100BaseT Fast Ethernet

In 1995, the IEEE approved the IEEE 802.3u, the 100BaseT Ethernet standard. It defines the Physical and Data Link layers, uses the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol, and is 10 times faster than 10BaseT. Some of the new technology stars are:

100BaseFX 100BaseFX is Ethernet over fiber-optic cable at 100Mbps using 802.3 specs. It uses a two-strand, 50/125-micron or 62.5/125-micron multimode fiber-optic cable.

100BaseT4 100BaseT4 uses 802.3 specs, 100Mbps over Category 3, 4, or 5 cabling with a standard RJ-45 connector.

100BaseTX 100BaseTX is Fast Ethernet over Category 5 cabling. It’s compatible with, and adheres to, 802.3 specifications. It can also use two- pair, 100-ohm shielded twisted-pair (STP) cable or Type 1 STP cable.

100BaseX 100BaseX refers to either the 100BaseTX or 100BaseFX media. This standard was approved to ensure compatibility between the Ethernet CSMA/CD and ANSI X3T9.5 standard.

100VG AnyLan 100VG AnyLan is IEEE movement into Fast Ethernet and Token Ring that appears to be going nowhere fast, mostly because it’s not compatible with the 802.3 standards and Cisco doesn’t support it.

The Advantages of Fast Ethernet

Migrating or upgrading to 100BaseT from 10BaseT can substantially improve network throughput and overall performance. Because 100BaseT uses the same signaling techniques as 10BaseT, a gradual migration to 100BaseT doesn’t have to be expensive or time-consuming. Partially converting your LAN is a viable alternative to converting all clients simultaneously. The advantages of 100BaseT over 10BaseT are as follows:

• 100BaseT has 10 times the performance of 10BaseT.

• Existing cabling and network equipment can be used.

• 100BaseT can use 10Mbps and 100Mbps together.

• 100BaseT uses tried-and-true CSMA/CD technology.

• Migration is easy.

And now, the catch: There can be some cost involved in replacing old NICs with new 10/100 cards, and it’s possible that rewiring the building or floor might be necessary if the existing equipment is too old and doesn’t meet the 100BaseT specifications.

100BaseT Specifications

100BaseT networks use the same time slots that 10BaseT networks do. What is meant by “time slots”? Time slots require a station to transmit all its bits before another station can transmit its packet. For 100BaseT networks to transmit in the same time slots, the distance must be reduced. This means that instead of the 5-4-3 rule that the standard Ethernet uses (5 network segments, 4 repeaters, only 3 segments populated) you can use only two Class II repeaters in a 100BaseT network. The timing in Fast Ethernet is shorter (10 percent of Ethernet). The maximum frame size (time slot) is 1,518 bytes. The physical distance is reduced because both Fast Ethernet and regular Ethernet specifications state that the round-trip time must not exceed 512 bit times. Since Fast Ethernet transmits faster, a signal of 512 bits covers a shorter distance.

100BaseT Repeaters

You can still use repeaters in your network to extend the distance of your shared Ethernet network, or in switches with dedicated segments. Repeaters can actually reduce 100BaseFX maximum distances, because the repeater delays eat up the timing budget. This repeater will, however, extend 100BaseTX distances. The different types of repeaters available are

Class I A translational repeater that can support both 100BaseX and 100BaseT4 signaling. The allowable delay for a Class I repeater is 140 bit times.

Class II A transparent repeater has shorter propagation delay, but supports either 100BaseX or 100BaseT4, not both at the same time. The allowable delay for a Class II repeater is only 92 bit times.

FastHub 300 A repeater compatible with the IEEE 802.3u standard for Fast Ethernet. The FastHub 300 delivers 10 times the performance of a 10BaseT hub.

Table 2.1 shows the cable type, the connector type, and the maximum distance between end nodes.

Table 2.1: Cable Type, Connector Type, and Maximum Distance between End Nodes

Port Type	Cable	Connector Type	Distance
100BaseTX	Category 5	RJ 45	100 meters
100BaseFX	50/125 or 62.5/125	SC/ST/MIC	512 meters. Half- duplex restricted to 512 meters. No restric tions for full-duplex. (Distance restrictions due to signal attenua tion still apply.)

Table 2.2 shows the maximum distance between end nodes with repeaters.

Table 2.2: Maximum Distance between End Nodes with Repeaters

Standard or Repeater Type	Number of Repeaters	UTP Medium	UTP and Fiber Media (TX/FX)
802.3u	One Class I repeater	200 meters	261 meters
	One Class II repeater	200 meters	308 meters
	Two Class II repeaters	205 meters	216 meters
FastHub 300	One Class II repeater	200 meters	318 meters
	Two Class II repeaters	223 meters	236 meters
FastHub 300, plus one third- party 100BaseT class II repeater	Two Class II repeaters	214 meters	226 meters