Issues in ad-hoc networks

1. Issues in ad-hoc networks Miguel Sanchez Nov-2000

2. Contents • Introduction to ad-hoc networks • Media access control in wireless networks • Routing in ad-hoc networks • Wireless Local Area Network standards • Research project

3. What an ad-hoc networks is? • It is a network made of wireless nodes • No network infrastructure is present • Transmission range is limited, not all the destinations can be reached in one hop • Multi-hop forwarding is needed to provide any-to-any communication • Nodes act as both, end-points and routers

4. What an ad-hoc networks is for? • Cellular networks are infrastructure-based networks • When there is no network available, ad-hoc networks can be of help: • Disaster recovery (floods, earthquakes, …) • Space exploration (multiple probes) • Network of sensors (NOS) • Even if a network is available: • Meeting rooms (education or business) • Military applications (armies communication)

5. Why not to have a longer transmission range? • A long transmission range can avoid the multi-hop routing • By using low power transmissions a double benefit is obtained: • Reduced power consumption (batteries lasting longer) • Increased spectrum reuse (more network throughput)

6. Contents • Introduction to ad-hoc networks • Media access control in wireless networks • Routing in ad-hoc networks • Wireless Local Area Network standards • Research project

7. Media access in wireless networks • Dashed circle is a transmitting node • Arrows show receiving nodes • Dashed-line big circle is the transmission range • Nodes than can hear a given transmitter are called neighbors of it

8. Carrier sense algorithms suffer the hidden terminal problem Some protocols establish a reservation handshake before actually sending the data to avoid this problem New MAC problems

9. RTS/CTS reservation dialog • Unicast only • Transmitter sends a RTS packet to the destination node • Destination node acknowledges by sending back a CTS packet • If CTS is successfully received, then transmitter sends a data packet. If not, backs off and it retries later

10. Leading idea of our research at the MAC level • We question the current approach of letting the hardware features to decide • We study both, static and dynamic settings for the transmission range: • Optimal transmission range • Adaptive power control

11. Transmission range adjustment • We have proposed an heuristic to adjust the transmission range • With this method, we can estimate a transmission range for best throughput results • In this work we obtain a static value for the transmission range

12. Our power control algorithm • RTS/CTS dialog is a reservation mechanism • This dialog always reserve the same area for each transmission • Our algorithm uses a variable transmission range and to adapt it depending on the distance to the destination node • Each node can use a different power level for each transmission

13. Contents • Introduction to ad-hoc networks • Media access control in wireless networks • Routing in ad-hoc networks • Wireless Local Area Network standards • Research project

14. IETF MANET working group • The primary focus of the working group is to develop and evolve MANET routing specification(s) and introduce them to the Internet Standards track • Several proposals are available for end-to-end and multicast traffic

15. Ad-hoc routing 101 • Topology changes can happen anytime and at a high pace • Two basic approaches: • Proactive (i.e. table-driven routing) • Reactive (i.e. source routing) • Some hybrid algorithms, too • Mobility scenario not defined • Multicast protocols also proposed

16. More ad-hoc routing problems • Scalability, or lack of. Some protocols work ok for tens of nodes but they degrade for hundreds or thousands of nodes • Providing QoS in such an scenario seems to be quite a challenge (some proposals exist) • Power-conserving routing is also under study (there are some proposals too)

17. Contents • Introduction to ad-hoc networks • Media access control in wireless networks • Routing in ad-hoc networks • Wireless Local Area Network standards • Research project

18. IEEE 802.11 • This working group developed the 1 and 2 Mbps specification • Now a new revision, called 802.11b, is supported in some new devices, raising the speed to 11 Mbps • Two modes of operation (alternative): • Access point based (infrastructure) • Ad-hoc (infrastructure-less)

19. Wireless LANs • It is a commercial reality • It is being used as an installation-free setup for some offices • Access points provide the necessary connectivity to the wired backbone • Ad-hoc mode is supported but not very often used (user has to choose the operating mode when installing the driver)

20. Contents • Introduction to ad-hoc networks • Media access control in wireless networks • Routing in ad-hoc networks • Wireless Local Area Network standards • Research project

21. Research project • Operational mode is alternative. Or it is access point based or it is ad-hoc • A combined mode will allow better coverage with less access points • A mobile node could act as a “helper node” to nearby nodes unable to reach the access point

22. Some extensions • We envision an scenario where a convergence of cellular and ad-hoc networks can bring some benefits for the user • This new scenario also brings the necessity of rethink billing systems • Multiple access technologies can be present in smarter, new mobile devices, that could use some of these mobile extensions