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collision domain. Regardless of the actual network topology, the Collision Domain diameter must be
less than 510 BT (Bit Times). Bit Times are related to media type as shown in the table below.
Table 2.2.2a:Worst case round-trip delay for Fast Ethernet media*
*Worst case delays taken from IEEE Std 802.3u-1995, actual delays may be less for a particular cable.
Each shared Fast Ethernet network device also has an associated BT delay. Table 2.2.2b shows
typical Fast Ethernet device components and the associated BT delay.
Note:
There is only one DTE pair associated with any device-to -device path.
Table 2.2.2b: Worst case round-trip delay for Fast Ethernet device components*
* Worst case delays taken from IEEE Std 802.3u1995
To determine whether a prospective network topology adheres to the collision domain diameter
specification, the following formula should be applied to 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 and 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 11 Repeater has a PDV of about 90 to 95 BTs, and twisted-pair or fibre media has a PDV of
about 1 BT per meter of length. The
RS
Media Converters has a PDV of 80 BT. Therefore, in shared
Fast Ethernet applications, the converters uses about 80 meters of equivalent cable distance to
convert from TX media to fibre FX media, i.e., it consumes almost as much of the available PDV as a
Class 11 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 Converter and
a length of fibre media indicates that the available fibre length will be much less than the 412 meters
that is the known maximum for fibre. Therefore, in shared environments, the RS Media Converters
will be of benefit when they allow the user of fibre media, but not to gain distance by facilitating use
of fibre media instead of twisted pair.
As a sample calculation, consider the question of what fibre cable distance (connected by a pair of
Media Converters 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 11 repeaters leaves 232 BTs,
minus 20 BT for two 10-meter TP cables for hubs to users leaves 212 BTs.
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 1 repeater
140
Class 11 repeater with any combination of TX and FX ports
92
Class 11 repeater with T4 ports
67
Media type
Round-trip delay in
Bit Time per meter (BT/m)
Fibre optic
1.000
Shielded TP cable
1.112
Category 5 cable
1.112
Category 4 cable
1.140
Category 3 cable
1.140
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When installing the Media Converters in a full-duplex segment, it is important to consider the
combined overall segment length of both the attached media types. The overall segment length is
calculated by adding together the segment lengths on both sides of the Media Converters. The
figure below illustrates how the Media Converter is used to connect a multi-mode fibre (100BASE-
FX) with a twisted pair (100BASE-T) segment.
Segment length on each side of the Converter is measured as a percentage of the maximum
allowable standard media distance for the given media type.
The percentages, when added together, must not exceed 100%.
Media distance formula, Full-duplex:
X% + Y%
≤
100%
Where X = The segment distance on one side of the media converter divided by standard
maximum media distance for that media type, x 100%
Where Y =
The segment length on the other side of the media converter divided by the standard
maximum media distance for that media cabling type, x 100%
I
n the example shown above, the length of the fibre segment X is 1500m (4920 ft). This is 75% of the
maximum allowable distance for multi-mode 100BASE-FX fibre full-duplex media (2000m) [75/2000
x 100% = 75%]. The length of twisted pair segment Y is 10m (33 ft). This is 10% of the maximum
allowable distance for 100BASE-TX full-duplex twisted-pair media (100m) [10/100 x 100% = 10%].
The total of two percentages (75% + 10%) is 85%, which is allowable.
Note 1:
Where more than one media converter is used in one segment run, the percentages for all
the cabling lengths in the run must be added together and must not exceed 100%.
In another distance, an RS media converter is used to connect a single mode fibre (100BASE-FX)
with a twisted pair (100BASE-T) segment. In this example, the length of fibre segment X is 8500m
(27,880 ft). This is 57% of the maximum allowable distance for single mode 100BASE-FX fibre full-
duplex media (15,00m) [57/15,000 x 100% = 57%]. The length of twisted pair segment Y is 12m (40
ft).
This is 12% of the maximum allowable distance for 100BASE-TX full-duplex twisted-pair media
(100m) [12/100 x 100% = 12%]. The total of the two percentages (57% + 12%) is 69%, which is
allowable.
2.2.2 Segment distance, half-duplex
Fast ethernet shared bandwidth devices operate with multiple nodes in a traffic domain. When a
node attempts to sends a packet, it may hit another packet passing by, i.e., a collision may occur. This
is normal and does not cause a problem because the Ethernet protocol provides for this situation
and requires that the sender wait and try again. When installing the Media converters in a half-duplex
segment, it is important to consider the collision domain of the segment, including the Converter
itself, repeaters and hubs present, and the lengths of both of the attached media types.
Collision domain
A collision domain is defined in the IEEE 802.3u standard as a cluster of network devices that,
regardless of topology, must be less than 512 BT (Bit Times) of signal delay (PDV or Path Delay Value)
in diameter between any two nodes. Nodes in a collision domain are connected by means of a
repeater or repeaters such that no bridging or switching devices are present between two nodes in
the cluster. The RS Fast Ethernet Media Converters has a PDV of about eighty Bit Times (80 BT), and
this value must be included in the overall collision domain diameter PDV calculation as applicable
for the placement of the Converters in the topology of the collision domain.
Collision domain diameter
The collision domain diameter is the length of the longest path between any two devices in a single
100 Mbps
LINK
PWR
LINK
Segment Y
10m (33ft)
Segment X
1500m (4920ft)
Fibre multimode
(100BASE-FX)
Twisted pair
(100BASE-TX)
Connectivity between 100BASE-TX and 100BASE-FX Ethernet media
