Local Area Networks: The Basics

1
Data Communications and Computer Networks A
Business Users Approach Third Edition

• Chapter 7
• Local Area Networks The Basics

2
Topics

• Local Area Network defined
• Advantages and disadvantages of LANs
• Physical and logical topologies
• Characteristics of wireless LANs
• Medium access control protocols
• IEEE 802 frame formats
• Common LAN systems

3
Local Area Networks Characteristics

• A network that connects a variety of data
  communicating devices within a small geographic
  area
• Broadcasts data at high data transfer rates with
  very low error rates
• Has become widespread in commercial and academic
  environments over the past 20 years

4
Primary Function of a LAN

• Provides users access to hardware and software
  resources such as
• File servers
• large disk drives that act as a central data
  storage repository
• Print servers
• Provide access for authorized users a particular
  printer
• Manage multiple print jobs
• Allow canceling or pausing jobs in print queue

5
LAN Applications

• Video transfers
• Support for video image and live video transfers
• Manufacturing
• Support manufacturing and industrial processes
• Academic support
• classrooms, labs, and wireless access Internet
• E-mail support

6
Advantages of LANs

• Hardware and software resource sharing
• Workstations function independently of LAN
• Component and system evolution
• Supports dissimilar hardware and software
• Access to other LANs and WANs
• Private ownership
• Secure transfers at high speeds with low error
  rates

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LAN Interconnectivity

8
LAN Disadvantages

• Equipment and support is costly
• Maintenance costs
• IT Expertise
• Hardware, software, and data incompatibility

9
LAN Physical Topologies

• Bus
• Star
• Ring
• Tree
• Hybrid

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Bus Physical Topology

• An early topology
• A coaxial cable system using special connectors
  called taps or BNC connectors
• Workstations require a network interface card or
  NIC
• Data transferred using either
• Baseband (digital) signals
• Broadband (analog) signals

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Bus Topology Illustrated

12
Bus Connections Illustrated

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Types of LAN Signaling

• Baseband
• Digital signaling
• Bidirectional (Broadcast) on bus
• Broadband
• analog signaling
• Usually uni-directional
• Special wiring is required

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Bus Physical Topology Illustrated

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Tree Topology

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Star Physical Topology

• Stations wired to central repeater or hub
• Logically operates as a bus
• Typically unshielded twisted pair wire is used
• Hub broadcasts signals out on all connected links
• Hubs can be interconnected to extend network

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Star Physical Topology Illustrated

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Extending a Star Topology

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Star Design Considerations

• Modular connectors and twisted pair facilitate
  LAN installation and maintenance
• Hubs may be interconnected with UTP, coaxial, or
  fiber optic cable
• Bandwidth is shared

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Ring Physical Topology

• Stations wired to central MAU (multi-station
  access unit) that contains the ring
• The MAU passes the signal around the ring
• MAUs can be interconnected to expand the network

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Ring Physical Topology Illustrated

22
MAU Operation

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Wireless LANs

• Workstation
• must be within transmitting distance of a
  wireless access point
• must have a NIC capable of transmitting to the
  access point
• IEEE 802.11x standard defines various wireless
  LAN configurations

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Basic Wireless Components

• Client Radio
• PC Network Interface Card with integrated antenna
• Access Point (AP)
• Ethernet port plus transceiver
• AP acts as bridge between wired and wireless
  networks
• Can perform basic routing functions

25
Wireless Service Sets

• Workstations with client radio cards reside
  within a basic service set
• Multiple basic service sets create an extended
  service set
• Wireless LANs may also be configured without
  access point
• These configurations are called ad-hoc

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Wireless LANs Illustrated

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Wireless LANs Service Sets

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Ad Hoc Wireless LAN

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IEEE Wireless Standards

• IEEE 802.11
• an early standard
• capable of transmitting data at 2 Mbps
• IEEE 802.11b
• a later standard
• capable of transmitting data at 11 Mbps
• Transmission speeds are often half of their rated
  speed

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Transmission Distances

• The transmission range may be little as 100 feet
  for 802.11b devices using a common
  omni-directional antenna to a typical AP
• 802.11b devices using a point-to-point
  directional antennae can transmit about 10 miles

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Newer/Other Standards

• IEEE 802.11a
• Capable of transmitting data at 54 Mbps using 5
  GHz frequency range
• Not compatible with 802.11b
• IEEE 802.11g
• Capable of transmitting data at 54 Mbps but using
  802.11b frequencies (2.4 GHz)
• Backwards compatible with 802.11b
• HiperLAN/2
• European standard, 54 Mbps in 5 GHz band

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Wireless Security

• Wired Equivalent Privacy (WEP)
• provides either 40- or 128-bit key protection
• WPA
• a more-secure standard
• More details are provided in Chapter 13

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Comparison of Topologies

34
Media access

• The two protocols used to transmit data onto a
  LAN are
• Contention-based protocols
• Round robin protocols

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Contention-based Protocols

• A first-come first-served approach
• Media is checked by workstation
• If clear, workstation can transmit
• If another station is transmitting, workstation
  waits, then transmits
• Carrier sense multiple access with collision
  detection (CSMA/CD)
• is the most common contention-based protocol
• also referred to as a non-deterministic protocol

36
Collisions

• The media detection process is not perfect
• it is affected by timing, distance, and heavy
  network traffic
• Collisions occur
• when two workstations transmit at nearly the same
  time and their messages interfere with each other
• When a collision occurs
• transmission stops
• Each workstation waits a random amount of time
  and retransmits its message
• Hopefully another collision will not occur

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Contention-Based Protocols Illustrated

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Round-Robin Protocols

• Workstations take turns transmitting data
• The workstation must possess a circulating
  token before it can transmit a message
• token is released to network after message
  arrives at its destination
• Token Ring is the most common RR protocol
• TR Controller Issues
• token is lost
• token is duplicated
• token not released after transmission

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Token Ring Illustrated

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OSI Data Link Sub-layers

• Data link layer of the OSI model is divided into
  two sub-layers
• Logical link control
• Defines DL to network layer interface standards
• Medium access control
• defines DL to physical layer interface standards
  (frame layouts)

41
Data Link Sub-layers Illustrated

42
Common Frame Formats

• Frame formats describe how data are packaged for
  transmission on media
• CSMA/CD is defined by IEEE 802.3 standards
• Token ring is defined by IEEE 802.5
• Frame formats differ significantly
• Interconnected networks may require frame
  conversion

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IEEE 802.3 Frame Format

44
IEEE 802.5 Frame Format

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Ethernet

• Most LANs today are Ethernet LANs
• Physical topology is either a star or bus
• Ethernet LANs use the CSMA/CD protocol
• Different Ethernet standards exist depending upon
• Medium used
• Transmission speed
• Technology used

46
Ethernet Speeds

• Originally 10 Mbps
• 100 Mbps became available in 1990s
• Most modern NICs support both 10 and100 Mbps
  speeds
• 1000 Mbps (1 Gbps) have been available for about
  5 years
• 10 Gbps is now beginning to appear

47
Gigabit Ethernet

• 1000 Mbps Ethernet supports
• Full duplex transmissions using separate transmit
  and receive wire pairs
• Prioritization is possible using 802.1p protocol
• Topology can be star or mesh (for trunks)
• 10 Mbps Ethernet has less than 30 utilization
  due to collisions
• 1000 Mbps is limited only by traffic queuing
• Distance with 10 Mbps is limited by CSMA/CD
  propagation time
• 1000 Mbps limited only by media

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10 Gigabit Ethernet Issues

• Cabling can be either UTP or optical fiber
• May not work over UTP due to radio frequency
  interference

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Ethernet Standards Summary

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IBM Token Ring

• Deterministic LAN offered at speeds of 4 or 16
  Mbps originally.
• Newer 100 Mbps versions are available
• Very good throughput under heavy loads
• More expensive components than CSMA/CD
• Being replaced by Ethernet LANs, especially on
  new installations

51
Fiber Distributed Data Interface (FDDI)

• A token ring design using 100 Mbps fiber optic
  cabling
• Supports for two counter-rotating concentric
  rings
• Inner ring can support data travel in direction
  opposite outer ring
• In case of ring failure, rings join to form a
  single ring, until failure is corrected
• Token is released before message is delivered, or
  multiple tokens may circulate to speed message
  delivery

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FDDI Illustrated

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LANs In Action A Small Office Solution

• What type of system will interconnect twenty
  workstations in one room and fifteen workstations
  in another room to a central server, offering
• Internal e-mail
• A database that contains all customer information
• High quality printer access

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LANs In Action A Small Office Solution

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LANs In Action A Small Office Solution

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Home Network

• What if you have two computers at home and want
  both to share a printer and connection to the
  Internet?
• Some type of Small Office/Home Office (SOHO)
  solution might solve this problem
• LAN with 2- or 3-port hub, connecting cables, and
  software
• In some models, hub also acts as a router to the
  Internet

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Home Network

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Summary

• Local area networks
• Medium access control techniques
• IEEE 802 frame formats