ATM inverse multiplexing system

Details ATM inverse multiplexing system ATM inverse multiplexing system

1999-02-22

2004-01-20

Pham, Chi (Department: 2667)

Multiplex communications

Communication techniques for information carried in plural...

Assembly or disassembly of messages having address headers

C370S535000

Reexamination Certificate

active

06680954

ABSTRACT:

FIELD
The present invention relates to a cell-based ATM Inverse Multiplexing System which reduces the incidence of error-multiplication due to error events in the transmitted cell stream.
BACKGROUND
In an ATM Inverse Multiplexing System (formerly referred to as AIMUX but presently referred to as IMA, for Inverse Multiplexing for ATM), ATM cell traffic is transported by means of time-division multiplexing over several channels (typically T
1
or E
1
data links). In a cell based IMA system, these ATM cells (defined here as “payload cells”) are sent on each channel in a round-robin fashion as depicted in FIG.
1
. In
FIG. 1
, an incoming cell stream from an ATM layer on bus interface
21
is received by IMA device
11
of IMA system
10
and is coupled to another IMA device
12
by three channels or links
14
,
16
, and
18
. Incoming cells
20
on incoming bus interface
21
enter the IMA device
11
and are time-division multiplexed over links
14
,
16
, and
18
. In each channel or link
14
,
16
, or
18
, a sequence number “S” cell is inserted periodically, as determined by a specified time interval or by a given number of cells. In this case an “S” cell precedes each series of cells from the incoming cell stream over a selected time interval or for each of a selected number of incoming cells. Another “S” cell is inserted on each channel after the last cells of the given set of cells in the incoming cell stream has been transmitted. At the IMA device
12
, the egress payload cell stream is reconstructed by assembling incoming cells on each channel in the same order as they were transmitted.
A more specific configuration is shown in
FIG. 2
, in which one ATM Layer Device
18
is coupled through an IMA system
10
to another ATM Layer Device
19
. The ATM Layer Device
18
connects to IMA device
11
via Utopia/SCI-PHY bus interface
21
which, in turn, is coupled to a number of physical layer devices
30
by Utopia/SCI-PHY bus interface
14
. Each physical layer (PHY-layer) device
30
is coupled by links
34
and
36
to another corresponding physical layer device
32
. Each physical layer device
32
is connected to IMA device
12
by one of several Utopia/SCI-PHY bus interface
16
. IMA device
12
connects to ATM Layer device
19
by Utopia/SCI-PHY bus line
22
.
Cells from ATM Layer Device
18
are sent to IMA device
11
which multiplexes them together with “S” cells, onto bus lines
14
. A transmitting PHY-layer ATM device
30
sends idle cells over its link for the purpose of cell rate decoupling whenever its transmit FIFO-buffer (not shown) is empty. This can occur when the incoming ATM traffic is either bursty or if the cells arrive at a rate slower than the transmit rate of the PHY device
30
.
The receiving IMA device
12
must reconstruct its output stream from cells received over the constituent channels, in such a way that cell sequence integrity is preserved. Referring to
FIG. 3
, an input cell stream
13
is multiplexed over three links
15
(link #
1
),
17
(link #
2
), and
19
(link #
3
). An S cell precedes cell #
1
followed by cell #
1
. Similarly, an S cell precedes cell #
2
followed by an idle cell containing an error (error(
2
)) and an S cell on link
19
is followed by cell #
3
. The remaining cells are sent in the order shown in
FIG. 3
with cells #
4
and #
8
containing errors (error(
1
) and error(
3
), respectively). Suppose a payload (or S) cell is lost due to an HEC error, as shown in
FIG. 3
(event error (
1
)). If the cells are simply reassembled directly according to the successfully received payload cell sequence in each channel or link, then the output cells will no longer be delivered in the correct sequence to the ATM-layer device
12
, as can be seen in FIG.
4
. These errors will not be detected until an S cell is subsequently received (error-free), at which point the IMA device will realize the problem because the number of payload cells received between that and the previous S cell will be different from what is expected.
It will be seen that mis-sequencing occurs from a combination of 1)idle cells inserted in a manner that disrupts the ordered arrival sequence of payload cells and, 2) ambiguities as to where these idle cells may be in the received cell stream (and guessing wrong). These are consequences of using channels that operate in an asynchronous manner, where each channel may be operating at slightly (but nevertheless significantly) different frequencies and phase differences relative to the other channels at any given time. The mis-sequencing problem can be solved by having the channels operate synchronously with each other, but that may not always be feasible as it depends very much on the underlying telephone network infrastructure.
ATM PHY devices
32
are typically configured to discard idle cells and cells with HEC errors. If these devices can be programmed to pass HEC-errored cells on to the higher layers, additional information can be used to assist in the decoding process (see ATMF 95-1659, “Synchronous Links, Cell Loss Handling, and Control Cells in AIMUX”, December 1995). However, the IMA device
12
will still need to guess whether or not a HEC-errored cell corresponds to an idle cell. Now suppose that the number of payload cells framed between two S cells is fixed. The IMA device
12
can now use this information to identify errored cells by buffering all subsequently received cells for the remainder of the frame, and count the number of payload cells (see ATMF 95-1659, supra.). If the number of payload cells is less than expected, then it may be possible to determine the position of any missing payload cell by determining the locations of the HEC-errored cells. Furthermore, cell arrival timing information can be used to reduce the range of possibilities. This can be done by either explicitly recording the arrival times or by having the PHY devices pass idle cells through. Even if all these measures are taken, certain error patterns can produce unresolved ambiguities that lead to error multiplication. An example is shown in
FIG. 3
for events error (
2
) and error (
3
), where the corresponding decoded sequence is shown in FIG.
5
. Moreover, buffering schemes such as this one will not work if the number of payload cells in a given frame is unknown.
Accordingly, it is an object of the invention to provide a more robust solution to reduce error multiplication, as compared to the known schemes. It is a further object of the invention to provide error free multiplication which does not require additional PHY-layer signaling information from known schemes which work over asynchronous multiplexed channels. It is yet a further object of the invention to provide a solution to reduce error multiplication with no additional transmission overhead compared to existing schemes and which does not require an inordinate increase in implementation complexity or memory requirement.
SUMMARY OF THE INVENTION
According to the invention there is provided a method of reducing error-multiplication due to error events in a cell stream transmitted as a plurality of cell sub-streams which includes the steps of receiving an incoming cell stream in the form of an ordered sequence of cells including payload cells, transmitting the incoming cell stream in a round robin fashion on a plurality of physical links such that the ordered sequence of cells is transmitted as a plurality of cell sub-streams, with each cell sub-stream having a multiplexed set of cells from the incoming cell stream, and inserting stuff (st) cells into the cell sub-streams so as to form continuous streams of data. Sequence number (S) cells which are inserted periodically into each cell sub-stream, are used to align the cell sub-streams in frames. Sets of cell location information for the cell sub-streams are encoded and contain the location of payload cells and st cells located within a corresponding cell sub-stream. The sets of cell location information are passed on to a receiving end where they are re-constructed at th

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