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Magnum DS8016 Dual-Speed Stackable Hubs Installation and User Guide (07/06)
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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 3.2.3b: 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 DS8016, 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 may be a value between 0 and 5. This margin can be used to accommodate unexpected
delays such as extra long patch cable. A safety margin of about 2 to 4 BT is recommended.
A typical example of a PDV calculation is shown below, and is illustrated in Figure
3.2.3a. Here, an integrator wishes to cascade the 100Mbps collision domains of two
Magnum DS8016 Hubs (each having a PDV of 80 BT) for use with standard 100m Category
5 user cable segments (i.e. from computer to hub) and needs to know how long the inter-
repeater Category 5 cable segment, used to cascade the hubs, can be. The variable “X”
represents the unknown maximum cascade cable delay.
PDV = X + ((111 + 111) + (80 + 80) + (100) + 2 ) BT < 512 BT
512 > (X + 486) BT
X < (512 - 486 ) BT, X < 28 BT
Twisted Pair cable length, TL < (28 BT) / (1.112 m/BT)
TL < 25 meters
25 m maximum total length for Category 5 inter-repeater segment,