GarrettCom manual Magnum 8000X Mixed-Media Fiber Hubs

Magnum 8000X Mixed-Media Fiber Hubs Installation and User Guide (05/ 02)

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

Cat. 5 cable for cascading

100 m Cat. 5 cables

to user devices 100 m Cat. 5 cables

to user devices

safety margin of 2 was used. Figure 3.6.1a: Two cascaded Magnum 8000X hubs

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