lect17

Information about lect17

Published on January 1, 2008

Author: Moorehead

Source: authorstream.com

Content

Ethernet:  Ethernet Widely deployed because: First LAN technology Simpler and less expensive than token LANs and ATM Kept up with the speed race: 10, 100, 1000 Mbps Many E-net technologies (cable, fiber etc). But they all share common characteristics Ethernet Frame Structure:  Ethernet Frame Structure Sending adapter encapsulates an IP datagram in Ethernet Frame which contains a Preamble, a Header, Data, and CRC fields Preamble: 7 bytes with the pattern 10101010 followed by one byte with the pattern 10101011; used for synchronizing receiver to sender clock (clocks are never exact, some drift is highly likely) Header contains Destination and Source Addresses and a Type field Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk) CRC: checked at receiver, if error is detected, the frame is simply dropped CSMA/CD:  CSMA/CD A: sense channel, if idle then {transmit and monitor the channel; If detect another transmission then { abort and send jam signal; update # collisions; delay as required by exponential backoff algorithm; go to A } else {done with the frame} } else {wait until ongoing transmission is over and go to A} CSMA/CD (Cont.):  CSMA/CD (Cont.) Jam Signal: to make sure all other transmitters are aware of the collision; 48 bits; Exponential Backoff: Goal is to adapt the offered rate by transmitters to the estimated current load (ie backoff when load is heavy) After the first collision Choose K from {0,1}; delay is K x 512 bit transmission times After second collision choose K from {0,1,2,3}… After ten or more collisions, choose K from {0,1,2,3,4,…,1023} Under this scheme a new frame has a chance of sneaking in the first attempt, even in heavy traffic Ethernet Efficiency: under heavy traffic and large number of nodes: Ethernet Technologies: 10Base2:  Ethernet Technologies: 10Base2 10=>10Mbps; 2=>under 200 meters maximum length of a cable segment; also referred to as “Cheapnet” Uses thin coaxial cable in a bus topology Repeaters are used to connect multiple segments (up to 5); a repeater repeats the bits it hears on one interface to its other interfaces, ie a physical layer device only! 10BaseT and 100BaseT:  10BaseT and 100BaseT 10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Hub to which nodes are connected by twisted pair, thus “star topology” CSMA/CD implemented at the Hub Max distance from node to Hub is 100 meters Hub can disconnect a “jabbering adapter”; 10base2 would not work if an adapter does not stop transmitting on the cable Hub can gather monitoring information and statistics for display to LAN administrators Gbit Ethernet:  Gbit Ethernet Use standard Ethernet frame format Allows for Point-to-point links and shared broadcast channels In shared mode, CSMA/CD is used Full-Duplex at 1 Gbps for point-to-point links Hubs, Bridges and Switches:  Hubs, Bridges and Switches Used for extending LANs in terms of geographical coverage, number of nodes, administration capabilities, etc. Differ in regards to: collision domain isolation layer at which they operate Hubs:  Hubs 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 a backbone hub at its top Each connected LAN is referred to as a LAN segment Hubs do not isolate collision domains: a node may collide with any node residing at any segment in the LAN Hubs (Cont.):  Hubs (Cont.) Hub Advantages: + Simple, inexpensive device + Multi-tier provides graceful degradation: portions of the LAN continue to operate if one of the hubs malfunction + Extends maximum distance between node pairs (100m per Hub) + Interdepartmental Communication Hub Limitations: - Single collision domain results in no increase in max throughput; the multi-tier throughput same as the the single segment throughput - Cannot connect different Ethernet types (eg 10BaseT and 100baseT) Bridges:  Bridges Link Layer devices: operate on Ethernet frames, examining the frame header and selectively forwarding a frame based on its destination Bridge isolates collision domains since it buffers frames When a frame is to be forwarded on a segment, the bridge uses CSMA/CD to access the segment and transmit 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 type Ethernet since it is a store and forward device + Transparent: no need for any change to hosts LAN adapters Backbone Bridge:  Backbone Bridge Interconnection Without Backbone:  Interconnection 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 segment Bridge Filtering:  Bridge Filtering Bridges learn which hosts can be reached through which interfaces and maintain filtering tables (bridge tables) A filtering table entry: (Node LAN Address, Bridge Interface, Time Stamp) where Node LAN Address is the 6 byte physical address Filtering procedure: if destination is on LAN on which frame was received then drop the frame else { lookup filtering table if entry found for destination then forward the frame on interface indicated; else flood; /* forward on all but the interface on which the frame arrived*/ } Bridge Learning:  Bridge Learning When a frame is received, the bridge “learns” from the source address and updates its filtering table (Node LAN Address, Bridge Interface, Time Stamp) Stale entries in the Filtering Table are dropped (TTL can be 60 minutes) Bridges Spanning Tree:  Bridges Spanning Tree For increased reliability, it is desirable to have redundant, alternate paths from a source to a destination With multiple simultaneous paths however, cycles result on which bridges may multiply and forward a frame forever Solution is organizing the set of bridges in a spanning tree by disabling a subset of the interfaces in the bridges: Disabled Bridges Vs. Routers:  Bridges Vs. Routers Both are store-and-forward devices, but Routers are Network Layer devices (examine network layer headers) and Bridges are Link Layer devices Routers maintain routing tables and implement routing algorithms, bridges maintain filtering tables and implement filtering, learning and spanning tree algorithms Routers Vs. Bridges (Cont):  Routers Vs. Bridges (Cont) Bridges + 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) Routers + and - + Arbitrary topologies can be supported, cycling is limited by TTL counters + 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 are required in large networks (thousands of hosts) Ethernet Switches:  Ethernet Switches A switch is a device that incorporates bridge functions as well as point-to-point ‘dedicated connections’ A host attached to a switch via a dedicated point-to-point connection; will always sense the medium as idle; no collisions ever! Ethernet Switches provide a combinations of shared/dedicated, 10/100/1000 Mbps connections Some E-net switches support cut-through switching: frame forwarded immediately to destination without awaiting for assembly of the entire frame in the switch buffer; slight reduction in latency Ethernet switches vary in size, with the largest ones incorporating a high bandwidth interconnection network Ethernet Switches (Cont):  Ethernet Switches (Cont) Dedicated Shared

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