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Fast Ethernet

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Fast Ethernet is traditional CSMA/CD (carrier sense multiple access/collision detection) access control at 100 Mbits/sec over twisted-pair wire. The original Ethernet data rate was 10 Mbits/sec.

During the early development of Fast Ethernet, two different groups worked out standards proposals-and both were finally approved, but under different IEEE committees. One standard became IEEE 802.3u Fast Ethernet and the other became 100VG-AnyLAN, which is now governed by the IEEE 802.12 committee. The latter uses the "demand priority" medium access method instead of CSMA/CD.

100VG-AnyLAN has not caught on, while Fast Ethernet enjoyed great success in enterprise LAN environments. At about the same time, switching technologies came on the scene to further improve network performance. More recently, Gigabit Ethernet has come on the scene to provide a high-speed upgrade path for Fast Ethernet users. It provides high-speed backbones that interconnect Fast Ethernet LANs. See "Gigabit Ethernet."

Fast Ethernet takes advantage of the scalability of CSMA/CD hierarchical networking. It is designed after the 10Base-T standard and can be built into hierarchical networking topologies like that shown in Figure F-1. This type of configuration is compatible with structured wiring strategies, as discussed under "TIA/EIA Structured Cabling Standards."

The primary concern of Fast Ethernet developers was to preserve the CSMA/CD medium access method of 802.3 Ethernet while boosting the data rate. In addition, the developers kept the frame format. Because of this, Fast Ethernet fits in well with traditional Ethernet installations. One version of the Fast Ethernet standard will run on older Category 3 cable installations. Multiple Ethernet types (10 and 100 Mbits/sec) may coexist. A Fast Ethernet-compatible hub simply needs to perform speed matching when exchanging frames. An autonegotiate feature allows devices to detect the speed of incoming transmissions and adjust appropriately.

There are four Fast Ethernet schemes:

  • 100Base-TX    Runs on two pairs of Category 5 data-grade twisted-pair wire with a maximum distance of 100 meters between hub and workstation.

  • 100Base-T4    Runs on four pairs of cable, including Category 3 cable, with a maximum distance of 100 meters between hub and workstation.

  • 100Base-FX    Runs on optical cable at distances up to 2 kilometers, and is used to connect hubs over long distances in a backbone configuration (e.g., a building in a campus environment).

  • 100Base-SX    Called Short Wavelength Fast Ethernet, this is a proposed standard (as of this writing) for a Fast Ethernet over fiber-optic cable using 850 nm wavelength optics. Refer to FOLS (Fiber Optics LAN Section) of the TIA at

The higher frequency used in the Fast Ethernet standard is prone to attenuation, so cable distance is more limited than in the Ethernet 10Base-T specification. If the encoding scheme of traditional Ethernet were used with Fast Ethernet, the high-end frequency would be above 200 Mhz. That is double the maximum frequency rating of Category 5 cable. To get around this, new encoding schemes were implemented to allow higher-frequency transmissions. Refer to the paper called "Specifications for Data Encoding in Fast Ethernet" at the Optimized Engineering Web site. See the related entries and Web links page.

Note that the collision domain is 1/10th the size of the collision domain in 10-Mbit/sec Ethernet. This is because of the 10-times increase in speed of the network. It also means that Fast Ethernet networks can only be one-tenth the physical size of a 10Base-T network.

Fast Ethernet can support full-duplex switched networking modes to provide even better performance. A full-duplex nonshared link (i.e., workstations are attached to switches, not hubs) doesn't even need CSMA/CD, because no other stations are trying to use the link, and each end system has its own channel to transmit on. Collision detection and loopback functions can be disabled. This arrangement is especially useful for backbone connections. If both end systems are transmitting at the same time, the combined data rate is 200 Mbits/sec.


100Base-TX works with two pairs of UTP (unshielded twisted pair) or STP (shielded twisted pair). Transmission takes place on one pair of wires and collision detection takes place on the other pair of wires. Two types of cable can be used with this specification: Category 5 UTP and IBM's Type 1 STP. Category 5 cable has four pairs of wire. Since only two of the pairs are used, the other two are available for other uses or future expansion. However, it is not recommended that another high-speed network be used on the pairs.

Category 5 cable is designed to cancel out the effects of EMI and, therefore, has very stringent installation requirements. All the components in the cabling system must be Category 5 compliant, including connectors, patch panels, punchdown blocks, and hubs/switches. The twists in the cable must be maintained all the way up to within 1/2 inch of a connector. The twists in the cable help maintain proper signaling, especially at high data rates. Other 100Base-TX requirements are

  • Cable distance    A link segment is the connection between a workstation and a hub or switch. The maximum link segment distance is 100 meters; but if faceplates are used, the specification recommends not exceeding 90 meters from hub to faceplate. This allows 10 meters for the faceplate-to-station connection.

  • Maximum collision domain size    First, understand that a collision domain is a segment of a network in which all the stations on that segment hear the same broadcast. The maximum end-to-end distance between two systems within a 100Base-TX twisted-pair broadcast/collision domain segment is approximately 205 meters. This is illustrated in Figure F-2. These distances are flexible as long as the maximum is not exceeded. For example, the inter-repeater distance could be increased if the workstation-to-repeater distance is decreased.

ANCHOR HERE: Figure 2 (see book)

  • Repeater hubs    Fast Ethernet repeaters are relatively sophisticated devices compared to standard Ethernet repeaters. They detect improper signals, perform data translations between different types of Fast Ethernet, monitor the network for faults, disconnect faulty ports, and partition off part of a network that is having problems so that the network can continue running. There are two types of repeater hubs in the 100Base-TX scheme:

  • Class I repeaters should be used if the network has mixed Fast Ethernet types (100Base-TX, 100Base-FX, and 100Base-T4) because they will change the data encoding between network types on different ports. One class I repeater is allowed per collisions domain (see "Collisions and Collision Domains" for an explanation).

  • Class II repeaters are strictly devices that send all incoming signals to all other ports without translation. They should be used when all the ports are supporting the same network type. Two class II repeaters are allowed per collision domain, but the link between the repeaters cannot exceed 5 meters.

  • Switching hubs    To get around the hub count limitation just described, you can install switching hubs, which basically segment the network as a bridge would segment a network. Refer to "Ethernet" and "Switching and Switched Networks" for a discussion of switches and network design. Beyond plain switching is multilayer routing, which builds high-speed inexpensive routing right into the switches and allows network managers great flexibility in designing their networks. See "Multilayer Switching."

  • Full-Duplex mode    Full-duplex mode allows simultaneous data transmission between two end nodes without collisions. A 100Base-TX link operating in full-duplex mode has an effective bandwidth of 100 Mbits/sec. All components, including the hubs, must be full-duplex capable.


The 100Base-T4 standard has the same cabling specifications as 100Base-TX in terms of distances and hub configuration. The difference is that 100Base-T4 uses all four wires in a half-duplex signaling scheme. Three pairs are used to either transmit or receive data, and the other pair is used for collision detection. 100Base-T4 also uses a special three-level encoding scheme (as opposed to two levels in other media) to reduce the clock rate. Spreading the 100-Mbit/sec signal over three pairs reduces the signal frequency and allows it to run on older Category 3 cable. Basically, 33.33 Mbits/sec is transmitted over each of the three pairs.

Higher-grade cable such as Category 5 is recommended for future expansion. Like 100Base-TX, the 100Base-T4 specification has a maximum hub-to-station cable length of 100 meters and an end-to-end maximum of 250 meters. 100Base-T4 also uses the same repeater classing scheme as 100Base-TX. Class I hubs allow a mix of different Fast Ethernet network types, and class II hubs allow only one scheme (refer to the description in the previous section).


100Base-FX is the fiber-optic cable implementation of the Fast Ethernet standard. It is ideal for building backbone connections. Cable distance is limited to 412 meters; but if full-duplex mode is used, cable runs may be as long as 2 km. While fiber-optic cable can span longer distances, the limit is imposed to account for packet round-trip timing.

Fiber-optic cable is not prone to interference. It does not emanate a signal, so it is more secure (especially for wiring across public areas). In addition, fiber-optic cable can scale up to higher transmission rates for future expansion.

100Base-FX requires a cable with two strands of 62.5/125-micron fiber. One strand is used for signal transmission while the other is used to receive and detect collisions. 100Base-FX also uses the same repeater classing scheme as 100Base-TX. Class I hubs allow mixed Fast Ethernet networks, and class II hubs allow only one scheme (refer to the description in the previous section).

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.