Multiplex communications – Data flow congestion prevention or control
Reexamination Certificate
1996-09-18
2002-12-03
Patel, Ajit (Department: 2664)
Multiplex communications
Data flow congestion prevention or control
C370S395420, C370S415000
Reexamination Certificate
active
06490248
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a packet transfer device such as a cell multiplexing device and a cell buffer device in an ATM communication network. Since a cell switch can be formed by combining a plurality of cell multiplexing devices, the present invention is also applicable to a cell switch which has each output port in a configuration of a cell multiplexing device according to the present invention. The cell buffer device of the present invention is not necessarily limited to handling of cells and is applicable to a buffer device in general. The present invention is also not necessarily limited to an application to the ATM communication network and is applicable to a packet switching network as well.
2. Description of the Background Art
Currently, researches on an ATM (Asynchronous Transfer Mode) communication scheme are actively conducted by researchers of the communication technology throughout the world. In an ATM communication scheme, fixed length packets called cells are used to transmit and switch information. In an ATM communication scheme, a high speed cell switching can be realized by a cell switch provided by the hardware inside a switch node, so that it is possible to realize an information transfer performance per unit time much superior than those of the existing communication networks.
An ATM communication scheme can set up a plurality of logical connections (Virtual Connection: VC) in one physical transmission link by using a connection identifier information called VPI (Virtual Path Identifier) and VCI (Virtual Channel Identifier) to be provided in a header of a cell. At each switch node within the network, routes are set up in advance for each VC, and the switch node determines an output route for outputting the cell according the connection identifier VPI and VCI of the cell. VPI and VCI are uniquely assigned at each physical transmission link between the switch nodes, so that the switch node has a function for rewriting values of VPI and VCI of a passing cell.
Up to now, the VC for which the quality is guaranteed in the ATM network has been either a CBR (Constant Bit Rate) connection or a VBR (Variable Bit Rate) connection. The CBR connection is a VC for transmitting the traffic in which the cell transmission rate (that is, a number of transmission cells per unit time, which is also referred to as a cell rate or a bandwidth) is constant and known in advance. The VBR connection is a VC for transmitting the traffic in which the cell transmission rate is not constant but some traffic characteristics such as a maximum value (peak rate) and an average value (average rate) are known in advance.
Basically, in a case of multiplexing a plurality of VCs in one physical transmission link while maintaining a sufficient quality, it is sufficient for a sum of the peak rates of all the VCs to be not greater than the bandwidth of the physical transmission link. This scheme is called a peak rate allocation. It is possible for the peak rate allocation to realize a sufficiently high physical transmission link utilization efficiency in a case of dealing only with the CBR connections, but a physical transmission link utilization efficiency cannot be very high in a case of dealing with the VBR connections. For this reason, there are many studies on a technique for improving the physical transmission link utilization efficiency by using the statistical multiplexing effect according to the traffic characteristics known in advance while maintaining the quality.
However, the ATM communication between computers is characterized by the fact that the traffic characteristics such as an average rate cannot be predicted in advance, and the characteristic called burstiness which indicates a coexistence of some timing at which a large amount of cells are transmitted instantaneously and some other timing at which no cell is transmitted. Consequently, for the ATM communication between computers, it is difficult to improve the network utilization efficiency while maintaining the quality by using the techniques used for CBR and VBR.
More specifically, for data transferred between computers, when the peak rate allocation is utilized to guarantee the quality, the network utilization efficiency will be lowered considerably, whereas when the statistical multiplexing effect is utilized as in a case of VBR, a large amount of cells arrive simultaneously at some output port of the cell switch because of the burstiness of the traffic, and the cell loss due to the buffer overflow will be caused unless a sufficient buffer capacity is provided at the cell switch. In addition, when the cell loss is caused, a re-transmission in unit of a packet formed by a plurality of cells will be carried out, so that the effective throughput will be lowered.
For this reason, in recent years, a service class called ABR (Available Bit Rate) has been proposed and studied. In this ABR, the network utilization efficiency is improved while guaranteeing the cell transfer quality (especially a quality concerning cell loss) by applying a flow control between terminals and switch nodes. In the ABR connection, when the switch node is about to fall into the cell congestion state, the source terminals are controlled to suppress cell transmissions before the cell congestion occurs at the switch node. Here, the traffic control information to be given to terminals is mainly carried by a cell called RM (Resource Management) cell. This traffic control from the switch node to the source terminal in the ABR has an unignorable delay time, so that there is a need to implement a large capacity buffer at the cell switch so as not to discard cells until the traffic control begins to function effectively.
Apart from these CBR, VBR and ABR, there is a service class called UBR (Unspecified Bit Rate) which is a class of the so called best effort service in which a terminal is not required to declare the characteristics of traffic to be outputted in detail to the network, but instead the network does not guarantee the transfer quality at all.
As described above, data between computers have the burstiness, so that in order to realize a satisfactory level of the cell loss rate for the UBR connections, it is considered that the implementation of a large capacity cell buffer in the cell switch is necessary.
Fortunately, the transfer delay time and the transfer delay jitter required by the traffic between computers are often not so severe compared with those required for the CBR and VBR. By implementing a large capacity cell buffer in the cell switch, the delay time and the delay jitter for the cell transmission increase, but it is considered that there are many applications that can tolerate such an increase of the delay time and the delay jitter.
In particular, in a case of the ABR and UBR services, it is considered necessary to provide means for avoiding the congestion of the network. One known means for avoiding the congestion is the so called EFCI (Explicit Forward Congestion Indication). In the EFCI, a header of each cell has an EFCI bit as an indication of a congestion experienced by that cell, and a cell switch within the network marks the EFCI but according to the congestion state. As each terminal utilizes the information of the EFCI, it becomes possible to avoid the congestion.
Next, the conventional per-VC fair queueing in the cell multiplexing device and the cell switch will be described.
First, the per-VC fair queueing which is required as the robustness in the ABR service will be described.
The ABR service works as the source terminal controls the transmission of cells according to the traffic control information given from the network. If some terminal ignores (either by fault or intentionally) the traffic control information given from the network, it may be difficult to recover from the congestion of the network.
This problem can be resolved by carrying out the per-VC fair queueing at a cell multiplexing device or a cell switch, so as to realize the fair cell multiplexing schedu
Kabushiki Kaisha Toshiba
Patel Ajit
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