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Data Communicationsand
Computer NetworksA Business Users Approach
Parviz KermaniPolytechnic UniversityParviz KermaniPolytechnic University
Chapter 7 Local Area Networks (LAN):
The Basics
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Acknowledgement
The original contents of this presentation wereprovided by the publisher, Course
Technology. Additional materials from othersources were added
William Stallings, Business Data Communications,
4th Edition, Prentice Hall publisher
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Objectives
State the definition of a local area network
List the primary function, activities, andapplication areas of a local area network
Cite the advantages and disadvantages of
local area networks
Identify the physical and logical topologies
of local area networks
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Objectives (continued)
Cite the characteristics of wireless local
area networks and their medium accesscontrol protocols
Specify the different medium access
control techniques Recognize the different IEEE 802 frame
formats Describe the common local area network
systems
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Introduction
Local area network - communication network
Interconnects a variety of data communicating
devices within a small geographic area Broadcasts data at high data transfer rates with very
low error rates.
Since the local area network first appeared inthe 1970s, its use has become widespread in
commercial and academic as well homeenvironments.
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Basic Characteristics of LANs
Small geographic area
Broadcast: transmission heard by all
High data rates
Low error rates
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Functions of a Local Area Network
Resource Sharing - To provide access to hardwareand software resources that will allow users to performone or more of the following activities
File server - A large storage disk drive that acts as a centralstorage repository.
Printer server - Provides the authorization to access aparticular printer, accept and queue print jobs, and provides auser access to the print queue to perform administrative duties.
Interconnection - A LAN can provide an interconnection toother systems, LANs, and to wide area networks.
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Functions of a Local Area Network
Video transfers - High speed LANs are capable ofsupporting video image and live video transfers.
Manufacturing support - LANs can supportmanufacturing and industrial environments.
Distributed processing - LANs can support networkoperating systems which perform the operations ofdistributed processing.
Academic support In classrooms, labs, and wireless.
E-mail support.
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Advantages of Local Area Networks
Ability to share hardware and softwareresources.
Individual workstation might survive networkfailure.
Component and system evolution are possible. Support for heterogeneous forms of hardware
and software.
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Advantages of Local Area Networks
Access to other LANs and WANs.
Private ownership.
Secure transfers at high speeds with low errorrates.
Di d t f L l A
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Disadvantages of Local AreaNetworks
Equipment and support can be costly.
Level of maintenance continues to grow.
Private ownership.
Some types of hardware may not interoperate.
Just because a LAN can support two differentkinds of packages (e.g. databases) does notmean their data can interchange easily.
A LAN is only as strong as it weakest link, andthere are many links.
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Basic Network Topologies
Stations on a local area networks are
interconnected using one of four basic
configurations:
1. Bus/tree
2. Star-wired bus3. Star-wired ring
4.
Wireless
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Bus/Tree Topology
The original topology
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Bus/Tree Topology
Workstation has a network interface card (NIC)that attaches to the bus (a coaxial cable) via a
tap.
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Bus/Tree Topology
Data can betransferred using
either baseband digital
signals
Bidirectional
broadband analogsignals
Unidirectional
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Bus/Tree Topology
Basebandsignals are bidirectional and moveoutward in both directions from the workstation
transmitting.
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Bus/Tree Topology
Broadbandsignals are usually unidirectionaland transmit in only one direction. Because of
this, special wiring considerations arenecessary.
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Bus/Tree Topology
In broadband topology, buses can be split and joined,creating trees.
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Advantages of Broadband LAN
Broadband signals easy to amplify
Network extends for 100s to 1000s of meters
Wide bandwidth: support for video and
radio signal transmissions
Multiple concurrent channels
Possible to split and join broadband
cables
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Disadvantages of Broadband LANs
More susceptible to noise
More expensive (than baseband)
Analog transmission
Long average propagation delay
Difficult to plan
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Star-wired Bus Topology
Logicallyoperates as a bus, but physically lookslike a star.
Star design is based on hub. All workstations attach to hub.
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Star-wired Bus Topology
Unshielded twisted pair usually used to connectworkstation to hub.
Hub takes incoming signal and immediatelybroadcasts it out all connected links.
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Star-wired Bus Topology
Hubs can be interconnected to extend size ofnetwork.
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Star-wired Bus Topology
Modular connectors and twisted pair
make installation and maintenance of
star-wired bus better than standard bus.
Hubs can be interconnected with twisted
pair, coaxial cable, or fiber optic cable. Biggest disadvantage(?): when one
station talks, everyone hears it. This is called a shared network. All devices
are sharing the network medium.
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Star-wired Ring Topology
Logically operates as aring
But physically appearsas a star
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Ring Topology
NIC operates as a repeater
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Ring Topology
Star ring topology is based on MAU (multi-station access unit) which functions similarly to
a hub.
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Ring Topology
Where a hub immediately broadcasts allincoming signals onto all connected links, the
MAU passes the signal around in a ring fashion.
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Ring Topology
Like hubs, MAUs can be interconnected toincrease network size.
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Wireless LANs
Not really a specific topology Workstation in a wireless LAN can be
anywhere as long as it is within transmittingdistance to an access point.
Newer IEEE 802.11 standard defines
various forms of wireless LANconnections.
Speed of 11/54 Mbps Speeds up to 20 Mbps available on other
technologies.
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Wireless LANs
Workstations reside within a basic service set,while multiple basic service sets create an
extended service set.
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Wireless LANs
Two basic componentsnecessary:
Client radio: usually a PCcard with an integratedantenna, and
Access point (AP): an
Ethernet port plus atransceiver
The AP acts as a bridgebetween the wired andwireless networks
Can perform basic routingfunctions.
Workstations with clientradio cards reside within abasic service set,
Multiple basic service setscreate an extendedservice set.
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Wireless LANs
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Wireless LANs
With directional antennae designed for
point-to-point transmission (rare), 802.11b
can work for more than 10 miles.
With an omni-directional antenna on a
typical AP, range may drop to as little as100 feet.
Distance is inversely proportional totransmission speed - as speed goes up,
distance goes down.
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Wireless LANs
In actual tests, 11 Mbps 802.11b devices
managed 5.5 Mbps (from a July 2000 test
by Network Computing).
To provide security, most systems use
Wired Equivalent Privacy (WEP), whichprovides either 40- or 128-bit keyprotection.
What will Bluetooths impact be on
802.11b?
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Other Wireless Standards
IEEE 802.11 (older 2 Mbps)
IEEE 802.11b (11 Mbps, 2.4 GHz)
IEEE 802.11a (54 Mbps, 5 GHz, in 2002)
IEEE 802.11g (54 Mbps, 2.4 GHz, in
2002), same frequency as 802.11b
Backward compatible
HiperLAN/2 (European standard, 54 Mbpsin 5 GHz band)
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Comparison of Technologies
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Peer-to-Peer LANs (Ad-Hoc Nets)
Wireless LANs may also be configured
without access point
These configurations are called ad-hoc
Not as common as server-based LANs
Most peer-to-peer LANs still use one ormore servers
Interesting collaborative-type applications(world-wide law firm)
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Medium Access Control Protocols
How does a workstation get its data onto
the LAN medium?
A medium access control protocol is the
software that allows workstations to
take turns at transmitting data. Three basic categories:
1. Contention-based protocols2. Round robin protocols (not in use)
3. Reservation protocols (not in use)
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Medium Access Protocols
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Contention-Based Protocols
Most common example is carrier sense multiple accesswith collision detection (CSMA/CD).
CSMA/CD protocol is based on the ALOHA protocol.
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Contention-Based Protocols
If no one is transmitting, a workstation cantransmit.
If someone else is transmitting, the workstationbacks off and waits.
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Contention-Based Protocols
If two workstationstransmit at the sametime, a collision occurs.
When the twoworkstations hear thecollision, they stoptransmitting immediately.
new packets
old packets
channel
Collision?
No
Yes
GS
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ALOHA Performance
Pure ALOHA:
Send the packet when it isgenerated
Period of vulnerability is 2time slots
Maximum efficiency= 18%
Slotted ALOHA
Time is slotted.
Stations can transmit onlyat the beginning of slots
Maximum efficiency= 36%
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ALOHA Performance
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Contention-Based Protocols
Each workstation backs off a random
amount of time and tries again.
Hopefully, both workstations do not try
again at the exact same time.
CSMA/CD is an example of a non-deterministic protocol.
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Contention-Based Protocols
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Round Robin Protocols
Each workstation takes a turn transmitting Turn is passed around the network from workstation to
workstation.
Most common example is token ring LAN in which asoftware token is passed from workstation toworkstation.
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Round Robin Protocols
Token ring is an example of a deterministic protocol.
Token ring is more complex than CSMA/CD. What
happens if token is lost? Duplicated? Hogged? Token ring LANs are losing (have lost!) the battle with
CSMA/CD LANs.
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Round Robin Protocols
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Reservation Protocols
Workstation places a reservation with centralserver.
Workstation cannot transmit until reservationcomes up.
Under light loads, this acts similar to CSMA/CD.
Under heavy loads, this acts similar to tokenring.
Though a powerful access method but again
losing out to CSMA/CD. Most common example of reservation protocol
is demand priority protocol.
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Medium Access Control Sublayer
Shortcoming of the OSI model for LANs
Routing at the network layer
Interdependence of physical & DLC layers
Need for medium access
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Medium Access Control Sublayer
To better support local area networks, the datalink layer of the OSI model was broken into two
sublayers: Logical link control sublayer
Medium access control sublayer
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Medium Access Control Sublayer
Medium access control sublayer defines the framelayout and is more closely tied to a specific medium atthe physical layer.
Thus, when people refer to LANs they often refer to itsMAC sublayer name, such as 10BaseT.
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IEEE 802 Frame Formats
The IEEE 802 suite of protocols defines
the frame formats for CSMA/CD (IEEE
802.3) and token ring (IEEE 802.5).
Each frame format describes how thedata package is formed.
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IEEE 802 Frame Formats
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IEEE 802 Frame Formats
Note how the two frames are different. If
a CSMA/CD network connects to a token
ring network, the frames have to beconverted from one to another.
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LAN Systems: Ethernet
Also known as CSMA/CD.
Most common form of LAN today.
Star-wired bus is most common topology, butbus topology also available.
Ethernet comes in many forms dependingon
medium used
transmission speed
technology.
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Ethernet
Originally, CSMA/CD was 10 Mbps.
Then 100 Mbps was introduced. MostNICs sold today are 10/100 Mbps.
Then 1000 Mbps (1 Gbps) was
introduced. 10 Gbps is now beginning to appear.
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Ethernet
1000 Mbps introduces a few interesting
wrinkles:
Transmission is full duplex (separate transmitand receive), thus no collisions.
Prioritization is possible using 802.1pprotocol.
Topology can be star or mesh (for trunks).
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Ethernet
A few more interesting wrinkles:
Cabling can be either UTP or optical (but 10
Gbps Ethernet may not work over UTP dueto radio frequency interference).
Where 10 Mbps Ethernet has less than 30%utilization due to collisions, 1000 Mbps islimited only by traffic queueing.
Distance with 10 Mbps is limited byCSMA/CD propagation time, whereas 1000Mbps is limited only by media.
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Ethernet Standards
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Wired Ethernet (continued)
One of the latest features is power overEthernet (PoE)
What if you have a remote device that hasan Ethernet connection? It will require a power connection
What if you dont have an electrical outletnearby?
Use PoE Power to drive Ethernet NIC is sent over wiring
along with usual Ethernet signals
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LAN Systems: IBM Token Ring
Deterministic LANoffered at speeds of 4,16 and 100 Mbps.
Experimental 100Mbpsnever made it to market.
Very good throughput
under heavy loads. More expensive
components than
CSMA/CD. Losing (lost!) ground
quickly to CSMA/CD.
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LAN Systems: FDDI
Fiber Distributed Data Interface
Based on the token ring design using 100 Mbps fiberconnections.
Allows for two concentric rings - inner ring can supportdata travel in opposite direction or work as backup.
Token is attached to the outgoing packet, rather thanwaiting for the outgoing packet to circle the entire ring.
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FDDI Dual Ring Topology
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FDDI Reliability
Reconfigured dual-ring network.
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Wireless Ethernet
As we have already seen, IEEE hascreated the 802.11b, 802.11a, and
802.11g wireless standards IEEE 802.11n (100 Mbps) will be ratified
soon and should start appearing inproduct form in 2006 (maybe?)
Latest wireless Ethernet is using MIMO
technology (multiple input multiple output) Sender and receiver have multiple antennas
for optimum reception
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LAN Systems: 100VG-AnyLAN
Deterministic LAN based on demand
priority access method (IEEE 802.12).
Transmits data at 100 Mbps
Uses and supports Ethernet and TR
technologies (Any). Originally designed to run on category 3
(Voice Grade, VG) wires Category 5 is now the preferred cable
S G
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LAN Systems: 100VG-AnyLAN
Similar to hub topology (star design).
Similar to Token Ring, uses round robin
scheduling (enforced by the root hub)
Two levels of priority - normal and high.
Supports a wide-variety of media types.
Losing (has lost!) ground to CSMA/CD.
00VG A LAN T l
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100VG-AnyLAN Topology
LANs In Action : A Small OfficeS l i
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Solution
What type of system will interconnect
twenty workstations to a central server,
which offers: Electronic mail
A database that contains all customerinformation
Internet access
High quality printer access
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Solution
LANs In Action : A Small OfficeS l ti
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Solution
LANs In Action : A Small OfficeS l ti
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Solution
LANs In Action : A Small OfficeS l ti
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Solution
LANs In Action: A Home OfficeL l A N t k S l ti
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Local Area Network Solution
What if you have two computers at home and want bothto share a printer and a connection to the Internet.
Some type of network-in-a-box solution might solvethis problem.
Essentially a LAN with a 2- or 3-port hub, connectingcables, and software.
In some models, the hub also acts as a router to theInternet.
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Local Area Network Solution
Wh t l d i thi h t
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What we learned in this chapter
State the definition of a local area network. List the primary function, activities, and application
areas of a local area network.
Cite the advantages and disadvantages of local areanetworks. Identify the physical and logical local area network
topologies. Cite the characteristics of wireless local area networks
and their medium access control protocols. Recognize the difference between client/server
networks and peer-to-peer networks. Specify the different medium access control techniques. Recognize the different IEEE 802 frame formats Describe the common local area network systems