Ethernet is the underlying basis for the technologies most widely used in LANs. In the 1980s and early 1990s, most networks used 10Mbps 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 1000Mbps. Table 4-2 describes the specifications. The table also includes some physical (100BASE-T) specifications for Fast Ethernet. Table 4-2 provides scalability information that you can use when provisioning IEEE 802.3 networks.
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 cor-rectly. 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 micro-seconds because the bit time on a 100-Mbps Ethernet network is 0.01 micromicro-seconds as opposed to 0.1 microseconds on a 10-Mbps Ethernet network.
10-Mbps Fiber Ethernet Design Rules
Table 4-3 provides some guidelines for fiber-based 10-Mbps Ethernet media for network designs.
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 4-2 Scalability Constraints for IEEE 802.3
10BASE5 10BASE2 10BASE-T 100BASE-T
See the details in the section
“100Mbps Fast Ethernet Design Rules” later in this chapter.
100-Mbps Fast Ethernet Design Rules
The IEEE introduced the IEEE 802.3u-1995 standard to provide Ethernet speeds of 100Mbps over UTP and fiber cabling. The 100BASE-T standard is similar to 10Mbps 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.
100Mbps 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 con-form to the specifications for the network to work properly. This section discusses these issues and provides sample calculations.
The overriding design rule for 100Mbps Ethernet networks is that the round-trip collision delay must not exceed 512 bit times. However, the bit time on a 100Mbps Ethernet network is 0.01 micro-seconds, as opposed to 0.1 microseconds on a 10Mbps Ethernet network. Therefore, the maximum round-trip delay for a 100Mbps Ethernet network is 5.12 microseconds, as opposed to the more lenient 51.2 microseconds in a 10Mbps Ethernet network.
The following are specifications for Fast Ethernet, each of which is described in the following sections:
■ 100BASE-TX
■ 100BASE-T4
■ 100BASE-FX
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
Table 4-3 Scalability Constraints for 10-Mbps Fiber Ethernet (Continued)
10BASE-FP 10BASE-FB 10BASE-FL
Topology Passive star Backbone or
repeater-fiber system
Link
Maximum Segment Length 500 m 2000 m 1000 or 2000 m
Allows Cascaded Repeaters? No Yes No
Maximum Collision Domain 2500 m 2500 m 2500 m
LAN Media 81
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 or CAT 1 shielded twisted-pair (STP) wire.
■ RJ-45 connector (same as in 10BASE-T).
■ Punchdown blocks in the wiring closet must be CAT 5 certified.
■ 4B5B coding.
100BASE-T4 Fast Ethernet
The 100BASE-T4 specification was developed to support UTP wiring at the CAT 3 level. This spec-ification takes advantage of higher-speed Ethernet without recabling to CAT 5 UTP. This implemen-tation is not widely deployed. The specifications are as follows:
■ Transmission over CAT 3, 4, or 5 UTP wiring.
■ Three pairs are used for transmission, and the fourth pair is used for collision detection.
■ No separate transmit and receive pairs are present, so full-duplex operation is not possible.
■ 8B6T coding.
100BASE-FX
The 100BASE-FX specification for fiber is as follows:
■ Operates over two strands of multimode or single-mode fiber cabling
■ Can transmit over greater distances than copper media
■ Uses media interface connector (MIC), Stab & Twist (ST), or Stab & 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 10Mbps Ethernet. For repeater networks, there is no five-hub rule; Fast Ethernet is limited to two repeaters. The general rule is that a 100Mbps Ethernet has a maximum diameter of 205 meters (m) with UTP cabling, whereas 10Mbps 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 4-4 shows the maximum size of collision domains, depending on the type of repeater.
Again, for switched networks, the maximum distance between the switch and the host is 100 m.