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Chapter 11 Extending LANs: Fiber Modems, Repeaters, Bridges, & Switches

Chapter 11 Extending LANs: Fiber Modems, Repeaters, Bridges, & Switches. Repeater Hub Bridge Switch. Each LAN technology is designed for a specific combination of speed, distance, and cost.

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Chapter 11 Extending LANs: Fiber Modems, Repeaters, Bridges, & Switches

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  1. Chapter 11 Extending LANs: Fiber Modems, Repeaters, Bridges, & Switches • Repeater • Hub • Bridge • Switch

  2. Each LAN technology is designed for a specific combination of speed, distance, and cost. • A maximum length specification is a fundamental part of LAN technology. LAN hardware is engineered for a fixed maximum length cable and the hardware will not work correctly over wires that exceed the bound • Fiber Optic Extensions (fig 11.1) • a pair of fiber transceivers and optical fibers can be used to provide a connection between the AUI on a hub and a remote AUI on a router on a remote LAN.

  3. Repeater (Hub) • a hardware device used to extend a LAN. • Provides connectivity between two cable segments • amplifies and sends all electrical signals that occur on one segment to the other segment (fig 11.2)(ANIM09_1.MOV) • A hub is essentially a multiport repeater • Speed of signals in copper cable = 2 x 108 m/s • any two stations cannot be separated by more than 4 repeaters (fig 11.3) • Drawback of repeaters • all signals are repeated, including those overlapping signals that correspond to collisions and those due to electrical noise. Therefore repeaters cause the same problem to occur on all other segments.

  4. Bridge • a hardware device that extends a LAN by forwarding complete, correct frames from one segment to another (fig 11.4)(ANIM09_2.MOV) • Reads each frame in promiscuous mode and verifies data integrity • bridge will not forward collisions or interference from one segment to another • Computers do not know whether a bridge separates them • Bridges operate at data-link (layer 2) layer. • A bridge filters, forwards, or floods an incoming frame based on the MAC address of that frame • Frame filtering function of bridge: frames are not forwarded across bridge unless necessary. • Using bridges and fiber modem(transceiver) (fig 11.6)

  5. Adaptive (learning) bridge • looks at the physical addresses in the header of each frame it receives. • source address used to determine location of sender • destination address used to determine whether to forward a frame • Since bridge does not know location of computers at bootup, frames are forwarded until location of computer can be determined (fig 11.4)(fig 11.5) • Broadcast and multicast frames are always forwarded • Broadcast storms can cripple network

  6. Switch (Layer 2) • Multiport bridge • Segment • a section of a network that is bounded by bridges, switches, or routers • switching simulates a bridged LAN with one computer per segment while a hub simulates a single shared medium (fig 11.10) • Flooding • forwarding packets out all ports(segments) except source segment • Occurs at power up and for unknown destinations • occurs for all broadcast packets

  7. Maximum bandwidth of a switch = RN/2 R is data transmission rate on a given port N is the total number of switch ports.

  8. Memory buffering in switches • port-based (separate I/O queues per port) • shared memory buffering

  9. Switching methods • store & forward • cut-through switching • fast forward [wait 8-bytes] • collision fragment-free [wait 64-bytes]

  10. Collision domain • the network area within which frames that have collided are propagated. • Repeaters and hubs propagate collisions. • LAN bridges, switches, and routers do not.

  11. Broadcast domain • the set of all devices that will broadcast frames originating from any device within the set • crosses layer 2 switches • bounded by layer 3 devices (router)

  12. Advantages of bridged(switched) network over repeaters and hubs • Larger aggregate bandwidth • allows simultaneous communication between more than one pair of computers • Traffic isolation • Packets forwarded only when needed • Fewer collisions • Bridging across longer distances • point-to-point and special bridge hardware at each end. Both sites filters packets due to bandwidth constraints. Buffering is done on bridge to accommodate speed differences between bridge ports. • leased serial line to connect sites • leased satellite channel

  13. Cycle of Bridges • Occurs when multiple paths between two machines exist • may cause infinite loops • To prevent infinite loops in a bridge network that contains a cycle of bridge segments, some bridges must agree not to forward frames (fig 11.9)

  14. Spanning-Tree protocol • used to insure loop-free path between any two nodes on network. • protocol detects and breaks loops by placing some connections in standby mode • a bridge does not forward frames if the bridge finds that each segment to which it attaches already contains a bridge that has agreed to forward frames • Spanning-tree protocol is used in fault-tolerant networks

  15. VLAN • A VLAN is a broadcast domain. • VLANS group users by logical association instead of physical location. • broadcast domain of VLAN A is separate from that of VLAN B • VLAN is a logical grouping of network devices/users not restricted to a physical switch segment • Frame tagging (802.1Q) in trunk connections • Types of VLAN • port-based (layer1) • mac-based (layer2) • IP-based VLANS (layer3)

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