LAN Technologies and Network Topology

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LAN Technologies and Network Topology
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Sharing

• LANs typically consist of a share cable
• As all computers are on the single cable, it must
  be shared

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P2P

• Alternative point to point connections
• Can have different types of connections
• Speed, frames, error detection can vary
• Can change independent of other connections
• Might have increased security as only two see
  traffic

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Squared

• The total number of connections (wires) varies as
  the square of the number of computers
• Adding computer n require n-1 lines
• So, directly connecting n computers needs
  n(n-1)/2 connections O(n2)

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Squared Continued

• 2 1
• 3 3
• 4 6
• 5 10
• 6 15
• 7 21
• 8 28
• 9 36
• 10 45
• 20 190
• 100 4950

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Connections

• Also sets of computers are connected so multiple
  wires would run next to each other
• If medium shared, cost greatly reduced
• However, sharing requires coordination and this
  reduces available bandwidth

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Locality

• LANs also good due to locality of reference
• Computers (humans) dont communicate randomly,
  but exhibit locality

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Locality Continued

• Temporal time if two humans/compouters
  communicate now, its likely that they will again
  soon
• Email conversation
• Physical or spatial two humans/computers are
  more likely to communicate if they a physically
  close to each other
• Sharing documents

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Topologies

• LANs are connected using a variety of topologies
• We differentiate between logical and physical
  topologies
• Logical how we think about it
• Physical how it is wired
• LAN technologies differ in voltages, timing,
  frame format/size, protocol for sharing, etc.)

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Star
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Ring
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Driving the Bus
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Bus

• All computers see all the communication
• Have to coordinate

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Pluses and Minuses

• Ring easy coordination, fails with one
  disconnect
• Star more robust, more wires than bus, switch at
  center can get expensive
• Bus few wires, less robust

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Ethernet

• Invented at Xeroxs PARC
• Codified by DEC, Intel, Xerox (DIX)
• Now IEEE controls standard 802.3
• Originally used coax
• Used term segment to specify a single length of
  coax
• Originally 10Mbs, now 100 (Fast) and 1000
  (Gigabit)

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Manchester

• For self-clocking and reliability bits are
  encoded using a Manchester encoding scheme
• Edges of signals used to encode 0s and 1s
• Always a transition mid-bit
• Falling edge 0, rising 1
• Preamble used to get synched up 64 bits of
  alternating 0s and 1s (can you picture the from?)

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Manchester Picture
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Sharing the Ether
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Sending All zeros!
0 0 0 0 0 0
Timeslot
Timeslot
Timeslot
Timeslot
Timeslot
Timeslot
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Sharing Continued

• Only one frame can be on wires at a time
• All other machines must wait
• Ethernets do not have a centralized controller
• Has distributed control system

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CSMA

• Carrier Sense Multiple Access
• Carrier Sense each machine can detect when
  another is transmitting can detect any
  electrical signal on line
• Multiple Access if no one else transmitting (no
  carrier sensed) a station can try

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CSMA/CD

• Prevents all others from talking once
  transmission started
• However what if no carrier and two start talking
  at same time?
• Signals garbled
• Called collision
• Must consider end to end propagation delay (speed
  of light)

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CSMA/CD Continued

• Stations required to monitor line while
  transmitting
• Each endpoint can sense collision
• Comparison what is being sent is not what is
  being detected
• Called collision detection
• Stations stop transmitting immediately

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Collisions

• Stations wait for a random amount of time, listen
  again, and if empty try again
• Random important so stations dont repeatedly
  collide
• Standard must specify maximum cable length and
  minimum frame size
• Known upper bound for non-collision knowledge

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Collisions Continued

• If another collision occurs, wait for 2 x time, 4
  x, 8 x, etc.
• Binary exponential backoff

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Wireless

• Similar approach taken in wireless LANs
• Assumption not all stations can see each other
• Cannot use CSMA/CD

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CSMA/CA

• Use collision avoidance
• Sender sends request to talk
• Receiver sends response
• Now all stations that see receiver know not to
  transmit
• When sender receives response, starts to transmit

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CSMA/CA Continued

• If request collides senders back off

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Token Ring

• Coordinated using token passing
• Rings call token passing ring or simply token
  rings
• Have single shared channel
• When computer wants to send data, must capture
  token
• Only that computer can transmit

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Token Ring Continued

• Data transmitted around the ring and returned to
  sender
• All stations forward all data
• Sender must drain data
• Receiver copies data
• Does not use CSMA/CD
• Has a special bit pattern called token

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Token Ring Continued

• Token a special sequence of bits
• Must be different that data
• Bit stuffing used
• Must only be a single token on the ring at a time
• Hardware assures this
• Must be able to regenerate token

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Token Ring Continued

• To transmit, computer removes token
• Sends one frame
• Reissues token
• Token may then be captured by other computers
• If no computer has data to send token circulates
  quickly

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Token Ring Continued

• Only transmit one frame with token
• Round robin
• Good for heavy loads

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FDDI

• Ring downside failure due to single link
  breakage
• Token ring NICs designed to pass even if not
  powered
• Another ring Fiber Distributed Data Interconnect
  (FDDI)
• 100Mbps

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FDDI Continued

• Contains two complete rings
• One that send data
• One as a backup
• Sends data but is typically ignored
• Rings counter rotating
• Going in opposite directions
• Helps in the case of a break
• Can self heal

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FDDI Continued

• Typically, if one fiber breaks, both do
• Each end can forward bits around and ring remains
  intact

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ATM

• Asynchronous Transfer Mode
• Uses switching technology
• Typically 155Mbps or greater
• Typically optical fiber

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ATM Switch
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ATM Switch Connection
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Exercise

• Go through this entire chapter and make a list of
  each specific LAN technology
• Name the specific technology
• List its data speed (bandwidth)
• Describe its topology
• Describe in sharing scheme
• Scheme for collision avoidance or handling
• Be able to make a comparison table of pros and
  cons