3-MAC and 802.11
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Transcript of 3-MAC and 802.11
Advanced Topics in Next-Generation Wireless Networks
Qian ZhangDepartment of Computer Science
HKUST
MAC and 802.11
Multiple Access Links and Protocols
Two types of “links”:• point-to-point
– PPP for dial-up access– point-to-point link between Ethernet switch and host
• broadcast (shared wire or medium)– old-fashioned Ethernet– upstream HFC– 802.11 wireless LAN
shared wire (e.g., cabled Ethernet)
shared RF (e.g., 802.11 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air, acoustical)
Multiple Access protocols
• Single shared broadcast channel • Two or more simultaneous transmissions by nodes:
interference – Collision if node receives two or more signals at the same
time
Multiple access protocol• Distributed algorithm that determines how nodes
share channel, i.e., determine when node can transmit
• Communication about channel sharing must use channel itself! – no out-of-band channel for coordination
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1. When one node wants to transmit, it can send at
rate R.2. When M nodes want to transmit, each can send
at average rate R/M3. Fully decentralized:
– no special node to coordinate transmissions– no synchronization of clocks, slots
4. Simple
MAC Protocols: a Taxonomy
Three broad classes:• Channel Partitioning
– divide channel into smaller “pieces” (time slots, frequency, code)
– allocate piece to node for exclusive use
• Random Access– channel not divided, allow collisions– “recover” from collisions
• “Taking turns”– nodes take turns, but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access • access to channel in "rounds" • each station gets fixed length slot (length =
pkt trans time) in each round • unused slots go idle • example: 6-station LAN, 1,3,4 have pkt, slots
2,5,6 idle
1 3 4 1 3 4
6-slotframe
Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access • channel spectrum divided into frequency bands• each station assigned fixed frequency band• unused transmission time in frequency bands go idle • example: 6-station LAN, 1,3,4 have pkt, frequency
bands 2,5,6 idle
frequ
ency
bands time
FDM cable
Random Access Protocols
• When node has packet to send– transmit at full channel data rate R.– no a priori coordination among nodes
• Two or more transmitting nodes ➜ “collision”,• Random access MAC protocol specifies:
– how to detect collisions– how to recover from collisions (e.g., via delayed
retransmissions)
• Examples of random access MAC protocols:– slotted ALOHA– ALOHA– CSMA, CSMA/CD, CSMA/CA
Slotted ALOHA
Assumptions:• all frames same size• time divided into
equal size slots (time to transmit 1 frame)
• nodes start to transmit only slot beginning
• nodes are synchronized
• if 2 or more nodes transmit in slot, all nodes detect collision
Operation:• when node obtains fresh
frame, transmits in next slot– if no collision: node
can send new frame in next slot
– if collision: node retransmits frame in each subsequent slot with prob. p until success
Slotted ALOHA
Pros• single active node can
continuously transmit at full rate of channel
• highly decentralized: only slots in nodes need to be in sync
• simple
Cons• collisions, wasting slots• idle slots• nodes may be able to
detect collision in less than time to transmit packet
• clock synchronization
Slotted Aloha Efficiency
• suppose: N nodes with many frames to send, each transmits in slot with probability p
• prob that given node has success in a slot = p(1-p)N-1
• prob that any node has a success = Np(1-p)N-1
• max efficiency: find p* that maximizes Np(1-p)N-1
• for many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives:
Max efficiency = 1/e = .37
Efficiency : long-run fraction of successful slots (many nodes, all with many frames to send)
At best: channelused for useful transmissions 37%of time!
!
Pure (unslotted) ALOHA
• unslotted Aloha: simpler, no synchronization• when frame first arrives
– transmit immediately
• collision probability increases:– frame sent at t0 collides with other frames sent in [t0-
1,t0+1]
Pure Aloha Efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [p0-1,p0] .
P(no other node transmits in [p0-1,p0]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
… choosing optimum p and then letting n -> infty ...
= 1/(2e) = .18
even worse than slotted Aloha!
CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit:If channel sensed idle: transmit entire frame• If channel sensed busy, defer transmission
• human analogy: don’t interrupt others!
CSMA Collisions
collisions can still occur:propagation delay means two nodes may not heareach other’s transmissioncollision:entire packet transmission time wasted
spatial layout of nodes
note:role of distance & propagation delay in determining collision probability
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA– collisions detected within short time– colliding transmissions aborted, reducing channel
wastage
• collision detection: – easy in wired LANs: measure signal strengths,
compare transmitted, received signals– difficult in wireless LANs: received signal strength
overwhelmed by local transmission strength
• human analogy: the polite conversationalist
CSMA/CD Collision Detection
“Taking Turns” MAC protocols
channel partitioning MAC protocols:– share channel efficiently and fairly at high load– inefficient at low load: delay in channel access,
1/N bandwidth allocated even if only 1 active node!
Random access MAC protocols– efficient at low load: single node can fully
utilize channel– high load: collision overhead
“taking turns” protocolslook for best of both worlds!
“Taking Turns” MAC protocols
Polling: • master node
“invites” slave nodes to transmit in turn
• typically used with “dumb” slave devices
• concerns:– polling overhead – latency– single point of
failure (master)
master
slaves
poll
data
data
“Taking Turns” MAC protocols
Token passing:• control token
passed from one node to next sequentially.
• token message• concerns:
– token overhead – latency– single point of failure
(token)
T
data
(nothingto send)
T
Summary of MAC protocols
• channel partitioning, by time, frequency or code– Time Division, Frequency Division
• random access (dynamic), – ALOHA, S-ALOHA, CSMA, CSMA/CD– carrier sensing: easy in some technologies (wire),
hard in others (wireless)– CSMA/CD used in Ethernet– CSMA/CA used in 802.11
• taking turns– polling from central site, token passing– Bluetooth, FDDI, IBM Token Ring
Fixed-Assignment vs. Random Access
• Fixed-assignment methods – Make relatively efficient use of communications resources
when each user has a steady flow of information to be transmitted during each session
– Could be a waste of resources for intermittent traffic– Require an “arbitrator”
• Random access methods– Evolved around bursty data applications in computer networks– Provide more flexible and efficient ways of managing medium
access for short bursty messages– Could think of them as distributed statistical multiplexing
techniques
Why Not CSMA/CD?
• Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is used in IEEE 802.3 but not IEEE 802.11. Why?
• CSMA/CD– “Sense” the medium, send as soon as the medium is free, and
listen to the medium to “detect collisions”
• CSMA/CD does not work in wireless networks – Signal strength decreases proportional to the square of the
distance Hidden Terminal and Exposed Terminal problems
– The sender would apply CS and CD, but the collisions happen at the receiver
– Might be the case that a sender cannot “hear” the collision or cannot “sense” another carrier at the receiver
Hidden Terminal Problem
• Hidden terminals– A and C cannot hear each other– A sends to B, C cannot receive A– C wants to send to B, C senses a “free”
medium (CS fails)– Collision occurs at B– A cannot hear the collision (CD fails)– A is “hidden” for C
• Solution?– Hidden terminal is peculiar to wireless (not found in wired)– Need to sense carrier at receiver, not sender!– “Virtual carrier sensing”: Sender (node C) “asks” receiver
(node B) whether it senses the channel is busy. If so, behave as if channel busy
Exposed Terminal Problem
• Exposed terminals– A valid comm can not take place because the sender is
exposed– A starts sending to B– C senses carrier, finds medium in use and has to wait for A->B
to end– D is outside the range of A, therefore waiting is not necessary– A and C are “exposed” terminals, i.e. A and C could
communicate to their receivers at the same time, but because A and C can exposed to each other, one of them needlessly refrains from transmitting
D
C
A B
• A->B and C->D transmissions could have taken place in parallel without collisions
CSMA/CA Algorithm
• Control packet transmissions precede data packet transmissions to facilitate collision avoidance
• Basic procedure– Sense channel (CS)– If busy
• Back-off to try again later– Else
• Send RTS (optional) • If CTS not received (optional)
– Back-off to try again later• Else
– Send Data– If ACK not received
» Back-off to try again later– Next packet processing
CSMA/CA Algorithm (Cont.)
• Maintain a value CW (Contention Window)
• If Busy,– Wait till channel is idle. Then choose a random
number between 0 and CW and start a back-off timer for proportional amount of time
– If transmissions within back-off amount of time, freeze back-off timer and start it once channel becomes idle again
• If Collisions (Control or Data)– Binary exponential increase (doubling) of CW
RTS/CTS Solution of Hidden Terminal Problem
•C knows B is listening to A•Will not attempt to transmit to B
• Can there be collisions?– Control packet collisions (C transmitting RTS at the same time as A)– C does not register B’s CTS – C moves into B’s range after B’s CTS
Basic of IEEE 802.11
IEEE 802.11
• IEEE working group 802.11 formed in 1990– Now the most popular and pervasive Wireless LAN
standard
Basic service set
Access point
Distribution system
• An isolated BSS is called an Independent BSS
• A set of AP’s connected with a LAN are called an Extended BSS
Infrastructure vs. Independent Mode
Infrastructure mode:All communications must be relayedby access point
Independent mode:Nodes communicate directly witheach other
Main Requirements
• Single MAC to support multiple PHYs– Support single and multiple channel PHYs, and
– PHYs with different Medium Sense characteristics
• Should allow overlap of multiple networks in the same area and channel space– Need to be able to share the medium
– Allow re-use of the same medium
• Need to be Robust for Interference– Microwave interferers
– Other un-licensed spectrum users
– Co-channel interference
• Need mechanisms to deal with Hidden Nodes
IEEE 802.11: Network Hierarchy
Layers 1 and 2
Layer
Application
TCP
IP
LLC
MAC
PHY802.11
One MAC and Multiple PHYs
MAC
DS FH IR .11a OFDM
.11b CCK
.11g OFDM
1 & 2 Mbps
5.5 & 11 Mbps
6 ~ 54 Mbps
.11n OFDM
6 ~ 54 Mbps
150 (?) Mbps
2.4 GHz5 GHz
e/h/i/k
Complementary Code Keying
A set of 64 eight-bit code words used to encode data of 5.5 and 11Mbps data rates
Applies sophisticated mathematical formulas to the DSSS codes, permitting the codes to represent a greater volume of information per clock cycle
IEEE 802.11 Wireless MAC
• Distributed and centralized MAC components– Centralized – Point Coordination Function (PCF)
• In infrastructure mode
• Contention-free access protocol with a controller (AP) called a point coordinator within the BSS
– Distributed – Distributed Coordination Function (DCF)• In ad-hoc mode
• DCF is a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocol (distributed contention based protocol)
• Both the DCF and PCF can operate concurrently within the same BSS to provide alternative contention and contention-free periods
PCF in 802.11 MAC
• Its objective is to provide QoS guarantees – E.g. bound the max access delay, bound the minimum
guaranteed txmt rate
• Centralized MAC – infrastructure mode
• Key idea– The AP polls the nodes in its BSS
– A PC (point coordinator) at the AP splits the access time into super frame periods
– A super frame period consists of alternating contention free periods (CFPs) and contention periods (CPs)
– The PC then determines which station transmits at any point in time
DCF in 802.11 MAC
• The AP doesn’t control the medium access
• Use collision avoidance techniques, in conjunction with a (physical or virtual) carrier sense mechanism
• Carrier sense:
– When a node wishes to transmit a packet, it first waits until the channel is idle
• Collision avoidance:
– Once channel becomes idle, the node waits for a randomly chosen duration before attempting to transmit
– Nodes hearing RTS or CTS stay silent for the duration of the corresponding transmission
DCF Illustration
• Before a node transmits, it listens for activity on the network
• If medium is busy, node waits to transmit• After medium is clear, don't immediately start
transmitting...• Otherwise all nodes would start talking at the same time!• Instead, delay a random amount of time (random backoff)
Interframe space (IFS)
Sender
Receiver
Time
Reliability and Hidden Terminal
• Uses ACK to achieve reliability– Receiver sends ACK after each successful transmission
– Sender will retransmit if no ACK is heard, after waiting for a random interval
• Uses RTS-CTS exchange to avoid hidden terminal problem– Network Allocation Vector (NAV): each message includes
length of time other nodes must wait to send
– Any node receiving the RTS cannot transmit for the duration of the transfer
– Any node overhearing a CTS cannot transmit for the duration of the transfer
C FA B EDRTS
RTS = Request-to-Send
RTS-CTS and NAV
Pretending a circular range
C FA B EDRTS
RTS = Request-to-Send
RTS-CTS and NAV
NAV = 10
NAV = remaining duration to keep quiet
C FA B EDCTS
CTS = Clear-to-Send
RTS-CTS and NAV
C FA B EDCTS
CTS = Clear-to-Send
RTS-CTS and NAV
NAV = 8
C FA B EDDATA
•DATA packet follows CTS•Successful data reception acknowledged using ACK
RTS-CTS and NAV
C FA B EDACK
RTS-CTS and NAV
C FA B EDACK
RTS-CTS and NAV
Reserved area
Backoff Interval
• 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
• When backoff interval reaches 0, transmit RTS
B1 = 25
B2 = 20
Backoff Interval Example
data
wait
B1 = 5
B2 = 15
data
wait
B1 and B2 are backoff intervals at nodes 1 and 2
cw = 31
B2 = 10
Backoff Interval (Cont.)
• The time spent counting down backoff intervals is a part of MAC overhead
• Contention windows, cw, selection– Large cw leads to large backoff intervals and can result in
larger overhead
– Small cw leads to a larger number of collisions (two nodes count down to 0 simultaneously)
• Need mechanism to manage contention– The number of nodes attempting to transmit simultaneously
may change with time
– IEEE 802.11 DCF: cw is chosen dynamically depending on collision occurrence
Binary Exponential Backoff in DCF
• cw follows a sawtooth curve• When a node fails to receive CTS in
response to its RTS, it increases the cw– cw is doubled (up to an upper bound)
• When a node successfully completes a data transfer, it restores cw to cwmin
• Potential inefficiency in 802.11– Backoff interval should be chosen
appropriately for efficiency– Backoff interval with 802.11 far from optimum
Other Topics
• Energy consumption– Power saving– Power control
• Directional antenna• User diversity
Power Management
• Mobile devices are battery powered
• Current LAN protocols assume stations are always ready to receive– Idle receive state dominates LAN adapter power consumption
over time
• How can we power off during idle periods, yet maintain an active session?
• 802.11 Power Management protocol– Allows transceiver to be off as much as possible
– Is transparent to existing protocols
– Is flexible to support different applications
• Possible to trade off throughput for battery life
Power Management Approach
• Allow idle stations to go to sleep– Station’s power save mode stored in AP
• APs buffer packets for sleeping stations– AP announces which stations have frames buffered
– Traffic Indication Map (TIM) sent with every Beacon
• Power saving stations wake up periodically– Listen for Beacons
• Time Synchronization Function (TSF) assures AP and power save stations are synchronized– Stations will wake up to hear a Beacon
– Synchronization allows extreme low power operation
Infrastructure Power Management
• Broadcast frames are also buffered in AP– Broadcasts/multicasts are only sent after DTIM– DTIM interval is a multiple of TIM interval
• Stations wake up prior to an expected (D)TIM• If TIM indicates frame buffered
– Station sends PS-Poll and stays awake to receive data
TIM
TIM-Interval
Time-axis
Busy Medium
AP activityTIM TIM TIM DTIMDTIM
DTIM interval
BroadcastBroadcast
PS Station
Tx operationPS-Poll
Power Control
• Power control has (at least) two potential benefits– Reduced interference & increased spatial reuse– Energy saving
Basic of Power Control
• When C transmits to D at a high power level, B cannot receive A’s transmission due to interference from C
A B C D
Basic of Power Control (Cont.)
• If C reduces transmit power, it can still communicate with D– Reduces energy consumption at node C– Allows B to receive A’s transmission (spatial reuse)
A B C D
More on Power Control
• Received power level is proportional to inverse square of distance
• If power control is utilized, potential energy savings
• Shorter hops typically preferred for energy consumption
• Transmit to C from A via B, instead of directly from A to C
A
B
C×
Tradeoff between energy consumption and overall throughput
• Directional antenna gain higher than omni-directional antenna gain
• Increased range by limiting energy waste in unnecessary directions
• Number of hops to a destination may be smaller• Reach a given neighbor with less power than
omni-directional transmission
Directional Antennas
Directional Antennas (Cont.)
• Potential benefits– Higher spatial reuse– Larger range (for given transmit power)– Reduction in energy consumption
• Need new MAC protocols to best utilize directional antennas– Deafness
• Unable to hear RTS because directional in some other directions
– New hidden terminal problem• Directional CTSs may not be heard (the listener node in
omni mode/directional in another direction
User Diversity
• Location dependent and bursty errors– For the same wireless channel, a mobile station might
experience a clean channel while another might experience high error rates (not correlated)
APclient
client
client
client
• Negative efforts of channel variations– Head-of-Line blocking– False link breakage, thus
unnecessary rerouting– Rate adaptation or power control
try to mitigate rather than utilize channel variations (inefficient)
bestcurrent
User Diversity (Cont.)
• Using multiuser diversity– Node access the instantaneous channel condition of
each of multiple receivers – Select the receiver whose channel condition is the
best
• Improve overall network throughput
• How to leverage such diversity among multiple users, while maintain the user fairness– Both short-term and long-term fairness important