1. An Adaptive, High Performance MAC for Long- Distance Multihop Wireless Networks Presented by Jason Lew

2. An Adaptive, High Performance MAC for Long-Distance Multihop Wireless Networks Authors: Sergiu Nedevschi, UC Berkeley Rain K. Patra, UC Berkeley Sonesh Surana, UC Berkeley Sylvia Ratnasamy, Intel Research Berkeley Lakshminarayanan Subramanian, NYU Eric Brewer, UC Berkeley

3. Overview Background Improvement JazzyMac Design JazzyMac Evaluation Conclusion

4. Background: WiLD Networks Multi-hop WiFi long-distance networks are able to provide connectivity to thinly- populated or rural regions Networks have long links and use directional antennas Therefore, there are long propagation delays and increased collision probability

5. Background: Using SynOp to Avoid Interference a) Mix-Tx-Rx Not feasible because of the physical proximity between the radios and the presence of antenna side-lobes b) SynRx Must satisfy (1) c) SynTx Must satisfy (1) (1)

6. Background: Using SynOp to Avoid Interference Thus, SynOp can allow multiple adjacent WiLD links to simultaneously use the same wireless channel if: Links are separated by large enough α AND Radio transmit powers satisfy (1) (1)

7. Background: MAC Protocols for WiLD Links CSMA-based MAC protocols do not perform well in networks with long distance links Links are much too long to carrier sense TDMA is a better option

8. Background: MAC Protocols for WiLD Links (a) is an example of a bipartite network used in long distance WiLD Links A transmits on all links Then, B transmits on all links

9. Improvement: Improving Throughput Adapt to traffic demand Allocate time slots according to demand Allow neighboring, overlapping transmissions If A only needs a portion of transmission time to B, allow B to transmit to D during A’s slot

10. Improvement: Improving the Bandwidth-Delay Tradeoff Bandwidth vs. delay tradeoff is fixed with fixed slots Fix by using dynamic slot adaptation Low utilization link  small TDMA slots  lower per-hop delay High utilization link  large TDMA slots  higher bandwidth

11. JazzyMac Design: Features Adaptive slots Allow parallel neighboring-but- independent transmissions Generalized topologies (not limited to bipartite)

12. JazzyMac Design: Protocol Each node has a mode of operation mode is either Tx or Rx Each link AB has a token Only the node (A or B) holding the token can transmit Each token has timeout value Controls when the node with the token can transmit over the link Network-wide parameter, max_slot, bounds the maximum transmission slot length

13. JazzyMac Design: 4 Protocol Rules Token Exchange Rule: After transmitting, node tells connected node when it will be able to receive traffic Passes this along with token Mode Rule: A node in receive mode can move to transmit mode only when it has the token for all of its links A node in transmit mode can move to receive mode when it has release the tokens for all of its links

14. JazzyMac Design: 4 Protocol Rules Transmission Rule: Node can transmit over link only if: Node is in transmit mode AND Node holds token Token is valid Slot Rule: Node can transmit on link for no longer than max_slot

15. JazzyMac Design: Operation t = 0: A has Token AB and Token AC Can transmit on both links

16. JazzyMac Design: Operation t = 15: A’s Tx to B ends Token AB is passed to B Token AB is passed with timeout = 35 because A’s Tx to C needs more time

17. JazzyMac Design: Operation t = 15: B has Token AB and Token BC Token AB is not valid because it was passed with timeout = 35 B transmit to C only

18. JazzyMac Design: Operation t = 50: A releases Token AC A moves into Rx mode Token AB is now valid B can now transmit over AB

19. JazzyMac Design: Operation t = 60: C transitions to Tx

20. JazzyMac Design: Dealing with Loss Packet loss can cause the loss of link tokens Implementations of JazzyMac can piggyback tokens or send multiple copies of the token Loss will still occur rarely Solution: add sequence number to each token Increment every time token is exchanged After timeout, every node resends tokens Duplicate tokens ignored; lost tokens recovered Drawback: token retransmissions can interfere with other token retransmissions or other packets

21. JazzyMac Evaluation Improves maximum throughput by 15-100% in typical topologies Improvements are consistent across many topologies and traffic patterns Reduces gap between network throughput from practical approaches and the optimal theoretical network throughput (assuming idealized transmission slots) Dynamic slot sizing improves the delay-throughput trade- off Throughput increases of as large as 80% for asymmetric (non-bipartite) traffic distributions

22. JazzyMac Evaluation: Performance in Random Topologies

26. JazzyMac Evaluation: Effect of Traffic and Topology

30. Conclusion WiLD networks provide services to many users in rural areas around the world JazzyMac can improve throughput, reduce latency, and enable operation on general topologies JazzyMac outperforms existing WiLD MAC protocols and can operate on any network topology

31. An Adaptive, High Performance MAC for Long- Distance Multihop Wireless Networks