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 transferUsually limited distance, few dropsSpeed more important than cost Attachment costs in the thousands of

dollars

Storage Area Networks

High-Speed Office Networks

Increased processing and transfer requirements in many graphics-intensive applications now require significantly higher transfer ratesDecreased cost of storage space leads to program and file bloat, increased need for transfer capacityTypical office LAN runs at 1-20mbps, high-speed alternatives run at 100+

Backbone Local Networks

Used instead of single-LAN strategyBetter reliabilityHigher capacityLower cost

Factory Networks

High capacityAbility to handle a variety of data trafficLarge geographic extentHigh reliabilityAbility to specify and control transmission delays

LAN Configuration - Tiered LANs

Cost of attachment to a LAN tends to increase with data rateAlternative to connecting all devices is to have multiple tiersMultiple 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 togetherTop-down strategy: management develops an organization-wide networking plan

Tiered LAN Diagram

Topologies & Transmission Media

Key Elements of a LAN Topology

Bus, Ring, or Star Transmission Medium

Twisted Pair, Coaxial Cable, or Optical Fiber Layout

Linear, or Star Medium Access Control

CSMA/CD, or Token Passing

LAN Topology

Arrangement of workstations in a shared medium environmentLogical arrangement (data flow)Physical arrangement (cabling scheme)

LAN Topologies: Bus

Multipoint mediumStations attach to linear medium (bus) using tapFull-duplex between station and tapTransmission from any stations travels entire medium (both directions)Termination required at ends of bus

Bus LAN Diagram

LAN Topologies: Tree

Generalization of bus topologyBranching cable with no closed loopsCable(s) begin at headend, travel to branches which may have branches of their ownEach 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 point-to-point links in a ringAs a frame circulates past a receiver, the receiver checks its address, and copies those intended for it into a local bufferFrame 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 linksCentral node can broadcast info, or can switch frames among stations

Star LAN Diagram

Choosing a Topology

Factors to consider include reliability, flexibility/expandability, and performanceBus/tree is most flexibleTree topology easy to lay outRing provides high througput, but reliability problemsStar can be high speed for short distances, but has limited expandability

Transmission Media Options

Twisted pair--digital signalingOptical fiber--analog signalingBaseband coax--digital signalingBroadband 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/TIA-568 and ISO 11801)Includes cabling for all applications, including LANs, voice, video, etcVendor and equipment independentDesigned to encompass entire building, so that equipment can be easily relocatedProvides guidance for pre-installation in new buildings and renovations

Wiring Layouts

Wiring layout is different from logical topologyLinear layout minimizes amount of cableStar 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 regenerates signals for transmission)

Horizontal Cabling

Backbone Cabling(Vertical)

Also refer to Fig. 14.7 (p.383)

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 standardsUse a three-layer protocol hierarchy: physical, medium access control (MAC), and logical link control (LLC)

p. 385

Logical Link Control

Specifies method of addressing and controls exchange of dataIndependent of topology, medium, and medium access controlUnacknowledged connectionless serviceUnacknowledged connectionless service (higher layers handle error/flow control, or simple apps)Connection-mode serviceConnection-mode service (devices without higher-level software)Acknowledged connectionless serviceAcknowledged connectionless service (no prior connection necessary)

Medium Access Control

Provides a means of controlling access to a shared mediumTwo 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 WANsOperates at Layer 2 (Data Link Layer) of OSIUsed between networks using identical physical and link layer protocolsProvide 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 networkAccept frames from sender on one network that are addressed to a receiver on the other networkRetransmit frames from sender using MAC protocol for receiverMust 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

Shared medium hubs

Switched LAN hubs

x

Advantages of Switched Hubs

No modifications needed to workstations when replacing shared-medium hubEach device has a dedicated capacity equivalent to entire LANEasy 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 packet-forwarding logic of the router in hardware.