Embed Size (px)
Citation preview
Link Layer:Partitioning Dimensions;
Media Access Control using Aloha
Y. Richard Yang
11/01/2012
2
Outline
Admin. and recap Intro to link layer Media access control
ALOHA protocol
Admin.
Assignment 3 Due date: coming Monday Check web site for office hours and FAQs
Project proposal (due Friday) Please send email to [email protected] by
end of day Subject: Project Proposal Content:
• Team members:• Topic: • One paragraph rough idea:
Exam: next Tuesday in class
3
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
App Layer: Logical Communications
5
E.g.: application
provide services to users
application protocol: send
messages to peer
6
Physical Communication
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
networklink
physical
data
data
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
o encapsulate datagram into frame, adding header, trailer and error detection/correction
Multiplexing/demultiplexingo use frame headers to identify src, hop dest
Media access control Reliable delivery between adjacent nodes
o seldom used on low bit error link (fiber, some twisted pair)
o common for wireless links: high error rates
10
Link Adaptors
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
sendingnode
frame
receivingnode
datagram
frame
adapter adapter
link layer protocol
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
frequ
ency
bands time
5
1
4
3
2
6
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
higher GSM frame structures
935-960 MHz124 channels (200 kHz)downlink
890-915 MHz124 channels (200 kHz)uplink
frequ
ency
time
1 2 3 4 5 6 7 8
GSM TDMA frame
4.615 ms
GSM - TDMA/FDMA
GSM time-slot (normal burst)
546.5 µs577 µs
tail user data TrainingSguardspace S user data tail
guardspace
3 bits 57 bits 26 bits 57 bits1 1 3
S: indicates data or control
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
#user supported: = 80030
24
KHz
MHz
1 2
3 4
1 2
3 4
1 2
3 4
1 2
3 4
Using cell #user supported: = 320016*2001630
6
KHz
MHz
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
user data d(t)
code c(t)
resultingsignal
1 -1
-1 1 1 -1 1 -1 1 -11 -1 -1 1 11
X
=
tb
tc
tb: bit periodtc: chip period
-1 1 1 -1 -1 1 -1 11 -1 1 -1 -11
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(2ft)
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 j’s 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
0 ij cc
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
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
1)1( 0
M
N
NPM
NPSNR For IS-95 CDMA,
N = 1.25M/4800 = 260
B
29
Generating Orthogonal Codes
The most commonly used orthogonal codes in current CDMA implementation are the Walsh Codes
nn
nnn WW
WWW
W
2
0 )1(
30
Walsh Codes
1
1,1
1,-1
1,1,1,1
1,1,-1,-1
X
X,X
X,-X 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,-1,1,-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,-1,-1,-1,-1
1 2 4 8
n 2n
...
...
...
...
31
Orthogonal Variable Spreading Factor (OSVF) Variable codes: Different users use different
lengths spreading codes Orthogonal: diff. users’ codes
are orthogonal
1
1,1
1,-1
1,1,1,1
1,1,-1,-1
X
X,X
X,-X 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,-1,1,-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,-1,-1,-1,-1
SF=1 SF=2 SF=4 SF=8
SF=n SF=2n
...
...
...
...
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
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)
call setup from an MSBTSMS
RACH (request signaling channel)
AGCH (assign signaling channel)
SDCCH (request call setup)
SDCCH (assign TCH)
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.
Success (S), Collision (C), Empty (E) slots
A
B
37
Slotted Aloha EfficiencyQ: What is the fraction of successful
slots?suppose n stations have packets to sendsuppose 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 CurveS =
thro
ughput
= “
goodput
” (
succ
ess
rate
)
Define G = offered load = np0.5 1.0 1.5 2.0
Slotted Aloha
when G (=p*n) < 1, as p (or n) increases probability of empty slots reduces probability of collision is still low, thus goodput increases
when G (=p*n) > 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 (=p*n) = 1
39
Maximum Efficiency vs. n
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
2 7 12 17 n
ma
xim
um
eff
icie
nc
y1/e = 0.37
At best: channeluse for useful transmissions 37%of time!
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
Modeln backlogged
each transmits with prob. p
m-n: unbacklogged
each transmits with prob. pa
42
Dynamics of Aloha: Effects of Fixed Probability
n: number of backlogged stations
0 m
successful transmission rate at
offered load np + (m-n)pa
new arrival rate:(m-n) pa
desirable stable point
undesirable stable point
Lesson: if we fix p, but n varies, we may have an undesirable stable point
offered load = 1
- assume a total ofm stations- pa << p- success rate is thedeparture rate, the rate the backlog is reducing
dep.andarrivalrateofbackloggedstations
Backup Slides: Error Corrections Codes
43
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:
)1)(1(1
2)1(2
110
1
)1(21
42
210
2
11
2210
0
...)(
...
...)(
...)(
)0(
knk
nnn
kk
kk
xxxxP
xxxxP
xxxxP
xP
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-1+s equations, and the # of votes it can get is at most:
k
sn
k
sk 1
48
Reed-Solomon Codes
If
or (n-s > k – 1 + s) or (n-k > 2s – 1) or (n-k s), it can correct any s errors
k
sk
k
sn 1
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 Euclid’s 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,
t0+1]
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, t0+1]
= 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 LoadS =
thro
ughput
= “
goodput
” (
succ
ess
rate
)
G = offered load = Np0.5 1.0 1.5 2.0
0.1
0.2
0.3
0.4
Pure Aloha
protocol constrainseffective channelthroughput!
Slotted Aloha
