7-1 Business Data Communications and Networking, 6 th ed. FitzGerald and Dennis

7-2 Copyright © 1999 John Wiley & Sons, Inc. All rights reserved. Reproduction or translation of this work beyond that permitted in Section 117 of the 1976 United States Copyright Act without the express written permission of the copyright owner is unlawful. Request for further information should be addressed to the Permissions Department, John Wiley & Sons, Inc. The purchaser may make back-up copies for his/her own use only and not for redistribution or resale. The Publisher assumes no responsibility for errors, omissions, or damages, caused by the use of these programs or from the use of the information contained herein.

7-3 Local Area Networks Chapter 7

7-4 Objectives of Chapter 7 Understand... u the three major components of LANs, u how Ethernet LANs operate, u how Token Ring LANs operate, u how to improve LAN performance. Become familiar with… u the roles of LANs in organizations, u other types of LANs, u issues involved in selecting a LAN.

7-5 INTRODUCTION

7-6 Introduction Most organizations have local area networks (LANs) connected by backbone networks. In many cases, these LANs also provide access to the organization’s mainframe computer.

7-7 Why Use a LAN? There are two basic reason for developing a LAN: u Information sharing refers to having users who access the dame data files, exchange information via electronic mail, or search the Internet for information. u Resource sharing refers to one computer sharing a hardware device (e.g. printer) or software package with other computers on the network in order to save costs.

7-8 Why Use a LAN? An alternative to purchasing a single copy of software for each user is to install the software on the network for all to use. In most cases, not all users would need to access the software package simultaneously, allowing a reduced number of licenses to be purchased.

7-9 Why Use a LAN? One approach to controlling the number of copies of a particular software package is to use LAN metering software that prohibits using more copies of a package than there are installed licenses. The Software Publishers Association (SPA) estimates that about 40% of all the software in the world is used illegally. SPA has recently undertaken an aggressive software audit program to check the number of illegal software copies on LANs.

7-10 Dedicated Server vs. Peer-to-Peer LANs One common way to categorize LANs is by whether they have a dedicated server of whether they operate as a peer-to-peer LAN without a dedicated server.

7-11 Dedicated Server LANs A dedicated server LAN has one or more computers that are permanently assigned to being the network server(s). In a dedicated server LAN, the server’s usual operating system is replaced by a network operating system.

7-12 Dedicated Server LANs A LAN can have many different types of dedicated servers. Four common types are: u File servers - allow many users to share the same set of files on a common, shared disk drive. u Database servers - not only provides shared access to the files on the server, but also can perform database processing on those files associated with client-server computing.

7-13 Dedicated Server LANs u Print servers -handle print requests on the LAN. u Communications servers - are dedicated to performing communications processing. There are three fundamental types: Fax servers - manage fax boards Modem servers - for dialing out of the network Access servers - for dialing into the network

7-14

7-15 Peer-to-Peer Networks Peer-to-peer networks do not require a dedicated server. All computers run special network software that enables them to function as both a client and as a server. In general, peer-to-peer LANs have less capability, support a more limited number of computers, provide less sophisticated software, and can prove more difficult to manage than dedicated server LANs.

7-16 LAN COMPONENTS

7-17 LAN Components There are five basic components to a LAN u Client u Server u Network Interface Cards u Network Cables and Hubs u Network Operating Systems

7-18 LAN Components

7-19 Network Interface Cards Most computers are not delivered with a built-in network interface. The Network Interface Card (or NIC) allows the computer to be physically connected to the network cable, which provides the physical layer connection among computers in the network. Most NICs are installed inside the computer.

7-20 Network Cables and Hubs Each computer must be physically connected by network cable to the other computers in the network. The selection of a LAN can be influenced greatly by the type of cable that already exists where the LAN is to be installed.

7-21 Network Cable Most LANs are formed with a blend of unshielded twisted pair (UTP) wire, shielded twisted pair (STP), coaxial cable, and fiber optic cable. Many LANs use a combination of shielded and unshielded twisted pair. Coax cable used to be the most common type of LAN cable, but today is less commonly used than UTP or STP.

7-22 Network Cable Fiber optic cable is even thinner (and lighter) than unshielded twisted pair and therefore takes far less space when cabled throughout a building. Because of its high capacity, fiber is perfect for BNs, although it is beginning to be used in LANs. While most LANs use only one type of cable, it is possible to buy devices that permit different type of cables to be connected together.

7-23 Network Hubs Network hubs go by many names depending on the type of network and the specific vendor. Concentrator, Multistation access unit, Transceiver, Repeater

7-24

7-25 Network Hubs Network hubs serve two purposes: First they provide an easy way to connect network cables. A better approach is to use a hub in any area in which the network might expand. Second, many hubs act as repeaters or amplifiers to prevent attenuation. Some hubs are “smart”, because they can detect and respond to network problems.

7-26 Network Cable Plans In the early days of LANs, it was common practice to install network cable wherever it was convenient. With today’s explosion in LAN use, it is critical to plan for effective installation and use of LAN cabling. Most buildings under construction have a separate LAN cable plan as they do for telephone cables and electrical cables.

7-27 Network Operating Systems The network operating system is the software that controls the network. Every NOS provides two sets of software: u one that runs the network server(s), and u one that runs on the network client(s).

7-28 Network Operating Systems The server version of the NOS provides the software the performs the functions associated with the data link, network and application layers and usually the computer’s own operating system. The client version of the NOS provides the software the performs the functions associated with the data link and the network and must interact with the application layers and the computer’s own operating system.

7-29 Network Operating Systems The NOS server software enables the file server, print server, or database server to operate. NOS server software typically replaced the normal operating system on the server. The NOS client software running at the client computers provides the data link layer and the network layer. A Network profile specifies what resources on each server are available for network use by computers and which devices or people (user profile) are allowed what access to the network.

7-30 ETHERNET (IEEE 802.3)

7-31 Ethernet (IEEE 802.3) Almost 70 percent of all LANs in the world use Ethernet. The Ethernet LAN standard was originally developed by DEC, Xerox, and Intel, but has since become a formalized standard by the Institute of Electrical and Electronics Engineers as IEEE 802.3

7-32 Topology Topology is the basic geometric layout of the network -- the way in which the computers on the network are interconnected.

7-33

7-34 Topology Ethernet uses a bus topology (a high speed circuit and a limited distance between the computers, such as within one building). From the outside, an ethernet LAN appears to be a star, because all cables connect to the central hub. Most ethernet LANs span sufficient distance to require several hubs, but some ethernet LANs are build without the use of hubs (coax bus).

7-35 Topology

7-36 Media Access Control When several computers share the same communication circuit, it is important to control their access to the media. If two computers on the same circuit transmit at the same time, their transmission become garbled. These “collisions” must be prevented, or means to recover established.

7-37 Media Access Control Ethernet uses a contention-based technique called Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Like all contention-based techniques, CSMA/CD is very simple; wait until the bus is free and then transmit. If two computers attempt to transmit at the same time, they detect the collision, send a jamming signal, wait a random amount of time, then re-broadcast.

7-38 Types of Ethernet Baseband Ethernet uses digital signaling. It treats the cable as one single channel, so it can carry only a single channel at any one moment. Broadband Ethernet uses analog signaling and splits the cable into many different channels (using FDM), so more than one transmission can occupy the LAN cable at the same time; allowing an intermix of voice, data and image signals.

7-39 Types of Ethernet The original ethernet specification was a 10 Mbps data rate using baseband signaling on thick coaxial cable called 10base5 or thick ethernet, capable of running 500 meters between hubs. Today, 10base2 or thin ethernet coaxial cable is replacing the 10base5 because it is considerably cheaper and easier to work with, although it is limited to 185 meters between hubs.

7-40 Types of Ethernet 10baseT is the most common type of ethernet, and uses unshielded twisted pair wire. The name means 10Mbps, using “T” twisted-pair wiring. Broadband ethernet is also available as 10broad3600 is with a maximum distance of 3600 meters.

7-41 TOKEN RING (IEEE 802.5)

7-42 Token Ring (IEEE 802.5) The second most popular type of LAN is token ring. Almost 25 percent of all LANs worldwide are token ring LANs. Token Ring was originally developed by IBM, and have since been standardized by IEEE as IEEE

7-43 Topology Token ring uses a ring topology, connecting all computers on the LAN in one closed loop circuit with each computer connected to the next. From the outside, a token ring appears to be a star, because all cables flow into the central hub, but it is truly a ring, with messages passing from one computer to the next.

7-44 Topology

7-45 Media Access Control All computers in a token ring LAN share the same common circuit, so it is essential that access to the media be controlled by the data link layer. Token ring uses a controlled-access technique called token passing. The “token” a series of bits, travels between the computers in a predetermined sequence.

7-46 Media Access Control A computer with a message waits to transmit waits until it receives a free token. The computer changes the free token to a busy token attaches its message to it, and retransmits it on the circuit to the next computer in the sequence. The computer receiving the message, changes the acknowledgement to ACK (or NAK) and sends the message back to the sender, who creates a new free token.

7-47 Media Access Control

7-48 Media Access Control

7-49 Media Access Control One problem with token-passing protocols is dealing with “lost” tokens. The solution is to designate one computer in the network to be the token monitor. If no token circulated through the network for a certain length of time or if a busy token circulates too often, the token monitor will create a new free token (and destroy the busy token if necessary.)

7-50 Types of Token Ring There are two common types of token ring u The original token ring was Token-ring-4 (TR4), operating at 4 Mbps, over UTP. u Newer token ring is Token-ring-16 (TR16) which operates at 16 Mbps over higher quality UTP cable.

7-51 OTHER TYPES OF LANs

7-52 Arcnet Attached Resource Computing Network is another popular peer-to-peer LAN, developed by Datapoint Corp. in Arcnet is a baseband token-passing bus or star architecture. Its low cost has made Arcnet very popular -- so much so that ANSI is expected to designate Arcnet as ANSI standard Arcnet originally transmitted over a coaxial cable at 2.5 Mbps. Newer Arcnet Plus can transmit at 20 Mbps.

7-53 AppleTalk Macintosh computers have a built-in network feature called AppleTalk, which is a nonstandard set of protocols similar to ethernet, that works with the Apple LocalTalk cabling system (230 Kbps). Like ethernet, LocalTalk uses CSMA, but uses collision avoidance (CA) (not CD), where the LocalTalk link access protocol (LLAP) sends out a 3K “warning message” to let other computers know a message is forthcoming.

7-54 Wireless LANs (IEEE802.11) Wireless LANs transmit data through the air (space) rather than through wire or cable. Wireless LANs use CSMA/CA similar to ethernet and AppleTalk. There are three commonly used types of wireless LANs: Infrared Direct-sequence spread spectrum radio Frequency hopping spread spectrum radio.

7-55 Infrared Wireless LANs In general, infrared LANs are the least flexible because most require direct line of sight between transmitters and receivers. The primary advantage of a wireless LAN is the reduction of wiring. The primary disadvantage is in the low speed (1-4 Mbps). A new version of infrared, called diffuse infrared, operates without a direct line of sight by bouncing infrared light around a room.

7-56 Infrared Wireless LANs

7-57 Radio Wireless LANs Radio waves travel in all directions and through non-metal objects and thus are more flexible that infrared systems. Most radio LANs have a range of feet and may even reach 1000 feet in open areas. Wireless LANs are also being used increasingly with laptop computers, permitting new capabilities for mobile computing.

7-58 Radio Wireless LANs

7-59 Radio Wireless LANs Direct-sequence spread-spectrum (DSSS) systems transmit signals through a wide spectrum of radio frequencies simultaneously. Frequency-hopping spread-spectrum (FHSS) systems transmit signals through the same wide spectrum of radio frequencies, but use each frequency in turn.

7-60 Radio Wireless LANs Radio LANs (both DSSS and FHSS) have two disadvantages compared to traditional wired LANs. u The increased opportunity for noise to disrupt transmissions. u Wireless LANs have a much slower data transmission rates.

7-61 IMPROVING LAN PERFORMANCE

7-62 Improving LAN Performance When most computers in an organization are on LANs, performance can be a problem. In order to improve performance, you must locate the bottleneck, the part of the network that is restricting the data flow. Generally speaking, this bottleneck is in one of two places: The network server The network circuit

7-63 Improving LAN Performance The first step in improving performance is to identify whether the bottleneck lies in the circuit or the server. If the server utilization during periods of poor performance is high (60-100%), then the bottleneck is in the server, it cannot process all the requests it receives in a timely. If the server utilization during periods of poor performance is low (10-40%), then the problem lies in the network circuit.

7-64 Improving LAN Performance Performance Checklist Increase Server Performance u Software Upgrade to a faster network operating system Fine-tune the network operating system settings u Hardware Add more servers and spread the network applications across the servers to balance the load Upgrade to a faster computer Increase the server’s memory Increase the number and speed of the server’s hard disk(s) Upgrade to a faster NIC

7-65 Improving LAN Performance Performance Checklist (cont.) Increase Circuit Capacity u Upgrade to a faster circuit u Change protocols u Segment the network Reduce Network Demand u Move files from the server to the client computers u Increase the use of disk caching on client computers u Change user behavior

7-66 Improving Server Performance u Software: Faster NOS Disk caching Disk elevatoring NOS software settings u Hardware Second server Upgrade server (CPU speed, Memory) Number and speed of hard drives in the server. NIC Improving server performance can be approached from two directions simultaneously:

7-67 Improving Circuit Capacity Improving the capacity of the circuit means increasing the volume of simultaneous messages the circuit can transmit from clients to the server(s).

7-68 Ethernet vs. Token Ring Token ring should be about 60 percent better than ethernet, because token ring provides a 16Mbps data transmission rate vs. Ethernet’s10Mbps In general, contention approaches such as CSMA/CD work better for small networks that have low usage The opposite is true for large networks with high usage; token passing works better.

7-69 Ethernet vs. Token Ring A second performance consideration is response time. Generally, response time in token rings is more consistent than ethernet. There are limits on how long a computer can transmit in token ring networks, so that no one monopolizes the network. Ethernet is so popular because the cables, NICs, and hubs cost much much less than their token ring counterparts. Ethernet can also be segmented to relieve overloading.

7-70 Ethernet vs. Token Ring

7-71 Network Segmentation If there is more traffic on a LAN than the network circuit and media access protocol can handle, the solution is to divide the LAN into several smaller segments (network segmentation). Most servers can handle up to as many as 16 separate networks or network segments, simply by adding one NIC into the server for each network. Adding NICs, however, slows processor speed as the server monitors and manages each NIC.

7-72 Network Segmentation

7-73 Network Segmentation

7-74 Reducing Network Demand Performance can also be improved by reducing network demand: Moving files from the server to clients. Using disk caching software on the client machines, to reduce the client’s need for to access disk files stored on the server. Attempting to move user demands from peak times to off-peak times.

7-75 SELECTING A LAN

7-76 Selecting a LAN Start with basic questions: u How many users are expected? u How much data will be stored and transmitted? u How easy will it be to add workstations? u What cabling is needed? u Whose software should be selected? u How much security is needed?

7-77 LAN Selection Checklist Network Needs Number of client computers Number of dedicated servers Distance between computers Internetworking requirements Specific application needs User training, documentation, and network policies Future growth

7-78 LAN Selection Checklist Technology Protocols Cabling/Wireless Network Operating System v Reliability v Ease of use v Performance LAN management software LAN backup software and hardware Vendor Experience with network hardware and software Experience with your network application Vendor service and support

7-79 Network Needs u The maximum number of client computers must be planned for. u Whether or not a dedicated file server(s) will be used, and if so, how many. u Distance between computers. u Whether the LAN is going to internetwork with another network or backbone. u Future growth.

7-80 Network Needs u Specific needs for initial applications. u Proper written documentation should be provided for all the hardware, software, users, etc.

7-81 Technology Issues u One of the first issues is the protocol: Ethernet or Token Ring u Which cabling to use? unshielded twisted pair, shielded twisted pair, coaxial cable, fiber optics or wireless u Which Network Operating System? Novell, NT, Unix?

7-82 Technology Issues u Is it possible to actively manage the LAN and all of the related network elements with LAN management software? u Proper backup and recovery hardware, software and procedures must be provided.

7-83 Vendor Issues u Vendor’s Experience with network hardware and software. u Vendor’s experience with application software. u Levels of service and support provided. u Is it safer to purchase all network components from one vendor?

7-84 End of Chapter 7