Figure 1: A Conventional Bus-based System
Switcherland is a modular and scalable computer architecture applicable to low-end as well as to high-end systems. Its switch-based interconnection structure allows a system to be easily extended as the users' demands increase. System components can be added whereby the communication infrastructure grows as the system is expanded and, in contrast to bus-based systems, never becomes a performance bottleneck. Furthermore, Switcherland provides real-time guarantees making it particularly well suited for running multimedia applications that process audio and video data.
Systems in use today typically use busses to communicate. As shown in Figure 1 busses are used at different system levels: a peripheral bus is used inside a workstation to connect I/O devices (I/O) such as storage devices or network interfaces to the processor (P) and memory (M); outside the workstations, another bus, the local area network is used to transfer data between machines. Busses exchange data by broadcasting messages on a shared set of wires. Their implementation is simple and low cost, however, due to the physical properties of a shared medium the performance is limited: the number of devices as well as the bandwidth is fixed. As busses can be expected to not be able to keep up with increasing bandwidth requirements much longer, alternative interconnection structures have to be explored.
Figure 1: A Conventional Bus-based System
As suggested by its name, Switcherland systems communicate with the help of switches rather than busses. Figure 2 shows an example of a Switcherland system. A port of a switch (S) connects either to a node or to another switch. A node corresponds to a processor (P) with local memory (M) or to an I/O device (IO). The switches have two purposes. Within a workstation, they serve as an I/O interconnection structure, whereby a workstation can contain one or several switches, and, within a cluster of workstations, they are used as a network interconnection structure. When cascading switches any arbitrary topology is allowed. By using the same interconnection structure for inter- and intra-workstation communication the boundaries of the workstations are less clear than in a traditional networked system since the logical grouping of nodes can easily be different from the physical arrangement.
Figure 2: The Switcherland System
Since such an interconnection structure uses multiple links rather than one shared link, it cannot compete in terms of cost. However, it offers scalability and high bandwidth. As indicated in Figures 1 and 2, a bus can execute only one data transfer at a time while a switch-based interconnection structure can accommodate multiple data transfers simultaneously. Since switches can be cascaded easily, the aggregate bandwidth can be many times the link bandwidth and also many times the switch bandwidth. In addition, a switch-based interconnection structure does not present a single point of failure and offers the possibility to improve availability by adding redundant links and switches.
The implementation of the switch is shown in Figure 3. Logically, the switch constitutes a non-blocking crossbar switch with output buffering while its implementation uses a shared memory switch. Four ports are provided, each of which connects to a 265.625 Mbit/s full-duplex serial link. The physical layer of the links is compatible with the FibreChannel standard. Links use shielded twisted-pair wiring and can span distances of up to 50 m. Taking the overhead of the physical layer's 8B/10B encoding into account a 4 by 4 switch offers an aggregate bandwidth of 0.85 Gbit/s.
Figure 3: A 0.85 Gbit/s Switcherland Switch
(he, August 18, 1997)