Friday, May 18, 2007

Description of Quick Ring

The basic Quick Ring system consisted of seven serial links, six of them carrying data, and one a 50 MHz clock signal. Two physical media were specified, sets of twisted-pair copper wiring embedded in a thin plastic strip for use inside a computer, or the same signals using frequency-division multiplexing in a single fiber optic cable for longer links between machines.

The data lines were clocked at seven times the clock signal, so each clock "tick" moved 42 bits of data over the bus for a raw data rate of 2.1 Gbit/s. Ten bits of the 42 were used for signaling and control, leaving the other 32 for data, resulting in a net data transfer rate of 1.6 Gbit/s, or 200 MBytes/s.

Each Quick Ring interface enclosed two of these 200 MB/s links, one for "upstream" and one for "downstream" connections in a point-to-point ring. Since the system was not a bus, machines could talk up and downstream at the same time without interfering with other users. The drawback was that each hop over an intervening point added a latency of up to 1.3 ┬Ás.

Since Quick Ring was built in a ring topology there was no need for a dedicated switch, or router, making the system lower cost. It used a circuit switching system, in which the message path is set up before the data is sent, and once set up the connection is very lightweight. This is as opposed to packet switching, in which every message contains all of the data needed to reach the destination, this is more flexible, but adds overhead. Of the 10 bits of control data, four were used to specify a circuit number, allowing for a total of 16 devices per ring.

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