Multiplex communications – Data flow congestion prevention or control – Control of data admission to the network
Reexamination Certificate
1998-08-27
2002-09-10
Marcelo, Melvin (Department: 2663)
Multiplex communications
Data flow congestion prevention or control
Control of data admission to the network
C370S232000, C370S395430
Reexamination Certificate
active
06449253
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to asynchronous transfer mode (ATM) switching systems, and more particularly, to a method and system for dynamically allocating bandwidth to Available Bit Rate (ABR) virtual circuits in ATM switching systems.
In an ATM network, a virtual source (VS) transmits data in the form of fixed sized cells to a virtual destination (VD) through a connection (referred to as virtual circuit) established between the virtual source and the virtual destination. The virtual source and virtual destination may be a telephone, video equipment, facsimile, computer, edge-router, edge-switch, etc. The cells may include any type of digitized information, including audio, computer data, video, multimedia, Internet data, etc. For example, in a network that uses Transmission Control Protocol/Internet Protocol (TCP/IP) over ATM, a virtual source may be an edge-router at the entry to an ATM network. An entry edge-router segments the incoming TCP/IP data packets into one or more ATM cells before transmitting each cell to the ATM network. Similarly, a virtual destination may be an edge router at the exit of the ATM network. An exit edge-router reassembles incoming ATM cells into TCP/IP data packets before transmitting each packet to its destination.
When establishing a virtual circuit through an ATM network, a virtual source can select one of five different categories of service: Constant Bit rate (CBR), Variable Bit Rate—Real Time (VBR-RT), Variable Bit Rate—Non Real Time (VBR-NRT), Available Bit Rate (ABR), and Unspecified Bit Rate (UBR). ATM Forum Traffic Management Standard af-tm-0056.00 describes each of these services.
The ABR service determines excess bandwidth in the network and uses network management methods to reallocate the excess bandwidth among the virtual circuits in the network to reduce network congestion and cell loss. In negotiating an ABR virtual circuit, a virtual source negotiates a peak cell rate (PCR) and a minimum cell rate (MCR) with the ATM network. PCR is the maximum cell rate a virtual circuit can support. MCR is the minimum cell rate that a virtual circuit must support. The ABR service uses the negotiated PCR and MCR parameters to provide a guaranteed quality of service concerning bandwidth availability and cell loss in a virtual circuit.
When a virtual source selects the ABR service, the virtual source periodically generates a resource management. (RM) cell to get feedback from the network on the rate at which the virtual source can transmit cells on a virtual circuit without causing loss of cells due to network congestion. Typically, a virtual source generates an RM cell for every thirty-one cells it transmits or at the expiration of a fixed time interval, whichever occurs first. The network processes the RM cell, updates virtual circuit bandwidth information in the RM cell, and returns the RM cell to the virtual source. The virtual source then dynamically adjusts its rate of cell transmission based on the bandwidth information contained in the RM cell.
An RM cell generated by a virtual source is referred to as a Forward RM cell. The Forward RM cell passes through one or more switching systems in the network before reaching a virtual destination. The virtual destination processes the Forward RM cell and returns a Backward RM cell to the virtual source. The Backward RM cell passes through one or more switching systems in the network before reaching the virtual source.
A virtual source maintains the MCR, current Allowed Cell Rate (ACR), and the PCR associated with the virtual circuit. ACR is the rate at which the network allows the virtual source to transmit cells on a virtual circuit. When a virtual source receives a Backward RM cell, based on the bandwidth information in the Backward RM cell, the virtual source computes a new ACR. Consequently, ACR dynamically changes as the network traffic changes and as the virtual source receives feedback from the network.
A Forward RM cell includes an MCR field, current cell rate (CCR) field, and an explicit rate (ER) field. CCR is the rate at which a virtual source is transmitting cells on a virtual circuit at the time the virtual source generates a Forward RM cell. ER is the rate at which the virtual source wishes to transmit cells on a virtual circuit. A virtual source cannot set the ER field in a Forward RM cell to be greater than PCR. After generating a Forward RM cell and setting the MCR, CCR, and ER fields in the Forward RM cell, the virtual source transmits the Forward RM cell to the network.
When a virtual source transmits a Forward RM cell, the Forward RM cell passes through each switching system on the path of the virtual circuit to the virtual destination. Each switching system on the path can either keep the ER in the Forward RM cell the same or decrease the ER to a lower rate. However, according to ATM Forum Traffic Management Standard, af-tm-0056.00, a switching system cannot decrease the ER below the MCR for the virtual circuit. Furthermore, a switching system cannot increase the ER for the virtual circuit. A switching system that allocates bandwidth to a virtual source by setting the ER field in an RM cell is referred to as an ABR Explicit Rate (ABR-ER) switching system.
When a Forward RM cell associated with a virtual circuit arrives at an ABR-ER switching system, the switching system determines an upper threshold (referred to as “cutoff”) for the bandwidth that can be made available to the virtual circuit in the switching system. If the switching system determines that the computed cutoff for the virtual circuit sets the ACR in the virtual source (i.e, the switching system computes the smallest cutoff among all of the switching systems on the path of the virtual circuit), then the switching system considers the virtual circuit to be “bottlenecked here” or bottlenecked in the switching system. If the switching system determines that the computed cutoff does not set the ACR in the virtual source (i.e, the switching system does not compute the smallest cutoff among all of the switching systems on the path of the virtual circuit), then the switching system identifies the virtual circuit as “bottlenecked elsewhere.”
The switching system determines a new bandwidth that it can allocate to the virtual circuit by determining a new ER for the virtual circuit and setting the new ER in the Forward RM cell. The switching system then determines an estimated rate (Exp_Rate) at which the switching system “expects” the virtual source to transmit data cells after the virtual source adjusts its ACR based on the newly set ER. Finally, the switching system sends the Forward RM cell to the next switching system on the path of the virtual circuit.
When the Forward RM cell reaches the virtual destination, the virtual destination returns the Forward RM cell as a Backward RM cell. The Backward RM cell passes through one or more switching systems on the path of the virtual circuit without any further modification to the bandwidth information set in the Forward RM cell. When the Backward RM cell reaches the virtual source, the virtual source uses the new ER in the RM cell to determine a new ACR. Based on the new ACR, the virtual source adjusts the rate at which it transmits cells.
Every time a switching system determines a cutoff and ER for a virtual circuit, the switching system also recomputes certain global bandwidth parameters for the virtual circuits that the switching system handles. These global bandwidth parameters include the total bandwidth available to all ABR-ER virtual circuits, the total Exp_Rate for ABR-ER virtual circuits that are bottlenecked elsewhere, the total number of ABR-ER virtual circuits that are bottlenecked elsewhere, and the total number of ABR-ER virtual circuits that are bottlenecked at the switching system.
Methods for determining and updating the global bandwidth parameters in an ABR-ER switching system are known. However, these methods have the disadvantage that every time a switching system recomputes the global bandwidth parameters that the swi
Abelson Ronald
Giordano Joseph
Marcelo Melvin
Schoneman William A.
Telcordia Technologies Inc.
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