BASIC TOKEN RING NETWORKS
Page A-8
ARL is calculated by combining the lengths of all the trunk cables,
subtracting the length of the shortest trunk cable (typically an 8-foot
patch cable within a wiring closet) and then doubling the result (Lobe
cabling is not part of the ARL calculation). The impact of ARL on
drive distance is apparent when we examine the resulting calculation
for drive distance. The ARL plus the Longest Lobe Length is now the
maximum drive length. This maximum drive length is the limit of
reliable data transmission. (Appendix C provides procedures and
tables to assist in calculating cable lengths, giving consideration to
ARL.)
Figure A-6 shows the impact of a broken trunk segment on overall
ring length. Without the break, the ring length is 275 meters and the
drive distance for the station at the longest lobe is 575 meters (275 +
(2 x 150)). With the 50-meter section of trunk cable between
Concentrator #2 Ring-Out and Concentrator #3 Ring-In broken, the
two ring ports must be wrapped, connecting the primary ring to the
backup ring, and bypassing the broken trunk cable. In this condition,
the ring length increases from 275 meters to 450 meters (2 x (75 +
150)) and the drive distance becomes 750 meters.
Figure A-6. Adjusted Ring Length
50
METERS
75
METERS
WRAP
LONGEST LOBE
(150 METERS)
CONCENTRATOR #1
PRIMARY RING
BACKUP RING
150
METERS
CONCENTRATOR #3
CONCENTRATOR #2
RING-IN
RING-OUT