Chapter Seven

Local Area Networks (LAN): The Basics

Data Communications and Computer Networks: A Business User’s Approach,

Fourth Edition

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Outline• Introduction• Primary tasks of LAN• Pros and Cons of LAN• Basic Network Topologies• Current LAN topologies• Medium Access Control Protocols• Data layers• Industrial LAN products

– Wired Ethernet– IBM Token Ring– FDDI– Wireless Ethernet

• Business Application

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Introduction

• A LAN – is a communication network that interconnects a

variety of data communicating devices within a small geographic area and broadcasts data at high data transfer rates with very low error rates

• Since the LAN first appeared in the 1970s, its use has become widespread in commercial and academic environments

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Primary Function of Local Area Networks• To provide access to hardware and software resources that will

allow users to perform one or more of the following activities:– File serving

• A large storage disk drive acts as a central storage repository– Print serving

• Providing the authorization to access a particular printer, accept and queue print jobs, and providing a user access to the print queue to perform administrative duties

– Video transfers • High speed LANs are capable of supporting video image and live

video transfers– Manufacturing support

• LANs can support manufacturing and industrial environments– Academic support

• In classrooms, labs, and wireless– E-mail support– Interconnection between multiple systems

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Advantages of Local Area Networks

• Advantages– Ability to share hardware and software resources– Individual workstation might survive network failure– Component and system evolution are possible– Support for heterogeneous forms of hardware and

software– Access to other LANs and WANs (Figure 7-1)– Private ownership– Secure transfers at high speeds with low error rates

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Primary Function of Local Area Networks (continued)

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Advantages and Disadvantages of Local Area Networks (continued)

• Disadvantages – Equipment and support can be costly– Level of maintenance continues to grow– Private ownership?– Some types of hardware may not interoperate– Just because a LAN can support two different kinds of

packages does not mean their data can interchange easily

– LAN is only as strong as its weakest link, and there are many links

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Basic Network Topologies

• 1. Star network • 2. Hierarchical network• 3. Mesh network• 4. Bus network• 5. Ring Network• 6. Hybrid Network

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1. Star network

– All circuits radiate from a central node, typically a host computer

– can be either or both point-to-point or multi-point circuits

– See Figure 11-2

– Example:• PBX is an example of star network

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FIGURE 11-2 Star network.

Applications (to p11)

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FIGURE 11-22 Dow Corning’s U.S. data communications network has a classic star topology with the center at the company’s headquarters in Midland, Michigan.

Another application (to p12)

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FIGURE 11-22 Continued (to p9)

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– Disadv:• if host computer fails, entire network is down• may become overloaded and unable to keep up

with all messages that need to transmit

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2. Hierarchical network

– has a tree structure, with a top node called root node

– See Figure 11-3

– most likely implemented where the lower level nodes at the second or third level of computers

– transmit is done from bottom node to the root node and vice-versa for replying information

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FIGURE 11-3 Hierarchical network. (to p14)

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– Advantage:• there is no single point of failure in the network

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3. Mesh network

– Similar to hierarchical network except there are more interconnections between nodes at different levels

– In this respect, the higher level of node may not be exited at all

– See Figure 11-4

– Example:– Public telephone network

– mobile or cellular phone system

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FIGURE 11-4 Mesh network. (to p17)

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4. Bus network

– is a telecommunications medium to which multiple nodes are attached

– usually implemented in situations where the distance between all nodes is limited

– Figure 11-5

– Problems:• limited to number of devices as each node would

contribute a certain amount of signals loss on the cable

• when problem occurred, fault is difficult to detect

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FIGURE 11-5 Bus network. (to p19)

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5. Ring Network

– Associated with networks where nodes are relatively close together and each device is connected to the ring

» Figure 11-6

– signals are passed from one device to another one until destination is reached

– this network is less subjected to attenuation (why?)

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FIGURE 11-6 Ring network. (to p21)

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FIGURE 11-7 Ring network with two channels. (to p21)

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– to activate a device, all nodes must work together. Thus, if one node fails, potential exits that the entire ring network will be out of service

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6. Hybrid Network

– various network typology discussed are used to combine into hybrid network

– if two or more networks are operated under different protocols, then a gateway device is needed to connect them together

» Figure 11-8 (to p24)

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FIGURE 11-8 Hybrid network.(to p25)

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Basic Local Area Network Topologies

• Local area networks are interconnected using one of four basic configurations:– Bus/tree

– Star-wired bus

– Star-wired ring

– Wireless

– Comparison

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Bus/Tree Topology• The original topology• Workstation has a network interface card (NIC) that

attaches to the bus (a coaxial cable) via a tap• Data can be transferred using either baseband digital

signals or broadband analog signals• Baseband signals are

– bidirectional and more outward in both directions from the workstation transmitting

• Broadband signals are – usually uni-directional and transmit in only one direction – Because of this, special wiring considerations are

necessary• Buses can be split and joined, creating trees

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Bus/Tree Topology (continued)

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Bus/Tree Topology (continued)

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Bus/Tree Topology (continued)

Need toResolve this!

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Bus/Tree Topology (continued)

Need aTurnaroundtechnique

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Star-Wired Bus Topology• Logically operates as a bus, but physically looks like a star• Star design is based on hub

– All workstations attach to hub• Modular connectors and twisted pair make installation and

maintenance of star-wired bus better than standard bus• Unshielded twisted pair usually used to connect workstation to

hub, but can be connected using– twisted pair, coaxial cable, or fiber-optic cable

• Hub takes incoming signal and immediately broadcasts it out all connected links, thus one talks everyone can listen (Disadv in security)– It also known as the shared network

• All devices are sharing the network medium

• Hubs can be interconnected to extend size of network

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Star-Wired Bus Topology (continued)

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Star-Wired Bus Topology (continued)

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Star-Wired Ring Topology

• Logically operates as a ring but physically appears as a star

• Based on MAU (Multistation Access Unit) which functions similarly to a hub– Where a hub immediately broadcasts all

incoming signals onto all connected links, the MAU passes the signal around in a ring fashion

– Like hubs, MAUs can be interconnected to increase network size

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Star-Wired Ring Topology (continued)

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Star-Wired Ring Topology (continued)

MAU takesPlace here!

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Wireless LANS

• Not really a specific topology (Mesh!)

– Workstation in a wireless LAN can be anywhere as long as it is within transmitting distance to an access point

• Several versions of IEEE 802.11 standard define various forms of wireless LAN connections

• Workstations reside within Basic Service Set, while multiple basic service sets create an Extended Service Set

• How it works? (to p40)

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Wireless LANS (continued)

• Two basic components necessary: – Client radio

• Usually a PC card with an integrated antenna installed in a laptop or workstation

– Access point (AP)• An Ethernet port plus a transceiver• AP acts as a bridge between the wired and wireless

networks and can perform basic routing functions

• Workstations with client radio cards reside within Basic Service Set, while multiple basic service sets create an Extended Service Set

• Their standards

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Wireless LANS (continued)

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Wireless LANS (continued)

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Wireless LANS standards (continued)• IEEE 802.11

• Original wireless standard, capable of transmitting data at 2 Mbps

• IEEE 802.11b• Second wireless standard, capable of transmitting data at 11

Mbps• In actual tests, 11 Mbps 802.11b devices managed 5.5 Mbps

(from a July 2000 test by Network Computing)– With directional antennae designed for point-to-point

transmission (rare), 802.11b can transmit for more than 10 miles

– With an omni-directional antenna on typical AP, range may drop to as little as 100 feet

• Recent standards (to p44)

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• IEEE 802.11a • One of the more recent standards• Capable of transmitting data at 54 Mbps

(theoretical) using the 5-GHz frequency range

• IEEE 802.11g • The other recent standard• Also capable of transmitting data at 54 Mbps

(theoretical) but using the same frequencies as 802.11b (2.4-GHz)

• Is backwards compatible with 802.11b

• Other standards (to p45)

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Wireless LANS (continued)

• HiperLAN/2 (European standard, 54 Mbps in 5-GHz band)

• To provide security, most systems use either: – Wired Equivalent Privacy (WEP) – provides either

40- or 128-bit key protection

– WPA or some other more advanced standard

• Wireless LANs may also be configured without an access point – These configurations are called “ad-hoc”

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Wireless LANS (continued)

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Comparison of Bus, Star-Wired Bus, Star-Wired Ring, and Wireless Topologies

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Medium Access Control Protocols

• How does a workstation get its data onto the LAN medium?

• A medium access control protocol is the software that allows workstations to “take turns” at transmitting data

• Two basic categories:– Contention-based protocols

– Round-robin protocols

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Contention-Based Protocols• Essentially first-come, first-served• Most common example is carrier sense multiple access

with collision detection (CSMA/CD)• If no one is transmitting, workstation can transmit• If someone else is transmitting, workstation “backs off” and

waits– If two workstations transmit at same time, collision occurs

• When two workstations hear collision, they stop transmitting immediately

• Each workstation backs off a random amount of time and tries again

• Hopefully, both workstations do not try again at exact same time

• CSMA/CD is an example of a nondeterministic protocol• Another one CSMA/CA (to p51)

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Contention-Based Protocols (continued)

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Contention-Based Protocols (continued)

• CA (Collision avoidance) – Protocol does not listen and detect collisions– Instead, tries to avoid collisions before they happen

• How does CSMA/CA do this?– All devices, before they transmit, must wait an

amount of time called an interframe space (IFS)– Some applications have a short IFS, while others

have a long IFS• If two applications want to transmit at same time, the

application with shorter IFS will go first

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Round-Robin Protocols

• Each workstation takes a turn transmitting and the turn is passed around the network from workstation to workstation

• Most common example is token ring LAN in which a software token is passed from workstation to workstation – Token ring is an example of a deterministic protocol– Token ring more complex than CSMA/CD

• What happens if token is lost? Duplicated? Hogged?

– Token ring LANs are losing the battle with CSMA/CD LANs

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Round Robin Protocols (continued)

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IEEE 802

• To better support local area networks, the data link layer of the OSI model was broken into two sublayers:– Logical link control sublayer

– Medium access control sublayer

• Medium access control sublayer defines frame layout and is more closely tied to a specific medium at the physical layer– Thus, when people refer to LANs they often refer to its

MAC sublayer name, such as 10BaseT

– IEEE 802.3 Frame Format

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IEEE 802 (continued)

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IEEE 802.3 Frame Format

• IEEE 802 suite of protocols defines frame formats for CSMA/CD (IEEE 802.3) and token ring (IEEE 802.5)

• Each frame format describes how data package is formed

• The two frames do not have the same layout – If a CSMA/CD network connects to a token ring

network, the frames have to be converted from one to another

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IEEE 802.3 Frame Format (continued)

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Wired Ethernet

• Most common form of LAN today• Star-wired bus is most common topology but bus

topology still not totally dead yet• Comes in many forms depending upon medium

used and transmission speed and technology• Originality and its development• Ethernet Standards• Its trends

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Wired Ethernet (continued)

• Originally, CSMA/CD was 10 Mbps• Then 100 Mbps was introduced

– Most NICs sold today are 10/100 Mbps

• Then 1000 Mbps (1 Gbps) was introduced– 1000 Mbps introduces a few interesting wrinkles:

• Transmission is full-duplex (separate transmit and receive), thus no collisions

• Prioritization is possible using 802.1p protocol• Topology can be star or mesh (for trunks)

• 10 Gbps is now being installed in high-end applications

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Wired Ethernet (continued)

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Wired Ethernet (continued)

• One of the latest features is power over Ethernet (PoE) – ie supply electricity current over the medium

• What if you have a remote device that has an Ethernet connection? – It will require a power connection

• What if you don’t have an electrical outlet nearby?– Use PoE

• Power to drive Ethernet NIC is sent over wiring along with usual Ethernet signals

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IBM Token Ring

• Deterministic LAN offered at speeds of 4, 16 and 100 Mbps

• Very good throughput under heavy loads• More expensive components than CSMA/CD• Losing ground quickly to CSMA/CD

– May be extincted soon

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Fiber Distributed Data Interface (FDDI)

• Based on token ring design using 100 Mbps fiber connections

• Allows for two concentric rings– Inner ring can support data travel in opposite

direction or work as backup

• Token is attached to outgoing packet, rather than waiting for outgoing packet to circle entire ring

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Wireless Ethernet

• As we have already seen, IEEE has created the 802.11b, 802.11a, and 802.11g wireless standards

• IEEE 802.11n (100 Mbps) will be ratified soon and started appearing in product form in 2007

• Latest wireless Ethernet is using MIMO technology (multiple input multiple output)

– Sender and receiver have multiple antennas for optimum reception

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LANs In Action: A Small Office Solution

• What type of system will interconnect 20 workstations in one room and 15 workstations in another room to a central server, which offers:– Internal e-mail

– A database that contains all customer information

– High-quality printer access

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LANs In Action: A Small Office Solution (continued)

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LANs In Action: A Small Office Solution (continued)

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LANs In Action: A Home Office Solution

• What if you have two computers at home and want both to share a printer and a connection to the Internet?– Some type of SOHO solution might solve this

problem• Essentially a LAN with a 2- or 3-port hub,

connecting cables, and software

– In some models the hub also acts as a router to the Internet

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LANs In Action: A Home Office Solution (continued)