Back-pressure type ATM switch

Details Back-pressure type ATM switch Back-pressure type ATM switch

1998-06-30

2003-03-11

Yao, Kwang Bin (Department: 2664)

Multiplex communications

Pathfinding or routing

Switching a message which includes an address header

Reexamination Certificate

active

06532234

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a back-pressure type ATM (Asynchronous Transfer Mode) switch, and particularly to an economical and flexible configuration of the back-pressure type ATM switch for guaranteeing fair throughput making use of an internal rate control.
In conventional back-pressure type ATM switches such as those disclosed in a Japanese patent application entitled “A Method of Priority Control in an Output-buffer Type ATM Switch” and laid open as a Provisional Publication No. 297840/'95, or a Japanese patent application entitled “A Back-pressure Type ATM Switch” and laid open as a Provisional Publication No. 223173/'96, cells toward a congestion node are suspended to be read out from input buffers according to a “back-pressure” signals which are generated at the congestion node in the ATM switch to be fed-back to the input buffers.
FIG. 7
is a block diagram schematically illustrating an example of the back-pressure control applied in a conventional I/O (In/Out) buffer type ATM switch.
In the example of
FIG. 7
, there are comprised m input buffers
71
-
1
to
71
-m each corresponding to each of m input ports, n output buffers
72
-
1
to
72
-n each corresponding to each of n output ports, and a switch
73
for switching each of cells read out from the input buffers
71
-
1
to
71
-m into one of output buffers
72
-
1
to
72
-n according to header information of the cell (m and n being positive integers).
In each of the input buffers
71
-
1
to
71
-m, s×n input queues (s being an positive integer) are prepared each corresponding to each of s QoS (Quality of Service) classes of cells to be switched to each of the n output buffers
72
-
1
to
72
-n, while s output queues are prepared each corresponding to each of s QoS classes in each of the output buffers
72
-
2
to
72
-m.
From each of the output buffers
72
-
1
to
72
-n, s back-pressures are generated to be fed-back to every of the input buffers
71
-
1
to
71
-m. Each of the s×n back-pressures suspends read-out of cells from corresponding each of s×n input queues of every of the input buffers
71
-
1
to
71
-m, when it is enabled.
Thus, a back-pressure control is realized in the ATM switch of
FIG. 7
for obtaining high robustness against instantaneous excessive traffic.
However, according to above back-pressure control, there arise unfair throughputs among the input ports. Consider, for example, only one connection of a certain QoS class toward a certain output port is established in a first input port, while ten connections of the same QoS class toward the same output port are established in a second input port. Then, throughput given for each connection of the second input port becomes {fraction (1/10)} of that of the first input port, when read-out of cells from concerning two input queues are controlled in the same way, resulting in unfair bandwidth allocation for connections requiring the same QoS among different input ports.
For dealing with this unfair bandwidth allocation, an “internal rate control” is proposed by Shinohara, et al., in “Large Scale ATM Switch with Multiple QoS Guarantee”, pp. 73 to 78, TECHNICAL REPORT OF IEICE, SSE96-55 (1996-08), and in “Internal Congestion Contrl in Large Scale ATM Switch”, pp. 13 to 18, TECHNICAL REPORT OF IEICE, SSE96-131, CQ96-41 (1996-12). In the internal rate control, an open-loop control is applied to bandwidth guaranteed QoS classes like CBR (Constant Bit Rate) and VBR (Variable Bit Rate) wherein a bandwidth is allocated for each connection, and a closed-loop control according to internal congestion is applied to no bandwidth guaranteed QoS classes like ABR (Available Bit Rate) and UBR (Unspecified Bit Rate).
FIG. 8
is a block diagram schematically illustrating the above closed-loop control applied to the no bandwidth guaranteed QoS classes in a conventional I/O buffer type ATM switch.
For performing the internal rate control, a rate shaper S having a virtual queue (virtual and not depicted in the drawing) is provided for each of n×s input queues, corresponding to each of n output ports and s QoS classes, in each of m input buffers
81
-
1
to
81
-m. In the internal rate control, the rate shaper S virtually reads out cells from its input queue at a rate, or a read-out speed, calculated according to rate information delivered from a rate calculator
84
, and the cells read out by the rate shaper S are virtually transferred in the virtual queue to be read out by the read controller when corresponding back-pressure is not enabled.
The rate information is calculated for each of n×s output queues, that is, for each of s QoS class toward each of n output ports, according to queue length of a concerning output queue (hatched output queue
821
of
FIG. 8
, for example), global queue length, namely, total queue length of every virtual queue (in hatched rate shapers accompanying input queues
811
-
1
to
811
-m, in the example) corresponding to the concerning output queue (
821
) and cells on a switch
83
toward the concerning output queue (
821
), and a total number of connections, that is, a sum of active VCs (Virtual Channels) assigned to every input queue (
811
-
1
to
811
-m) corresponding to the concerning output queue (
821
). The read controllers are controlled by back-pressures generated by the switch
83
The rate information defines a read-out speed per a VC, and the rate shaper S shifts cells from the output queue to the virtual queue at a speed calculated by (rate information)×(number of active VCs of the input queue). From every of the virtual queues not suspended by corresponding back-pressure, cells are read out according to rotation priority read-out control (reading out cells from queues in turn).
Thus, fair bandwidth allocation according to closed-loop control is realized for no bandwidth guaranteed QoS classes like ABR and UBR, in the I/O buffer type ATM switch of FIG.
8
.
As to bandwidth guaranteed QoS classes like CBR and VBR, an open-loop control, that is, a fixed rate cell-transfer in proportion to the guaranteed bandwidth is performed according to fixed priority read-out control (reading out cells from queues in order of priority).
However, in the above prior art, the fixed rate cell-transfer and the fixed priority read-out control is required for queues assigned to bandwidth guaranteed QoS classes to be open-loop-controlled, while the variable speed rate shaper and the rotation priority read-out control is needed for queues assigned to no bandwidth guaranteed QoS classes to be closed-loop-controlled.
Therefore, for enabling to assign a queue flexibly to any of the bandwidth guaranteed QoS classes and the no bandwidth guaranteed QoS classes, the queue should be prepared with every one of the fixed rate cell-transfer, variable speed rate shaper, fixed priority read-out control and the rotation priority read-out control, which requires large and complicated hardware. In other words, number of QoS classes of the open-loop control and the closed-loop control are both fixed because of hardware restriction, in the above prior art.
Furthermore, a fixed buffer space must be allocated for each of the bandwidth guaranteed QoS classes, and for performing the closed-loop control effectively, certain amount of buffer space must be also allocated for each of no bandwidth guaranteed QoS classes, otherwise the cell rejection rate increases and the acceptable connection number is restricted. Therefore, the number of QoS classes are fixed, efficiency of buffer space is inevitably lowered.
SUMMARY OF THE INVENTION
Therefore, a primary object of the present invention is to provide a back-pressure type ATM switch wherein desired QoS class setting is realized by enabling to assign any queue to any QoS class without needing large hardware. Another object of the invention is to increase the acceptable connection number and decrease the cell rejection rate by improving efficiency of buffer space making use of a common memory.
In order to achieve the object, in a back-pressure ty

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