LAN/WAN Interconnectivity
Learning Objectives
Explain the OSI reference model, which
sets standards for LAN and WAN
communications
 Discuss communication between OSI
stacks when two computers are linked
through a network
 Apply the OSI model to realistic
networking situations
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Learning Objectives
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Describe the types of networks as represented
through LAN topologies
Describe major LAN transmission methods,
including Ethernet, token ring, and FDDI
Explain basic WAN network communications
topologies and transmission methods, including
telecommunications, cable TV, and satellite
technologies
LAN/WAN Interconnectivity
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Intense competition between three
sectors:
Telecommunications companies
 Cable TV companies
 Satellite communications companies
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OSI Reference Model
Foundation that brings continuity to LAN
and WAN communications
 Product of two standards organizations:
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ISO
 ANSI
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Developed in 1974
 Set of communication guidelines for
hardware and software design
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OSI Guidelines Specify…
How network devices contact each other;
how devices using different protocols
communicate
 How a network device knows when to
transmit and not transmit data
 How physical network network devices are
arranged and connected
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OSI Guidelines Specify…
Methods to ensure that network
transmissions are received correctly
 How network devices maintain a
consistent rate of data flow
 How electronic data is represented on
network media
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OSI Layers
OSI Layers
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Bottom layers
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Middle layers
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Support for physical connectivity, frame formation,
encoding, and signal transmission
Establish and maintain a communication session
between two network nodes
Monitor for error conditions
Uppermost layers
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Application/software support for encrypting data and
assuring interpretation/presentation of data
Physical Layer Functions
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Provides transfer medium (eg, cable)
Translates data into a transmission signal
Sends signal along the transfer medium
Includes physical layout of network
Monitors for transmission errors
Determines voltage levels for data signal
transmissions and to synchronize
transmissions
Determines signal type (eg, digital or analog)
Analog Signals
Digital Signals
Data Link Layer Functions
Constructs data frames
 Creates CRC information; checks for
errors
 Retransmits data if there is an error
 Initiates communications link; makes sure
it is not interrupted (ensures node-to-node
physical reliability)
 Examines device addresses
 Acknowledges receipt of a frame
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Data Link Layer
Data link frame contains fields consisting
of address and control information
 Two important sublayers
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Logical link control (LLC)
 Media access control (MAC)
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Connectionless service versus connectionoriented service
Network Layer Functions
Determines network path for routing
packets
 Helps reduce network congestion
 Establishes virtual circuits
 Routes packets to other networks,
resequencing packet transmissions when
needed
 Translates between protocols
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Transport Layer Functions
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Ensures reliability of packet transmissions
Ensures data is sent and received in the same
order
Sends acknowledgement when packet is
received
Monitors for packet transmission errors and
resends bad packets
Breaks large data units into smaller ones and
reconstructs them at the receiving end for
networks using different protocols
Session Layer Functions
Establishes and maintains
communications link
 Determines which node transmits at any
point in time
 Disconnects when communication session
is over
 Translates node addresses
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Presentation Layer Functions
Translates data to a format the receiving
node understands (eg, from EBCDIC to
ASCII)
 Performs data encryption
 Performs data compression
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Application Layer Functions
Enables sharing remote drivers and
printers
 Handles e-mail messages
 Provides file transfer services
 Provides file management services
 Provides terminal emulation services
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Communicating Between
Stacks
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OSI model provides standards for:
Communicating on a LAN
 Communicating between LANs
 Internetworking between LANs and WANs
and between WANs and WANs
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Peer Protocols
Primitives
Layered Communications
Applying the OSI Model
Types of Networks
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Three main topologies
Bus
 Ring
 Star
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Bus Topology
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Built by running cable
from one PC or file
server to the next
Terminators signal the
physical end to the
segment
Advantages of Bus Topology
Works well for small networks
 Relatively inexpensive to implement
 Easy to add to it
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Disadvantages of
Bus Topology
 Management
costs can be high
 Potential for congestion with network
traffic
Ring Topology
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Continuous path for
data with no logical
beginning or ending
point, and thus no
terminators
Advantages of Ring Topology
Easier to manage; easier to locate a
defective node or cable problem
 Well-suited for transmitting signals over
long distances on a LAN
 Handles high-volume network traffic
 Enables reliable communication
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Disadvantages of
Ring Topology
Expensive
 Requires more cable and network
equipment at the start
 Not used as widely as bus topology
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Fewer equipment options
 Fewer options for expansion to high-speed
communication
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Star Topology
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Oldest and most
common network
design
Multiple nodes
attached to a central
hub
Advantages of Star Topology
Good option for modern networks
 Low startup costs
 Easy to manage
 Offers opportunities for expansion
 Most popular topology in use; wide variety
of equipment available
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Disadvantages of
Star Topology
Hub is a single point of failure
 Requires more cable than the bus
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Bus Networks in a Physical Star
Layout
No exposed terminators
 Capability for connecting multiple hubs to
expand network in many directions
 Expansion opportunities for implementing
high-speed networking
 Popular design; wide range of equipment
available
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LAN Transmission Methods
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Ethernet
IEEE 802.3 specifications
 Broadest options for expansion and highspeed networking
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Token ring
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IEEE 802.5 specifications
FDDI (Fiber Distributed Data Interface)
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High-speed variation of token ring
Ethernet
Uses CSMA/CD access method for data
transmission on a network
 Typically implemented in a bus or bus-star
topology
 Carrier sense
 Collision
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Ethernet Communications
Ethernet II
Ethernet Standards
Token Ring
Developed by IBM in the 1970s; remains a
primary LAN technology
 Employs physical star topology with logic
of ring topology
 Each node connects to a central hub, but
the frame travels from node to node as
though there were no starting or ending
point
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Token Ring Frame
Token Ring Terms
Multistation access unit (MAU)
 Beaconing
 Broadcast storms
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FDDI
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Fiber-optic data transport method capable of a
100-Mbps transfer rate using a dual ring
topology
Synchronous versus asynchronous
communications
Nodes monitor network for error conditions
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Long periods of no activity
Long periods where the token is not present
Class A and Class B nodes
WAN Network Communications
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Typical providers of WAN network services
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Newer sources of WAN connectivity
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Telecommunications companies
Cable TV companies
Satellite providers
Cable television networks
Satellite TV companies
Wireless WANs
Wide use of star topology
Telecommunications WANs
Earliest source of WAN connectivity
 Regional telephone companies, also
called:
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Telcos
 Regional bell operating companies (RBOCs)
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Long-distance telecommunications
companies
 Plain old telephone service (POTS) or
public switched telephone network (PSTN)
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General Topology Linking LATA
and IXC Lines
Connecting LANs through
a T-Carrier Line
T-Carrier Services and Data
Rates
Cable TV WANs
Also called cablecos or multiple system
operators (MSOs)
 Use a distributed architecture that consists
of several star-shaped centralized
locations
 Headend is the main focal point in the star
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Wireless WANs
Use radio, microware, and satellite
communications
 Packet radio communications
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Topology of a Radio Wave WAN
Joining Two LANs
WAN Transmission Methods
Use different switching techniques to
create data paths (channels) for
transmitting data
 Switching
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Enables multiple nodes to simultaneously
transmit and receive data, or
 Enables data to be transmitted over different
routes to achieve maximum efficiency in terms
of speed and cost
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Switching Techniques
Time division multiple access
(TDMA)
Divides channels into distinct time
slots
Frequency division multiple
access (FDMA)
Divides channels into frequencies
Statistical multiple access
Dynamically allocates bandwidth
based on application need
Circuit switching
Uses a dedicated physical circuit
Message switching
Uses store-and-forward method of
data transmission
Packet switching
Combines circuit and message
switching
Chapter Summary
Open Systems Interconnection (OSI)
model
 Basic network topologies
 Key LAN transmission methods
 WAN communications options
 WAN transmission methods
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