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.