Study on Window-Based Reliable Multicast Protocols for Wireless LANs

Study on Window-Based Reliable Multicast Protocols for Wireless LANs

Huei-Wen Ferng, Ph.D.

Assistant ProfessorDepartment of Computer Science and Information Engineering (CSIE)Nation Taiwan University of Science and Technology (NTUST)Wireless Communications and Networking Engineering (WCANE) LabE-mail: hwferng@mail.ntust.edu.tw

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Outline

Introduction Description of the proposed protocols Performance study and numerical

examples Conclusions

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Unicast vs. Multicast Wired vs. Wireless Unreliable vs. Reliable We deal with the issue of incorporating th

e reliability into the multicast of wireless LANs.

Two major problems: ACK/NAK implosion and media access

Introduction (1/2)

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Introduction (2/2)

Existing approaches (Kuri and Kasera [6]): Delay feedback-based protocol (DBP) Probabilistic feedback-based protocol (PBP) Leader-based protocol (LBP)

Based on LBP, we further propose LBP with a sliding window (LBPW) LBP with a sliding window and n-fold acknowledgement

reduction (LBPR(n))

To achieve reliability, automatic repeat request (ARQ) is applied in this paper.

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Outline

Introduction Description of the proposed protocols Performance study and numerical

examples Conclusions

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Scenario

Basic network architecture: AP and several mobile hosts

We split the communication link into Sender to APs An AP to group members (GMs)

Merits of such an arrangement Scalability Local error recovery

LBPW and LBPR(n) are designed for the basic network architecture.

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Event PCW1 - AP to GMs (starting in slot k): Send an RTS to all GMs

Event PCW2 - Leader/GMs to AP (in slot k+1): Leader

Send a CTS if it is ready to receive data frames; otherwise, do nothing.

Other GMsSend an NCTS if it is not ready to receive data frames; otherwise, do nothing.

LBPWPhase of RTS/CTS exchange

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Event PTW1 - AP to GMs (in slot k+2): If a CTS was received by the AP in slot k + 1, start to transmit contiguously available na (<= WS) data frames with labels, say, 1, 2, . . . , na; otherwise, go back to event PCW1.

Event PTW2 - Leader/GMs to AP (during slot k + 2 + ⌈(fl * ttr * na + tpc + tpp)/tst⌉ and slot k + 1 + ⌈(fl * ttr * na + tpc + tpp)/tst ⌉ + na) :

LeaderIf the leader received the ith frame correctly, it sends an ACK in slot k+1+ ⌈(fl * ttr * na + tpc + tpp)/tst ⌉ +i; otherwise, it sends a NAK.

LBPWPhase of data frames transfer

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Other GMsIf the ith frame was received with error bits by any

GM, it sends a NAK in slot k + 1 + ⌈(fl * ttr * na + tpc + tpp)/tst ⌉ + i; otherwise, it does nothing.

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LBPW

Based on feedbacks from GMs, AP should make a decision.

Three cases AP faces: An ACK is received Nothing is received A collision occurs

Case I: frame is correctly received.Other cases: retransmission is required.

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Event PCW1 - AP to GMs (starting in slot k): Send an RTS to all GMs

Event PCW2 - Leader/GMs to AP (in slot k+1): Leader

Send a CTS if it is ready to receive data frames; otherwise, do nothing.

Other GMsSend an NCTS if it is not ready to receive data frames; otherwise, do nothing.

LBPR(n) Phase of RTS/CTS exchange

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LBPR(n)Phase of data frames transfer

Event PTR1 - AP to GMs (in slot k+2): If a CTS was received by the AP in slot k + 1, start to transmit contiguously available na (<= WS) data frames with labels, say, 1, 2, . . . , na; otherwise, go back to event PCW1.

Event PTR2 - Leader/GMs to AP (during slot k + 2 + ⌈(fl * ttr * na + tpc + tpp)/tst⌉ and slot k + 1 + ⌈(fl * ttr * na + tpc + tpp)/tst ⌉ + ⌈ na/n ⌉) :

LeaderSend an acknowledgement in a bit map, including the receiving status for at most n frames at a time. Hence ⌈ na/n ⌉ times of ACKs are

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required to send during slot k+2+ ⌈(fl * ttr * na + tpc + tpp)/tst ⌉ and slot k+1+ ⌈(fl * ttr * na + tpc + tpp)/tst ⌉ + ⌈ na/n .⌉

Other GMsBreak the na frames into ⌈ na/n ⌉ subsegments (eac

h including exactly n frames except the last one). If one of frames for subsegment i was received with error bits by any GM, it sends a NAK directly in slot k + 1 + ⌈(fl * ttr * na + tpc + tpp)/tst ⌉ + i; otherwise, it does nothing.

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LBPR(n)

Based on feedbacks from GMs, AP should make a decision.

Two cases AP faces: An ACK in a bit map is received A collision occurs

Case I: erroneous frames are retransmitted.

Case II: all frames are retransmitted.

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Outline

GSM/GPRS system Description of the proposed protocols Performance study and numerical

examples Conclusions

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Assumptions

A minimal wireless LAN is considered. MAC is neglected. Perfect time synchronization is assumed. One multicast group is considered. Each GM is assumed to be always ready to

receive data frames. Data frames may be corrupted but not lost. Control frames are always correctly received. Frames are generated according to Batch

Poisson.

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Performance Metrics

Costthe average time lasting since the AP contends the cha

nnel until the AP ascertains that all group members correctly receive the frame.

Exposureratio of the number of mobile hosts actually receiving t

he frame and the number of mobile hosts who do need the frame.

Average queueing delay/queue length, Number of ACKs or NAKs.

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LBPW vs. LBP (1/2)

•The reductions when WS = 2 and WS = 10 compared to LBP are 4.3% and 7.0%, when fl = 20 and nGM = 107.3% and 13.3%, when fl = 10 and nGM = 10

•The increase of the window size WS causes a lower cost, i.e., higher throughput.

•These results evidently show that LBPW with a large window size, say 10, performs much better than LBP.

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LBPW vs. LBP (2/2)

Exposure is not affected by the increase of the window size but it increases as the group grows up.

We see that the queueing delay goes down as the window size increases or the number of group members decreases.

NCNQPDCA

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LBPR(n) vs. LBPW (LBP) (1/3)

LBPR(n) achieves ACKs/NAKs reduction approximately by a factor of n compared to LBP or LBPW.

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LBPR(n) vs. LBPW (LBP) (2/3)

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LBPR(n) vs. LBPW (LBP) (3/3)

•LBPR(n) performs better than LBPW due to the saving of ACKs/NAKs.•The cost reduction is more obviously when FEP is high.

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Outline

GSM/GPRS system Description of the proposed protocols Performance study and numerical

examples Conclusions

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Conclusions The cost of LBPR(n) is lower than that of LBPW which is

subsequently lower than LBP. The attainable cost reduction of LBPW compared to LBP

can be over 10%. Both LBPW and LBPR(n) perform better than LBP in ter

ms of queueing delay. LBPW mostly performs better than LBPR(n) for n ≥3, whi

le LBPR(2) performs better than LBPW when the frame loss probability is low.

As for the exposure metric, LBPW is the same as LBP and smaller than LBPR(n). For larger n, the exposure of LBPR(n) becomes higher.

So, we suggest LBPW and LBPR(2) to be used.

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Thank You!