Application of T9000 to CPLEAR
The technique used to route packets through a C104, to select the required output link, is that of interval labelling. In this technique each link of a C104 is assigned a range of device labels (a device interval) which indicates the physical devices that are accessible via that link. Each physical device has a unique label associated to it. When a packet enters a C104, the device label contained in the packet header is compared to the device intervals. The output link whose device interval contains the required device label is selected to route the packet out of the C104.
The C104 uses wormhole routing (see Fig. 3). A routing decision is made as soon as a packet header enters the C104. This routing decision leads to the creation of a temporary circuit through the C104 which vanishes as the packet terminator passes through. As a consequence of wormhole routing a single packet may pass through multiple C104s at any one time and the header may be received at the destination before the whole packet has been transmitted, thus minimizing communication latency.
In switching networks there will often be many possible routes that a packet may take to reach a specified destination. Should one of these links be in use or in error then it is desirable that an alternative link be chosen. To fulfil this requirement the C104 supports grouped adaptive routing. Output links can be grouped so that packets routed to the first link of a group can be routed to the other links of that group should the first link not be available.
The inter-connectivity offered by the C104 in combination with the VCP of the T9000 removes any requirement for through-routing software and virtual channel multiplexing, which was necessary with previous generations of the Transputer.
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