1. Reliable MAC Layer Multicast in IEEE 802.11Wireless Networks Min-Te Sun, Lifei Huang, Anish Arora, Ten-Hwang Lai Department of Computer and Information Science The Ohio State University ICPP’02 Speaker: Chien-Wen Chang

2. 2003/01/07 Outline  Introduction  Existing Multicast MAC Protocols  Problems with Existing Multicast MAC Protocols  Batch Mode Multicast MAC Protocol  Location Aware Multicast MAC Protocol  Simulation  Conclusion

3. 2003/01/07 Introduction  many random access MAC protocols do not deal directly with multicast  There is no MAC-level recovery on multicast frame  treat broadcast as a special case of multicast  several higher layer protocols rely on reliable MAC layer multicast/broadcast  delivery acceptable quality of service in many applications

4. 2003/01/07 Existing Multicast MAC Protocols(1)  In IEEE 802.11, the RTS/CTS extension is not used for broadcast/multicast  In [19] attempts to extend the IEEE 802.11 broadcast/multicast protocol with RTS/CST  In [20] augments the MAC protocol in [19] with the NAK frame and additional rules  In [21] is introduced to provide a reliable broadcast MAC

5. 2003/01/07 Existing Multicast MAC Protocols(2)  In [19]  the basic DCF MAC  adding the RTS/CTS handshaking  In [20], the Broadcast Support Multiple Access (BSMA)  adding NAK and the following rules  After the sender transmits a data frame, it waits for NAK  If a receiver does not receive the data frame after it transmitted the CTS frame, it transmits a NAK frame.

6. 2003/01/07 Existing Multicast MAC Protocols(3)  In [21], the Broadcast Medium Window (BMW)  basic idea is to treat each broadcast request as multiple unicast requests.  Each unicast is processed using the reliable IEEE 802.11 DCF MAC protocol  each node maintains three lists:  NEIGHBOR BUFFER  SEND BUFFER  RECEIVER BUFFER.

7. 2003/01/07 Existing Multicast MAC Protocols(4)  NEIGHBOR BUFFER list contains the current neighbors.  SEND BUFFER list contains the ongoing broadcast messages.  RECEIVE BUFFER list contains the sequence numbers of the data frames received by the node.  based on RTS/CTS/DATA/ACK  serves the nodes in NEIGHBOR BUFFER list

8. 2003/01/07 Problems with Existing Multicast MAC Protocols (1)  In [19]  the MAC protocol does not coordinate the transmission of CTS  an assumption is made in [19] that the sender’s radio has the Direct Sequence (DS) capture ability.  In [23] is reported that this probability quickly drops to 0.2 at the presence of more than 5 nodes

9. 2003/01/07 Problems with Existing Multicast MAC Protocols (2)  In [20], BSMA  The additional NAK frame does not help resolve the collision of the CTS frame  In [21], BMW  BMW is inefficient for following reasons:  Contention phase u at least n contention phases for each multicast data frame u some other node wins the contention

10. 2003/01/07 Problems with Existing Multicast MAC Protocols (3)  In [21], BMW (cont.)  Timeout  the prolonged multicast process can easily lead to a timeout in the higher layer

11. 2003/01/07 Batch Mode Multicast MAC Protocol (1)  consolidate the n contention phases into one  the required time to serve a multicast can be greatly reduced  To achieve this goal,  coordinate the transmissions of the control frames  uses RTS frames to sequentially instruct CTS transmissions  uses RAK frames to coordinate the ACK transmissions

12. 2003/01/07 Batch Mode Multicast MAC Protocol (2)  several advantages:  reduces the number of contention phases  does not modify any control frame format  co-exist with the other IEEE 802.11 protocols

13. 2003/01/07 Location Aware Multicast MAC Protocol(1)  When the size of S is large, it may be desirable to reduce S’s size  Find a cover set, S’, of S, and send RTS only to the addresses of nodes in S’  expect only those nodes in S’ to return a CTS and ACK  the sender is able to conclude that all nodes in S have received the multicast data frame

14. 2003/01/07 Location Aware Multicast MAC Protocol(2)  To achieve the above goal  use the location information obtained by GPS  some theorems  location information  US Federal Communications Commissions (FCC) mandated wireless service carriers to provide the location service  including the location information in beacons

15. 2003/01/07 Location Aware Multicast MAC Protocol(3)  some theorems  Theorem 1 Let S be the set of all intended receivers of the multicast data frame. In the Batch Mode Procedure(), suppose that the sender receives an ACK from every node in a subset S’ of S. A node p in S \ S’ is guaranteed to have received the data frame without collision if and only if S’ is a cover set of S.

16. 2003/01/07 T d f c b e p Neighbors ={a,b, c, d, e, f, p} a S’={b, e, c} S\S’={p} S={b, e, c, p}

17. 2003/01/07 Location Aware Multicast MAC Protocol(4)  some theorems (cont.)  Theorem 2 ([18]) The minimum cover set of a neighbor set S can be computed in O(n 4/3 ) time  Theorem 3 Let S be the set of all intended receivers of the multicast data frame. In the Batch Mode Procedure(), suppose that the sender receives an ACK from every node in a subset S ACK S’, where S’ is a cover set of S. Under the assumption that the primary transmission error is caused by collision, a node p in S\S’ is guaranteed to have received the data frame without collision if and only if A(p) A(S ACK )

18. 2003/01/07 Location Aware Multicast MAC Protocol(5)  some theorems (cont.)  Theorem 4 Assume that all nodes have the same transmission radius R. Given a node p and a set of nodes C, if the union of the cover angles is p’s cover angle for the transmission area of p, A(p), is completely covered by C  angle-based scheme

19. 2003/01/07 Location Aware Multicast MAC Protocol(6) Cover Angle of node p for node q ([α,β]) α β

20. 2003/01/07 T d f c b e p S={b, e, c, p} a S’={b, e, c} S\S’={p} x y z t v doesn’t return ACK

21. 2003/01/07 Location Aware Multicast MAC Protocol(7)  a refinement of the Batch Mode Multicast MAC protocol  MCS(S) the minimum cover set computation procedure  denote UPDATE(S, S’) the angle-based procedure  Batch Mode Procedure (S, S ACK ) => (MCS(S), S ACK )  S = UPDATE(S, S ACK ) S’ returns the set of nodes in S that are not completely covered by S ACK

22. 2003/01/07 Simulation(1)

23. 2003/01/07 Simulation(2)

24. 2003/01/07 Simulation(3) A multicast message transmission is considered successful if the message reaches a certain percentage of the intended receivers

25. 2003/01/07 Simulation(4)

26. 2003/01/07 Simulation(5)

27. 2003/01/07 Conclusion  co-exist with the current unreliable IEEE 802.11 multicast MAC protocol  reduce the number of contention phases  decreases the average total time required to complete a multicast request  Future work  multicast MAC protocol that solves both the hidden and exposed terminal problems