Cautionary Aspects of Cross Layer Design: Context, Architecture and Interactions

Transcription

1 Cationary Aspects of Cross Layer Design: Context, Architectre and Interactions Vikas Kawadia and P. R. Kmar Dept. of Electrical and Compter Engineering, and Coordinated Science Lab University of Illinois, Urbana-Champaign Phone , (Fax) Web ICC 2003, Panel on Defining Cross-Layer Design for Wireless Networking, Anchorage, Alaska, May 14, 2003

2 Themes Setting the context Importance of Architectre Interactions and the Law of Unintended Conseqences Conclding remarks

3 Wireless networks don t come with links They are formed by nodes with radios There is no a priori notion of links Nodes simply radiate energy

4 Nodes can cooperate in complex ways Nodes in Grop A cancel interference of Grop B at Grop C C X A B while Nodes in Grop D amplify and forward packets from Grop E to Grop F F D E while. SINR = Signal One strategy: Decode and forward Interference + Noise Instead, why not: Amplify and Forward One strategy: Increase Signal for Receiver Instead, why not: Redce Interference at Receiver

5 How shold nodes cooperate? Some obvios choices Shold nodes relay packets? Shold they amplify and forward? Or shold they decode and forward? Shold they cancel interference for other nodes? Or shold they boost each other s signals? Shold nodes simltaneosly broadcast to a grop of nodes? Shold those nodes then cooperatively broadcast to others? What power shold they se for any operation? Or shold they se mch more sophisticated nthoght of strategies? There are more things in heaven and earth, Horatio, Than are dreamt of in yor philosophy. Hamlet

6 On the optimality of the crrent proposal for ad hoc networks Theorem: Xie and Kmar (2002) Mlti-hop transport where» A packet is flly decoded at each hop» All interference from all other nodes is simply treated as noise Is optimal p to a known mltiplicative constant. ( Interference + Noise Interference + Noise Interference + Noise The crrent proposal Interferenc e + Noise Properties Simple receivers Simple mlti-hop packet relaying scheme Simple abstraction of wires in space This choice for the mode of operation gives rise to Roting problem Media access control problem Power control problem

7 The importance of architectre Sccess of Internet is de to its architectre Hierarchy of layers Peer-to-peer protocols Allows plg-and-play Longevity Important for proliferation of technology Application Layer Presentation Layer Session Layer Transport Layer Network Layer Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Data Link Layer Physical Layer Physical Layer Sccess of serial compting von Nemann bridge (Valiant) Hardware designers and software designers need only to conform to abstractions of each other Control system paradigm Plant and controller separation Hardware Plant Controller Software

8 Digital commnication Shannon s architectre Sorce code (Compression) Encode for the channel Channel Decode Sorce decode (Decompression)

9 Ever present tension between Performance and Architectre Performance: The short term vision Ptting a link between layer A and layer B can improve performance by x% Conseqences of this approach» Spaghetti code» Not modlar» Not pgradeable» No longevity» High per nit cost _ Vale of a commnication medim = Nmber of adoptees Architectre: The long term view» Mass prodction = Redced cost over long term Tension between Performance and Architectre

10 Interactions The Law of Unintended Conseqences Illstration by examples Example 1: Minimm hop roting over an Adaptive rate MAC Example 2: End-to-end feedback and topology control

11 Example 1

12 Adaptive Rate MAC Idea: Adapt transmission rate according to channel qality Change modlation to get higher rate if channel is good Cold send mltiple packets at higher rates (A sggested scheme) Protocol details RTS/CTS and Broadcast packets sent at lowest rate Receiver measres strength of RTS Commnicates rate to sender in CTS DATA and ACK at that rate

13 One conseqence: Interaction with Min Hop Roting Protocol Most crrent roting protocols are min hop Consider DSDV for example Chooses long hops Bt long hops => low signal strength => low rates

14 Switching off adaptation is better In fact, plain at 11Mbps gives higher end-to-end throghpt

15 Verification by ns2 simlations Channel parameters sch that at a fixed power level (0.28 W): Distance of 0-99m Û11 Mbps m Û 5.5 Mbps m Û 2 Mbps Scheme 1: Adaptive scheme (A sggested scheme) Eqal time share to each rate Send 5 packets at 11Mbs, 3 at 5.5 Mbps or 1 at 2 Mbps Scheme 2: 11 Mbps (no cross layer) Send packets only if distance is < 100m

16 A linear topology 18 nodes eqally spaced in a 1500m long rectangle, 400m wide Adaptive scheme One TCP connection from node 0 to 17, starting at time 50 DSDV roting protocol Carrier sensing disabled

17 Random topology 50 randomly located nodes in 1000x200m 5 simltaneos TCP connections, between distant nodes DSDV roting protocol No carrier sensing Adaptive scheme

18 Example 2

19 End-to-end feedback and topology control Two loops Loop 1: Power control to drive Nmber of 1-hop neighbors to Target_degree Loop 2: Target_degree controlled to increase Average endto-end network throghpt Loosely based on a sggested scheme

20 Adaptation rles Loop 1: Adjst transmit power every 15s, to maintain Ot_degree = Target_degree Loop 2: Target_degree adaptation done every 90s Rles for changing the Target_degree» Repeat change (+1 or -1) if throghpt increased on previos iterate» Reverse action if throghpt decreased on previos iterate» Increase Target_degree if throghpt is zero

21 Details of system tested by simlation 23 nodes in 500x500 m area Each node has 5 discrete power levels Corresponding to ranges of 100m, 140m, 180m, 220m and 250m When the TwoRayGrond propagation model is sed One TCP connection from Node 0 to Node 3 Power level 1 achieves Target_degree 5, even thogh the network is disconnected

22 Simlation topology o 1 2 3

23 Conseqence: Interaction with TCP dynamics Network may oscillate between connectivity and disconnectivity Bad for TCP

24 A random sample of cross layer design proposals Several sggestions for cross-layer design Signal stability based roting Transmit power based roting Battery life based roting Topology control sing transmit power adjstment Topology control sing angle of arrival information Power control by monitoring end-to-end throghpt Power control for energy efficiency Traffic based sleeping strategies TCP modifications for energy efficiency Roting for improving network lifetime Adaptive rate, adaptive power MAC protocols QoS schemes based on roting and MAC parameters What are the conseqences? What interactions are possible? What does the reslting code look like? What is the reslting architectre or lack of it? Longevity? Upkeep?

25 Themes Setting the context for cross-layer design optimization: Infinitde of choices for operating wireless networks: What s best? A Theorem: The good news Importance of Architectre Proliferation Tension between performance and architectre Longevity isse» Short term verss long term perspective Interactions and the Law of Unintended Conseqences Layers can interact Loops can be formed What is the natre of the interactions? Commnity needs to exercise cation before leaping into Cross-layer design

26 Conclding remarks Architectre is important Often neglected Interactions exist Often neglected Need to change the thrst of the work in cross-layer design to a more holistic perspective Design schemes which have no adverse interactions against other layer or cross-layer design now (holistic across layers) Design schemes good against other ftre bright ideas in any cross-layer (holistic across time)

27 To obtain papers Papers can be downloaded from For hard copy send to