Design and Implementation of a Reservation-based MAC Protocol for Voice/Data over IEEE Ad-Hoc Wireless Networks Shiann-Tsong Sheu, Tzu-Fang Sheu IEEE ICC ’ 2001 IEEE INFORCOM 2001 Selected Areas in Communications (jsac)2001

Outline Motivation Introduction Distributed Bandwidth Reservation Protocol (DBRP) Simulation Conclusion

Motivation 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 STA V intends to access the channel, it first listens to the reservation frame (RF) in a interval D max D max 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 D max, it means there is no active voice station. STA v 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

If VBT becomes zero, STA v will enter the Send_RTS procedure. If no collision occurs, STA v will receive a CTS frame. Then this STA v 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 p p. With a probability q = 1- p p,it will defer at least one Slot_time VBT = rand(c+1, d) × Slot_time

Reservation State If STA V detects a RF frame during D max,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 × T voice

Reservation State During WT, STA V 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, STA V will follow the backoff scheme.

Reservation State To prevent the transmission of the asynchronous MPDU from stretching the voice repetition period Dmax : D max ≧ RP max + T maxMPDU RP max is sum of the maximum voice packet reservation period and voice station contention period.

Reservation State

Transmission State A STA V duration is over the boundary of RPmax. If SID of STA V is equal to one (RCG), it deliver a RF frame and its voice packet immediately. Other STA V 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 STA V 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

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

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?