Download Chapter 4

Local Area Networks, 3rd Edition
David A. Stamper
Part 2: Hardware
Chapter 4
Topologies and Media
Access Control
© 2001 by Prentice Hall
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Chapter Preview
In this chapter you will study:
• LAN topologies
• Media access control protocols
• Common ways in which topologies
and media access control protocols
are combined
• LAN standards
• Strengths and weaknesses of
different LAN configurations
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LAN Topologies
• The term topology derives from a
mathematics field that deals with
points and surfaces in space—that
is, with the layout of objects in
space. The LAN topology is the
physical layout of the network.
• LANs have three basic topologies:
ring, bus, and star.
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Ring Topology
Data Flow
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Bus Topology
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Star Topology
Wiring Hub
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Data Link Protocols
• In general, a data link protocol establishes
the rules for gaining access to the medium
and exchanging messages. To do this, the
protocol describes several aspects of the
message-exchange process. Six of the
most important are:
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Media Access
Delineation of Data
Error Control
Addressing
Transparency
Code Independence
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Data Link Protocols (cont.)
• Media Access
– Media access defines how a node gains the right to
transmit data on the medium.
• Delineation of Data
– A data link protocol must define or delineate where the
data portion of the transmitted message begins and
ends. This can be accomplished in two basic ways: by
framing the data with certain control characters or by
using a standard message format wherein data is
identified by its position within the message.
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Data Link Protocols (cont.)
• Error Control
– Error control is used to determine whether data has been
corrupted during the transmission.
• Addressing
– Communication between two network nodes is
accomplished through an addressing scheme. Networks
use a hierarchical addressing scheme, with the hierarchy
being application, network node, and network.
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Data Link Protocols (cont.)
• Transparency
– Transparency is the ability of the data link to transmit any
bit combination. We like protocols to be transparent
because they can be used to transfer binary data such as
object programs as well as text data. LAN data link
protocols ordinarily provide transparency.
• Code Independence
– Code independence means that any data code, such as
ASCII or EBCDIC, can be transmitted.
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MAC Protocols
• LAN technology adheres to two primary
MAC protocols: token passing and
contention.
• Contention
– In a pure contention MAC protocol, each network node has equal
access to the medium. Variations of this protocol exist, some of
which allow for node priorities.
• Token Passing
– Token passing is used on both bus and ring topologies. Token
passing is a round-robin protocol in which each node gets an equal
opportunity to transmit. With token passing, the right to transmit is
granted by a token that is passed from one node to another.
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LAN Standards
• Standards exist covering most aspects of
LAN technology
• Implementing according to established
standards generally results in the
availability of components from multiple
vendors, competition among vendors, and
lower prices.
• The organizations most active in setting
standards for LAN topologies and MAX
protocols are the IEEE and ANSI.
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IEEE Standards
• The 802 Committee established by the IEEE is divided into
subcommittees, each of which addresses specific LAN
issues and architecture.
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High-Level Interface
Logical Link Control
CSMA/CD
Token Bus
Token Ring
MANs
Broadband Technical Advisory Group
Fiber Optic Technical Advisory Group
Integrated Data and Voice Networks
LAN Security
Wireless LANs
Demand Priority Access Method
Data Transport over Cable TV
Short-Distance Wireless Networks
Broadband Wireless Access
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LLC and MAC Sublayers of the
OSI Data Link Layer
Application Layer
Presentation Layer
Session Layer
Transport Layer
Logical Link Control
Network Layer
Media Access Control
Data Link Layer
Media Signaling
Physical Layer
Bus Interface Unit
OSI Reference Model
Layers
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Communications
Interface Unit (CIU)
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ANSI Distributed Queue Dual Bus
LAN
Bus A, Unidirectional
Bus B, Unidirectional
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Suggested Broadband Frequency
Allocations
280
Video Transmission Channel
Frequency (Nhz)
240
200
High-Speed Data Channel
160
120
80
Voice or Data Channel
40
0
Low-Speed Data Channel
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Comparison of Token-Passing and
CSMA/CD Media Access Control Protocols
Token Passing
CSMA/CD
Equal access for all nodes.
Equal access for all nodes.
Predictable access window.
Access window can be unpredictable.
Maximum wait time to transmit is token
circulation time.
Maximum wait time to transmit is
unpredictable and depends on collisions
Average wait time to transmit is
predictable (half the maximum
circulation time).
Average wit time to transmit is
unpredictable.
Network congestion does not
adversely affect network efficiency.
Network congestion may cause collisions
and reduce network efficiency.
A node needs to wait for the token
before being able to transmit.
A node may be able to transmit
immediately.
One node cannot monopolize the
network.
One node may be able to monopolize the
network.
Large rings can cause long delays
before a node obtains a token.
A node can transmit when the network is
quiet.
Consistent performance for large, busy
networks.
Unpredictable performance for large, busy
networks due to possibility of collisions.
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Topology and Protocol Trade-Offs
• There are two primary LAN implementations in
use, CSMA/CD buses (ethernet) and tokenpassing rings.
• Ethernet has the major market share between the
two.
• Ethernet provides fair access for all nodes, but he
exact time a node waits before sending a
message may vary.
• Ethernet provides higher speeds and lower costs
than token rings.
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Broadband and Baseband
Technologies
• Broadband transmission and
baseband transmission are different
ways in which you can use a
medium.
– Broadband transmission divides the medium into several
channels, thus allowing the medium to be used for
distinct transmission needs.
– Baseband transmission dedicates the entire datacarrying capacity of the medium to the LAN.
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