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					     Chapter Seven

Local Area Networks: The Basics
          After reading this chapter,
            you should be able to:
• State the definition of a local area network
• List the primary function, activities, and
  application areas of a local area network
• Cite the advantages and disadvantages of local
  area networks
• Identify the physical and logical topologies of
  local area networks

         After reading this chapter,
     you should be able to (continued):
• Cite the characteristics of wireless local area
  networks and their medium access control
• Specify the different medium access control
• Recognize the different IEEE 802 frame formats
• Describe the common local area network

                Food for thought

• Your car probably has more computing power
  than did the Apollo 11 ship that went to the
• Your car is a LAN, if you think about it.
• Each wheel is monitored for ABS
• Wheels and fuel control are monitored for
  stability control
• Wheels, fuel and tranny monitored for traction
• Heaven forbid, you add cruise control to the mix.

• A local area network 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 local area network first appeared in
  the 1970s, its use has become widespread in
  commercial and academic environments

  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
            – The UM Banner system on which I post your grades
            – My NAV updates
            – Not the least, my email and Web resources
   – 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
             – Classroom 118, and five others

  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 (continued):
   – Manufacturing support
      • LANs can support manufacturing and industrial environments
          – Milwaukee robotics exhibit…
   – Academic support
      • In classrooms, labs, and wireless
   – E-mail support
   – Interconnection between multiple systems

Primary Function of Local Area Networks

      Advantages and Disadvantages 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
  – Access to other LANs and WANs
  – Private ownership
  – Secure transfers at high speeds with low error rates

     Advantages and Disadvantages of
      Local Area Networks (continued)
• Disadvantages
  – Equipment and support can be costly
     • 10K rule of thumb
  – Level of maintenance continues to grow
  – Private ownership?
  – Some types of hardware may not interoperate
     • Can you say Mac? Is there Big Iron in the mix?
  – Just because a LAN can support two different kinds of
    packages does not mean their data can interchange
  – LAN is only as strong as its weakest link, and there
    are many links
      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

              Bus/Tree Topology

• The original topology
• Workstation has a network interface card (NIC)
  that attaches to the bus (a coaxial cable) via a
• Data can be transferred using either baseband
  digital signals or broadband analog signals

Bus/Tree Topology (continued)

      Bus/Tree Topology (continued)

       Bus/Tree Topology (continued)

• Baseband signals are bidirectional and move
  outward in both directions from the workstation
• Broadband signals are usually uni-directional
  and transmit in only one direction
  – Because of this, special wiring considerations are
• Buses can be split and joined, creating trees

Bus/Tree Topology (continued)

           Bus/Tree Topology (continued)

obsolescence alert!                        17
            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
• Unshielded twisted pair usually used to connect
  workstation to hub
• Hub takes incoming signal and immediately
  broadcasts it out all connected links
   – Both and advantage and disadvantage of the lowest
     OSI level connection.
• Hubs can be interconnected to extend size of
                        Has supplanted the Bus as the    18
                        standard LAN topology
Star-Wired Bus Topology (continued)

      Star-Wired Bus Topology (continued)

Address header is critical, since every
station sees ALL the data
    Star-Wired Bus Topology (continued)

• Modular connectors and twisted pair make
  installation and maintenance of star-wired bus
  better than standard bus
• Hubs can be interconnected with twisted pair,
  coaxial cable, or fiber-optic cable
• Biggest disadvantage = when one station talks,
  everyone hears it
  – This is called a shared network
     • All devices are sharing the network medium
     • Broadcast storms are the unintended result

          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
• Why would you want to do this?
  – Token passing?
  – Traffic management?                           22
Star-Wired Ring Topology (continued)

Star-Wired Ring Topology (continued)

                   Wireless LANS

• Not really a specific topology
   – 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

            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
   – Why ROUTING, in particular? Routing is “network aware,”
• Workstations with client radio cards reside within Basic
  Service Set, while multiple basic service sets create an
  Extended Service Set
   – Is this wireless LAN vs wireless WAN? Or not?
Wireless LANS (continued)

Wireless LANS (continued)

         Wireless LANS (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)

          Wireless LANS (continued)

• 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
  – With anything solid in the way, be happy with 20-

           Wireless LANS (continued)

• IEEE 802.11a
  – One of the more recent standards
  – Capable of transmitting data at 54 Mbps (theoretical) using
    the 5-GHz frequency range
  – Apple Air-Port. Is that why it’s called “a”?
• 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
     • But James suggests “not really”

           Wireless LANS (continued)

• HiperLAN/2 (European standard, 54 Mbps in 5-GHz
• 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
      • My hacker pals say use this, not WEP
• Wireless LANs may also be configured without an
  access point
   – These configurations are called “ad-hoc”
   – The MIT “$100” laptop for developing world. Ad-hoc P2P

Wireless LANS (continued)

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

Switched LANS – LAN segmentation, and LAN emulation. Was
the hottest thing going when I was a student.
      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

        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
• If someone else is transmitting, workstation
  “backs off” and waits
• How do they know?
  – My interference example from the other day. The
    “CD” part of the protocol.

  Contention-Based Protocols (continued)

• 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

Contention-Based Protocols (continued)

   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
• This is WiFi’s modality

             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
  – 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
Round Robin Protocols (continued)

                        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
   – And why we have “MAC addresses” based on the unique
     identity of our Ethernet NICS.
Obsolescence alert! 10baseT was what
I used as a doctoral student 10 years                          42
IEEE 802 (continued)

         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
  – Gateway required…a connecting device that also
    translates each frame format to the other.
IEEE 802.3 Frame Format (continued)

                     Ensures an
                     accurate data


               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

         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
• 10 Gbps is now being installed in high-end

          Wired Ethernet (continued)

• 1000 Mbps introduces a few interesting wrinkles:
  – Transmission is full-duplex (separate transmit and
    receive), thus no collisions
  – Cabling comes in pairs, though.
  – Prioritization is possible using 802.1p protocol
     • Topology can be star or mesh (for trunks)
        – Mesh is when each node is connected to every

Wired Ethernet (continued)

           Wired Ethernet (continued)

• One of the latest features is power over Ethernet
• 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
   – Use PoE
      • Power to drive Ethernet NIC is sent over wiring
        along with usual Ethernet signals

                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 extinct soon

   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

               Wireless Ethernet

• As we have already seen, IEEE has created the
  802.11b, 802.11a, and 802.11g wireless
• IEEE 802.11n (100 Mbps) will be ratified soon
  and should start appearing in product form in
• Latest wireless Ethernet is using MIMO
  technology (multiple input multiple output)
  – Sender and receiver have multiple antennas for
    optimum reception

   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

         LANs In Action:
A Small Office Solution (continued)

         LANs In Action:
A Small Office Solution (continued)

   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
      • Wait! Let’s not mix our metaphors. A hub does not route,
        although a router can do hub work. Axiomatic requirement of

        LANs In Action:
A Home Office Solution (continued)


• Primary functions of a LAN are to enable sharing of
  data, software, and peripherals and to provide
  common services such as file serving, print serving,
  support for electronic mail, and process control and
  monitoring in office, academic, and manufacturing
• Local area networks have numerous advantages
  and disadvantages
• A LAN can be configured as a bus/tree topology, a
  star-wired bus topology, a star-wired ring topology,
  or a wireless network

               Summary (continued)

• With both baseband and broadband buses, expansion in
  the form of adding a new workstation is difficult when a
  tap is not available
• For a workstation to place data onto a LAN, network
  must have a medium access control protocol
   – Two basic forms of medium access control protocols are:
      • Contention-based
      • Round-robin
• CSMA/CD works on first-come, first-served basis,
  supports half-duplex and full-duplex connections, and is
  clearly the most popular access protocol

            Summary (continued)

• Round-robin protocols are good under heavy
  loads but require more software support
• To standardize the medium access control
  protocols, IEEE created the 802 series of
  network standards
• The most popular types of LAN systems are
  Ethernet (CSMA/CD) and wireless Ethernet


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