Link-Adaptable Polling-based MAC Protocol for Wireless LANs Byung-Seo Kim, Sung Won Kim, Yuguang Fang and Tan F. Wong Department of Electrical and Computer Engineering University of Florida GlobeCom 2004

Ju-Mei Li Outline Introduction Related Work Two-Step Multi-Polling Scheme (TS-MP) Performance Evaluation Conclusions

Ju-Mei Li Introduction Point Coordination Function (PCF) –supports time-bound services PC A BC D BSS Beacon: parameters to control the superframe After Beacon: PC use Round-Robin scheme to poll stations polling Data polling Data polling Data

Ju-Mei Li Introduction Disadvantage of PCF –Overhead of polling frames, the throughput is low –Round-Robin scheme is inefficient –Collision may be caused by stations in neighboring BSS –Transmission time is unpredictable

Ju-Mei Li Related Work e Hybrid Coordination Function (HCF) in the CFP use PCF with two enhancements –RTS/CTS handshaking Collision avoidance –The station in neighboring BSS –Use Transmission Opportunity (TXOP) Transmission time of a station will be bounded

Ju-Mei Li Related Work: multi-poll [8] CP-MP protocol –After broadcasting beacon frame, PC sends a multipolling frame containing the allocated TXOP and initial backoff time for each station to be polled –Each station send packet to PC in its TXOP CSMA/CA with backoff time RTS/CTS handshaking in order to avoid collision

Ju-Mei Li Motivation Polling schemes in HCF and CP-MP solve –Collision problem caused by a station in neighboring BSS –But more overheads due to the RTS/CTS exchanges But CP-MP scheme –May introduce collision in the same BSS There are no rate-adaptive MAC protocol for the PCF proposed in the current literture

Ju-Mei Li Two-Step Multi-Polling Scheme (TS-MP) Basic idea Polling scheduler –First scheduler for SRMP –Second scheduler for DTMP

Ju-Mei Li TS-MP: basic idea PC A BC D E Status-Request Multi-Poll (SRMP) Status-Response (SR) Overhearing SR of C: set the Tentative-NAV Data Transmission Multi-Poll (TDMP) BSS When E hears the data frame from C, E will reset its NAV

Ju-Mei Li TS-MP: basic idea PC A B C D E Beacon SRMPDTMP NAV Status Response (SR) (Date Rate, Buffer Status) Data Transmission (TXOP) Status Collection Period (SCP) Data Transmission Period (DTP) CFPCP PC A B C D E BSS

Ju-Mei Li TS-MP: frame structure Byte 2 Frame Control 6 BSSID 1224 Polling Count (N) AID 1AID NFCS (a) SRMP Byte 2 Frame Control 6 BSSID 2124 Tentative NAV Buffer Status AIDFCS 1 Down- Rate (b) SR Byte 2 Frame Control 6 BSSID 15 * Polling Count (N)4 Polling Count (N) AID FCS Polling Control Up-RateTXOP (c) DTMP

Ju-Mei Li SPi –inter-arrival time of frames at station i –w i related to SPi in order to manage the polling time of station i –is the normalized number of transmitted frames TS-MP: polling scheduler

Ju-Mei Li P i : payload M i : the average data arrival rate in the MAC layer at node i T SF : time duration of superframe T i d : from the time instant when frame is generated in MAC layer to the time instant when frame is transmitted by station i The number of transmitted frames at station i in superframe j Frame delay

Ju-Mei Li TS-MP: polling scheduler First scheduler for SRMP –If number of polled stations is very large large amount of time is spent during SCP –Overhead –Poor performance –In order to avoid this problem The polled stations N i in the current CFP i –Is determined from previous CFPs If PC found shortage of DTP in previous CFP –The number of polled stations in SRMP is reduced by one If PC found that DTP is long enough in previous CFP –The number of polled stations in SRMP is increased by one CFP SCPDTP

Ju-Mei Li TS-MP: polling scheduler First scheduler for SRMP (cont.) –Lower w i value, higher priority Low value of w i means –Station i has high probability of having frames to transmit –Lower value, higher priority Station with low has high probability to have pending frames PC chooses N i stations with low w i Stations with the same w i –PC choose stations with lower

Ju-Mei Li TS-MP: polling scheduler Second scheduler for DTMP –TXOP i = (T pre +T phy_hdr +T MAC_hdr +2T SIFS +T ACK +L payload /R i )*Q i –R i : is data rate in the physical layer –Q i : number of frames in buffer station i

Ju-Mei Li Performance Evaluation One BSS with diameter of 250m Two rear-time traffic –CBR On: 1 sec Off: 1.35 sec Generate a frame of 200bytes at every 0.1 sec –VBR Bit rate of 53Kbps ~ 900Kbps Size of each video packet: 800 bytes

Ju-Mei Li Performance Evaluation CBR

Ju-Mei Li Performance Evaluation VBR

Ju-Mei Li Performance Evaluation

Ju-Mei Li Conclusions New polling-based MAC protocol for the PCF in IEEE WLAN –First multipolling PC obtain the information about The polling sequence for data transmission –Second multipolling Coordinates data transmission to avoid collision –Reduced the overhead caused by polling frames –More efficient scheduling schemes as well as rate adaptation

Thank You !!