4.3 ETHERNET

• Two kinds of Ethernet exist: classic Ethernet, which solves the multiple access problem using the techniques we have studied in this chapter; and switched Ethernet, in which devices called switches are used to connect different computers.
• They called the system Ethernet after the luminiferous ether, through which lectromagnetic radiation was once thought to propagate.
• The story of Ethernet starts about the same time as that of ALOHA, when a student named Bob Metcalfe got his bachelor’s degree at M.I.T. and then moved up the river to get his Ph.D. at Harvard.
• When the 19th-century British physicist James Clerk Maxwell discovered that electromagnetic radiation could be described by a wav e equation, scientists assumed that space must be filled with some ethereal medium in which the radiation was propagating. Only after the famous Michelson-Morley experiment in 1887 did physicists discover that electromagnetic radiation could propagate in a vacuum.)
• The Xerox Ethernet was so successful that DEC, Intel, and Xerox drew up a standard in 1978 for a 10-Mbps Ethernet, called the DIX standard. With a minor change, the DIX standard became the IEEE 802.3 standard in 1983
• Classic Ethernet snaked around the building as a single long cable to which all the computers were attached. This architecture is shown in Fig. 4-13. The first variety, popularly called thick Ethernet, resembled a yellow garden hose, with markings every 2.5 meters to show where to attach computers.
• The 802.3 standard did not actually require the cable to be yellow, but it did suggest it.) It was succeeded by thin Ethernet, which bent more easily and made connections using industry-standard BNC connectors. Thin Ethernet was much cheaper and easier to install, but it could run for only 185 meters per segment (instead of 500 m with thick Ethernet), each of which could handle only 30 machines (instead of 100).
• To allow larger networks multiple cables can be connected by repeaters. A repeater is a physical layer device that receives, amplifies (i.e., regenerates), and retransmits signals in both directions.
• First comes a Preamble of 8 bytes, each containing the bit pattern 10101010 (with the exception of the last byte, in which the last 2 bits are set to 11). This last byte is called the Start of Frame delimiter for 802.3.
• When a frame is sent to a group address, all the stations in the group receive it. Sending to a group of stations is called multicasting. The special address consisting of all 1 bits is reserved for broadcasting
• To do this, the first 3 bytes of the address field are used for an OUI (Organizationally Unique Identifier).
• Ethernet uses a Type field to tell the receiver what to do with the frame.
• The Type field specifies which process to give the frame to
• Now the rule is that any number there less than or equal to 0x600 (1536) can be interpreted as Length, and any number greater than 0x600 can be interpreted as Type.
• If the data portion of a frame is less than 46 bytes, the Pad field is used to fill out the frame to the minimum size.
• The final field is the Checksum. It is a 32-bit CRC of the kind we studied in Sec. 3.2. In fact, it is defined exactly by the generator polynomial we gav e there, which popped up for PPP, ADSL, and other links too
• Classic Ethernet uses the 1-persistent CSMA/CD algorithm
• This algorithm, called binary exponential backoff, was chosen to dynamically adapt to the number of stations trying to send. If the randomization interval for all collisions were 1023, the chance of two stations colliding for a second time would be negligible, but the average wait after a collision would be hundreds of slot times, introducing significant delay.
• The problems associated with finding breaks or loose connections drove it tow ard a different kind of wiring pattern, in which each station has a dedicated cable running to a central hub. A hub simply connects all the attached wires electrically, as if they were soldered together.
• Fortunately, there is an another way to deal with increased load: switched Ethernet. The heart of this system is a switch containing a high-speed backplane that connects all of the ports
• In a hub, all stations are in the same collision domain.
• Most LAN interfaces have a promiscuous mode, in which all frames are given to each computer, not just those addressed to it
• The work was done quickly (by standards committees’ norms), and the result, 802.3u, was approved by IEEE in June 1995. Technically, 802.3u is not really a new standard, but an addendum to the existing 802.3 standard (to emphasize its backward compatibility). This strategy is used a lot. Since practically everyone calls it fast Ethernet, rather than 802.3u, we will do that, too
• The Category 3 UTP scheme, formally called 100Base-T4, used a signaling speed of 25 MHz, only 25% faster than standard Ethernet’s 20 MHz.
• 100Base-T4 fell by the wayside as many off ice buildings were rewired with Category 5 UTP for 100Base-TX Ethernet, which came to dominate the market. This design is simpler because the wires can handle clock rates of 125 MHz
• Neither straight binary coding (i.e., NRZ) nor Manchester coding is used. Instead, the 4B/5B encoding we described in Sec 2.4.3 is used
• The last option, 100Base-FX, uses two strands of multimode fiber, one for each direction, so it, too, can run full duplex with 100 Mbps in each direction. In this setup, the distance between a station and the switch can be up to 2 km.
• To make upgrading easy, the standard itself provides a mechanism called auto-negotiation that lets two stations automatically negotiate the optimum speed (10 or 100 Mbps) and duplexity (half or full).
• The ink was barely dry on the fast Ethernet standard when the 802 committee began working on a yet faster Ethernet, quickly dubbed gigabit Ethernet.
• The first feature, called carrier extension, essentially tells the hardware to add its own padding after the normal frame to extend the frame to 512 bytes.
• The second feature, called frame bursting, allows a sender to transmit a concatenated sequence of multiple frames in a single transmission.
• To send bits over these versions of gigabit Ethernet, the 8B/10B encoding we described in Sec. 2.4.3 was borrowed from another networking technology called Fibre Channel.
• Jumbo frames allow for frames to be longer than 1500 bytes, usually up to 9 KB.
• 10GBase-ER can run for distances of 40 km, making it suitable for wide area applications. All of these versions send a serial stream of information that is produced by scrambling the data bits, then encoding them with a 64B/66B code. This encoding has less overhead than an 8B/10B code
• Much effort is being put into carrier-grade Ethernet to let network providers offer Ethernet-based services to their customers for metropolitan and wide area networks