Chapter 7 Local Area Networks: The Basics


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Chapter 7 Local Area Networks: The Basics

  1. 1. Chapter 7 Local Area Networks: The Basics
  2. 2. Learning Objectives <ul><li>State the definition of a local area network </li></ul><ul><li>List the primary function, activities, and application areas of a local area network </li></ul><ul><li>Cite the advantages and disadvantages of local area networks </li></ul><ul><li>Identify the physical and logical layouts (topologies) of local area networks </li></ul><ul><li>Cite the characteristics of wireless local area networks and their medium access control protocols </li></ul><ul><li>Specify the different medium access control techniques </li></ul><ul><li>Recognize the different IEEE 802 frame formats </li></ul><ul><li>Describe the common local area network systems </li></ul>
  3. 3. Primary Function of a LAN <ul><li>File serving – large storage disk drive acts as a central storage repository </li></ul><ul><li>Print serving – Providing authorization to access a particular printer, accept and queue print jobs, and user access to print queue to perform administrative duties </li></ul><ul><li>Video transfers – High speed LANs are capable of supporting video image and live video transfers </li></ul><ul><li>Manufacturing support – LANs can support manufacturing and industrial environments </li></ul><ul><li>Academic support – In classrooms, labs, and wireless </li></ul><ul><li>E-mail support </li></ul><ul><li>Interconnection between multiple systems </li></ul>
  4. 4. Advantages of LAN <ul><li>Ability to share hardware and software resources </li></ul><ul><li>Individual workstation might survive network failure </li></ul><ul><li>Component and system evolution are possible </li></ul><ul><li>Support for heterogeneous forms of hardware and software </li></ul><ul><li>Access to other LANs and WANs </li></ul><ul><li>Private ownership </li></ul><ul><li>Secure transfers at high speeds with low error rates </li></ul>
  5. 5. Disadvantages of LAN <ul><li>Equipment and support can be costly </li></ul><ul><li>Level of maintenance continues to grow </li></ul><ul><li>Private ownership? </li></ul><ul><li>Some types of hardware may not interoperate </li></ul><ul><li>Just because a LAN can support two different kinds of packages does not mean their data can interchange easily </li></ul><ul><li>A LAN is only as strong as it weakest link, and there are many links </li></ul>
  6. 6. Basic LAN Topologies <ul><li>Bus/tree </li></ul><ul><li>Star-wired bus </li></ul><ul><li>Star-wired ring </li></ul><ul><li>Wireless </li></ul>
  7. 7. Bus/Tree Topology <ul><li>The original topology. </li></ul><ul><li>Workstation has a network interface card (NIC) that attaches to the bus (a coaxial cable) via a tap. </li></ul><ul><li>Data can be transferred using either baseband digital signals or broadband analog signals. </li></ul><ul><li>Baseband signals are bidirectional (broadcast) and move outward in both directions from the workstation transmitting. </li></ul><ul><li>Broadband signals are usually uni-directional and transmit in only one direction. Because of this, special wiring considerations are necessary. </li></ul><ul><li>Buses can be split and joined, creating trees. </li></ul>
  8. 8. Baseband Broadband
  9. 9. Star-wired Bus Topology <ul><li>Logically operates as a bus, but physically looks like a star </li></ul><ul><li>Star design is based on hub. All workstations attach to hub </li></ul><ul><li>Unshielded twisted pair usually used to connect workstation to hub </li></ul><ul><li>Hub takes incoming signal and immediately broadcasts it out all connected links </li></ul><ul><li>Hubs can be interconnected to extend network size </li></ul><ul><li>Modular connectors and twisted pair make installation and maintenance of star-wired bus better than standard bus </li></ul><ul><li>Hubs can be interconnected with twisted pair, coaxial cable, or fiber optic cable </li></ul><ul><li>Biggest disadvantage: when one station talks, everyone hears it. This is called a shared network. All devices are sharing the network medium </li></ul>
  10. 10.
  11. 11. Star-wired Ring Topology <ul><li>Logically operates as a ring but physically appears as a star </li></ul><ul><li>Based on MAU (multi-station access unit) which functions similarly to a hub </li></ul><ul><li>Where a hub immediately broadcasts all incoming signals onto all connected links, the MAU passes the signal around in a ring fashion </li></ul><ul><li>Like hubs, MAUs can be interconnected to increase network size </li></ul>
  12. 12.
  13. 13. Wireless LANs (I) <ul><li>Not really a specific topology </li></ul><ul><li>Workstation in wireless LAN can be anywhere as long as within transmitting distance to access point </li></ul><ul><li>Several versions of IEEE 802.11 standard defines various forms of wireless LAN connections </li></ul><ul><li>Two basic components necessary: </li></ul><ul><ul><li>Client Radio - usually PC card with integrated antenna installed in a laptop or workstation </li></ul></ul><ul><ul><li>Access Point (AP) - Ethernet port plus transceiver </li></ul></ul><ul><li>AP acts as bridge between wired and wireless networks </li></ul><ul><ul><li>Can perform basic routing functions </li></ul></ul><ul><li>Single-cell - Workstations reside within a basic service set </li></ul><ul><li>Multiple-cell - Multiple basic service sets create an extended service set </li></ul><ul><li>Ad-hoc - Wireless LANs configured without access point </li></ul>
  14. 14. Wireless LANs (II) <ul><li>IEEE 802.11 – The original wireless standard, transmitting data at 2 Mbps </li></ul><ul><li>IEEE 802.11b – The second wireless standard, transmitting data at 11 Mbps </li></ul><ul><ul><li>In actual tests, 11 Mbps 802.11b devices managed 5.5 Mbps </li></ul></ul><ul><ul><li>With directional antennae designed for point-to-point transmission (rare), 802.11b can transmit for more than 10 miles </li></ul></ul><ul><ul><li>With an omni-directional antenna on a typical AP, range may drop to as little as 100 feet </li></ul></ul><ul><li>IEEE 802.11a – One of the more recent standards, transmitting data at 54 Mbps using 5 GHz frequency range </li></ul><ul><li>IEEE 802.11g – The other recent standard, also transmitting data at 54 Mbps but using the same frequencies as 802.11b (2.4 GHz) </li></ul><ul><ul><li>Backwards compatible with 802.11b </li></ul></ul><ul><li>HiperLAN/2 (European standard, 54 Mbps in 5 GHz band) </li></ul><ul><li>To provide security, most systems use either: </li></ul><ul><ul><li>Wired Equivalent Privacy (WEP) – provides either 40- or 128-bit key protection </li></ul></ul><ul><ul><li>WPA or some other more advanced standard </li></ul></ul>
  15. 15.
  16. 16. Medium Access Control Protocols <ul><li>How does a workstation get its data onto the LAN medium? </li></ul><ul><li>Medium access control protocol - software that allows workstations to “take turns” at transmitting data </li></ul><ul><li>Two basic categories: </li></ul><ul><ul><li>Contention-based protocols </li></ul></ul><ul><ul><li>Round robin protocols </li></ul></ul>
  17. 17. Contention-Based Protocols (I) <ul><li>Essentially first come first served </li></ul><ul><li>Most common example: </li></ul><ul><ul><li>Carrier sense multiple access with collision detection (CSMA/CD) </li></ul></ul><ul><li>If no one is transmitting, a workstation can transmit </li></ul><ul><li>If someone else is transmitting, workstation “backs off” and waits </li></ul><ul><li>If two workstations transmit at same time, collision occurs </li></ul><ul><ul><li>When two workstations hear collision, they stop transmitting immediately </li></ul></ul><ul><ul><li>Each workstation backs off a random amount of time and tries again </li></ul></ul><ul><ul><li>Hopefully, both workstations do not try again at exact same time </li></ul></ul><ul><li>CSMA/CD is an example of a nondeterministic protocol </li></ul>
  18. 18.
  19. 19. Contention-Based Protocols (II) <ul><li>Wireless CSMA/CA (Collision avoidance) </li></ul><ul><ul><li>Protocol does not listen and detect collisions </li></ul></ul><ul><ul><li>Instead, tries to avoid collisions before they happen </li></ul></ul><ul><li>How does CSMA/CA do this? </li></ul><ul><ul><li>All devices, before they transmit, must wait an amount of time called an interframe space (IFS) </li></ul></ul><ul><ul><li>Some applications have a short IFS, while others have a long IFS </li></ul></ul><ul><ul><li>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. </li></ul></ul>
  20. 20. Round Robin Protocols <ul><li>Each workstation takes turn transmitting: turn is passed around the network from workstation to workstation </li></ul><ul><li>Most common example is token ring LAN in which a software token is passed from workstation to workstation </li></ul><ul><ul><li>Token ring is an example of a deterministic protocol </li></ul></ul><ul><ul><li>Token ring more complex than CSMA/CD </li></ul></ul><ul><ul><ul><li>What happens if token is lost? Duplicated? Hogged? </li></ul></ul></ul><ul><ul><li>Token ring LANs are losing the battle with CSMA/CD LANs </li></ul></ul>
  21. 21.
  22. 22. IEEE 802 <ul><li>To better support local area networks, data link layer of the OSI model was broken into two sublayers: </li></ul><ul><ul><li>Logical link control sublayer </li></ul></ul><ul><ul><li>Medium access control sublayer </li></ul></ul><ul><li>Medium access control sublayer defines the frame layout </li></ul><ul><ul><li>More closely tied to specific medium at physical layer </li></ul></ul><ul><li>Thus, when people refer to LANs they often refer to its MAC sublayer name, such as 10BaseT </li></ul>
  23. 23. IEEE 802 Frame Formats <ul><li>IEEE 802 suite of protocols defines frame formats for CSMA/CD (IEEE 802.3) and token ring (IEEE 802.5) </li></ul><ul><li>Each frame format describes how data package is formed </li></ul><ul><li>If a CSMA/CD network connects to a token ring network, frames have to be converted from one to another </li></ul>
  24. 24. Frame Formats IEEE 802.3 CSMA/CD IEEE 802.5 Token Ring
  25. 25. LAN Systems <ul><li>Ethernet or CSMA/CD </li></ul><ul><li>IBM Token Ring </li></ul><ul><li>FDDI (Fiber Distributed Data Interface) </li></ul>
  26. 26. Ethernet <ul><li>Originally, CSMA/CD was 10 Mbps. </li></ul><ul><li>Then 100 Mbps was introduced. Most NICs sold today are 10/100 Mbps. </li></ul><ul><li>Then 1000 Mbps (1 Gbps) was introduced. </li></ul><ul><ul><li>Transmission is full duplex (separate transmit and receive), thus no collisions. </li></ul></ul><ul><ul><li>Prioritization is possible using 802.1p protocol. </li></ul></ul><ul><ul><ul><li>Topology can be star or mesh (for trunks). </li></ul></ul></ul><ul><ul><li>Cabling can be either UTP or optical. </li></ul></ul><ul><ul><li>Where 10 Mbps Ethernet has less than 30% utilization due to collisions, 1000 Mbps is limited only by traffic queuing. </li></ul></ul><ul><ul><li>Distance with 10 Mbps is limited by CSMA/CD propagation time, whereas 1000 Mbps is limited only by media. </li></ul></ul><ul><li>10 Gbps is now beginning to appear. </li></ul>
  27. 27.
  28. 28. Power & Ethernet <ul><li>What if you have a remote device that has an Ethernet connection? </li></ul><ul><ul><li>It will require a power connection </li></ul></ul><ul><li>What if you don’t have an electrical outlet nearby? </li></ul><ul><ul><li>Use PoE </li></ul></ul><ul><ul><li>Power to drive Ethernet NIC is sent over wiring along with usual Ethernet signals </li></ul></ul><ul><li>Ethernet over power line </li></ul><ul><ul><li>Uses existing power lines in the building </li></ul></ul><ul><ul><li>No new wiring needed </li></ul></ul><ul><ul><li>Slower </li></ul></ul>
  29. 29. IBM Token Ring <ul><li>Deterministic LAN offered at speeds of 4, 16 and 100 Mbps. </li></ul><ul><li>Very good throughput under heavy loads. </li></ul><ul><li>More expensive components than CSMA/CD. </li></ul><ul><li>Losing ground quickly to CSMA/CD. May be extinct soon. </li></ul>
  30. 30. FDDI <ul><li>Based on the token ring design using 100 Mbps fiber connections. </li></ul><ul><li>Allows for two concentric rings - inner ring can support data travel in opposite direction or work as backup. </li></ul><ul><li>Token is attached to the outgoing packet, rather than waiting for the outgoing packet to circle the entire ring. </li></ul>
  31. 31. Wireless Ethernet <ul><li>As we have already seen, IEEE has created the 802.11b, 802.11a, and 802.11g wireless standards </li></ul><ul><li>IEEE 802.11n (100 Mbps) will be ratified soon and should start appearing in product form in 2006 (maybe?) </li></ul><ul><li>Latest wireless Ethernet is using MIMO technology (multiple input multiple output) </li></ul><ul><ul><li>Sender and receiver have multiple antennas for optimum reception </li></ul></ul>
  32. 32. Steps in setting up a network <ul><li>Design the network (decide on and buy the components) </li></ul><ul><li>Install the NICs if necessary </li></ul><ul><li>Establish connections </li></ul><ul><ul><li>Attach the cable (UTP Cat5, UTP Cat3, Fiber optic) </li></ul></ul><ul><ul><li>Attach required hubs, switches, routers </li></ul></ul><ul><li>Configure network software </li></ul>
  33. 33. Types of configuration 1 <ul><li>PCs connected using a cross-over cable, each PC is a peer, no server (<3 PCs) </li></ul>Cross-over cable
  34. 34. Types of configuration 2 <ul><li>Peer-to-peer networks: </li></ul><ul><ul><li>All computers function as both a client and as a server </li></ul></ul><ul><ul><ul><li>Only useful for small (<10) device networks </li></ul></ul></ul>Switch
  35. 35. Types of configuration 3 <ul><li>Dedicated server LAN: </li></ul><ul><ul><li>One or more computers are permanently assigned to being the network server(s) </li></ul></ul><ul><ul><ul><li>File, Database, Print, Communications servers </li></ul></ul></ul>Switch
  36. 36. Types of configuration 4 <ul><li>Virtual private networks (VPNs): </li></ul><ul><ul><li>Devices assigned to a logical network </li></ul></ul><ul><ul><ul><li>using software and third-party network (telephone company or ISP) </li></ul></ul></ul>Switch
  37. 37. The 5-4-3 Rule <ul><li>Between two nodes </li></ul><ul><ul><li>5 segments </li></ul></ul><ul><ul><li>4 repeaters </li></ul></ul><ul><ul><li>3 segments contain user connections </li></ul></ul>R R R R 1 2 3 4 5
  38. 38. Interconnection <ul><li>Necessary to connect a local area network to another local area network or to a wide area network. </li></ul><ul><ul><li>LAN-to-LAN connections are often performed with a bridge-like device. </li></ul></ul><ul><ul><li>LAN-to-WAN connections are usually performed with a router. </li></ul></ul><ul><ul><li>A switch can be used to interconnect segments of a local area network. </li></ul></ul>
  39. 39. Why Segment or Interconnect? <ul><li>To separate / connect one corporate division with another </li></ul><ul><li>To connect two LANs with different protocols </li></ul><ul><li>To connect a LAN to the Internet </li></ul><ul><li>To break a LAN into segments to relieve traffic congestion </li></ul><ul><li>To provide a security wall between two different types of users </li></ul>
  40. 40. Hubs <ul><li>Interconnects two or more workstations into a local area network. </li></ul><ul><li>When a workstation transmits to a hub, the hub immediately resends the data frame out all connecting links. </li></ul><ul><li>A hub can be managed or unmanaged. </li></ul><ul><ul><li>A managed hub possesses enough processing power that it can be managed from a remote location. </li></ul></ul>
  41. 41. Hub issues <ul><li>Maximum distance between devices (100m in 10Base-T) </li></ul><ul><li>Must avoid loops between connected hubs </li></ul><ul><ul><li>message would circulate endlessly </li></ul></ul><ul><li>Number of devices on network increases collision risks </li></ul><ul><ul><li>collisions during peak traffic periods can crash the network (200 devices) </li></ul></ul>
  42. 42. Bridges <ul><li>Connect two similar LANs, such as two CSMA/CD LANs. </li></ul><ul><li>Connect two closely similar LANs, such as a CSMA/CD LAN and a token ring LAN. </li></ul><ul><li>Examines the destination address in a frame and either forwards this frame onto the next LAN or does not. </li></ul><ul><li>Examines the source address in a frame and places this address in a routing table, to be used for future routing decisions. </li></ul>
  43. 43.
  44. 44.
  45. 45. Transparent Bridge <ul><li>Does not need programming but observes all traffic and builds routing tables from observation. </li></ul><ul><ul><li>The observation is called backward learning. </li></ul></ul><ul><li>Each bridge has two connections (ports) and there is a routing table associated with each port. </li></ul><ul><li>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. </li></ul><ul><li>Found with CSMA/CD LANs. </li></ul><ul><li>Can also convert one frame format to another. </li></ul><ul><ul><li>Sometimes refereed to as a gateway or sometimes a router. </li></ul></ul><ul><li>Removes the headers and trailers from one frame format and inserts (encapsulates) the headers and trailers for the second frame format. </li></ul>
  46. 46.
  47. 47. Remote Bridge <ul><li>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. </li></ul><ul><li>Takes the frame before it leaves the first LAN and encapsulates the WAN headers and trailers. </li></ul><ul><ul><li>When the packet arrives at the destination remote bridge, that bridge removes the WAN headers and trailers leaving the original frame. </li></ul></ul>
  48. 48. Switches (I) <ul><li>Combination of a hub and a bridge. </li></ul><ul><li>Can interconnect two or more workstations, but like a bridge, it observes traffic flow and learns. </li></ul><ul><li>When a frame arrives at a switch, the switch examines the destination address and forwards the frame out the one necessary connection. </li></ul><ul><li>Workstations that connect to a hub are on a shared segment. </li></ul><ul><li>Workstations that connect to a switch are on a switched segment. </li></ul>
  49. 49. Switches (II) <ul><li>The backplane of a switch is fast enough to support multiple data transfers at one time. </li></ul><ul><li>A switch that employs cut-through architecture is passing on the frame before the entire frame has arrived at the switch. </li></ul><ul><li>Multiple workstations connected to a switch use dedicated segments. </li></ul><ul><ul><li>This is a very efficient way to isolate heavy users from the network. </li></ul></ul><ul><li>A switch can allow simultaneous access to multiple servers, or multiple simultaneous connections to a single server. </li></ul><ul><li>Using a pair of routers, it is possible to interconnect to switched segments, essentially creating one large local area network </li></ul>
  50. 50. Virtual LANs <ul><li>Logical subgroup within a LAN that is created via switches and software rather than by manually moving wiring from one network device to another </li></ul><ul><li>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” </li></ul><ul><li>A relatively new standard, IEEE 802.1Q, was designed to allow multiple devices to intercommunicate and work together to create a virtual LAN </li></ul><ul><li>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 </li></ul>
  51. 51. Full Duplex Switches <ul><li>Allows for simultaneous transmission and reception of data to and from a workstation </li></ul><ul><li>This full duplex connection helps eliminate collisions </li></ul><ul><li>To support a full duplex connection to a switch, at least two pairs of wires are necessary </li></ul><ul><ul><li>One for the receive operation </li></ul></ul><ul><ul><li>One for the transmit operation </li></ul></ul><ul><ul><li>Most people install four pairs today, so wiring is not problem </li></ul></ul>
  52. 52. Routers <ul><li>Router - device that connects a LAN to a WAN or a WAN to a WAN </li></ul><ul><li>Router: </li></ul><ul><ul><li>Accepts outgoing packet </li></ul></ul><ul><ul><li>Removes any LAN headers and trailers </li></ul></ul><ul><ul><li>Encapsulates necessary WAN headers and trailers </li></ul></ul><ul><li>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 </li></ul><ul><li>Routers are often called “layer 3 devices” </li></ul><ul><ul><li>Operate at the third layer, or OSI network layer, of the packet </li></ul></ul><ul><li>Often incorporate firewall functions </li></ul>