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Unformatted text preview: Data and Computer Communications Communications
Chapter 15 – Local Area Network Overview Overview
Eighth Edition by William Stallings Lecture slides by Lawrie Brown Local Area Network Overview Local
The whole of this operation is described in The minute detail in the official British Naval History, and should be studied with its excellent charts by those who are interested in its technical aspect. So complicated is the full story that the lay reader cannot see the wood for the trees. I have endeavored to render intelligible the broad effects. the —The World Crisis, Winston Churchill LAN Applications (1) LAN personal computer LANs low cost limited data rate iinterconnecting large systems (mainframes and large nterconnecting storage devices) storage
• • • • • high data rate high speed interface distributed access limited distance limited number of devices back end networks LAN Applications (2) LAN storage area networks (SANs) separate network handling storage needs detaches storage tasks from specific servers shared storage facility
• eg. hard disks, tape libraries, CD arrays accessed using a high-speed network
• eg. Fibre Channel improved client-server storage access direct storage to storage communication for backup Storage Area Networks Storage LAN Applications (3) LAN high speed office networks desktop image processing high capacity local storage interconnect low speed local LANs reliability capacity cost backbone LANs LAN Architecture LAN topologies transmission medium layout medium access control LAN Topologies LAN Bus and Tree Bus used with multipoint medium transmission propagates throughout medium transmission heard by all stations full duplex connection between station and tap allows for transmission and reception to avoid collisions and hogging need to regulate transmission terminator absorbs frames at end of medium tree a generalization of bus headend connected to branching cables Frame Transmission Transmission on Bus LAN Ring Topology Ring a closed loop of repeaters joined by point to closed point links point receive data on one link & retransmit on another links unidirectional stations attach to repeaters circulate past all stations destination recognizes address and copies frame frame circulates back to source where it is removed data in frames media access control determines when a station media can insert frame can Frame Transmission Transmission Ring LAN Star Topology Star each station connects to central node usually via two point to point links physical star, logical bus only one station can transmit at a time either central node can broadcast or central node can act as frame switch Choice of Topology Choice reliability expandability performance needs considering in context of: medium wiring layout access control Bus LAN Transmission Media (1) Transmission twisted pair early LANs used voice grade cable didn’t scale for fast LANs not used in bus LANs now uses digital signalling original Ethernet baseband coaxial cable Bus LAN Transmission Media (2) Transmission broadband coaxial cable as in cable TV systems analog signals at radio frequencies expensive, hard to install and maintain no longer used in LANs expensive taps better alternatives available not used in bus LANs optical fiber less convenient compared to star topology twisted pair coaxial baseband still used but not often in new coaxial installations installations Ring and Star Usage Ring ring very high speed links over long distances single link or repeater failure disables network uses natural layout of wiring in building best for short distances high data rates for small number of devices star Choice of Medium Choice constrained by LAN topology capacity reliability types of data supported environmental scope Media Available Media Voice grade unshielded twisted pair (UTP) Cat 3 phone, cheap, low data rates more expensive, higher data rates even more expensive, higher data rate Cat 5+, very high data rates, witched star topology Cat security, high capacity, small size, high cost Shielded twisted pair / baseband coaxial Broadband cable High performance UTP Optical fibre LAN Protocol Architecture LAN IEEE 802 Layers (1) IEEE Physical encoding/decoding of signals preamble generation/removal bit transmission/reception transmission medium and topology IEEE 802 Layers (2) IEEE Logical Link Control Logical interface to higher levels flow and error control on transmit assemble data into frame on on receive disassemble frame govern access to transmission medium for same LLC, may have several MAC options Media Access Control LAN Protocols in Context LAN Logical Link Control transmission of link level PDUs between transmission stations stations must support multiaccess, shared medium but MAC layer handles link access details addressing involves specifying source and addressing destination LLC users destination referred to as service access points (SAP) typically higher level protocol LLC Services LLC based on HDLC unacknowledged connectionless service connection mode service acknowledged connectionless service LLC Protocol LLC modeled after HDLC asynchronous balanced mode asynchronous connection mode (type 2) LLC service connection using unnumbered information PDUs (type 1) using using 2 new unnumbered PDUs (type 3) unacknowledged connectionless service acknowledged connectionless service permits multiplexing using LSAPs MAC Frame Format MAC Media Access Control Media where central
• greater control, single point of failure distributed
• more complex, but more redundant how synchronous
• capacity dedicated to connection, not optimal asynchronous
• in response to demand Asynchronous Systems Asynchronous round robin each station given turn to transmit data divide medium into slots good for stream traffic all stations contend for time good for bursty traffic simple to implement tends to collapse under heavy load reservation contention MAC Frame Handling MAC MAC layer receives data from LLC layer fields MAC control destination MAC address source MAC address LLC CRC MAC layer detects errors and discards frames LLC optionally retransmits unsuccessful frames Bridges Bridges connects similar LANs identical physical / link layer protocols minimal processing can map between MAC formats reasons for use reliability performance security geography Bridge Function Bridge Bridge Design Aspects Bridge no modification to frame content or format no encapsulation exact bitwise copy of frame minimal buffering to meet peak demand contains routing and address intelligence may connect more than two LANs bridging is transparent to stations Bridge Protocol Architecture Bridge IEEE 802.1D MAC level MAC bridge does not need LLC layer can pass frame over external comms system capture frame encapsulate it forward it across link remove encapsulation and forward over LAN link e.g. WAN link Connection of Two LANs Connection Bridges and Bridges LANs with LANs Alternative Routes Fixed Routing Fixed complex large LANs need alternative routes for load balancing and fault tolerance bridge must decide whether to forward frame bridge must decide LAN to forward frame to can use fixed routing for each source-destination can pair of LANs pair done in configuration usually least hop route only changed when topology changes widely used but limited flexibility Spanning Tree Spanning bridge automatically develops routing bridge table table automatically updates routing table in automatically response to changes response three mechanisms: frame forwarding address learning loop resolution Frame Forwarding Frame maintain forwarding database for each port lists station addresses reached through each port search forwarding database to see if MAC address is search listed for any port except X listed if address not found, forward to all ports except X iif address listed for port Y, check port Y for blocking f or forwarding state or if not blocked, transmit frame through port Y for a frame arriving on port X: Address Learning Address can preload forwarding database when frame arrives at port X, it has come form when the LAN attached to port X the use source address to update forwarding use database for port X to include that address database have a timer on each entry in database if timer expires, entry is removed each time frame arrives, source address each checked against forwarding database checked if present timer is reset and direction recorded if not present entry is created and timer set Spanning Tree Algorithm Spanning address learning works for tree layout in general graph have loops for any connected graph there is a spanning tree for maintaining connectivity with no closed loops maintaining IEEE 802.1 Spanning Tree Algorithm finds this each bridge assigned unique identifier exchange info between bridges to find spanning tree automatically updated whenever topology changes Loop of Bridges Loop Interconnecting LANs - Hubs Interconnecting active central element of star layout each station connected to hub by two UTP lines hub acts as a repeater llimited to about 100 m by UTP properties imited to optical fiber may be used out to 500m physically star, logically bus transmission from a station seen by all others if two stations transmit at the same time have a collision collision Two Level Hub Topology Two Buses, Hubs and Switches Buses, bus configuration bus configuration all stations share capacity of bus (e.g. 10Mbps) only one station transmitting at a time transmission from any station received by hub and retransmitted on all outgoing lines retransmitted only one station can transmit at a time total capacity of LAN is 10 Mbps total capacity can switch multiple frames between separate ports multiplying capacity of LAN hub uses star wiring to attach stations can improve performance using a layer 2 switch Shared Medium Bus and Hub Hub Layer 2 Switch Benefits Layer no change to attached devices to convert bus LAN or hub LAN to switched LAN LAN e.g. Ethernet LANs use Ethernet MAC protocol e.g. assuming switch has sufficient capacity to keep up with all devices with additional devices attached to switch by increasing additional capacity of layer 2 have dedicated capacity equal to original LAN scales easily Types of Layer 2 Switch Types store-and-forward switch accepts frame on input line, buffers briefly, routes to accepts frame destination port destination see delay between sender and receiver better integrity use destination address at beginning of frame switch begins repeating frame onto output line as soon as destination address recognized soon destination highest possible throughput risk of propagating bad frames risk of cut-through switch Layer 2 Switch vs Bridge Layer Layer 2 switch can be viewed as full-duplex hub incorporates logic to function as multiport bridge differences between switches & bridges: bridge frame handling done in software switch performs frame forwarding in hardware bridge analyzes and forwards one frame at a time bridge analyze switch can handle multiple frames at a time can bridge uses store-and-forward operation switch can have cut-through operation operation hence bridge have suffered commercially Layer 2 Switch Problems Layer broadcast overload broadcast overload users share common MAC broadcast address broadcast frames are delivered to all devices broadcast frame are connected by layer 2 switches and/or bridges connected broadcast frames can create big overhead broadcast overhead broadcast storm from malfunctioning devices devices limits performance & reliability lack of multiple links lack of Router Problems Router typically use subnetworks connected by routers limits broadcasts to single subnet supports multiple paths between subnet high-speed LANs and high-performance layer 2 high-speed switches pump millions of packets per second switches software-based router only able to handle well under -based a million packets per second million routers do all IP-level processing in software Layer 3 Switches Layer Solution: layer 3 switches iimplement packet-forwarding logic of router in mplement hardware hardware packet by packet packet flow based flow based two categories Packet by Packet or Flow Based Flow packet by packet operates like a traditional router order of magnitude increase in performance order compared to software-based router compared enhances performance by identifying flows of enhances IP packets with same source and destination IP by observing ongoing traffic or using a special by flow label in packet header (IPv6) a predefined route is used for identified flows flow-based switch -based Typical Large LAN Organization Organization Diagram Summary Summary LAN topologies and media LAN protocol architecture bridges, hubs, layer 2 & 3 switches ...
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- Spring '09