1 Introduction to Wireless Networks Michalis Faloutsos

2 What is an ad hoc network A collection of nodes that can communicate with each other without the use of existing infrastructure Each node is a sender, a receiver, and a relay There are no “special nodes” (in principal) No specialized routers, no DNS servers Nodes can be static or mobile Can be thought of us: peer-to-peer communication

3 Example: Ad hoc network Nodes have power range Communication happens between nodes within range

4 Some Introductory Things The MAC layer Typical Simulations The routing protocols TCP and ad hoc networks

5 What Is Different Here? Broadcasts of nodes can “overlap” -> collision How do we handle this? A MAC layer protocol could be the answer If one node broadcasts, neighbors keeps quite Thus, nearby nodes compete for air time This is called contention

6 Contention in ad hoc networks A major difference with wireline networks Air-time is the critical resource Fact 1: connections that cross vertically interfere Fact 2: connections that do not share nodes interfere Fact 3: a single connection with itself interferes!

7 Example of contention Yellow connection bothers pink connection Yellow bothers itself When A-E is active E-F is silent F-G is silent (is it?) A B C D E G F H

8 The MAC protocol Introduced to reduce collisions Sender sends Request To Send (RTS): ask permission Case A: Receiver gives permission Clear To Send (CTS) Sender sends Data Receiver sends ACK, if received correctly Case B: Receiver does not respond Sender waits, times out, exponential back-off, and tries again A D C B RTS CTS RTS CTS

9 Why is this necessary? A: RTS, and B replies with a CTS C hears RTS and avoids sending anything C could have been near B (not shown here) D hears CTS so it does not send anything to B A D C B RTS CTS

10 Some numbers for Typical radius of power-range: 250m Interference range: 500m At 500m one can not hear, but they are bothered! RTS packet 40 bytes CTS and ACK 39 bytes MAC header is 47 bytes

11 Typical Simulation Environment A 2-dimensional rectangle Fixed number of nodes Static: uniformly distributed Dynamic: way-point model Pick location, move with speed v, pause Power range: fixed or variable Sender-receivers uniformly distributed

12 Various Communication Paradigms Broadcasting: one nodes reaches everybody Multicasting: One node reaches some nodes Anycasting: One node reaches a subset of some target nodes (one) Application Layer protocols and overlays Applications like peer-to-peer

13 Layered and Cross Layer Protocols Layering: Modular Isolates details of each layer Cross Layer: Information of other layers is used in decisions Pros: efficiency Cons: deployability and compatibility application transport Network Link physical application transport Network Link physical

14 Example: application layer multicast Source unicasts data to some destinations Destinations unicast data to others Pros: easy to deploy, no need to change network layer Cons: not as efficient

15 Example: application layer multicast II Members need to make multiple copies It would happened anyway Link A B gets two packets Similarly in wireline multicast Node B sends and receives packet 4 times s A B

16 Some major assumptions The way-point model is a good model for mobility Homogeneity is a good assumption Links are bidirectional: I hear U, U hear me Uniform distribution of location is good will be used at the MAC layer Space is two dimensional

17 Some “proven” claims The smallest the range, the better the throughput Mobility increases the capacity of a network A node should aim for 6-7 neighbors We can challenge these claims

18 End of Introduction Resources: Google: Citeseer: C. Perkins book: Ad Hoc Networking

19 Modeling Contention (based on Nandagopal et al MOBICOM 200) Seminar 260 Michalis Faloutsos

20 Problem: Find Hotspot in a graph Given a graph and source- destinations Where is the bottleneck? Or how much bandwidth can each connection have?

21 Solution: Find areas of contention Intuition Step 1: create graph “range connectivity” Step 2: create graph of flows (route flows on graph) Step 3: find which flows contend for airtime (find areas where only one flow can be active)

22 Clarification: interference When C->D A-B, B-C, D-E, E-F can not be active!

23 Clarification: Dual graph Each edge becomes a node in G’ An edge exists between two nodes in G’ iff the edges have a common node edge Interference

24 In more detail 1.Find topological graph 2.Find dual graph: edges -> nodes 3.Consider “interference” between non adjacent edges 4.Find Maximal cliques

25 Contention Modeling: conclusion Elegant approaches and tools are available The realism of the modeling must be considered Do not over-generalize results when heavy assumptions have been made

26 Considering Connections If we know which pairs want to communicate, we consider only these flows as contenders Routing could be independent of contention of an area If routing is contention aware, then we have a closed loop system: Routing -> contention -> routing -> ….

27 Question: what is optimal routing? Given a graph, source-destination pairs How do I route the flows to minimize contention? What happens if I do not know the connections ahead of time (online version of problem)?

28 Modeling the Physical Channel There are several ways depending on degree of accuracy Binary, simplified: in one prange you communication In two prange you interfere but do not communicate

29 Considering the power: path loss P_R: received power P_t: transmission power d: distance alpha: constant

30 The physical model Node Y hears node i, iff received power of i is above a threshold beta Needs to rise above noise and other transmissions Pi Noise + SUM_k Pk =

31 A more optimistic channel model Node Y hears i, if i is the “loudest” Interference from other nodes: per pair comparison Delta>0 is a protocol specified “guard zone”

32 Channel Modeling: Conclusion Several different models You need to find and justify the model you use

33 Topology Control We cannot always control the mobility We can control the network topology Power control Deciding to ignore particular neighbors From a given graph G of possible connections we keep a subset G’ of these connections What is good topology? …

34 Topology Control Metrics What is good topology? Energy efficiency, Robustness to mobility, Throughput - capacity

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36 Topics Of Interest - Wireless Characterizing the ad hoc topology A snapshot Its evolution Mobility Realistic mobility models Effect of topology/mobility in Routing Multicasting in ad hoc networks

37 Topics of Interest - Wireline Generating a realistic directed graph Reducing a real (directed) graph to a small realistic Survey on graph generation models Measuring the Internet topology Router level AS level

38 We need to model contention First the obvious Adjacent edges Second, one edge away, considering RTS CTS Third, interference (500m instead of 250m) Modeling issue

39 Typical “Errors” Mobility: too slow or too fast Mobility speed may not be the expected Homogeneity may “hide” issues Few nodes are responsible for most traffic Some spots are more popular than others Power range is too large for the area Ie radius 250m, a grid of 1Km -> one broadcast covers “half” the area

40 What’s the problem? There is no systematic way to model and simulate such networks No clue what are the right assumptions Not sure how the assumptions affect the results

41 Consequences Simulation results are Meaningless Unrepeatable Incomparable between different analysis Prone to manipulation Claim: give me any statement, I can create simulations to prove it

42 What Will We Do Here? Identify assumptions Some of them are subtle Characterize the scenarios Study their effect on the performance results