Multiplex communications – Data flow congestion prevention or control – Control of data admission to the network
Reexamination Certificate
1998-12-28
2001-12-18
Olms, Douglas (Department: 2732)
Multiplex communications
Data flow congestion prevention or control
Control of data admission to the network
C370S395430
Reexamination Certificate
active
06331970
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the policing of Available Bit Rate (ABR) traffic in Asynchronous Transfer Mode (ATM) communication systems.
BACKGROUND OF THE INVENTION
In an ATM system, the traffic from a source is transmitted in fixed-length packets called cells, containing data and header information. The cells from different sources are interleaved and transmitted along a single link. The interleaving is non-uniform, in that more cells from a particular source may be interleaved per unit time than from a second source. This allows traffic from different types of sources to use only as much bandwidth as is needed, and provides much more efficient use of the total available bandwidth. The ATM network should allocate bandwidth to the different sources of traffic according to how fast each source needs to transmit information and the desired Quality of Service (QoS). This will depend on the type of information being transmitted, and on what the user is willing to pay for.
The Connection Admission Control (CAC) mechanism of the network is responsible for determining if a connection request can be accepted. When a source attempts to make a connection to the ATM network it will specify a service category and negotiate a traffic contract with the network. The traffic contract consists of a connection traffic description, a service category, a set of QoS parameters, and a conformance definition. The connection traffic description contains two parts: a source traffic descriptor which characterizes the source traffic using a set of rate and burstness related parameters, including the Peak Cell Rate (PCR) and the Minimum Cell Rate (MCR); and the Cell Delay Variation Tolerance (CDVT) which is to be used by the network at the User-Network Interface (UNI) to account for the amount of Cell Delay Variation (CDV) when monitoring the user traffic. The negotiated QoS parameters (some parameters are not negotiated) can include the Peak-to-peak Cell Delay Variation (CDV), the Maximum Cell Transfer Delay (CTD), and the Cell Loss Ratio (CLR). The conformance definition identifies a set of predefined rules which unambiguously specify the conforming cells of the connection. If the source and the network reach agreement on the QoS for the requested traffic descriptor and service category, a connection is established.
The service category specifies broadly the manner in which the cell transmission rate is to vary, if at all. One of the possible service categories is Available Bit Rate (ABR). In ABR additional parameters are negotiated at call set-up: the Initial Cell Rate (ICR), the ADTF (described below), the RIF (described below), and the RDF (described below). In ABR the user requests a Minimum Cell Rate (MCR) and specifies a Peak Cell Rate (PCR) and an Initial Cell Rate (ICR). The CAC evaluates the capacity of the network and the current traffic on the network to determine if this minimum cell rate can be guaranteed to the user. In the meantime, network elements may directly modify the ICR to a lower value if needed, and provide delay and buffer space information which enables the source to recompute an updated ICR which should be no higher than the ICR originally requested by the source. If the MCR can be guaranteed, a connection will be established and the source can begin transmitting at the updated ICR. During transmission the available bandwidth in the network may decrease due to higher bandwidth consumption by services that enjoy a higher service priority than ABR, and the source will be instructed to lower its cell transmission rate. At other times more capacity may become available due to the dropping of other calls or the variation of cell rate of other connections. In effect, the source is always allowed to transmit cells at the MCR, but can transmit cells at a higher rate as bandwidth becomes available. ABR is not intended to support real-time applications. The QoS can remain unspecified since Cell Delay Variation and Cell Transfer Delay are unimportant, and the Cell Loss Ratio is expected to be very low as long as the source transmits within the cell rate specified at any given time by the network. The negotiation at connection set-up of an ABR call consists simply of a request by the source for a connection based on a specified MCR and PCR, and a determination by the network as to whether it can offer a very low CLR for the specified MCR. If it can, the CAC system will inform the source that it can offer a cell transmission rate at least as great as the MCR, and a very low CLR as long as the source complies with the cell transmission rate specified by the network from time to time.
During the transmission, the source must be informed of any changes in its Allowed Cell Rate (ACR), the rate at which the source is allowed to transmit cells at any given time. This is done actively by the source, which periodically polls the network to determine the ACR. The frequency with which this is done is determined by the value of Nrm, a parameter negotiated during call set-up. This is the maximum number of cells that the source can transmit for each Forward Resource Management (F-RM) cell. A F-RM cell is transmitted by the source to convey resource management information between the source, the network elements, and the destination. It carries a requested cell rate value (typically the PCR) in the cell's Explicit Rate (ER) field. When the F-RM cell reaches the destination, it is turned around and sent back to the source as a Backward Resource Management (B-RM) cell, carrying feedback information provided by the network elements and the destination. As the B-RM cell passes through each network element on its way back to the source, the network element compares the value in the ER field of the B-RM cell with the cell rate that the network element can offer. If the network element can only offer a cell rate less than the value contained in the ER field, the network element substitutes its offered cell rate for that in the B-RM cell. One type of network element, a binary switch, is not capable of providing exact ER information. Instead, a binary switch examines its buffer to derive congestion information and indicates to the source indirectly that it should perform a relative rate increase or decrease. If the binary switch detects congestion it sets the Explicit Forward Congestion Indication (EFCI) state of the data cell to “1” by altering the Payload Type Identifier field in the header of the data cell. The EFCI of each data cell is monitored by the destination. When the destination receives a new F-RM cell it checks whether the EFCI state of the immediately preceding data cell was “1”. If so, the destination sets the Congestion Indication (CI) and No Increase (NI) fields of the B-RM cell to “1”.
By the time the B-RM cell returns to the source it carries the value of the lowest cell rate offered by any of the network elements and carries binary flags indicating if there is congestion in any binary switches. The source computes its ACR as the minimum of the Explicit Rate (ER) offered by the network and a locally computed rate which is a function of the previous ACR, the presence of congestion in the network as indicated by the CI and NI fields in the B-RM cell, and the Rate Increase Factor (RIF) and Rate Decrease Factor (RDF) negotiated during call set-up. The computed ACR is the cell rate at which the source should then transmit, as it is the highest bandwidth that the network as a whole can guarantee for the connection.
An honest source will adapt its transmission rate to match the ACR, thereby ensuring a low CLR and usage of its fair share of the overall available bandwidth. However non-compliant sources will transmit cells at a rate higher than the ACR in hopes of obtaining a higher share of the bandwidth than that to which they are entitled. To prevent this the network must carry out a function called policing. Policing is carried out at the User Network Interface (UNI) Usage Parameter Control (UPC) which a set of traffic monitor and control functions. When a B-RM
Guttman Jacob
Marchand Bernard
Nieh Benjamin B.
Pillar John F.
Rabie Sameh
Nortel Networks Limited
Olms Douglas
Vanderpuye Ken
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