Special Topics in Wireless Networking MAC design and

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

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)

Important PHY Parameters • Wireless MAC’s need to be designed to deal with the following PHY parameters – Propagation delay (span of 1 m, 10 m or 1 Km? ) – Bit-rate (1 Mbps 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

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 – Qo. S needs, if any – Packet formats and fragmentation – Level of reliability required at layer 3

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)

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

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

Impact of Modem Synch • Critical parameter for control packet overhead • Typical control pkt ~16 B payload • Sync overhead ~16 B can be tolerated, lesser is better… • 802. 11 params: sync hdr 24, RTS 20 = 352 ms @1 Mbps • WATM params: sync hdr 16, control 8, 8 ms @25 Mbps

TDMA/TDD MAC Protocol • Important wireless MAC category with variations used in Ø Hiperlan/WATM Ø Bluetooth Ø 802. 15. 3 and possibly 802. 16 Ø Detail of implementation varies Modem preamble TDM Downlink TDMA Frame Burst from Access Point -> Mobiles D-TDMA Uplink S-ALOHA control Burst from User A To Access Point User B User C

TDMA/TDD MAC • MAC protocol used in some broadband wireless scenarios (Hiperlan, 802. 16, WATM) supports flow Qo. S, etc. Modem preamble TDM Downlink TDMA Frame (~1 -2 ms) S-ALOHA control Burst from Base Station -> Mobiles AP Identifier TDMA Frame No. # Dn Control Slots # Up contention slots # Dn data cells Superframe size Reserved (8 Bytes) CRC-16 Frame Header D-TDMA Uplink Cell Sequence No. VPI GFC VPI VCI PTI CLP VCI HEC Standard ATM Payload (48 bytes) CRC-16 WATM Cell Burst from User A To Base Station User B # Data Cells # Control Pkts Mobile ID Reserved CRC-16 Uplink Subframe Header User C Type RSN Mobile ID # Slots Req/Start. . . Slot RT Resvd/# Slots Alloc CRC-16 BW Req/Alloc Pkt

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 VC x AAL 5 packet for UBR/ABR MAC Interface 5 4 3 2 1 3 Receive DLC buffers MAC Interface 5 4 3 2 1 Transmit DLC buffers ACK (1, 2, 4, 5) 5 4 3 2 1 VC x ACK(3) • Involves buffering & re-sequencing for each service flow. . .

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

UWB: Ad-hoc MAC example • Potential MAC/link layer based on DS/CDMA UWB PHY: Ø Continuous beacon for synchronization Ø Low bit-rate, high-spreading gain common channel Ø Handshake protocol for setting achievable link bit-rate Beacon S 1 Beacon S 2 S 1 S 2 Link establishment signal (S 1, S 2, C 12) S 1 Common code Control Code A Rate adaptation, ARQ Link ACK (S 1, S 2, C 12) S 2 Code B

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

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

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

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

Impact of Channel Quality • Scheduling in 802. 11 R=5. 5 Mbps R=11 Mbps User 1 SNR=20 d. B R=1 Mbps User 3 SNR=15 d. B User 2 SNR=8 d. B

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 Channel time (without scheduling) 1 3 Channel time (with scheduling) …. 2 @5. 5 Mbps 2

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

Network Layer Considerations: Supporting flow Qo. S • Wireless MAC’s studied have various levels of Qo. S 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

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.

MAC Project • 802. 11 protocol extensions AP test harness for application level MAC development Client test harness for application level MAC development

MAC Project • Design MAC reservation and scheduling extensions to provide flow Qo. S 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

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.