Winter 2004 UCSC CMPE252B1 CMPE 257: Wireless and Mobile Networking SET 3p: Medium Access Control Protocols

Spring 2005CMPE257 UCSC2 MAC Protocol Topics n Time synchronization n Power saving

Spring 2005CMPE257 UCSC3 IEEE Time Sync. Function n Bandwidth: p Up to 54 Mbps p Good for a few hundred nodes n Time Synchronization Function (TSF) p Not scalable p How to fix it? n Note: Only single-hop ad hoc networks are dealt with here ([HL02]).

Spring 2005CMPE257 UCSC4 IEEE TSF n Time divided into beacon intervals, each containing a beacon generation window. n Each station: p Waits for a random number of slots; p transmits a beacon (if no one else has done so). n Beacon: several slots in length. window beacon interval

Spring 2005CMPE257 UCSC5 IEEE TSF n Beacon contains a timestamp. On receiving a beacon, STA adopts beacon ’ s timing if T(beacon) > T(STA). n Clocks move only forward. faster adopts 12:0112:00 slower not adopts 12:01 12:02 12:01

Spring 2005CMPE257 UCSC6 Problems with ’ s TSF n Faster clocks synchronize slower clocks. n Equal opportunity for nodes to generate beacons. 1:10 1:11 1:12 1:13 1:14 1:15 1:13 1:14 1:15 1:16 1:17 1:18 1:19 1:21 1:23 1:18 1:19 1:21 1: :21 1:22 1:23 1:25 1:28 1:31 1:23 1:25 1:28 1:31

Spring 2005CMPE257 UCSC7 The Out-of-Sync Problem When number of stations increases Fastest station sends beacons less frequently Stations out of synchronization

Spring 2005CMPE257 UCSC8 Two Types of Out-of-Sync Fastest-station out-of-sync – fastest station is out of sync with all others. k-global out-of-sync – k percent of links are out of sync. n Questions: How often? For how long?

Spring 2005CMPE257 UCSC9 Fastest-station out-of-sync (1) n Clock1 and Clock2: two fastest clocks n d = their difference in accuracy n T = length of beacon interval (0.1 sec.) n Clock drift: d*T per beacon interval. n In  /(d*T) intervals, fastest-station will be out of sync with all others. T

Spring 2005CMPE257 UCSC10 Fastest-station out-of-sync (2) n n = number of stations. w = size of beacon window. P ’ (n,w) = prob(fastest station wins beacon contention)

Spring 2005CMPE257 UCSC11 Prob(Fastest station sends a beacon)

Spring 2005CMPE257 UCSC12 Fastest-station out-of-sync (3) n H = # beacon intervals with F.S. out-of- sync. n L = # beacon intervals between async periods. n E(R) = E(H)/[E(H)+E(L)] = percent of time in which the fastest station is out of sync with all others. LH

Spring 2005CMPE257 UCSC13 How often does fastest-node get out of sync with others?

Spring 2005CMPE257 UCSC14 Percentage of time fastest station out of sync with all others a 54 Mbps ∆ = 224  s d = 0.003%

Spring 2005CMPE257 UCSC15 How often does 25%-async occur?

Spring 2005CMPE257 UCSC16 Percentage of time with 25 percent of links out-of-sync a 54 Mbps ∆ = 224  s d = 0.01%

Spring 2005CMPE257 UCSC17 How to fix it? n Desired properties: simple, efficient, and compatible with current TSF. n Causes of out-of-sync p Unidirectional clocks p Equal beacon opportunity p Single beacon per interval p Beacon contention (collision)

Spring 2005CMPE257 UCSC18 Improve fastest station ’ s chance n Let the fastest station contend for beacon generation more frequently than others.

Spring 2005CMPE257 UCSC19 Adaptive Clock Sync Protocol n Station x participates in beacon contention once every C(x) intervals. n Initially, C(x) =1. Always, 1 < C(x) < Cmax. n Dynamically adjust C(x): x faster C(x) +1 x slower C(x) -1

Spring 2005CMPE257 UCSC20 Once the protocol converges Fastest station, C(x) =1 Other stations, C(x) = Cmax (Cmax= ?)

Spring 2005CMPE257 UCSC21 What if the fastest node leaves the IBSS? n The previously second fastest now becomes the fastest. Its C(x) will decrease to 1.

Spring 2005CMPE257 UCSC22 What if a new fastest node enters the IBSS? n The previously fastest now no longer the fastest. Its C(x) will increase to Cmax.

Spring 2005CMPE257 UCSC23 Performance n Performance of TSFPerformance of TSF n ATSP Performance of ATSPPerformance of ATSP n TATSP Performance of Modified TSFPerformance of Modified TSF

Spring 2005CMPE257 UCSC24 Modified TSF n Divide stations into three groups: p Group 1: C(x) = Cmax1 = 1 p Group 2: C(x) = Cmax2 = a small number p Group 3: C(x) = Cmax3 = a large number

Spring 2005CMPE257 UCSC25 Performance of TSF

Spring 2005CMPE257 UCSC26 Performance of ATSP

Spring 2005CMPE257 UCSC27 Performance of Modified TSF

Spring 2005CMPE257 UCSC28 Summary n Showed: the IEEE Timing Sync Function (TSF) is not scalable. n Proposed: a simple remedy compatible with the current TFS. n Choice of Cmax?

Spring 2005CMPE257 UCSC29 What ’ s Next? n IBSS: single-hop n MANET: multi-hop transmission range

Spring 2005CMPE257 UCSC30 Comments n Need simulations with data traffic p Some data transmissions may go beyond the Target Beacon Transmission Time (TBTT) n More realistic analysis p Nodes may be still in defer state when in beacon window time: independent, uniform assumption doesn’t hold.

Spring 2005CMPE257 UCSC31 Power Saving Protocols n Various aspects of solution for saving power p Transmission power control p Power aware routing p Low-power mode n Power saving modes in IEEE p Active mode p Power saving mode (PS) p Protocols under Infrastructure and ad hoc network are different

Spring 2005CMPE257 UCSC32 Power Saving at MAC Layer awake sleep Beacon window ATIM window Beacon interval

Spring 2005CMPE257 UCSC33 Challenges n MANET (Mobile ad hoc networks) p Multi-hop, unpredictable mobility, no plug-in power, no clock synchronization p Clock synchronization r Radio interference r Variable packet delay (unpredictable mobility) r Lack of central control p Neighbor discovery r Because PS host will reduce its transmitting/receiving activity r Routing problem p Network partitioning and merging

Spring 2005CMPE257 UCSC34 Design guidelines n More beacon  A PS host should not inhibit its beacon in ATIM window even if it has heard other beacons  Inaccurate-neighbor problem prevention  Multiple beacon in a ATIM window n Overlapping Awake interval  No clock synchronization  Overlapping of Wake-up pattern of two PS host n Wake-up prediction  PS host’s wake-up pattern based on their time difference

Spring 2005CMPE257 UCSC35 Infrastructure and Ad Hoc Protocols n Access Point (AP) monitors each host n PS mode host wakes up periodically for incoming packet from AP. n Periodic beacon frames. n In each beacon frame, a Traffic Indication Map(TIM) will be delivered, which contains ID’s of those PS host with buffered unicast packet in the AP. n PS hosts wakes up periodically n ATIM window : short interval that PS hosts wake up n In the beginning of each ATIM window, each mobile host will contend to transmit a beacon frame. n Successful beacon synchronizes mobile host’s clock and prevents other hosts from sending their beacon

Spring 2005CMPE257 UCSC36 Dominating-awake-interval n PS host stay awake sufficiently long so as to ensure that neighboring host can know each other. n Dominating awake property p AW >= BI/2 + BW n Alternatively labeled odd and even sequence of beacon intervals

Spring 2005CMPE257 UCSC37 Periodically–fully-awake-interval n Two types of beacon interval p Low power intervals r Length of active window is reduced to minimum r Starts with an active window which contains a beacon window followed by a MTIM window AW = BW + MW, in the rest of the time, the host can go to the sleep mode. p Fully awake intervals r Length of active window is extended to the maximum r Arrives periodically, interleaved between low power intervals r AW = BI, rest of the time must remain awake n Suitable for slowly mobile environments

Spring 2005CMPE257 UCSC38 Periodically–fully-awake-interval Rest of active window

Spring 2005CMPE257 UCSC39 Properties n Each PS host’s beacon window overlaps with any neighbor’s fully-awake intervals in every T beacon intervals. n More power saving than previous protocol when T > 2. n Remark: Every node chooses the same T.

Spring 2005CMPE257 UCSC40 Quorum-based n Quorum p A set of identities one need to obtain before doing sth. p Two quorums have non-empty intersection to ensure atomicity

Spring 2005CMPE257 UCSC41 Quorum-based n PS host only needs to send beacon O(1/n) of all the beacon intervals n Quorum interval p Beacon + MTIM, AW = BI n Non quorum intervals p Starts with an MTIM window, after that, host may go to sleep mode, AW=MW p Amount of awaking time is less than 50%, provided n >=4 n Suitable for expensive transmission cost

Spring 2005CMPE257 UCSC42 Communication with PS hosts n Unicast p Predict PS host’s wakeup time and send MTIM packet during that time n Broadcast p Divide them into groups p Hosts within the same group have overlapping MTIM window p Need multiple transmissions

Spring 2005CMPE257 UCSC43 Summary n Three power saving protocol for asynchronous MANETs: p Dominating awake interval r Most power consumption, r Lowest neighbor discovery time. p Periodically-fully-awake interval r Balance both power consumption and neighbor discovery time. p Quorum based r The most power saving r Longest neighbor discovery time

Spring 2005CMPE257 UCSC44 Comments on Simulations n A custom-built simulator p Many details omitted n Carrier sensing and transmission range n Star-topology n Packet delivery delay (tradeoff?)

Spring 2005CMPE257 UCSC45 Future Work n More MANET scenarios n Adaptive beacon intervals?

Spring 2005CMPE257 UCSC46 References  [HL02] Lifei Huang and Ten-Hwang Lai, On the Scalability of IEEE Ad Hoc Networks, in ACM MobiHoc  [THH02] Tseng et al., Power-Saving Protocols for IEEE Based Multi- Hop Ad Hoc Networks, in IEEE INFOCOM 2002.

Spring 2005CMPE257 UCSC47 Acknowledgments n Parts of the presentation are adapted from the following sources: p Moses Pawar, USC, 04/slides/Mac_protocols.ppt 04/slides/Mac_protocols.ppt p Ten H. Lai, Ohio State University r scalibility.pdf scalibility.pdf r Power%20Saving.ppt Power%20Saving.ppt