CMPE 257: Wireless and Mobile Networking SET 3b:

1. CMPE 257: Wireless and Mobile NetworkingSET 3b: Medium Access Control Protocols CMPE257

2. Channel Access Schemes • Contention based schemes • ALOHA, CSMA/CA (FAMA, MACA, MACAW, IEEE 802.11) : with/without RTS/CTS handshakes. • Difficulties: not scalable, fairness, QoS. • Scheduled schemes • FDMA/TDMA/CDMA in multi-hop networks: graph coloring problem — UxDMA. • Node/link activation based on NCR (Neighbor-aware Contention Resolution) UCSC CMPE257

3. UxDMA [R01] • Channel assignments (code in CDMA, time-slot in TDMA and frequency in FDMA) are abstracted as graph coloring problems. • Several atomic constraints are identified. Node-based constraint Edge-based constraint B B A A 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. UCSC CMPE257

4. UxDMA (Cont’d) • Channel assignments can be classified based on certain sets of constraints. • (T/F)DMA broadcast schedule/assignment • RTS/CTS protocols • Then a unified algorithm for efficient (T/F/C)DMA channel assignments is proposed using global topology. UCSC CMPE257

5. Scheduled Access • Problem description: • Given a set of contenders Mi of an entity i in contention context t, how does i determine whether itself is the winner during t ? • Topology dependence: • Exactly two-hop neighbor information required to resolve contentions. • In ad hoc networks, two-hop neighbors are acquired by each node broadcasting its one-hop neighbor set. UCSC CMPE257

6. Goals to Achieve • Collision-free — avoid hidden terminal problem, no waste on transmissions; • Fair — the probability of accessing the channel is proportional to contention; • Live — capable of yielding at least one transmission each time slot. UCSC CMPE257

7. 4 9 e 6 5 c a 2 b Contention Floor d Neighbor-Aware Contention Resolution (NCR) • In each contention context (time slot t ): • Compute priorities • i is the winner for channel access if: UCSC CMPE257

8. Channel Access Probability: • Dependent on the number of contenders in the neighborhood. • Channel access probability: • Bandwidth allocation general formula to i UCSC CMPE257

9. NAMA: Node Activation Multiple Access (Broadcast) • Channel is time-slotted. • Transmissions are broadcasts via omni-directional antenna: all one-hop neighbors can receive the packet from a node. • The contenders of a node for channel access are neighbors within two hops because of direct and hidden terminal contentions. UCSC CMPE257

10. Algorithm UCSC CMPE257

11. 5 1 8 A B F 3 6 4 D E C 2 9 H G Illustration of NAMA UCSC CMPE257

12. 4 10 8 e b a 6 5 d c 7 3 1 g h f NAMA Improvements • Inefficient activation in certain scenarios. • 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. UCSC CMPE257

13. Node + Link (Hybrid) Activation • Additional assumption • Radio transceiver is capable of code division channelization (DSSS —— direct sequence spread spectrum) • Code set is C . • Code assignment for each node is per time slot: i .code = i .prio mod |C | UCSC CMPE257

14. Hybrid Activation Multiple Access (HAMA) • Node state classification per time slot according to their priorities. • Receiver (Rx): intermediate prio among one-hop neighbors. • Drain (DRx): lowest prio amongst one-hop. • BTx: highest prio among two-hop. • UTx: highest prio among one-hop. • DTx: highest prio among the one-hop of a drain. UCSC CMPE257

15. HAMA (cont.) • Transmission schedules: • BTx —> all one-hop neighbors. • UTx —> selected one-hops, which are in Rx state, and the UTx has the highest prio among the one-hop neighbors of the receiver. • DTx —> Drains (DRx), and the DTx has the highest prio among the one-hops of the DRx. UCSC CMPE257

16. 4-DRx 8-Rx 10-BTx e b a 5-DTx 6-Rx d c 7-UTx 3-DRx 1-DRx g h f HAMA Operations • Suppose no conflict in code assignment. • Nodal states are denoted beside each node: • Node D converted from Rx to DTx. • Benefit: one-activation in NAMA to four possible activations in HAMA. UCSC CMPE257

17. Other Channel Access Protocols • Other protocols using omni-directional antennas: • LAMA: Link Activation Multiple Access • PAMA: Pair-wise Activation Multiple Access • Protocols that work when uni-directional links exist. • Node A can receive node B’ s transmission but B cannot receive A’ s. • Protocols using direct antenna systems. UCSC CMPE257

18. Channel Access Probability Analysis of NAMA • The channel access probability for a single node i is given by • We are interested in average probability of channel access in multi-hop ad hoc networks. UCSC CMPE257

19. Ad Hoc Network Settings • Equal transmission range; • Each node knows its one- and two-hop neighbors — Mi . • Nodes are uniformly distributed on an infinite plane with density . • A node may have different numbers of neighbors in one-hop and two-hop. UCSC CMPE257

20. Counting One-Hop Neighbors • The prob of having k nodes in an area of size S is a Poisson distribution: • Average one-hop neighbors is: • Note: the mean of r.v. with Poisson dist is UCSC CMPE257

21. Counting Two-hop Neighbors • Two nodes become two-hop nbrs if they share at least one one-hop neighbor. • Average number in B(t): UCSC CMPE257

22. Counting Two-hop Neighbors • Probability of becoming two-hop: • Prob of a node staying at tr is 2t. • Summation of nodes in ring (r,2r) times the corresponding prob of becoming two-hop --- number of two-hop neighbors: UCSC CMPE257

23. Total One- and Two-hop Neighbors • Sum: • This is average number of one-hop and two-hop neighbors. UCSC CMPE257

24. Average Probability of Channel Access • Apply Poisson distribution with the mean (number of one- and two-hop neighbors) UCSC CMPE257

25. Plotting Channel Access Probability UCSC CMPE257

26. Comparison of Channel Access Probability UCSC CMPE257

27. Delay per Node • Delay is related with the probability of channel access and the load at each node. • Channel access probability can be different at each node. • Delay is considered per node. UCSC CMPE257

28. Packet Arrival and Serving: • M/G/1 with server vacation: Poisson arrival (exponential arrival interval), service time distribution (any), single server. • FIFO service strategy: head-of-line packet waits for geometric distributed period Yi with parameter 1-qi ̶ qi is the channel access probability of node i. UCSC CMPE257

29. Service Time: • Service time: Xi = Yi + 1. • The mean and second moment of service time: • Server vacation: V=1, UCSC CMPE257

30. Delay in The System • Pollaczek-Kinchin formula: • Take in Xi and Vi: • Delay in the system: (q>) UCSC CMPE257

31. Plotting System Delay UCSC CMPE257

32. System Throughput • Multi-hop networks have concurrent transmissions >1. • The system can carry as many packets at a time as all nodes can be activate. • Simple! UCSC CMPE257

33. Comparisons with CSMA CSMA/CA by Analysis • Different slotting: • NAMA long slots • CSMA CSMA/CA short slots • CSMA(CA) assumptions: • Heavy load (always have packets waiting) • Channel access regulated by back-off probability p’ in each slot. • Convert the load to comparable one in NAMA. UCSC CMPE257

34. Convert Load in CSMA(CA) to the Load in NAMA • Each attempt to access channel is a packet arrival p’. • Packet duration is geometric with average 1/q. • Two state Markov chain to compute the load. UCSC CMPE257

35. NAMA Load • Relation: • i=b is the load for each node. • qmis the channel access probability of each node. UCSC CMPE257

36. Protocol Throughput Comparison UCSC CMPE257

37. Simulations • Two scenarios: • Fully connected: 2, 5, 10, 20 nodes. • Multi-hop network: • 100 nodes randomly placed in 1000x1000 area. • Transmission range: 100, 200, 300, 400. • Compare with UxDMA: UCSC CMPE257

38. Fully Connected Network (Throughput) UCSC CMPE257

39. Fully Connected Network (Delay) UCSC CMPE257

40. Multi-hop Network (Throughput) UCSC CMPE257

41. Multi-hop Network (Delay) UCSC CMPE257

42. Conclusions • NCR ensures collision-free transmissions. • Only two-hop topology information is needed. • HAMA performs better than static scheduling algorithms (UxDMA). • HAMA performs better than contention-based protocols. • The use of directional antennas can improve performance further. (Next topic) UCSC CMPE257

43. Comments • Scheduled-access protocols are evaluated in static environments and what about their performance in mobile networks? • Neighbor protocol will also have impact on the performance of these protocols • Need comprehensive comparison of contention-based and scheduled access protocols. UCSC CMPE257

44. References • [R01] S. Ramanathan, A unified framework and algorithm for channel assignment in wireless networks, ACM Wireless Networks, Vol. 5, No. 2, March 1999. • [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. • [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. UCSC CMPE257