Installation guide

ProMedia-100 Manual Page 6
full-duplex media (15,000 m) [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 (100 m)
[12/100 x 100% = 12%]. The total of the two percentages (57% + 12%) is 69%, which is allowable.
3.2.2 Segment Distances, Half duplex
Fast Ethernet shared bandwidth devices operate with multiple nodes in a traffic domain. When a
node attempts to send 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 waits and try again. When installing the ProMedia-100 in a half-duplex segment,
it is important to consider the collision domain of the segment, including the ProMedia-100 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 any two nodes in the cluster.
A ProMedia-100 has a PDV of about eighty Bit Times (80 BT), and this value must be included in the
overall collision domain diameter PDV calculations as applicable for the placement of the -100 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 collision domain. Regardless of the actual network topology, the Collision Domain Diameter must
be less than 512 BT (Bit Times). Bit Times are related to media type as shown in Table 3.2.2a.
Table 3.2.2a: Worst case round-trip delay for Fast Ethernet media*
Media Type Round-trip delay in Bit
Time per Meter (BT/m)
Fiber Optic 1.000
Shielded TP cable 1.112
Category 5 Cable 1.112
Category 4 Cable 1.140
Category 3 Cable 1.140
*Worst case delays taken from IEEE Std 802.3u-1995, actual delays may be less for a particular
cable. Contact your cable supplier for exact cable specifications.
Each shared Fast Ethernet network device also has an associated BT delay. Table 3.4b shows
typical Fast Ethernet device components and the associated BT delay. Note that there is only one DTE
pair associated with any device-to-device path.