Installation guide

ProMedia-100 Manual Page 7
Table 3.4b: Worst case round-trip delay for Fast Ethernet device components*
Component Round-trip delay in Bit
Times (BT)
2 TX DTEs 100
2 FX DTEs 100
1 FX and 1 TX DTE 100
2 T4 DTEs 138
1 T4 and 1 TX or FX DTE 127
Class I Repeater 140
Class II Repeater with any
combination of TX and FX
ports
92
Class II Repeater with T4 ports 67
*Worst case delays taken from IEEE Std 802.3u-1995.
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 delay time.
PDV = (sum of cabling delays) + (sum of repeater & media converter delays) + (DTE pair
delays) + (safety margin)
PDV is the Path Delay Value of the worst-case path. For the network to adhere to IEEE 802.3u
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 an extra long
patch cable. A safety margin of at least 4 BT is recommended.
“Rules-of-thumb” Collision Domain Calculations
Rules-of-thumb, while inexact, may be helpful in planning network topology. As a rule-of-thumb,
a Class II Repeater has a PDV of about 90 to 95 BTs, and twisted-pair or fiber media has a PDV of about
1 BT per meter of length. The ProMedia-100 has a PDV of 80 BT. Therefore, in shared Fast Ethernet
applications, the ProMedia-100 uses about 80 meters of equivalent cable distance to convert from TX
media to fiber FX media, i.e., it consumes almost as much of the available PDV as a Class II repeater.
Since a 512BT collision domain will almost always include at least one repeater and two media
segments, the remaining amount of Bit Times left after allowing for a ProMedia-100 and a length of fiber
media indicates that the available fiber length will be much less than the 412 meters that is the known
maximum for fiber. Therefore, in shared environments, ProMedia-100 media converters will be of benefit
when they allow the use of fiber media, but not to gain distance
by facilitating use of fiber media instead
of twisted pair.
As a sample calculation, consider the question of what fiber cable distance (connected by a pair
of ProMedia-100s on each end) can be obtained that will interconnect two 100Mbps hubs where the
twisted pair cables to the user nodes are 10 meters in length. The solution is:
512 = total available Bit Times in a collision domain diameter,
minus 100 BT for two DTEs on each end leaves 412 BTs,
minus 180 BT for two Class II repeaters leaves 232 BTs,
minus 20 BT for two 10-meter TP cables for hubs to users leaves 212 BTs,
minus 10 BT for two short TP cables from the hubs to -100’s leaves 202 BTs,
minus 160 BT for two ProMedia-100s leaves 42 BTs for fiber cable,
which indicates a fiber cable length of about 40 meters
.