Network Diameter

CSMA/CD is subject to another potential network issue—the time it takes for the signal to travel across the medium between the two endpoints. If two LAN hosts were far enough apart, neither host would see traffic on the medium, and both could transmit at the same time, resulting in a collision. This anomaly is related to cable length and issues related to bit times.

The worst-case scenario occurs when the two most-distant hosts on the network both need to send a frame and when the second host does not begin transmitting until just before the frame from the first host arrives. The collision will be detected almost immediately by the second host, but it will not be detected by the first host until the corrupted signal has propagated all the way back to that host. The maximum time that is required to detect a collision (the collision window, or "slot time") is approximately equal to twice the signal propagation time between the two most-distant hosts on the network:

CollisionDetectTime = 2(H| - Hj)

where Hi and H2 are the two most distant hosts on the network.

Slot time is the maximum time that can elapse between the first and last network host's receipt of a frame. To ensure that a network host, or node, can determine whether the frame it transmitted has collided with another frame, a frame must be longer than the number of bits that can be transmitted in the slot time. In Ethernet networks, this time interval is about half a microsecond, which is long enough to transmit at least 512 bits.

This Collision Detect Time means that both the minimum frame length and the maximum collision diameter are directly related to the slot time. Longer minimum frame lengths translate to longer slot times and larger collision diameters; shorter minimum frame lengths correspond to shorter slot times and smaller collision diameters.

Table 3-2 demonstrates the different segment lengths allowed, based on the respective Ethernet implementation.

Table 3-2. Ethernet/802.3 Collision Domain Distance Limitations

10Base5

10Base2

10BaseT

100BaseT

Topology

Bus

Bus

Star

Star

Maximum segment length (meters)

500

185

100 from hub to host

100 from hub to host

Maximum number of attachments per segment

100

30

2 (hub and host or hub-hub)

2 (hub and host or hubhub)

Maximum collision domain

2500 meters of 5 segments and 4 repeaters; only 3 segments can be populated

2500 meters of 5 segments and 4 repeaters; only 3 segments can be populated

2500 meters of 5 segments and 4 repeaters; only 3 segments can be populated

100BaseT table

Table 3-3 demonstrates maximum distance allowed in 100BaseT, based on cabling technology used—for example, copper or fiber—and the number of repeaters, if any, that are implemented.

Table 3-3. 100BaseT Maximum Distance Table

Copper

Mixed Copper and Multimode Fiber

Multimode Fiber

DTE-DTE (or switch-switch)

100 m

412 m (2000 if full duplex)

One Class I repeater

200 m

260 m

272 m

One Class II repeater

200 m

308 m

320 m

Two Class II repeaters

205 m

216 m

228 m

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