Black Box LH8050A, Installation And User Manual

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Black Box Mixed-Media Fiber Hubs Installation and User Guide (4/99)
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 LH8050A
Mixed-Media Fiber 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
LH8050A , 100 is the BT
delay for the TX DTE pair,
and a safety margin of 2
was used.
512 > (X + 508) BT, X < 4 BT
25
Figure 3.6.1a: Two cascaded LH8050A hubs
The resulting value tells us that a Category 5 cable with a length of up to 25 meters
may be used to cascade the LH8050As. 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 LH8050As (serving up to 120 users
with full-length 100m 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 3.6.1b, where the hub-to-user cable
lengths are up to 75 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
100 m Cat. 5 cables
to user devices
100 m Cat. 5 cables
to user devices
Cat. 5 cable for
cascading
10 100
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