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Link Layer: Partitioning Dimensions; Media Access Control using Aloha

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Partitioning Dimensions; Media Access Control using Aloha Y. Richard Yang 11/01/2012 – PowerPoint PPT presentation

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Title: Link Layer: Partitioning Dimensions; Media Access Control using Aloha


1
Link LayerPartitioning DimensionsMedia Access
Control using Aloha
  • Y. Richard Yang
  • 11/01/2012

2
Outline
  • Admin. and recap
  • Intro to link layer
  • Media access control
  • ALOHA protocol

3
Admin.
  • Assignment 3
  • Due date coming Monday
  • Check web site for office hours and FAQs
  • Project proposal (due Friday)
  • Please send email to cs434ta_at_cs.yale.edu by end
    of day
  • Subject Project Proposal
  • Content
  • Team members
  • Topic
  • One paragraph rough idea
  • Exam next Tuesday in class

4
Recap
  • App layer
  • Handles issues such as UI
  • Uses lower-layer provided abstractions, e.g.,
  • network abstraction
  • e.g., HttpURLConnection in GoogleSearch Example
  • location service abstraction
  • e.g., Location Manager, and Location Listener in
    Assignment 3
  • Physical layer capability
  • sender sends a seq. of bits to a receiver

5
App Layer Logical Communications
  • E.g. application
  • provide services to users
  • application protocol
  • send messages to peer

6
Physical Communication
7
Protocol Layering and Meta Data
  • Each layer takes data from above
  • adds header (meta) information to create new data
    unit
  • passes new data unit to layer below

8
Link layer Context
  • Data-link layer has responsibility of
    transferring datagram from one node to another
    node
  • Datagram may be transferred by different link
    protocols over different links, e.g.,
  • Ethernet on first link,
  • frame relay on intermediate links
  • 802.11 on last link
  • transportation analogy
  • trip from New Haven to San Francisco
  • taxi home to union station
  • train union station to JFK
  • plane JFK to San Francisco airport
  • shuttle airport to hotel

9
Main Link Layer Services
  • Framing
  • encapsulate datagram into frame, adding header,
    trailer and error detection/correction
  • Multiplexing/demultiplexing
  • use frame headers to identify src, hop dest
  • Media access control
  • Reliable delivery between adjacent nodes
  • seldom used on low bit error link (fiber, some
    twisted pair)
  • common for wireless links high error rates

10
Link Adaptors
datagram
receiving node
link layer protocol
sending node
adapter
adapter
  • link layer typically implemented in adaptor
    (aka NIC)
  • Ethernet card, modem, 802.11 card
  • adapter is semi-autonomous, implementing link
    physical layers
  • sending side
  • encapsulates datagram in a frame
  • adds error checking bits, rdt, flow control, etc.
  • receiving side
  • looks for errors, rdt, flow control, etc
  • extracts datagram, passes to receiving node

11
Outline
  • Recap
  • Introduction to link layer
  • Wireless link access control

12
Wireless Access Problem
  • Single shared broadcast channel
  • thus, if two or more simultaneous transmissions
    by nodes, due to interference, only one node can
    send successfully at a time

13
Wireless Access Problem
  • The general case is challenging and largely still
    open
  • We start with the simplest scenario (e.g., 802.11
    or cellular up links) in this class
  • a single receiver (the Access Point)

14
Multiple Access Control Protocol
  • Protocol that determines how nodes share channel,
    i.e., determines when nodes can transmit
  • Communication about channel sharing must use
    channel itself !
  • Discussion properties of an ideal multiple
    access protocol.

15
Ideal Mulitple Access Protocol
  • Efficient
  • Fair/rate allocation
  • Simple

16
MAC Protocols a Taxonomy
  • Goals
  • efficient, fair, simple
  • Three broad classes
  • channel partitioning
  • divide channel into smaller pieces (time slot,
    frequency, code)
  • non-partitioning
  • random access
  • allow collisions
  • taking-turns
  • a token coordinates shared access to avoid
    collisions

17
Outline
  • Recap
  • Introduction to link layer
  • Wireless link access control
  • partitioning dimensions

18
FDMA
  • FDMA frequency division multiple access
  • Channel divided into frequency bands
  • A transmission uses a frequency band

19
TDMA
  • TDMA time division multiple access
  • Time divides into frames frame divides into
    slots
  • A transmission uses a slot in a frame

20
Example GSM
  • A GSM operator uses TDMA and FDMA to divide its
    allocated frequency
  • divide allocated spectrum into different physical
    channels each physical channel has a frequency
    band of 200 kHz
  • partition the time of each physical channel into
    frames each frame has a duration of 4.615 ms
  • divides each frame into 8 time slots (also called
    a burst)
  • each slot is a logical channel
  • user data is transmitted through a logical channel

21
GSM - TDMA/FDMA
22
SDMA
  • SDMA space division multiple access
  • Transmissions at different locations, if far
    enough, can transmit simultaneously (same freq.)
  • Example the cellular technique

Suppose 24 MHz spectrum, 30 K per user
Q why not divide into infinite small cells?
23
CDMA
  • CDMA (Code Division Multiple Access)
  • Unique code assigned to each user i.e., code
    set partitioning
  • All transmissions share the same frequency and
    time each transmission uses DSSS, and has its
    own chipping sequence (i.e., code) to encode
    data
  • e.g. code -1 1 1 -1 1 -1 1

Examples Sprint and Verizon, WCDMA
24
DSSS Revisited
tb
user data d(t)
1
-1
X
tc
code c(t)
-1
1
1
-1
1
-1
1
-1
1
-1
-1
1
1
1

resulting signal
-1
1
1
-1
-1
1
-1
1
1
-1
1
-1
-1
1
tb bit period tc chip period
25
Illustration CDMA/DSSS Using BPSK
  • Assume BPSK modulation using carrier frequency f
  • yi(t) A xi(t)ci(t) sin(2? ft)
  • A amplitude of signal
  • f carrier frequency
  • xi(t) data of user i in 1, -1
  • ci(t) code of i (a chipping sequence in 1,
    -1)
  • Suppose only i transmits
  • y(t) yi(t)
  • Decode at receiver i
  • incoming signal multiplied by ci(t) sin(2? ft)
  • since, ci(t) ci(t) 1, yi(t)ci(t) sin(2?
    ft) A xi(t) sin2(2?ft)

26
Illustration Multiple User CDMA
  • Assume M users simultaneously transmit y(t)
    y1(t) y2(t) yM(t)
  • At receiver i, incoming signal y(t) multiplied by
    ci(t) sin(2? ft)
  • consider the effect of js transmission
  • yj(t)ci(t) sin(2? ft) A cj(t)ci(t)xj(t)
    sin2(2? fct)

27
CDMA Deal with Multiple-User Interference
  • Two codes Ci and Cj are orthogonal, if
  • , where we use . to denote inner
    product, e.g.
  • If codes are orthogonal, multiple users can
    coexist and transmit simultaneously with
    minimal interference

C1 1 1 1 -1 1 -1
-1 -1 C2 1 -1 1 1
1 -1 1 1 ------------------------
-------------------------- C1 . C2 1 (-1)
1 (-1) 1 1 (-1)(-1)0
Analogy Speak in different languages!
28
Capacity of CDMA
B
  • In realistic setup, cancellation of others
    transmission is incomplete
  • Assume the received power at base station from
    all nodes is the same P (how?)
  • The power of the transmission with known code is
    increased to N P, where N is chipping expansion
    factor
  • The others remain on the order of P
  • Assume a total of M users
  • Then

For IS-95 CDMA, N 1.25M/4800 260
29
Generating Orthogonal Codes
  • The most commonly used orthogonal codes in
    current CDMA implementation are the Walsh Codes

30
Walsh Codes
1,1,1,1,1,1,1,1
...
1,1,1,1
1,1,1,1,-1,-1,-1,-1
1,1
1,1,-1,-1,1,1,-1,-1
...
1,1,-1,-1
X,X
1,1,-1,-1,-1,-1,1,1
1
X
1,-1,1,-1,1,-1,1,-1
X,-X
...
1,-1,1,-1
1,-1,1,-1,-1,1,-1,1
1,-1
n
2n
1,-1,-1,1,1,-1,-1,1
...
1,-1,-1,1
1,-1,-1,1,-1,1,1,-1
1
2
4
8
31
Orthogonal Variable Spreading Factor (OSVF)
  • Variable codes Different users use different
    lengths spreading codes
  • Orthogonal diff. users codesare orthogonal

1,1,1,1,1,1,1,1
...
1,1,1,1
1,1,1,1,-1,-1,-1,-1
1,1
1,1,-1,-1,1,1,-1,-1
...
1,1,-1,-1
X,X
1,1,-1,-1,-1,-1,1,1
1
X
1,-1,1,-1,1,-1,1,-1
X,-X
...
1,-1,1,-1
1,-1,1,-1,-1,1,-1,1
1,-1
SFn
SF2n
1,-1,-1,1,1,-1,-1,1
...
1,-1,-1,1
1,-1,-1,1,-1,1,1,-1
SF1
SF2
SF4
SF8
If user 1 is given code 1,1, what orthogonal
codes can we give to other users?
32
WCDMA Orthognal Variable Spreading Factor (OSVF)
  • Flexible code (spreading factor) allocation
  • up link SF 4 256
  • down link SF 4 - 512

WCDMA downlink
33
Summary
  • SDMA, TDMA, FDMA and CDMA are basic media
    partitioning techniques
  • divide media into smaller pieces (space, time
    slots, frequencies, codes) for multiple
    transmissions to share
  • A remaining question is how does a network
    allocate space/time/freq/code?

34
Outline
  • Recap
  • Introduction to link layer
  • Wireless link access control
  • partitioning dimensions
  • media access protocols

35
GSM Logical Channels and Request
  • call setup from an MS
  • Control channels
  • Broadcast control channel (BCCH)
  • from base station, announces cell identifier,
    synchronization
  • Common control channels (CCCH)
  • paging channel (PCH) base transceiver station
    (BTS) pages a mobile host (MS)
  • random access channel (RACH) MSs for initial
    access, slotted Aloha
  • access grant channel (AGCH) BTS informs an MS
    its allocation
  • Dedicated control channels
  • standalone dedicated control channel (SDCCH)
    signaling and short message between MS and an MS
  • Traffic channels (TCH)

BTS
MS
SDCCH message exchange
Communication
36
Slotted Aloha Norm Abramson
  • Time is divided into equal size slots ( pkt
    trans. time)
  • Node with new arriving pkt transmit at beginning
    of next slot
  • If collision retransmit pkt in future slots with
    probability p, until successful.

A
B
Success (S), Collision (C), Empty (E) slots
37
Slotted Aloha Efficiency
  • Q What is the fraction of successful slots?
  • suppose n stations have packets to send
  • suppose each transmits in a slot with probability
    p
  • - prob. of succ. by a specific node p
    (1-p)(n-1)
  • - prob. of succ. by any one of the N nodes
  • S(p) n Prob (only one transmits)
  • n p (1-p)(n-1)

38
Goodput vs. Offered Load Curve

S throughput goodput (success rate)
1.5
2.0
0.5
1.0
Define G offered load np
  • when G (pn) lt 1, as p (or n) increases
  • probability of empty slots reduces
  • probability of collision is still low, thus
    goodput increases
  • when G (pn) gt 1, as p (or n) increases,
  • probability of empty slots does not reduce much,
    but
  • probability of collision increases, thus goodput
    decreases
  • goodput is optimal when G (pn) 1

39
Maximum Efficiency vs. n
1/e 0.37
40
Dynamics of (Slotted) Aloha
  • In reality, the number of stations backlogged is
    changing
  • we need to study the dynamics when using a fixed
    transmission probability p
  • Assume we have a total of m stations (the
    machines on a LAN)
  • n of them are currently backlogged, each tries
    with a (fixed) probability p
  • the remaining m-n stations are not backlogged.
    They may start to generate packets with a
    probability pa, where pa is much smaller than p

41
Model
n backlogged each transmits with prob. p
m-n unbacklogged
each transmits with prob. pa
42
Dynamics of Aloha Effects of Fixed Probability
  • - assume a total of
  • m stations
  • pa ltlt p
  • success rate is thedeparture rate, the rate
    the backlog is reducing

dep. and arrival rate of backlogged stations
n number of backlogged stations
m
0
offered load 1
Lesson if we fix p, but n varies, we may have an
undesirable stable point
43
Backup Slides Error Corrections Codes
44
Reed-Solomon Codes
  • Very commonly used, send nsymbols for k data
    symbols
  • e.g., n 255, k 223
  • We will discuss the original version (1960)
  • modern versions are slightly different they use
    generator polynomial, but the idea is essentially
    the same
  • If the data we want to send is (x0, x1,, xk-1),
    where xi are data symbols, define polynomial
    P(t) x0 x1t x2t xk-1tk-1
  • Assume ? is a generator of the symbol field
    (i.e., ?i not equal to ?j if i not equal to j)
  • Then for the data sequence, send
  • P(0), P(?), P(?2), P(?n-1) to
    receiver

45
Reed-Solomon Codes
  • Receive the message P(0), P(?), P(?2), P(?n-1)
  • If no error, can recover data from any k
    equations

since any k equations are independent, they have
a unique solution
46
Reed-Solomon Codes Handling Errors
  • But, what if s errors occur during transmission?
  • Keep a counter (vote) for each solution
  • Enumerate all combinations of k equations, for
    each combination, solve it, and increase the
    counter of the solution
  • Identify the solution which gets the largest of
    votes

47
Reed-Solomon Codes
  • The transmitted data is the correct solution for
    n-s equations, and thus gets
  • votes (i.e., combinations of k equations)
  • An incorrect solution can satisfy at most k-1s
    equations, and the of votes it can get is at
    most

48
Reed-Solomon Codes
  • If
  • or (n-s gt k 1 s) or (n-k gt 2s 1) or (n-k ?
    s), it can correct any s errors

49
Reed-Solomon Codes
  • The voting-based decoding algorithm proposed in
    1960 is inefficient
  • 1967 - Berlekamp introduced first truly efficient
    algorithm for both binary and nonbinary codes.
    Complexity increases linearly with number of
    errors
  • 1975 - Sugiyama, et al. Showed that Euclids
    algorithm can be used to decode R-S codes
  • Below is a typical current decoder

50
Pure (unslotted) Aloha
  • Unslotted Aloha simpler, no clock
    synchronization
  • Whenever pkt needs transmission
  • send without awaiting for the beginning of slot
  • Collision probability increases
  • pkt sent at t0 collide with other pkts sent in
    t0-1, t01

51
Pure Aloha (cont.)
  • Assume a node transmit with probability p in one
    unit of time
  • P(success by a given node) P(node transmits)

  • P(no other node transmits in t0-1,t0

  • P(no other node transmits in t0, t01
  • p .
    (1-p)n-1 . (1-p)n-1
  • p .
    (1-p)2(n-1)
  • P(success by any of N nodes) n p . (1-p)2(n-1)

  • - Bound 1/(2e) .18

52
Goodput vs. Offered Load

0.4
0.3
S throughput goodput (success rate)
0.2
0.1
1.5
0.5
1.0
2.0
G offered load Np
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