CS455 Introduction to Computer Networks

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CS455 Introduction to Computer Networks
WSU Vancouver

• Dr. Wenzhan Song
• Assistant Professor, Computer Science

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Course roadmap

• Introduction
• Application Layer WWW, FTP, email, DNS,
  multimedia
• Transport Layer reliable end-end data transfer
  principles, UDP, TCP
• Network Layer IP addressing, routing and other
  issues
• Data Link Layer framing, error control, flow
  control
• Medium Access Control (MAC) Layer
  multiple-access, channel allocation
• Physical Layer wired, wireless, satellite
• Other Topics network security, social issues,
  hot topics, research directions

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Data Link Layer Road Map

• Data link layer design issues
• Framing
• Error Control
• Reliable data transfer and flow control
• Example data link protocols
• HDLC and PPP
• Multiple Access Protocols
• Static channel allocation
• Dynamic channel allocation
• LAN technologies and their MAC protocols
• Ethernet
• WiFi and WiMax

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Multiple Access Links and Protocols

• Two types of links
• point-to-point
• PPP for dial-up access
• point-to-point link between switch and host
• broadcast (shared wire or medium)
• traditional Ethernet
• upstream HFC
• 802.11 wireless LAN

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Multiple Access protocols

• single shared broadcast channel
• two or more simultaneous transmissions by nodes
  interference
• collision if node receives two or more signals at
  the same time
• multiple access protocol
• distributed algorithm that determines how nodes
  share channel, i.e., determine when node can
  transmit
• communication about channel sharing must use
  channel itself!
• no out-of-band channel for coordination

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Ideal Multiple Access Protocol

• Broadcast channel of rate R bps
• 1. When one node wants to transmit, it can send
  at rate R.
• 2. When M nodes want to transmit, each can send
  at average rate R/M
• 3. Fully decentralized
• no special node to coordinate transmissions
• no synchronization of clocks, slots
• 4. Simple

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MAC Protocols a taxonomy

• Static Channel Allocation
• divide channel into smaller pieces (time slots,
  frequency, code)
• allocate piece to node for exclusive use
• Dynamic Channel Allocation
• channel not divided, allow collisions
• recover from collisions

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Static Channel Allocation TDMA

• TDMA time division multiple access
• access to channel in "rounds"
• each station gets fixed length slot (length pkt
  trans time) in each round
• unused slots go idle
• example 6-station LAN, 1,3,4 have pkt, slots
  2,5,6 idle

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Static Channel Allocation FDMA

• FDMA frequency division multiple access
• channel spectrum divided into frequency bands
• each station assigned fixed frequency band
• unused transmission time in frequency bands go
  idle
• example 6-station LAN, 1,3,4 have pkt, frequency
  bands 2,5,6 idle

time
frequency bands
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Dynamic Channel Allocation

• ALOHA, slotted ALOHA
• CSMA, CSMA/CD, CSMA/CA in future slides
• Collision free protocols
• Bit-map protocol
• Binary countdown
• Limited contention protocols
• Adaptive tree walk
• Wavelength division multiple access protocol

Not currently used in major system yet
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Pure (unslotted) ALOHA

• unslotted Aloha simpler, no synchronization
• when frame first arrives
• transmit immediately, if collision, then
  retransmit with p, or waits for another frame
  time with 1-p
• collision probability increases
• frame sent at t0 collides with other frames sent
  in t0-1,t01

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Pure Aloha efficiency

• P(success by given node) P(node transmits) .
• P(no
  other node transmits in t0-1,t0 .
• P(no
  other node transmits in t0,1t0
• p .
  (1-p)N-1 . (1-p)N-1
• p
  . (1-p)2(N-1)
• choosing optimum
  p and then letting N -gt infty ...
• 
  1/(2e) .18

Even worse !
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Slotted ALOHA

• Assumptions
• all frames same size
• time is divided into equal size slots, time to
  transmit 1 frame
• nodes start to transmit frames only at beginning
  of slots
• nodes are synchronized
• if 2 or more nodes transmit in slot, all nodes
  detect collision

• Operation
• when node obtains fresh frame, it transmits in
  next slot
• no collision, node can send new frame in next
  slot
• if collision, node retransmits frame in each
  subsequent slot with prob. p until success

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Slotted ALOHA
C collisionE emptyS - successful

• single active node can continuously transmit at
  full rate of channel
• highly decentralized only slots in nodes need to
  be in sync
• simple

• collisions, wasting slots
• idle slots (even with data)
• nodes may be able to detect collision in less
  than time to transmit packet
• clock synchronization

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Slotted Aloha efficiency

• For max efficiency with N nodes, find p that
  maximizes Np(1-p)N-1
• For many nodes, take limit of Np(1-p)N-1 as N
  goes to infinity, gives 1/e .37

Efficiency is the long-run fraction of
successful slots when there are many nodes, each
with many frames to send

• Suppose N nodes with many frames to send, each
  transmits in slot with probability p
• prob that node 1 has success in a slot
  p(1-p)N-1
• prob that an arbitrary node has a success
  Np(1-p)N-1

At best channel used for useful transmissions
37 of time!
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ALOHA

• Throughput versus offered traffic for ALOHA
  systems.

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CSMA (Carrier Sense Multiple Access)

• CSMA listen before transmit
• If channel sensed idle may transmit entire frame
• If channel sensed busy, defer transmission
• Human analogy dont interrupt others!

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Nonpersistent CSMA

• Station wishing to transmit listens and obeys
  following
• If medium is idle, transmit otherwise, go to 2
• If medium is busy, wait amount of time drawn from
  probability distribution (retransmission delay)
  and repeat 1
• Consequences
• Random delays reduces probability of collisions
• Consider two stations become ready to transmit at
  same time
• While another transmission is in progress
• If both stations delay same time before retrying,
  both will attempt to transmit at same time
• Capacity is wasted because medium will remain
  idle following end of transmission
• Even if one or more stations waiting

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1-persistent CSMA

• Station wishing to transmit listens and obeys
  following 
• If medium idle, transmit otherwise, go to step 2
• If medium busy, listen until idle then transmit
  immediately
• Consequences
• avoid idle channel time
• 1-persistent stations selfish
• If two or more stations waiting, collision
  guaranteed
• Gets sorted out after collision

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P-persistent CSMA

• Station wishing to transmit listens and obeys
  following 
• If medium idle, transmit with probability p, and
  delay one time unit with probability (1 p)
• If medium busy, listen until idle and repeat step
  1
• If transmission is delayed one time unit, repeat
  step 1
• Consequences
• Compromise that attempts to reduce collisions
• Like nonpersistent
• And reduce idle time
• Like 1-persistent
• What is an effective value of p? depend on load

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Persistent and Nonpersistent CSMA
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Persistent and Nonpersistent CSMA

• Comparison of the channel utilization versus load
  for various random access protocols.

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Collision-Free Protocols

• The basic bit-map protocol.

• Contention period, consists N slots
• Station j want to transmit frame, insert a 1 bit
  into slot j
• Each station has complete knowledge after listen
  contention period
• Transmit frame in numerical order
• Never has collisions

Overhead is 1 bit per station
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Collision-Free Protocols (2)

• The binary countdown protocol

• Station want to transmit, broadcasts its address
  as a binary string, starting with the highest bit
• Bits in each address position from different
  stations are Boolean ORed together
• The result address is the winner of the bidding,
  it may now transmit a frame
• Higher address has a higher priority (good or
  bad)

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Limited-contention protocols

• Adaptive tree walk protocol

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Wavelength Division Multiple Access Protocols
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Wireless LAN Protocols

• (a) Hidden station problem (b) Exposed station
  problem

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MACA protocol (Multiple Access with Collision
Avoidance )

• (a) A sending an RTS (Request to Send) to B.
• (b) B responding with a CTS (Clear to Send) to A.

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Wireless LAN Protocols

• Multiple Access with Collision Avoidance (MACA)
• The sender stimulate the receiver into outputting
  a short frame, so stations nearby can detect this
  transmission and avoid transmitting for the
  duration of the upcoming data frame
• Multiple Access with Collision Avoidance for
  Wireless (MACAW)
• Introduce ACK to enable retransmit
• Add carrier sense to avoid conflict with RTS
• Run backoff algorithm for each pair, instead of
  each station
• Exchange congestion info between neighbors

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Summary of MAC protocols

• What do you do with a shared media?
• Channel Partitioning, by time, frequency or code
• Time Division, Frequency Division
• Random partitioning (dynamic),
• ALOHA, S-ALOHA
• CSMA
• carrier sensing easy in some technologies
  (wire), hard in others (wireless)
• CSMA/CD used in Ethernet
• Collision-free and limited contention protocols
• WDMA
• MACA, MACAW wireless
• CSMA/CA used in 802.11

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Reading Assignment

• Chapter 5.3 and 5.5
• Chapter 6.3