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News and New Products
New spec for HyperTransport enhances robustness
By Nicholas Cravotta -- EDN, 11/13/2003
The new HyperTransport 1.1 specification introduces native packet handling, retry protocols, peer-to-peer routing, and virtual channels. Native packet handling addresses many of the issues, such as complex channelization and data structures, that standard load/store approaches introduce. The spec stipulates encapsulating user packets before sending them over channels, thus easing the implementation of back pressure and traffic arbitration. Instead of adding a layer to the specification, HyperTransport approached the problem by redefining its posted-write-control packet to avoid adding additional overhead.
Two new VCSets (virtual-channel sets) are the StreamVC, which supports 16 channels of streaming traffic with back pressure and end-to-end flow control, and the NonFC (non-flow-controlled), or insertion-buffered, virtual channel for inserting traffic into available buffers.
The HyperTransport Consortium claims that the error checking mechanism of Hyper-Transport 1.05 is sufficient because the 1.x electrical interface is highly reliable. The planned 2.x Electrical Interface, however, is expected to have a low but measurable error rate. The Consortium added an error-retry capability using a per-packet CRC (cyclic-redundancy code) to the 1.1 Version of the spec as a move to anticipate the necessity for this capacity in future specs. To reduce latency, a node may speculatively forward a packet before fully receiving it; the node can invert the CRC to mark the packet if the CRC turns out to be bad.
To maintain the PCI-ordering model, all peer-to-peer packets are routed through the host. For applications that require no PCI ordering, the 1.1 spec now allows peers to communicate directly. The HyperTransport Consortium plans no more protocol updates but does plan a major speed increase, still under discussion, for the 2.0 Version of the spec, which it expects to release next year.
HyperTransport Consortium, www.hypertransport.org.
