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Quick Reference Guide to 100-Mbps Multi-Segment Configuration

14.2 Collision Domain

The scope of the multi-segment configuration guidelines is limited to a single Ethernet, or collision domain. A collision domain is formally defined as a single CSMA/CD network in which there will be a collision if two computers attached to the system both transmit at the same time.

An Ethernet system composed of a single segment or multiple segments linked to each other with a combination of repeaters is a network that functions as a single collision domain.

FIGURE 14.1 Repeater hubs create a single collision domain

Figure 14.1 shows two repeater hubs connecting three computers. Since only repeater connections are used between segments in this network, all of the segments and computers are in the same collision domain.

FIGURE 14.2 Switching hub creates separate collision domains

The repeaters and DTEs in Figure 14.2 are instead separated by a packet switch (switching hub, bridge, or router), and are therefore in separate collision domains, since packet switches do not forward collision signals from one segment to another. Packet switches contain multiple Ethernet interfaces, and are designed to receive a packet on one Ethernet port and transmit the data onto another Ethernet port in a new packet. Instead of propagating collision signals between Ethernet segments, packet switches interrupt the collision domain and allow the Ethernets they link to operate independently. Therefore, you can use packet switching hubs to build larger network systems by interconnecting individual Ethernet systems.

The configuration guidelines described here are from the 802.3 standard, which describes the operation of a single Ethernet LAN. Therefore, the guidelines apply to a single collision domain only and have nothing to say about combining multiple Ethernets with packet switches. As long as each collision domain is configured properly it will function correctly, and you can link many such networks together using packet switching hubs.

Quick Reference Guide to 100-Mbps Multi-Segment Configuration - 09 SEP 95
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