View
2
Download
0
Category
Preview:
Citation preview
Wireless Networking & Mobile Computing
CS 752/852 - Spring 2012
Tamer Nadeem Dept. of Computer Science
Lec #4: Medium Access Control - II
Page 2 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
IEEE 802.11 Standards
Page 3 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
IEEE 802.11 MAC
• Very popular wireless MAC protocol
• Two Architectures
IEEE 802.11
Medium Access Control (PCF+DCF)
FHSS DSSS Infrared OFDM
MA
C
PH
Y
SSID
BSSID
Page 4 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 4
802.11 PHY Sublayers
• Physical layer convergence protocol (PLCP)
• Provides common interface for MAC
• Offers carrier sense status & CCA (Clear channel assesment)
• Performs channel synchronization / training
• Physical medium dependent sublayer (PMD)
• Functions based on underlying channel quality and characteristics
• E.g., Takes care of the wireless encoding
Page 5 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 5
PLCP
• PLCP has two structures.
• All 802.11b systems have to support Long preamble.
• Short preamble option is provided to improve efficiency when transmitting
voice, VoIP, streaming video.
• PLCP Frame format
• PLCP preamble
• SFD: start frame delimiter
• PLCP header
• 8-bit signal or data rate (DR) indicates how fast data will be transmitted
• 8-bit service field reserved for future
• 16-bit length field indicating the length of the ensuing MAC PDU (MAC sublayer’s
Protocol Data Unit)
• 16-bit Cyclic Redundancy Code
Page 6 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 6
PLCP (802.11b)
long
preamble
192us
short
preamble
96us
(VoIP, video)
Page 7 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• The IEEE 802.11 channelization scheme.
• The 2.4-GHz band is broken down into 11 in USA. However, at most there
is 3 non-overlapping channels.
802.11 Channels
Page 8 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
802.11 Channels
Page 9 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
IEEE 802.11 MAC
• Two modes:
• DCF (distributed coordination function)
• PCF (point coordination function)
• IEEE 802.11 DCF is based on CSMA/CA
• Physical Carrier Sense
• Explicit ACK from receiver (for unicast transmission)
• RTS/CTS reservation frames (Virtual Carrier Sensing)
• Retry Counters
• Different Timing Intervals for priorities
Page 10 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• RTS-CTS used for frames longer than a Threshold
• RTS-CTS overhead not efficient for short frames
• Some environments may not find RTS-CTS useful, e.g. many
infrastructure networks
• Threshold variable can be tuned
• Virtual carrier sensing
• Duration field in all frames, including RTS and CTS, monitored by
every station
• Duration field used to construct a network access vector (NAV)
• Inhibits transmission, even if no carrier detected
IEEE 802.11 DCF Basics – RTS/CTS & Virtual Carrier Sense
Page 11 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• Counter and timer for each frame
• Short or long retry counter
• Lifetime timer
• Retry counter
• Incremented for each transmission attempt
• Use of short versus long retry counter based on Threshold variable
• Threshold limit
• ShortRetryLimit for short retry counter ‰
• LongRetryLimit for long retry counter ‰
• If threshold exceeded, frame is discarded and upper
layer is notified via MAC interface
IEEE 802.11 DCF Basics – Retry Counters
Page 12 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• Timing intervals are defined that control a station’s access to the
medium
• Slot time (SlotTime)
• Specific value depends on PMD layer
• Derived from propagation delay, transmitter delay, etc. (20micro-sec for
DSSS and 50 for FHSS)
• Basic unit of time for MAC, e.g. for backoff time is a multiple of slot time
• Short Inter-Frame Space (SIFS)
• Shortest interval -- SIF < SlotTime e.g. 10 microsec for FHSS
• Used for highest priority access to the medium, e.g., for ACK and CTS
• Allows Data-ACK and RTS-CST to be atomic transactions
IEEE 802.11 DCF Basics – Timing Intervals
Page 13 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• Priority (or PCF) Inter-Frame Space (PIFS)
• PIFS = SIFS + SlotTime
• Used for Point Coordination Function (PCF) access to the medium
• Allows priority based access to the medium after ACKs but before
contention based access
• Distributed (or DCF) Inter-Frame Space (DIFS)
• DIFS = SIFS + 2×SlotTime
• Used for Distributed Control Function (DCF) access to the medium
• Results in lower priority access than using SIFS or PIFS
• Extended Inter-Frame Space (EIFS)
• EIFS = SIFS + (8×ACK) + PreambleLength + PLCPHeaderLength + DIFS
• Used in the event that the MAC receives a frame with an error
• Provides an opportunity for a fast retransmit of the error frame
• In summary …
• SIFS < SlotTime < PIFS < DIFS << EIFS
IEEE 802.11 DCF Basics – Timing Intervals
Page 14 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• When a sender has a data to transmit, it picks a random wait period.
The wait period is decremented if the channel is idle
• When this period expires, the node tries to acquire the channel by
sending a RTS packet
• The Receiving node (destination) responds with a CTS packet
indicating that its ready to receive the data
• The sender then completes the packet transmission
• If the packet is received without errors, the destination node responds
with an ACK
• If an ACK is not received, the packet is assumed to be lost and the
packet is retransmitted
802.11 DCF Mode Principles
Page 15 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• If RTS fails, the node attempts to resolve the collision by doubling the
wait period. (This is known as binary exponential back-off (BEB)).
• Station trying to send an ACK is given preference over a station that
is acquiring a channel (Different waiting intervals are specified)
• A node needs to sense channel for Distributed Inter- Frame Space
(DIFS) interval before making an RTS attempt and a Short Inter
Frame Space (SIFS) interval before sending an ACK packet
802.11 DCF Mode Principles
Page 16 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
DIFS
RTS
CTS
DATA
ACK
NAV (CTS)
NAV (RTS)
SIFS SIFS SIFS
B
A
C
Contention
Window
DIFS
802.11 DCF Mode
RTS
Deferred CW
A B
C
D
D
Page 17 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• Because SIFS is shorter than the DIFS interval, the station sending an
ACK attempts transmission before a station sending a data packet
• In addition to physical channel sensing, virtual carrier sensing is
achieved due to NAV (Network allocation vector) field in the packet
• NAV indicates the duration of current transmission
• Nodes listening to RTS, or CTS messages back off NAV amount of
time before sensing the channel again
• Several papers describe this protocol and even suggest
enhancements.
802.11 DCF Mode Notes
Page 18 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• Control Frame:
• RTS, CTS, ACK
• Data Frame
• Management Frame:
• Beacon
• Probe Req, Probe Resp
• Assoc Req, Assoc Resp
• Reassoc Req, Reassoc Resp
• Disassociation
• Authentication
• Deauthentication
802.11 Frames Type
Page 19 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• Ver - The Protocol Version number is always 0
• Type - indicates whether the frame is a
Management, Control or Data frame.
802.11 Data Frame Format
• Subtype - describe the detail
of the frame type.
• To DS - set if the frame is to
be sent by the AP to the
Distribution System
• From DS - set if the frame
is from the Distribution
System
• More Frag - set if this
frame is a fragment of a
bigger frame and there are
more fragments to follow.
• Retry - set if this frame is a retransmission, maybe through the loss of an ACK
Page 20 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
802.11 Data Frame Format
• Power Mgmt - indicates what power mode ('save' or 'active') the station is to be
in once the frame has been sent
• More Data - set by the AP to indicate that more frames are destined to a
particular station that may be in power save mode. These frames will be buffered
at the AP ready for the station should it decide to become 'active'.
• WEP - set if WEP is being used to encrypt the body of the frame
• Duration & ID - In Power save poll messages this is the station ID, whereas in
all other frames this is the duration used when calculating the NAV
• Address 1 - The recipient station address on the BSS. If To DS is set, this is the
AP address; if From DS is set then this is the station address
• Address 2 - The transmitter station address on the BSS. If From DS is set, this
is the AP address; if To DS is set then this is the station address
• Address 3 - If Address 1 contains the destination address then Address 3 will
contain the source address. Similarly, if Address 2 contains the source address
then Address 3 will contain the destination address.
Page 21 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
802.11 Data Frame Format
• Address 4 - If a Wireless Distribution System (WDS) is being used (with AP to
AP communication), then Address 1 will contain the receiving AP address;
Address 2 will contain the transmitting AP address; Address 3 will contain the
destination station address and Address 4 the source station address.
• Sequence Control - contains the Fragment Number and Sequence
Number that define the main frame and the number of fragments in the frame
• Frame Body - contains the actual data e.g. IP datagrams and can be up to 2312
octets in size
• CRC - 32-bit Cyclic Redundancy Check on the whole 802.11 frame.
Page 22 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
802.11 Control Frame Format
Page 23 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
802.11 Contention Window
• Random number selected from [0,cw]
• If transmission was successful, set CW = CWmin
• If transmission fails (i.e., no ACK), CW =
min{2(CW+1)-1, CWmax}
• Small value for cw
• Less wasted idle slots time
• Large number of collisions with multiple senders
(two or more stations reach zero at once)
• Optimal CW for known number of contenders & know packet size
• Computed by minimizing expected time wastage (by both collisions and
empty slots)
• Tricky to implement because number of contenders is difficult to estimate
and can be VERY dynamic
• Project Idea: • Evaluate literature for CW calculation schemes under different scenarios
• Enhance/New adaptive CW scheme
Page 24 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
802.11 Fragmentation
t
SIFS
DIFS
data
ACK1
other
stations
receiver
sender frag1
DIFS
contention
RTS
CTS SIFS SIFS
NAV (RTS) NAV (CTS)
NAV (frag1) NAV (ACK1)
SIFS ACK2
frag2
SIFS
Page 25 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Physical Carrier Sense Mechanisms
• Energy detection threshold
• Monitors channel during “idle” times between packets to measure floor noise
• Energy levels above this floor noise by a threshold trigger carrier sense
• DSSS correlation threshold
• Monitors the channel for Direct Sequence Spread Spectrum (DSSS) coded signal
• Triggers carrier sense if the correlation peak is above a threshold
• More sensitive than energy detection (but only works for 802.11 transmissions)
• High BER disrupts transmission but not detection
• Carrier can be sensed at lower levels than
packets can be received
• Results in larger carrier sense range than transmission range
• More than double the range in NS2 802.11 simulations
Receive Range
Carrier Sense Range
Page 26 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• RTS/CTS & Carrier Sense • When RTS/CTS is useful?
• Should Carrier Sensing replace RTS/CTS?
• Interference Range vs. Carrier Sense Range • How effective CSMA carrier sense?
• BER & Date rate and Transmission Range (data rate affect the SNR
threshold and hence the transmission range but not the physical CS)
• Contention Window Size
• Is ACK necessary? • MACA said no ACKs. Let TCP recover from losses
On 802.11 Issues
The search for the best MAC protocol is still on. However, 802.11 is being optimized too.
Thus, wireless MAC research still alive
Page 27 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 27
On RTS/CTS & Carrier Sense
• Does RTS/CTS (Virtual CS) solve hidden terminals ?
• Assuming carrier sensing zone = communication zone
C F
A B
E
D
CTS RTS
E does not receive CTS successfully Can later initiate transmission that
interferes with D
Hidden terminal problem remains
Page 28 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
On RTS/CTS & Carrier Sense
• Hidden Terminal: How about increasing Physical Carrier Sense range ??
• E will defer on sensing carrier no collision !!!
C B D Data
A
E
CTS
RTS F
Page 29 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
On RTS/CTS & Carrier Sense
• Exposed Terminal: B should be able to transmit to A
• Carrier sensing makes the situation worse
C A B
E
D
CTS
RTS
Page 30 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
On RTS/CTS & Carrier Sense
• 802.11 does not solve HT/ET completely
• Only alleviates the problem through RTS/CTS and recommends larger CS zone
• Large CS zone aggravates exposed terminals
• Spatial reuse reduces A tradeoff
• RTS/CTS packets also consume bandwidth
• Moreover, backing off mechanism is also wasteful
• Carrier sense relies on channel measurements at the sender to infer the probability of reception at the receiver!
• Project Idea: • Evaluation of the benefits and drawbacks of carrier sense
• Scheme to intelligently choose a Carrier sensing threshold
• Evaluate tracking correlation between channel conditions at the
sender and at the receiver.
Page 31 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
On Contention Window Size
• Optimal CW for known number of
contenders & know packet size
• Computed by minimizing expected time wastage (by
both collisions and empty slots)
• Tricky to implement because number of contenders is
difficult to estimate and can be VERY dynamic
• 802.11 adaptive scheme is unfair • Under contention, unlucky nodes will use larger cw than lucky nodes (due to
straight reset after a success)
• Lucky nodes may be able to transmit several packets while unlucky nodes are
counting down for access
• Fair schemes should use same cw for all contending nodes
• Project Idea: • Evaluate literature for CW calculation schemes under different scenarios
• Enhance/New adaptive CW scheme
Page 32 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• 802.11 physical layer (e.g., Direct Sequence Spread Spectrum (DSSS)
used in 802.11b)
Capture effect: two transmissions received by the same receiver, the
signals of the stronger transmission will capture the receiver radio, and
signals of the weaker transmission will be rejected as noise.
Frame 2
Frame 1
Received Frame
Frame 2 Frame 1
Received Frame
• Simple and widely accepted model:
• Capturing stronger signal ≠ Capturing stronger frame
On Interference Range vs. Carrier Sense Range
Page 33 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Inefficiency:
• Interference Range:
R
I 1 2
I
C
d
• Power path loss model:
• Capture model:
Given:
R=250m, C=550, l =2, α=5
On Interference Range vs. Carrier Sense Range
Page 34 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
On Interference Range vs. Carrier Sense Range
• Project Idea: • How to estimate interference range (distance)
• Propagation Delay?
• Interference Aware MAC Scheme
Page 35 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
On Transmission Date rate
Floor Noise Data Rate
Received Power
Channel Bandwidth
• Bit error (p) for BPSK and
QPSK :
SNR
Page 36 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
On Transmission Date rate
Page 37 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
On ACKnowledgment
• APs typically backlogged with traffic
• Persistent traffic possibility of optimization
• Use implicit ACK optimization
• Piggyback the CTS with ACK for previous dialog
802.11
Implicit
ACK
Gain
Page 38 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
On ACKnowledgment
• The optimization timeline
T R
RTS
CTS
Data
ACK
RTS
CTS
Data
ACK
T R
RTS
CTS
Data
RTS
CTS +ACK
Data
T R
RTS
CTS
Data
Poll +ACK
Data
RTS
CTS +ACK
Ba
cko
ff
Ba
cko
ff
Ba
cko
ff
Ba
cko
ff
Poll +ACK
Data
Ba
cko
ff
Ba
cko
ff
802.11 Implicit ACK Hybrid Channel Access
Page 39 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Performance Analysis of the
IEEE 802.11 Distributed
Coordination Function (Giuseppe Bianchi)
Page 40 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
802.11 DCF Throughput Analysis (Bianchi)
• Objective:
• Analytical Evaluation of Saturation Throughput
• Assumptions:
• Fixed number of stations having packet for transmission
• Each packet collide with constant and independent probability
• Model bi-dimensional process {s(t) , b(t)} with discrete-
time Markov chain
• Analysis divided into two parts:
• Study the behavior of single station with a Markov model
• Study the events that occur within a generic slot time & expressed
throughput for both Basic & RTS/CTS access method
Page 41 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Markov Chain Model
Page 42 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• Closed form solution for Markov chain
Markov Chain Model
• Stationary Probability
Page 43 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• In general τ depends on conditional collision
probability p
Markov Chain Model
• Probability τ that a station transmits in randomly
chosen slot time
• When m =0 no exponential backoff is considered
probability τ results independent of p
Page 44 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Throughput Analysis
• Normalized system throughput S
• Probability of transmission Ptr
• Probability of successful transmission Ps
Page 45 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Normalized system throughput
Throughput Analysis
Specify Ts and Tc to compute throughput for DCF access mechanism
Page 46 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• Considering System via Basic Access mechanism
• Packet header H = PHYhrd +MAChrd
• Propagation delay δ
Throughput Analysis
Page 47 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• Packet transmission via RTS/CTS Access
mechanism
Throughput Analysis
Page 48 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Model Validation
• Compared analytical results with that obtained by
means of simulation
• Analytical model extremely accurate
• Analytical results (lines) coincide with simulation results
(symbols) in both Basic Access & RTS/CTS cases
Saturation throughput
analysis vs. simulation
Page 49 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Performance Evaluation
Saturation throughput vs. initial window
size for Basic Access mechanism
• Greater the network size lower is the throughput for basic access
Page 50 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• Throughput of Basic Access mechanism depends
on W
• W depends on number of terminals
• High value of W gives
excellent throughput
performance
Performance Evaluation
Saturation throughput vs. initial window
size for Basic Access mechanism
Page 51 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• Throughput obtained with RTS/CTS mechanism
• Independent of value of W
Performance Evaluation
Saturation throughput vs. initial window
size for RTS/CTS mechanism
Page 52 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
• Number of transmissions per packet increases as W
reduces & network size n increases.
Performance Evaluation
Average number of transmissions
per packet
Page 53 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing
Questions
Recommended