3-MAC and 802.11


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  • A and C are “exposed” terminals, i.e. A and C could communicate to their receivers at the same time (since their receivers are far apart), but because A and C can exposed to each other (I.e. can hear each other), one of them needlessly refrains from transmitting.
  • 3
  • DS: direct sequence spread sprectrum FH: frequency hopping spread spectrum: data signal is modulated with a narrowband carrier signal that “hops” in a random but predictable sequence from frequency to frequency as a function of time over a wide band of frequencies. 802.11k: radio resource measurement 802.11r: fast roaming/ fast BSS transition IR CCK OFDM Frequency hopping PHY Direct sequence PHY Infrared light PHY
  • DCF suitable for multi-hop ad hoc networking DCF is a Carrier Sense Multiple Access/Collision Avoidance ( CSMA/CA ) protocol
  • Power Management is important for mobility
  • TSF timer keeps running when stations are sleeping
  • Wireless channel capacities are scarce Efficient utilize the channel capacity is critical
  • 3-MAC and 802.11

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