1. A Scalable MAC Protocol for Next-Generation Wireless LANs Zakhia (Zak) Abichar, J. Morris Chang, and Daji Qiao Dept. of Electrical and Computer Engineering Iowa State University IEEE International Symposium on a World of Wireless, Mobile and Multimedia Networks (WoWMoM), June 2006

2. Slide 2/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Outline Introduction System model Limitations of DCF in Next-Generation WLANs Group-Based Medium Access Control (GMAC) Simulation Results

3. Slide 3/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Introduction The performance of current wireless LANs is not sufficient for our needs – With higher rates, we can enable new applications Multimedia-oriented (video, voice) IEEE 802.11n – Currently a draft (Pre-N products) – Aims at a throughput higher than 100 Mbps – Backed by Enhanced Wireless Consortium (EWC), an industry group – Requirement: 802.11n and 802.11b/g can operate in the same WLAN Simply increasing Tx rates doesn’t provide a higher throughput – The overhead of the MAC should be reduced

4. Slide 4/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs System Model Infrastructure-based WLAN with high data rates – Rates up to 270 Mbps A large number of users in a cell – Enough bandwidth to serve many users Hidden nodes Stations are able to localize themselves – Error upperbound δ

5. Slide 5/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Limitations of DCF DCF has a high overhead especially in next-generation WLANs Overhead of DCF: – Interframe spaces (IFS), backoff slots, control packets – Weight of overhead becomes magnified with high rates Collision rate becomes high when the number of station is large CTSRTSDATAACK Current WLANs CTSRTSACK Next-Generation WLANs DATA DIFS contention SIFS

6. Slide 6/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Overhead of DCF Control packets are still transmitted at low rate – They should be received by all the stations – Maintain interoperability with 802.11b/g stations In the figure, there are no collisions – i.e. assume there is one station only Cases: – Rate: 27, Throughput: 15 – Rate: 81, Throughput: 25 – Rate: 135, Throughput: 28 – Rate: infinity, Throughput: 36 Throughput upperbound of DCF

7. Slide 7/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Collision Rate of DCF Collision rate of DCF is high with a large number of stations In the figure: – collision rate with no hidden nodes 10 stations: coll. rate=15% 30 stations: coll. rate=26% 100 stations: coll. rate=40% Collision rate of DCF

8. Slide 8/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Other Limitations of DCF Hidden nodes – Use RTS/CTS (further control overhead) Fairness in a multi-rate environment – With DCF each station transmits one packet upon access: Throughput-based fairness – Stations with low rate occupy the channel for a long time – We use time-based fairness: a station is allocated a time to transmit one packet at the lowest data rate Stations with high rates aggregate multiple MPDUs CTSRTSDATAACK Low-rate station CTSRTS ACK High-rate station DATA

9. Slide 9/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Group-Based Medium Access Control (GMAC) Stations are divided into groups – Each group has a leader – Groups are free of hidden nodes Only group leaders contend using CSMA/CA – A winning leader reserves time for all its group (RTS/CTS) – Then it transmits a polling packet Non-leader stations transmit following the polling packet Interoperable with 802.11b/g AP Group leader: Non-leader: Contention Group 1 Group 2 Group 3

10. Slide 10/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Details of GMAC Group formation Contention of group leaders Polling of non-leaders Group maintenance

11. Slide 11/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Group formation Groups should be free of hidden nodes Group leaders broadcast their location in the polling packet A station joins a group if: (d(sta,leader)< R/2 - 2δ) && (groupSize < maximumSize) – Otherwise, the stations becomes a leader A station indicates a leader in the Association Request Implementation: – Several localization schemes based on Received Signal Strength (RSS) – Localization based on GPS Polling Packet Group formation

12. Slide 12/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Contention of Group Leaders Group leaders contend using CSMA/CA Reserves time using RTS/CTS – Time to transmit one data packet at the lowest rate – Stations aggregate multiple packets if they have a high rate Leader monitors all the transmissions – Check if some stations skip Release the reserved time early by the End-NAV packet Reserved time for 7 stations

13. Slide 13/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Polling of Non-leaders A winning leader polls all the stations in its group Non-leaders transmit – SIFS between consecutive transmissions Within a group, no hidden nodes – A station can detect when other stations have missed transmission

14. Slide 14/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Group Maintenance Leader withdraws: group is disbanded – Form a new group according to the initial procedure – We cannot relegate the role of leader to another station, this cannot guarantee the group remains free of hidden nodes Non-leader withdraws: it is removed from the polling list Leader fails: – Group leader appends its backoff counter – Non-leaders can track the transmission of their leader and detect failure Non-leader fails: – The leader uses a timer Timer-Skip-Max – If a station skips many times, its timer is reset – Station is removed from the polling list Data of Leader4 Backoff used in next contention CTSRTS

15. Slide 15/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Simulation Results Custom MAC-level simulation for GMAC and DCF PHY characteristics from 802.11n draft Two configurations for the data rates: – Single-stream rates. Two antennas at each station. Rates from 13.5 Mbps to 135 Mbps – Double-stream rates. Four antennas at each station. Rates from 27 Mbps to 270 Mbps Nodes uniformly distributed in the WLAN cell

16. Slide 16/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Throughput with High Data Rates Number of stations: 20 – Low collision rate Did we reduce the control overhead? Single-stream rates (Mbps) – Average rate: 13.5 to 62 – GMAC: 8.9 to 47.3 – DCF: 4.5 to 6.1 Double-stream rates (Mbps) – Average rate: 27 to 125 – GMAC: 19 to 94 – DCF: 5.4 to 6.5 Single-stream rates Double-stream rates

17. Slide 17/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Throughput with a Large Number of Stations Large number of stations Did we reduce the collision rate? Single-stream rates (Mbps) – Average rate: 56 to 49 – GMAC: close to 40 – DCF: 6.2 to 4.1 Double-stream rates (Mbps) – Average rate: 113 to 98 – GMAC: 81 to 74 – DCF: 6.5 to 4.3 Single-stream rates Double-stream rates

18. Slide 18/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Dependency on the Average Data Rate Number of stations: 20 Single-stream rates Simulation with various positions of the stations – Average rate changes The throughput is sensitive to the average rate Single-stream rates Throughput Average rate

19. Slide 19/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Conclusion We need new MAC protocols for Next-Generation Wireless LANs – Focus on reducing the overhead and collision rate In GMAC: – A hierarchical approach reduces the overhead – Maintains a high throughput Scales with data rates Scales with the number of stations

20. Slide 20/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Thank you

21. Slide 21/20 – A Scalable MAC Protocol for Next-Generation Wireless LANs Limitations of DCF DCF has a high overhead especially in next-generation WLANs Overhead of DCF: – Interframe spaces (IFS), backoff slots, control packets – Weight of overhead becomes magnified with high rates Collision rate becomes high when the number of station is large