1 / 28

Background and Motivation

Medium Access Control Protocols Using Directional Antennas in Ad Hoc Networks CIS 888 Prof. Anish Arora The Ohio State University.

rafe
Download Presentation

Background and Motivation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Medium Access Control Protocols Using Directional Antennas in Ad Hoc NetworksCIS 888Prof. Anish AroraThe Ohio State University

  2. A paper by Young-Bae Ko, Vinaychandra Shankarkumar and Nitin.H.VaidyaDepartment of Computer ScienceTexas A&M universityhttp://www.cs.tamu.edu/faculty/vaidya/papers/mobile-computing/infocom00.ps

  3. Background and Motivation • Wireless mobile networks • Omnidirectional antennas • Wastage of network capacity • Directional antennas • Improve network capacity • Improve routing performance • Physical size limitations

  4. Related work: • Packet Radio Networks • Slotted ALOHA using RTDMA • SDMA for performance improvement • Mobile Broadband Systems • Dynamic slot assignment protocol RTDMA – Random Time Division Multiple Access SDMA – Space Division Multiple Access (simultaneous multiple receptions)

  5. Network Model: • Shared wireless channel • Multiple Directional antennas • Interference assumption • Hidden terminal problem • Fixed transmission range • Unidirectional transmission

  6. IEEE 802.11 MAC Protocol: • RTS/CTS mechanism • Sender Broadcasts RTS packet • Intended Receiver replies with CTS packet • Sender transmits data packet • Receiver sends an ACK RTS – Request To Send CTS – Clear To Send

  7. RTS/CTS Mechanism in 802.11 A B C D E RTS RTS CTS CTS DATA DATA ACK ACK

  8. RTS/CTS (Contd…) • RTS and CTS contain proposed duration of data transmission • All in-range nodes MUST wait for this duration before transmitting • Adv – Elimination of Hidden terminals • Disadv – Wastage of network capacity (D cannot send anything to E)

  9. D-MAC Schemes (Overview) • Similar to IEEE 802.11 – only on a per antenna basis. • 802.11 • if node N is aware of an on-going transmission, N cannot send or receive itself • D-MAC • if antenna A at node N is aware, N cannot send or receive using antenna T.

  10. Contd… • Even if one antenna is blocked, the node may transmit using unblocked antennas. • Leads to performance enhancement • Can be used omnidirectionally as well

  11. Scheme 1 – Using DRTS packets • Directional RTS packets • Omnidirectional CTS packets • DRTS – sender’s location • OCTS – sender’s and receiver’s locations • Data packet and ACK sent using Directional antennas

  12. Scheme 1 (contd…) • Other nodes CAN transmit A B  C xD  E A  B C  D xE • All DRTS may not get an OCTS reply (D & E in the above scenarios cannot send OCTS if anyone sends them a DRTS because one of their antennas is blocked) • Control packets may collide A B- -> C  D E

  13. Scheme 2 – Using DRTS/ORTS • Send either DRTS or ORTS based on rule • “if all D-antennas are unblocked, send ORTS but if any D-antenna is blocked, send DRTS” A  B  C  D E • C sends ORTS but B can only send DRTS • Reduces, not eliminates control packet collisions

  14. Performance evaluation • Modified ns-2 simulator to include D-antennas (90°) and location information • Simulation model • 5 x 5 mesh – 200m apart • 250m range for each node • 2Mbps wireless link bandwidth

  15. Network Topology 5 10 15 20 25 4 9 14 19 24 3 8 13 18 23 2 7 12 17 22 21 1 6 11 16

  16. Case 1: Use of DRTS allows simultaneous transmissions hence throughput for Scheme 1 is better than 802.11 Fairness is also much better in Scheme 1

  17. Case 2: Directions of data transfers differ, hence fewer collisions Best case scenario for D-antennas

  18. Case 3: Scheme 2 performs better than Scheme 1 because probability of control packet collisions decreases.

  19. Case 4: Border connections have much higher throughput Percentage performance enhancement of DMAC not so high because of increase in number of connections Reasonable fairness (esp. to conn. 9) in DMAC schemes

  20. Case 5:

  21. Case 5 (contd…) • DMAC 1 outperforms DMAC 2 • The use of ORTS in scheme 2 reduces the possibility of simultaneous transmissions by neighbouring nodes • Trade-off between probability of collisions and loss of simultaneous transmissions

  22. Case 5(contd…) Scheme 1 Scheme 2 E F E F DRTS C D C D ORTS A B A B DRTS ORTS

  23. Optimization: Using DWTS • DWTS – Directional Wait-To-Send • Aim: to avoid DRTS retransmissions A B C D E DRTS(B) DRTS(E) OCTS(B,C) OCTS(B,C) DATA ACK

  24. DWTS (contd…) • DWTS – short control packet, includes time-to-wait before RTS retransmission A B C D E DRTS(B) DRTS(E) OCTS(B,C) OCTS(B,C) DWTS (D) DATA ACK DRTS(E)

  25. Conflict-Free ACK • 802.11 – Immediate ACK for reliability • Minimal ACK collisions due to reserved transmission range • D-MAC – No guarantee on ACK collisions • Possible solutions – • Use separate channels for DATA/ACK & RTS/CTS • Use RTS/CTS for ACK packets

  26. Location Information • DRTS – accurate node locations required • Hard to achieve in mobile nodes • Solution: • If location unknown - send ORTS (no loss of correctness) • If location known – send DRTS

  27. (Contd…) • Stale location data: • Include location information in RTS/CTS • Set a threshold for DRTS transmissions • If no response, switch to ORTS

  28. Conclusions: • Current MAC protocols – wasted bandwidth • D-MAC – utilizes directional transmissions • Scheme 1 – DRTS/OCTS • Scheme 2 – DRTS, ORTS / OCTS • Optimization using DWTS • D-MAC outperforms 802.11 by allowing simultaneous transmissions

More Related