Introduction
1-3
token is available for use. In Token Ring LANs interconnected by
bridges, a subfield directs token routes. With the ring operated at 16
megabits per second, you can have frame sizes of up to 18,000 bits.
With the ring operated at four megabits per second, the maximum
frame size is 2,000 bits.
A workstation wishing to send data captures an available token and
adds the source and destination addresses along with the other fields
to the frame. A workstation receiving a token frame checks the
destination address to see if it is the intended destination. If not, the
workstation sends the token to the next downstream workstation. If
so, the workstation copies the data field into its input buffer,
modifies the ending delimiter field to indicate reception of the data,
and then sends the token downstream. When received by the source
workstation, the ending delimiter is examined to check reception by
the destination workstation. After verifying reception, the source
workstation strips off the address and data fields and sends the token
downstream. The source workstation cannot immediately recapture
the token, because ring protocol prevents a workstation from
monopolizing data transmissions.
Multistation Access Units
MAUs serve as the connecting hubs for a group of workstations.
Each MAU has a group of typically eight or more connectors that
connect to the workstations and two connectors that allow expansion
to other MAUs (called Ring In and Ring Out). Basic MAUs are
passive devices that serve to control access of workstations into a
Token Ring LAN. By using Type 1 shielded-twisted-pair cable
(STP) or Category 5 unshielded twisted pair cable (UTP), you can
have up to 260 devices on a ring.
In token ring LANs, MAUs control access to the ring by monitoring
each lobe port to a workstation. If a DC “Phantom” voltage is
present from a workstation, the MAU knows the workstation is
active. On this basis, the MAU gives the workstation access to the
ring.
When the “Phantom” voltage is removed due to a disconnected cable
or workstation power down, the MAU then closes the port. This
maintains ring integrity for the remaining devices.
