Distributed-Queue Access for Wireless Ad Hoc Networks

1. Distributed-Queue Access for Wireless Ad Hoc Networks Authors: V. Baiamonte, C. Casetti, C.-F. Chiasserini Dipartimento di Elettronica, Politecnico di Torino, Torino, Italy From: Energy-Efficient Wireless Communications and Networks (EWCN 2004) Yuhe-yi Wang Jan 3, 2006

2. Outline • 802.11 Wireless MAC • 802.6 DQDB • Proposed Method -DQDC • Simulation Result • Conclusion

3. Defines MAC and PHY layers for a LAN with wireless connectivity 802.11 Wireless MAC-Overview

4. 802.11 Architecture • Can be with/without infrastructure support • With • A centralized controller for each cell, AP • Without • Each node is a Station. • Each communicates directly with each other. • Mobile ad-hoc configuration mode

5. 802.11- Two access control services • contention-based • DCF –Distributed Coordination Function • contention-free access control services • PCF -- Point Coordination Function • polling principle • Centralized MAC algorithm

6. DCF • Basic access method of IEEE 802.11 • Specifies the use of CSMA with CA • CSMA/CA • Carrier Senses: • Every node senses the carries before transmitting • If the node detects carrier then defers transmitting • Multiple Access • Transmissions by one node are generally “received” by all other nodes using the medium

7. CSMA/Collision Avoidance • Each node must inform other nodes of an intent to transmit • CSMA/CA With RTS/CTS • When station A wishes to transmit to station B, it sends a Request-to-Send (RTS) packet to B; • Destination + Length of Message • If station Bhears the RTS, and it is not currently deferring, it immediately replies with a Clear-to-Send(CTS) packet to A

8. RTS CTS Data ACK CSMA/CA with RTS/CTS • Any station overhearing an RTS defers all transmissions until some time after the associated CTS packet world have finished • random backoff period • NAV (Network Allocation Vector) • alerts all others to back off for a duration of the transmission 4-way handshake

9. Introduction to Interframe Space Concept • DIFS: Distributed InterFrame Space • SIFS: Short InterFrame Space

10. No NAV=0? Yes Sense the medium (perform physical channel assessment) Random Backoff Time No Medium Idle? Yes Transmit Frame Yes Collision? No Basic Transmission Algorithm

11. slot source Bus A slot sink â â â â â 1 4 3 5 2 á á á á ß slot sink Bus B slot source DQDB (Distributed Queue Dual Bus) • MAC layer specified in IEEE 802.6 standard used in wired MANs. • Can be 30 miles long with 34~155 Mbps • Composed of 2 bus lines with stations attached to both

12. slot source Bus A slot sink â â â â â 1 4 3 5 2 á á á á ß slot sink Bus B slot source Function of DQDB • Transmitting Data • Node acquires slot • Sets header • Copies data into slot • Cells propagate to end of bus • (absorbed by sink) • Copied by intended destination on way

13. Proposed Method-DQDC • Distributed Queue Dual Channel • Propose a MAC protocol for wireless ad hoc networks • Key idea • relies on DCF scheme with DQDB protocol • 2 separate channels: a control and a data • Objective: • to achieve 100% utilization of the data channel, • minimizing the collision probability on it. • Simulation result by ns-2

14. DQDC Overview • Data channel • Data frames, and ACKs • Control channel • STA contend for future access to data channel • Successful STA stored into a virtual distributed queue system • only switch to data channel when at the top of the queue

15. DQDC scheme

16. DQDC in Detail • each STA maintains 2 counters • Access Counter (AC) • Countdown Counter (CC) • Access Counter (AC) • a global counter that • ++ every time a successful contention on control channel • -- each transmission on data channel

17. DQDC in Detail (contd.) • Countdown Counter (CC) • associated with a single data frame waiting to be transmitted • reset to current AC value while winning a control-channel contention. • -- when start of a transmission on data channel • 1: the station is up next for transmits

18. More Than one Frame to Send • How about STA with several frames to send? • Allowed to occupy more entries at once in the virtual queue. • local vector to store each pending AC. • CC will be reset to 0 or set to the value for the next entry.

19. Data Channel Access Scheme

20. Control Channel Access Scheme

21. Low-traffic Contention • AC counting down to 0 • Original counter mechanism doesn’t work for contention • Solution: • post-backoff phase • backoff counter to down count if NO transmission on DATA • decrease to 0 allows to transmit

22. Missed Transmission Opportunities • What if STA that won the contention is turned off? • Can be detected when • idle channel more than SIFS+SIFS following the ACK • AC is > 0 • Solution: • Decrement AC, CC as if the transmission had occurred.

23. Simulation Results-Scenario • Simple Network Scenario: • 4 stations, which communicate in pairs. • Ad Hoc Mode • All within radio proximity • Trans. Rate: • Data: 11 Mbps • Control: 1 Mbps • No multihop trans

24. Simulation Results-Throughput

25. Simulation Results-Packet Delay

26. Simulation Results-Energy per Successful Packet

27. Conclusions • Proposed DQDC: • bases DCF + DQDB (two buses) key idea • Goal: • achieving 100% utilization of the data channel, minimizing the collision probability on it. • DQDC Performance in throughput, delay, energy better than standard DCF • Issue: • Simulation needs to consider more complex scenarios