Multiplex communications – Data flow congestion prevention or control – Control of data admission to the network
Reexamination Certificate
1998-02-27
2004-09-14
Phunkulh, Bob A. (Department: 2661)
Multiplex communications
Data flow congestion prevention or control
Control of data admission to the network
C370S253000, C370S395100
Reexamination Certificate
active
06791943
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to digital communications and, more particularly, to estimating cell bandwidth in digital communications networks.
2. Discussion of Related Art
Integrated Services Digital Network (ISDN) systems have been evolving for the past several decades. Since its inception, ISDN has provided a wide variety of services, including voice and data services, with bit rates of up to 64 Kbps, integrated within a single network. For voice communications and many text and data applications, the 64 Kbps ISDN rate has been found to be sufficient. However, there are increasing demands for broadband communications with substantially higher bit rates, such as high-speed data communications, video and high resolution graphics communications. A second generation of ISDN, referred to as Broadband ISDN (B-ISDN), has developed to support these latter types of communications while continuing to provide the same advantages as the first generation ISDN.
The first generation ISDN utilizes a synchronous transfer mode wherein, for the duration of a connection, a synchronous channel with a constant bit rate (CBR) is allocated to that connection. Although suitable for certain applications, synchronous transfer mode generally is unsuitable for integration of service types that have bit rate requirements above 2 Mbs. This inadequacy resulted in the development of the Asynchronous Transfer Mode (ATM) protocol as a preferred method for transferring information in a B-ISDN system. In ATM networks, a user's data is partitioned into fixed-length cells consisting of a 5 byte header and a 48 byte payload. The use of short, fixed-length cells while transmitting only the necessary number of cells per unit time provides the flexibility needed to support variable transmission rates, making ATM particularly suited for integrated traffic consisting of, for example, voice, data and video.
An ATM network is connection-oriented. A Virtual Channel Connection (VCC) is established between a source and destination node before information is transmitted. The VCC traverses one or more intermediate nodes in the communications network. The VCC includes one or more virtual paths (VPs) and virtual channels (VCs). During transmission, an ATM cell stream is switched among VCs and VPs by the intermediate network elements. VP and VC Identifier (VPI/VCI) values included in the cell header are set when the VCC is first established, and altered as the cell stream passes through each switch in the network to identify the next selected virtual path and virtual channel of the VCC. Because of the connection-oriented nature of ATM and the distribution of cell transfer control across the nodes of a virtual channel connection, each network element or node in a virtual path operates as a source and destination of a virtual path, and is generally referred to as an ATM source and ATM destination, depending upon the operations it is performing. For ease of discussion, the source node and any intermediate ATM source are generally referred to herein as an ATM source. Likewise, the destination node and any intermediate ATM destination are generally referred to herein as an ATM destination or ATM destination node.
Each switch in an ATM network has limited resources with which to service large groups of individual network nodes. That is, although the capabilities of ATM switches are increasing rapidly, each switch nevertheless has a limited bandwidth and limited buffer space. With the advent of optical transmission systems such as Synchronous Optical Network (SONET) and an ever increasing number of communicating nodes, these limits often are exceeded. As a result, congestion at the switches occurs quite rapidly. If left unchecked, such congestion can result in buffer overflow and cell loss.
When a VCC is established between a source and destination node, a set of parameters are negotiated, primarily based upon the current bandwidth availability of the network. These parameters, commonly referred to as source-control parameters, are intended to discipline the behavior of the source node so as to avoid congestion on the network. For example, the source node may be provided maximum values for the rate at which it may transmit data instantaneously (Peak Cell Rate or PCR); the average rate over an extended time interval that it may transmit cells (Sustained Cell Rate or SCR); and the burst size that may be sent at the peak rate (Burst Tolerance or BT).
The source node is expected to operate within the limits defined by these negotiated parameters to prevent unexpected traffic from occurring on the network. The presence of unexpected network traffic could cause the network to become congested which, as noted, may cause ATM cells to be lost. It therefore has been established as a protocol violation for a source node to exceed one or more of these negotiated source-control parameters.
Recently, the ATM forum has established a traffic class referred to as the Available Bit rate (ABR) traffic category. ABR is a complex B-ISDN protocol, consisting of a one-directional information flow and a bi-directional control flow. Specifically, a bi-directional VCC is established between a source and destination node. Forward-going ATM (source-to-destination), and a backward-going ATM (destination-to-source) cell streams are established over this VCC. According to an established ABR protocol, specialized control cells called Resource Management (RM) cells are periodically inserted into the forward-going ATM stream and the backward-going ATM stream. The RM cells are generated by the source and destination nodes and, if necessary, modified by the intermediate nodes.
The RM cells inserted into the forward-going stream by the source node contain transmission-related information, including the rate that the source node is currently transmitting cells. To avoid loss of data and to generally maintain the efficiency of the network, the actual transmission rate through all of the virtual paths in a virtual channel connection should be identical to the source transmission rate indicated in the forward-going RM cells. If an ATM source transmits at a rate other than the rate that is indicated in its forward-going RM cells, additional, unexpected traffic may occur. As noted, the extra traffic from a source node can cause the network to become congested, and ATM cells to be lost. It is therefore also a significant protocol violation for these two values not to be the same.
The RM cells inserted into the backward-going ATM stream contain source-control information that instructs the source, node to either decrease or increase its transmission rate, depending on the current state of the network. That is, if additional bandwidth is available on the network, then a backward-going RM cell will indicate to the source that it may increase its transmission rate so as to fully utilize the available bandwidth. Conversely, if the bandwidth of the network is close to or above its maximum capacity, the backward-going RM cells may instruct an ATM source node to decrease its transmission rate so as to avoid cell loss caused by excessive congestion.
The degree by which the source node increases or decreases its transmission rate may be explicitly identified in the backward-going RM-cell. Alternatively, the rate of change is pre-negotiated when a VCC is established so that only a binary indication of whether the source should decrease or increase its transmission rate may be used to provide the above feedback.
Because the consequences of such protocol violations can be so severe, conventional network switches and network test equipment have been developed to monitor the actual rate at which cells are transmitted by a source to identify when the source is operating beyond the negotiated source-control parameters. Traditionally, is source transmission bandwidth has been measured by counting the number of cells received at an ATM virtual destination during sequential fixed periods of time. The number of received cells is
Agilent Technologie,s Inc.
Phunkulh Bob A.
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