Multiplex communications – Wide area network – Packet switching
Patent
1990-10-16
1994-06-07
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
370 8512, H04J 324
Patent
active
053196430
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns a method for shortening the distributed queue which exists over a number of nodes connected to a digital transmission line.
2. Description of the Related Art
In "The QPSX MAN", R. M. Newman et al., IEEE Communications Magazine, Vol. 26 (April 1988) No.4, p. 20-28, an arrangement is introduced which digital information is transmitted between several nodes over two non-branching parallel buses. Nodes are situated one after the other and are attached to both buses. Contiguous, address-labelled packets run in one direction only on each of the two buses at the same rate. Every packet contains, in the header part, two bit segments which are necessary to create distributed queues over all nodes. One of these bit segments is used to signify "empty" (E) or "busy" (B). The other bit segment is used to signify "request" (R) or "no request" (N). According to the values of these bits, packets will subsequently be referred to as "empty packets" (E), "information packets" (B), "request packets" (R), and "non-request packets" (N).
The distributed queue enables every node to access the buses at more-or-less any time, which is particularly important when short, urgent messages (e.g. alarms) must be sent. A queue is created when there is a lot of traffic on the bus, by means of a continuously operating protocol. A partial representation (image) of the queue is found in each node. This protocol is described as follows: whenever a request packet (R) is forwarded, and an entry is read out of the same FIFO memory whenever an empty packet (E) is forwarded in the opposite direction. Through this action, the FIFO memory creates the aforementioned image of the distributed queue. signal in the opposite direction to that in which it wants to send the information. The node does this by changing the first non-request packet (N) it receives into a request packet (R) and sending this out. It also adds a later-recognisable entry into the FIFO memory. read out of the FIFO memory, the node is authorised to send its information. The node does this by filling the next arriving empty packet (E) with its information and sending the packet out as an information packet (B). The next request to send information can then be actioned.
This described protocol operates satisfactorily and balances the access requirements between nodes at all times. A disadvantage, however, is that it is not possible to extend this protocol to genuine ring-structured systems (without special `head-of-bus` units).
SUMMARY OF THE INVENTION
The purpose of this invention is, on one hand, to eliminate this limitation and, on the other hand, to minimize the length and range (distance along the bus) of the respective distributed queue, thereby also possibly allowing several separate queues to exist next to each other.
In accordance with the present invention, in a method for shortening a distributed queue which exists across a number of modes connected to a digital transmission line, wherein on one of two non-branching parallel bases, contiguous empty packets and information packets run in one direction and on the other of the two bases, request packets and non-request packets run in the other direction and with the sam transmission rate, wherein at one place, empty packets are generated on one bus and non-request packets on one bus and non-request packets are generated on the other bus, wherein the nodes are connected to both buses and every node receives and transmits information packets, wherein every mode which wants to send information, successively changes an arriving non-request packet into a request packet, transmits it, and inserts its own request into its own distributed queue image and as soon as the authorization to send has been signaled, fills an empty packet with the information, marks it as an information packet and transmits it, the improvements comprising that at least one node generates empty packets and non-request packets and that every node, which has either itself generated
REFERENCES:
patent: 5181202 (1993-01-01), Walser et al.
Beeler Reto
Potts Martin
Rao Sathyanarayana
ASCOM Tech AG
Olms Douglas W.
Ton Dang
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