Chapter 7 Local Area Networks: The Basics
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Chapter 7 Local Area Networks: The Basics






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Chapter 7 Local Area Networks: The Basics Chapter 7 Local Area Networks: The Basics Presentation Transcript

  • Chapter 7 Local Area Networks: The Basics
  • Learning Objectives
    • 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 layouts (topologies) of local area networks
    • Cite the characteristics of wireless local area networks and their medium access control protocols
    • Specify the different medium access control techniques
    • Recognize the different IEEE 802 frame formats
    • Describe the common local area network systems
  • Primary Function of a LAN
    • File serving – large storage disk drive acts as a central storage repository
    • Print serving – Providing authorization to access a particular printer, accept and queue print jobs, and user access to 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
  • Advantages of LAN
    • 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
    • Private ownership
    • Secure transfers at high speeds with low error rates
  • Disadvantages of LAN
    • 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
    • A LAN is only as strong as it weakest link, and there are many links
  • Basic LAN Topologies
    • 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 tap.
    • Data can be transferred using either baseband digital signals or broadband analog signals.
    • Baseband signals are bidirectional (broadcast) and move 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.
  • Baseband Broadband
  • 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
    • Hubs can be interconnected to extend network size
    • 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
  • Star-wired Ring Topology
    • Logically operates as a ring but physically appears as a star
    • Based on MAU (multi-station 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
  • Wireless LANs (I)
    • Not really a specific topology
    • Workstation in wireless LAN can be anywhere as long as within transmitting distance to access point
    • Several versions of IEEE 802.11 standard defines various forms of wireless LAN connections
    • Two basic components necessary:
      • Client Radio - usually PC card with integrated antenna installed in a laptop or workstation
      • Access Point (AP) - Ethernet port plus transceiver
    • AP acts as bridge between wired and wireless networks
      • Can perform basic routing functions
    • Single-cell - Workstations reside within a basic service set
    • Multiple-cell - Multiple basic service sets create an extended service set
    • Ad-hoc - Wireless LANs configured without access point
  • Wireless LANs (II)
    • IEEE 802.11 – The original wireless standard, transmitting data at 2 Mbps
    • IEEE 802.11b – The second wireless standard, transmitting data at 11 Mbps
      • In actual tests, 11 Mbps 802.11b devices managed 5.5 Mbps
      • 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 a typical AP, range may drop to as little as 100 feet
    • IEEE 802.11a – One of the more recent standards, transmitting data at 54 Mbps using 5 GHz frequency range
    • IEEE 802.11g – The other recent standard, also transmitting data at 54 Mbps but using the same frequencies as 802.11b (2.4 GHz)
      • Backwards compatible with 802.11b
    • 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
  • Medium Access Control Protocols
    • How does a workstation get its data onto the LAN medium?
    • Medium access control protocol - software that allows workstations to “take turns” at transmitting data
    • Two basic categories:
      • Contention-based protocols
      • Round robin protocols
  • Contention-Based Protocols (I)
    • Essentially first come first served
    • Most common example:
      • Carrier sense multiple access with collision detection (CSMA/CD)
    • If no one is transmitting, a 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
  • Contention-Based Protocols (II)
    • Wireless CSMA/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. If medium is idle after IFS, a random backoff counter is selected and transmission starts after the countdown.
  • Round Robin Protocols
    • Each workstation takes turn transmitting: 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
  • IEEE 802
    • To better support local area networks, 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 the frame layout
      • More closely tied to specific medium at physical layer
    • Thus, when people refer to LANs they often refer to its MAC sublayer name, such as 10BaseT
  • IEEE 802 Frame Formats
    • 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
    • If a CSMA/CD network connects to a token ring network, frames have to be converted from one to another
  • Frame Formats IEEE 802.3 CSMA/CD IEEE 802.5 Token Ring
  • LAN Systems
    • Ethernet or CSMA/CD
    • IBM Token Ring
    • FDDI (Fiber Distributed Data Interface)
  • Ethernet
    • 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.
      • 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).
      • Cabling can be either UTP or optical.
      • Where 10 Mbps Ethernet has less than 30% utilization due to collisions, 1000 Mbps is limited only by traffic queuing.
      • Distance with 10 Mbps is limited by CSMA/CD propagation time, whereas 1000 Mbps is limited only by media.
    • 10 Gbps is now beginning to appear.
  • Power & 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 nearby?
      • Use PoE
      • Power to drive Ethernet NIC is sent over wiring along with usual Ethernet signals
    • Ethernet over power line
      • Uses existing power lines in the building
      • No new wiring needed
      • Slower
  • 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.
  • FDDI
    • Based on the 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 the outgoing packet, rather than waiting for the outgoing packet to circle the entire ring.
  • 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 should start appearing in product form in 2006 (maybe?)
    • Latest wireless Ethernet is using MIMO technology (multiple input multiple output)
      • Sender and receiver have multiple antennas for optimum reception
  • Steps in setting up a network
    • Design the network (decide on and buy the components)
    • Install the NICs if necessary
    • Establish connections
      • Attach the cable (UTP Cat5, UTP Cat3, Fiber optic)
      • Attach required hubs, switches, routers
    • Configure network software
  • Types of configuration 1
    • PCs connected using a cross-over cable, each PC is a peer, no server (<3 PCs)
    Cross-over cable
  • Types of configuration 2
    • Peer-to-peer networks:
      • All computers function as both a client and as a server
        • Only useful for small (<10) device networks
  • Types of configuration 3
    • Dedicated server LAN:
      • One or more computers are permanently assigned to being the network server(s)
        • File, Database, Print, Communications servers
  • Types of configuration 4
    • Virtual private networks (VPNs):
      • Devices assigned to a logical network
        • using software and third-party network (telephone company or ISP)
  • The 5-4-3 Rule
    • Between two nodes
      • 5 segments
      • 4 repeaters
      • 3 segments contain user connections
    R R R R 1 2 3 4 5
  • Interconnection
    • Necessary to connect a local area network to another local area network or to a wide area network.
      • LAN-to-LAN connections are often performed with a bridge-like device.
      • LAN-to-WAN connections are usually performed with a router.
      • A switch can be used to interconnect segments of a local area network.
  • Why Segment or Interconnect?
    • To separate / connect one corporate division with another
    • To connect two LANs with different protocols
    • To connect a LAN to the Internet
    • To break a LAN into segments to relieve traffic congestion
    • To provide a security wall between two different types of users
  • Hubs
    • Interconnects two or more workstations into a local area network.
    • When a workstation transmits to a hub, the hub immediately resends the data frame out all connecting links.
    • A hub can be managed or unmanaged.
      • A managed hub possesses enough processing power that it can be managed from a remote location.
  • Hub issues
    • Maximum distance between devices (100m in 10Base-T)
    • Must avoid loops between connected hubs
      • message would circulate endlessly
    • Number of devices on network increases collision risks
      • collisions during peak traffic periods can crash the network (200 devices)
  • Bridges
    • Connect two similar LANs, such as two CSMA/CD LANs.
    • Connect two closely similar LANs, such as a CSMA/CD LAN and a token ring LAN.
    • Examines the destination address in a frame and either forwards this frame onto the next LAN or does not.
    • Examines the source address in a frame and places this address in a routing table, to be used for future routing decisions.
  • Transparent Bridge
    • Does not need programming but observes all traffic and builds routing tables from observation.
      • The observation is called backward learning.
    • Each bridge has two connections (ports) and there is a routing table associated with each port.
    • Observes each frame that arrives at a port, extracts the source address from the frame, and places that address in the port’s routing table.
    • Found with CSMA/CD LANs.
    • Can also convert one frame format to another.
      • Sometimes refereed to as a gateway or sometimes a router.
    • Removes the headers and trailers from one frame format and inserts (encapsulates) the headers and trailers for the second frame format.
  • Remote Bridge
    • Passing a data frame from one LAN to another when the two LANs are separated by a long distance and there is a wide area network connecting the two LANs.
    • Takes the frame before it leaves the first LAN and encapsulates the WAN headers and trailers.
      • When the packet arrives at the destination remote bridge, that bridge removes the WAN headers and trailers leaving the original frame.
  • Switches (I)
    • Combination of a hub and a bridge.
    • Can interconnect two or more workstations, but like a bridge, it observes traffic flow and learns.
    • When a frame arrives at a switch, the switch examines the destination address and forwards the frame out the one necessary connection.
    • Workstations that connect to a hub are on a shared segment.
    • Workstations that connect to a switch are on a switched segment.
  • Switches (II)
    • The backplane of a switch is fast enough to support multiple data transfers at one time.
    • A switch that employs cut-through architecture is passing on the frame before the entire frame has arrived at the switch.
    • Multiple workstations connected to a switch use dedicated segments.
      • This is a very efficient way to isolate heavy users from the network.
    • A switch can allow simultaneous access to multiple servers, or multiple simultaneous connections to a single server.
    • Using a pair of routers, it is possible to interconnect to switched segments, essentially creating one large local area network
  • Virtual LANs
    • Logical subgroup within a LAN that is created via switches and software rather than by manually moving wiring from one network device to another
    • Even though employees and their actual computer workstations may be scattered throughout the building, LAN switches and VLAN software can be used to create a “network within a network”
    • A relatively new standard, IEEE 802.1Q, was designed to allow multiple devices to intercommunicate and work together to create a virtual LAN
    • Instead of sending technician to a wiring closet to move a workstation cable from one switch to another, an 802.1Q-compliant switch can be remotely configured by a network administrator
  • Full Duplex Switches
    • Allows for simultaneous transmission and reception of data to and from a workstation
    • This full duplex connection helps eliminate collisions
    • To support a full duplex connection to a switch, at least two pairs of wires are necessary
      • One for the receive operation
      • One for the transmit operation
      • Most people install four pairs today, so wiring is not problem
  • Routers
    • Router - device that connects a LAN to a WAN or a WAN to a WAN
    • Router:
      • Accepts outgoing packet
      • Removes any LAN headers and trailers
      • Encapsulates necessary WAN headers and trailers
    • Because router has to make wide area network routing decisions  router has to dig down into the network layer of the packet to retrieve network destination address
    • Routers are often called “layer 3 devices”
      • Operate at the third layer, or OSI network layer, of the packet
    • Often incorporate firewall functions