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This section identifies some of the constraints you should consider when provisioning various LAN media types. It covers the physical specifications of Ethernet, Fast Ethernet, and Gigabit Ethernet. It also covers the specifications for Token Ring, because you may find this technology on existing networks.
You must also understand the design constraints of wireless LANs in the campus network. Specifications for wireless LANs are covered in Chapter 4, "Wireless LAN Design."
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.
| 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.
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
- 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 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:
- RJ-45 connector (the 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 specification takes advantage of higher-speed Ethernet without recabling to CAT 5 UTP. This implementation is not widely deployed. The specifications are as follows:
- Transmission over CAT 3, 4, or 5 UTP wiring.
- 8B6T coding.
100BASE-FX Fast Ethernet
- It can transmit over greater distances than copper media.
- 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) 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 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.
Gigabit Ethernet Design Rules
Gigabit Ethernet was first specified by two standards: IEEE 802.3z-1998 and 802.3ab-1999. The IEEE 802.3z standard specifies the operation of Gigabit Ethernet over fiber and coaxial cable and introduces the Gigabit Media-Independent Interface (GMII). These standards are superseded by the latest revision of all the 802.3 standards included in IEEE 802.3-2002.
The IEEE 802.3ab standard specified the operation of Gigabit Ethernet over CAT 5 UTP. Gigabit Ethernet still retains the frame formats and frame sizes, and it still uses CSMA/CD. As with Ethernet and Fast Ethernet, full-duplex operation is possible. Differences appear in the encoding; Gigabit Ethernet uses 8B10B coding with simple nonreturn to zero (NRZ). Because of the 20 percent overhead, pulses run at 1250 MHz to achieve a 1000 Mbps throughput.
The following are the physical specifications for Gigabit Ethernet, each of which is described in the following sections:
- 1000BASE-LX
- 1000BASE-SX
- 1000BASE-CX
- 1000BASE-T
1000BASE-LX Long-Wavelength Gigabit Ethernet
IEEE 1000BASE-LX uses long-wavelength optics over a pair of fiber strands. The specifications are as follows:
- Maximum lengths for multimode fiber are
- - 62.5-micrometer fiber: 440 m
- - 50-micrometer fiber: 550 m
-
- Maximum length for single-mode fiber (9 micrometers) is 5 km
- Uses 8B10B encoding with simple NRZ
1000BASE-SX Short-Wavelength Gigabit Ethernet
IEEE 1000BASE-SX uses short-wavelength optics over a pair of multimode fiber stands. The specifications are as follows:
- Uses short wave (850 nm)
- Use on multimode fiber
- Maximum lengths:
- - 62.5-micrometer fiber: 260 m
- - 50-micrometer fiber: 550 m
-
- Uses 8B10B encoding with simple NRZ
1000BASE-CX Gigabit Ethernet over Coaxial Cable
IEEE 1000BASE-CX standard is for short copper runs between servers. The specification is as follows:
- Used on short-run copper
- Maximum length is 25 m
- Mainly for server connections
1000BASE-T Gigabit Ethernet over UTP
The IEEE standard for 1000-Mbps Ethernet over CAT 5 UTP was IEEE 802.3ab; it was approved in June 1999. It is now included in IEEE 802.3-2002. This standard uses the four pairs in the cable. (100BASE-TX and 10BASE-T Ethernet use only two pairs.) The specifications are as follows:
- CAT 5, four-pair UTP
- Maximum length is 100 m
- Encoding defined is a five-level coding scheme
- 1 byte is sent over the four pairs at 1250 MHz
10 Gigabit Ethernet (10GE) Design Rules
The IEEE 802.3ae supplement to the 802.3 standard, published in August 2002, specifies the standard for 10 Gigabit Ethernet. It is defined only for full-duplex operation over optical media. Hubs or repeaters cannot be used because they operate in half-duplex mode. It allows the use of Ethernet frames over distances typically encountered in metropolitan-area networks (MAN) and WANs. Other uses include data centers, corporate backbones, and server farms.
10GE Media Types
10GE has seven physical media specifications based on different fiber types and encoding. Multimode fiber (MMF) and single-mode fiber (SMF) are used. Table 3-6 describes the different 10GE media types.
Short-wavelength multimode fiber is 850 nm. Long-wavelength is 1310 nm, and extra-long-wavelength is 1550 nm. The WIS is used to interoperate with Synchronous Optical Network (SONET) STS-192c transmission format.
Fast EtherChannel
The Cisco EtherChannel implementations provide a method to increase the bandwidth between two systems by bundling Fast Ethernet or Gigabit Ethernet links. When bundling Fast Ethernet links, use Fast EtherChannel. EtherChannel port bundles allow you to group multiple ports into a single logical transmission path between the switch and a router, host, or another switch. EtherChannels provide increased bandwidth, load sharing, and redundancy. If a link fails in the bundle, the other links take on the traffic load. You can configure EtherChannel bundles as trunk links.
Depending on your hardware, you can form an EtherChannel with up to eight compatibly configured ports on the switch. The participating ports must have the same speed and duplex mode and belong to the same VLAN.
Token Ring Design Rules
Token Ring is not a CCDA test subject but this section is included for reference because you might find Token Ring on existing networks. IBM developed Token Ring in the 1970s. In the 1980s, Token Ring and Ethernet competed as the preferred medium for LANs. The IEEE developed the IEEE 802.5 specification based on the IBM Token Ring specifications. The 802.5 working group is now inactive. The most recent specification is IEEE 802.5-1998. You can find more information at http://www.8025.org.
Table 3-7 lists some media characteristics for designing Token Ring segments.
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