Wireless Medium Access Control Protocols

1. Wireless Medium Access Control Protocols A Survery by Ajay Chandra V. Gummalla and John O. Limb

2. Introduction • Survey • Distributed vs. Centralized Networks • Wireless MAC Issues • Low Power Sensor Nodes • Random Access • Guaranteed Access • Hybrid Access

3. Introduction Cont’d. • Distributed MAC Protocols • Distributed Foundation Wirelesss MAC (DFWMAC) • Eliminate Yield – Non-Preemptive Priority Multiple Access (EY-NPMA)

4. Introduction Cont’d. • Centralized MAC Protocols • Random Access • Idle Sense Multiple Acces (ISMA) • Randomly Addressed Polling (RAP) • Resource Auction Multiple Access (RAMA) • Guaranteed Access • Zhang’s and Acampora’s Proposals • Disposable Token MAC Protocol (DTMP)

5. Introduction Cont’d. • Hybrid Access • Random Reservation Protocols (RRA) • Packet Reservation Multiple Access (PRMA) • Random Reservation Access – Independent Stations Algorithm (RRA-ISA) • Distributed Queuing Request Updated Multiple Access (DQRUMA) • Moble Access Scheme based on Contention and Reservation for ATM (MASCARA)

6. Introduction Cont’d. • Dynamic Slot Assignment ++ (DSA++)

7. Distributed Wireless Network • ad hoc network • No central administration • Multi-hop wireless networks • Wireless Sensor Nets

8. Centralized Wireless Network • Last Hop Network • Very common • Corporate, Academic, and Cellular uses. • Has a controlling Base Station, with variable intelligence • Wireless Access Point • Cellular Tower

9. Wireless MAC Issues • Half-Duplex • No Collision Detection • Uplink and Downlink must be multiplexed • Time Varying Channel • Reflection, Diffraction, and Scattering • Different version of signal are superimposed on each other • Multipath Propagation • Coherence Time = time signal strength changes by 3dB

10. Wireless MAC Issues Cont’d. • Burst Channel Errors • Higher BER • Errors occur in long bursts • Link Layer retransmission based on immediate ACKs

11. Wireless MAC Issues Cont’d. • Location Dependent Carrier Sensing • Hidden Nodes: Node A doesn’t know Node B is also talking to BS • Exposed Nodes: Node A knows node B is talking, but doesn’t know that it will not affect Node A’s conversation with BS • Capture: Node A and B are both transmitting to BS, but since Node A’s signal strength is stronger Node A’s transmission is used no collision is detected.

12. Random Access • Random Access is based on a “Talk whenever you want” way of thinking • Collisions are resolved by a contention resolution algorithm • Distributed Networks

13. Guaranteed Access • Access to medium is scheduled • Round Robin • Master/Slave (Polling) • Tokens • TDMA, FDMA

14. Hybrid Access • Melds best qualities of Random and Guaranteed Access • Request-Grant mechanisms • Requests are Random Access, and once reserved transmission is guaranteed • Random Reservation Access • Demand Assignment

15. Distributed Foundation Wireless MAC (DFWMAC) • 802.11 Standard • 4-way exchange: RTS-CTS-DATA-ACK • No ACK causes sender to retransmit • No CTS causes exponential backoff • RTS and CTS contain a NAV which details how much data is to be sent

16. Elimination Yield – Non-Preemptive Priority Multiple Access (EY-NPMA) • HIPERLAN • Sense channel for time to send (TTS) 1700 bits, if clear, then send • If busy, N slots; When done listen again • If still busy, abort; Else listen again, and if not busy then transmit until finished

17. Idle Sense Multiple Access (ISMA) • Carrier Sensing and Collision detection are performed by the BS • When medium is idle BS broadcasts idle signal (IS) • Nodes with data send • If collision BS cannot decode signal, does not send ACK and broadcasts IS again • Otherwise BS sends ACK/ISA (ISA) • Efficiency is improved by using small Reservation packets

18. Randomly Address Polling (RAP) • Nodes with data broadcast orthogonal “codes” simultaneously • BS receives all codes, using a CDMA receiver • BS then polls each code • All nodes with that code transmit • If only one node the BS sends ACK • More than one node with code causes BS to send NACK • Reservation RAP supports nodes with streaming traffic

19. RAP

20. Resource Auction Multiple Access (RAMA) • Each node has and N-bit ID and transmits it, in contention phase • BS then echos back ID it heard bit-by-bit • Once a node receives a bit it did not transmit, it drops out • Since BS does an OR operation on received IDs then node with highest ID always wins

21. Zhang’s Proposal • BS polls each node for data, round robin • Node responds with data request, or a keep alive if queue is empty • BS then polls each node that responded with a data request

22. Disposable Token MAC Protocol (DTMP) • Improves on Zhang’s proposal • When polling nodes BS indicates if it has data to send to nodes • If no data, then remain silent • Otherwise send short message • Transmit any data to send • Channel is assumed to be reciprocal

23. Acampora’s Proposal • Poll, request, data phases • BS polls each node, if the node has data to sends it responds • The BS the broadcasts this nodes ID so that all nodes know the order in which to send • BS then polls nodes each node in turn for its data

24. Various Proposals

25. Random Reservation Protocols (RRA) • Uplink is time slotted • Each slot large enough to carry one voice packet • Downlink is broadcast channel • Nodes use random access to request reservations for data to send • BS enforces a policy of reservations • Stream Reservation • Complete BS scheduling

26. Packet Reservation Multiple Access (PRMA) • A node with a back-logged voice packet transmits with probability p • If successful, reserves that slot for following packets • Data is similar, though no reservations are made • Different access probabilities are used for voice and data • Introduction of data packets into voice only network decreases efficiency • Improvements include limited data reservations, separating voice and data channels (FRMA), separating request and data channels (PRMA++) • Centralized PRMA uses scheduling to achieve QOS

27. Random Reservation Access – Independent Stations Algorithm (RRA-ISA) • BS polls a subset of all nodes • Subset is defined by the probability of a single transmission in a slot is maximized • BS uses channel history to compute subsets

28. Distributed-Queuing Request Update Multiple Access (DQRUMA) • Uplink and Downlink are duplexed • Uplink has request channel and packet channel • Request channel is for contention requests • Packet channel is for data (and piggyback new contention requests) • Downlink has 3 messages: ACK for current slot, transmission permission for node to use next uplink slot, and data to the nodes • Better than RAMA and PRMA

29. Mobile Access Scheme based on Contention and Reservation for ATM (MASCARA) • Frame consists of three periods: broadcast, reserved, and contention • Broadcast informs nodes of structure of current frame and scheduled uplink transmissions • Reserved period consists of downlink data, and uplink data as defined in broadcast period • Contention is random access and used to send new requests to BS

30. Dynamic Slot Assignment++ (DSA++) • MAC on uplink is TDMA • Both uplink and downlink are slotted • Each downlink slot contains some data and a MAC message • MAC message contains ACK for transmission on previous uplink slot and a reservation for next uplink slot • BS collects all requests and schedules uplink transmissions

31. Hybrids

32. Comparison Summary

33. Comparisons • QoS guarantees are not suited to Random Access protocols because delay cannot be bounded • Demand Assignment protocols are best suited to multimedia applications • Random Access lends itself to large networks • Polling protocols are efficient only for smaller networks • TDD protocols perform poorly at high data rates due to increase in switching