head-of-line blocking and/or output queue buffer overflow problems to which the technique illustrated in
FIG. 3
is subject. Furthermore, by allowing access controller
18
B to handle insertion of cell
24
B into processing queue
22
, the loop-back technique avoids the timing and logic problems to which the
FIG. 4
“reservation” technique is subject.
However, the
FIG. 5
loop-back technique is subject to a further problem. After generating Fwd PM cell
24
B, PM
20
B immediately clears and restarts its count of the total number of cells for the corresponding the ATM connection. But, by the time access controller
18
B inserts cell
24
B into processing queue
22
, additional cells corresponding to connection “
12
” may have been admitted into the cell stream within processin

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