Special Topics in Wireless Networking: MAC design and cross-layer issues

1. Special Topics in Wireless Networking: MAC design and cross-layer issues D. Raychaudhuri & N. Mandayam 2/18/04

2. Today’s Lecture • Wireless MAC • Understanding the relationship between PHY and MAC • Some design examples from 802.11, Bluetooth, Hiperlan, UWB,… • A cross-layer design project (MAC flow scheduling in 802.11)

3. Important PHY Parameters • Wireless MAC’s need to be designed to deal with the following PHY parameters • Propagation delay (span of 1m, 10m or 1Km?) • Bit-rate (1 Mbps vs. 10 vs. 100?) • Modem acquisition & training delay • Radio coverage (hidden/exposed nodes) • Carrier sensing (yes/no), and its threshold • Spreading codes (yes/no), and capture • Link reliability

4. Higher Layer Considerations • MAC also needs to consider higher layer requirements such as • Centralized AP vs. ad-hoc modes • Single vs. multi-hop usage • Flows and connectionless packets • Latency constraints • QoS needs, if any • Packet formats and fragmentation • Level of reliability required at layer 3

5. MAC Design Options • Several design options for wireless MAC • Slotted channel vs. asynchronous • Pure contention (ALOHA) • Carrier sensing (CS) • Collision detection (CD) • Collision avoidance (CA) • Locally synchronous scheduling • Time division multiple access (TDMA) • Code division multiple access (CDMA) • Polling, Reservations • RLC (reliable retransmission protocols)

6. Impact of Prop Del & Bit Rate • Effect of increasing propagation delay or bit-rate • R=100m ~ 1ms, 1 Km ~ 10ms, 10 Km ~ 100ms • Pkt size = 50B -> tx time @ 10Mbps = 40 ms, @100 Mbps = 4 ms • Pkt size = 1000B -> tx time @ 10Mbps = 800 ms, @100 Mbps = 80 ms span r a = (span/c)/(pkt size/R)

7. Impact of Prop Del & Bit Rate • 802.11 uses CS/CA which works only for a <<1 (WPAN and WLAN) • Think about outdoor mesh 802.11 with ~1-10 Km average spacing between nodes and R~10-100 Mbps • 802.16 with similar parameters • Alternative TDMA based access protocols proposed for this scenario…  slides showing CSMA/CA, Bluetooth & DTDMA

8. Impact of Modem Synch • Critical parameter for control packet overhead • Typical control pkt ~16B payload • Sync overhead ~16B can be tolerated, lesser is better… • 802.11 params: sync hdr 24, RTS 20 = 352 ms @1Mbps • WATM params: sync hdr 16, control 8, 8 ms @25Mbps

9. TDMA/TDD MAC Protocol • Important wireless MAC category with variations used in • Hiperlan/WATM • Bluetooth • 802.15.3 and possibly 802.16 • Detail of implementation varies TDMA Frame TDM Downlink D-TDMA Uplink Modem preamble S-ALOHA control User B Burst from User A To Access Point User C Burst from Access Point -> Mobiles

10. TDMA Frame (~1-2 ms) TDM Downlink D-TDMA Uplink Modem preamble S-ALOHA control User B Burst from User A To Base Station User C Burst from Base Station -> Mobiles Cell Sequence No. # Data Cells Type RSN VPI GFC AP Identifier # Control Pkts VPI VCI TDMA Frame No. Mobile ID Mobile ID # Dn Control Slots VCI CLP PTI # Slots Req/Start.. VCI # Up contention slots .. ..Slot HEC # Dn data cells RT Reserved Resvd/# Slots Alloc Superframe size Standard ATM Payload (48 bytes) CRC-16 CRC-16 Reserved (8 Bytes) BW Req/Alloc Pkt Uplink Subframe Header CRC-16 CRC-16 Frame Header WATM Cell TDMA/TDD MAC • MAC protocol used in some broadband wireless scenarios (Hiperlan, 802.16, WATM) supports flow QoS, etc.

11. D-TDMA RLC Protocol • RLC with group ACK for error recovery on a per-flow basis, both UBR and CBR selective retx initial data tx 5 4 3 2 1 3 Transmit DLC buffers Receive DLC buffers 5 4 3 2 1 5 4 2 1 3 MAC Interface MAC Interface VC x VC x AAL5 packet for UBR/ABR ACK (1,2,4,5) ACK(3) • Involves buffering & re-sequencing for each service flow...

12. UWB: MAC protocol example • MAC optimized for bulk data transfer • Utilizes multi-code CDMA capability in UWB • Simplifies synch requirements for UWB PHY Downlink Beacon (for synch) Code 1D (high PG) Access Zone coverage Code 2D Timing marker Control packet (with allocations) Donwlink access control channel (multicast) Code 1U Uplink access control channel (asynch random access) TDM downlink Code 3D Service Zone coverage Scheduled (TDMA) uplink PHY bit rate may be Adapted dynamically Allocations relative to timing markers Code 2U (low PG)

13. UWB: Ad-hoc MAC example • Potential MAC/link layer based on DS/CDMA UWB PHY: • Continuous beacon for synchronization • Low bit-rate, high-spreading gain common channel • Handshake protocol for setting achievable link bit-rate Beacon S1 Beacon S2 S1 S2 Link establishment signal (S1,S2, C12) Link ACK (S1,S2, C12) S1 S2 Rate adaptation, ARQ Common code Control Code B Code A

14. Impact of Radio Coverage • Radio propagation effects result in “hidden node” and “exposed node” problems • Arises in both centralized and ad-hoc network architectures • Hidden node solutions: • Broadcast of control information from AP • RTS/CTS procedure in 802.11 • Separate node discovery phase

15. Impact of Radio Coverage • Exposed node solutions (…more difficult problem) • MACA-P (Arup Acharya, 2002) • D-LSMA (Zhibin Wu, work in progress)

16. Impact of Channel Quality • Variations in link SNR have an impact on MAC in terms of: • Adaptive link bit-rate may vary in certain systems (e.g. 802.11). Results in major changes in pkt tx time, control overhead, etc. • Packets may experience high error rates, resulting in repeated retransmission, and hence poor throughput • Combination of the above may occur as well

17. Impact of Channel Quality • Typical solutions are: • Small packets or adaptive packet fragmentation • Built-in radio link control (RLC) protocol, e.g. ACK in 802.11 or group ACK in DTDMA • Increased backoff for retransmitting users • MAC scheduling based on link quality

18. Impact of Channel Quality • Scheduling in 802.11 R=11Mbps User 1 SNR=20 dB R=5.5 Mbps R=1 Mbps User 3 SNR=15 dB User 2 SNR=8 dB

19. Impact of Channel Quality • Scheduling in 802.11 prioritizing by channel quality, flow rate needs, etc. 2 2 Retx 1 1 1 2 Retx 2 2 Channel time (without scheduling) 3 2 1 …. @5.5 Mbps Channel time (with scheduling)

20. Network Layer Considerations:MAC for multi-hop service • MAC optimizations needed for different types of service, for example ad-hoc multihop vs. centralized single hop • An example is the DCMA protocol for “cut-through” ad-hoc routing + MAC • See Acharya’s paper

21. Network Layer Considerations:Supporting flow QoS • Wireless MAC’s studied have various levels of QoS support • Explicit support in D-TDMA schemes (connection oriented) • Hybrid contention/reservation method in 802.11 spec (not supported by most vendors) • PCF (point co-ordination function) See slide on PCF from 802.11 tutorial

22. MAC Project • Design and Prototyping project • Test harness for adding limited application level MAC features to 802.11 provided (with RTS/CTS and ACK’s turned off) • Linux PC AP and client platforms with modified drivers, etc. provided by Z. Wu • Design document (full description, flow charts, pseudo-code, etc.) and prototype demo as deliverable by April 15.

23. MAC Project AP test harness for application level MAC development • 802.11 protocol extensions Client test harness for application level MAC development

24. MAC Project • Design MAC reservation and scheduling extensions to provide flow QoS for 1,2..n users. Test with 3 UDP sources generating ~0.5, 1, 2 Mbps. Measure connection setup delay, flow rate, overhead, net throughput and packet loss rates. OR

25. MAC Project • Design MAC reservation, error control and scheduling extensions to provide fast downloading of large files to 1,2..n users with simultaneous requests. Prototype with 3 users each downloading a 10 MB file using TCP. The goal is to minimize total elapsed time for delivering all 3 files to completion.