HIPPI

HIPPI (HIGH PERFORMANCE PARALLEL INTERFACE)

ABSTRACT:

The High-Performance Parallel Interface is a point-to-point connection technology, designed to connect two devices (typically a supercomputer and a peripheral) at either 800 or 1600 Mb/s over a cable no more than 25 meters long (An implementor’s agreement called "Serial HIPPI" exists for operating HIPPI over fiber to distances of 10 kilometers). But HIPPI has turned out to be very important for experimentation in gigabit data communications, because it was the first standard way to connect devices at high data rates and because it can be used in conjunction with a switch to provide a gigabit LAN service.

HIPPI is a standard produced by the ANSI X3T9.3 committee. It defines a simplex packet framing protocol, based on a connection like scheme. To send data, a device requests that it be connected to the destination device. HIPPI does not allow multiple connections to be multiplexed over a line at the same time. When a device is done sending, it asks to end the connection. If a connection cannot be established, because the destination is already connected to another system, HIPPI allows the requesting device to either wait for the destination to become free or be notified that a conflict exists and it should try again. For simple point-to-point links between two devices, HIPPI can just establish a single, permanent connection.

Packets are sent as a sequence of bursts, where each burst is a group of 1 to 256 words. At 800 Mb/s the words are 32 bits wide. At 1600 Mb/s the words are 64 bits wide. Only the first or last burst in a packet (but not both) can be less than 256 words. The HIPPI cables are very wide, so the clock rate on the cable is just 25MHz. Packets can be up to two bytes short of four gigabytes long. An indeterminated packet mode is also supported. Every HIPPI packet contains a higher layer protocol identifier and up to 1016 bytes of control information.

When sending variable length packets through a switch, the usual procedure is to establish and tear down the connection after every packet, to allow maximum multiplexing of the switch. As a result, part of the packet overhead is the time required to get a connection request to the switch and get the answer back and for the sending device to handle the connection setup and teardown. The connection setup and teardown times depend on the length of the HIPPI cable.
HIPPI also lacks support for multicasting. For protocols that use multicasting to find services, HIPPI can support a special multicast port that actually just directs multicast transmissions to the port of a server which has agreed to provide the service. But this workaround does not comfortably serve multicast services like video and audio conferencing.

Overall, two things make HIPPI interesting. First, that it exists and is widely used - it will be a gigabit attachment technology for some years to come. Second, the HIPPI designers were relentless about keeping the technology simple, to make implementations straightforward. As a result, HIPPI is a good example of just how easy it can be to implement gigabit speeds. Certainly, a 32-bit data path clocked at 25 MHz is very comfortably within the state of the art.

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