GarrettCom 8000X - Manual (Page 32)

Magnum 8000X Mixed-Media Fiber Hubs Installation and User Guide (05/ 02)
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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 3.6.1a. Here, an integrator wishes to cascade the 100Mbps collision domains of
two Magnum 8000X Mixed-Media 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,
where 111 is the BT delay for 100m of Category 5 cable (1.112 BT/m* 100m), 80 is the
BT delay for each Magnum 8000X , 100 is the BT delay for the TX DTE pair, and a
safety margin of 2 was used. Figure 3.6.1a: Two cascaded Magnum 8000X hubs
100 m Cat. 5 cables
to user devices
100 m Cat. 5 cables
to user devices
Cat. 5 cable for
cascading
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Magnum 8000X Mixed Media Hub
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