Download logical topology

Chapter 8
LAN Architectures
Part II: Understanding Internet Access Technologies
Topics Addressed in Chapter 8
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What is meant by LAN architecture?
LAN topologies
Data link and media access control protocols
Physical layer data encoding in LANs
Ethernet architectures
Token ring LANs
FDDI and 100VG-AnyLAN
ATM LANs
Virtual LANs
Wireless LAN architectures
LAN architecture selection criteria
IEEE LAN standards
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What is Meant By LAN
Architecture?
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LAN architecture is the overall design of a LAN.
It includes:
 LAN hardware
 LAN software
 LAN topology
 Media access control (MAC) protocol
The LAN’s network operating system is
sometimes also considered to be part of LAN
architecture
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LAN Topologies
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There are two types of LAN topologies: physical
and logical
Physical LAN topology refers to the physical
layout of the network
 The way in which the communication is
configured and how nodes attach to the network
 Because the focus is on physical connections
among hardware component, physical
topologies correspond to the physical layer of
the OSI reference model
Logical topology is concerned with how messages
are passed from node to node within the network
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Physical Topologies
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LAN’s have three basic physical topologies:
 Bus: all nodes attach to a common communication
pathway or channel
 Ring: the medium forms a loop to which all nodes are
attached
 Star: uses a central station (hub or switch) to which all
other nodes have point-to-point connections; all
communication among nodes occurs through this
central station
These are illustrated in Figure 8-1
Physical star topologies are most common in today’s LANs
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Figure 8-1
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Bus Topologies
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In a classic bus topology, the medium consists of a single wire or cable
to which other nodes are attached via connectors or transceivers
 Variations include a primary bus with spurs (see Figure 8-2)
 Disadvantages include the potential for loose connections or breaks
in the bus to disrupt the entire network
Early Ethernet LAN implementations were typically physical bus
architectures; today, most Ethernet implementations are physical stars
 However, an Ethernet shared media hub is sometimes called a “bus
in a box”
Both IEEE 802.3 standard and IEEE 802.4 standards and their
protocols address communication over LANs with bus topologies
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Ring Topologies
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In a physical ring topology, the communication medium
forms a closed loop (ring) and all stations are connected to
the loop
 Data is transmitted node-to-node in one direction on the
ring (see Figure 8-3)
 Similar to a linear bus, the entire network could be
disrupted if one of the connectors or links in the ring
should fail
Physical ring topologies are less common than bus or star
topologies
Token ring and FDDI LANs have physical ring topologies
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Figure 8-3
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IEEE 802.5 and 802.6 LANs
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The most widely used microcomputer ring network is the
token passing ring. It conforms to the IEEE 802.5 standard
Token ring networks physically look like a star topology,
but technically they are physical rings
 Token ring nodes attach to multistation access units
(MAUs) – see Figure 8-4
 MAUs can be described as “a ring in a box”, because
nodes attach to the physical ring by connecting to the
MAU (see Figure 8-5)
 MAUs can be interconnected to form larger rings (see
Figure 8-6)
IEEE 802.6 addresses dual-ring metropolitan area network
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(MAN) architectures (see Figures 8-24 and 8-25)
Figure 8-4
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Figure 8-5
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Figure 8-6
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Star Topologies
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In LANs with star topologies, all nodes are connected to some kind of
wiring center such as a hub or switch (see Figure 8-7)
 Today, most LAN implementations physically resemble star
topologies
Each node is isolated on its own network segment in a physical star
topology which minimizes the possibility of total network disruption
by a malfunctioning connector, NIC, or link
 However, the network is vulnerable to wiring center failure
The use of central connection points also facilitates network traffic
monitoring and network management, including network security
management
ARCnet was one of the first LAN architectures with a star topology
(see Figure 8-8b)
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Figure 8-7
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Logical Topologies
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Every LAN has both a physical and logical topology
A LAN’s logical topology specifies how messages are
passed from node to node within the network
 It corresponds to the media access control (MAC)
protocol used in the LAN
Two logical LAN topologies exist:
 Sequential (or logical ring): data is passed from one
node to another in a ring-like sequence
 Token passing in token ring and FDDI LANs are
examples
 Broadcast: nodes transmit frames/packets to all other
nodes in the network; only the intended recipient
processes the entire frame/packet
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Data Link Protocols
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Data link protocols, including those used in LANs, are
responsible for establishing the rules by which nodes gain
access to a network’s communication medium and
exchange messages. Such protocols describe several
important aspects of the message exchange process
including:
 Delineation of data
 Error control
 Addressing
 Transparency
 Code independence
 Media access--this is governed by media access control
(MAC) protocols
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LLC and MAC Sublayers
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LANs employ two primary data link protocols:
contention and token passing
In IEEE 802 standards, the data link layer is
divided into two sublayers LLC and MAC (see
Figure 8-10)
 LLC (logical link control) is responsible for
flow control, message sequencing, message
acknowledgement, and error checking
 MAC (media access control) enables network
nodes to access the communication medium via
contention or token passing
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Figure 8-10
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CSMA/CD
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CSMA/CD (Carrier Sense Multiple Access and Collision Detection) is
most widely used contention-based MAC used in LANs
 It is the MAC protocol used in Ethernet LANs
In a true contention MAC (like CSMA/CD), each node has equal
access to the medium
As noted in Table 8-1, each node monitors the medium for data traffic
and if none is detected, it begins transmitting data
A collision occurs when two or more nodes begin to transmit at the
same time
To avoid collision recurrence, each node waits a random time interval
(hardwired in its NIC) before attempting to retransmit
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Table 8-1
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CSMA/CA
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CSMA/CA (Carrier Sense Multiple Access and
Collision Avoidance) is a variation of CSMA/CD
used in wireless LANs because it is difficult to
detect collisions in such networks
When CSMA/CA is used, each node must wait a
random time interval (hardwired in the wireless
NIC) after detecting a clear medium before
transmitting
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Token Passing
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Token passing is the other major MAC protocol found in LANs
It is used in token ring and FDDI LANs and other networks with
logical ring topologies
The token is a pre-defined bit pattern that is passed among network
attached computers until one of them wants to use the medium to
transmit data
Token passing is summarized in Table 8-2
In token ring networks that resemble physical star topologies, token
passing takes place within MAUs (see Figure 8-12)
Token passing can be used in bus topologies as well as in physical ring
topologies (see Figure 8-11)
Table 8-3 compares token passing and CSMA/CD
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Table 8-2
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Figure 8-12
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Table 8-3
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Physical Layer Data Encoding
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Baseband transmission is common in LANs
When LAN nodes use the communication medium, their
NICs transmit digital signals to represent the bits in data
link layer protocols frames directly onto the medium
Some of the encoding schemes used in LANs are
illustrated in Figure 8-13. These include:
 Manchester encoding (used in 10 mbps Ethernet)
 4B5B (used in 100 mbps Ethernet)
 Differential Manchester encoding (used in token ring
LANs)
 NRZI (used in FDDI LANs)
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Figure 8-13
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Ethernet LAN Architectures
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IEEE 802.3-compliant LANs are better known as Ethernet
LANs
There are a variety of IEEE 802.3-compliant LANs (see
Table 8-4)
Today, most Ethernet LANs have physical star topologies;
some have physical bus topologies
All Ethernet LANs have broadcast logical topologies and
use CSMA/CD as the MAC protocol
Figure 8-9 illustrates widely used Ethernet frame formats
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Table 8-4
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Figure 8-9
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Some Key Ethernet Implementations
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Particularly important Ethernet implementations
include:
 Fast Ethernet (e.g. 100BaseT and 100BaseFX)
 The IEEE 802.3u specification covers Fast
Ethernet
 Gigabit Ethernet (e.g. 1000BaseT,
1000BaseSX, and 1000BaseLX)
 The IEEE 802.3z specification addresses
Gigabit Ethernet
 Iso-Ethernet enables Ethernet LANs at different
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geographic locations to be connected over
Figure 8-14
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Token Ring Architectures
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Token ring networks are addressed in the IEEE
802.5 specification
Physically, token ring LANs resemble star
topologies, but technically they are rings
 Ring is physically implemented in MAUs
 UTP is the most common cabling
Speed is typically 16 mbps, however, 4 mbps and
100 mbps token ring networks exist
IEEE 802.5 frame formats are illustrated in Figure
8-15
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Figure 8-15
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FDDI LANs
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Fiber Distributed Data Interface (FDDI) was first
recognized in ANSI’s X3T9.5 specification
Physically, it has a dual ring topology
It has a sequential/ring logical topology and uses a
variation of token passing as the MAC protocol
Key FDDI technologies are identified in Figure 8-16
 These include single attached stations (SAS), dual
attached stations (DAS), FDDI concentrators, and
FDDI/Ethernet bridges
FDDI is often used as a backbone network architecture
(see Figure 8-18)
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Figure 8-16
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100VG-AnyLAN
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100VG-AnyLAN (aka 100BaseVG) is capable of
transporting both IEEE 802.3 and IEEE 802.5
frames
It provides a mechanism for interconnecting 100
mbps token ring and 100BaseT Ethernet LANs via
specialized hubs and routers (see Figure 8-20)
 100VG-AnyLAN-compliant adapters are also
needed
It uses demand priority access (DPA) rather than
CSMA/CD as the MAC protocol in order to
enable real-time voice and video frames to be
given priority over other data frames
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Figure 8-20
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ATM LANs
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ATM (asynchronous transfer mode) is a switched network
architecture that employs 53-octet cells to transmit data
Two data link layers are defined:
 ATM adaptation layer (AAL)
 ATM
ATM physical topologies are stars
ATM NICs with 25 speeds of 25, 100, or 155 mbps are
available for workstations
Ethernet and token ring LANs can interface with an
organization’s ATM backbone network via ATM
gateway/access switches (see Figure 8-21)
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Figure 8-21
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Wireless LAN Architectures
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IEEE 802.11x standards are the most important wireless LAN
(WLAN) specifications that exist today (see Table 8-6)
WLANs are typically implemented as physical stars
 Nodes connect to wireless hubs called access points
CSMA/CA is the MAC protocol for IEEE 802.11-compliant LANs
IEEE 802.11 addresses FHSS (frequency hopping spread spectrum),
DSSS (direct sequence spread spectrum), and diffuse infrared
transmission
 User “roaming” capabilities are also addressed
WiFi (Wireless Fidelity) certification has been developed to promote
interoperability among WLAN products
The WISPR (Wireless ISP Roaming) standard is designed to enable
users to roam from one publicly accessible WLAN to another
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Table 8-6
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Choosing Among LAN
Architectures
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A number of factors should be considered when selecting among LAN
architectures
Some of the major factors are described in Table 8-7; others are described in
Table 8-10)
Especially important factors to consider include:
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Immediate and recurring LAN costs (see Table 8-8)
Total cost of ownership (TCO)
Number of concurrent users that can be supported
Transmission speed and data throughput
Vendor support
Manageability
Scalability/expandability
Security
Adherence to widely accepted standards
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Table 8-7
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IEEE LAN Standards & Committees
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802.1 High-Level
Interface
802.2 Logical Link
Control
802.3 CSMA/CD
802.4 Token Bus
802.5 Token Ring
802.6 Metropolitan Area
Networks (MANs)
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802.7 Broadband
Technical Advisory Group
802.8 Fiber Optic
Technical Advisory Group
802.9 Integrated Data and
Voice Networks
802.10 LAN Security
802.11 Wireless LANs
802.12 Demand Priority
Access Method
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Chapter 8
LAN Architectures
Part II: Understanding Internet Access Technologies