IEEE 1355 - What IT Is

What It Is

The work of the Institute of Electrical and Electronics Engineers was sponsored by the Bus Architecture Standards Committee as part of the Open Microprocessor Systems Initiative. The chair of the group was Colin Whitby-Strevens, co-chair was Roland Marbot, and editor was Andrew Cofler. The standard was approved 21 September 1995 as IEEE Standard for Heterogeneous InterConnect (HIC) (Low-Cost, Low-Latency Scalable Serial Interconnect for Parallel System Construction) and published as IEEE Std 1355-1995. A trade association was formed in October 1999 and maintained a web site until 2004.

The family of standards use similar logic and behavior, but operate at a wide range of speeds over several types of media. The authors of the standard say that no single standard addresses all price and performance points for a network. Therefore, the standard includes slices (their words) for single-ended (cheap), differential (reliable) and high speed (fast) electrical interfaces, as well as fiber optic interfaces. Long-distance or fast interfaces are designed so that there is no net power transfer through the cable.

Transmission speeds range from 10 megabits per second to 1 gigabit per second. The network's normal data consists of 8-bit bytes sent with flow control. This makes it compatible with other common transmission media, including standard telecommunications links.

Each link defines a full-duplex (continuous bidirectional transmission and reception) point-to-point connection between two communicating pieces of electronics. Every transmission path has a flow control protocol, so that when a receiver begins to get too much data, it can turn down the flow. Every transmission path's electronics can send link control data separately from normal data. When a link is idle, it transmits NULL characters. This maintains synchronization, finishes any remaining transmission quickly, and tests the link.

Some Spacewire users are experimenting with half-duplex versions. The general scheme is that half-duplex uses one transmission channel rather than two. In space, this is useful because the weight of wires is half as much. Controllers would reverse the link after sending an end-of-packet character. The scheme is most effective in the self-clocking electrical systems, such as Spacewire. In the high speed optical slices, half-duplex throughput would be limited by the synchronization time of the phase locked loops used to recover the bit clock.

Interface electronics perform most of the packet-handling, routing, housekeeping and protocol management. Software is not needed for these tasks.

A switching node reads the first few bytes of a packet as an address, and then forwards the rest of the packet to the next link without reading or changing it. This is called "wormhole switching" in an annex to the standard. Wormhole switching requires no software to implement a switching fabric. Simple hardware logic can arrange fail-overs to redundant links.

When there is an error, the two ends of a link exchange an interval of silence or a reset, and then restart the protocol as if from power-up.

The connectors are defined so that if a plug fits a jack, the connection is supposed to work. Cables have the same type of plug at both ends, so that each standard has only one type of cable. "Extenders" are defined as two-ended jacks that connect two standard cables.

The maximum length of the different data transmission media range from one meter to 3 kilometers. The 3 km standard is the fastest. The others are cheaper.