Design and Implementation of aReservation-based MAC Protocolfor Voice/Data over IEEE 802.11Ad-Hoc Wireless Networks

Shiann-Tsong Sheu, Tzu-Fang SheuIEEE ICC’2001IEEE INFORCOM 2001Selected Areas in Communications (jsac)2001

Outline Motivation Introduction Distributed Bandwidth Reservation

Protocol (DBRP) Simulation Conclusion

Motivation 802.11 includes DCF & PCF in MAC layer. PCF supports real-time traffic in a centralized scheme.

Introduction Time-sensitive voice packets have the higher priority to access the channel Frames are divided into three priorities

SIFS is used by control frame. PIFS is used by real-time frames. DIFS is used by non-real-time frames.

data station regulates their access according to CSMA/CA protocol.

Introduction voice station follows a modified CSMA/CA protocol. voice station could transmit its packets without contention if any voice station leaves, the reserved bandwidth is released immediately.

DBRP Medium access procedure for voice station is separated into three states

Reservation State Transmission State Initial State

A sequence ID (SID) and an active count (AC) are used to maintain access sequence.

Reservation State If a STAV intends to access the channel, it

first listens to the reservation frame (RF) in a interval Dmax

Dmax is the maximum tolerance delay of voice packet (25ms)

The RF frame only carries the information of the number of active voice stations (AN) in the BSS

Reservation State If a RF frame is not received during the interval Dmax, it means there is no active voice station. STAv go to Backoff procedure if the channel is idle in the interval (t+ Dmax, t+ Dmax+PIFS)

Reservation State Reservation cycle generator (RCG) is the first active voice station,has to issue RF frame and initiate the real-time period. The voice backoff time of a voice station (VBT) is defined as VBT = rand(c,d) × Slot_time c and d are set to 0 and 3 ,respectively.

Reservation State

Reservation State If VBT becomes zero, STAv will enter the Send_RTS procedure. If no collision occurs, STAv will receive a CTS frame. Then this STAv becomes the RCG and enter Transmission State. It sets both its SID and AC to one and transmits RF and voice packet .

Reservation State If collision occurs when transmitting the RTS, P-persistent scheme is used. The collided voice station will retransmit RTS in the following Slot_time with a probability pp . With a probability q = 1- pp ,it will defer at least one Slot_time VBT = rand(c+1, d) × Slot_time

Reservation State If STAV detects a RF frame during Dmax ,it enters RF_received procedure and sets it’s AC to the AN field of RF frame. AC is used to generate proper SID. To avoid disturbing the reserved voice stations, the new voice station will wait for WT: WT = RF.AN × Tvoice

Reservation State During WT, STAV will enter Wait_to_contend procedure. If the channel idles a Slot_time during WT, we consider a voice disconnects and the WT should be decreased. After duration of WT, STAV will follow the backoff scheme.

Reservation State To prevent the transmission of the asynchronous MPDU from stretching the voice repetition period Dmax : Dmax ≧ RPmax + TmaxMPDU RPmax is sum of the maximum voice packet reservation period and voice station contention period.

Reservation State

Transmission State A STAV duration is over the boundary

of RPmax. If SID of STAV is equal to one (RCG) , it

deliver a RF frame and its voice packet immediately.

Other STAV s keep monitor the channel and silence until receiving a RF frame.

Transmission State After a RF frame is received , a important cd_timer is used : cd_timer = (SID – 1) × Tvoice A STAV will transmit its voice packet if its cd_timer is decreased to zero.

Transmission State If the channel is idle for a Slot_time (no RF frame received), we assume the RCG has left. Every following station with larger SID and AC will decrease both its SID and AC by one.

Transmission State During the waiting period, the station keeps monitoring the channel If the channel is idle for a Slot_time, we assume the station that should transmit its voice packet at this moment has disconnected

Decrease both SID and AC by everyone to shift its access sequence in the virtual channel list

Transmission State

Simulation

Simulation Goodput is defined as the percentage of the bandwidth used to transmit their pure payloads.

Simulation

Simulation PDP is defined as the ratio of the

discarded voice packets to total voice packets

Simulation

Conclusion A reservation-based MAC protocol is proposed. The voice station can reserve/release the channel resource dynamically Starvation?