BASIC TOKEN RING NETWORKS
Page A-7
A.2 DESIGN CONSIDERATIONS
A major design consideration is ring length. The ring propagation
must be long enough to accommodate an entire token (24 bit times)
and still be short enough for the transmitting devices to reliably send
information to the next station. The AM inserts an artificial delay
that prevents the ring from appearing too short. Overly long ring
lengths create a problem as well, but these problems are best solved
through restraints in the network design.
Drive Distance is the limit of reliable signal propagation around the
ring. The cable length between a sending and receiving station must
not exceed the drive distance. The cable lengths that make up the
drive distance include the lobe from the sending station to the
concentrator, the sum of the trunk cable segments around the ring
and, since the sending station must ultimately remove the original
message from the ring, the lobe from the concentrator to the sending
station. Figure A-5 illustrates the cables that make up the drive
distance in a token ring.
Figure A-5. Cable Lengths in a Token Ring
When a wrap occurs that bypasses the shortest segment of trunk
cable on the ring, it produces a “worst case” ring length referred to as
the Adjusted Ring Length (ARL). Since the lobe is easily bypassed
by the concentrator’s trunk coupling units, lobe cabling does not
provide a backup path. Consequently, a wrap affects only the main
ring (trunk cables). The lobe cable length is not included in the ARL.
LONGEST LOBE
(150 METERS)
DRIVE DISTANCE
LOBE CABLING
TRUNK CABLING
75
METERS
150
METERS
50
METERS
CONCENTRATOR #3
CONCENTRATOR #2
CONCENTRATOR #1