Multiplex communications – Data flow congestion prevention or control – Control of data admission to the network
Reexamination Certificate
1997-09-10
2001-04-24
Hsu, Alpus H. (Department: 2662)
Multiplex communications
Data flow congestion prevention or control
Control of data admission to the network
C370S395430, C370S468000, C370S477000
Reexamination Certificate
active
06222823
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
This invention relates to a broadband switching system for the switching of asynchronously transferred data cells, and to a method of switching asynchronously transmitted data cells.
2. Related Art
Broadband switching networks for switching asynchronously transferred cells are known, in which a predetermined level of bandwidth is allocated to a transmission channel connecting a first customer to a second customer. In some of these known systems, a communications channel is provided over a significant period of time, effectively of the leased-line type, and manual measures are implemented in order to set up such a connection or to modify a connection according to the particular terminations and the level of traffic being conveyed. Consequently, it is usual for customers to incur a fixed rate charge as part of the overall charge for the connection, resulting in payment being made irrespective as to whether the connection is being used or not.
Alternative systems have been proposed or are available. In particular, it is possible for connections to be established on a dial-up basis, requiring termination equipment to be provided with facilities for establishing connections by issuing signalling commands and responding to similar commands issued by the network.
The use of permanent circuits to support a private communications network is widespread. The demand for such circuits is expected to grow to include broadband rates above 2 Mbit/s, the circuits carrying traffic multiplexed from sources which are inherently bursty, possibly together with traffic which is transmitted at constant bit rates and is delay sensitive, such as voice transmission and constant bit-rate video.
Asynchronous transfer mode (ATM) cells all have a fixed information field of forty eight octets which can carry customer traffic or customer-originating control information (signalling). These two types of data transmission are distinguished by setting virtual path (VP) and vertical circuit (VC) values in the cell headers. Another field provided in the ATM header is known as cell loss priority, which enables low priority cells to be distinguished from high priority cells. In the event of congestion, the low priority cells may be discarded first.
For private circuits within an ATM based network, the desired route, the required bandwidth, and the quality of service (QOS) are set up using network management procedures. The private circuits are known as permanent virtual circuits (PVCs) because there is no actual physical circuit, only a VP/VC value or “label” which is associated with information stored in the switches to determine the route and preserve the bandwidth and QOS requirements.
A disadvantage of all known permanent circuits is that the bandwidth remains assigned to the circuit, even when the customer has nothing to transmit. This means that the customer may have to pay higher charges than would be obtained if the bandwidth was only made available when needed. The assumption being made here is that charging is related to reserved bandwidth, and this is not necessarily correct in terms of the way public network operators may choose to charge for virtual circuits. However, it is expected that charging based on reserved bandwidth will become a significant factor in the future.
A common practice is to set up a permanent virtual circuit so that it is only available during certain hours of the day, or during certain days of the week. A difficulty with this approach is that it does not allow the customer to change the pattern of usage quickly, and it may only crudely reflect the usage required by the customer.
A second proposal has been to provide the customer with a separate communications channel to the network management plane, thereby allowing a permanent virtual circuit to be reconfigured. A difficulty with this approach is that some time delay will be incurred before the customer can start to use the virtual circuit.
A third proposal is to introduce equipment at every switching point in the network that recognises a fast resource management cell, indicating that bandwidth should now be assigned to the circuit. A difficulty with this approach is that there is no internationally agreed standard for a bandwidth-requesting cell that would be recognised by the switching equipment produced by the various manufacturers.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, a broadband switching system having at least one ingress for connection to a respective signal source and at least one egress for connection to a receiving system, the switching system having at least one switch for transmitting information-carrying asynchronously transferred data cells from the ingress to the egress, system control means for accepting and establishing a connection between the ingress and egress via the said switch, and bandwidth control means arranged to feed a indicator signal back to the ingress for transmission to the signal source, to detect incoming cells supplied to the ingress from the signal source, and, automatically in response to such cell detection, to cause the system control means to allocate a predetermined bandwidth for transmission of the cells to the egress.
It will be appreciated that in practice, the automatic allocation of the predetermined bandwidth may occur some time after incoming cells are detected. This is because the required bandwidth may not immediately be available for allocation.
A connection established in this way is particularly suitable for bandwidth sensitive transmissions such as data communications between networked personal computers. This is because if bandwidth is allocated, it will always be at the predetermined level. The above types of transmission do not operate satisfactorily when operating below the predetermined level of bandwidth (which is a pre-registered level that has been calculated as being sufficient for satisfactory operation) and thus if that level is not available, no bandwidth is allocated, i.e. lower bandwidth amounts are not allocated.
However, in some circumstances the bandwidth control means may be arranged to cause allocation of bandwidth to a signal source at a level less than the predetermined bandwidth associated with that signal source and in conjunction with downgrading the priority of cells received at the ingress such that the bandwidth of non-downgraded cells supplied to the switch does not exceed the allocated bandwidth. This will typically occur when the predetermined level of bandwidth is no longer available on the system.
Once the cells have been downgraded, the system may delete the cells if the system becomes overloaded i.e. has more cells for transmission than can be transmitted in the available bandwidth on the system. Thus the user is taking a conscious risk that some of a message may not be transmitted when transmitting in this optional priority-downgrading mode. Conveniently, the cell deletion technique used is an “intelligent” technique which does not randomly delete cells (thereby corrupting an unknown number of messages) but deletes cells wherever possible, only from a single message.
The bandwidth control means may include feedback means arranged to transmit a cell rate value derived from a bandwidth-indicating signal provided by a stored table associating fixed bandwidth levels with signal sources. The stored table be located in the system control means and/or the bandwidth control means. The cell rate value is fed back to the ingress for transmission to a recognised signal source to indicate to the signal source a permitted rate of supply of cells to the ingress.
The feedback means may also be arranged to transmit a “predetermined bandwidth not available” signal to the ingress when the system control means determines that system needs to operate in the priority-downgrading mode for a signal source. Having received such a signal, a signal source may choose to take advantage of the opportunity of transmitting at a lower priority (with the risk of lost cells) but at the sa
Adams John L.
Smith Avril J.
British Telecommunications public limited company
Hsu Alpus H.
Nixon & Vanderhye P.C.
Qureshi Afsar M.
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