GarrettCom DS 880 - Manual (Page 21)

Magnum DS880 Dual-Speed Stackable Hubs Installation and User Guide (05/01)
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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
3.2.2a. Here, an integrator wishes to cascade the 100Mbps collision domains of two Magnum
DS880 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,
1 2 3 4 5 6 7 8
COL ACT PWR LK RX
BR UPLINK 1 2
7 8 3 4 5 6
100
to u
m Cat. 5 cables
ser devices
100 m Cat. 5 cables
to user devices
Cat. 5 cable for
cascading
1 2 3 4 5 6 7 8
COL ACT PWR LK RX
BR UPLINK 1 2 3 4 5 6
7 8
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 DS880, 100 is
the BT delay for the TX DTE pair,
and a safety margin of 2 was used.
Figure 3.2.2a: Two cascaded Magnum DS880 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 Magnum DS880s. 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 Magnum DS880s (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