Features and Characteristics of Ethernet 802.3 Standards

• Specify the characteristics (e.g., speed, length, topology, cable type, etc.) of the following 802.3 (Ethernet) standards:

  • 10BaseT

  • 10BaseTX

  • 10Base2

  • 10Base5

  • 100BaseFX

  • Gigabit Ethernet

As outlined in Chapter 1, the IEEE 802 standards specify the characteristics of LAN systems. Of all the standards detailed by the IEEE, the most widely implemented is the 802.3 standard. Since the introduction of the original 802.3 standards, many substandards have been developed, each of which has its own specifications.

Each 802.3 standard specifies the signaling mode (baseband or broadband), topology, data rates, and media type used. The following sections describe the characteristics of the 802.3 standards.

10Base2

10Base2, sometimes called Thinnet or Thin Ethernet, is the 802.3 specification for a network that uses thin coaxial cable (that is, RG-58 cable). 10Base2 specifies a maximum speed of 10Mbps; although 10Mbps has been sufficient in the past, it is now considered too slow for many network implementations. Because it uses thin coaxial cable, 10Base2 specifies BNC barrel and BNC T-connectors to connect the cable and computers. At the physical ends of each cable segment, a 50-ohm terminator absorbs the signal, thus preventing signal reflection. One of the terminators must be grounded. Figure 2.11 shows the basic layout of a 10Base2 network.

The 10Base2 standard specifies a limit of 185 meters per segment (that is, approximately 600 feet). Thinnet cable is prone to breaks, and a break anywhere in the cable renders the entire network unusable. For this reason, and due to speed and distance limitations, 10Base2 networks have largely been replaced. You may find an existing 10Base2 network, but you are unlikely to install one. Regardless of its low profile in the networking arena, you will be expected to know the characteristics of 10Base2 for the Network+ exam and be able to identify why such a network may be used.

In a 10Base2 network, only 30 networked devices can be attached to a single segment. A maximum of 3 segments can have network devices connected, or populated (although the network can have a total of 5 segments). This means that a 10Base2 network can have a maximum of 90 computers attached, but you can extend this limit if you use repeaters. The minimum distance between two devices on a 10Base2 network is .5 meters.

Table 2.4 summarizes the features of 10Base2.

Table 2.4. Summary of 10Base2 Characteristics

Characteristic	Description
Transmission method	Baseband
Speed	10Mbps
Total distance	185 meters
Cable type	Thin coax
Number of systems/segment	30
Min. distance between nodes	.5 meter

10Base5

10Base5 networks use thick coaxial cable (that is, RG-8 cable), also known as Thicknet or Thick Ethernet, and devices attach to it by using external transceivers and AUI ports. The transceivers attach to the thick coax cable via vampire taps, which penetrate the outer sheath of the cable and make a connection to the inside core.

10Base5 uses baseband transmission, has a maximum transfer rate of 10Mbps, and has a cable distance of 500 meters per segment (that is, about 1,600 feet). A maximum of 100 devices can be attached per segment, and the total number of populated segments allowed is 3. The minimum distance between each vampire tap on the network must be no less than 2.5 meters, and the maximum allowed distance between the tap and the networked device is 5 meters. As with 10Base2, each end of the cable must be terminated with a 50-ohm terminator. Figure 2.12 shows an example of a 10Base5 network.

At one time, because of the distances 10Base5 allowed, it was relatively popular; however, today it is rarely used. Its transfer speeds and implementation difficulty exclude 10Base5 from being a viable network solution. Table 2.5 summarizes the characteristics of the 10Base5 specification.

Table 2.5. Summary of 10Base5 Characteristics

Characteristic	Description
Transmission Method	Baseband
Speed	10Mbps
Total distance	500 meters
Cable type	Thick coax
Number of systems/segment	100
Min. distance between nodes	2.5 meters

10BaseT

The 10BaseT LAN standard specifies an Ethernet network that commonly uses UTP cable; however, in some implementations that require a greater resistance to interference and attenuation, STP can be used. Remember from the section “Twisted-Pair,” earlier in this chapter, that STP has extra shielding to combat interference.

10BaseT uses baseband transmission and has a maximum segment length of 100 meters. As with the coaxial standards, repeaters are sometimes used to extend the maximum segment length, although the repeating capability is now often built in to networking devices used in twisted-pair networks. 10BaseT specifies transmission speeds of 10Mbps and can use several categories of UTP cable, including Categories 3, 4, and 5 (all of which use RJ-45 connectors). 10BaseT takes advantage of the multiple wires inside twisted-pair cable to create independent transmit and receive paths, which means that a full-duplex mode can be optionally supported. The maximum number of computers supported on a 10BaseT network is 1,024.

All 10BaseT networks use a point-to-point network design, with one end of the connection attaching to the network card and the other to a hub or switch. These point-to-point connections result in a physical star topology. Chapter 3 provides more information on the devices used in twisted-pair networks.

Table 2.6 summarizes the characteristics of the 10BaseT standard.

Table 2.6. Summary of 10BaseT Characteristics

Characteristic	Description
Transmission Method	Baseband
Speed	10Mbps
Total distance/segment	100 meters
Cable type	Category 3, 4, or 5 UTP or STP
Connector	RJ-45

IN THE FIELD: MAKE OR BUY? During your networking career, you will most certainly encounter the debate about whether to crimp your own RJ-45 network cables or buy them. The arguments for making cables always seems to hinge on cost savings. The arguments against crimping cable are often much more solid. Purchasing cables from a reputable maker ensures that the cables you install will work every time. The same cannot be said of homemade cables. In addition, when you factor in the time it takes to make a cable or troubleshoot a poorly made one, the cost savings are lessened. However, in some instances you'll have no choice but to make cables—for instance, when special-distance cables are required or when your local Cables-R-Us is closed.

Fast Ethernet: 100BaseX

Many networks demand more bandwidth than is available with 10Mbps network solutions. For such networks, Fast Ethernet is the most commonly used network design. Fast Ethernet standards are specified in the IEEE 802.3u standard. Three standards are defined by 802.3u: 100BaseTX, 100BaseT4, and 100BaseFX.

100BaseTX

100BaseTX is a Fast Ethernet networking design and is one of three 802.3u standards. As its name suggests, 100BaseTX transmits network data at speeds up to 100Mbps, the speeds at which most LANs operate today. 100BaseTX is most often implemented with UTP cable, but it can use STP; therefore, it suffers from the same 100 meter distance limitations as other UTP-based networks. 100BaseTX uses Category 5 UTP cable and, like 10BaseT, it uses independent transmit and receive paths and can therefore support full-duplex operation. 100BaseTX is without question the most common Fast Ethernet standard.

100BaseT4

100BaseT4 is the second Fast Ethernet standard specified under 802.3u. It can use Category 3, 4, and 5 UTP cable, and it uses all four of the available pairs of wires within the cable, limiting full-duplex transfer. 100BaseT4 is similar in other respects to 100BaseTX: Its cable distance is limited to 100 meters, and its maximum transfer speed is 100Mbps. 100BaseT4 is not widely implemented, but it is sometimes used in environments where existing cable, such as Category 3 cable, exists. In such a situation, you can use 100BaseT4 instead of replacing the Category 3 cable with Category 5 UTP.

100BaseFX

100BaseFX is the IEEE standard for running Fast Ethernet over fiber-optic cable. Due to the expense of fiber implementations, 100BaseFX is largely limited to use as a network backbone.

100BaseFX can use two-strand multimode fiber or single-mode fiber media. The maximum segment length for half-duplex multimode fiber is 412 meters, but this maximum increases to an impressive 10,000 meters for full-duplex single-mode fiber. 100BaseFX often uses SC or MIC fiber connectors. To see where 100BaseFX compares with other 100Base technologies refer to Table 2.7.

REVIEW BREAK: Fast Ethernet Comparison

Table 2.7 summarizes the characteristics of the 802.3u Fast Ethernet specifications.

Table 2.7. Summary of 802.3u Fast Ethernet Characteristics

	100BaseTX	100BaseT4	100BaseFX
Transmission Method	Baseband	Baseband	Baseband
Speed	100Mbps	100Mbps	100Mbps
Distance	100 meters	100 meters	412 meters (multimode, half-duplex); 10,000 meters (single-mode, full-duplex)
Cable Type	Category 5 UTP, STP	Category 3, 4, 5	Fiber-optic
Connector Type	RJ-45	RJ-45	SC, MIC

Gigabit Ethernet: 1000BaseX

Gigabit Ethernet, 1000BaseX, is another variation on the 802.3 standard and is given its own identifier: 802.3z. Gigabit Ethernet offers transfer rates of up to 1000Mbps and is most often associated with fiber cable. 1000BaseX refers collectively to three distinct standards: 1000BaseLX, 1000BaseSX, and 1000BaseCX.

Both 1000BaseSX and 1000BaseLX are laser standards used over fiber. LX refers to long wavelength laser, and SX refers to short wavelength laser. Both the SX and LX wave lasers can be supported over two types of multimode fiber: fibers of 62.5 micron and 50 micron diameters. Only LX wave lasers support the use of single-mode fiber.

At the end of the day, the differences between 1000BaseLX and the 1000BaseSX have to do with cost and transmission distance. 1000BaseLX can transmit over 316 meters in half-duplex for both multimode fiber and single-mode fiber, 550 meters for full-duplex multimode fiber, and 5,000 meters for full-duplex single-mode fiber. Although 1000BaseSX is less expensive than 1000BaseLX, it cannot match the distances achieved by 1000BaseLX.

1000BaseCX moves away from the fiber cable and uses shielded copper wire. Segment lengths in 1000BaseCX are severely restricted; the maximum cable distance is 25 meters. Because of the restricted cable lengths, 1000BaseCX networks are not widely implemented. Table 2.8 summarizes the characteristics of Gigabit Ethernet 802.3z standards.

Table 2.8. Summary of IEEE 802.3z Gigabit Ethernet Characteristics

	1000BaseSX	1000BaseLX	1000BaseCX
Transmission Method	Baseband	Baseband	Baseband
Speed	1000Mbps	1000Mbps	1000Mbps
Distance	Half-duplex 275 (62.5 micron multimode fiber); half-duplex 316 (50 micronl8u multimode fiber); full-duplex 275 (62.5 micron multimode fiber); full-duplex 550 (50 micron multimode fiber)	Half-duplex 316 (multimode and single-mode fiber); full-duplex 550 (multimode fiber); full-duplex 5000 (single-mode fiber)	25 meters for both full-duplex and half-duplex operations
Cable Type	62.5/125 and 50/125 multimode fiber	62.5/125 and 50/125 multimode fiber; two 10-micron single-mode optical fibers	Shielded copper cable
Connector Type	SC connector	SC connector	9-pin shielded connector, 8-pin fiber channel type 2 connector

1000BaseT

1000BaseT is another Gigabit Ethernet standard, and it is given the IEEE 802.3ab designation. The 802.3ab standard specifies Gigabit Ethernet over Category 5 UTP cable. The standard allows for full-duplex transmission using the four pairs of twisted cable.

To reach speeds of 1000Mbps over copper, a data transmission speed of 250Mbps is achieved over each pair of twisted-pair cable. Table 2.9 summarizes the characteristics of 1000BaseT.

Table 2.9. Summary of 1000BaseT Characteristics

Characteristic	Description
Transmission method	Baseband
Speed	1000Mbps
Total distance/segment	100 meters
Cable type	Category 5 or better
Connector type	RJ-45