Introduction to Wireless Ad Hoc and Sensor Networks: From IEEE to Berkeley Motes Wireless Ten-Hwang Lai Ohio State University

Outline  Wireless LANs  Ad Hoc Networks  IEEE  Bluetooth  Berkeley Motes

Wireless LANs  IEEE  Bluetooth  HiperLan (Europe)

History of IEEE  standard first ratified in LAN emulation 1 & 2 Mbps in the 2.4 GHz band  Two high rate PHY’s ratified in a: 6 to 54 Mbps in the 5 GHz band b: 5.5 and 11 Mbps in the 2.4 GHz band

The Beat Goes On  d: new support for frames  c: support for frames  e: QoS enhancement in MAC  f: Inter Access Point Protocol  g:2.4 GHz extension to 22 Mbps  h:channel selection and power control  i:security enhancement in MAC  j:5 GHz globalization

Can Bluetooth Compete with ?  IEEE already has been widely accepted.  What’s Bluetooth chance of success stacking against ?

BSS  Basic Service Set (BSS) --- a basic LAN  Infrastructure BSS  Independent BSS (Ad Hoc LAN) Access point

ESS  Extended Service Set (ESS) Distributed System

Bluetooth Piconet & Scatternet Master Slaves Master Slaves Master Slaves M S Piconet Scatternet

Comparison of Bluetooth to b ParameterBluetooth802.11b Bandwidth1 Mbps11 Mbps Range10 meters100 meters ProfilesAlmost unlimitedAP, STA Current consumption60mA300mA AudioPCM channelsvoice/ Cable replacementSerial, USB, Audio802.3 Circuit cost (9/2001)$11.00$46.00 Ad hoc networkingmulti-hopsingle-hop

Bluetooth or ?

Can Bluetooth Compete with ?  IEEE already has been widely accepted.  What’s Bluetooth chance of success stacking against ? Answer: ? WLAN Bluetooth-- WPAN

Ad Hoc Networking  BT Scatternet --- multihop?  single hop? Master Slaves Master Slaves M S

BT Scatternet Formation  Problem: design a protocol that given a set of bluetooth nodes organizes the nodes into a scatternet.  Still an interesting research problem.

A Sensor Node Processor Sensor Actuator Network Interface Memory (Application)

Berkeley Mote: a sensor device prototype  Atmel ATMEGA103 4 Mhz 8-bit CPU 128KB Instruction Memory 4KB RAM  RFM TR1000 radio 50 kb/s  Network programming  51-pin connector Analog compare + interrupts

Berkeley DOT Mote  Atmel AVR MHz 8KB of Memory 0.5KB of RAM  Secondary store  Low power radio  Power consumption Active 5mA Standby 5μA

Tightly-Coupled Sensor Array

Artificial Retina

Smart Clothing & Wearable Computing  Smart Underwear  Smart Eyeglasses  Smart Shoes  …

Berkeley Smart Dust  bi-directional communications  sensor: acceleration and ambient light  11.7 mm 3 total circumscribed volume  4.8 mm 3 total displaced volume

Is IEEE Suitable for Supporting Large-Scale Multihop Ad Hoc Networks? Ten-Hwang Lai Ohio State University

Approach to the Problem  Now: single-hop, small-scale  Future: multi-hop, large scale? Single-hop, Small-scale Single-hop, Large-scale Multi-hop, Large-scale

Topics  Is IEEE (single-hop) scalable?  Time sync in multihop ad hoc networks.  Constructing connected dominating sets by way of clock synchronization.

Is IEEE Scalable?

Problem Statement  Can support a large-scale ad hoc network?  Large scale – say, a few hundred nodes

Timers (Clocks)  Timer: 64 bits, ticking in microseconds.  Accuracy: within %, or +100 ppm.  Time synchronization needed for: Frequency hopping Power-saving mode  ∆ = max tolerable difference between clocks.

802.11’s Time Sync Function (I)  Time divided into beacon intervals, each containing a beacon generation window.  Each station: waits for a random number of slots; transmits a beacon if no one else has done so.  Beacon: several slots in length. window beacon interval

802.11’s Time Sync Function (II)  Beacon contains a timestamp.  On receiving a beacon, STA adopts beacon’s timing if T(beacon) > T(STA).  Clocks move only forward. faster adopts 12:0112:00 slower not adopts 12:01 12:02 12:01

Problems with ’s TSF  Faster clocks synchronize slower clocks.  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

The Out-of-Sync Problem When the number of stations increases  More beacon contention  Fastest station send beacons less frequently Stations out of sync

Performance of TSF

How to fix it?  Desired properties: simple, efficient, and compatible with current TSF.  Causes of out-of-sync Unidirectional clocks Equal beacon opportunity Single beacon per interval Beacon contention (collision) 1n1n Prob <

Improve fastest station’s chance  Let the fastest station contend for beacon generation more frequently than others.

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

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

What if the fastest node leaves the IBSS?  The previously second fastest now becomes the fastest. Its C(x) will decrease to 1.

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

Compatible with current TSF  Suppose some nodes do not implement the new protocol.

Performance of Modified TSF

Summary  Showed: the IEEE Timing Sync Function (TSF) is not scalable.  Proposed: a simple remedy compatible with the current TFS.

What’s Next?  IBSS: single-hop  MANET: multihop ?? transmission range

Time Synchronization in based MANET

Out-of-sync problem in MANETs  More sever than in IBSS because of hidden terminals.  Recall: causes of out-of-sync Unidirectional clocks Equal beacon opportunity Single beacon per interval Beacon contention (collision)

Basic Idea  Select a subset of nodes to generate beacons more frequently than the rest.  What subset? fastest node + (connected) dominating set

Dominating Set  A set of nodes that covers the entire graph. connected dominating set

Constructing CDS  Many existing algorithms.  Layer 3 algorithms – useful for routing, useless for our purpose.

A New CDS Algorithm  Embedded in TSF (time sync function)  Node exchanging info via beacons  Overhead: 3 bits per beacon (550 bits)  Assumption: unique fastest clock window beacon interval

DS, Bridges, Covered, Uncovered Nodes DS

Constructing a DS: basic idea  Initially, DS contains a single node.  The fastest node enters DS.  Bridges keep entering DS until no more bridges. DS

Example  Fastest node enters DS.  Bridges keep entering DS until no more bridges.

Design Issue #1  How to recognize the fastest node, bridges, DS nodes, covered nodes, uncovered nodes thru beacons? SA Timestamp Beacon

Design Issue #2  How to minimize the number of bridges entering DS?

Design Issue #3  Cope with topology change and node mobility. B A A B

Design Issue #4  How to merge two subnets? Easy & hard. ?

Design Issue #5: MANET Formation  How to form a MANET from scratch? ?

Another way of MANET formation ?

Assumptions  Formation: MANET initiated by a single node.  Connectivity: MANET remains connected.

Summary of Design Issues 1. How to recognize the fastest node and bridges? 2. How to control the number of bridges entering DS? 3. How to cope with topology change and node mobility? 4. How to merge subnets?

Initialization Rule 1:  Let the starting node enter the DS.

Rule 2:  A node x recognizes itself as the fastest if T(beacons) < T(x) for the last k received beacons.  The fastest enters DS Am I the Fastest? 1:00 12:01 3: : :59 1:33 1:32 1:31 1: :01 1:35

Solution for Design Issue #1  How to recognize fastest node and bridges, DS nodes, covered nodes, uncovered nodes thru beacons? SA Timestamp Beacon

Adding Bridges to DS Rule 3:  In each beacon interval, let bridge i enter DS with probability P(i).  Desired properties of P(i)? DS

Does it construct a CDS? R1. The starting node enters DS. R2. The fastest node enters DS. R3. Each bridge enters DS with a probability. DS, yes. CDS, not necessarily.

How to make it connected?  Gateway: a covered node receiving a beacon from a with a far smaller timing. Rule 4:  Let gateways enter DS. 12:05 12:04 12:03 12:32 12:30 12:20

How fast can gateways be recognized?  Depends on the drift rate difference between fastest node and A.  The higher the drift rate, the easier and faster to recognize gateways. A

Is the resulting DS always connected?  Not necessarily  Not a problem as far as clock sync is concerned.

What if we do need a connected DS?  Is it possible to always construct a CDS using only beacons? Yes.

A problem: entrance only, no exit. R1. The starting node enters DS. R2. The fastest node enters DS. R3. Each bridge enters DS with a probability. R4. Each gateway enters DS.

Exit Rules R1. The starting node enters DS. R2. The fastest node enters DS. R3. Each bridge enters DS with a probability. R4. Each gateway enters DS. R2’. If no longer the fastest, leaves the DS.

Exit Rules R1. The starting node enters DS. R2. The fastest node enters DS. R3. Each bridge enters DS with a probability. R4. Each gateway enters DS. R3’ & R4’. Leaves DS after a random amount of time.

Maximum Clock Drift – b vs. DS-based b DS-based

Summary  Proposed: a DS-based clock sync protocol  By-product: an algorithm for constructing DS.  DS: mostly connected, occasionally not.  What’s next?

Constructing Connected Dominating Sets in Mobile Ad Hoc Networks

Requirements  Nodes exchange info via beacons.  Info added to beacons is fixed in size and as little as possible. SA Time Beacon (~550 bits)

Assumption  MANET remains connected

Constructing Connected DS  Important to recognize gateways.  Use time difference between components to recognize gateways. DS

Dilemma  Time synchronization: Good if every node relays time info. Good if clocks are (almost) identical in accuracy.  Constructing CDS: Good if nodes in DS only relay time info. Good if clocks are quite different in accuracy.

Resolving the Dilemma

What If the Initiator Crashes?

What if MANET gets disconnected?