Filter
Top Products
APPENDIX B 100BASE-T Topology Rules and Guidelines
45
After you calculate values for each network device in the path, use
these values to determine the PDV for all paths in your network. If
your path delays exceed 512 bit times, your network may suffer from
late collisions or cyclic redundancy check (CRC) errors.
To calculate the worst case PDV, follow these steps:
1. Determine the delay value for each link segment (LSDV),
including inter repeater links.
The LSDV is a delay value associated with a particular network
segment. LSDV is similar to the segment delay value (SDV) used
in calculating PDV for 10BASE-T networks, except that it does
not include any delay values associated with the attached DTEs or
repeaters. These delay values are calculated separately and applied
to the total PDV calculation. The formula for calculating the
LSDV for a particular segment is:
LSDV = 2 (round trip delay) x segment length (in meters) x cable
delay (in bit times per meter) for the segment
To determine the total segment length, use the following formula:
Segment length = sum of all cable lengths between the physical
layer interfaces at the repeater and the farthest DTE for end
segments + sum of the cable lengths between the physical layer
interfaces for inter repeater links. Use meters for measurements.
Use the DTE delay values shown with step 4 to find the round trip
delay value in bit times per meter (bt/m) for the type of cable used
for a particular segment. That table also provides the round trip
delay value in bit times. You can use this value if you do not know
the actual cable lengths or propagation delays for each link
segment. However, because the round trip delay value of 412 bit
times for fiber optic cables will not apply to most applications,
delays for fiber optic cables should always be calculated.
Cable type Round trip delay
value in bit
times per meter
Round trip delay value
in bit times
Category 3 UTP 1.14 bt/m 114 bit times (100 meters)
Category 4 UTP 1.14 bt/m 114 bit times (100 meters)
Category 5 UTP 1.112 bt/m 111.2 bit times(100 meters)
STP 1.112 bt/m 111.2 bit times (100 meters)
Fiber optic 1.0 bt/m 412 bit times (412 meters)