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Chapter 9: Local Area Networks

Chapter 9: Local Area Networks. Principles of Computer Networks and Communications M. Barry Dumas and Morris Schwartz. Objectives. Describe how different forms of LANs originated and how they evolved Differentiate LAN physical and logical topologies

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Chapter 9: Local Area Networks

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  1. Chapter 9:Local Area Networks Principles of Computer Networks and Communications M. Barry Dumas and Morris Schwartz

  2. Objectives • Describe how different forms of LANs originated and how they evolved • Differentiate LAN physical and logical topologies • Identify LAN addressing issues and the role of MAC addresses • Describe the role of LAN segmentation and its impact on performance • Compare and contrast Ethernet, Token Ring, and FDDI LAN models • Describe the role of VLANs and LANE configurations in networking schemes Principles of Computer Networks and Communications

  3. “Despite the traditional classification of LANs by span, a more relevant classification is link ownership.” Overview • LAN decisions (configuration, speed, O/S, access, etc.) are made by businesses (LAN owners) • WAN links are owned by public carriers Principles of Computer Networks and Communications

  4. Overview • Two basic LAN classifications • Dedicated-server (server-centric or client-server) • Servers function only as servers with specialized functions (printing, database, websites, etc.) • One server must be a file-server • Used by vast majority of businesses! • Peer-to-peer • Each station is a functional equal of every other station • Any computer can access files from any other computer • Any computer can be a server (i.e., take on special functions) Principles of Computer Networks and Communications

  5. Higher layers get involved only when interconnecting LANs LAN Hardware and Software “LAN hardware and software are the concern of the two lowest layers of the Open Systems Interconnection Model (OSI) and TCP/IP model architectures: Layer 1, the physical layer, and Layer 2, the data link layer.” The two lower layers handle all the protocols and specifications needed to run the LAN Principles of Computer Networks and Communications

  6. A device must have a NIC to be a LAN node LAN Hardware and Software • Network Interface Card (NIC) • Hardware/firmware combination containing almost all of the LAN protocols • Contains port(s) to accommodate medium (e.g., CAT 6 copper, fiber, etc.) • Can provide device LAN address • Required for each node on the LAN; a node is a device • Directly connected to the LAN • Directly addressable by the LAN Principles of Computer Networks and Communications

  7. LAN Hardware and Software • Medium Access Control (MAC) address • Physical address—different for each NIC • Defined (and assigned) by IEEE • Hard-coded by manufacturer • Flat addresses contain no location or sequencing • 48 bits long • First 24 bits—IEEE Organizationally Unique Identifier (OUI) • Second 24 bits—manufacturer ID [224 = 16,777,216 addresses] • Stored in read-only memory (ROM) on the NIC Principles of Computer Networks and Communications

  8. LAN Hardware and Software • Network operating system (NOS) Mediates between • LAN workstations • LAN resources • LAN processes • Computer operating system (OS) • Mediates individual workstation resources Full-blown NOS:MS Windows Server Novell Netware Partial NOS:(newer) WindowsMacUNIXLinux Principles of Computer Networks and Communications

  9. LAN Hardware and Software • NOS functions • Contains redirector that determines whether actions are local (for workstation) or network • Incorporates LAN protocols • Enables LAN software to use LAN hardware • Controls server operations • Manages network storage, disk access, and memory • Provides LAN management tools for administrators Principles of Computer Networks and Communications

  10. LANs do not guarantee error-free delivery! Ethernet: The Once and Future King • LAN protocols are designed for best effort delivery • Data frames have a “good chance” of surviving • Receiver determines whether a frame has errors • Higher-layer protocols might provide more precise error detection and recovery Principles of Computer Networks and Communications

  11. Ethernet: The Once and Future King • Ethernet—802.3 • Was not the first [Arcnet was first in 1977] • Is currently the most widely installed • Is considered a contention protocol (stations contend for access) • Uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) • Station desiring access must listen • If a transmission is detected—carrier sensed—station waits • If no transmission is detected—bus is idle—station transmits • Simultaneous transmissions cause collisions Principles of Computer Networks and Communications

  12. Ethernet: The Once and Future King If a collisionis detected Fig 9.1CSMA/CD Principles of Computer Networks and Communications

  13. Ethernet: The Once and Future King • The Ethernet frame • Max frame size = 1,518 bytes [data = 1,500 bytes] • Min frame size = 64 bytes [data = 46 bytes] • 5 data fields • Destination address • Source address • Network protocol or data length (if < 1,518) • Data PDU (higher layer data) • Frame check sequence (error detection) • 2 synchronization fields • Preamble for frame synchronization • Start frame delimiter indicating frame start for receiver Principles of Computer Networks and Communications

  14. Ethernet: The Once and Future King • The Ethernet frame • Max frame size = 1,518 bytes (data = 1,500 bytes) • Min frame size = 64 bytes (data = 46 bytes) Fig 9.2 Data Synchronization Principles of Computer Networks and Communications

  15. Ethernet: The Once and Future King • Ethernet collision window (“slot time”) • Length of time for frame to travel from one end of the LAN to the other • Requires frame limits to work (64 byte min frame size) • For 10 Mbps • Key factors • Bit rate—time for a station to transmit a complete frame • Propagation speed—time for 1 bit to travel to the end of the bus 512 bit times = 512 bits/8 bits per byte = 64 bytes Max length = 500 m Principles of Computer Networks and Communications

  16. Improving Traditional Ethernet • Bus and hub comparison hub bus Fig 9.3 Principles of Computer Networks and Communications

  17. Improving Traditional Ethernet • Bus and star cabling comparison (8 nodes) bus Node Fig 9.4 star Principles of Computer Networks and Communications

  18. Ethernet: The Once and Future King • Thicknet • 10BASE5 • 10 Mbps data rate • Baseband signaling over thick coaxial copper • Max segment length: 500 m • Up to 100 nodes • Up to 4 repeaters • Physical bus • Connected by medium attachment unit (MAU) Principles of Computer Networks and Communications

  19. Improving Traditional Ethernet • Thinnet • 10Base2 • 10 Mbps data rate • Baseband signaling over pencil-thin coaxial copper • Max segment length: 185 m • Up to 30 nodes • Up to 4 repeaters • Physical bus • Connected by NICs (MAU function moved to NICs) Principles of Computer Networks and Communications

  20. Improving Traditional Ethernet • Ethernet(with media type indicators) • 10BASE-T • 10 Mbps data rate • Baseband signaling over twisted pair copper • Max segment length: 185 m • Node limits dictated by ports available on hubs • Hubs could be repeaters (“active hubs”) • Physical star operating as a logical bus • Connected by hubs Principles of Computer Networks and Communications

  21. Improving Traditional Ethernet • Ethernet(with media type indicators) • Advantages • Reliability improved—bus disruptions don’t take down LAN • Management improved—simple network management protocol (SNMP) installed on hub • Maintenance improved—easier to add workstations • Disadvantages • Physical stars require more cabling • Hub becomes single point of failure Principles of Computer Networks and Communications

  22. Improving Traditional Ethernet • Replacing the hub with a switch • How it works • Switch connects workstations in pairs • Will not connect transmitting stations to a busy one • LAN no longer operates as a bus—no contention! • Advantages • No collisions—each station has own link to switch • Compatibility with CSMA/CD is maintained • Multiple workstations can transmit simultaneously • Simple to upgrade—replace hub with switch Principles of Computer Networks and Communications

  23. Improving Traditional Ethernet • Fast Ethernet • 100BASE-TX • 100 Mbps data rate • Baseband signaling, cat 5 UTP • Max segment length: 100 m (span limit: 250m) • Node limits dictated by ports available on hubs • Hubs could be repeaters (“active hubs”) • Physical star operating as a logical bus • Connected by switches 100BASE-FX is multimode fiber-optic version Principles of Computer Networks and Communications

  24. Improving Traditional Ethernet • Fast Ethernet • 100BASE-TX LAN side device side Fig 9.5 Principles of Computer Networks and Communications

  25. Designed to run on cat 3 UTP Improving Traditional Ethernet • Fast Ethernet • 100BASE-T4 LAN side device side Fig 9.6 Principles of Computer Networks and Communications

  26. Improving Traditional Ethernet • Fast Ethernet (100BASE-TX) • Advantages • Speed boost (10 Mbps to 100 Mbps) • Backward compatible—10/100 Mbps on same LAN • Easy device upgrade • Upgrade switch • With CAT 5 UTP or STP, swap NICs • Disadvantages • Maximum segment length is 100 m [total span limit: 250 m] • Switch is single point of failure Principles of Computer Networks and Communications

  27. Improving Traditional Ethernet • Gigabit Ethernet • 1000BASE-T • 1000 Mbps data rate • Baseband signaling over cat 5UTP • Max segment length: 100 m (span limit: 100 m) • Min frame size: 512 bytes (up from 64 byte) • Connected by switches Principles of Computer Networks and Communications

  28. Improving Traditional Ethernet • Gigabit Ethernet(other classifications) • 1000BASE-X • 1000BASE-CX • Copper over twinax or quad cabling • Max span: 25 m • 1000BASE-LX • Fiber-optic (1,300 nm signals) • Max span: 550 m (multimode) • Max span: 3,000 m (single-mode) • 1000BASE-SX • Fiber-optic (850 nm signals) • Max span: 550 m (multimode) • Max span 3,000 m (single-mode) Principles of Computer Networks and Communications

  29. Improving Traditional Ethernet • 10 Gigabit Ethernet • 10GBASE-X • 10 Gbps data rate • Full duplex signaling over fiber-optic media • 7 versions • 10GBASE-SR (short-range) and –SW (short-wavelength) • 10GBASE-LR (long-range) and –LW (long-wavelength) • 10GBASE-ER (extended-range) and –EW (extra-long wavelength) • 10GBASE-LX4 (carries signals on 4 light wavelengths) Principles of Computer Networks and Communications

  30. Token Ring • Token ring – 802.5 • Patented by Olof Söderblom in the late 60s—licensed to IBM • Practically no new installations • Speeds typically 4/16Mbps (100 Mbps standard exists) • A round-robin protocol (stations take turns in order) • Most commonly configured as a physical star/logical ring • Each station is connected to a multistation access unit (MAU) • Logically, each station is connected point to point to a predecessor node and a successor node • A small packet (token) controls medium access • A station can transmit data only when it has the token • Only one token is in circulation at any time Principles of Computer Networks and Communications

  31. Token Ring Fig 9.7 Principles of Computer Networks and Communications

  32. Token Ring • The token ring frame • Three frame types • Token frame • Data frame • Control frame • Data and control frames have the same format Principles of Computer Networks and Communications

  33. Token Ring • The token ring frame data and control frames have the same format Fig 9.8 Principles of Computer Networks and Communications

  34. LAN Segmentation Segmentation in action LAN segmentation • Goal • Reduce congestion by grouping stations according to traffic • Approach • Segments include workstations that often communicate with • On another • Common data source • Common resource • Each segment becomes a LAN in itself • Segments can later be interconnected to share resources 40 stations : 10 Mbps LAN  10 Mbps/40 = 250 Kbps(2) 20 stations: 10 Mbps LAN `10 Mbps/20 = 500 Kbps Principles of Computer Networks and Communications

  35. LAN Segmentation • Bridge operation and bridge types • What is a bridge? • A traffic monitor between two LANs • A filter to keep local traffic from crossing between LANs • A segment device that keeps local traffic off other LANs • Bridge address tables • Track device addresses on both sides • How they are created distinguishes types of bridges • Types of bridges • Manual bridge—addresses are manually loaded into a table • Learning bridge—automatically creates its own tables • Bridge can flood both sides of a LAN to learn what devices respond • Bridge can learn device addresses when new source addresses appear in frames Principles of Computer Networks and Communications

  36. LAN Segmentation • Using backbones to interconnect LANs • Instead of directly connecting LANs and bridges, all interLAN links traverse the backbone • Backbones may be • Linked to LANs by bridges • Based on routers • LANs themselves • LAN stations connect to the backbone via their LAN hubs or switches Principles of Computer Networks and Communications

  37. LAN Segmentation • Bridged backbone • Each server has • one port connection to the backbone bus • One port connection to its LAN switch • Bridge forwards only to the bus frames from its LAN destined for a nonlocal LAN Fig 9.11 Principles of Computer Networks and Communications

  38. If the router failsthe backbone fails LAN Segmentation • Star-wired (collapsed) backbone • Each LAN switch is connected to a router that sends frames according to frame destination addresses • Backbone is considered to be shrunk (collapsed) into the router itself Fig 9.11 Principles of Computer Networks and Communications

  39. LAN Segmentation • Backbone LAN • Same as star-wired backbone except a LAN takes the place of a router • Each connected LAN becomes a node on the backbone LAN Fig 9.12 Principles of Computer Networks and Communications

  40. LAN Segmentation • FDDI (Fiber Distributed Data Interface) • Token-passing protocol • 100 Mbps • Station separation up to 2 Km (1.25 mi) on single-mode fiber • Originally used as MAN backbone • Superseded by higher speed Ethernet Principles of Computer Networks and Communications

  41. LAN Segmentation • FDDI (Fiber Distributed Data Interface) Fig 9.13 Principles of Computer Networks and Communications

  42. VLANs • Virtual LAN (VLAN) • 802.3ac • Grouped by • Station characteristics • Switch characteristics • Frame protocols • Physical LAN memberships or links are not changed Principles of Computer Networks and Communications

  43. VLANs • Virtual LAN (VLAN) • Benefits • Security • Traffic reduction • Flexibility • Cost savings • Caveats • Ease in setup does not presume well-designed • Be wary of too many members on too many physical LANs • Stations with occasional communications should not be members • Problems • Congestion • Network management difficulty Principles of Computer Networks and Communications

  44. VLANs Fig 9.14 Principles of Computer Networks and Communications

  45. VLANs • Attribute-based VLANs • Configured by creating list mappings (access lists) • Switches discern which ports belong to which VLANs • Membership can be assigned • Mostly manual • Partly manual • Mostly automatic • Protocol-based VLANs • Membership determined on a frame-by-frame basis • Participation based on individual transmissions instead of port assignment Principles of Computer Networks and Communications

  46. VLANs • Tagged Ethernet • Enables workstations to belong to several VLANS at same time • First 20 bytes are same as Ethernet frame • Four tag bytes are inserted between the source address and the type/length field Fig 9.15 Principles of Computer Networks and Communications

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