Multistation Access Units (MSAUs)

MSAUs are used in IBM and similar token ring networks.
MSAUs are also known as 'wiring centers' or 'concentrators'.
These devices enable token ring networks to be physically wired like star topology.
However the logical topology of a ring will remain unchanged within the MSAU.
Computer A receives the token and removes it from the network.
Computer A places a frame addressed to computer B onto the media.
The frame is received by the MSAU at port 2.
The MSAU places the frame onto its ring.
The frame moves around the ring bypassing port 3 because no node is attached.
The frame passes out of port 4 to be examined by computer C and then returns in at port 4.
The MSAU in turn will pass the frame to the next device on the ring.
The device will pass it back to the MSAU.
This continues till every device attached to the ring has seen the frame.
Features

The MSAU uses electrical relays to allow devices to join the ring.

A device in order to keep the ring connection open must supply a voltage of at least 5 volts.

Incase due to some failure the device cannot supply the requisite voltage.

The connection ring is lost and the device can no longer see any network activity.

This feature is extremely important since the frame sent to a non-functioning device is not returned to the MSAU and results in network failure.
The ports labeled 'ring out' (RO) and 'ring in' (RI) allow MSAUs to be connected together.

This enables it to form a greater ring with many more devices attached to it.

MSAUs normally have RJ-45 connectors or less frequently IBM Data Connectors.
Token ring cards may show a device failure on system startup if they are not connected to a port on the MSAU.