Ethernet Design Rules

Ethernet is the underlying basis for the technologies most widely used in LANs. In the 1980s and early 1990s, most networks used 10-Mbps Ethernet, defined initially by Digital, Intel, and Xerox (DIX Ethernet Version II) and later by the IEEE 802.3 working group. The IEEE 802.3-2002 standard contains physical specifications for Ethernet technologies through 10 Gbps.

Table 3-2 describes the physical Ethernet specifications up to 100 Mbps. It provides scalability information that you can use when provisioning IEEE 802.3 networks. Of these specifications, 10BASE5 and 10BASE2 are no longer used but are included for completeness.

Table 3-2 Scalability Constraints for IEEE 802.3

Specification

10BASE5

10BASE2

10BASE-T

100BASE-T

Physical Topology

Bus

Bus

Star

Star

Maximum Segment Length (in Meters)

500

185

100 from hub to station

100 from hub to station

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 (m) of five segments and four repeaters; only three segments can be populated

2500 m of five segments and four repeaters; only three segments can be populated

2500 m of five segments and four repeaters; only three 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. This 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.

10-Mbps Fiber Ethernet Design Rules

Table 3-3 provides some guidelines for fiber-based 10-Mbps Ethernet media for network designs. These specifications are not part of the CCDA test but are included for reference. The 10BASE-FP standard uses a passive-star topology. The 10BASE-FB standard is for a backbone or repeater-based system. The 10BASE-FL standard provides specifications on fiber links.

Table 3-3 Scalability Constraints for 10-Mbps Fiber Ethernet

Specification

10BASE-FP

10BASE-FB

10BASE-FL

Topology

Passive star

Backbone or repeater-fiber system

Link

Maximum Segment Length

1000 m

2000 m

2000 m

Allows Cascaded Repeaters?

No

Yes

No

Maximum Collision Domain

2500 m

2500 m

2500 m

100-Mbps Fast Ethernet Design Rules

IEEE introduced the IEEE 802.3u-1995 standard to provide Ethernet speeds of 100 Mbps over UTP and fiber cabling. The 100BASE-T standard is similar to 10-Mbps Ethernet in that it uses carrier sense multiple access collision detect (CSMA/CD); runs on Category (CAT) 3, 4, and 5 UTP cable; and preserves the frame formats. Connectivity still uses hubs, repeaters, and bridges.

100-Mbps Ethernet, or Fast Ethernet, topologies present some distinct constraints on the network design because of their speed. The combined latency due to cable lengths and repeaters must conform to the specifications for the network to work properly. This section discusses these issues and provides sample calculations.

The overriding design rule for 100-Mbps Ethernet networks is that the round-trip collision delay must not exceed 512-bit times. However, 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. Therefore, the maximum round-trip delay for a 100-Mbps Ethernet network is 5.12 microseconds, as opposed to the more lenient 51.2 microseconds in a 10-Mbps Ethernet network.

The following are specifications for Fast Ethernet, each of which is described in the following sections:

100BASE-TX Fast Ethernet

The 100BASE-TX specification uses CAT 5 UTP wiring. Like 10BASE-T, Fast Ethernet uses only two pairs of the four-pair UTP wiring. If CAT 5 cabling is already in place, upgrading to Fast Ethernet requires only a hub or switch and network interface card (NIC) upgrades. Because of the low cost, most of today's installations use switches. The specifications are as follows:

■ Transmission over CAT 5 UTP wire.

■ Punchdown blocks in the wiring closet must be CAT 5 certified.

100BASE-T4 Fast Ethernet

The 100BASE-T4 specification was developed to support UTP wiring at the CAT 3 level. This specification takes advantage of higher-speed Ethernet without recabling to CAT 5 UTP. This implementation is not widely deployed. The specifications are as follows:

■ Three pairs are used for transmission, and the fourth pair is used for collision

■ No separate transmit and receive pairs are present, so full-duplex operation is

100BASE-FX Fast Ethernet

The 100BASE-FX specification for fiber is as follows:

■ It operates over two strands of multimode or single-mode fiber cabling.

■ It can transmit over greater distances than copper media.

■ It uses media interface connector (MIC), Stab and Twist (ST), or Stab and Click (SC) fiber connectors defined for FDDI and 10BASE-FX networks.

■ 4B5B coding. 100BASE-T Repeaters

To make 100-Mbps Ethernet work, distance limitations are much more severe than those required for 10-Mbps Ethernet. Repeater networks have no five-hub rule; Fast Ethernet is limited to two repeaters. The general rule is that 100-Mbps Ethernet has a maximum diameter of 205 meters (m)

detection. not possible.

with UTP cabling, whereas 10-Mbps Ethernet has a maximum diameter of 500 m with 10BASE-T and 2500 m with 10BASE5. Most networks today use switches instead of repeaters, which limits the length of 10BASE-T and 100BASE-TX to 100 m between the switch and host.

The distance limitation imposed depends on the type of repeater.

The IEEE 100BASE-T specification defines two types of repeaters: Class I and Class II. Class I repeaters have a latency (delay) of 0.7 microseconds or less. Only one repeater hop is allowed. Class II repeaters have a latency of 0.46 microseconds or less. One or two repeater hops are allowed.

Table 3-4 shows the maximum size of collision domains, depending on the type of repeater. Table 3-4 Maximum Size of Collision Domains for 100BASE-T

Repeater Type

Copper

Mixed Copper and Multimode Fiber

Multimode Fiber

DTE-DTE (or Switch-Switch)

100 m

Not applicable

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

Again, for switched networks, the maximum distance between the switch and the host is 100 m.

Again, for switched networks, the maximum distance between the switch and the host is 100 m.

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