1. Cog-Fi: A Cognitive Wi-Fi Channel Hopping Architecture for Urban MANETs Sung Chul Choi and Mario Gerla WONS 2012 Presentation

2. Motivation 2

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4. Network Model 4 Mobile node Fixed Interfering source

5. Network Model 5 3 1 4 5 8 1 Network node Interfering source ch Goal: Avoid the channels used by interfering sources using a cognitive multi-channel scheme.

6. Problem Statement  Develop a multi-channel solution for P2P networks that operate in unlicensed bands, where external interference exists.  Requirements  Interfering sources should be avoided.  Yet, this must not impact P2P connectivity. i.e., logical partition must be avoided. 6 1 1 2 13 ? 2 ? 3 ! channel rendezvous

7. Problem Statement  Develop a multi-channel scheme for P2P networks that operate in unlicensed bands, where external interference exists.  Requirements  Interfering sources should be avoided.  Yet, this should not impact P2P connectivity. i.e., logical partition must be avoided. 7

8. Solution Preview  To avoid interfering sources:  Use Cognitive radio technology to sense channel load and discover lightly loaded channels  To maintain P2P network connectivity in spite of unpredictable interferers:  Exploit multi channel diversity: a node can receive on multiple channels via Cognitive Channel Hopping  Guarantee neighbor discovery and rendezvous in a finite # of steps(using the QUORUM set)  Design routing algorithm that accounts for “multichannel links” and Channel Hopping 8

9. Cognitive Channel Hopping  Cognitive Channel Hopping (CCH)  Single-radio, channel-hopping solution in which each node picks its channels based on the load sensed on them 9 t f t f t f t f t f t f

10. CCH: Design Choice #1  Multi-radio nodes?  Pros Can have each radio to tune to a different channel. Enables simultaneous transmission and reception. Can observe multiple channels simultaneously.  Cons Costly. Device form factor. Power consumption. Requires a significant amount of low-layer changes.  Assume single radio. 10

11. CCH: Design Choice #2  Channel coordination  Control channel-based explicit coordination? A single point of failure. Control channel saturation problem. Control overhead. For single-radio nodes, time-synchronization is required.  Use channel hopping.  with distributed channel rendezvous. 11 13 ? 2 ? 3 ! channel rendezvous

12. CCH vs. Bluetooth Scatternet 12  Doesn't Scatternet already do this?  In CCH, channel hopping occurs at a much more slower rate -- tight time- synchronization is not required.  In Scatternet, each slave node syncs to its master's hopping sequence, and can only talk to its master(s). A direct link is not established between two slaves. In CCH, pairwise rendezvous is achieved. M M M S S S S S S S

13. CCH: Protocol Operation  A node x periodically triggers Channel Quality Assessment (CQA).  A channel availability vector a = {a 1, …, a |C| } is produced. In this work, a i = 1 - [channel load in i ].  Based on channel availabilities, x picks a channel set Q = {q 1, …, q k } from a predefined Quorum list (any two Q-sets have at least one common element) 13 Example list of channel sets, each with size k = 5.  It picks the channel set with the highest combined channel quality, defined as: C = {0, 1, 2, …, 11, 12}

14. CCH: Protocol Operation  Given Q, x generates two hopping sequences, u tx and u rx. 14 129310 9310 9310 9310 9310 93100 93110 93310 99310 M tx M rx Q = {0, 1, 3, 9, 12} 129310 9310 9310 310 … 93100 93110 93 0 9 … u tx u rx k = 5 |u tx | = |u rx | = k 2 = 25

15. CCH: Channel Rendezvous Property  Claim: A channel rendezvous of a pair of nodes is guaranteed to occur within k 2 slots. 15 Q x = {0, 1, 2} Q y = {2, 3, 4} M tx (x) 012012012 M rx (y) 234342423 Q x = {0, 1, 2} Q y = {2, 3, 4} 012012012234342423 2 appears in the same column, every row. 2 appears exactly once in each column. By the property of a quorum system, there exists at least one common channel. 012012012234342423 …… … … u tx (x) u rx (y)

16. CCH: Channel Rendezvous Property  This still holds when two sequences are not in sync. 16 Q x = {0, 1, 2} Q y = {2, 3, 4} M tx (x) 012012012 M rx (y) 234342423 Q x = {0, 1, 2} Q y = {2, 3, 4} 012012012232343424 2 appears in the same column, every row. 2 appears exactly once in each column. By the property of a quorum system, there exists at least one common channel. 012012012234342423 …… … … u tx (x) u rx (y)

17. CCH: Channel Rendezvous Property  When the slot boundaries are not aligned (i.e., in a complete asynchrony)... 17 Q x = {0, 1, 2} Q y = {2, 3, 4} M tx (x) 012012012 M rx (y) 234342423 Q x = {0, 1, 2} Q y = {2, 3, 4} 012012012232343424 2 appears in every column. 2 appears exactly once in each column. By the property of a quorum system, there exists at least one common channel. 012012012234342423 …… … … u tx (x) u rx (y) α fraction of a slot (1-α) fraction of a slot

18. 1 2 3 4 5 6 7 8 9 1010 1 1212 0 CCH: Protocol Operation  When x has no packet to transmit, it follows u rx (x).  When x has packets to transmit, it follows u tx (x) to locate the neighbor.  A channel rendezvous is guaranteed within the length of u tx (x), k 2. 18 time channel quality assessment Has packets to send to y. tx slot rx slot slot No more packets to send.

19. CCH: Protocol Operation  Within a slot, a conventional RTS/CTS-based packet exchange is made.  By default, a slot is 10ms, enough to fit in tens of MAC frames.  Retransmissions occur within a slot and over multiple slots. 19 time x y backoff RT S CT S DATA AC K

20. CCH: Learning of u tx and u rx  Learning hopping sequences  Every CCH frame includes information about the hopping sequences that the transmitter is using.  If node x has received a frame from y, it can later use its cache to predict which channel y will be without scanning channels with u tx (x). 20 yx

21. CCH: MAC-level Broadcast  Broadcast function is critical in making upper-layer mechanisms to work (e.g., routing).  Not all neighbors are in the same channel as you! 21 11 32 4 12 ? ? ? ?  Each broadcast frame is kept in a separate buffer and transmitted in the transmitting channel (specified in u tx ) in the beginning of the slot, for multiple slots.

22. CCH: Multi-channel Hidden Terminals  Channel rendezvous property breaks if:  Node x wants to send a frame to y, so switches to x’ s transmitting channel, while y is also trying to transmit to another node z, thus not in its receiving channel. 22 y’s rx: 1 z’s rx: 2 tx x y y ’ s rx channel. tx rx x has a packet for y. where is y? y is involved in a transmission. tx y z 2 1 x ?

23. CCH: Recovery  CCH mitigates this effect by forcing each node to "yield" for a small amount of time after each frame transmission.  During a yield session, the node waits in its receiving channel.  The yield length is dynamically adjusted based on the level of congestion the node sees.  This simple trick turns out to be very effective. 23 y’s rx: 1 z’s rx: 2 y z 2 1 x ? txrxtxrx x y y ’ s rx channel. tx rx yield y ’ s rx rx x has a packet for y. x finds y, and starts transmitting to y. y is involved in a transmission.

24. CCH: Recovery  A link is set up between two nodes x and y, and saturated CBR streams are configured between them. 24 xy y x Uni-directional Bi-directional

25. Solution: Cog-Fi Architecture  Cog-Fi is a cross-layer architecture with these modules: 25 CCH 802.11 PHY CH-LQSR IP PHY MAC Routin g Coordinate channel access. Store and maintain channel status. Make a routing decision. Regular 802.11 PHY. channel load, link rate, BER SNR/BER CH-LQSR

26. CH-LQSR: Motivation  Conventional on demand routing protocols like AODV and DSR are not well-suited.  Problem 1: Not all hops are equal. 26 S T

27. CH-LQSR: Motivation  Conventional, hop-count based routing protocols like AODV and DSR are not well- suited.  Problem 1: Not all hops are equal. 27 S T 2 1 18Mbps 54Mbps 11Mbps

28. CH-LQSR: Motivation  Conventional, hop-count based routing protocols like AODV and DSR are not well- suited.  Problem 2: Broadcast does not occur simultaneously. 28 S T

29. CH-LQSR: Motivation  Conventional, hop-count based routing protocols like AODV and DSR are not well- suited.  Problem 1: Not all hops are equal. Use the channel load and link rates to quantifying the quality of each hop, and factor this in when computing routes.  Problem 2: Broadcast does not occur simultaneously. Modify Route Discovery procedure. 29

30. CH-LQSR: ETT CH Metric  Extend ETX and ETT [4, 5].  p : prob. that the packet transmission is not successful: p = 1 – (1 – p f ) · (1 – p r )  s(m) : prob. that the packet is delivered at m -th attempt. s(m) = p m – 1 · (1 – p)  The expected transmission count (ETX) of link e = (u, v) is:  Factoring in the link bandwidth and packet size, one can define the expected transmission time (ETT) of e as: c : channel index S d : data packet size B : bandwidth (data rate) of the channel 30 v u e p f and p r, the forward and backward packet error probabilities, are computed based on the link BER reported from the PHY module. B, the bandwidth, is computed by taking the channel load and link rates into account.

31. CH-LQSR: ETT CH Metric  (cont'd. from the previous slide)  The multi-channel ETT of e, ETT(e), is: which is the avg. of ETT c (e) values over the channels in the channel set the receiver v is using.  Finally, Channel Hopping ETT of a path P is: 31 v u e

32. CH-LQSR: Protocol Operation  Extension of DSR  Route Discovery involving RREQ/RREP.  Once a route is discovered, source routing is used. 32 S A D T B C E How do I reach T? Here I am!

33. CH-LQSR: Protocol Operation  Extension of DSR  Route Discovery involving RREQ/RREP 33 S A D T B C E RREQ(T) Path: S RREQ(T) Path: S-A ETT CH : 0.012 RREQ(T) Path: S-A-B ETT CH : 0.032 RREQ(T) Path: S-A-B-C ETT CH : 0.076 RouteCache(E) src des t ETT CH S T 0.076 path S-A-B- C RREQ(T) Path: S RREQ(T) Path: S-D ETT CH : 0.011 RouteCache(E) src des t ETT CH S T 0.011 path S-D

34. Cog-Fi: Evaluation Setup  QualNet 4.5  CCH: implemented as a full-fledged MAC protocol.  CH-LQSR: implemented as a full-fledged routing protocol.  Channel environment  13 orthogonal channels in the 5-GHz band.  Interfering source: ( x, y, tx_power, channel, active_% ).  CCH parameters  Use RBAR for rate adaptation [8], using 802.11a rates.  Channel set size k = 5.  Channel switching delay: 80 µ s.  Slot size: 10ms.  CQA Period: 3 seconds. 34

35. Cog-Fi: Evaluation Setup  List of schemes compared for evaluation 35 SymbolDescription CCH+CH-LQSR Our Cog-Fi solution. CCH+DSR CCH with DSR. CCH+AODV CCH with AODV. 802.11a Single-channel 802.11a, routed using DSR. COG A conventional cog radio scheme, with DSR. RH+DSR Random hopping and DSR. time [COG] CONTROL for single-radio nodes communicati on contr ol scannin g interferen ce

36. Cog-Fi: 25-node (5x5) Grid Topology  5 saturated 1500-byte CBR streams for 5 random node pairs. 36 … … … … …… 40 m 1 3 5 2 2 7 4 8

37. Cog-Fi: 100-node (10x10) Grid  5 saturated 1500-byte CBR streams for 5 random node pairs. 37 … … … … …… 30 m 1 3 5 2 2 7 4 8

38. Cog-Fi: Route Request count  5 saturated 1500-byte CBR streams for 5 random node pairs. 38 … … … … …… 40 m 1 3 5 2 2 7 4 8 25-node grid

39. Summary  Goal: Devise a multi-channel multi-hop mechanism with the following requirements.  Interfering sources should be avoided A CCH node employs a cognitive radio-like channel sensing to identify lightly loaded channels.  The network connectivity must be maintained. Exploit multi channel diversity: a node can receiver on multiple channels via Cognitive Channel Hopping Guarantee neighbor discovery and rendezvous in a finite # of steps(using the QUORUM set)  Performance is further improved by CH-LQSR, ie by using a link metric that factors in channel load and link rates. 39

40. Thank You! 40