ATM switch and congestion control method

Details ATM switch and congestion control method ATM switch and congestion control method

1997-12-04

2001-07-10

Chin, Wellington (Department: 2664)

Multiplex communications

Pathfinding or routing

Switching a message which includes an address header

C370S229000, C370S230000, C370S235000

Reexamination Certificate

active

06259696

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an asynchronous transfer mode (ATM) switch and a congestion control method in an asynchronous transfer mode, and particularly to an ATM switch capable of packet level discard.
2. Description of the Related Art
Heretofore, as a cell transmission method of an ATM network comprising an ATM switch using an asynchronous transfer mode (hereinafter simply referred to as “ATM”), there are standardized the following four kinds of traffic classes. That is, a CBR (Constant Bit Rate) and a VBR (Variable Bit Rate) are standardized as a traffic class of a bandwidth-reserved type in which a bandwidth is reserved when a call is set between a network and an end system. An ABR (Available Bit Rate) and a UBR (Unspecified Bit Rate) are standardized as a traffic class in which a bandwidth cannot be reserved when a call is set because a traffic occurs in a burst fashion like a LAN (local Area Network) traffic. In the ABR and UBR traffic classes, the ATM network hardly guarantees a quality of service, and hence the ABR and UBR traffic classes are called “Best Effort type” traffic classes.
In the bandwidth-reserved type traffic classes, a surveillance mechanism called a UPC (Usage Parameter Control) provided at the entrance of the source end system side of a user network interface detects whether or not data is transmitted at a rate under a previously-reserved bandwidth. If it is determined by the UPC that there is a non-conforming cell, then such non-conforming cell is discarded or a discard priority is set to be high one more time, and data is transferred to the next ATM switch. Accordingly, it is possible to prevent a network congestion from being caused by a traffic from the source end system.
On the other hand, in the Best Effort type traffic class, the bandwidth is not reserved, and hence the above-mentioned UPC cannot be applied thereto. Further, a traffic occurs in a burst fashion, and a traffic which exceeds an available rate of an ATM switch provided within the network tends to be received, thereby resulting in a congestion. There is then a large possibility that a cell will be discarded. The ATM switch includes an internal cell buffer to temporarily accumulate cells when a congestion state is a slight congestion state, thereby preventing a cell from being discarded. However, if an excess traffic concentration is continued so that a congestion state becomes serious and accumulated cells exceed a storage capacity of the cell buffer in the ATM switch, then a cell discard will occur.
As a method of preventing a transmission efficiency of an ATM network from being lowered by minimizing a cell discard influence exerted by a congestion, there is known a method (packet level discard) which discards cells at the packet level. JP-A-6-334681 (“CELL SWITCH SYSTEM AND APPARATUS”), for example, discloses a packet level discard. Although a cell discard occurs in the ATM switch which has fallen into the congestion state, a destination end system cannot rearrange a packet if any one of cells composing the same packet is missing. Therefore, when a cell discard occurs over a number of packets, a number of packets are transmitted again from the source end system to the destination end system, resulting in a throughput of packet level (goodput) being lowered unnecessarily. The packet level discard function is able to discard cells composing the same packet continuously transmitted from the connection up to the interval of packets when the ATM switch, which has fallen into the congestion state, executes a cell discard. Owing to the packet level discard, the cell discard is concentrated on lesser packets so that an invalid packet in which any one of cells is missing can be prevented from being transmitted. Therefore, it becomes possible to improve a goodput of an ATM network.
SUMMARY OF THE INVENTION
The above-mentioned packet level discard has hitherto been proposed as a method of preventing the goodput of the ATM network from being lowered by suppressing an influence of a congestion unavoidably caused in the Best Effort type traffic class to the minimum. However, as shown in
FIG. 2
, when a plurality of ATM switches not having the packet level discard function exist in the upstream of the connection of other ATM switches having a packet level discard function (case
1
), there arise the following problems.
As shown in
FIG. 2
, an ATM network comprises ATM switches A
201
and C
203
not having a packet level discard function, an ATM switch B
202
with a packet level discard function, an end system A
211
, an end system B
212
, an end system C
213
, an end system D
214
and an end system E
215
. A cell flows from the end system A
211
to the end system E
215
, a cell flows from the end system B
212
to the end system E
215
, a cell flows from the end system C
213
to the end system E
215
, and a cell flows from the end system D
214
to the end system E
215
, respectively. Let us now consider the case in which a congestion occurs in the ATM switch A
201
and no congestion occurs in the ATM switch C
203
. There is a large possibility that a packet transmitted from the ATM switch A
201
to the ATM switch B
202
will be an imperfect packet having a missing cell (hereinafter referred to as “cell-missing-packet”). On the other hand, there is a large possibility that a packet transmitted from the ATM switch C
203
to the ATM switch B
202
will be a packet of which the cell is not missing (hereinafter referred to as “perfect packet”). However, when the ATM switch B
202
is placed in the congestion state, there is then a large possibility that, in order to equivalently packet-level-discard the packet sent from the ATM switch A
201
and the packet sent from the ATM switch C
203
, the ATM switch B
202
will transmit the cell-missing-packet sent from the ATM switch A
201
to the end system E
215
but instead the ATM switch B
202
will packet-level-discard the perfect packet from the ATM switch C
203
. In this case, since both of the cell-missing packet and the perfect packet are transmitted again, the bandwidth between the ATM switch B
202
and the end system E
215
is utilized uselessly, resulting in the goodput of the ATM network being lowered.
Further, as shown in
FIG. 3
, when the ATM switch with the packet level discard function implemented and the ATM switch not having the packet level discard function exist in the upstream of the connection of the other ATM switch having the packet level discard function (case
2
), there arises a similar problem.
As shown in
FIG. 3
, in the ATM network, the ATM switch C
203
not having the packet level discard function in
FIG. 2
is replaced having an ATM switch D
204
with a packet level discard function. Similarly to
FIG. 2
, as shown in
FIG. 3
, a cell flows from the end system A
211
to the end system E
215
, a cell flows from the end system B
212
to the end system E
215
, a cell flows from the end system C
213
to the end system E
215
, and a cell flows from the end system D
214
to the end system E
215
, respectively. When a congestion occurs in the ATM switch A
201
, there is then a large possibility that a packet transmitted from the ATM switch A
201
to the ATM switch B
202
will be a cell-missing-packet. On the other hand, a packet transmitted from the ATM switch D
204
to the ATM switch B
202
is a perfect packet. However, when the ATM switch B
202
is placed in the congestion state, the ATM switch B
202
equivalently packet-level-discards the packet transmitted from the ATM switch A
201
and the packet transmitted from the ATM switch D
204
so that a problem similar to the above-mentioned (case
1
) arises.
FIG. 8
is a table showing classified results of the ATM switches placed in the upstream of the ATM switch having the packet level discard function from a stand-point of the implementation of packet level discard function and the existence of a congestion. An ATM switch having a packet level discard function and which is no

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