Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1998-06-09
2003-03-25
Hsu, Alpus H. (Department: 2665)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S412000, C370S428000, C370S466000, C709S225000, C709S229000, C709S236000, C709S238000, C709S249000, C709S250000, C709S251000, C709S253000
Reexamination Certificate
active
06539020
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a bridge device for connecting a plurality of self-contained communication networks around which data is transmitted in frames which include control information defining at least the identity of the destination for data in the frame, each network having at least one end station.
DESCRIPTION OF THE PRIOR ART
Examples of networks to which the invention apply include token passing rings as well as FDDI and ATM networks.
In typical local area network installations, it has been possible in the past to use a single network to achieve communication between a number of end stations such as PCs, file servers and the like. However, with the need to provide more and more end stations, it has been the practice to divide the overall system up into a number of individual local area networks (for example one per floor) and to connect the networks together via bridge and other devices. Most of the time, an end station on a network will only wish to transmit to another end station on the same network. However, there will be occasions when communication between end stations on different networks will be required. For example, an end station on a network may require access to a file server on another network. To achieve this, the end station constructs a data frame including control information which is addressed to the final destination on the other network. This frame is then sent around the first network until it reaches the bridge which recognises that the frame is destined for an end station on another network and so diverts the frame into its memory. A controller within the bridge device reviews the frame once it is received in the memory to determine on which output port it should be directed (since typically a bridge device will be connected to more than two networks), and in some cases will make changes in the control information, following which the frame is sent out onto the desired output port for onward transmission. The destination end station may be on the output network or the frame may have to traverse one or more further bridge devices.
The routing of a frame through a bridge device is generally carried out using one of two techniques. In Source Routing, prior to transmitting a frame, an end station transmits a special Source Route Broadcast frame (to which data could also be attached) which is transmitted to all end stations on all networks and, as it passes through each bridge device, is modified to indicate the network on to which it is being transmitted. The source route broadcast frame includes the desired destination end station identifier and once that end station receives the broadcast frame it indicates its presence and the frame is returned to the originating end station. The originating end station can determine from the information now contained on the returned frame the identities of the. various networks through which the frame has passed and this information is incorporated into the data frame which is to be sent so as to control the routing of the data frame.
In Transparent Bridging, each bridge device contains a look-up table which is continuously updated with information defining for each end station on all the networks the output port of the bridge concerned to which a frame directed to that end station should be coupled. This information is built up by the bridge by monitoring source addresses on incoming frames.
Recently, it has been proposed to incorporate a common memory or storage means through which all data is transmitted. Examples include WO-A-9504970, U.S. Pat. No. 5,307,345, and U.S. Pat. No. 5,400,326.
This enables certain types of data to be transmitted through the bridge much more quickly than has previously been possible. This is achieved by providing common storage means through which all the data passes irrespective of the incoming and destination networks concerned and control means which can route incoming data to a destination network without waiting for all the data to be stored in the common storage means. Furthermore, by routing data to the destination network before the data frame has been fully stored in the common storage means, the amount of time the bridge device takes to transfer data from one network to another can be considerably reduced. This technique is known as “cut-through”.
Problems can arise, however, with the use of a common memory in handling large volumes of data.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the, present invention, a bridge device for connecting a plurality of communication networks around which data is transmitted in frames which include control information defining at least the identity of a destination for data in the frame, each network having at least one end station, comprises a corresponding plurality of data coupling means for connection to respective ones of the networks, each data coupling means including receive and transmit interfaces for respectively receiving data from and transmitting data onto the connected network; common storage means connected to all the data coupling means for receiving data being transmitted from one network to another; and control means for controlling the routing of data through the bridge device via the common storage means, the control means causing incoming data to be routed to a destination network before the data has been fully stored in the common storage means, if predetermined conditions are satisfied, the control means being adapted to monitor the amount of free space in the common storage means and to prevent the supply of data to the common storage means if the available free space is less than a predetermined amount.
We have devised a significant improvement upon the known bridge device by causing the control means to monitor the amount of free space in the common storage means and thus avoid breakdown of the device and/or the connected networks when too much data is stored in the common storage means.
The control means could be implemented in a variety of ways but in the preferred example includes a free buffer pointer queue store which contains a set of pointers indicating addresses in the common storage means which are available for storing incoming data. Typically, this queue store will constitute a FIFO which is replenished with pointers relating to the addresses of frames which have been transmitted out of the bridge device from the common storage means. This provides a particularly simple way of utilising the common storage means.
When the control means is implemented in this manner, the content of the common storage means can be determined by monitoring the free buffer pointer queue store.
The predetermined conditions may include, for example, ensuring that the input and output networks operate at compatible speeds so that the leading end of a data frame can be output on a destination network while the trailing end is still being input to the common storage means. Typically, this will require the output network to operate s at least the same speed or a slower speed than the input network.
Another predetermined condition is to ensure that the data coupling means associated with the desired output network is enabled.
There may also be circumstances in which the device does not permit cut-through. This would be, for example, where there is a need to check the integrity of the frame before it is transmitted or where some processing needs to be carried out on the data in the frame before it is transmitted.
Although each data coupling means could be connected individually to the common storage means, preferably the device further comprises a receive bus connected to the receive interfaces of all the data coupling means, the receive bus being connected to an input of the common storage means; and a transmit bus connected to the transmit interfaces of the data coupling means and to an output of the common storage means. In this way, the control means and common storage means of the bridge device are separated from the data coupling means by receive and transmit data buses which enable th
Barber Simon Davie
Curnyn Jonathan
Dent-Young Crispin Nicholas
Pearce David Andrew James
Sabey Christopher Guy
Hsu Alpus H.
Madge Networks Limited
Stevens Roberta
Sughrue & Mion, PLLC
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