15-441 Computer NetworkingLAN SwitchingSwitched Network AdvantagesInterconnecting LANsHubsHubs (more)Hub limitationsBridgesBridges (more)Bridges: frame filtering, forwardingBackbone BridgeInterconnection Without BackboneBridge FilteringSlide 14Bridge Learning: exampleSlide 16Bridges Spanning TreeWWF Bridges vs. RoutersRouters vs. BridgesSlide 20Ethernet SwitchesSlide 22Ethernet Switches (more)IEEE 802.11 Wireless LANAd Hoc NetworksIEEE 802.11 MAC Protocol: CSMA/CAIEEE 802.11 MAC ProtocolHidden Terminal effectCollision Avoidance: RTS-CTS exchangeSlide 30Point-to-Point Data Link ControlPPP Design Requirements [RFC 1557]PPP non-requirementsPPP Data FrameSlide 35Byte StuffingSlide 37PPP Data Control Protocol15-441 Computer NetworkingBridges/Switches, 802.11, PPPLecture #6: 9-13-01 2LAN Switching•Extend reach of a single shared medium•Connect two or more “segments” by copying data frames between them•Switches only copy data when needed key difference from repeatersLAN 1 LAN 2Lecture #6: 9-13-01 3Switched Network Advantages•Higher link bandwidth•Point to point electrically simpler than bus•Much greater aggregate bandwidth•Separate segments can send at once•Improved fault tolerance•Redundant paths•Challenge•Learning which packets to copy across links•Avoiding forwarding loopsLecture #6: 9-13-01 4Interconnecting LANsQ: Why not just one big LAN? •Limited amount of supportable traffic: on single LAN, all stations must share bandwidth •limited length: 802.3 specifies maximum cable length •large “collision domain” (can collide with many stations)•limited number of stations: 802.5 have token passing delays at each stationLecture #6: 9-13-01 5Hubs•Physical Layer devices: essentially repeaters operating at bit levels: repeat received bits on one interface to all other interfaces•Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its topLecture #6: 9-13-01 6Hubs (more)•Each connected LAN referred to as LAN segment•Hubs do not isolate collision domains: node may collide with any node residing at any segment in LAN •Hub Advantages:•simple, inexpensive device•Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions•extends maximum distance between node pairs (100m per Hub)Lecture #6: 9-13-01 7Hub limitations•single collision domain results in no increase in max throughput•multi-tier throughput same as single segment throughput•individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage •cannot connect different Ethernet types (e.g., 10BaseT and 100baseT)Lecture #6: 9-13-01 8Bridges•Link Layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination•Bridge isolates collision domains since it buffers frames•When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmitLecture #6: 9-13-01 9Bridges (more)•Bridge advantages:•Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage•Can connect different types of Ethernet since it is a store-and-forward device•Transparent: no need for any change to hosts LAN adaptersLecture #6: 9-13-01 10Bridges: frame filtering, forwarding•bridges filter packets •same-LAN -segment frames not forwarded onto other LAN segments •forwarding: •how to know which LAN segment on which to forward frame?•looks like a routing problem (more shortly!)Lecture #6: 9-13-01 11Backbone BridgeLecture #6: 9-13-01 12Interconnection Without Backbone•Not recommended for two reasons:- single point of failure at Computer Science hub- all traffic between EE and SE must path over CS segmentLecture #6: 9-13-01 13Bridge Filtering•bridges learn which hosts can be reached through which interfaces: maintain filtering tables•when frame received, bridge “learns” location of sender: incoming LAN segment•records sender location in filtering table•filtering table entry: •(Node LAN Address, Bridge Interface, Time Stamp)•stale entries in Filtering Table dropped (TTL can be 60 minutes)Lecture #6: 9-13-01 14Bridge Filtering•filtering procedure:if destination is on LAN on which frame was receivedthen drop the frameelse { lookup filtering table if entry found for destinationthen forward the frame on interface indicated;else flood; /* forward on all but the interface on which the frame arrived*/}Lecture #6: 9-13-01 15Bridge Learning: exampleSuppose C sends frame to D and D replies back with frame to C•C sends frame, bridge has no info about D, so floods to both LANs •bridge notes that C is on port 1 •frame ignored on upper LAN •frame received by DLecture #6: 9-13-01 16Bridge Learning: example•D generates reply to C, sends •bridge sees frame from D •bridge notes that D is on interface 2 •bridge knows C on interface 1, so selectively forwards frame out via interface 1Lecture #6: 9-13-01 17Bridges Spanning Tree•for increased reliability, desirable to have redundant, alternate paths from source to dest•with multiple simultaneous paths, cycles result - bridges may multiply and forward frame forever•solution: organize bridges in a spanning tree by disabling subset of interfacesDisabledLecture #6: 9-13-01 18WWF Bridges vs. Routers•both store-and-forward devices•routers: network layer devices (examine network layer headers)•bridges are Link Layer devices•routers maintain routing tables, implement routing algorithms•bridges maintain filtering tables, implement filtering, learning and spanning tree algorithmsLecture #6: 9-13-01 19Routers vs. BridgesBridges + and - + Bridge operation is simpler requiring less processing bandwidth- Topologies are restricted with bridges: a spanning tree must be built to avoid cycles - Bridges do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a bridge)Lecture #6: 9-13-01 20Routers vs. BridgesRouters + and -+ arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols)+ provide firewall protection against broadcast storms- require IP address configuration (not plug and play)- require higher processing bandwidth•bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)Lecture #6: 9-13-01
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