Chapter 6. Ethernet

Ethernet will play a very important role in home networking. It can be directly installed within a room with a bunch of twisted pair patch cables or in a house with home run wired Category 5 twisted pair cables. Furthermore, other existing wiring based home networking systems are also based on the Ethernet frame format and the Media Access and Control protocol. Operation principles of wireless Ethernet and its home networking variations are very close to that of the original Ethernet. The concept of Ethernet was originated by Bob Metcalfe and his Xerox PARC colleagues in late 1972 to interconnect personal workstations. Their first experimental network was called the Alto Aloha Network. In 1973, Metcalfe changed the name to Ethernet, to make it clear that the system could support any computer and to point out that his new network mechanisms had evolved well beyond the Aloha system. He chose to base the name on the word "ether" as a way of describing an essential feature of the system: the physical medium (i.e., a cable) carries bits to all stations, much the same way that the old luminiferous ether was once thought to propagate electromagnetic waves through space.

After some refinement, the second generation called Ethernet II was widely used. Ethernet from this period is often called DIX after its corporate sponsors Digital, Intel, and Xerox. As the holder of the trademark, Xerox established and published the original standards for a coaxial cable based Ethernet in 1980. The Institute of Electrical and Electronics Engineers (IEEE) was assigned the task of developing formal international standards for all Local Area Network technology. It formed the "802" committee to look at Ethernet, Token Ring, Fiber Optic, and other LAN technology. The thick coaxial media system was the first media system specified in the IEEE Ethernet, CSMA/CD, standard of 1985. The thick coaxial cable based Ethernet is also called 10Base5 and can carry a transmission data rate of 10 Mbps. 10Base5 uses relatively inflexible coaxial cable with a diameter of 1 centimeter. A thin coaxial cable version of Ethernet, named 10Base2, was subsequently developed in 1987. The diameter of a thin coaxial cable is 0.5 centimeter (cm). This thin coaxial cable is also known as RG-58 and has an impedance of 50 ohms. In comparison, coaxial TV cables are either RG-6 or RG-59 and have an impedance of 75 ohms.

10Base5 and 10Base2 have maximum transmission distances of 500 and 185 m, respectively. Although 10Base2 coaxial cables are relatively flexible, they are still not as easy to handle as twisted pair cables. The IEEE standards for a twisted pair version of Ethernet, known as 10BaseT, was released during 1990. 10BaseT has a transmission data rate of 10 Mbps over two pairs of Category 3 or Category 5 twisted pair cable. Multiple pairs, 2 or 4, are usually included in a Category 3 or Category 5 twisted pair cable. Twisted pair based Ethernet has a star topology with an active hub at the center in contrast to the bus topology of coaxial cable based Ethernet. The first 100-Mbps version of twisted pair based Ethernet, known as 100BaseTX, was standardized during 1995. 100BaseTX utilizes two out of four pairs of a Category 5 twisted pair cable. Another version of 100-Mbps Ethernet using all four pairs of a Category 3 twisted pair cable was also standardized during 1995 and known as 100BaseT4. A 100-Mbps Ethernet using only two pairs of a Category 3 twisted pair cable was standardized during 1997 and known as 100BaseT2. The 1000-Mbps Ethernet using all four pairs of a Category 5 cable was standardized during 1999 and known as 1000BaseT. Today most sites use twisted pair media for Ethernet connections to the desktop.

In this chapter we will examine details of twisted pair based Ethernet systems. They are becoming the most used types. We can witness an evolution of transmission efficiency and transceiver complexity in terms of Digital Signal Processing (DSP) and error correction coding techniques by reviewing different versions starting from 10BaseT. The first twisted pair based Ethernet 10BaseT uses no DSP except the Manchester line code. 100BaseTX uses 4B5B and MLT3 line codes and requires a channel equalizer that can be implemented with either analog operational amplifiers or digitally for a better performance. 100BaseT4 uses 8B6T half duplex line code and also requires some channel equalization. On the contrary, 100BaseT2 uses extensive DSP techniques including echo cancellation, NEXT cancellation, and channel equalization. 1000BaseT achieves its very high transmission throughput by using error correction coding on top of all these DSP techniques over four pairs of a Category 5 twisted pair cable. All different types of Ethernet use the same CSMA/CD MAC protocol. All twisted pair based Ethernet uses the same RJ45 connector. Different versions of twisted pair based Ethernet can be identified with the standardized autonegotiation process.

We start this chapter with frame structure and MAC protocol, which are common features for all versions of Ethernet. We then look into each twisted pair based Ethernet version individually in the sequence of 10BaseT, 100BaseT4, 100BaseTX, and 100BaseT2. The discussion of 1000BaseT is postponed as a future topic. For each version, we highlight features of the standards, examine its typical transceiver structure, and study corresponding transmission performance. Other common features known as autonegotiation and Media Independent Interface are also discussed.