1.  Field of the Invention
This invention relates to priority selection means for data switching apparatus for use in computer-controlled digital data transmission systems. 
2.  Description of the Related Art
Many types of data transmission apparatus are known, all having their own particular features and systems.  In all cases the intention is to allow data switching and transmission to be achieved as rapidly as the apparatus will allow.  It is common for data to be sent in xe2x80x9cpacketsxe2x80x9d consisting of a predetermined number of bits of data plus control information indicating certain parameters about the data or its mode of transmission.  One of the parameters often defined is the priority of that particular packet, that is the precedence which it should be given over other packets of data awaiting transmission.  Apparatus for ensuring that, wherever possible, such a priority indication is observed may conveniently be referred to as priority selection means, and this term is used throughout this specification. 
It is an object of the invention to provide priority selection means for data transmission apparatus operable to ensure that, over a period of time, data is transmitted with the precedence indicated by the priority level contained in its associated control information, independent of any other parameters to be observed. 
According to the present invention there is provided priority selection means for digital data transmission apparatus, which means includes buffer means operable to receive control information relating to data to be transmitted through the data transmission apparatus, the buffer means comprising a separate buffer arranged to store control information having each of a predetermined number of priority levels and each buffer being arranged to deliver an override signal when its contents reach a predetermined level, storage means operable to store digital priority symbols representative of the different priority levels denoted by control information received by the buffer means, a random number generator operable to generate a succession of numbers each of which produces a corresponding priority symbol from the storage means, gating means responsive to the receipt of the override signal from one of the buffers to change the priority symbol generated by the storage means, and selector means responsive to the output of the gating means to select an output from that buffer designated by that output. 
The priority selection means may, for example, be used with the data switching apparatus described and claimed in our co-pending British patent application No.  971742.2.