Network+ Certification Training Kit, Second Edition by Microsoft Corporation

From the Publisher

The Network+ certification offered through CompTIA, the Computing Technology Industry Association, measures industry-standard knowledge of networking technology and practices for computer service technicians. And with the help of this competency-based, self-paced training kit, professionals can advance their real-world expertise as they prepare for the corresponding skill areas of the Network+ exam. The Network+ Certification Training Kit delivers a thorough, vendor-neutral study of baseline networking knowledge, including in-depth coverage of TCP/IP. The kit is modular and self-paced, with hands-on, skill-building exercises; the entire book is featured on CD-ROM for easy searches and reference. The Network+ credit is accepted by Intel, Novell, and Lotus towards their certification tracks, and it can be an important first step toward earning certification through the Microsoft® MCSE program. With the Network+ Certification Training Kit, IT professionals can demonstrate essential knowledge and set their own pace for career advancement!

Booknews

An introduction to the basics of computer networking, for beginning IT professionals interested in careers as network administrators or support technicians. It's designed to prepare readers to take the Network+ Certification exam administered by the Computing Technology Industry Association. Covers planning, installation, configuration, maintenance, and trouble-shooting of industry-standard networking technology, including TCP/IP networking. The guide assumes familiarity with the workings of personal computers and the uses of a data network. Annotation c. Book News, Inc., Portland, OR (booknews.com)

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Table of Contents

Read an Excerpt

Chapter 2: Network Hardware

Lesson 1: Network Cables

• List the cabling topologies used to build networkss
• Name the types of cables used to build local area networks
• Understand the grading systems used for the various cable types

Estimated lesson time: 40 minutes

Cable Topologies

The three primary topologies used to build LANs are as follows:

• Bus
• Star
• Ring

The Bus Topology

A bus network is one in which the computers are connected in a single line, with each system logically cabled to the next system. Bus networks are illustrated in Figure 2.1. Early Ethernet systems used the bus topology with coaxial cable, a type of network that is rarely seen today. The cabling of a bus network can take two forms: thick and thin. Thick Ethernet networks use a single length of coaxial cable and connect the computers to it using smaller individual cables called transceiver cables, as shown on the top half of Figure 2.1. Thin Ethernet networks use separate lengths of a narrower type of coaxial cable, and each length of cable connects one computer to the next, as shown in the bottom half of Figure 2.1.

Run the c02dem01 video located in the Demos folder on the CD-ROM accompanying this book for a demonstration of Thin Ethernet bus topology connections.

Click to view graphic

Figure 2.1 Bus topology cabling options

When any one of the computers on the network transmits data, the signals travel down the cable in both directions, reaching all of the other systems. A bus network always has two open ends, which must be terminated. Termination is the process of installing a resistor pack at each end of the bus to negate the signals that arrive there. Without terminators, the signals reaching the end of the bus would reflect back in the other direction and interfere with the newer signals being transmitted.

Run the c02dem02, c02dem03, c02dem04, c02dem05, and c02dem06 videos located in the Demos folder on the CD-ROM accompanying this book for a demonstration of bus topology communications, signal bounce, and termination.

The main problem with the bus topology is that a single faulty connector, terminator, or break in the cable affects the functionality of the entire network. Signals that cannot pass beyond a certain point fail to reach all of the computers beyond that point. In addition, the break in the cable is also unterminated. On the half of the network that does receive the transmitted signals, the data can be affected by reflected signals. This is one of the primary reasons that bus networks are almost never used nowadays.

Run the c02dem07 and c02dem08 videos located in the Demos folder on the CD-ROM accompanying this book for a demonstration of a bus topology failure.

The Star Topology

While the bus topology has the computers in a network logically connected directly to each other, the star topology uses a central cabling nexus called a hub or concentrator. In a star network, each computer is connected to the hub using a separate cable, as shown in Figure 2.2. Most LANs installed today use the star topology. LANs can use several different cable types, including various twisted pair and fiber optic configurations. The main advantage of the star network is that each computer has its own dedicated connection to the hub. If a single cable or connector should fail, only one computer is affected.

Run the c02dem09, c02dem10, and c02dem11 videos located in the Demos folder on the CD-ROM accompanying this book for a demonstration of star topology.

Click to view graphic

Figure 2.2 The star topology uses an individual connection for each computer to provide a greater measure of fault tolerance than the bus topology.

The disadvantage of the star topology is that an additional piece of hardware, the hub, is required to implement it. If the hub should fail, the entire network goes down. However, this is a relatively rare occurrence, since hubs are usually found in a protected environment, such as a data center or server closet.

The Ring Topology

As far as signal transmissions are concerned, a ring network is like a bus in that each computer is logically connected to the next. The difference is that in a ring network the two ends are connected instead of being terminated. This enables a signal originating on one computer to travel around the ring to all of the other computers and eventually back to its point of origin. Networks such as Token Ring, which use token passing for their media access control (MAC) mechanism (as explained in Lesson 2: The OSI Reference Model, in Chapter 1, "Networking Basics"), are wired using a ring topology. The most important thing to understand about the ring topology, however, is that it is strictly a logical construction, not a physical one. Or, to be more precise, the ring exists in the wiring of the network, but not in the cabling.

When you look at a network that uses the ring topology, you may be puzzled to see what looks like a star. In fact, the cables for a ring network connect to a hub and take the form of a star. The ring topology is actually implemented logically, using the wiring inside the cables. Ring networks use a special type of hub, called a multistation access unit (MAU), which receives data through one port and transmits it out through the next. This process continues until the MAU has transmitted the signals to each computer on the ring. If you were to remove the wires from the cable sheath, you would have a circuit that runs from the MAU to each computer and back to the MAU, as shown in Figure 2.3.

Run the c02dem12, c02dem13, c02dem14, and c02dem15 videos located in the Demos folder on the CD-ROM accompanying this book for a demonstration of the ring topology.

Click to view graphic

Figure 2.3 A ring network uses a ring topology in a logical sense only. The cables are actually arranged in the form of a star.

The design of the star topology used by the ring makes it possible for the network to function even when a cable or connector fails. The MAU contains special circuitry that removes a malfunctioning workstation from the ring. By comparison, a network that is literally cabled as a ring would have no MAU, but that would cause the network to cease to function in the event of a cable failure.

Run the c02dem16 video located in the Demos folder on the CD-ROM accompanying this book for a demonstration of a ring topology failure.

Cable Types

Coaxial Cable

Coaxial cable is so named because it contains two conductors within the sheath. Unlike other two-conductor cables, however, coaxial cable has one conductor inside the other. This is illustrated in Figure 2.4. At the center of the cable is the copper core, which actually carries the electrical signals. The core can be solid copper or composed of braided strands of copper. Surrounding the core is a layer of insulation, and surrounding that is the second conductor, which is typically made of braided copper mesh. This second conductor functions as the cable’s ground. Finally, the entire assembly is encased in an insulating sheath made of PVC or Teflon.

Click to view graphic

Figure 2.4 Coaxial cable consists of two electrical conductors sharing the same axis, with insulation in between and encased in a protective sheath.

There are two types of coaxial cable that have been used in local area networking: RG8, also known as Thick Ethernet, and RG58, which is known as Thin Ethernet. These two cables are similar in construction but differ primarily in their thickness (0.405 inches for RG8 versus 0.195 inches for RG58) and in the types of connectors they use (N connectors for RG8 and BNC connectors for RG58). Both cable types are wired using a bus topology.

Coaxial cable is used today for many applications, most noticeably on cable television networks, but it has fallen out of favor as a LAN medium. This is due to the bus topology’s fault-tolerance problems and the size and relative inflexibility of the cables, which make them difficult to install and maintain.

Twisted Pair Cable

Twisted pair cable wired in a star topology is the most common type of network medium used in LANs today. Most of the LANs installed today use unshielded twisted pair (UTP) cable, but there is also a shielded twisted pair (STP) variety for use in environments more prone to electromagnetic interference. UTP cable contains eight separate conductors, as opposed to the two used in coaxial cable. Each conductor is a separate insulated wire, and the eight wires are arranged in four pairs of twisted conductors. The twists prevent the signals on the different wire pairs from interfering with each other (called crosstalk) and also provide resistance to outside interference. The four wire pairs are then encased in a single sheath, as shown in Figure 2.5. The connectors used for twisted pair cables are called RJ45s; they are the same as the connectors used on standard telephone cables, except that they have eight electrical contacts instead of four.

Click to view graphic

Figure 2.5  UTP cable has four separate wire pairs, each pair individually twisted, enclosed in a protective sheath.

Twisted pair cable has been used for telephone installations for decades; its adaptation to LAN use is relatively recent. Twisted pair cable has replaced coaxial cable in the data networking world, because it has several distinct advantages. First, because it contains eight separate wires, the cable is more flexible than the more solidly constructed coaxial cable. This makes it easier to flex, which simplifies installation. The second major advantage is that there are thousands of qualified telephone cable installers who can easily adapt to installing LAN cables as well. In new construction, telephone and LAN cables are often installed at the same time, by the same contractor.

UTP Cable Grades

UTP cable comes in a variety of different grades, called "categories" by the Electronics Industry Association (EIA) and the Telecommunications Industry Association (TIA), the combination being referred to as EIA/TIA. These categories are listed in Table 2.1. The two most significant UTP grades for LAN use are Category 3 and Category 5. Category 3 cable was designed for voice-grade telephone networks and eventually came to be used for Ethernet. Category 3 cable is sufficient for 10 Mbps Ethernet networks (where it is called 10BaseT), but it is generally not used for Fast Ethernet (except under certain conditions). If you have an existing Category 3 cable installation, you can use it to build a standard Ethernet network, but virtually all new UTP cable installations today use at least Category 5 cable.

Table 2.1  EIA/TIA UTP cable grades

Category 1	Used for voice-grade telephone networks only; not for data transmissions
Category 2	Used for voice-grade telephone networks, as well as IBM dumb-terminal connections to mainframe computers
Category 3	Used for voice-grade telephone networks, 10 Mbps Ethernet, 4 Mbps Token Ring, 100BaseT4 Fast Ethernet, and 100VG AnyLAN
Category 4	Used for 16 Mbps Token Ring networks
Category 5	Used for 100BaseTX Fast Ethernet, SONet, and OC-3 ATM
Category 5e	Used for Gigabit (1000 Mbps) Ethernet protocols

Category 5 UTP is suitable for 100BaseTX Fast Ethernet networks running at 100 Mbps, as well as for slower protocols. In addition to the officially ratified EIA/TIA categories, there are other UTP cable grades available that have not yet been standardized. A cable standard called Level 5 by a company called Anixter, Inc. is currently being marketed using names such as Enhanced Category 5. This cable increases the bandwidth of Category 5 from 100 to 350 MHz, making it suitable to run the latest Gigabit Ethernet protocol at 1,000 Mbps (1 Gbps). Level 6 cable increases the bandwidth even further.

STP Cable Grades

Shielded twisted pair cable is similar in construction to UTP, except that it has only two pairs of wires and it also has additional foil or mesh shielding around each pair. The additional shielding in STP cable makes it preferable to UTP in installations where electromagnetic interference is a problem, often due to the proximity of electrical equipment. The various types of STP cable were standardized by IBM, who developed the Token Ring protocol that originally used them. STP networks use Type 1A for longer cable runs and Type 6A for patch cables. Type 1A contains two pairs of 22 gauge solid wires with foil shielding, and Type 6A contains two pairs of 26 gauge stranded wires with foil or mesh shielding. Token Ring STP networks also use large, bulky connectors called IBM data connectors (IDCs). However most Token Ring LANs today use UTP cable.

Fiber Optic Cable

Fiber optic cable is a completely different type of network medium. Instead of carrying signals over copper conductors in the form of electrical voltages, fiber optic cables transmit pulses of light over a glass or plastic conductor. Fiber optic cable is completely resistant to the electromagnetic interference that so easily affects copper-based cables. Fiber optic cables are also much less subject to attenuation than are copper cables. Attenuation is the tendency of a signal to weaken as it travels over a cable. The longer the cable, the weaker the signal gets. On copper cables, signals weaken to the point of unreadability after 100 to 500 meters (depending on the type of cable). Some fiber optic cables, by contrast, can span distances up to 120 kilometers without excessive signal degradation. This makes fiber optic the medium of choice for installations that span long distances or that connect buildings on a campus. Fiber optic cable is also inherently more secure than copper, because it is not possible to tap into a fiber optic link without affecting the normal communication over that link.

A fiber optic cable, as illustrated in Figure 2.6, consists of a clear glass or clear plastic core that actually carries the light pulses, and is surrounded by a reflective layer called the cladding. Around the cladding is a plastic spacer layer, a protective layer of woven Kevlar fibers, and an outer sheath.

Click to view graphic

Figure 2.6 Fiber optic cable has a glass or plastic core surrounded by cladding that reflects the light pulses back and forth along the cable’s length.

There are two primary types of fiber optic cable, called singlemode and multimode. The difference between the two is in the thickness of the core and the cladding. The measurements are the primary specifications used to identify each type of cable. Singlemode fiber typically has a core diameter of 8.3 microns, and the thickness of the core and cladding together is 125 microns. You will generally see this referred to as 8.3/125 singlemode fiber. Multimode fiber is usually rated as 62.5/125.

Singlemode fiber uses a single-wavelength laser as a light source, and as a result, it can carry signals for extremely long distances. For this reason, singlemode fiber is more commonly found in outdoor installations that span long distances, such as telephone and cable television networks. This type of cable is less suited to LAN installations because it is much more expensive than multimode and has a higher bend radius, meaning that it cannot be bent around corners as tightly. Multimode fiber, by contrast, uses a light emitting diode (LED) as a light source instead of a laser and carries multiple wavelengths. Multimode fiber cannot span distances as long as singlemode, but it bends around corners better and is much cheaper.

Installing fiber optic cable is very different from any copper cable installation. The tools and testing equipment required for installation are different, as are the cabling guidelines. Generally speaking, fiber optic cable is quite a bit more expensive than twisted pair or coaxial in every way, although prices have come down in recent years.

Exercise 2.1: Network Cable Types

Read a Sample Chapter

• About This Chapter
• Before You Begin
• Lesson 1: Ethernet
• Ethernet Standards
• Physical Layer Specifications
• The Ethernet Frame
• CSMA/CD
• Exercise 1: CSMA/CD Procedures
• Lesson Review
• Lesson Summary
• Lesson 2: Token Ring
• Physical Layer Specifications
• Token Passing
• Token Ring Frames
• Exercise 1: IEEE Standards and Technologies
• Exercise 2: Selecting a Data-Link Layer Protocol
• Lesson Review
• Lesson Summary
• Lesson 3: FDDI
• The FDDI Physical Layer
• The FDDI Frames
• Exercise 1: FDDI Concepts
• Lesson Review
• Lesson Summary
• Lesson 4: Wireless Networking
• The IEEE 802.11 Physical Layer
• The IEEE 802.11 MAC Layer
• Exercise 1: IEEE 802.11 Concepts
• Lesson Review
• Lesson Summary

About This Chapter

The protocol operating at the data-link layer of the Open Systems Interconnection (OSI) reference model describes the nature of the network medium and performs the final preparation of outgoing data before it is transmitted. This protocol also receives incoming data, evaluates it, and, if necessary, passes it on to the appropriate network layer protocol. This chapter examines the protocols most commonly found at the data-link layer and how they affect the performance of the network. These protocols are vital to any study of computer networking, as they determine how the network is constructed and how computers actually transmit and receive data.

Before You Begin

This chapter requires a basic understanding of the OSI reference model, as described in Chapter 1, "Networking Basics," and familiarity with the hardware components of the network, as examined in Chapter 2, "Network Hardware."

Lesson 1: Ethernet

Ethernet is the most popular local area network (LAN) protocol operating at the data-link layer; it has been for decades. In most cases, when people talk about a LAN, they are referring to an Ethernet LAN. The Ethernet protocol was developed in the 1970s and has since been upgraded repeatedly to satisfy the changing requirements of networks and network users. Today's Ethernet networks run at speeds of 10, 100, and 1000 Mbps (1 Gbps), enabling them to fulfill roles ranging from home and small business networks to high-capacity backbones.

• List the Ethernet physical layer standards
• Describe the functions of the Ethernet frame
• Understand the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Media Access Control (MAC) mechanism

Estimated lesson time: 50 minutes

Ethernet Standards

There have been two sets of Ethernet standards over the years. The first was the original Ethernet protocol, as developed by Digital Equipment Corporation, Intel, and Xerox, which came to be known as DIX Ethernet. The DIX Ethernet standard was first published in 1980 and defined a network running at 10 Mbps using RG-8 coaxial cable in a bus topology. This standard is known as thick Ethernet, ThickNet, or 10Base5. The DIX Ethernet II standard, published in 1982, added a second physical layer option to the protocol using RG-58 coaxial cable. This standard is called thin Ethernet, ThinNet, Cheapernet, or 10Base2.

Around the same time that these standards were published, an international standards-making body called the Institute of Electrical and Electronic Engineers (IEEE) set about creating an international standard defining this type of network, which would not be held in private hands, as was the DIX Ethernet standard. In 1980, the IEEE assembled what they called a working group with the designation IEEE 802.3 that began the development of an Ethernet-like network standard. They couldn't call their network Ethernet because Xerox had trademarked the name, but in 1985, they published IEEE 802.3 Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications. This document included specifications for networks using the same two coaxial cable options as DIX Ethernet and, after further development, added a specification for an unshielded twisted pair (UTP) cable option known as 10Base-T. Additional documents published by the IEEE 802.3 working group in later years include IEEE 802.3u in 1995, which defines the 100-Mbps Fast Ethernet specifications, and IEEE 802.3z and IEEE 802.3ab, which are the 1000-Mbps Gigabit Ethernet standards.

The IEEE 802.3 standard differs only slightly from the DIX Ethernet standard. The IEEE standard contains additional physical layer options, as already noted, and some differences in the frame format. Despite the continued use of the name Ethernet in the marketplace, however, the protocol that networks use today is actually IEEE 802.3, because this version provides the additional physical layer options and the Fast Ethernet and Gigabit Ethernet standards. Development of the DIX Ethernet standards ceased after Ethernet II, and coaxial cable Ethernet is all but obsolete. When people use the term Ethernet today, it is understood that they actually mean IEEE 802.3. The only element of the DIX Ethernet standard still in common use is the Ethernet II frame format, which contains the Ethertype field that is used to identify the network layer protocol that generated the data in each packet.

Both the IEEE 802.3 and DIX Ethernet standards consist of the following three basic components:

• Physical layer specifications
• Frame format
• CSMA/CD MAC mechanism

Physical Layer Specifications

The physical layer specifications included in the Ethernet standards describe the types of cables you can use to build the network, define the topology, and provide other crucial guidelines, such as the maximum cable lengths and the number of repeaters you can use. The basic specifications for the Ethernet physical layer options are listed in Table 5.1. Observing these guidelines is an important part of building a reliable Ethernet network, because they limit the effects of problems like attenuation and crosstalk, which are common to all networks and can inhibit the functionality of the CSMA/CD mechanism. The precise timing involved in Ethernet's collision detection mechanism makes the length of the network cables and the number of repeaters used to build the network highly significant to its smooth operation.

Table 5.1  Ethernet Physical Layer Specifications

Designation	Cable Type	Topology	Speed	Maximum Segment Length
10Base5	RG-8 coaxial	Bus	10 Mbps	500 meters
10Base2	RG-58 coaxial	Bus	10 Mbps	185 meters
10Base-T	Category 3 UTP	Star	10 Mbps	100 meters
Fiber Optic Inter-Repeater Link (FOIRL)	62.5/125 multimode fiber optic	Star	10 Mbps	1000 meters
10Base-FL	62.5/125 multimode fiber optic	Star	10 Mbps	2000 meters
10Base-FB	62.5/125 multimode fiber optic	Star	10 Mbps	2000 meters
10Base-FP	62.5/125 multimode fiber optic	Star	10 Mbps	500 meters
100Base-TX	Category 5 UTP	Star	100 Mbps	100 meters
100Base-T4	Category 3 UTP	Star	100 Mbps	100 meters
100Base-FX	62.5/125 multimode fiber optic	Star	100 Mbps	412 meters
1000Base-LX	9/125 singlemode fiber optic	Star	1000 Mbps	5000 meters
1000Base-LX	50/125 or 62.5/125 multimode fiber optic	Star	1000 Mbps	550 meters
1000Base-SX	50/125 multimode fiber optic (400 MHz)	Star	1000 Mbps	500 meters
1000Base-SX	50/125 multimode fiber optic (500 MHz)	Star	1000 Mbps	550 meters
1000Base-SX	62.5/125 multimode fiber optic (160 MHz)	Star	1000 Mbps	220 meters
1000Base-SX	62.5/125 multimode fiber optic (200 MHz)	Star	1000 Mbps	275 meters
1000Base-LH	9/125 singlemode fiber optic	Star	1000 Mbps	10 km
1000Base-ZX	9/125 singlemode fiber optic	Star	1000 Mbps	100 km
1000Base-CX	150-ohm shielded copper cable	Star	1000 Mbps	25 meters
1000Base-T	Category 5 (or 5E) UTP	Star	1000 Mbps	100 meters

Coaxial Ethernet

The coaxial Ethernet standards (10Base5 and 10Base2) are the only ones that call for a bus topology. The maximum segment length indicates the length of the entire bus, from one terminator to the other, with all of the computers in between, as shown in Figure 5.1. A cable segment that connects more than two computers is called a mixing segment. The coaxial standards are no longer in use, except on a few older networks, because coaxial cable is more difficult to install and maintain than UTP and it is limited to a maximum speed of 10 Mbps.

Figure 5.1  Ethernet's coaxial cable specifications use a mixing segment to connect multiple computers to the network (Image unavailable)

UTP Ethernet

All of the other Ethernet physical layer specifications use the star topology, in which a separate cable segment connects each computer to a hub. A cable segment that connects only two devices is called a link segment. UTP is the most popular type of cable used on Ethernet networks today because it is easy to install and it is upgradeable from 10 Mbps to 100 or even 1000 Mbps. 10Base-T Ethernet uses link segments up to 100 meters long to connect computers to a repeating hub, which enables the incoming signals to go out to a computer another 100 meters away, as shown in Figure 5.2. 10Base-T uses only two of the four wire pairs in the cable, one pair for transmitting data and one pair for receiving it.

Figure 5.2  UTP cables can connect Ethernet systems to a hub 100 meters away, and the hub repeats the signal to another hub or computer (Image unavailable)

The Fast Ethernet standard (IEEE 802.3u) includes two UTP cable specifications, known collectively as 100Base-T, both of which retain the 100-meter maximum segment length. 100Base-TX does this by requiring a higher grade of cable, Category 5 (the current industry standard), which provides better signal transmission capabilities. 100Base-T4, however, provides increased speed using the same Category 3 cable as older Ethernet and telephone networks. The difference between the two is that 100Base-TX uses only two pairs of wires, just like 10Base-T, whereas 100Base-T4 uses all four wire pairs. In addition to the transmit and receive pairs, 100Base-T4 uses the other two pairs for bidirectional communications.

Most of the physical layer specifications for Gigabit Ethernet defined in the IEEE 802.3z standard use fiber optic cable, but there is one UTP option, defined in a separate document called IEEE 802.3ab, that does not. The 1000Base-T standard, designed specifically as an upgrade for existing UTP networks with 100-meter cable segments, calls for Category 5 cable, but is better serviced by the higher performance cables now being marketed as Enhanced Category 5 or Category 5E. The Electronics Industry Association and Telecommunications Industry Association (EIA/TIA) have officially ratified the Category 5E cable rating, but the rating does not increase the performance of the cable substantially. The bandwidth of Category 5E is the same as that of Category 5, although its requirements for resistance to certain types of crosstalk are increased and some new performance parameters have been added. 1000Base-T achieves its great speed using all four wire pairs, like 100Base-T4, and by using a different signaling scheme called Pulse Amplitude Modulation-5 (PAM-5).

Fiber Optic Ethernet

Fiber optic cable has been an Ethernet physical layer option since its early days. The FOIRL was part of the DIX Ethernet II standard, and the IEEE 802.3 standards later included the 10Base-FL, 10Base-FB, and 10Base-FP specifications that were intended for various types of networks. None of these solutions were extremely popular because running a fiber optic network at 10 Mbps is a terrible waste of potential. Fiber Distributed Data Interface (FDDI, which is not a form of Ethernet) running at 100 Mbps soon became the fiber optic backbone protocol of choice. Later, Fast Ethernet arrived with its own 100 Mbps fiber optic option, 100Base-FX. 100Base-FX uses the same hardware as 10Base-FL, but it limits the length of a cable segment to 412 meters.

Gigabit Ethernet is the newest form of Ethernet, raising network transmission speed to 1000 Mbps. Gigabit Ethernet relies heavily on fiber optic cabling and provides a variety of physical layer options using different types of cable to achieve different segment lengths. Singlemode fiber cable is designed to span extremely long distances, making Gigabit Ethernet suitable for connecting distant networks or large campus backbones.

Cabling Guidelines

Repeating is an essential part of most Ethernet networks, and the standards include rules regarding the number of repeaters you can use on a single LAN. For the original 10-Mbps Ethernet standard, the use of repeaters is governed by the 5-4-3 rule, which states that you can have up to five cable segments, connected by four repeaters, with no more than three of these segments being mixing segments. In the days of coaxial cable networks, this meant that you could have up to three mixing segments of 500 or 185 meters each (for 10Base5 and 10Base2, respectively) populated with multiple computers and connected by two repeaters. You could also add two additional repeaters to extend the network with another two cable segments of 500 or 185 meters each, as long as these were link segments connected directly to the next repeater in line with no intervening computers, as shown in Figure 5.3. A 10Base2 network could therefore span up to 925 meters and a 10Base5 network up to 2500 meters.

Figure 5.3  Coaxial Ethernet networks consist of up to three mixing segments and two link segments, all connected by repeaters (Image unavailable)

On networks using the star topology, all of the segments are link segments, meaning that you can connect up to four repeating hubs using their uplink ports and still adhere to the 5-4-3 rule (see Figure 5.4). As long as the traffic between the two most distant computers doesn't pass through more than four hubs, the network is configured properly. Because the hubs function as repeaters, each 10Base-T cable segment can be up to 100 meters long, for a maximum network span of 500 meters.

Because Fast Ethernet networks run at higher speeds, they can't support as many hubs as 10-Mbps Ethernet. The Fast Ethernet standard defines two types of hubs, Class I and Class II, which must be marked with the appropriate Roman numeral in a circle. Class I hubs connect Fast Ethernet cable segments of different types, such as 100Base-TX to 100Base-T4 or UTP to fiber optic, whereas Class II hubs connect segments of the same type. You can have as many as two Class II hubs on a network, with a total cable length (for all three segments) of 205 meters when using UTP cable and 228 meters using fiber optic cable. Because Class I hubs must perform an additional signal translation, which slows down the transmission process, you can have only one hub on the network, with maximum cable lengths of 200 and 272 meters for UTP and fiber optic, respectively.

Figure 5.4  10Base-T Ethernet networks can have up to four repeating hubs connected together (Image unavailable)

The 1000Base-T cabling guidelines are simple. Because of the high transmission speed, only one repeater is permitted on the network. Although Gigabit Ethernet theoretically supports half-duplex operation with the use of hubs, there are no products like this on the market. All Gigabit Ethernet implementations are full-duplex and use switches to connect the network nodes together.

The Ethernet Frame

One of the primary functions of the Ethernet protocol is to encapsulate the data it receives from the network layer protocol in a frame, in preparation for its transmission across the network. The frame consists of a header and a footer that are divided into fields containing specific information needed to get each packet to its destination. Regular, Fast, and Gigabit Ethernet all use the same frame, the format of which is shown in Figure 5.5.

Figure 5.5  The Ethernet/IEEE 802.3 frame (Image unavailable)

The functions of the Ethernet frame fields are as follows:

• Preamble (7 bytes).  This field contains 7 bytes of alternating 0s and 1s, which the communicating systems use to synchronize their clock signals.
• Start Of Frame Delimiter (1 byte).  This field contains 6 bits of alternating 0s and 1s, followed by two consecutive 1s, which is a signal to the receiver that the transmission of the actual frame is about to begin.
• Destination Address (6 bytes).  This field contains the 6-byte hexadecimal address of the network interface adapter on the local network to which the packet will be transmitted.
• Source Address (6 bytes).  This field contains the 6-byte hexadecimal address of the network interface adapter in the system generating the packet.
• Ethertype/Length (2 bytes).  In the DIX Ethernet frame, this field contains a code identifying the network layer protocol for which the data in the packet is intended. In the IEEE 802.3 frame, this field specifies the length of the data field (excluding the pad).
• Data And Pad (46 to 1500 bytes).  This field contains the data received from the network layer protocol on the transmitting system, which is sent to the same protocol on the destination system. Ethernet frames (including the header and footer, except for the Preamble and Start Of Frame Delimiter) must be at least 64 bytes long; so if the data received from the network layer protocol is less than 46 bytes, the system adds padding bytes to bring it up to its minimum length.
• Frame Check Sequence (4 bytes).  The frame's footer is a single field that comes after the network layer protocol data and contains a 4-byte checksum value for the entire packet. The sending computer computes this value and places it into the field. The receiving system performs the same computation and compares it to the field to verify that the packet was transmitted without error.

Ethernet Addressing

The Destination Address and Source Address fields use the 6-byte hardware addresses coded into network interface adapters to identify systems on the network. Every network interface adapter has a unique hardware address (also called a MAC address), which consists of a 3-byte value called an organizationally unique identifier (OUI), which is assigned to the adapter's manufacturer by the IEEE, plus another 3-byte value assigned by the manufacturer itself.

Ethernet, like all data-link layer protocols, is concerned only with transmitting packets to another system on the local network. If the packet's final destination is another system on the LAN, the Destination Address field contains the address of that system's network adapter. If the packet is destined for a system on another network, the Destination Address field contains the address of a router on the local network that provides access to the destination network. It is then up to the network layer protocol to supply a different kind of address (such as an Internet Protocol [IP] address) for the system that is the packet's ultimate destination.

Ethertypes

The 2-byte field after the Source Address field is the primary difference between the DIX Ethernet and IEEE 802.3 standards. For any network that uses multiple protocols at the network layer, it is essential for the Ethernet frame to somehow identify which network layer protocol has generated the data in a particular packet. The DIX Ethernet frame does this simply by specifying an Ethertype in this field, using values like those shown in Table 5.2. The IEEE 802.3 standard uses this field to specify the length of the data field.

Table 5.2  Common Ethertype Values, in Hexadecimal

Network Layer Protocol	Ethertype
IP	0800
X.25	0805
Address Resolution Protocol (ARP)	0806
Reverse ARP	8035
AppleTalk on Ethernet	809B
NetWare Internetwork Packet Exchange (IPX)	8137

IEEE 802.3 takes a different approach. In this frame, the field after the Source Address specifies the length of the data in the packet. The frame uses an additional component, the Logical Link Control (LLC), to identify the network layer protocol. The IEEE's 802 working group is not devoted solely to the development of Ethernet-like protocols. In fact, there are other protocols that fit into the IEEE 802 architecture, the most prominent of which (aside from IEEE 802.3) is IEEE 802.5, which is a Token Ring–like protocol. To make the IEEE 802 architecture adaptable to these various protocols, the data-link layer is split into two sublayers, as shown in Figure 5.6.

Figure 5.6  The IEEE 802 protocols split the data-link layer into two sublayers, the MAC layer and the LLC layer (Image unavailable)

The MAC sublayer is the part that contains the elements particular to the IEEE 802.3 specification, such as the Ethernet physical layer options, the frame, and the CSMA/CD MAC mechanism. The functions of the LLC sublayer are defined in a separate document, published as IEEE 802.2. This same LLC sublayer is also used with the MAC sublayers of other IEEE 802 protocols, such as 802.5.

The LLC standard defines an additional 3-byte or 4-byte subheader that is carried within the Data field, which contains service access points (SAPs) for the source and destination systems. These SAPs identify locations in memory where the source and destination systems store the packet data. To provide the same function as the Ethertype field, the LLC subheader can use a SAP value of 170, which indicates that the Data field also contains a second subheader called the Subnetwork Access Protocol (SNAP). The SNAP subheader is 5 bytes long and contains a 2-byte Local Code that performs the same function as the Ethertype field in the Ethernet II header.

It is typical for computers on a Transmission Control Protocol/Internet Protocol (TCP/IP) network to use the Ethernet II frame because the Ethertype field performs the same function as the LLC and SNAP subheaders and saves 8 to 9 bytes per packet. Microsoft Windows servers and clients automatically negotiate a common frame type when communicating, and when you install a Novell NetWare server, you can select the frame type you want to use. There are two crucial factors to be aware of when it comes to Ethernet frame types. First, computers must use the same frame type to communicate. Second, if you are using multiple network layer protocols on your network, such as TCP/IP for Windows networking and IPX for NetWare, you must use a frame type that contains an Ethertype or its functional equivalent, such as Ethernet II or Ethernet SNAP.

CSMA/CD

The MAC mechanism is the single most defining element of the Ethernet standard. A protocol that is very similar to Ethernet in other ways, such as the short-lived 100Base-VG-AnyLAN, is placed in a separate category because it uses a different MAC mechanism. CSMA/CD may be a confusing name, but the basic concept is simple. Only when you get into the details do things become complicated.

When an Ethernet system has data to transmit, it first listens to the network to see if it is in use by another system. This is the carrier sense phase. If the network is busy, the system does nothing for a given period and then checks again. If the network is free, the system transmits the data packet. This is called the multiple access phase because all of the systems on the network are contending for access to the same network medium.

Run the CSMA video located in the Demos folder on the CD-ROM accompanying this book for a demonstration of the carrier sense and multiple access phases.

Even though an initial check is performed during the carrier sense phase, it is still possible for two systems on the network to transmit at the same time, causing a collision. For example, when a system performs the carrier sense, another computer has already begun transmitting, but its signal has not yet reached the sensing system. The second computer then transmits and the two packets collide somewhere on the cable. When a collision occurs, both packets are discarded and the systems must retransmit them. These collisions are a normal and expected part of Ethernet networking, and they are not a problem unless there are too many of them or the computers are unable to detect them.

Run the Collision video located in the Demos folder on the CD-ROM accompanying this book for a demonstration of a collision.

The collision detection phase of the transmission process is the most important part of the operation. If the systems can't tell when their packets collide, corrupted data may reach the destination system and be treated as valid. Ethernet networks are designed so that packets are large enough to fill the entire network cable with signals before the last bit leaves the transmitting computer. This is why Ethernet packets must be at least 64 bytes long, systems pad out short packets to 64 bytes before transmission, and the Ethernet physical layer guidelines impose strict limitations on the lengths of cable segments.

As long as a computer is still in the process of transmitting, it is capable of detecting a collision on the network. On a UTP or fiber optic network, a computer assumes that a collision has occurred if it detects signals on both its transmit and receive wires at the same time. On a coaxial network, a voltage spike indicates the occurrence of a collision. If the network cable is too long or if the packet is too short, a system might finish transmitting before the collision occurs.

When a system detects a collision, it immediately stops transmitting data and starts sending a jam pattern instead. The jam pattern serves as a signal to each system on the network that a collision has taken place, that it should discard any partial packets it may have received, and that it should not attempt to transmit any data until the network has cleared. After transmitting the jam pattern, the system waits a specified period of time before attempting to transmit again. This is called the backoff period, and both of the systems involved in a collision compute the length of their own backoff periods using a randomized algorithm called truncated binary exponential backoff. They do this to try to avoid causing another collision by backing off for the same period of time.

Because of the way CSMA/CD works, the more systems you have on a network or the more data the systems transmit over the network, the more collisions there are. Collisions are a normal part of Ethernet operation, but they cause delays, because systems have to retransmit packets. When the number of collisions is minimal, the delays aren't noticeable, but when network traffic increases, the number of collisions increases, and the accumulated delays can begin to have a palpable effect on network performance. For this reason, it is not a good idea to run an Ethernet network at high traffic levels. You can reduce the traffic on the network by installing a bridge or switch or by splitting it into two LANs and connecting them with a router.

Using CSMA/CD may seem to be an inefficient way of controlling access to the network medium, but the process by which the systems contend for access to the network and recover from collision occurs many times per second, so rapidly that the delays caused by a moderate number of collisions are negligible.

Run the Contention video located in the Demos folder on the CD-ROM accompanying this book for a demonstration of how Ethernet systems contend for access to the network.

Exercise 1: CSMA/CD Procedures

Place the following steps of the CSMA/CD transmission process in the proper order.

1. System begins transmitting data
2. System retransmits data
3. System detects incoming signal on receive wires
4. System backs off
5. System listens to the network
6. System stops transmitting data
7. System transmits jam pattern
8. System detects no network traffic

Lesson Review

1. What does an Ethernet system generate when it detects a collision?

1. A beacon frame
2. An error message
3. A jam signal
4. None of the above

2. Which of the following is not a required component of a 10Base-T Ethernet network?

1. Network interface adapters or NICs
2. Cables
3. A hub
4. Computers

3. To achieve 100 Mbps speed over Category 3 cable, 100Base-T4 Ethernet uses which of the following?

1. PAM-5 signaling
2. Quartet signaling
3. CSMA/CD
4. All four wire pairs

4. In which of the following standards is Gigabit Ethernet defined?

1. IEEE 802.2
2. IEEE 802.3
3. IEEE 802.3u
4. IEEE 802.3z

5. List the hardware components that you have to replace when upgrading a 10-year-old 10Base-T network to 100Base-TX.
6. How could you upgrade a 10-year-old 10Base-T network to Fast Ethernet without replacing the cables?
7. Which Fast Ethernet physical layer option is best suited for a connection between two campus buildings 200 meters apart? Why?
8. Which of the following is a valid MAC address?

1. 00:B0:A1:8C:32:65:BB
2. 01:DB:7F:86:E4:6G
3. 00:D0:B7:AD:1A:7B
4. 03:BC:5A:E6:E4

Lesson Summary

• There are two sets of Ethernet standards: DIX Ethernet and IEEE 802.3, which differ primarily in their frame formats.
• Ethernet supports many different physical layer configurations, using various types of cables: coaxial, twisted pair, and fiber optic.
• Ethernet uses the CSMA/CD MAC mechanism, which relies on the ability of the computers to detect packet collisions when they occur.

Lesson 2: Token Ring

Token Ring is a protocol that contains the same basic elements as Ethernet: physical layer options, a frame format, and a MAC mechanism. However, it approaches the tasks of transmitting and receiving data on a shared network medium in a completely different manner. IBM originally designed Token Ring, but it was standardized in the IEEE 802.5 document, and there are many manufacturers now producing Token Ring hardware. Token Ring networks were originally designed to run at 4 Mbps, but later implementations increased the speed to 16 Mbps. Most of the Token Ring NICs sold today support both speeds. 16 Mbps is faster than standard Ethernet, but nowhere near the 100-Mbps speed of Fast Ethernet. However, it's important to note that Token Ring networks experience no collisions (under normal circumstances) like Ethernet, which improves the network's overall efficiency.

• List the physical layer options for Token Ring networks
• Diagram the Token Ring frames
• Understand the token-passing MAC mechanism

Estimated lesson time: 30 minutes

Token Ring is far less commonly used than Ethernet, and one of the major reasons is the price of Token Ring hardware, which is substantially higher than that of Ethernet equipment. You can build a simple Ethernet network by purchasing NICs for as little as $20 and a hub for less than $75. Token Ring multistation access units (MAUs) are considerably more complex than Ethernet hubs, however, and start at around $250, and Token Ring NICs generally cost $120 and more.

Physical Layer Specifications

As described in Lesson 1: Network Cables, in Chapter 2, "Network Hardware," Token Ring networks use a ring topology, which is implemented logically inside the MAU, the Token Ring equivalent of a hub. The network cables take the form of a star topology, but the MAU forwards incoming data to the next port only, not to all of the ports at the same time, as in an Ethernet hub. This topology enables data packets to travel around the network from one workstation to the next until they arrive back at the system that originally generated them.

Token Ring networks still use a shared medium, however, meaning that every packet is circulated to every computer on the network. When a system receives a packet from the MAU, it reads the destination address from the Token Ring header to determine if it should pass the packet up through that computer's networking stack. However, no matter what the address, the system returns the packet to the MAU so that it can be forwarded to the next computer on the ring.

The physical layer specifications for Token Ring networks are not as numerous as are those for Ethernet, and they are not as precisely standardized. The IEEE 802.5 document contains no physical layer specifications at all. Cabling guidelines are derived from practices established by IBM and they can differ when you are working with products made by other manufacturers.

Originally, the medium for Token Ring networks was a cable known as IBM Type 1, also called the IBM Cabling System. Type 1 is a heavy, shielded twisted pair (STP) cable that is sold in various lengths, generally with connectors attached. The connector at the MAU end of the cable is a large, proprietary jack called an IBM data connector (IDC) or a universal data connector (UDC), as shown in Figure 5.7. The NICs in the computers use standard DB-9 connectors. Cables with one IDC and one DB-9 connector, which are used to connect a computer to a MAU, are called lobe cables. Cables with IDC connectors at both ends, used for connecting MAUs together, are called patch cables.

Figure 5.7  A Type 1 cable with an IDC attached (Image unavailable)

Type 1 cable is thick, relatively inflexible, and difficult to install in walls and ceilings because of its large, preattached connectors. Type 1 MAUs also require a special IDC "key," which is a separate device that you plug into each MAU port and remove to initialize the port before connecting a lobe cable to it. Today, most Token Ring networks use Category 5 UTP cable with standard RJ-45 connectors at both ends, known in the Token Ring world as Type 3 cabling. Type 3 networks use the same connectors for both computers and MAUs, so only one type of cable is needed. In addition, it's possible to install the network inside walls and ceilings using bulk cable and attach the connectors afterward. Type 3 MAUs also don't require a separate key, as the ports are self-initializing.

The only advantages Type 1 networks have over Type 3 networks are that they can span longer distances and connect more workstations. A Type 1 lobe cable can be up to 300 meters long, whereas Type 3 cables are limited to 150 meters. Type 1 networks can have up to 260 connected workstations, whereas Type 3 networks can have only 72.

Token Passing

The MAC mechanism of a Token Ring LAN, called token passing, is the single most defining element of the network, just as CSMA/CD is for Ethernet. Token passing is an inherently more efficient MAC mechanism than CSMA/CD because it provides each system on the network with an equal opportunity to transmit its data without generating any collisions and without diminished performance at high traffic levels. Other data-link layer protocols, like FDDI, also use token passing as their MAC mechanism.

Token passing works by circulating a special packet called a token around the network. The token is only 3 bytes long and contains no useful data. Its only purpose is to designate which system on the network is allowed to transmit its data. In their idle state, computers on a Token Ring network are in what is known as repeat mode. While in this state, the computer systems receive packets from the network and immediately forward them back to the MAU for transmission to the next port. If a system doesn't return the packet, the ring is effectively broken and network communication ceases. After a designated system (called the active monitor) generates it, the token circulates around the ring from system to system. When a computer has data to transmit, it must wait for a free token to arrive before it can send its data. No system can transmit without being in possession of the token, and because there is only one token, only one system on the network can transmit at any one time. This means that there can be no collisions on a Token Ring network unless something is seriously wrong.

Run the TokenPassing video located in the Demos folder on the CD-ROM accompanying this book for a demonstration of how token passing works.

When a computer takes possession of the token, it changes the value of one bit (called the monitor setting bit) and forwards the packet back to the MAU for transmission to the next computer on the ring. At this point, the computer enters transmit mode. The new value of the monitor setting bit informs the other computers that the network is in use and that they can't take possession of the token themselves. Immediately after the computer transmits the "network busy" token, it transmits its data packet.

As with the token frame transmitted immediately before it, the MAU forwards the data packet to each computer on the ring in turn. Eventually, the packet arrives back at the computer that generated it. At the same time that the sending computer goes into transmit mode, its receive wire pair goes into stripping mode. When the data packet traverses the entire ring and returns to its source, it is the responsibility of the sending computer that generated the packet to strip it from the network. This prevents the packet from circulating endlessly around the ring.

Run the TokenRingNetwork video located in the Demos folder on the CD-ROM accompanying this book for a step-by-step illustration of the path that packets take on a Token Ring network.

The original Token Ring network design calls for the system transmitting its data packet to wait for the last bit of data to arrive back at its source before it generates a new token by modifying the monitor setting bit in the token frame back to its original value and transmitting it. Today, most 16-Mbps Token Ring networks have a feature called early token release, which enables workstations to transmit a free token immediately after their data packets. This way, another system on the network can receive a data packet, take possession of the token, and begin transmitting its own data frame before all of the data from the first packet has returned to its source. There are parts of two data frames on the network at the same time, but there is never more than one free token.

Token Ring Frames

Unlike Ethernet, which uses one frame format for all communications, Token Ring uses four different frames: the data frame, the token frame, the command frame, and the abort delimiter frame. The largest and most complex of the Token Ring frames is the data frame, shown in Figure 5.8. This is the frame that is most

Figure 5.8  The Token Ring data frame (Image unavailable)

comparable to the Ethernet frame, because it encapsulates the data received from the network layer protocol using a header and a footer. The other three frames are strictly for control functions, such as ring maintenance and error notification.

The functions of the fields in the data frame are as follows:

• Start Delimiter (1 byte).  This field contains a bit pattern that signals the beginning of the frame to the receiving system.
• Access Control (1 byte).  This field contains bits that can be used to prioritize Token Ring transmissions, enabling certain systems to have priority access to the token frame and the network.
• Frame Control (1 byte).  This field contains bits that specify whether the frame is a data or a command frame.
• Destination Address (6 bytes).  This field contains the 6-byte hexadecimal address of the network interface adapter on the local network to which the packet will be transmitted.
• Source Address (6 bytes).  This field contains the 6-byte hexadecimal address of the network interface adapter in the system generating the packet.
• Information (up to 4500 bytes).  This field contains the data generated by the network layer protocol, including a standard LLC header, as defined in IEEE 802.2.
• Frame Check Sequence (4 bytes).  This field contains a 4-byte checksum value for the packet (excluding the Start Delimiter, End Delimiter, and Frame Status fields) that the receiving system uses to verify that the packet was transmitted without error.
• End Delimiter (1 byte).  This field contains a bit pattern that signals the end of the frame, including a bit that specifies if there are further packets in the sequence yet to be transmitted and a bit that indicates that the packet has failed the error check.
• Frame Status (1 byte).  This field contains bits that indicate whether the destination system has received the frame and copied it into its buffers.

The token frame is 3 bytes long (as shown in Figure 5.9), and contains only the Start Delimiter, Access Control, and End Delimiter fields. The Start Delimiter and End Delimiter fields use the same format as in the data frame, and the token bit in the Access Control field is set to a value of 1.

Figure 5.9  The Token Ring token frame (Image unavailable)

The command frame (also called a MAC frame because it operates at the MAC sublayer, whereas the data frame operates at the LLC sublayer) uses the same basic format as the data frame, differing only in the value of the Frame Control field and the contents of the Information field. The Information field, instead of containing network layer protocol data, contains a 2-byte major vector ID, which specifies the control function the packet is performing, followed by the actual control data itself, which can vary in length. The following major vector IDs indicate some of the most common control functions performed by these packets:

• 0010—Beacon.  Beaconing is a process by which systems on a Token Ring network indicate that they are not receiving data from their nearest active upstream neighbor, presumably because a network error has occurred. Beaconing enables a network administrator to more easily locate the malfunctioning computer on the network.
• 0011—Claim Token.  This vector ID is used by the active monitor system to generate a new token frame on the ring.
• 0100—Ring Purge.  This vector ID is used by the active monitor system in the event of an error to clear the ring of unstripped data and to return all of the systems to repeat mode.

The abort delimiter frame consists of only 2 bytes, the same Start Delimiter and End Delimiter fields, and uses the same values for those fields as the data and command frames. When a problem occurs, such as an incomplete packet transmission, the active monitor system generates an abort delimiter frame to flush all existing data from the ring.

Exercise 1: IEEE Standards and Technologies

Match the standard in the left column with the most suitable technology in the right column.

IEEE 802.2 IEEE 802.3 IEEE 802.3u IEEE 802.3z IEEE 802.3ab IEEE 802.5 DIX Ethernet DIX Ethernet II

Gigabit Ethernet Fast Ethernet Thick Ethernet LLC 10Base-T Thin Ethernet 1000Base-T Token Ring

Exercise 2: Selecting a Data-Link Layer Protocol

For each of the following scenarios, specify which data-link layer protocol you think is preferable, Ethernet or Token Ring, and give reasons why. In some cases, either protocol would be suitable; the reasons you provide are more significant than the protocol you select.

1. A family with two computers in the home wants to network them to share a printer and an Internet connection.
2. A small graphics design firm wants to build a 10-node network to handle the extremely large image files that they must transfer between systems and to a print server.
3. A company with a 50-node LAN used by its order entry staff will be going public in the near future and is expected to grow enormously over the next year.

Lesson Review

1. The Frame Check Sequence field in a data-link layer protocol header is used for _________________.
2. Which data-link layer protocol is preferred on a network with high levels of traffic, Ethernet or Token Ring? Why?
3. Which of the following Token Ring cables has both IDC and DB-9 connectors?
1. A Type 3 cable
2. A patch cable
3. A lobe cable
4. A token cable

4. Most Token Ring networks today run at what speed?
1. 4 Mbps
2. 16 Mbps
3. 100 Mbps
4. 1000 Mbps

5. A Token Ring system that is waiting to capture a free token is said to be in what mode?
1. Transmit mode
2. Passive mode
3. Repeat mode
4. Stripping mode

Lesson Summary

• Token Ring supports two physical layer options, a shielded twisted pair cable called Type 1 and an unshielded twisted pair cable called Type 3.
• Token Ring uses the token passing Media Access Control mechanism, in which only the system in possession of a special token frame is permitted to transmit data.
• Token Ring uses four different types of frames, whereas Ethernet uses only one.

Lesson 3: FDDI

Until the introduction of Fast Ethernet, FDDI (pronounced "fiddy"), was the only data-link layer protocol in common use to offer 100-Mbps transmission speeds using fiber optic cable. Standardized by the American National Standards Institute (ANSI), FDDI is primarily a protocol used on backbone networks, but there was also a desktop version of the protocol designed to use copper cables called Copper Distributed Data Interface (CDDI, or "siddy") that never achieved widespread deployment. Like Token Ring, FDDI networks are cabled using a ring topology and employ the token passing MAC mechanism, but there are several important differences between FDDI and Token Ring.

• Describe the characteristics of the FDDI protocol
• Distinguish between the various types of FDDI network connections
• Diagram a FDDI frame

Estimated lesson time: 15 minutes

The FDDI Physical Layer

Apart from its speed, which was unprecedented at the time of its introduction, the use of fiber optic cable was the primary reason for FDDI's commercial success. Like other fiber optic protocols, FDDI networks can span much longer distances than copper-based networks and are completely resistant to electromagnetic interference. FDDI supports several different types of fiber optic cable, including the 62.5/125 micron multimode cable that is the industry standard for fiber optic LANs, which provides for network segments up to 100 kilometers long with up to 500 workstations placed as far as 2 kilometers apart. Singlemode fiber optic cables provide even longer segments, with up to 60 kilometers between workstations.

The original FDDI standard calls for a ring topology, but unlike Token Ring networks, this ring is not strictly a logical one implemented in the hub. The computers are actually cabled together in a ring. To provide fault tolerance in the event of a cable break, the network is a double ring that consists of two independent rings, a primary and a secondary, with traffic flowing in opposite directions. A computer that is connected to both rings is called a dual attachment station (DAS), and when one of the rings is broken by a cable fault, the computer switches to the other ring, providing continued full access to the entire network. A double ring FDDI network in this condition is called a wrapped ring.

It's also possible to cable a FDDI network in a star topology using a hub called a dual attachment concentrator (DAC). The DAC creates a single logical ring, like a Token Ring MAU. A computer connected to the DAC is called a single attachment station (SAS). A FDDI network can be deployed using the double ring, the star topology, or both. The double ring is better suited to use as a backbone network, and the star to a segment network connecting desktop computers. To construct an entire enterprise network using FDDI, you create a double ring back-bone, to which you connect your servers and other vital computers as DASes. You then connect one or more DACs to the double ring, which you use to attach your workstations, as shown in Figure 5.10. This is sometimes called a dual ring of trees. The DAS servers have full advantage of the double ring's fault tolerance, as do the DACs, whereas the SAS computers attached to the DACs are connected to the primary ring only. If a cable connecting a workstation to a DAC fails, the DAC can remove it from the ring without disturbing communications to the other computers, as on a Token Ring network. To expand the network further, you can connect additional DACs to ports in existing DACs without limit, as long as you remain within the maximum number of computers permitted on the network.

Figure 5.10  An enterprise FDDI network (Image unavailable)

The FDDI Frames

Like Token Ring, FDDI uses several different types of frames in its communications. The most common of these is the data frame, shown in Figure 5.11. The functions of the fields in the FDDI data frame are as follows:

Figure 5.11  The FDDI data frame (Image unavailable)

• Preamble (PA, 8 bytes).  Contains series of alternating 0s and 1s, used for clock synchronization.
• Starting Delimiter (SD, 1 byte).  Indicates the beginning of the frame.
• Frame Control (FC, 1 byte).  Indicates the type of data found in the Data field. Some of the most common values are as follows:
• 41, 4F—Station Management (SMT) Frame.  Indicates that the Data field contains an SMT Protocol Data Unit.
• C2, C3—MAC Frame.  Indicates that the frame is either a MAC Claim frame (C2) or a MAC Beacon frame (C3), which are use to recover from token passing errors.
• 50, 51—LLC Frame.  Indicates that the Data field contains application data in a standard IEEE 802.2 LLC frame.

• Destination Address (DA, 6 bytes).  Specifies the hardware address of the computers that will receive the frame.
• Source Address (SA, 6 bytes).  Specifies the hardware address of the system sending the frame.
• Data (variable).  Contains network layer protocol data, or an SMT header and data, or MAC data, depending on the function of the frame.
• Frame Check Sequence (FCS, 4 bytes).  Contains a cyclical redundancy check (CRC) value, used for error detection.
• Ending Delimiter (ED, 4 bits).  Indicates the end of the frame.
• End of Frame Sequence (FS, 12 bits).  Contains three indicators that may be modified by intermediate systems when they retransmit the packet, the functions of which are as follows:
• E (Error).  Indicates that an error has been detected, either in the FCS or in the frame format.
• A (Acknowledge).  Indicates that the intermediate system has determined that the frame's destination address applies to itself.
• C (Copy).  Indicates that the intermediate system has successfully copied the contents of the frame into its buffers.

Because it is a token passing protocol, FDDI also must have a token frame, which contains only the Preamble, plus the Starting Delimiter, Frame Control, and Ending Delimiter fields, for a total of 3 bytes. The token passing mechanism used by FDDI is virtually identical to that of Token Ring, except that the early token release feature that is optional in Token Ring is standard equipment for the FDDI protocol. The third type of frame used on FDDI networks is the station management frame, which is responsible for ring maintenance and network diagnostics.

Exercise 1: FDDI Concepts

Match the acronyms in the left column with the correct definitions in the right column.

DAS DAC SAS CDDI SMT

A version of FDDI that uses copper cable A computer connected to a FDDI network using the star topology A FDDI frame that performs ring management functions The hub used in a FDDI star network A computer connected to both rings of a double ring

Lesson Review

1. A FDDI double ring network that has experienced a cable failure is called what?

1. A wrapped ring
2. A truncated ring
3. A bifurcated ring
4. A dual ring of trees

2. Which FDDI physical layer option supports the longest network segments?

1. The double ring topology
2. The star topology
3. Singlemode fiber optic
4. Multimode fiber optic

3. Which of the following fields identifies the type of data carried in a FDDI data frame?

1. Starting Delimiter
2. Frame Control
3. Source Address
4. Frame Check Sequence

Lesson Summary

• FDDI is a token passing data-link layer protocol that was at one time a popular solution for backbone networks.
• FDDI uses either a physical double ring topology or a star topology.
• A FDDI workstation attached to both rings of a double ring is called a dual attachment station (DAS), and one that is attached to a single ring is called a single attachment station (SAS).

Lesson 4: Wireless Networking

When speaking of data networks, it's most common to think of a cable as the network medium. However, there have been wireless data networking technologies available for several years. Until recently, a wireless LAN was usually synonymous with slow transmission speeds and unreliable service, but there are now wireless LAN technologies available that provide reasonably reliable service at speeds that are at least acceptable to the average user accustomed to a cable network.

• Describe the two basic wireless topologies
• List the IEEE 802.11 physical layer options
• Describe the CSMA/CA MAC mechanism

Estimated lesson time: 20 minutes

Although wireless LANs can now reach respectable speeds (up to 11 Mbps), they are not likely to ever be as fast or reliable as a cabled LAN. For that reason, wireless LANs are typically used only in specific situations that require them, such as when users must be able to roam around a site with a portable computer while remaining connected to the LAN or when network access is required in places where it is impractical or impossible to install LAN cables. For example, many organizations use wireless LAN technologies to keep employees with portable computers or handheld devices in constant touch with the company LAN. Hotel and airport kiosks, rental car agents performing curbside check-ins, and retail workers scanning products on display for inventory control are just some of the many applications for this technology. There are also wireless LAN products designed for home use, where a cable installation would be inconvenient, expensive, or unsightly.

The dominant wireless LAN standard today is IEEE 802.11, developed by the same organization responsible for the current Ethernet and Token Ring standards. The IEEE 802.11 working group was convened in 1990 for the purpose of developing a global wireless networking standard with a transmission rate of 1 to 2 Mbps. This standard has come to be known as IEEE 802.11a. The later IEEE 802.11b standard provides transmission speeds of 5.5 and 11 Mbps.

The IEEE 802.11 Physical Layer

As mentioned in Chapter 2, "Network Hardware," wireless LANs support two topologies, an ad hoc topology and an infrastructure topology. The ad hoc or independent topology is one in which computers equipped with wireless network interface adapters communicate directly with each other on a peer-to-peer basis; there is no cabled network involved. This type of network is designed to support only a limited number of computers, such as those in a home or small business. The infrastructure topology is designed to extend the range and flexibility of a normal cabled network by enabling wireless-equipped computers to connect to it using a specialized module called an access point.

In some cases, an access point is a computer with a wireless network interface adapter as well as a standard adapter connecting it to a standard cabled LAN, or it can be a dedicated device. The wireless clients communicate with the cabled network using the access point as an intermediary. The access point is essentially a translation bridge because it converts between the wireless network signals and those of the cabled network, preserving the single broadcast domain. As with all wireless communication technologies, distance and environmental conditions can have great effects on the performance realized by the mobile workstations. A single access point can typically support 10 to 20 clients, depending on how heavily they use the LAN, as long as they remain within an approximately 100- to 200-foot radius of the access point. Intervening walls and interference can diminish this performance substantially.

To extend the range of the wireless part of the network and provide support for more clients, you can use multiple access points in different locations, or you can use an extension point. An extension point is essentially a wireless signal repeater that functions as a way station between wireless clients and an access point. An IEEE 802.11 LAN is divided into cells, each of which is controlled by a base station. The 802.11 standard refers to each cell as a basic service set (BSS) and to each base station as an access point. If the network uses multiple access points, they are connected by a backbone, which the standard calls a distribution system (DS). The DS is usually a cabled network, but it can conceivably be wireless as well.

Run the WirelessLANs video located in the Demos folder on the CD-ROM accompanying this book for a demonstration of the ad hoc and infrastructure topologies.

The IEEE 802.11 standard supports three different types of signals at the physical layer, which are as follows:

• Direct Sequence Spread Spectrum (DSSS).  A radio transmission method in which the outgoing signals are modulated using a digital code (called a chipping code) that uses a redundant bit pattern. The end result is that each bit of data is converted into multiple bits, enabling the signal to be spread out over a wider frequency band. The use of DSSS in combination with a technique called complimentary code keying (CKK) enables IEEE 802.11b systems to achieve their 11-Mbps transmission rates.
• Frequency Hopping Spread Spectrum (FHSS).  A radio transmission method in which the transmitter continuously performs rapid frequency shifts according to a preset algorithm. The receiver performs the exact same shifts to read the incoming signals. IEEE 802.11a systems can use FHSS, but IEEE 802.11b doesn't support it.
• Infrared.  Infrared communications use high frequencies, just below the visible light spectrum. Infrared is a "line of sight" technology, meaning that the signals cannot penetrate through opaque walls and objects. This severely limits the utility of infrared technology, which is why it is rarely used for LAN communications, outside of simple links between computers and peripherals, such as printers and handheld devices.

The IEEE 802.11 MAC Layer

Like all of the protocols developed by the IEEE 802 working groups, IEEE 802.11 splits the data-link layer into two sublayers, LLC and MAC. The LLC sublayer used to package the network layer data to be transmitted is the same for all of the IEEE 802 protocols. The IEEE 802.11 protocol's MAC sublayer defines the data, control, and management frames used by the protocol, as well as its MAC mechanism. IEEE 802.11 uses a variation on the CSMA/CD MAC mechanism used by Ethernet, called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).

CSMA/CA is similar to CSMA/CD in that computers listen to the network to see if it is in use before they send their data, and if the network is free, the transmission proceeds. Also like CSMA/CD, two computers can transmit at the same time on a CSMA/CA network, causing a collision. The difference between the two MAC mechanisms is that in a wireless environment, the CSMA/CD collision detection mechanism would be impractical, because it would require full-duplex communications. A computer on a twisted-pair Ethernet network assumes that a collision has occurred when an incoming signal arrives over its receive wire pair while it's sending data over the transmit wire pair. Making wireless LAN devices that can transmit and receive signals simultaneously is far more difficult.

Instead of detecting collisions as they occur, the receiving computer on a CSMA/CA network performs a CRC check on the incoming packets and, if no errors are detected, transmits an acknowledgment message to the sender. This acknowledgment serves as an indication that no collision has occurred. If the sender does not receive an acknowledgment for a particular packet, it automatically retransmits it until it either receives an acknowledgment or times out. If the sender still doesn't receive an acknowledgment after a specific number of retransmissions, it abandons the effort and leaves the error correction process to the protocols at the upper layers of the networking stack.

Exercise 1: IEEE 802.11 Concepts

Match the concepts in the left column with the correct definitions in the right column.

Extension point BSS Base station DS

An access point A backbone connecting access points Another term for a cell A repeater for wireless signals

Lesson Review

1. What MAC mechanism does an IEEE 802.11 network use?

1. DSSS
2. FHSS
3. CSMA/CA
4. CSMA/CD

2. Which of the following terms describes a wireless LAN that does not use access points?

1. Infrastructure
2. Distribution
3. Ad hoc
4. Basic

3. Which of the following is not a physical layer option supported by IEEE 802.11?

1. DSSS
2. Infrared
3. BSS
4. FHSS

Lesson Summary

• Wireless LAN technologies enable wireless computers to communicate among themselves or with a standard cabled network.
• The IEEE 802.11 protocol supports three physical layer options and provides transmission speeds up to 11 Mbps.
• IEEE 802.11 splits the data link layer into LLC and MAC sublayers and uses CSMA/CA as its MAC mechanism.