Chapter 2 Multiple Access Protocols

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Chapter 2Multiple Access Protocols

• Professor Rick Han
• University of Colorado at Boulder
• rhan_at_cs.colorado.edu

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Announcements

• Previous lecture now online
• Homework 1 is on the Web site, due Feb. 5
• Programming assignment 1 will be available on
  Web site on Tuesday, not today
• Next, Chapter 2, MAC protocols

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Recap of Previous Lecture

• Designing reliable ARQ protocols
• Acknowledgements
• Timeouts
• Sequence numbers
• Designing efficient reliable protocols
• Choose timeout wisely
• Keep the pipe full
• Stop-and-Wait one outstanding packet
• Go-Back-N sliding window, cumulative ACK
• Selective Repeat sliding window, selective ACK

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Direct-Link or Point-to-Point Networks

• Physical layer handles bits
• Data-link layer handles packets
• Framing
• Error detection
• Retransmission-based protocols
• How do I send to N hosts?
• N point-to-point links
• Other possibilities?

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Shared-Media or Broadcast Networks

• N senders and receivers connected by a shared
  medium (copper wire, atmosphere-TV!)
• Sharing access to the same media
• Analogy How do N persons converse in a room or
  at the dinner table? At once, or one by one?
  What is the communications protocol?
• Local Area Network (LAN)

802.11/Wireless Ethernet
Ethernet (802.3)
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Multiple Access Protocols

• Determine which host is allowed to transmit next
  to a shared medium
• Channel reservation TDMA, FDMA, CDMA, Token
  Ring,
• Random access ALOHA, CSMA/CD, CSMA/CA

802.11/Wireless Ethernet
Ethernet
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Multiple Access Protocols (2)

• Also called Medium-Access Control (MAC) protocols
• Before data link-layer packets can be sent, a
  sender has to gain access to the media
• MAC layer is often placed in the stack between
  layer 2 and layer 1

Host A
Data Link Layer
MAC Layer
Physical Layer
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Time Division Multiple Access (TDMA)

• Divide time into multiple slots
• Each host sends in a pre-determined slot
• Out-of-band reservation mechanism
• Compare to Time Division Multiplexing (TDM)

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Frequency Division Multiple Access (FDMA)

• Divide spectrum into frequency bins
• Each host sends in a pre-determined frequency bin
• Out-of-band reservation mechanism
• Also called Frequency Division Multiplexing (FDM)
• Example AM/FM radio, TV

Host 1
Host 2
Host 3
Freq. (Hz)
AM 500-1700 KHz
FM 88-108 MHz
Satellite GHz range
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Code Division Multiple Access (CDMA)

• Use multiple orthogonal codes to partition a
  range of spectrum
• Each host sends using a pre-determined code
• Also called spread spectrum
• Direct-sequence spread spectrum (802.11, cell)
• Frequency-hopping spread spectrum (Bluetooth)

Host 1
Freq
F1
F2
F3
Hopping sequence F1, F3, F2, F1, F3, F2,
Host 2
Host 1s Code 132, Host 2s Code 321, Host 3s
Code 213 all 3 codes orthogonal
Bluetooth
Host 3
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Random Access ALOHA Protocol

• Developed at University of Hawaii in 1971 by
  Abramson
• Ground-based UHF radios connect computers on
  several island campuses to main university
  computer on Oahu
• pure ALOHA hosts transmit whenever they have
  information to send form of random access
• Collision will occur when two hosts try to
  transmit packets at the same time
• Hosts wait a timeout1 RTT for an ACK.
• If no ACK by timeout, then wait a randomly
  selected delay to avoid repeated collisions, then
  retransmit

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Random Access ALOHA Protocol (2)

• Collision of packets can occur when a packet
  overlaps another packet

time
T0
Wasted Time Due to a Collision 2 packet
intervals
Collision
Collision
Wasted Time Colliding with B
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Random Access Slotted ALOHA

• Rather than sending a packet at any time, send
  along time slot boundaries
• Collision are confined to one time slot

time
T0
Wasted Time Due to a Collision 1 packet interval
No Collision
Collision
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Random Access Slotted ALOHA (2)

• How do hosts synchronize to begin transmitting
  along time slot boundaries?
• One central station transmits a synchronization
  pulse or beacon
• Slotted ALOHA is more efficient than ALOHA
  because when there is a collision, the wasted
  time is confined to one time slot
• Assuming Poisson packet arrivals (memoryless),
  can compute the maximum throughput of ALOHA to be
  18.
• Maximum throughput of Slotted ALOHA is 37
• Why are ALOHA slotted ALOHA so inefficient?

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Random Access CSMA

• ALOHA slotted ALOHA are inefficient because
  hosts dont take into account what other hosts
  are doing before they transmit
• Example at party, everyone speaks whenever they
  want to, regardless of whether another person is
  speaking
• Instead, listen before you talk Carrier Sense
  Multiple Access (CSMA)
• Sense for carriers (see if anyone else is
  transmitting) before you begin transmitting

time
Host B listens
delay
Collision still possible over long prop. delays
Host B sends
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Random Access 1-Persistent CSMA

• If channel is busy,
• A host listens continuously
• When channel becomes free, a host transmits its
  packet immediately (with probability 1)
• Collision scenarios
• Hosts A and B are far apart (long prop. delay).
  As signal takes a long time to reach B. So, B
  thinks channel is free, and begins transmitting.
• Hosts B and C transmit as soon as A finishes
• Still, CSMA is more efficient than ALOHA variants

time
Host B listens
Host B sends
Collision
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Random Access p-Persistent CSMA

• Generalization of 1-persistent CSMA
• Typically applied to slotted channels
• Slot length is chosen as maximum propagation
  delay
• A host senses the channel, and
• If slot is idle, transmit with probability p, or
  defer with probability q1-p
• If next slot is idle, transmit with probability
  p, or defer with probability 1-p, repeat
• If channel is busy, then sense channel
  continuously until it becomes free, begin again

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Random Access Non-Persistent CSMA

• Host does not sense channel continuously
• Instead, if channel is busy,
• Wait/sleep a random interval before sensing again
• As with 1-persistent CSMA, as soon as channel is
  idle, then send a packet
• Random interval reduces collisions
• Higher throughput than 1-persistent CSMA when
  many senders

time
Host B listens
Random Sleep
Host B sends
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Random Access Ethernet CSMA/CD

• Ethernet uses CSMA/CD, i.e. CSMA with Collision
  Detection (CD)
• Listen-while-talk protocol
• A host listens even while it is transmitting, and
  if a collision is detected, stops transmitting

time
Host B senses carrier
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Random Access Ethernet CSMA/CD (2)

• Can abort transmission sooner than end-of-packet
  if there is a collision
• Can happen if prop. delays are long
• Better efficiency than pure CSMA
• CSMA/CD doesnt require explicit acknowledgement
• Unlike CSMA, which requires an ACK or timeout to
  detect a collision
• Collision detection is built into the transmitter
• When collision detected, begin retransmission

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Random Access Ethernet CSMA/CD (3)

• Exponential backoff strategy
• When a collision is detected, a host waits for
  some randomly chosen time, then retransmits a
  packet
• If a second collision is detected, a host doubles
  the original wait time, then retransmits the
  packet
• Each time there is another collision, the wait
  time is doubled before retransmission
• Variants
• At each retransmission, choose a random value
  from the exponentially increasing wait time.
• At each retransmission, choose randomly from
  among a discrete set of values within
  exponentially increasing wait time
• Retransmit a finite of times

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Random Access Ethernet CSMA/CD (4)

• CSMA/CD can be used with nonpersistent,
  1-persistent, or p-persistent variants of CSMA
• Ethernet is synonymous with the IEEE 802.3
  standard
• Initial work on Ethernet at Xerox in early 70s
• Ethernet specifies 1-persistent CSMA/CD
• To extend an Ethernet, repeaters are placed.
• Start to run into propagation delay issues and
  noise amplification issues
• Ethernet keeps its maximum length to 2500 m to
  keep prop. delays tight, so that CSMA/CD responds
  well

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Random Access 802.11 Wireless Ethernet

• Employs CSMA/CA, i.e. CSMA with Collision
  Avoidance (CA)
• Hidden terminal effect
• Example B can hear A and C, but A and C cant
  hear each other. If A is sending B, C thinks
  channel is clear and starts sending gt collision!
• Doesnt happen in wired Ethernet, because hosts
  can hear each other

Host A
Host B
Host C
Collision
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Random Access 802.11 Wireless Ethernet (2)

• How to handle the hidden terminal effect?
• Host A sends a Request-To-Send (RTS)
• Host B sends a Clear-To-Send (CTS)
• Host C hears the CTS, and does not interrupt
  transmission between A and B
• This helps implement Collision Avoidance

RTS
Host A
Host B
Host C
CTS
CTS
Data
Host C Suppresses Its Data
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Random Access 802.11 Wireless Ethernet (3)

• Acknowledgements are still needed
• After Host A has finished sending its data, Host
  B sends an ACK
• Host C hears this ACK, and sends its RTS

Data
Host A
Host B
Host C
ACK
ACK
RTS
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Random Access 802.11 Wireless Ethernet (4)

• 802.11 does not support Collision Detection
• In a wired LAN, transmitter can check voltage
  levels to see if there is a collision. A remote
  transmitters power doesnt attenuate severely.
• In a wireless LAN, the transmitters power
  overwhelms a distant transmitters power, so its
  difficult to detect collision.
• What happens if two RTSs collide?
• Senders realize after timeout that CTS did not
  come back, and then practice exponential backoff
  in trying to send new RTSs