| An MSAU is a device used in Token Ring networks to physically attach multiple network stations in a star topology. These devices are internally wired to connect the workstations into a logical ring. The electrical layout of Token Ring networks uses a ring
topology, but the physical layout is actually a combination of a star and
ring topology.
A star topology has a single hub or concentrator in the center with satellite units radiating outward from the central node that rebroadcast all transmissions received from any peripheral node to all peripheral nodes on the network, which are located at the
points of the star. This is the most common topology used today.
The ring topology, based on IBM’s Token Ring networks, is not a physical arrangement of the network cable, but a logical arrangement. Each node pair is connected to its adjacent node in a ring, similar to a closed circuit loop, and each node is connected
on its own cable to the MSAU. The network behaves, however, as a ring, and the network signals travel around the ring to each node in turn.
The MSAU combines the functions of repeaters and hubs. The nodes are all physically wired to this device by a single twisted-pair cable with two wire pairs. One pair is the inbound or receive portion of the ring and the other pair is the outbound or transmit
wire. The workstations are connected directly to the MSAUs, which are then wired together using patch cables to form one large ring.
The purpose of the MSAU is to short out, or bypass, malfunctioning stations. Basically, the MSAU repeats the signal from the output of one node to the input of the next node, which lets the signals flow in a logical ring. On a Token Ring network, a single
faulty or inoperative node can break the ring, which crashes the network.
The MSAU contains active relays to short circuit, or effectively disconnect, the inoperative nodes and maintain the ring integrity (because it bypasses these inactive nodes). Basically,
an MSAU is
a special kind of hub, which is the central connection point of the star topology.
For large networks, multiple MSAUs can be connected into a larger ring (through their ring-in and ring-out connectors) to accommodate more users. This physical layout is the star portion of the topology, but since every message passes through every computer
one at a time, and each computer passes that message on to the next in a continuing circle, the logical configuration is actually a ring topology, hence the name star-wired ring. |
