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Transcript
Chapter 14: Local Area Network Technology Business Data Communications, 4e PC Networks Client/Server Communication Shared databases Shared hardware resources Shared Internet access Peer-to-Peer Communication Sharing work and information with colleagues Low cost is high priority Attachment costs in the hundreds of dollars Backend & Storage Area Networks “Computer room networks” Interconnect large systems (mainframes, supercomputers, etc) Key requirement is high-speed bulk transfer Usually limited distance, few drops Speed more important than cost Attachment costs in the thousands of dollars High-Speed Office Networks Increased processing and transfer requirements in many graphics-intensive applications now require significantly higher transfer rates Decreased cost of storage space leads to program and file bloat, increased need for transfer capacity Typical office LAN runs at 1-20mbps, high- Backbone Local Networks Used instead of single-LAN strategy Better reliability Higher capacity Lower cost Factory Networks High capacity Ability to handle a variety of data traffic Large geographic extent High reliability Ability to specify and control transmission delays Tiered LANs Cost of attachment to a LAN tends to increase with data rate Alternative to connecting all devices is to have multiple tiers Multiple advantages Higher reliability Greater capacity (less saturation) Better distribution of costs based on need Tiered LAN Strategies Bottom-up strategy: individual departments create LANs independently, eventually a backbone brings them together Top-down strategy: management develops an organization-wide networking plan Tiered LAN Diagram LAN Topology Arrangement of workstations in a shared medium environment Logical arrangement (data flow) Physical arrangement (cabling scheme) LAN Topologies: Bus Multipoint medium Stations attach to linear medium (bus) using tap Full-duplex between station and tap Transmission from any stations travels entire medium (both directions) Termination required at ends of bus Bus LAN Diagram LAN Topologies: Tree Generalization of bus topology Branching cable with no closed loops Cable(s) begin at headend, travel to branches which may have branches of their own Each transmission propagates through network, can be received by any station Tree LAN Diagram Bus/Tree Topology Problems How do you identify who the transmission is intended for? Data transmitted in frames Each frame has header with addressing info How do you regulate access? Stations take turns sending, by monitoring control information in frames LAN Topologies: Ring Repeaters are joined by unidirectional pointto-point links in a ring As a frame circulates past a receiver, the receiver checks its address, and copies those intended for it into a local buffer Frame circulates until it returns to source, which removes it from network Ring LAN Diagram LAN Topologies: Star Each station connected directly to central node, usually with two undirectional links Central node can broadcast info, or can switch frames among stations Star LAN Diagram Choosing a Topology Factors to consider include reliability, flexibility/expandability, and performance Bus/tree is most flexible Tree topology easy to lay out Ring provides high througput, but reliability problems Star can be high speed for short distances, but has limited expandability Transmission Media Options Twisted pair--digital signaling Optical fiber--analog signaling Baseband coax--digital signaling Broadband coax--analog signaling Uses FDM to carry multiple channels Can be used over longer distances Inherently unidirectional, due to amplifier limitations Selecting Transmission Media Capacity: Can it support expected network traffic? Reliability: Can it meet requirements for availability? Types of data supported: Is it well-suited to the applications involved? Environmental scope: Can it provide service in the environments required? Medium and Topology Structured Cabling System Standards for cabling within a building (EIA/TIA568 and ISO 11801) Includes cabling for all applications, including LANs, voice, video, etc Vendor and equipment independent Designed to encompass entire building, so that equipment can be easily relocated Provides guidance for pre-installation in new buildings and renovations Wiring Layouts Wiring layout is different from logical topology Linear layout minimizes amount of cable Star layout uses individual cable from concentration point to subscribers Can be used for bus and ring as well as star Concentration point can be wiring closet or hub (an active node that accepts frames and LAN Standards (802.x) Advantages of standards Assure sufficient volume to keep costs down Enable equipment from various sources to interconnect IEEE 802 committee developed, revises, and extends standards Use a three-layer protocol hierarchy: physical, medium access control (MAC), and logical link control (LLC) Link Control SpecifiesLogical method of addressing and controls exchange of data Independent of topology, medium, and medium access control Unacknowledged connectionless service (higher layers handle error/flow control, or simple apps) Connection-mode service (devices without higherlevel software) Acknowledged connectionless service (no prior connection necessary) Medium Access Control Provides a means of controlling access to a shared medium Two techniques in wide use CSMA/CD Token passing LLC frames data, passes it to MAC which frames it again MAC control (e.g. priority level) Destination physical address Source physical address Bridges Allow connections between LANs and to WANs Operates at Layer 2 (Data Link Layer) of OSI Used between networks using identical physical and link layer protocols Provide a number of advantages Reliability: Creates self-contained units Performance: Less contention Security: Not all data broadcast to all users Geography: Allows long-distance links Bridge Functions Read all frames from each network Accept frames from sender on one network that are addressed to a receiver on the other network Retransmit frames from sender using MAC protocol for receiver Must have some routing information stored in order to know which frames to pass Bridge Operation Hubs The active central element of the star layout. When a single station transmits, the hub repeats the signal on the outgoing line to each station. Physically a star; logically a bus. Hubs can be cascaded in a hierarchical configuration. Two-Level Star Topology Layer 2 Switches (Switching Hubs) Ethernet Hubs and Switches Shared medium hubs x Switched LAN hubs Advantages of Switched Hubs No modifications needed to workstations when replacing shared-medium hub Each device has a dedicated capacity equivalent to entire LAN Easy to attach additional devices to the network Types of Switched Hubs Store and forward switch Accepts a frame on input line Buffers it briefly Routes it to appropriate output line Cut-through switch Begins repeating the frame as soon as it recognizes the destination MAC address Higher throughput, increased chance of error Layer 3 Switches Problems With Layer 2 Switches Broadcast overload Lack of multiple links Can be solved with subnetworks connected by routers Layer 3 switches implement the packetforwarding logic of the router in hardware.
