Fast EtherHub-8s User’s Guide
Configuring the Network
3-3
Guidelines for Configuring Multiple Hubs
In a multiple hub configuration, the maximum overall segment length for inter-
hub cabling depends on the length of the cables used to connect station nodes to
the hub system and the number of hubs you cascade together. The IEEE 802.3u
standard (illustrated in the preceeding example) provides strict guidelines for
calculating the maximum network diameter. A quick formula for calculating
the maximum length of cabling that can be run from end-node to end-node is:
367m (1204 ft)
≥
S
(repeater equivalent length) +
S
(length of twisted-pair cable)
Remember that this 100BASE-TX Class II Repeater has an inherent repeater
delay of 80 bit time or 72 meters (236 feet) of twisted-pair cable. For the daisy
chain of three hubs shown in Figure 3.3, communications must be supported for
node pairs AB and AC. In this case, first determine the limitations on cable
length found on the longest path (i.e., the path with the longest delay time).
If node A and B are linked to repeaters 1 and 3 respectively in the system (each
using 50 meters (164 feet) of twisted-pair cable), then the maximum length
from node A to B is 367m - (3 x 72m) = 151m (495 feet) and the total interhub
cabling will be limited to 51 meters (167 feet), i.e., 151m - 2x50m = 51m.
Therefore, if you use 21 meters (68 feet) of twisted-pair cable to connect hubs 1
and 2, then 30 meters (98 feet) of cable can be used to connect hubs 2 and 3.
Then, if node A and C are linked to adjacent repeaters 1 and 2 in the hub
system, the maximum length of twisted-pair cable that can be used to connect
node C to hub 2 is 367m - 2*72m - 50m - 21m = 152 meters (498 feet).
However, the maximum length for any twisted-pair connection cannot exceed
100 meters (328 feet).
