1. Outline Wireless cellular (GSM, CDMA, UMTS) Wireless LANs, MAC layer
Wireless introduction
Wireless cellular (GSM, CDMA, UMTS)
Wireless LANs, MAC layer
IEEE
Bluetooth
ZigBee
Wireless Ad hoc networks
routing: proactive routing, on-demand routing, scalable routing, geo-routing
wireless Ad hoc multicast
TCP in ad hoc networks
QoS, adaptive voice/video apps
Sensor networks
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2. Wireless Protocol Layers
Data Plane
Control Plane
Application Processing
Propagation Model
Mobility
Frame Processing
Radio Status/Setup
CS/Radio Setup
RTS/CTS
Frame Wrapper
Ack/Flow Control
Clustering
Packet Store/Forward VC
Handle
Flow
Control
Routing
IP Wrapper
IP/Mobile IP
RSVP
Transport Wrapper
TCP/UDP Control
Channel
Radio
MAC Layer
Network IP
Transport
Application
RTP Wrapper
RCTP
Link Layer
Application Setup
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3. Architecture
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4. IEEE 802.11 Standard Why we study this standard: overall architecture
MAC layer spec.
channel access
mobility support
physical layer spec.
direct sequence
frequency hopping
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5. 802.11 Features CSMA/CA based MAC protocol
- DCF (Distributed Coordination Function)
support for both time-critical PCF( Point Coordination Function) and non-critical traffic (DCF)
support multiple priority levels
spread spectrum technology (no licensing)
power management allows a node to doze off
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6. 802.11 Protocol Entities MAC entity basic access mechanism
fragmentation & encryption
MAC layer management entity
synchronization
power management
roaming
Physical layer convergence protocol (PLCP)
PHY-specific, common PHY SAP support
provides carrier sense
Physical medium dependent sublayer (PMD)
modulation & coding
PHY layer management
channel tuning & PHY MIB
MAC
Sublayer
MAC layer
Management
PLCP
sublayer
PHY layer
Management
PMD sublayer
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7. PHY spec Infrared PHY (No products !) diffuse infrared 1 and 2Mbps
Radio PHY
Frequency hopping PHY
Direct Sequence PHY
CCA (clear channel assessment) - how to sense a channel is clear:
energy level is above a threshold
can detect a signal
use both
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8. Frequency Hopping
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9. Frequency Hopping Spread Spectrum
Pseudo-random frequency hopping
2.4Ghz ISM band, 1-2Mbps; 2GFSK (2 level Gaussian frequency shift keying), 4GFSK; hop over 79 channels
spreads the power over a wide spectrum -> spread spectrum
narrowband interference cannot jam
developed initially for military
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10. Direct Sequence Spread Spectrum
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11. Direct Sequence Spread Spectrum
Spreading factor = code bits/data bit, commercial (min 10 by FCC)
Signal bandwidth>10*data bandwidth
code sequence synchronization
correlation between codes -> interference: orthogonal
2.4Ghz band, 1,2Mbps; DBPSK (differential binary phase shift keying), DQPSK (differential quadrature phase shift keying); 11 chip barker sequence
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12. Multiple Access Control (MAC) Protocols
MAC protocol: coordinates transmissions from different stations to minimize/avoid collisions
(a) Channel Partitioning MAC protocols: TDMA, FDMA, CDMA
(b) Random Access MAC protocols: CSMA, MACA
(c) “Taking turns” MAC protocols: polling
Goal: efficient, fair, simple, decentralized
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13. Basic MAC Features
DCF: Carrier sense multiple access with collision avoidance (CSMA/CA) based
based on carrier sense function in PHY called Clear Channel Assessment (CCA)
CSMA/CA+ACK for unicast frames, with MAC level recovery
parameterized use of RTS/CTS to protect against hidden nodes
frame formats to support both infrastructure and ad-hoc networks
PCF (option, not been widely implemented)
centralized, polling based
restricted to infrastructure network
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14. CSMA/CA+ACK: 4-way handshake
MAC headers format differs per type
control frames: RTS, CTS, ACK
management frames, e.g. beacon, probe/probe response, (re)-association request/response,
data frames
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15. Frame Format Addressing: Address 1 Address 2 Address 3 Address 4
Frame Control Field
Addressing: Address 1 Address 2 Address 3 Address 4
Ad hoc: DA SA BSSID
From AP: DA BSSID SA
To AP: BSSID SA DA
AP to AP: RA TA DA SA
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16. frame priorities
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17. CSMA/CA+ACK explained
Reduce collision probability where mostly needed
defer access based on carrier sense
CCA from PHY and virtual carrier sense state
direct access when medium is sensed free
longer than DIFS, otherwise defer and backoff
receiver of directed frames to return ACK when
CRC correct
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18. •Defer on either NAV or “CCA” indicating Medium Busy
•Duration field in RTS and CTS frames distribute Medium Reservation information which is stored in a Net Allocation Vector(NAV)
•Defer on either NAV or “CCA” indicating Medium Busy
•Use of RTS/CTS is optimal but must be implemented
•Use is controlled by a RTS
-Threshold parameter per station
-To limit overhead for short frames
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19. Time-critical service via PCF
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20. PCF Access Procedure
Point Coordinator (PC) senses the medium at the beginning of each CFP
PC in Access Point transmits a beacon containing “CF parameter set element” when idle > PIFS
each station presets its NAV to the CFPMaxDuration from the CF Parameter Set Element in beacons from the PC
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21. PCF Access Procedure (cont)
after a SIFS period, PC sends one of the following: a data frame, CF-Poll frame, Data+CF-Poll frame, CF-end frame (when no traffic buffered & no polls to send at the PC)
PC maintains a polling list to select stations that are eligible to receive CF-Polls during contention-free periods.
A CF-Pollable station always responds to a CF-Poll: if no data from the station, responds with a Null Frame or a CF-ACK (no data) frame (when ACK is required);
“piggyback” ACK or Poll in the data frame whenever possible
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22. Further details
Alternating Contention free and contention operations under PCF control
NAV prevents contention traffic until reset by the last PCF transfer -> variable length contention free period per interval
both PCF and DCF defer to each other causing PCF burst start variations
CF-burst by polling bit in CF-down frame
immediate response by station on a CF_Poll
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23. Synchronization in 802.11 All stations maintain a local timer
Timing synchronization function (TSF)
keeps timers from all stations in synch
AP controls timing in infrastructure networks
timing conveyed by periodic beacons
beacons contain timestamp for the entire BSS
timestamp from beacons to calibrate local clocks
not required to hear every beacon to stay in synch
used for power management
beacons sent at well known intervals
all station timers in BSS are synchronized
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24. Roaming in
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25. Roaming Approach Station decides that link to its current AP is poor
station uses scanning function to find another AP
station sends Reassociation Request to new AP
if Reassociation Response is successful
then station has roamed to the new AP
else station scans for another AP
if AP accepts Reassociation Request
AP indicates Reassociation to the Distribution System
Distribution System information is updated
normally old AP is notified thru distributation system
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26. Scanning Scanning required for many functions
finding and joining a network
finding a new AP while roaming
initializing an ad hoc network
MAC uses a common mechanism
Passive scanning
by listening for Beacons
Active Scanning
probe + response
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27. Active scanning Steps to Association: Station sends Probe
APs send Probe Response
Station selects best AP:
Station sends Association Request to select AP
AP sends Association Response
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28. Power Management A station can be in one of three states:
- Transmitter on
- Receiver only on
- Dozing: Both transmitter and receivers off
Access point (AP) buffers traffic for dozing stations
AP announces which stations have frames buffered. Traffic indication map included in each beacon. All multicasts/broadcasts are buffered.
Dozing stations wake up to listen to the beacon. If there is data waiting for it, the station sends a poll frame to get the data.
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29. Congestion Avoidance: IEEE 802.11 DCF
Before transmitting a packet, randomly choose a backoff interval in the range [0,cw]
cw is the contention window
Direct access when medium is sensed free longer than DIFS, otherwise defer and backoff
“Count down” the backoff interval when medium is idle
Count-down is suspended if medium becomes busy
When backoff interval reaches 0, transmit packet (or RTS)
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30. DCF Example (count down)
Let cw = 31
B1 = 25
B2 = 20
B1 = 5
B2 = 15
data
wait
data
wait
B2 = 10
B1 and B2 are backoff intervals
at nodes 1 and 2
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31. Congestion Avoidance
The time spent counting down backoff intervals contributes to MAC overhead
Choosing a large cw leads to large backoff intervals and can result in larger overhead
Choosing a small cw leads to a larger number of collisions (more likely that two nodes count down to 0 simultaneously)
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32. Congestion Control
Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage congestion is needed
IEEE DCF: Congestion control achieved by dynamically adjusting the contention window cw
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33. Binary Exponential Backoff in DCF
When a node fails to receive CTS in response to its RTS, it increases the contention window
cw is doubled (up to an upper bound – typically 5 times)
When a node successfully completes a data transfer, it restores cw to CWmin
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34. MILD Algorithm in MACAW [Bharghavan94Sigcomm]
When a node fails to receive CTS in response to its RTS, it multiplies cw by 1.5
Less aggressive than , which multiplies by 2
When a node successfully completes a transfer, it reduces cw by 1
More conservative than , where cw is restored to Cwmin
reduces cw much faster than it increases it
MACAW: cw reduction slower than the increase
Exponential Increase Linear Decrease
MACAW can avoid wild oscillations of cw when congestion is high
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35. Fairness Issue Many definitions of fairness plausible
Simplest definition: All nodes should receive equal bandwidth
Observation: unfairness occurs when one node has backed off much more than some other node A B C D
Two flows
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36. Fairness Issue
Assume that initially, A and B both choose a backoff interval in range [0,31] but their RTSs collide
Nodes A and B then choose from range [0,63]
Node A chooses 4 slots and B choose 60 slots
After A transmits a packet, it next chooses from range [0,31]
It is possible that A may transmit several packets before B transmits its first packet A B C D
Two flows
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37. MACAW Solution for Fairness
When a node transmits a packet, it appends its current cw value to the packet
All nodes hearing that cw value use it for their future transmission attempts
The effect is to reset all competing nodes to the same ground rule
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38. Distributed Fair Scheduling (DFS) [Vaidya Mobicom00]
A fully distributed algorithm for achieving weighted fair queueing: Assign a weight to each node
Goal: bandwidth used by each node should be proportional to the weight assigned to the node
Chooses backoff intervals proportional to
(packet size / weight)
DFS attempts to mimic the centralized Self-Clocked Fair Queueing algorithm
Works well on a LAN
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39. Distributed Fair Scheduling (DFS)
data
wait
B1 = 15
B2 = 5
B1 = 10
B1 = 5
Collision !
Weight of node 1 = 1
Weight of node 2 = 3
Assume equal
packet size
B1 = 15 (DFS actually picks a random value　with mean 15)
B2 = 5 (DFS picks a value with mean 5)
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40. Performance Improvement for 802. 11-based Wireless Networks [L
Performance Improvement for based Wireless Networks [L. Zhang ICC06]
Problem with WLANs
Every packet need the AP to forward
The AP has the same priority with wireless stations to access the wireless channel
Motivation
Make the AP with higher priority
The AP send a frame immediately after receiving a frame from the WS
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41. Action for the AP The AP must be involved in any communication.
If the AP is the receiver, it will set its backoff time counter to be zero
the AP should obtain the channel immediately and send the data, since its backoff time counter is zero.
As all wireless stations has increased their backoff time counter by one after the communication, there is no collision.
As a result, the AP can send one frame, after any wireless station sending a frame. It will not be the bottleneck anymore.
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42. Action for Wireless Stations
In backoff procedure, the backoff counter is
decremented while the medium is sensed idle,
frozen when a transmission is detected on the channel.
increased by one If the sender is one of other wireless stations (except when the backoff counter is already at its maximum)
reactivated when the channel is sensed idle again
The station transmits a frame when the backoff counter reaches zero.
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43. Model: a discrete-time Markov chain for two-dimensional process {s (t), b (t)} s (t) - stochastic process - backoff stage b (t) - stochastic process - backoff-time counter q - probability that at least one station transmits
decremented while the medium is sensed idle,
increased by one If the sender is one of other wireless stations (except when the backoff counter is already at its maximum)
reactivated when the channel is sensed idle again
The station transmits a frame when the backoff counter reaches zero.
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44. Goodput Analysis Throughput
Goodput G – sum of the end-to-end throughput in WLAN
Because there is no collision when the AP access the wireless channel.
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45. Results - UDP Goodput performance compare for UCP pair scenario
Fairness performance compare
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46. MAC Enhancements for QoS: IEEE 802.11e
The major enhancement of e
Traffic differentiation
Concept of transmission opportunity (TXOP)
Enhanced DCF (contention-based)
HCF (Hybrid Coordination Function) controlled channel access (contention free)
Burst ACK (optional)
Direct link protocol (DLP)
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47. IEEE 802.11e MAC Architecture
Hybrid Coordination Function (HCF): TGe (Group E) proposes HCF to provide QoS for real-time applications
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48. HCF - Introduction
HCF combines functions from the DCF and PCF with enhanced QoS-specific mechanisms
HCF consists of
Enhance DCF (EDCF) for contention-based access: provides differentiated access to the WM (Wireless Mobility) for 8 priorities for non-AP STAs (stations)
Controlled Access for contention-free access
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