Editing QuickRing (section)

== Description ==

The basic QuickRing system consisted of a number of single-direction 1-bit [[serial communications|serial]] links carrying data, and one extra line carrying a 50&nbsp;MHz clock signal. Apple's implementation consisted of six data lines and the clock line using twisted-pair copper wiring (using [[LVDS]]) embedded in a thin plastic strip. National Semiconductor offered a variety of different implementations with up to 32 data lines,<ref name=enews/> as well as the same signals multiplexted using [[frequency-division multiplexing]] in a single [[fibre optic]] cable for longer links between machines.

The data lines were clocked at seven times the clock signal, so each clock "tick" moved 7 [[bit]]s of data over each of the bus lines. For the Apple implementation this meant 7 bits times 6 links at 50 million times a second, for a raw data rate of 2.1 [[gigabit|Gbit]]/s. Ten bits of the 42 were used for signalling and control, leaving 32 for data, resulting in a net data transfer rate of 1.6&nbsp;Gbit/s, or 200 [[megabyte|MB]]/s. This was only slightly faster than contemporary (1993) versions of PCI at ~130&nbsp;MB/s, but much faster than NuBus of the same era, at about 20&nbsp;MB/s.<ref>[http://www.tidbits.com/tb-issues/TidBITS-126.html#lnk4 QuickRing Speed]</ref>

Each QuickRing interface contained two such 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 μs. Since QuickRing was built in a ring topology there was no need for a dedicated switch or router, potentially making the system lower cost to deploy. Two rings could be connected together by putting the bus IC's "back to back" in a switch, allowing for larger networks.

QuickRing routing 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.