Ethernet Design Rules

Table 4-1 provides scalability information that you can use when provisioning IEEE 802.3 networks.

Table 4-1 Scalability Constraints for IEEE 802.3

10Base5

10Base2

10BaseT

100BaseT

Topology

Bus

Bus

Star

Segment

Length

(meters)

500

185

100 from hub to station

100 from hub to station

Table 4-1 Scalability Constraints for IEEE 802.3 (Continued)

10Base5

10Base2

10BaseT

100BaseT

Maximum Number of Attachments per Segment

100

30

2 (hub and station or hub-hub)

2 (hub and station or hub-hub)

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

See the details in the section "100 Mbps Fast Ethernet Design Rules" later in this chapter.

The most significant design rule for Ethernet is that the round-trip propagation delay in one collision domain must not exceed 512 bit times, which is a requirement for collision detection to work correctly. This rule means that the maximum round-trip delay for a 10 Mbps Ethernet network is 51.2 microseconds. The maximum round-trip delay for a 100 Mbps Ethernet network is only 5.12 microseconds because the bit time on a 100 Mbps Ethernet network is 0.01 microseconds as opposed to 0.1 microseconds on a 10 Mbps Ethernet network.

To make 100 Mbps Ethernet work, distance limitations are much more severe than those required for 10 Mbps Ethernet. The general rule is that a 100 Mbps Ethernet has a maximum diameter of 205 meters when unshielded twisted-pair (UTP) cabling is used, whereas 10 Mbps Ethernet has a maximum diameter of 500 meters with 10BaseT and 2500 meters with 10Base5.

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