6XGIZOIGR:)6/6GTJ+ZNKXTKZ4KZ]UXQOTM In LANs, polling provides a deterministic media access method in which the server polls each node in succession to determine whether that node wants to access the network. In some systems, the polling is done by means of software messages being passed to and fro, which could slow down the process. In order to overcome this problem, systems such as 100VG Any LAN employ a hardware-polling message, which uses voltage levels to determine whether a node wants to be serviced. 3 +ZNKXTKZTKZ]UXQY 5HPKIZO\KY When you have completed study of this chapter you should be able to: • Describe the major hardware components of an IEEE 802.3 CSMA/CD network • Explain the method of connection of 10Base5, 10Base2 and 10BaseT networks • Explain the operation of the CSMA/CD protocol • List the fields in the Ethernet data frames • Describe the causes of Ethernet collisions and how to reduce them • Demonstrate how to apply the Ethernet design rules /+++)93')*i+ZNKXTKZj The Ethernet network concept was developed by Xerox Corporation at its Palo Alto Research Center (PARC) in the mid-seventies. It was based on the work done by researchers at the University of Hawaii where there were campus sites on the various islands. Its ALOHA network was set up using radio broadcasts to connect the various sites. This was colloquially known as their ‘Ethernet’ since it used the ‘ether’ as the transmission medium and created a network ‘net’ between the sites. The philosophy was quite straightforward. Any station that wanted to broadcast to another station would do so immediately. The receiving stations then had a responsibility to acknowledge the message; thus advising the original transmitting station of a successful reception of the original message. This primitive system did not rely on any detection of collisions (two radio stations transmitting at the same time) but rather waited for an acknowledgment back within a predefined time. The initial system installed by Xerox was so successful that they soon applied the system to their other sites typically connecting office equipment to shared resources such as printers and large computers acting as repositories of large databases, for example. 6XGIZOIGR:)6/6GTJ+ZNKXTKZ4KZ]UXQOTM In 1980, the Ethernet Consortium consisting of Xerox, Digital Equipment Corporation and Intel (sometimes called the DIX consortium) issued a joint specification based on the Ethernet concepts and known as the Ethernet Blue Book 1 specification. This was later superseded by the Ethernet Blue Book 2 specification, which was offered to the IEEE as a standard. In 1983, the IEEE issued the 802-3 standard for Carrier Sense; Multiple Access; Collision Detect LANs based on the Ethernet standard, which gave this networking standard even more credibility. As a result of this, there are three standards in existence. The first – often-termed Ethernet Version 1 – can be disregarded as very little equipment based on this standard is still in use. Ethernet Version 2 or ‘Blue Book Ethernet’ is, however, still in use and there is a potential for incompatibility with the IEEE 802.3 standard. Whilst these differences are minor, they are nonetheless significant. Despite the generic term ‘Ethernet’ being applied to all CSMA/CD networks, it should be reserved for the original DIX standard. This book will continue with popular use and refer to all the LANs of this type as Ethernet, unless it is important to distinguish between them. The original Ethernet specification was developed around CSMA/CD. Later versions (from 100 Mbps upwards) also support full-duplex, although they support CSMA/CD for the sake of backward compatibility. Industrial versions of Ethernet typically operate at 100 Mbps and above in full-duplex mode, and support the IEEE 802.1p/Q modified frame structure. This allows highly deterministic operation. 6N_YOIGRRG_KX 802.3 standard defines a range of cable types that can be used for a network based on this standard. They include coaxial cable, twisted pair cable and fiber optic cable. In addition, there are different signaling standards and transmission speeds that can be utilized. These include both base band and broadband signaling, and speeds of 1 Mbps and 10 Mbps. The standard is continuing to evolve, and this manual will look at 100 Mbps CSMA/CD systems in the next chapter. The IEEE 802.3 standard documents (ISO 8802.3) support various cable media and transmission rates up to 10 Mb/s as follows: • 10Base2 Thin wire coaxial cable (0.25 inch diameter), 10 Mbps, single cable bus • 10Base5 Thick wire coaxial cable (0.5 inch diameter), 10 Mbps, single cable bus • 10BaseT Unscreened twisted pair cable (0.4 to 0.6 mm conductor diameter), 10 Mbps, twin cable bus • 10BaseF Optical fiber cables, 10 Mbps, twin fiber bus • 1Base5 Unscreened twisted pair cables, 1 Mbps, twin cable bus • 10Broad36 Cable television (CATV) type cable, 10 Mbps, broadband +ZNKXTKZTKZ]UXQY (GYKY_YZKSY This is a coaxial cable system and uses the original cable for Ethernet systems – generically called ‘Thicknet’. It is a coaxial cable, of 50-ohm characteristic impedance, and yellow or orange in color. The naming convention for 10Base5: means 10 Mbps; base band signaling on a cable that will support 500-meter segment lengths. It is difficult to work with, and so cannot normally be taken to the node directly. Instead, it is laid in a cabling tray etc and the transceiver electronics (medium attachment unit, MAU) is installed directly on the cable. From there an intermediate cable, known as an attachment unit interface (AUI) cable is used to connect to the NIC. This cable can be a maximum of 50 meters long, compensating for the lack of flexibility of placement of the segment cable. The AUI cable consists of 5 individually shielded pairs – two each (control and data) for both transmit and receive; plus one for power. Cutting the cable and inserting an N-connector and a coaxial Tee or more commonly by using a ‘bee sting’ or ‘vampire’ tap can make the MAU connection to the cable. This is a mechanical connection that clamps directly over the cable. Electrical connection is made via a probe that connects to the center conductor and sharp teeth, which physically puncture the cable sheath to connect to the braid. These hardware components are shown in Figure 3.1. Figure 3.1 10Base5 hardware components The location of the connection is important to avoid multiple electrical reflections on the cable, and the Thicknet cable is marked every 2.5 meters with a black or brown ring to indicate where a tap should be placed. Fan out boxes can be used if there are a number of nodes for connection, allowing a single tap to feed each node as though it was individually connected. The connection at either end of the AUI cable is made through a 25-pin D-connector, with a slide latch, often called a DIX connector after the original consortium. 6XGIZOIGR:)6/6GTJ+ZNKXTKZ4KZ]UXQOTM Figure 3.2 AUI cable connectors There are certain requirements if this cable architecture is used in a network. These include: • Segments must be less than 500 meters in length to avoid signal attenuation problems • No more than 100 taps on each segment i.e. not every potential connection point can support a tap • Taps must be placed at integer multiples of 2.5 meters • The cable must be terminated with a 50-ohm terminator at each end • One end of the cable shield must be earthed The physical layout of a 10Base5 Ethernet segment is shown in Figure 3.3. Figure 3.3 10Base5 Ethernet segment The Thicknet cable was extensively used as a backbone cable until recently but 10BaseT and fiber is becoming more popular. Note that when a MAU (tap) and AUI cable is used, the on board transceiver on the NIC is not used. Rather, there is a transceiver in the MAU and this is fed with power from the NIC via the AUI cable. Since the transceiver is remote from the NIC, the node needs to be aware that the termination can detect collisions if they occur. This confirmation is performed by a signal quality error (SQE), or heartbeat, test function in the MAU. The SQE signal is sent from the MAU to the node on detecting a collision on the bus. However, on completion of every . signaling standards and transmission speeds that can be utilized. These include both base band and broadband signaling, and speeds of 1 Mbps and 10 Mbps. The standard is continuing to evolve, and this. based on this standard is still in use. Ethernet Version 2 or ‘Blue Book Ethernet is, however, still in use and there is a potential for incompatibility with the IEEE 802.3 standard. Whilst. based on the Ethernet standard, which gave this networking standard even more credibility. As a result of this, there are three standards in existence. The first – often-termed Ethernet Version