Black Box Mixed-Media Fiber Hubs
Installation and User Guide (4/99)
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The 100Mbps Collision Domain Diameter is the length of the longest path between
any two devices in a single collision domain. A collision domain is defined as a cluster of
network devices which are connected by means of a repeater or repeaters such that no bridging
devices are present between any two devices in the cluster. In order to install an IEEE 802.3u
compliant Fast Ethernet network, the collision domain --- regardless of the actual network
topology --- must be less than 512 BT (Bit Times). Bit Times are related to media type as
shown in Table 3.6.1a.
Table 3.6.1a: Worst case round-trip delay for Fast Ethernet media*
Media Type
Round-trip delay in
Bit Time per Meter (BT/m)
Fiber Optic
1.000
Shielded TP cable
1.112
Category 5 Cable
1.112
Category 3, 4 Cable
1.140
*Worst case delays taken from IEEE Std 802.3u-1995, actual delays may be less for a particular cable.
Contact your cable supplier for exact cable specifications.
Each Fast Ethernet device component also has an associated BT delay which depends
on the physical signaling system employed. Table 3.6.1b shows each Fast Ethernet device
component and the associated BT delay. A “DTE” is an end node, such as a user station. Note
that there is only one DTE pair associated with any device-to-device path.
Table 3.6.1b: Worst case round-trip delay for Fast Ethernet device components*
Component
Round-trip delay in Bit Times (BT)
2 TX DTEs
100
2 FX DTEs
100
1 FX and 1 TX DTE
100
1 T4 and 1 TX or FX DTE
127
Class I Repeater
140
Class II Repeater with any
combination of TX and FX ports
92 **
**Note, the delay is only 80 Bit Times for
the LH8050A, front-port-to-front-port.
*Worst case delays taken from IEEE Std 802.3u -1995.
To determine whether a prospective network topology adheres to the collision
domain diameter specification, the following formula should be applied to the worst case path
through the network. The worst case path is the path between the two Fast Ethernet devices
(DTEs) which have the longest round-trip time.
PDV = (sum of cabling delays) + (sum of repeater delays) +
(DTE pair delay) + (safety margin)
PDV is the Path Delay Value of the worst case path. For the network to adhere to
IEEE standard, this value must be less than 512 BT. The safety margin is specified in BT and
