1. Media Access Control and 802.11
Y. Richard Yang
2/7/2011

2. Outline
Admin. and recap
Media access control

3. Admin. Start to think about project Admin: homework 2 linked online
2-3 students per group
Make appointment to talk to me
Admin: homework 2 linked online

4. Recap
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

5. 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

6. Recap
Media Access Control (MAC) scheduling problem: how does a network allocate space/time/freq/code
to maximize resource efficiency
to allocate resource fairly
to satisfy app requirements
Sharing or cellular up links:
time sharing the same frequency and code
a single receiver (the Access Point)
Time sharing techniques
fixed allocation
centralized authority, e.g., polling
taking turns, e.g., token passing
distributed random access

7. Success (S), Collision (C), Empty (E) slotsA
Recap: Slotted Aloha B
Time divided into slots
get a frame
repeat:
at each slot transmit with probability p;
until no collision
Success (S), Collision (C), Empty (E) slots

8. 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

9. 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

10. Pros and Cons of Slotted Aloha
Pros: simple
Cons:
low efficiency: utilization at ~1/e
undesirable stable point

11. Ethernet fix: CSMA/CD w/ Exp Backoff
Given collision detection, instead of wasting the whole packet transmission time (a slot), we waste only the time needed to detect collision.
Instead of a fixed probability, adapt probability according to # of collision
P: packet size, C: contention window C C C P

12. Does collision detection work well in wireless?
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
Does collision detection work well in wireless?

13. 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

14. 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 cause collapse!
Q: what is the outcome of CSMA/CD + hidden terminals?

15. Handling Hidden Terminals
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/ACK (congestion avoidance)
default in
even if media is not sensed busy, transmits with a probability
in real implementation, with a random delay
receiver ACK to make sure collision does not happen

16. 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
Only receiver can detect collision, to avoid,
B should tell C that it is receiving A B C

17. Hidden-Terminal Detection: Busy-tone
Used in CDPD (cellular digital packet data)
The base station sends a busy tone on the down link when receiving data

18. Hidden-Terminal Detection: Virtual Carrier Sense/ACK
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

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

20. 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 1997]; Digitan 2 Mbps WLAN 1:2:2.5
many measurements show that overhead hurts performance

21. Outline
Admin. and recap
MAC in wireless networks
802.11

22. IEEE Requirements
Design for small coverage (e.g. office, home) (implication?)
Low/no mobility (implications?)
High data-rate applications
Ability to integrate real time applications and non-real-time applications (implications?)
Use un-licensed spectrum

23. 802.11: Infrastructure Mode Architecture similar to cellular
LAN
Architecture similar to cellular
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

24. 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

25. 5000 + 5*channel number [MHz]
802.11a Physical Channels 36 40 44 48 52 56 60 64
channel#
5150
5180
5200
5220
5240
5260
5280
5300
5320
5350
[MHz]
center frequency =
*channel number [MHz]
149
153
157
161
channel#
5725
5745
5765
5785
5805
5825
[MHz]

26. 802.11a Modulation
Use OFDM to divide each physical channel (20 MHz) into 52 subcarriers (20M/64=312.5 KHz each)
48 data, 4 pilot
Adaptive modulation
BPSK: 6, 9 Mbps
QPSK: 12, 18 Mbps
16-QAM: 24, 36 Mbps
64-QAM: 48, 54 Mbps

27. 802.11 - MAC Layer Traffic services
Asynchronous Data Service (mandatory)
exchange of data packets based on “best-effort”
support of broadcast and multicast
Time-Bounded Service (optional)
exchange of bounded delay service

28. 802.11 MAC Layer: Access Methods
DFWMAC-DCF CSMA/CA (mandatory)
collision avoidance via randomized “back-off“
ACK packet for acknowledgements/detection
DFWMAC-DCF w/ RTS/CTS (optional)
additional virtual “carrier sensing: to avoid hidden terminal problem
DFWMAC- PCF (optional)
access point polls terminals according to a list

29. 802.11 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

30. 20 usec (mixed); 9 usec (g-only)
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. Congestion Avoidance: 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

32. – 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
DIFS
RTS
data
sender
SIFS
SIFS
CTS
SIFS
ACK
receiver
NAV (RTS)
DIFS
data
other
stations
NAV (CTS) t
defer access
new contention

33. direct access if medium is free  DIFS
– Inter Frame Spacing
Defined different inter frame spacing
SIFS (Short Inter Frame Spacing); 10 us in b
highest priority, for ACK, CTS, polling response
PIFS (PCF IFS); 30 us in b
medium priority, for time-bounded service using PCF
DIFS (DCF, Distributed Coordination Function IFS); 50 us in b
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. 802.11: PCF for Polling (Infrastructure Mode)
PIFS
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

36. 802.11b Frame Format preamble
Sync
SFD
PLCP header
MAC Data
CRC
Preamble (192 usec; or optional 96 short version)
- Sync: alternating 0s and 1s (DSSS 128 bits)
- SFD: Start Frame delimiter:
PLCH (Phsical Layer Convergence Procedure) Header
- payload length
- signaling field: the rate info.
- CRC: 16 bit protection of header

37. 802.11 – MAC Data Format Types Sequence numbers Addresses
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
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

39. Example: 802.11b Throughout Suppose TCP with 1460 bytes data payload
TCP data frame size (not including preamble)
1536 bytes ( TCP header header)
TCP ACK data frame size (not including preamble)
76 bytes
802.11b ACK frame size 14 bytes
Suppose b at the highest rate
8 bits per symbol
1.375 Msps
Q: What is TCP/802.11b throughput?
See page 4

40. 802.11b Throughout TCP Data TCP Ack DIFS (us) 50 802.11 Data (us)
/ = 1,310
/ = 248
SIFS (us) 10
ACK (us)
/ =203
203
Frame total (us)
1,573
511
Transactions total (us)
2,084

41. Example: g Throughout
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 symbols; ACK: 3 symbols
802.11b ACK frame size 14 bytes
1 symbol
Data: 28 (DIFS) + ( * 4 + 6) + 10 (SIFS) + ( * 4 + 6) = 322
Ack: 106

42. 802.11g Throughout TCP Data TCP Ack DIFS (us) 28 802.11 Data (us)
*4 + 6 = 254
* = 38
SIFS (us) 10
ACK (us)
* =30 30
Frame total (us)
322
106
Transactions total (us)
428

43. Example: TCP/802.11g + 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
Q: What is throughput?

44. 802.11g + CTS Throughout 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

45. 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

46. Outline
Admin. and recap
802.11
hidden-terminal revisited

47. The Hidden Terminal Problem
No ACK
Collision!
Alice
Bob

48. The Hidden Terminals Problem
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.