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GarrettCom DS 880 User Manual

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Magnum DS880 Dual-Speed Stackable Hubs Installation and User Guide (05/01)

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3.2.2

100Mbps Collision Domain Diameter, Cable Distances and PDV Calculations

The 100Mbps Collision Domain Diameter is the length of the longest path between

any two devices in a single collision domain. A collision domain is defined as a cluster of

network devices which are connected by means of a repeater or repeaters such that no bridging

devices are present between any two devices in the cluster. In order to install an IEEE 802.3u

compliant Fast Ethernet network, the collision domain . . regardless of the actual network

topology . . 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 3, 4 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 Fast Ethernet device component also has an associated BT delay which depends

on the physical signaling system employed. Table 3.2.2b shows each Fast Ethernet device

component and the associated BT delay. A “DTE” is an end node, such as a user station. Note

that there is only one DTE pair associated with any device-to-device path.

Table 3.2.2b: 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

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 **

**Note, the delay is only 80 Bit Times for

the DS880, front-port-to-front-port.

*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 time.