Transition Networks
SBFTF10xx-10x User Guide
33404 Rev. G
https://www.transition.com
Page 9 of 18
Half-Duplex Network
512-Bit Rule – 100Base-TX or 100Base-FX
Use the 512-Bit Rule to calculate the 100Base-TX or 100Base-FX half-duplex collision domain.
In a half-duplex network, the maximum cable lengths are determined by the round trip delay limitations
of each Fast Ethernet collision domain. (A collision domain is the longest path between any two terminal
devices, e.g., a terminal, switch, or router.)
The 512-Bit Rule determines the maximum length of cable permitted by calculating the round-trip delay
in bit-times (BT) of a particular collision domain. If the result is less than or equal to 512 BT, the path is
good. Note: The 512-Bit Rule applies separately to each collision domain.
To calculate the round-trip delay for a collision domain:
1. Find the collision domain, i.e., the longest path between any two terminal devices (e.g., terminal,
switch, and/or router).
2. Calculate the round-trip delay in bit-times for each length of cable.
3. Determine the bit-time values for each device (see below).
Class I hub
140 BT
Class II hub
92 BT
Terminal/router
50 BT
1 meter TP cable
1.11 BT
1 meter fiber cable
1 BT
Fast Ethernet switch 50 BT
SBFTF10xx-10x
50 BT
4. Add the bit-time values for each length of cable and the bit-times for each device.
100Base-TX twisted-pair example
In the example below, the SBFTF10xx-10x 2-Port bridging media converter acts as a switch. In this case,
the collision domain bound by the media converter on one end and a terminal on the other end.
Since the total of the bit-times in this example is less than 512 (see table below), the path is good.
Sum of the bit-times for the example collision domain:
Media converter
= 50.0 BT
100 m TP cable
(100m x 1.11 BT/m)
= 111.0 BT
Two Class II hubs
(2 x 92)
= 184.0 BT
Two 10 m TP cables
(2 x 10m x 1.11 BT/m) = 22.2 BT
Terminal
= 50.0 BT
Total
= 417.2 BT
