Medium Access Control

1
Medium Access Control
2
Channel Allocation

• Static channel allocation in LANs and MANs
• FDMA, TDMA, CDMA
• Dynamic channel allocation in LANs and MANs
• MAC protocols with collisions, polling, token

3
Static Channel Allocation

• Delay for one fast channel
• Delay for multiple FDM slower channels

4
Poisson Process

• Probability of k arrivals in time t
• Probability that packet duration exceeds t
• Note that ? is the average packet arrival rate,
  and 1/µ is the average packet duration.

5
M/M/1 Queue

• Queue equations, pj(t) is the probability that
  the number of packets at time t in a queue is j
• The solution of the previous recursion for
  stationary probabilities to which pj(t) converge

6
Delay

• Littles formula for average delay ED
• EQ is the average number of packets in a
  queue
• Delay is

7
Static Channel Allocation

• Delay for one fast channel
• where C is the channel bit-rate and EL is
  the average packet length.
• Delay for multiple FDM N times slower channels

8
Multiple Access Protocols

• ALOHA
• Carrier Sense Multiple Access (CSMA) protocols
• CSMA/CD
• CSMA/CA
• Collision-Free protocols
• Reservation based
• Token based

9
Pure ALOHA

• Vulnerable period for the shaded frame.

10
ALOHA Throughput

• Throughput is SGPs, where Ps is the
  probability of successful transmission.
• The k frames per f frame slots is
• Pse-fG
• For pure ALOHA f2, for slotted ALOHA f1,
  so

11
Pure and Slotted ALOHA

• Throughput versus offered traffic for ALOHA
  systems.

12
Carrier Sense Multiple Access

• 1-Persistant CSMA
• Nonpersistant CSMA
• P-Persistant CSMA

13
CSMA with Collision Detection

• CSMA/CD can be in one of three states
  contention, transmission, or idle.

14
Wireless LAN CSMA-CA

• The MACA protocol. (a) A sending an RTS to B.
• (b) B responding with a CTS to A.

15
DOCSIS (Data Over Cable Service Interface
Specification)
16
Collision-Free ProtocolsReservations

• The basic bit-map protocol.

17
Collision-Free Protocols Bidding

• The binary countdown protocol. A dash indicates
  silence.

18
Collision Free Protocols Fiber Distributed Data
Interface (FDDI)

• Station transmits only when it has a token
• Timers count the time while the token is away
• Two timers determine how much data a station may
  transmit, so that the token delay is limited

19
Ethernet

• Ethernet, IEEE 802.3
• 10Base (10Mbps)
• Fast Ethernet (100Mbps)
• Gigabit Ethernet

20
Ethernet MAC Sublayer Protocol

• Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.
• Preamble-synchronization, Type-upper layer
  protocol,
• Pad-to make the minimum packet size 64B

21
CSMA with Collision Detection

• CSMA/CD can be in one of three states
  contention, transmission, or idle.

22
Back-Off Mechanism

• After a collision, user accesses medium with
  probability 1/W where W is the window size.
• With each collision W doubles.

23
Ethernet Performance
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Throughput of CSMA/CD (Ethernet)

• Assume that requests form a Poisson process
  with rate g, T is time slot duration, and Tp is a
  packet duration. The throughput equals
  STp/(TpI), where I is the average time between
  packet transmissions.
• The probability of a packet transmission is
  equal to the probability that there is only one
  request in some previous time slot which is
  PsgTe-gT.
• The average time between transmissions is

25
Throughput of CSMA/CD

• The throughput is
• It tends to 0 when g increases .
• Protocol is unstable like ALOHA.

26
Throughput of CSMA/CD

• The throughput is
• If p is the packet generation probability and
  k is the number of active users
• Pskp(1-p)k-1
• The maximum throughput is achieved for p1/k
  and it is tends to e when k tends to infinity

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Ethernet Performance

• Efficiency of Ethernet at 10 Mbps with 512-bit
  slot times.

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10Mbps Ethernet Cabling

• The most common kinds of Ethernet cabling.

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10Mbps Ethernet Cabling

• Three kinds of Ethernet cabling.
• (a) 10Base5, (b) 10Base2, (c) 10Base-T.

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Ethernet Cabling

• Cable topologies. (a) Linear, (b) Spine, (c)
  Tree, (d) Segmented.

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10Mb Ethernet Coding

• (a) Binary encoding, (b) Manchester encoding,
  (c) Differential Manchester encoding.

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10 Mb Ethernet Collision Detection

• 10Base5 cabling,
• Kadambi, Crayford and Kalkunte, Gigabit Ethernet,
  Prentice Hall, 1998

33
10 Mb Ethernet Collision Detection

• 10Base2 and 10BaseT cabling,
• Kadambi, Crayford and Kalkunte, Gigabit Ethernet,
  Prentice Hall, 1998

34
Fast Ethernet

• The original fast Ethernet cabling.

35
Fast Ethernet

• Auto negotiation enables communication with 10Mb
  Ethernet
• Manchester code ? 4B/5B code
• Full duplex mode is optional with using PAUSE
  command

36
Switched Ethernet

• A simple example of switched Ethernet.

37
Gigabit Ethernet

• (a) A two-station Ethernet. (b) A multistation
  Ethernet.

38
Gigabit Ethernet

• Gigabit Ethernet cabling.

39
Gigabit Ethernet

• Prioritization of fiber over copper
• 4B/5B coding ? 8B/10B coding
• Full duplex mode is preferred with PAUSE message
• Carrier extension, and frame bursting introduced
  in half-duplex mode

40
IEEE 802.2 Logical Link Control

• (a) Position of LLC. (b) Protocol formats.

41
A Sample HFC System
Downstream 500 MHz shared by 50,000 (broadcast)
200 MHz by 1200 (narrowcast) Upstream
37 MHz shared by 300
HOME
o o
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lup lb ln (4ln/fiber)
Secondary Hub
Fiber Node
o o
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o o
o o
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RF Spectrum on coax return
80 broadcast channels 30 QAM
channels (150 video channels)
broadcast
5-42 MHz 550
MHz 750 MHz
Sheryl Woodward, ATT Labs-Research
42
Justification for Using Shared Medium

• Equivalent circuit rate (ECR) on a cable
  with many users is the rate of a dedicated link
  that would provide the same e.g. average delay
  (similar results is obtained for 90th percentile
  page delay). By Shankar, Jiang and Mishra
• where tON is the transmission tim, and tOFF is
  the think time, r is the channel rate,
  tON/(tONtOFF)ltlt1, on periods have an exponential
  distribution.

43
Justification for Using Shared Medium

• Lets calculate how many users can be
  allocated one DOCSIS channel of 32Mbps to get the
  same experience as DSL user with dedicated rate
  of 2Mbps. According to traffic statistics page
  size is 68KB on average, and tOFF is 14.5s on
  average,
• which is much more than 32/216 users. Price
  high user speed.

44
DOCSIS MAC Protocol

• Traffic that is transmitted downstream to the
  users is controlled by CMTS (cable modem
  termination system) in headend. It polices and
  shapes the traffic, and perform algorithms such
  are WFQ and RED.
• Users requests are resolved at headend, and
  they are informed about the resolution through
  the downstream channel. If there is a collision
  of requests, users repeat their requests
  according to exponential back-off mechanism,
  otherwise they send data in specified time
  slot(s).

45
QoS in DOCSIS
46
QoS in DOCSIS
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Performance for BE service in DOCSIS

• Assume that requests form a Poisson process
  with rate g, T is time slot duration, and Tp is a
  packet duration. The throughput equals
  STp/(TpI), where I is the average time between
  packet transmissions.
• The probability of a packet transmission is
  equal to the probability that there is only one
  request in some previous time slot which is
  gTe-gT.
• The average time between transmissions is

48
Performance for BE service in DOCSIS

• The throughput is
• It tends to 0 when g increases .
• Protocol is unstable like ALOHA.

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Wireless LANs

• Distributed coordination function (DCF)
• Point coordination function (PCF)

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The 802.11 Protocol Stack

• Part of the 802.11 protocol stack.

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The 802.11 MAC Sublayer Protocol

• (a) The hidden station problem.
• (b) The exposed station problem.

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Wireless LAN CSMA-CA

• The MACA protocol. (a) A sending an RTS to B.
• (b) B responding with a CTS to A.

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The 802.11 MAC Sublayer Protocol

• The use of virtual channel sensing using CSMA/CA.

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The 802.11 MAC Sublayer Protocol

• A fragment burst.

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The 802.11 MAC Sublayer Protocol

• Interframe spacing in 802.11.

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The 802.11 Frame Structure

• The 802.11 data frame.
• Address 3 and 4-for source and dest base
  stations, Seq-fragment sequence number, Type-data
  or control, Subtype-RTS or CTS, MF-more
  fragments, More-more frames, W-WEP, O-frame
  sequence maintained

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802.11 Services

• Association
• Disassociation
• Reassociation
• Distribution
• Authentication
• Integration
• Privacy