Fall 2005 LAN Technologies and Network Topology

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Fall 2005LAN Technologies and Network Topology

• Qutaibah Malluhi
• Computer Science and Engineering Department
• Qatar University

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Agenda

• LANs
• Topologies
• Bit Encoding
• Media Access Control
• Examples of LANs
• Ethernet emphasized

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Network Classification Terminology

• Network technologies classified into three broad
  categories
• Local Area Network (LAN)
• Metropolitan Area Network (MAN)
• Wide Area Network (WAN)
• LAN and WAN most widely deployed

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Point-to-Point Networks

• Computers connected by communication channels
  that each connect exactly two computers
• Point-to-point network
• Allows flexibility in communication hardware,
  packet formats, etc.
• Provides security and privacy because
  communication channel is not shared
• Number of wires grows as square of number of
  computers
• For N hosts,
• No. Connections (N2-N)/2
• Adding a new computer requires N - 1 new
  connections

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Local Area Network

• Developed in the 60s and 70s
• Interconnect a wide range of devices over short
  distances, e.g., within the same floor, building
  or campus (typically up to 10 km diameter).
• Key idea - reduce number of connections by
  sharing connections among many computers
• Computers take turns TDM
• Reduce cost but ... attached computers compete
  for use of shared connection
• Must include techniques for synchronizing use
• In practice
• Local communication almost exclusively LAN
• Long distance almost exclusively point-to-point
• E.g., ATM

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LAN (Cont.)

• Typically, all hosts on a LAN share a common
  medium.
• operate on a broadcast mode
• High throughput, low delay
• Many LAN technologies and standards exist
• E.g., Ethernet and FDDI are popular ones
• LAN standards are collectively known as the IEEE
  802 standards
• Must include techniques for synchronizing hosts

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Significance of LAN and Locality

• LANs are most popular types of networks WHY?
• Economical
• Principle of locality
• Principle of locality of reference helps predict
  computer communication patterns
• Spatial (or physical) locality of reference
• computers likely to communicate with other
  computers that are located nearby
• Temporal locality of reference
• computers are likely to communicate with the same
  computers repeatedly
• Thus - LANs are effective because of spatial
  locality of reference, and temporal locality of
  reference

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Different LAN Types

• LAN parameters
• topology
• shared medium (twisted pair, coaxial, fiber)
• medium access control technique
• governs the access to the LAN transmission
  medium.
• Topology
• Specifies general shape of a network
• Handful of broad categories
• Three most popular
• Star
• Ring
• Bus

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Star Topology

• Central component of network known as hub
• Hub repeats incoming signal to all outgoing
  links
• Each computer has separate connection to hub

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Star topology in practice

• Previous figure is idealized
• In practice

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Ring Topology

• No central facility
• Connections go directly from one computer to
  another
• In practice, there is a short connector cable
  from the computer to the ring
• Fault tolerance with two rings

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Bus Topology

• Single cable connects all computers
• Each computer has connector to shared cable
• Computers must synchronize and allow only one
  computer to transmit at a time

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Why Multiple Topologies?

• Each has advantages and disadvantages
• Ring ease synchronization may be disabled if any
  cable is cut
• Star easier to manage and more robust requires
  more cables
• Bus requires fewer cables may be disabled if
  cable is cut
• Industry is settling on star topology as most
  widely used

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Ethernet

• Widely used LAN technology
• Invented at Xerox PARC (Palo Alto Research
  Center) in 1970s
• Defined in a standard by Xerox, Intel and Digital
  - DIX standard
• Standard now managed by IEEE - defines formats,
  voltages, cable lengths, ...
• Uses bus topology
• Single coax cable - the ether
• Multiple computers connect to the ether
• One Ethernet cable is sometimes called a segment
• Limited to 500 meters in length
• Minimum separation between connections is 3
  meters
• Speed
• Originally 3Mbps (Obsolete)
• Popular standard is 10Mbps (Classic Ethernet)
• Fast Ethernet operates at 100Mbps
• Now Gigabit and 10 Gigabit

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Ethernet Operation

• One station transmits at a time
• Signal propagates across entire cable
• All stations receive transmission

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Manchester Encoding

• Ethernet standard uses Manchester encoding
• Uses rising and falling edges to encode data
• Edge triggered hardware
• Falling edge to encode 0, rising edge to encode 1
• Use preamble for synchronization
• Preamble consists of 64 alternating 1s and 0s

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Characteristics of Good Encoding Scheme

• Self synchronization
• Digital signal includes information about bit
  boundaries
• Transitions at the beginning, middle or end of
  the signal are used
• A signal with the same voltage level for a long
  period of time is bad
• No DC Component
• DC Component Useless extra energy residing on
  the line
• Signal with only positive voltage (unipolar) has
  a DC component
• Signal with positive and negative voltages
  (polar) reduce the DC component
• Bit rate versus pulse rate (baud rate)
• The first two (Self-Synch and No DC Comp) are
  provided by Manchester encoding

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Manchester Encoding Efficiency

• Classical Ethernet uses Manchester encoding
• Up to two signal transitions per-bit
• 1 Gbps Requires baud rate of 2 G
• Waste of bandwidth
• Newer Faster networks use a more efficient block
  encoding schemes (e.g. 4b/5b encoding for fast
  Ethernet and 8b/10b encoding for Gb Ethernet)

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Block Coding Schemes
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Careful Selection of Valid Codes

• Consider 4b/5b coding
• Select a subset of the 5-bit codes such that no
  more than three consecutive zeros are sent.
• Use a coding scheme that does not waste bandwidth
  (unlike Manchester Encoding)
• Advantages
• Synchronization
• No same voltage level for long period of time
• Error detection
• Errors may generate a non-valid 5 bit code
• Higher bit rate (than Manchester)

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CSMA/CD

• No central control managing when computers
  transmit on ether
• Ethernet employs CSMA to coordinate transmission
  among multiple attached computers
• Carrier Sense with Multiple Access
• Multiple access
• multiple computers are attached to shared media
• each uses same access algorithm
• Carrier sense
• computer wanting to transmit tests the media for
  carrier before transmitting
• Simultaneous transmission possible ? Collision

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Carrier-Sensing Effects
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CSMA/CD (contd)

• Even with CSMA, two computers may transmit
  simultaneously
• Both check ether at same time, find it idle, and
  begin transmitting
• Window for transmission depends on speed of
  propagation in ether
• Signals from two computers will interfere with
  each other
• Overlapping frames is called a collision
• Data from both frames is garbled
• Ethernet uses CSMA Collision Detection (CD) to
  coordinate transmission
• Ethernet interfaces include hardware to detect
  transmission
• Monitor outgoing signal
• Garbled signal is interpreted as a collision
• Listen to medium during transmission
• Detect whether another stations signal
  interferes
• Back off from interference (wait random amount of
  time) and try again

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Recovery from Collision

• Computer that detects a collision sends special
  signal to force all other interfaces to detect
  collision
• Computer then waits for ether to be idle before
  transmitting
• If both computers wait same length of time,
  frames will collide again
• Standard specifies maximum delay, and both
  computers choose random delay less than maximum
• After waiting, computers use carrier sense to
  avoid subsequent collision

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Exponential Backoff

• Even with random delays, collisions may occur
• Especially likely with busy segments
• Computers double delay with each subsequent
  collision
• Reduces likelihood of sequence of collisions
• Also called binary backoff

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CSMA/CD Algorithm
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Wireless LAN

• Use radio signals at 900 MHz
• Data rate of 2 Mbps
• Shared medium - radio instead of coax
• In contrast with wired LAN, not all participants
  may be able to reach each other
• Low signal strength
• Propagation blocked by walls, etc.
• Can't depend on CD not all participants may hear
  

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CSMA/CA

• CSMA Sense before sending. Only send if idle for
  IFS (Inter-Frame Space). O/W backoff.
• Wireless uses collision avoidance rather than
  collision detection
• Transmitting computer sends very short message to
  receiver
• Receiver responds with short message reserving
  slot for transmitter
• Response from receiver is broadcast so all
  potential transmitters receive reservation

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Handling Collisions

• Receiver may receive simultaneous requests
• Results in collision at receiver
• Both requests are lost
• Neither transmitter receives reservation both
  use backoff and retry
• Receiver may receive closely spaced requests
• Selects one
• Selected transmitter sends message
• Transmitter not selected uses backoff and retries
  

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Physical and Logical Ring Synchronization
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Token Ring

• Many LAN technologies that use ring topology use
  token passing for synchronized access to the ring
  
• Ring itself is treated as a single, shared
  communication medium
• Bits pass from transmitter, past other computers
  and are copied by destination
• Hardware must be designed to pass token even if
  attached computer is powered down

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Token Passing Synchronization

• Used with ring topology
• Guarantees fair access IEEE 802.5 standards
• Token Special small (a few bits) reserved (can
  not appear in data) message
• Only computer holding the token can transmit
• Because there is only one token, only one
  computer will transmit at a time
• Hardware must regenerate token if lost
• Token gives computer permission to send one frame
  
• If all ready to transmit, enforces
  round-robin'' access
• If none ready to transmit, token circulates
  around ring

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Token Ring Transmission

• Station waits for token before sending
• Signal travels around entire ring
• Sender receives its own transmission

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Token Release Mechanisms

• Release After Reception (RAR)
• Each station reissues the free token only after
  it receives the transmitted frame.
• Used on lower speed token rings ( lt 4Mbps).
• Release After Transmission (RAT)
• Each station attaches a free token at the end of
  its frame
• Possible multiple frames propagate in a ring
• Used on higher speed token rings (gt 16Mbps)

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Token Passing Ring Technologies

• IBM token ring
• Very widely used
• Originally 4mbps, now 16Mbps
• Fiber Distributed Data Interface (FDDI)
• Operates at 100 Mbps

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FDDI

• Fiber Distributed Data Interconnect (FDDI)
• Uses ring token passing synchronization (RAT
  token release)
• Uses fiber as transmission media
• Transmits data at 100Mbps
• Also suitable for MAN
• Can attach 1000 stations, can be up to 200 km
• Uses pairs of fibers to form two concentric rings
• FDDI uses counter-rotating rings in which data
  flows in opposite directions gt Reliability

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FDDI Self-healing

• In case of fiber or station failure, remaining
  stations loop back and reroute data through the
  spare ring
• All stations automatically configure loop back by
  monitoring the data ring

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ATM

• Asynchronous Transfer Mode technology consists of
  electronic packet switches to which computers can
  connect
• ATM switches form hub into which computers
  connect in a star topology
• Computers get point-to-point connections - data
  from transmitter is routed directly through hub
  switches to destination
• Transmits data at over 100Mbps
• Uses fiber optics to connect computer to switch
• Each connection includes two fibers

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ATM Switches
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IEEE 802.x LAN Standards

• 10 Mbps CSMA/CD (802.3)
• 100 Mbps CSMA/CD (802.3u)
• 1000 Mbps CSMA/CD (802.3z)
• Token Bus (802.4)
• Token Ring (802.5)
• Wireless LAN (IEEE 802.11)
• And many more

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Summary

• Local Area Networks
• Designed for short distance
• Use shared media
• Transmitting computer has exclusive use of
  communication medium
• computers must synchronize transmission
• Many technologies exist
• Ethernet, Wireless, IBM Token Ring, FDDI, ATM
• Topology refers to general shape
• Bus
• Ring
• Star
• Ethernet
• CSMA/CD
• Manchester encoding
• Exponential Binary backoff
• Synchronization by token passing in a ring
• IBM token ring and FDDI