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					Medium Access Control
       Zhibin Wu
                        Lecture Overview
   Introduction
   Random Access
    –   Aloha
    –   Slotted Aloha
    –   CSMA
    –   CSMA/CD
    –   CSMA/CA
   Scheduled Access
    – TDMA
    – Dynamic TDMA
   Spread-Spectrum/CDMA
                 Medium Access Sublayer

            network

                                     LLC        Link layer control
           Data link
                                     MAC        Medium access control
            physical

   Medium access (MAC) sublayer is not relevant on point-to-point links
   The MAC sublayer is only used in broadcast or shared channel networks
   All communication entities “share” a common channel
   Examples:
     – Wired networks: Ethernet LAN
     – Wireless & Mobile Networks: Satellite, Cellular, Wireless LAN,
     – Packet radio network?
                 Share a Channel Ideally


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
               Random Access Protocols

   Single channel shared by a large number of hosts
   No coordination between hosts
   Control is completely distributed
   Examples: ALOHA, CSMA, CSMA/CD
                 Scenarios of ALOHA

   A group of nodes trying
    to sending frames to a
    central node
   Star-topology.
   Not a complete solution
    for bi-directional
    communication
     – For half-duplex
       device, what if a data
       packet arrives while it
       is receiving?
                          Pure Aloha




   In Pure Aloha, frames are transmitted at completely arbitrary
    times.
                    Aloha Algorithm

1.   Transmit whenever you have data to send
2.   Listen to the broadcast (probably a separate channel)
     – Because broadcast is fed back, the sending host can
       always find out if its packet was destroyed just by
       listening to the downward broadcast one round-trip
       time after sending the packet
3.   If the packet was destroyed, wait a random amount of
     time and send it again
     – The waiting time must be random to prevent the same
       packets from colliding over and over again
                      Vulnerable Period




   Vulnerable period for the shaded frame is 2t
   Note that if the first bit of a new packet overlaps with the last bit
    of a packet almost finished, both packets are totally destroyed.
    (No capture effect)
                             Analysis of Aloha

   Packet Arrival is Poisson Process
   P [k arrivals in t seconds] = e  t (t ) k
                                          k!
   Let G be the total number of frames attempted in t seconds
     – P [k attempts in t seconds] = Gt k
                                               e   (Gt )
                                                   k!
   Conditional successful probability for one attempt is :
     – P0 = P [0 other attempts in 2t seconds] =e-2Gt
   Set t as unit frame time
   Let S be the mean number of successful attempts
   S=GP0=Ge-2G
   S is optimum at G=1/2
   S=1/2e = 0.184
                        Slotted Aloha




   Transmission of frames are synchronized slot by slot.
   Channel feedback about whether packet is received or not
             Slotted Aloha (Continued)

   Slotted ALOHA cuts the vulnerable period for packets
    from 2t to t.
   Time is slotted. Packets must be transmitted within a
    slot.
   Procedure
    1. If a host has a packet to transmit, it waits until the
       beginning of the next slot before sending
    2. Listen to the broadcast and check if the packet was
       destroyed
    3. If there was a collision, wait a random number of slots
       and try to send again
               Analysis of Slotted ALOHA

   Packet Arrival is Poisson Process
   P [k arrivals in t seconds] =   e  t (  t ) k
                                           k!

   Let G be the total number of frames attempted in t seconds
     – P [k attempts in t seconds] =  e  Gt (Gt ) k
                                                  k!
   Successful probability for each slot is :
     P [1 attempts in a t seconds slot] =Ge-Gt
   Set Slot time t as unit time, then S=Ge-G
   S is optimum at G=1
   S=1/e = 0.368
                    Performance of ALOHA




   Throughput versus offered traffic for ALOHA systems
   The main reason for poor channel utilization of ALOHA (pure or slotted) is
    that all stations can transmit at will, without paying attention to what the
    other stations are doing.
                        CSMA

   Protocols in which stations listen for a carrier
    (i.e., a transmission) and act accordingly are
    called carrier sense protocols.
   There are several types of CSMA protocols:
    – Non-Persistent CSMA
    – 1-Persistent CSMA
    – P-Persistent CSMA
       Assumptions with CSMA Networks

   Constant length packets
   No errors, except those caused by collisions
   No capture effect
   Each host can sense the transmissions of all
    other hosts
   The propagation delay is small compared to the
    transmission time
                     Propagation Delay




          A          B         C           D


   D only sense A’s transmission after a propagation delay τ
   If τ is larger than packet transmission time, there are too much
    time wastage.
   CSMA in satellite communication? No.

     The size (length) of the network must be limited!
                   Non-persistent CSMA

   To send data, a station first listens to the channel to see if
    anyone else is transmitting.
   If so, the station waits a random period of time (instead of
    keeping sensing until the end of the transmission) and repeats
    the algorithm. Otherwise, it transmits a frame.
   If a collision occurs, the station waits a random amount of time
    and starts all over again.
Assumption:
   propagation delay is a constant common to all nodes:
    –   a is the ratio of propagation delay to packet transmission time
     Analysis of Non-persistent CSMA
           Unsuccessful                       Successful
           transmission                       transmission
           period                             period
                                                                 Normalized Time

                          a
                                                             a
    Y          1                              1
     a                         Idle period   Busy period
         Busy period

   S= U/(B+I)
   B = Y + 1 + a , I = 1/G
   U = e-aG                                           Ge  aG
                                             S 
   FY(y)=P{no packet occur in an                G (1  2a )  e  aG
    duration of a-y } = e-G(a-y)
                          1
     E (Y )  a            (1  e  aG )
                          G
                 Discussion of Collisions

   What's the effect of signal propagation delay a?
    –   The longer the delay, the more the collisions, and the worse the
        performance of the protocol.


   How about zero propagation delay ?
    –   There still exist chances of collisions. S = G/(1+G)


   Is this protocol any better than ALOHA (both pure and
    slotted) ?
    –   Yes, because both stations have the decency to desist from
        interfering with the third station's frame.
                      1-persistent CSMA
1-persistent CSMA (Carrier Sense Multiple Access):
    1. To send data, a station first listens to the channel to see if anyone else is
       transmitting.
    2. If so, the station waits (keeps sensing it) until the channel becomes idle.
       Otherwise, it transmits a frame.
    3. If a collision occurs, the station waits a random amount of time and
       starts all over again.

   It is called 1-persistent because the station transmits with a
    probability of 1 whenever it starts sensing the channel and finds
    the channel idle. (Greedy)
   This protocol has worse channel utilization than non-persistent
    CSMA.
    Tradeoff between Non-persistent and 1-persistent

   If B and C become ready in the middle of A’s
    transmission,
     – 1-Persistent: B and C collide
     – Non-Persistent: B and C probably do not collide


   If only B becomes ready in the middle of A’s
    transmission,
     – 1-Persistent: B succeeds as soon as A ends
     – Non-Persistent: B may have to wait
                          P-persistent CSMA

   Assume channels are slotted
   One slot = contention period (i.e., one round trip propagation delay)


Algorithm
   Sense the channel
     – If channel is idle, transmit a packet with probability p
            if a packet was transmitted, go to step 2
            if a packet was not transmitted, wait one slot and go to step 1
     – If channel is busy, wait one slot and go to step 1.
     – In other words, wait until idle and then transmit with probability p
   Detect collisions
     – If a collision occurs, wait a random amount of time and go to step 1
Persistent and Non-persistent CSMA

Comparison of the channel utilization versus load for
          various random access protocols.
        CSMA with Collision Detection

CSMA/CD (Carrier Sense Multiple Access with Collision
   Detection) protocol further improves ALOHA by aborting
   transmissions as soon as a collision is detected.

The conceptual model:
•   To send data, a station first listens to the channel to see if anyone else
    is transmitting.
•   If so, the station waits until the end of the transmission (1-persistent)
    or wait a random period of time and repeats the algorithm (non-
    persistent). Otherwise, it transmits a frame.
•   If a collision occurs, the station will detect the collision, abort its
    transmission, waits a random amount of time, and starts all over again.
                      How to Detect Collision




   Prerequisite: A node can listening while talking   Tx   Rx
   Ethernet cables
                      CSMA/CD Continued




•   CSMA/CD can be in one of three states: contention, transmission, or
    idle
•   The minimum time to detect the collision is the time it takes the
    signal to propagate from one station to the other.
•   How long could the transmitting station be sure it has seized the
    network ? ( or 2 ? where  is time equal to the full propagation)
•   Model the contention interval as slotted aloha with slot width 2
                             CSMA/CA

   Wireless LAN
   How can detect collision if you cannot listening while talking?
   Collision Avoidance
    – Random Backoff (instead of 1-persistent)
    – RTS/CTS
   CS no longer works well
    – Rules:
           carrier    ==> do not transmit
           no carrier ==> OK to transmit
    – But the above rules do not always apply to wireless.
         Problems with carrier sensing

Hidden terminal problem



                               Z
                          Y
                                       W
                                      W finds that medium is free
                                      and it transmits a packet to Z



                             /
                no carrier ===> OK to transmit
         Problems with carrier sensing


Exposed terminal problem
                                      W
                                                Z is transmitting
                                                to W
                                  Z

            X              Y

        Y will not transmit to X
        even though it cannot interfere

                               /
         Presence of carrier ===> hold off transmission
    Solving Hidden Node problem with RTS/CTS


- listen RTS             RTS                         CTS
- wait long enough
  for the requested
                         X              Z
  station to respond
  with CTS                                              - listen CTS
- if (timeout) then            Y
     ready to transmit
                                                W       - wait long enough
                                                          for the transmitter
                                                          to send its data


   listen RTS ==> transmitter is close to me
   listen CTS ==> receiver is close to me


 Note: RTS/CTS does not solve exposed terminal problem. In the example above,
 X can send RTS, but CTS from the responder will collide with Y’s data.
                  RTS/CTS exchange example
                     SIFS
           DIFS


                  RTS                Frame
    Src


                        CTS                          ACK
    Dest
                  352    304          8192 s         304
                  µs 10 µs                         10 µs
                               10
                      µs       µs                  µs
Dest                                NAV (RTS)

                                       NAV (CTS)




    RTS + CTS + Frame + ACK exchange invoked when frame size is large
    NAV (Network Allocation Vector)
      – NAV maintains prediction of future traffic on the medium based on duration
        information that is announced in RTS/CTS frames prior to actual exchange of data
          Pros & Cons of Random Access

   Advantages
    – Short delay for bursty traffic
    – Simple (due to distributed control)
    – Flexible to fluctuations in the number of hosts
    – Fairness
   Disadvantages
    – Low channel efficiency with a large number of hosts
    – Not good for continuous traffic (e.g., voice)
    – Cannot support priority traffic
    – High variance in transmission delays
                     Scheduled Access

   TDMA
   Dynamic TDMA
   Widely used
    – cellular,
    – Wi-Fi (HyperLAN),
    – IEEE 802.16
    – Wireless ATM
                                TDMA
   Time Division Multiple Access
                      TDMA Continued

   access to channel in "rounds"
   each station gets fixed length slot (length = packet transmission
    time) in each round
   unused slots go idle
   example: 6-station LAN, 1,3,4 have packets, slots 2,5,6 idle
                                            Dynamic TDMA

   In dynamic time division multiple access, a scheduling
    algorithm dynamically reserves a variable number of timeslots
    in each frame to variable user data streams, based on the traffic
    demand of each user data stream.
   Negotiations (beforehand) to determine how to allocate slots
    dynamically.
                                                     TDMA Frame
                      TDM Downlink                                               D-TDMA Uplink
     Modem
                                                                                                     S-ALOHA
     preamble
                                                                                                       control




                                                             Burst from User A     User B        User C
                Burst from Access Point -> Mobiles
                                                              To Access Point
    Summary of Scheduled Access Protocols

   Avoid of contention/collision; better channel efficiency with a
    large number of hosts


   predetermined channel allocation
   Need centralized control
   Require global synchronization
   Guard time period to protect slots
   Delay?
             Spread Spectrum and CDMA

   What if not divide up the channel by time (as in TDMA), or
    frequency (as in FDMA)? Is collision inevitable?
   Not if collision is no longer damaging!
    – Is there any way to decode bits garbled by other overlapping
      frames?


CDMA based on Spread Spectrum
 A new perspective to solve multiple access problems

 Spread Spectrum is a PHY innovation, not a MAC technique.

   CDMA encodes data with a special code associated with each
    user and uses the constructive interference properties of the
    special codes to perform the multiplexing.
                   Spread Spectrum

   Idea
     – spread signal over wider frequency band than required
    – originally deigned to thwart jamming
   Frequency Hopping
     – transmit over random sequence of frequencies
    – sender and receiver share…
           pseudorandom number generator
           seed
                Spread Spectrum (cont)

   Direct Sequence
     – for each bit, send XOR of that bit and n random bits
     – random sequence known to both sender and receiver
     – called n-bit chipping code
     – 802.11 defines an 11-bit chipping code



1
0                                             Data stream: 1010

1
0                                             Random sequence: 0100101101011001

1
0                                             XOR of the two: 1011101110101001
      Code Division Multiple Access (CDMA)

   Multiplexing Technique used with spread spectrum
   Start with data signal rate D
     – Called bit data rate
   Break each bit into k chips according to fixed pattern specific to each user
     – User’s code
   New channel has chip data rate kD chips per second
   E.g. k=6, three users (A,B,C) communicating with base receiver R
   Code for A = <1,-1,-1,1,-1,1>
   Code for B = <1,1,-1,-1,1,1>
   Code for C = <1,1,-1,1,1,-1>
CDMA Example
                        CDMA Explanation

   Consider A communicating with base
   Base knows A’s code
   Assume communication already synchronized
   A wants to send a 1
     – Send chip pattern <1,-1,-1,1,-1,1>
            A’s code
   A wants to send 0
     – Send chip[ pattern <-1,1,1,-1,1,-1>
            Complement of A’s code
   Decoder ignores other sources when using A’s code to decode
     – Orthogonal codes
        Topics Not Covered in This Lecture

   Dynamic behavior of Aloha
    – Strict mathematical analysis
    – Stabilize Aloha systems with channel feedback
   Taking Turns MAC protocols
    – Token Ring
   FDMA

				
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