1. Spring 2005CMPE2571 CMPE 257: Wireless and Mobile Networking SET 3b: Medium Access Control Protocols

2. Spring 2005UCSC CMPE2572 Channel Access Schemes n Contention based schemes p ALOHA, CSMA/CA (FAMA, MACA, MACAW, IEEE 802.11) : with/without RTS/CTS handshakes. p Difficulties: not scalable, fairness, QoS. n Scheduled schemes p FDMA/TDMA/CDMA in multi-hop networks: graph coloring problem — UxDMA. p Node/link activation based on NCR (Neighbor-aware Contention Resolution)

3. Spring 2005UCSC CMPE2573 UxDMA [R01]R01 n Channel assignments (code in CDMA, time- slot in TDMA and frequency in FDMA) are abstracted as graph coloring problems. n Several atomic constraints are identified. A B Node-based constraintEdge-based constraint A B C E.g.: Two adjacent cells cannot use the same freq. set. E.g.: A node (A) cannot transmit and receive at the same time.

4. Spring 2005UCSC CMPE2574 UxDMA (Cont’d) n Channel assignments can be classified based on certain sets of constraints. p (T/F)DMA broadcast schedule/assignment p RTS/CTS protocols n Then a unified algorithm for efficient (T/F/C)DMA channel assignments is proposed using global topology.

5. Spring 2005UCSC CMPE2575 Scheduled Access n Problem description: p Given a set of contenders M i of an entity i in contention context t, how does i determine whether itself is the winner during t ? n Topology dependence: p Exactly two-hop neighbor information required to resolve contentions. p In ad hoc networks, two-hop neighbors are acquired by each node broadcasting its one-hop neighbor set.

6. Spring 2005UCSC CMPE2576 Goals to Achieve n Collision-free — avoid hidden terminal problem, no waste on transmissions; n Fair — the probability of accessing the channel is proportional to contention; n Live — capable of yielding at least one transmission each time slot.

7. Spring 2005UCSC CMPE2577 Neighbor-Aware Contention Resolution (NCR) n In each contention context (time slot t ): p Compute priorities p i is the winner for channel access if: a b c d e Contention Floor 6 9 5 4 2

8. Spring 2005UCSC CMPE2578 Channel Access Probability: n Dependent on the number of contenders in the neighborhood. n Channel access probability: p Bandwidth allocation general formula to i

9. Spring 2005UCSC CMPE2579 NAMA: Node Activation Multiple Access (Broadcast) n Channel is time-slotted. n Transmissions are broadcasts via omni- directional antenna: all one-hop neighbors can receive the packet from a node. n The contenders of a node for channel access are neighbors within two hops because of direct and hidden terminal contentions.

10. Spring 2005UCSC CMPE25710 Algorithm

11. Spring 2005UCSC CMPE25711 Illustration of NAMA A B C D E F H G 8 6 5 3 4 1 2 9

12. Spring 2005UCSC CMPE25712 NAMA Improvements n Inefficient activation in certain scenarios. p For example, only one node, a, can be activated according NAMA, although several other opportunities exist. —— We want to activate g and d as well. a f g c d e h b 10 1 6 4 7 3 8 5

13. Spring 2005UCSC CMPE25713 Node + Link (Hybrid) Activation n Additional assumption p Radio transceiver is capable of code division channelization (DSSS —— direct sequence spread spectrum) p Code set is C. n Code assignment for each node is per time slot: i.code = i.prio mod |C |

14. Spring 2005UCSC CMPE25714 Hybrid Activation Multiple Access (HAMA) n Node state classification per time slot according to their priorities. p Receiver (Rx): intermediate prio among one- hop neighbors. p Drain (DRx): lowest prio amongst one-hop. p BTx: highest prio among two-hop. p UTx: highest prio among one-hop. p DTx: highest prio among the one-hop of a drain.

15. Spring 2005UCSC CMPE25715 HAMA (cont.) n Transmission schedules: p BTx —> all one-hop neighbors. p UTx —> selected one-hops, which are in Rx state, and the UTx has the highest prio among the one-hop neighbors of the receiver. p DTx —> Drains (DRx), and the DTx has the highest prio among the one-hops of the DRx.

16. Spring 2005UCSC CMPE25716 HAMA Operations n Suppose no conflict in code assignment. n Nodal states are denoted beside each node: p Node D converted from Rx to DTx. p Benefit: one-activation in NAMA to four possible activations in HAMA. a f g c d e h b 10-BTx 1-DRx 6-Rx 4-DRx 7-UTx 3-DRx 8-Rx 5-DTx

17. Spring 2005UCSC CMPE25717 Other Channel Access Protocols n Other protocols using omni-directional antennas: p LAMA: Link Activation Multiple Access p PAMA: Pair-wise Activation Multiple Access n Protocols that work when uni-directional links exist. p Node A can receive node B’ s transmission but B cannot receive A’ s. n Protocols using direct antenna systems.

18. Spring 2005UCSC CMPE25718 Channel Access Probability Analysis of NAMA n The channel access probability for a single node i is given by n We are interested in average probability of channel access in multi-hop ad hoc networks.

19. Spring 2005UCSC CMPE25719 Ad Hoc Network Settings n Equal transmission range; n Each node knows its one- and two-hop neighbors — M i. n Nodes are uniformly distributed on an infinite plane with density . n A node may have different numbers of neighbors in one-hop and two-hop.

20. Spring 2005UCSC CMPE25720 Counting One-Hop Neighbors n The prob of having k nodes in an area of size S is a Poisson distribution: n Average one-hop neighbors is: p Note: the mean of r.v. with Poisson dist is

21. Spring 2005UCSC CMPE25721 Counting Two-hop Neighbors n Two nodes become two-hop nbrs if they share at least one one-hop neighbor. p Average number in B(t):

22. Spring 2005UCSC CMPE25722 Counting Two-hop Neighbors n Probability of becoming two-hop: n Prob of a node staying at tr is 2t. n Summation of nodes in ring (r,2r) times the corresponding prob of becoming two- hop --- number of two-hop neighbors:

23. Spring 2005UCSC CMPE25723 Total One- and Two-hop Neighbors n Sum: n This is average number of one-hop and two-hop neighbors.

24. Spring 2005UCSC CMPE25724 Average Probability of Channel Access n Apply Poisson distribution with the mean (number of one- and two-hop neighbors)

25. Spring 2005UCSC CMPE25725 Plotting Channel Access Probability

26. Spring 2005UCSC CMPE25726 Comparison of Channel Access Probability

27. Spring 2005UCSC CMPE25727 Delay per Node n Delay is related with the probability of channel access and the load at each node. n Channel access probability can be different at each node. n Delay is considered per node.

28. Spring 2005UCSC CMPE25728 Packet Arrival and Serving: n M/G/1 with server vacation: Poisson arrival (exponential arrival interval), service time distribution (any), single server. n FIFO service strategy: head-of-line packet waits for geometric distributed period Y i with parameter 1-q i ̶ q i is the channel access probability of node i.

29. Spring 2005UCSC CMPE25729 Service Time: n Service time: X i = Y i + 1. n The mean and second moment of service time: n Server vacation: V=1,

30. Spring 2005UCSC CMPE25730 Delay in The System n Pollaczek-Kinchin formula: n Take in X i and V i : n Delay in the system: (q> )

31. Spring 2005UCSC CMPE25731 Plotting System Delay

32. Spring 2005UCSC CMPE25732 System Throughput n Multi-hop networks have concurrent transmissions >1. n The system can carry as many packets at a time as all nodes can be activate. p Simple!

33. Spring 2005UCSC CMPE25733 Comparisons with CSMA CSMA/CA by Analysis n Different slotting: p NAMA long slots p CSMA CSMA/CA short slots n CSMA(CA) assumptions: p Heavy load (always have packets waiting) p Channel access regulated by back-off probability p’ in each slot. n Convert the load to comparable one in NAMA.

34. Spring 2005UCSC CMPE25734 Convert Load in CSMA(CA) to the Load in NAMA n Each attempt to access channel is a packet arrival p’. n Packet duration is geometric with average 1/q. n Two state Markov chain to compute the load.

35. Spring 2005UCSC CMPE25735 NAMA Load n Relation: n i =  b is the load for each node. n q m is the channel access probability of each node.

36. Spring 2005UCSC CMPE25736 Protocol Throughput Comparison

37. Spring 2005UCSC CMPE25737 Simulations n Two scenarios: p Fully connected: 2, 5, 10, 20 nodes. p Multi-hop network: r 100 nodes randomly placed in 1000x1000 area. r Transmission range: 100, 200, 300, 400. n Compare with UxDMA:

38. Spring 2005UCSC CMPE25738 Fully Connected Network (Throughput)

39. Spring 2005UCSC CMPE25739 Fully Connected Network (Delay)

40. Spring 2005UCSC CMPE25740 Multi-hop Network (Throughput)

41. Spring 2005UCSC CMPE25741 Multi-hop Network (Delay)

42. Spring 2005UCSC CMPE25742 Conclusions n NCR ensures collision-free transmissions. n Only two-hop topology information is needed. n HAMA performs better than static scheduling algorithms (UxDMA). n HAMA performs better than contention- based protocols. n The use of directional antennas can improve performance further. (Next topic)

43. Spring 2005UCSC CMPE25743 Comments n Scheduled-access protocols are evaluated in static environments and what about their performance in mobile networks? n Neighbor protocol will also have impact on the performance of these protocols n Need comprehensive comparison of contention-based and scheduled access protocols.

44. Spring 2005UCSC CMPE25744 References n [R01] S. Ramanathan, A unified framework and algorithm for channel assignment in wireless networks, ACM Wireless Networks, Vol. 5, No. 2, March 1999. n [BG01] Lichun Bao and JJ, A New Approach to Channel Access Scheduling for Ad Hoc Networks, Proc. of The Seventh ACM Annual International Conference on Mobile Computing and networking (MOBICOM), July 16-21, 2001, Rome, Italy. n [BG02] Lichun Bao and JJ, Hybrid Channel Access Scheduling in Ad Hoc Networks, IEEE Tenth International Conference on Network Protocols (ICNP), Paris, France, November 12-15, 2002.