1. Link Layer; Media Access Control
Y. Richard Yang
2/2/2011

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

3. Admin.
Hw2 to be posted later today

4. Summary: Challenges and Techniques of Wireless Design
Performance affected
Mitigation techniques
Shadow fading (large-scale fading)
Fast fading (small-scale, flat fading)
Delay spread (small-scale fading)
received signal strength
use fade margin—increase power or reduce distance
bit/packet error rate at deep fade
diversity
equalization; OFDM; directional antenna
ISI

5. Delay Spread as Transmit Diversity
Delay spread is really a type of transmit diversity

6. Multipath Diversity: Rake Receiver
Instead of considering delay spread as an issue, use multipath signals to recover the original signal
Used in IS-95 CDMA, 3G CDMA, and
Invented by Price and Green in 1958
R. Price and P. E. Green, "A communication technique for multipath channels," Proc. of the IRE, pp , 1958

7. DSSS Multipath
Choose chipping sequence with good autocorrelation, e.g., Barker code () used in

8. Multipath Diversity: Rake Receiver
Use several "sub-receivers" each delayed slightly to tune in to the individual multipath components

9. Rake Receiver Blocks
Correlator
Combiner
Finger 1
Finger 2
Finger 3

10. Outline
Recap
Introduction to link layer

11. Link Layer Services Framing: Flow control:
encapsulate bits into frame, adding header, trailer
multiplexing: frame header to identify source, dest
example: MAC address
error detection and correction
Flow control:
pacing between adjacent sending and receiving nodes
Link access (interference and quality of service control)
media access control (also called multiplexing)

12. Outline Recap Introduction to link layer
Error detection and correction

13. Example: Reed-Solomon Codes
Very commonly used, e.g.,
CD/DVD/BlueRay/DSL/WiMAX
Send n symbols 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
Why Error Detection and Correction?

14. Reed-Solomon Codes
If the data we want to send is (x0, x1,…, xk-1), where xi are data symbols, define polynomial P(t) = x0 + x1t + x2t2 + …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

15. since any k equations are independent, they have a unique solution
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

16. 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”

17. 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:

18. 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

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

20. Link Layer Services Framing: Flow control:
encapsulate bits into frame, adding header, trailer
multiplexing: frame header to identify source, dest
example: MAC address
error detection and correction
Flow control:
pacing between adjacent sending and receiving nodes
Link access (interference and quality of service control)
media access control (also called multiplexing)

21. Outline Recap Introduction to link layer
Error detection and correction
Media access control

22. Dimension for multiplexing
Multiple Access Control
Dimension for multiplexing
Frequency
Time
Space
Code
Two transmissions cannot conflict in all dimensions !

23. FDMA FDMA: frequency division multiple access
Channel divided into frequency bands
A transmission uses a frequency band
frequency bands
time 5 1 4 3 2 6

24. TDMA TDMA: time division multiple access
Time divides into frames; frame divides into slots
A transmission uses a slot in a frame

25. Q: why not divide into infinite small cells?
SDMA
SDMA: space division multiple access
Transmissions at different locations, if far enough, can transmit simultaneously (same freq.)
Example: the cellular technique 1 2 3 4
Suppose 24 MHz spectrum, 30 K per user
#user supported: =
Using cell #user supported: =
Q: why not divide into infinite small cells?

26. 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 =
Examples: Sprint and Verizon, 3G WCDMA and CDMA2000

27. DSSS Revisited tb: bit period tc: chip period tb user data d(t) 1 -1 X
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

28. 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)

29. 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)

30. 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:
C2:
C1 . C2 = (-1) (-1) (-1)+(-1)=0
Analogy: Speak in different languages!

31. 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

32. Generating Orthogonal Codes
The most commonly used orthogonal codes in current CDMA implementation are the Walsh Codes
Property of Walsh : every row is orthogonal to every other row and the log NOT of every other row

33. Walsh CDMA Code Assignment
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
If user 1 is given code [1,1,-1,-1], what codes can we give to other users?

34. WCDMA Orthognal Variable Spreading Factor (OSVF)
Flexible code (spreading factor) allocation
up link SF: 4 – 256
down link SF:
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
...
No parent-child on the code tree
1,-1,1,-1
1,-1,1,-1,-1,1,-1,1
1,-1
SF=n
SF=2n
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

35. W-CDMA Down Link Capacity

36. 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?

37. General Allocation Problem
Each transmission i (si->di) is specified by
transmission power pi
time ti
frequency band fi
code ci
Given any two transmissions (si->di; pi, ti, fi, ci) and (sj->dj; pj, tj, fj, cj), we can determine if they conflict
MAC scheduling objectives:
to maximize resource efficiency
to allocate resource fairly
to satisfy app requirements
e.g. real time app (such as VoIP) need access with a bounded delay
e.g. s wants to talk to d, but cannot reach d directly

38. MAC The general case is challenging and largely still open
We start with the simplest scenario (e.g., or cellular up links) in this class:
time sharing the same frequency and code
a single receiver (the Access Point)

39. How to Do Time Sharing? Fixed allocation
Centralized authority, e.g., polling
Taking turns, e.g., token passing
Distributed random access
discussion: compare the schemes in terms of
efficient utilization of resources
delay to access channel
quality of service (e.g., a guaranteed rate)

40. Slotted Aloha [Norm Abramson]
Used in GSM initial access channel: when you dial a number.
Time is divided into equal size slots (= pkt trans. time)
Node with new arriving request: transmit at beginning of next slot
If collision: retransmit request in future slots with probability p, until successful. A B

41. Success (S), Collision (C), Empty (E) slots
Slotted Aloha
Success (S), Collision (C), Empty (E) slots

42. Slotted Aloha Efficiency
S = throughput = “goodput”
(success rate)
G = offered load = np
0.5
1.0
1.5
2.0
Slotted Aloha
when p n < 1, as p (or n) increases
probability of empty slots reduces
probability of collision is still low, thus goodput increases
when 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 p n = 1

43. Pros and Cons of Slotted Aloha
Pros: simple
Cons:
need clock sync
low efficiency: utilization at ~1/e

44. Dynamics of Aloha: Effects of Fixed Probability
- assume a total of
m stations;
- An idle station starts a frame with prob. pa
pa << p
success rate is the departure rate, the rate the backlog is reducing
desirable stable point
successful transmission rate at offered load np + (m-n)pa
dep.
and
arrival
rate of
backlogged
stations
new arrival rate: (m-n) pa
undesirable stable point m
n: number of backlogged
stations
offered load = 1
Lesson: if we fix p, but n varies, we may have an undesirable stable point

45. Ethernet Fix: Carrier-Sense Multiple Access /Collision Detection/Exponential Backoff
The Ethernet algorithm
get a frame from upper layer;
K := 0; n := 0; // K: control wait time; n: no. of collisions
repeat:
wait for K * 512 bit-time;
while (network busy) wait;
wait for 96 bit-time after detecting no signal;
transmit and detect collision;
if detect collision
stop and transmit a 48-bit jam signal;
n ++;
m:= min(n, 10), where n is the number of collisions
choose K randomly from {0, 1, 2, …, 2m-1}.
if n < 16 goto repeat
else give up
else
declare success

46. Does collision detection work well in wireless?
Efficiency of CSMA/CD
Given collision detection, instead of wasting the whole packet transmission time (a slot), we waste only the time needed to detect collision.
P: packet size, C: contention window C C C P
Does collision detection work well in wireless?

47. The Hidden Terminal ProblemC
A is sending to B, but C cannot detect the transmission
Therefore C sends to B
In summary, A is “hidden” from C

48. CSMA/CD + Hidden Terminals
get a frame from upper layer;
K := 0; n := 0; // K: control wait time; n: no. of collisions
repeat:
wait for K * 512 bit-time;
while (network busy) wait;
wait for 96 bit-time after detecting no signal;
transmit and detect collision;
if detect collision
stop and transmit a 48-bit jam signal;
n ++;
m:= min(n, 10), where n is the number of collisions
choose K randomly from {0, 1, 2, …, 2m-1}.
if n < 16 goto repeat
else give up
else
declare success
Q: what is the outcome of CSMA/CD + hidden terminals?

49. Handling Hidden Terminals: Avoid
Hidden terminals -> collision detection is not reliable -> if do not detect collision, there is still a chance of collision -> transmit with only prob.
CSMA/CD -> CSMA/CA (congestion avoidance)
default in
even if media is not sensed busy, transmits with a probability
in real implementation, with a random delay

50. Handling Hidden Terminals: Detect
Develop techniques to detect hidden terminals
Why cannot C detect potential collision?
Collision is spatially dependent
C is at a different location than B
B should tell C that it is receiving A B C

51. Hidden-Terminal Detection
Busy-tone multiple access
used in CDPD (cellular digital packet data)
the base station sends a busy tone on the down link when receiving data
Virtual carrier sense

52. Virtual Carrier Sense: RTS/CTS
Short signaling packets (virtual carrier sense)
RTS (request to send) and CTS (clear to send)
contain sender address, receiver address, transmission duration, called network allocation vector (NAV)
A node keeps quiet for NAV in CTS
DATA
RTS A B C D
CTS

53. CSMA+RTS/CTS/DATA/ACK
Summary
Use RTS/CTS for virtual carrier sense
Use ACK to improve reliability
Missing CTS/ACK considered as collision
Protocol
begin:
while (channel busy) wait
if channel idle
send RTS
if receive CTS
send DATA
wait for ACK
if ACK
return
exponential backoff
wait for the backoff period
goto begin;

54. Comparisons: Media Access Techniques Handling Hidden Terminals
Slotted Aloha
very simple to implement but need clock sync; low efficiency
CSMA/CD (Ethernet alg.)
hidden terminal causes collapse
CSMA/CA
simple to implement
low efficiency

55. Comparisons: Media Access Techniques Handling Hidden Terminals
Busy tone
simple to implement but need a channel for busy signal
Virtual carrier sensing (RTS/CTS)
energy consumption can be high because a node needs to monitor the environment all the time
Idle:receive:send: 1:1.05:1.4 [Stemm and Katz 1997]; Digitan 2 Mbps WLAN 1:2:2.5
many measurements show that overhead hurts performance

56. Question for Thought: Is the preceding the best we can do?

57. CSMA/CD/ACK + Hidden Terminals Revisited
No ACK
Collision!
Alice
Bob
Assume CSMA/CD but add ACK; Missing ACK considers as collision

58. CSMA/CD/ACK + Hidden Terminals Revisited
Retransmission
One more Collision
Alice
Bob
The problem does not stop here. Alice and Bob retransmit their packets after increasing their contention window. But this does not help. Their transmissions still overlap causing more collisions.
Alice and Bob will continue increasing their contention window and retransmitting, until after many trials they get a packet through or they time out.
Question to think about during weekend: Anything we can do?

59. Backup Slides

60. Rake Receiver: Combiner
The weighting coefficients are based on the power or the SNR from each correlator output
If the power or SNR is small out of a particular finger, it will be assigned a smaller weight:

61. Forward Error Correction Code: Block Erasure Code
Basic idea: encode k symbols to n symbols so that if the receiver receives any k symbols, it can recover the original x y

62. FEC/Block: Example Implementation
Suppose data signal is x, and the encoded signal y = Gx, where G is the generator matrix; assume receives only a subset of y
The y-symbols in yellow are received;
other y-symbols are dropped by demodulator

63. FEC: An Example using Vandermonde Matrix
Suppose gij are the entries after the identify matrix
gij= aij-1, where ai are distinct numbers
Suppose k=3, and n=5

64. An Example using Vandermonde Matrix
Suppose y2 and y3 are dropped, then we have y1, y4, and y5. Given the relationship (we know they are y1, y4, y5)
Since the matrix is not singular, we can recover x1, x2, and x3 from y1, y4, y5

65. Other Codes
Many other types of codes are commonly used in networks, e.g., Reed-Solomon codes
where we consider each symbol (e.g., 8 bits) as the coefficient of a polynomial, c(x) is the parity check polynomial representing the parity correction symbols, d(x) the data polynomial, and g(x) the generator polynomial in the format: