1. (Phy-Link) Layers in the Brave New World
Nitin Vaidya
University of Illinois at Urbana-Champaign
August 2, 2005
NSF WMPG Workshop at Rutgers

2. Wireless Networks
Look Ma, no links ! C D E F B A

3. Wireless Networks
For the most part, the protocol stacks have pretended that wireless links are equivalent to wired links (with errors)

4. New Reality With denser deployment, per-user capacity degrading
Greater incentive for deploying more complex mechanisms
Simplicity
(“purer” layers)
Performance
(with cross-layer optimization)

5. However, if “raw” capacity improves, the balance may tilt the other way
Congnitive radios, Ultra-wide band ?
Performance
(with cross-layer
optimization)
Simplicity
(“purer” layers)

6. New Reality With denser deployment, greater incentive for
deploying more complex mechanisms
Other stuff
Transport
Network
Most
to lose
Link
Phy

7. New Reality With denser deployment, greater incentive for
deploying more complex mechanisms at lower layers
Other stuff
Rise of “cooperative” wireless networks
Adaptations aplenty
Transport
Network
Most
to lose
Link
Phy

8. Cooperative Wireless Networks

9. Ad Hoc, Mesh, Sensor … Networks
Hosts cooperating to route data or information on multi-hop wireless routes C D E F B A

10. (Without) Cooperative Diversity
Higher layer
F(PB) C B PB A P

11. (With) Cooperative Diversity
F(PB,PD)
Higher layer
f(PC) PD C P PB PC B A

12. Adaptations Aplenty … ?? Power/Rate Control Antenna beamforms Channel
Retransmissions/Coding
… ??

13. Cross-Layer Adaptations
Physical layer can perform many of these adaptations
But need cross-layer interactions to maximize benefits

14. Impact on Protocol Layers

15. The Vanishing Link What is a link anyway ?
Does a “link” span multiple “hops” ?
Higher layer
F(PB,PD)
f(PC) C PD PC B PB A P

16. The Vanishing Link What is a link anyway ?
Diversity muddles the notion of a link R0 D C R1
Rate
R0 < R1 < R2 B R2 A

17. The Vanishing Link Diversity muddles the notion of a link Channel
C0 = C1 = C2 C D C1 B C0 C2 A

18. The Vanishing Link Diversity muddles the notion of a link Beamform
B0 = B1 = B2 C D B1 B B2 B0 A

19. The Vanishing Link Physical layer plays an important role
Often need network-phy interaction for maximal performance
Other stuff
Transport
Network
Link
Phy

20. The Vanishing Boundary
Wired autonomous systems, even if in the same physical space, can be treated as separable
AS 2
AS 2
AS 1
AS 1

21. The Vanishing Boundary
Wireless autonomous systems are not so autonomous
Internal decisions in an AS can affect internal performance of other ASs
AS 2
AS 1

22. Issues Notions of link and boundary imprecise in wireless
Can (Should) we maintain meaningful separation between lower layers ?
Can we make the stack future-proof?
How not to want another clean slate
in another 20 years?

23. On a related note … Will you know an ad hoc network when you see one?F B A

24. On a related note … Will you know an ad hoc network when you see one?AP C AP D E F B A AP

25. Thanks!