Content Overview Ethernet has been the most successful LAN technology largely because of its simplicity of implementation compared to other technologies. Ethernet has also been successful because it has been a flexible technology that has evolved to meet changing needs and media capabilities. This module introduces the specifics of the most important varieties of Ethernet. The goal is not to convey all the facts about each type of Ethernet, but rather to develop a sense of what is common to all forms of Ethernet. Changes in Ethernet have resulted in major improvements over the 10-Mbps Ethernet of the early 1980s. The 10-Mbps Ethernet standard remained virtually unchanged until 1995 when IEEE announced a standard for a 100 Mbps Fast Ethernet. In recent years, an even more rapid growth in media speed has moved the transition from Fast Ethernet to Gigabit Ethernet. The standards for Gigabit Ethernet emerged in only three years. An even faster Ethernet version, 10 Gigabit Ethernet, is now widely available and still faster versions are being developed. In these faster versions of Ethernet, MAC addressing, CSMA/CD, and the frame format have not been changed from earlier versions of Ethernet. However, other aspects of the MAC sublayer, physical layer, and medium have changed. Copper-based network interface card (NICs) capable of 10/100/1000 operation are now common. Gigabit switch and router ports are becoming the standard for wiring closets. Optical fiber to support Gigabit Ethernet is considered a standard for backbone cabling in most new installations. Students completing this module should be able to:
Content 7.1 10-Mbps and 100-Mbps Ethernet 7.1.1 10-Mbps Ethernet 10BASE5, 10BASE2, and 10BASE-T Ethernet are considered Legacy Ethernet. The four common features of Legacy Ethernet are timing parameters, frame format, transmission process, and a basic design rule. 10BASE5, 10BASE2, and 10BASE-T all share the same timing parameters, as shown in Figure (1 bit time at 10 Mbps = 100 nsec = 0.1 µsec = 1 ten-millionth of a second.) 10BASE5, 10BASE2, and 10BASE-T also have a common frame format. The Legacy Ethernet transmission process is identical until the lower part of the OSI physical layer. The Layer 2 frame data is converted from hex to binary. As the frame passes from the MAC sublayer to the physical layer, further processes occur prior to the bits being placed from the physical layer onto the medium. One important process is the signal quality error (SQE) signal. SQE is always used in half-duplex. SQE can be used in full-duplex operation but is not required. SQE is active: All 10 Mbps forms of Ethernet take octets received from the MAC sublayer and perform a process called line encoding. Line encoding describes how the bits are actually signaled on the wire. The simplest encodings have undesirable timing and electrical characteristics. So line codes have been designed to have desirable transmission properties. This form of encoding used in 10 Mbps systems is called “Manchester.” Manchester encoding relies on the direction of the edge transition in the middle of the timing window to determine the binary value for that bit period. The top waveform has a falling edge, so it is interpreted as a binary 0. The second waveform shows a rising edge, which is interpreted as a binary 1. In the third waveform, there is an alternating binary sequence. With alternating binary data, there is no need to return to the previous voltage level. As can be seen from the third and fourth wave forms in the graphic, the binary bit values are indicated by the direction of change during any given bit period. The waveform voltage levels at the beginning or end of any bit period are not factors when determining binary values. Legacy Ethernet has common architectural features. Networks usually contain multiple types of media. The standard ensures that interoperability is maintained. The overall architectural design is of the utmost importance when implementing a mixed-media network. It becomes easier to violate maximum delay limits as the network grows. The timing limits are based on parameters such as: 10-Mbps Ethernet operates within the timing limits offered by a series of not more than five segments separated by no more than four repeaters. This is known as the 5-4-3 rule. No more than four repeaters may be connected in series between any two distant stations. There can also be no more than three populated segments between any two distant stations. Web Links Ethernet Encapsulation Cheat Sheet http://www.cisco.com/warp/public/105/ encheat.html
Content 7.1 10-Mbps and 100-Mbps Ethernet 7.1.2 10BASE5 The original 1980 Ethernet product 10BASE5 transmitted 10 Mbps over a single thick coaxial cable bus. 10BASE5 is important because it was the first medium used for Ethernet. 10BASE5 was part of the original 802.3 standard. The primary benefit of 10BASE5 was length. Today it may be found in legacy installations, but would not be recommended for new installations. 10BASE5 systems are inexpensive and require no configuration, but basic components like NICs are very difficult to find as well as the fact that it is sensitive to signal reflections on the cable. 10BASE5 systems also represent a single point of failure. 10BASE5 uses Manchester encoding. It has a solid central conductor. Each of the maximum five segments of thick coax may be up to 500 m (1640.4 ft) in length. The cable is large, heavy, and difficult to install. However, the distance limitations were favorable and this prolonged its use in certain applications. Because the medium is a single coaxial cable, only one station can transmit at a time or else a collision will occur. Therefore, 10BASE5 only runs in half-duplex resulting in a maximum of 10 Mbps of data transfer. Figure illustrates one possible configuration for a maximum end-to-end collision domain. Between any two distant stations only three repeated segments are permitted to have stations connected to them, with the other two repeated segments used only as link segments to extend the network. Lab Activity Lab Exercise: Waveform Decoding This lab is to integrate knowledge of networking media, OSI Layers 1, 2, and 3, and Ethernet, by taking a digital waveform of an Ethernet frame and decoding it. Interactive Media Activity Matching: 10BASE5 After completing this activity, the student will learn the characteristics of 10BASE5 technology. Web Links 10BASE5 http://www.usyd.edu.au/is/comms/networkcourse/ USydNet