Chapter 14:
Local Area Network Technology
Business Data Communications,
4e
PC Networks
Client/Server Communication
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Shared databases
Shared hardware resources
Shared Internet access
Peer-to-Peer Communication
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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
Storage Area Networks
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, highspeed 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
LAN Configuration - 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
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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
Topologies & Transmission Media
Key Elements of a LAN
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Topology
 Bus, Ring, or Star
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Transmission Medium
 Twisted Pair, Coaxial Cable, or Optical Fiber
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Layout
 Linear, or Star
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Medium Access Control
 CSMA/CD, or Token Passing
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?
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Data transmitted in frames
Each frame has header with addressing info
How do you regulate access?
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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
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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
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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
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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)
p. 385
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
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CSMA/CD
Token passing
LLC frames data,
passes it to MAC
which frames it again
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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
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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
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
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Accepts a frame on input line
Buffers it briefly
Routes it to appropriate output line
Cut-through switch
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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
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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.