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-speed alternatives run at 100+

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 point-to-

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/TIA-568 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 regenerates signals for transmission)

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)

Logical Link Control Specifies 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 higher-level 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

Switched LAN hubs

x

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 packet-

forwarding logic of the router in hardware.