1. CS434/534: Topics in Networked (Networking) Systems Wireless Foundation: Wireless MAC; MAC; New Designs Yang (Richard) Yang Computer Science Department Yale University 208A Watson

2. Admin.
PS2 questions
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3. Recap: Wireless PHY

4. Recap: Link Layer Services
Framing
separate bits into frames; each frame has header, trailer and error detection/correction
Multiplexing/demultiplexing
use frame headers to identify src, dest
Media access control
implement sharing of wireless resources
Reliable delivery between adjacent nodes
seldom used on low bit error link (fiber, some twisted pair)
common for wireless links: high error rates

5. Recap: MAC Protocols Taxonomy
Channel partitioning
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
Non-channel partitioning
random access
“taking-turns”

6. Recap: Aloha Protocol as a Random Access Protocol
call setup from an MS MS
BTS
RACH (request signaling channel) Slotted Aloha
AGCH (assign signaling channel)
SDCCH (request call setup)
SDCCH message exchange A
SDCCH (assign TCH)
Communication B

7. Recap: Slotted Aloha Advantages
Simple, decentralized random access protocol
Issues
Low efficiency
Only ~37% at optimal transmission rate
Even lower efficiency at non-optimal (fixed p)
No rate allocation/fairness

8. Recap: Ethernet Fix for Efficiency
Introduce carrier sense (CS): do not interrupt others
Introduce collision detection (CD): instead of wasting the whole frame transmission time (a slot), we waste only the time needed to detect collision.
Introduce adaptive probability using exponential backoff (EB): reduce probability #collisions increases
If more collisions => p is high => should reduce p P
P: packet size, C: contention window C

9. Q: Does Ethernet alg work well in wireless?
Ethernet Fix: Carrier-Sense Multiple Access /Collision Detection/Exponential Backoff
Carrier sense
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
Detect
Collision
Exponential backup
Q: Does Ethernet alg work well in wireless?

10. Outline Recap Wireless background Wireless MAC design Frequency domain
Modulation and demodulation
Wireless channels
Wireless PHY design
Wireless MAC design
wireless access problem and taxonomy
wireless resource partitioning dimensions
media access protocols
ALOHA protocol
The Ethernet protocol
Hidden terminals in wireless

11. 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

12. 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
Hidden terminals => 0 goodput!
Q: what is the outcome of CSMA/CD + hidden terminals, assume two senders with infinite backlog?

13. Hidden Terminals
Why cannot senders C and A detect collisions or potential collisions?
Collision is spatially dependent
C/A is at a different location than B
Only receiver can detect a collision happened or potential collisions A B C

14. Roadmap: Wireless MAC
Problem: single shared medium, hence if two transmissions overlap on all dimensions [time, space, frequency, and code], then it is a collision
+CS+CD+EB
Slotted ALOHA
Ethernet
Hidden-terminal Collision detection/ prevention ?

15. Solution I: Receiver Notifies Collision Happened
Solution: receiver sends ACK to sender to indicate a collision happened or not
If no ACK from receiver, sender assumes a collision

16. Solution II: Receiver Signals Potential Collision
Receiver sends busy-tone
Used in CDPD (cellular digital packet data)
The base station sends a busy tone on the down link when receiving data

17. Short signaling packets (virtual carrier sense)
Solution III: Receiver Signals Potential Collision Using Virtual Carrier Sense/ACK
Short signaling packets (virtual carrier sense)
Sender: RTS (request to send)
Receiver: 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

18. Comparisons: Media Access Techniques Handling Hidden Terminals
Slotted Aloha
very simple to implement but low efficiency
CSMA/CD (Ethernet alg.)
hidden terminals can cause 0 goodput
CSMA/CD + ACK
simple to implement
low efficiency (CD is not effective)

19. Comparisons: Media Access Techniques Handling Hidden Terminals
Busy tone
simple to implement but need a channel for busy signal
Virtual carrier sensing (RTS/CTS)
higher efficiency when a collision occurs (not waste the whole duration)
But 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]; Digitan 2 Mbps WLAN 1:2:2.5
many measurements show that overhead hurts performance

20. Outline Recap Wireless background Wireless MAC design Frequency domain
Modulation and demodulation
Wireless channels
Wireless PHY design
Wireless MAC design
wireless access problem and taxonomy
wireless resource partitioning dimensions
media access protocols
ALOHA protocol
The Ethernet protocol
Hidden terminals in wireless
IEEE

21. IEEE (Requirements)
Design for small coverage (e.g. office, home)
Use un-licensed spectrum
High data-rate applications
Ability to integrate real time applications and non-real-time applications (implications?)

22. 802.11: Infrastructure Mode Components 802.11 LAN
networks station (STA)
terminal with access mechanisms to the wireless medium and radio contact to the access point
access point (AP)
station integrated into the wireless LAN and the distribution system
basic service set (BSS)
group of stations using the same AP
portal
bridge to other (wired) networks
distribution system
interconnection network to form one logical network (EES: Extended Service Set) based on several BSS
802.x LAN
STA1
BSS1
Portal
Access
Point
Distribution System
Access
Point
ESS
BSS2
STA2
STA3
LAN 9

23. The IEEE 802.11 Family Protocol Release Data Freq. Rate (max)
Modulation
Range (indoor)
Legacy
1997
2.4 GHz
2Mbps
DSSS/FHSS
~20 m
802.11a
1999
5 GHz
54 Mbps
OFDM
~35 m
802.11b
11 Mbps
DSSS
~38 m
802.11g
2003
OFDM/DSSS
802.11n
2009
2.4/5 GHz
540 Mbps
~70 m

24. Long preamble & header (192 usec; or optional 96 short version)
802.11b PHY Format
Long preamble & header (192 usec; or optional 96 short version)
Preamble & header always transmitted at 1Mbps DBPSK
Preamble
- Sync: alternating 0s and 1s (DSSS 128 bits)
- SFD: Start Frame delimiter:
PLCH (Phsical Layer Convergence Procedure) Header
- SIGNAL: the rate info
- SERVICE: mostly future use
- payload length
- CRC: 16 bit protection of header

25. 802.11 – MAC Format Types Sequence numbers Addresses Miscellaneous
bytes 2 2 6 6 6 2 6
0-2312 4
Frame
Control
Duration/ ID
Address 1
Address 2
Address 3
Sequence
number
Address 4
Data
CRC
bits 2 2 4 1 1 1 1 1 1 1 1
Protocol
version
Type
Subtype To DS
From DS
More
Frag
Retry
Power
Mgmt
More
Data
WEP
Order
Types
control frames, management frames, data frames
Sequence numbers
important against duplicated frames due to lost ACKs
Addresses
receiver, transmitter (physical), BSS identifier, sender (logical)
Miscellaneous
sending time, checksum, frame control, data

27. 802.11 – MAC Asynchronous Data Service (ADS)
Objective: exchange data based on distributed random access
Approach: Implement ADS using distributed coordinate function (DCF):
DCF CSMA/CA (mandatory)
- collision avoidance via randomized “back-off“
ACK packet for acknowledgements/detection
DCF w/ RTS/CTS (optional)
additional virtual “carrier sensing
Time-Bounded Service (TBS)
Objective: Exchange data with bounded delay service
Approach: implemented TBS using point (access point) coordinated function (PCF)

28. 802.11 ADS/DCF: CSMA/CA CSMA: Listen before transmit
Collision avoidance
when transmitting a packet, choose a backoff interval in the range [0, CW]
CW is contention window
Count down the backoff interval when medium is idle
count-down is suspended if medium becomes busy
Transmit when backoff interval reaches 0

29. 802.11 ADS/DCF: CSMA/CA Example
busy
B1 = 25
B2 = 20
B1 = 5
data
wait
data
wait
B2 = 10
B2 = 15
busy
B1 and B2 are backoff intervals
at nodes 1 and 2
cw = 31

30. 802.11 ADS/DCF: CSMA/CA Backoff
IEEE contention window CW is adapted dynamically depending on collision occurrence
after each collision, CW is doubled
thus CW varies from CWmin to CWmax
802.11b
802.11a
802.11g
aSlotTime
20 usec
9 usec
20 usec (mixed); 9 usec (g-only)
aCWmin
31 slots
15 slots

31. ADS/DCF: RTS/CTS + ACK
Sender sends RTS with NAV (Network allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium)
Receiver acknowledges via CTS (if ready to receive)
CTS reserves channel for sender, notifying possibly hidden stations
Sender can now send data at once, acknowledgement via ACK
Other stations store NAV distributed via RTS and CTS
RTS
data
sender
CTS
ACK
receiver
NAV (RTS)
data
other
stations
NAV (CTS) t
defer access
new contention

32. 802.11 TBS/PCF: Polling (Infrastructure Mode)
point
coordinator U
polled
wireless
stations
NAV
NAV
contention free period t
medium
busy
contention
period
D: downstream poll, or data from point coordinator
U: data from polled wireless station

33. direct access if medium is free  DIFS
Integrating PCF and DCF
Using different inter frame spacing values to implement priority
SIFS (Short Inter Frame Spacing)
highest priority, for ACK, CTS, polling response
PIFS (PCF IFS)
medium priority, for time-bounded service using PCF
DIFS (DCF, Distributed Coordination Function IFS)
lowest priority, for asynchronous data service t
medium busy
SIFS
PIFS
DIFS
next frame
contention
direct access if medium is free  DIFS

34. 802.11 – Inter Frame Spacing 802.11b 802.11a 802.11g aSIFSTime 10 usec
aSlotTime
20 usec
9 usec
20 usec (mixed); 9 usec (g only)
aDIFTime (2xSlot+SIFS)
50 usec
34 usec
50 usec; 28 usec

35. Example: RTS-CTS-data-ACK
DIFS
RTS
data
sender
SIFS
SIFS
CTS
SIFS
ACK
receiver
NAV (RTS)
DIFS
data
other
stations
NAV (CTS) t
defer access
new contention

36. Example: PIFS D: downstream poll, or data from point coordinator
SIFS D D
point
coordinator
SIFS U
polled
wireless
stations
NAV
NAV
contention free period t
medium
busy
contention
period
D: downstream poll, or data from point coordinator
U: data from polled wireless station

37. Outline Recap Wireless background Wireless MAC design Frequency domain
Modulation and demodulation
Wireless channels
Wireless PHY design
Wireless MAC design
wireless access problem and taxonomy
wireless resource partitioning dimensions
media access protocols
ALOHA protocol
The Ethernet protocol
Hidden terminals in wireless
IEEE
Design
Timing example

38. Example: 802.11b/ACK Timing Analysis
Suppose TCP with 1460 bytes data payload
TCP data frame size (not including preamble)
1536 bytes ( TCP/IP header header)
TCP ACK data frame size (not including preamble)
76 TCP/IP header bytes
802.11b ACK frame size 14 bytes
Suppose b at the highest rate
8 bits per symbol
1.375 Msps
See page 4

39. 802.11b/ACK Timing (1460B data)
TCP Data
TCP Ack

40. 802.11b/ACK Timing (1460B data)
TCP Data
TCP Ack
DIFS (us) 50

41. 802.11b/ACK Timing (1460B data) TCP Data TCP Ack DIFS (us) 50
Data (us)
/ = 1,310
/ = 248

42. 802.11b/ACK Timing (1460B data) TCP Data TCP Ack DIFS (us) 50
Data (us)
/ = 1,310
/ = 248
SIFS (us) 10

43. 802.11b/ACK Timing (1460B data) TCP Data TCP Ack DIFS (us) 50
Data (us)
/ = 1,310
/ = 248
SIFS (us) 10
ACK (us)
/ =203
203

44. 802.11b/ACK Timing (1460B data) TCP Data TCP Ack DIFS (us) 50
Data (us)
/ = 1,310
/ = 248
SIFS (us) 10
ACK (us)
/ =203
203
Frame total (us)
1,573
511
Transactions total (us)
2,084

45. Example: g/ACK Timing
Suppose g at the highest rate (54Mbps)
symbol duration: 4 usec; 216 bits/symbol
20 usec preamble; 6 usec “signal extension time” at the end of each frame
Data: 28 (DIFS) + ( * 4 + 6) + 10 (SIFS) + ( * 4 + 6) = 322
Ack: 106

46. Example: g/ACK Timing
Suppose g at the highest rate (54Mbps)
symbol duration: 4 usec; 216 bits/symbol
20 usec preamble; 6 usec “signal extension time” at the end of each frame
Suppose TCP with 1460 bytes data payload
data: 57 (=1536*8/216) symbols; ACK: 3 (=76*8/216) symbols
802.11b ACK frame size 14 bytes
1 symbol
Data: 28 (DIFS) + ( * 4 + 6) + 10 (SIFS) + ( * 4 + 6) = 322
Ack: 106

47. 802.11g Basic Timing (1460B data) TCP Data TCP Ack DIFS (us) 28
Data (us)
*4 + 6 = 254
* = 38
SIFS (us) 10
ACK (us)
* =30 30
Frame total (us)
322
106
Transactions total (us)
428

48. Example: g + CTS
RTS/CTS uses b DIFS (50 usec) and long preamble (192 usec)
RTS/CTS uses b frame coding
20 bytes RTS
14 bytes CTS

49. 802.11g + CTS Timing (1460B data) TCP Data TCP Ack DIFS (us)
28 -> 50
CTS
/1.375 = 203
= 203
SIFS 10
Data (us)
*4 + 6 = 254
* = 38
SIFS (us)
ACK (us)
* =30 30
Frame total (us)
322
106
Transactions total (us)
428 -> 898

50. Summary Technology Transactions per sec Mbps of TCP
Relative to b
11b, 11Mbps
479
5.6 1
11a, 54 Mbps
2,336
27.3
4.9
11g, no CTS/RTS
11g, CTS
1,113
13.0
2.3
11g, RTS/CTS
750
8.8
1.6

51. Outline Recap Wireless background Wireless MAC design Frequency domain
Modulation and demodulation
Wireless channels
Wireless PHY design
Wireless MAC design
wireless access problem and taxonomy
wireless resource partitioning dimensions
media access protocols
ALOHA protocol
The Ethernet protocol
Hidden terminals in wireless
IEEE
New designs

52. Roadmap: Wireless MAC
Problem: single shared medium, hence if two transmissions overlap on all dimensions [time, space, frequency, and code], then it is a collision
+CS+CD+EB
Slotted ALOHA
Ethernet
Hidden-terminal Collision detection/ prevention
ACK/RTS-CTS
Contrarian design

53. Hidden Terminal with
No ACK
Collision!
Alice
Bob

54. Hidden Terminal with 802.11 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.

55. 802.11 Setting: Two Collisions of Two PacketsPb

56. Interference-free Bits
ZigZag Decoding
Exploits ’s behavior
Retransmissions
Same packets collide again
Senders use random jitters
 Collisions start with interference-free bits Pa Pa
…………
How do we leverage these interference free bits to decode the collisions? ∆1 Pb ∆2 Pb
Interference-free Bits

57. ZigZag Algorithm ∆1 ≠∆2 ∆1 ∆2 1 1
while (exists a chunk that is interference-free in one collision and has interference in the other) {
decode and subtract from the other collision }

58. How Does ZigZag Work? ∆1 ≠∆2 ∆1 ∆2 1 1 2
while (exists a chunk that is interference-free in one collision and has interference in the other) {
decode and subtract from the other collision }

59. How Does ZigZag Work? ∆1 ≠∆2 ∆1 ∆2 1 3 2 2
while (exists a chunk that is interference-free in one collision and has interference in the other) {
decode and subtract from the other collision }

60. How Does ZigZag Work? ∆1 ≠∆2 ∆1 ∆2 1 3 3 2 4
while (exists a chunk that is interference-free in one collision and has interference in the other) {
decode and subtract from the other collision }

61. How Does ZigZag Work? ∆1 ≠∆2 ∆1 ∆2 1 3 5 4 2 4
while (exists a chunk that is interference-free in one collision and has interference in the other) {
decode and subtract from the other collision }

62. How Does ZigZag Work? ∆1 ≠∆2 ∆1 ∆2 1 3 5 5 2 4 6
while (exists a chunk that is interference-free in one collision and has interference in the other) {
decode and subtract from the other collision }

63. How Does ZigZag Work? ∆1 ≠∆2 ∆1 ∆2 1 3 5 7 6 2 4 6
while (exists a chunk that is interference-free in one collision and has interference in the other) {
decode and subtract from the other collision }

64. How Does ZigZag Work? ∆1 ≠∆2 ∆1 ∆2 1 3 5 7 7 2 4 6 8
while (exists a chunk that is interference-free in one collision and has interference in the other) {
decode and subtract from the other collision }
Thus, even in the face of collisions, we are able to correctly receive 2 packets in 2 timeslots.
Delivered 2 packets in 2 timeslots
As efficient as if the packets did not collide