Black Box Dual-Speed Stackable Hubs Installation and User Guide
15
Speed LH8000A Hubs(each having a PDV of 80BT) 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 LH8000A’s, 100
is the BT delay for the TX DTE
pair, and a safety margin of 2
was used.
Figure 3.2.2a: Two cascaded LH8000A hubs
The resulting value tells us that a Category 5 cable with a length of up to 30 meters
may be used to cascade the LH8000As. Note that this inter-repeater cable length may be
increased by shortening the lengths of the 100m hub-to-user cable segments.
It is also possible to cascade a group of up to nine LH8000As (serving up to 56 users
with full-length 75m cables to each) by using short inter-repeater cable segments. The
following is an example of how to calculate the maximum allowable cascade cable segment
length. An illustration of this example is shown in Figure 3.2.2b, where the hub-to-user cable
lengths are up to 100 meters for Cat 5 twisted pair. The variable “X”, the maximum hub-to-hub
cable delay, is calculated as follows:
PDV = X + ((100) + (80 + 80 + 80 )+ (83 + 83) + 2 ) BT < 512 BT
512 > (X + 508) BT, X < 4 BT
Twisted pair user cable length, TL < (4 BT) / (1.112 m/BT)
TL < 3.6 meters for two hub-to-hub cascading cables
⇒
2
meters (6 ft.) length max. for each of the cascade segments,
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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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