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DPRMA (Distributed Packet Reservation Multiple Access) 2007. 5. 23 ( 수 ) 김 희 준

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Presentation on theme: "DPRMA (Distributed Packet Reservation Multiple Access) 2007. 5. 23 ( 수 ) 김 희 준"— Presentation transcript:

1 DPRMA (Distributed Packet Reservation Multiple Access) ( 수 ) 김 희 준

2 Contents Abstract Introduction Principle of DPRMA Approximate Performance Analysis Numerical Examples and Discussion Conclusion 2/37

3 Abstract Apply MAC scheme like TDMA for mobile ad hoc networks with emphasis on voice application support Major Effects ▲ simple slot reservation mechanism for voice traffic without relying on any central entity ▲ simple solution for the hidden and exposed terminal problems uniquely present in wireless ad hoc environments Performance test ▲ investigated by analysis and computer simulations in comparison with IEEE The results show that D-PRMA is much more suitable than IEEE for voice application 3/37

4 Introduction Existing difficult issue ▲ To design MAC scheme to support real-time applications  No fixed central entities can be used by the MAC layer in MANETs to coordinate communications  High dynamics of network topology caused by terminal mobility  real-time applications have requirements on QoS  the MAC scheme should be simple for implementation because terminals in such networks are portable and battery-operated personal devices Only focuses on MAC schemes based on channel in time ▲ No condition constantly frequency, frequency hopping 4/37

5 Introduction No central entities in mobile ad hoc environments ▲ Aspects of Unslotted MAC scheme  No useful “Jamming” mechanism is Like The MAC in HIPERLAN  Overhead problem that the reservation in MACA/PR is maintained by asking all neighbors to exchange their reservation tables ▲ Aspects of slotted MAC scheme  they can avoid difficulties in synchronization  Apply GPS problem (at providing global clock)  The same effort can also be found in code division multiple access (CDMA)- based third-generation cellular systems 5/37

6 Introduction Slotted-channel-based MAC schemes ▲ a successful contention process (FPRP / E-TDMA)  a long access delay for real-time applications if a slot is reserved by a terminal at the “talkspurt” level ▲ In the TDMA/FDD-based scheme  a slot is reserved for a voice terminal until the end of a call ▲ PRMA  a centralized and slotted MAC scheme  providing a mechanism for slot reservation at the “talkspurt” level for voice and data applications  with a base station as the fixed entity for the MAC operation So, Author discuss a simple extension of PRMA, termed “distributed PRMA” (D-PRMA) ▲ with emphasis on voice application support in mobile ad hoc environments 6/37

7 PRINCIPLE OF D-PRMA Slot Reservation Scheme Solution for the Hidden and Exposed Terminal Problems 7/37

8 PRINCIPLE OF D-PRMA Notations ▲ N: Total number of terminals in the system. ▲ F: Frame length in time units. ▲ O : Number of slots in one frame. ▲ m : Number of minislots in the payload of a slot. ▲ p : Contention probability. D-PRMA characteristics ▲ TDMA-based scheme ▲ uniformly attaches such fields to each slot ▲ tries to simplify the solution for the hidden and exposed terminal problems ▲ To facilitate a terminal to locate its reserved slot in the subsequent frames ▲ improve channel utilization( used several minislot for contention) 8/37

9 PRINCIPLE OF D-PRMA Minislot ▲ RTS/BI and CTS/BI  used by a terminal to both reserve a slot and prevent hidden terminals from colliding with transmission in the respective slots if a terminal wins the contention through the first minislot of a slot ▲ the extra minislots of this slot will be granted to the terminal as the payload ▲ the same slot in each subsequent frame can be reserved 9/37

10 Slot Reservation Scheme Reservation process is similar to the RTS/CTS used in IEEE ▲ sender detects the channel idle at the beginning of a minislot  some part of RTS/BI of each minislot is dedicated to channel sensing 10/37

11 Slot Reservation Scheme Guarantee voice traffic ▲ Define rule to prioritize voice terminals  voice terminals start the contention from minislot 0 with probability p=1 (data terminals p < 1)  Give same probability(p<1) through m extra minislots contention  to avoid unnecessary slot reservation ▲ the winner of a voice terminal can reserve the same slot in each subsequent frame until the end of the packet transmission  data terminal can only use one slot 11/37

12 Solution for the Hidden and Exposed Terminal Problems consider the following two cases ▲ When a terminal wins the contention in minislot 0, how to prevent other terminals from using any of the extra minislots in the same slot for contention? ▲ How to prevent a terminal from contending for a reserved slot in each subsequent frame? 12/37

13 Solution for the Hidden and Exposed Terminal Problems For case 1 ▲ use of RTS/CTS-like dialogue a part of solution  MACA consider for the same problems ▲ a winner through minislot 0 will transmit immediately from minislot 1 of the same slot  the neighbors of the sender will detect a busy channel before trying to send an RTS ▲ CTS/BI can be used  a terminal that receives RTS destined to it to transmit the respective CTS ▲ all terminals hearing the CTS sent by the receiver  not allowed to transmit during the remaining period of the same slot to avoid the hidden terminal problem ▲ Still transmit to avoid the exposed terminal problem  other terminals only hearing the RTS but not the CTS 13/37

14 Solution for the Hidden and Exposed Terminal Problems For case 1 ▲ to avoid the exposed terminal problem  duplex communication, where a sender may also be a receiver simultaneously and vice versa  the transmission of the sender’s neighbors should not be allowed either  a terminal hearing the RTS but not the CTS  not transmit anything during the remaining period of the same slot to avoid collision with the sender’s receiving ▲ If either the RTS and/or the CTS collide  the extra m minislots in the same slot can be still used for contention 14/37

15 Solution for the Hidden and Exposed Terminal Problems For case 2 ▲ define that the receiver of a reserved slot will send a busy indication (BI) immediately  through the RTS/BI of minislot 0 of the same slot in each subsequent frame without channel sensing, and so will the sender through the CTS/BI ▲ Letting the receiver transmit a BI signal first  also tries to avoid the hidden terminal problem  since not every neighbor of the receiver can hear from the sender while all neighbors of the sender can hear from the sender ▲ A terminal hearing a BI signal  not contend for the slot in the current frame 15/37

16 Approximate Performance Analysis Voice Traffic Model Analysis of System State Distribution Calculation of p i,j Calculation of p drop 16/37

17 Approximate Performance Analysis analyze the performance in an one-hop environment where all terminals can hear each other About Voice terminal ▲ only voice terminals can start contention from minislot 0 ▲ the bandwidth to be used by data terminals is mainly that which is not being used by voice terminals Voice packet dropping probability (p drop ) ▲ voice packet will be dropped if it is queued beyond a threshold  Generally, should be less than for an acceptable voice communication quality Leftover bandwidth for data traffic(L band ) ▲ left over by voice terminals can be used for data applications 17/37

18 Voice Traffic Model Voice terminal ▲ generates a pattern of talkspurt and silence periods as classified by its voice activity detector ▲ A terminal’s vocoder digitizes talkspurts into packets and suppresses silence periods  digitized packets have a fixed length Model for voice traffic described Markov process ▲ exponentially distributed talkspurt periods / silence periods 18/37

19 Voice Traffic Model Eqn. about periods ▲ Talkspurt periods ends within τ period ▲ t 1 = length of talkspurt ▲ slience periods ends within τ period ▲ t 2 = length of silence Author said ▲ Applying Two equations, can calculate p 0 and p 1 by setting τ equal to F 19/37

20 Analysis of System State Distribution durations of the talkspurt and silence periods are much longer than the length of a frame assume that terminal state transitions between “talkspurt” and “silence” occur only at a frame’s boundary Variable are defined to characterize system states observed at beginning and end of a frame ▲ R(R-) : Number of terminals in “reservation” state ▲ C(C-) : Number of terminals in “contention” state ▲ S(S-) : Number of terminals in “silence” state The system state 20/37

21 Analysis of System State Distribution Finite state space ▲ Modeled as as Markov process {Zi} ▲ probability of the system in steady-state Zi  is the dimension of the system state space ▲ Denote k as number of terminals with reservation  Its maximum is min(N,O)  Maximum number of contending terminals is N-k  N-k+1 is the maximum number of system states with respect to the number of contending terminals (+1 for zero contending terminal state)  where and 21/37

22 Analysis of System State Distribution Let p i,j the probability for a transition from system states to z i to z j denote the one step transition probability ▲ then, can have form with ▲ and Expectation Value ▲ and 22/37

23 Calculation of p i,j the “talkspurt–silence” transition is independent of the contention process a contention process in the frame ▲ numbers of terminals in the “reservation” state ( ) and in the “contention” state ( ) at the end of that frame and where s c,r is the number of terminals that have successfully made reservations in the frame ▲ numbers of terminals in the “reservation” state ( ) and in the “contention” state ( ) at the beginning of the next frame and where d c, d r, d s denote numbers of terminals that have departed from states “contention” and “reservation” as well as “silence” at the frame’s boundary ▲ Thus, r j and c j for state Z j that system at the beginning of the next frame 23/37

24 Calculation of p i,j p i,j and can be calculated from the distributions of D r, D s,D c and S c,r ▲ Where are random variable d r, d s, d c and s c,r Let ▲ Where x is the number of terminals successfully making reservation in a frame in the case of e free slot and c contending terminals available at the beginning of that frame aa ▲ T(c) the probability for a successful transmission of RTS/CTS in an available slot  c contending terminals available at the beginning of that slot ▲ Q(c) denote the probability of a successful transmission of CTS among c contending terminals with probability p through one of the m extra minislots of a free slot 24/37

25 Calculation of p i,j dd ▲ Where c=c i and e=O-r i for state Z i d The one-step state transmission probability 25/37

26 Calculation of p drop d ▲ d ▲ Where S’ c,r is the random variable for the number of terminals  Terminals that obtain reservations in minislot 0 of a frame ▲ computed as the ratio of the average number of voice packets dropped in a frame to the average number of packets generated per frame ▲ In the design, the frame length can be set to the queuing delay threshold for voice packets The average number of packets dropped per frame ▲ equal to the average number of contending terminals at the beginning of a frame minus the number of terminals that obtain reservation through minislot 0 in a frame 26/37

27 Calculation of L band A slot can be used ▲ by data terminals if and only if no voice terminal has reserved or contend for this slot ▲ E[S a ] is the average number of free slots  Slots available for contention in a frame ▲ E[S vc ] is the average number of voice terminals  Terminals start their contentions per frame 27/37

28 Numerical Examples and Discussion a voice terminal in “talkspurt” generates exactly one packet per frame and each payload of a slot carries one packet, the above parameters should satisfy ▲ r s = source rate of voice traffic ▲ h = physical and MAC layer headers for each digitized packet ▲ r s x F = amount of source information per packet ▲ r s x F + h = total packet length ▲ T o, T g = durations of minislot 0 and a slot guardband ▲ r c = channel transmission rate 28/37

29 Numerical Examples and Discussion ▲ Rx/Tx and Processing overhead set to zero ▲ N v,N d = The number of voice terminals and data terminals are denoted ▲ P v, P d = the contending probability of voice and data terminals are denoted 29/37

30 Analytical Results Versus Simulation Results 30/37

31 Performance of D-PRMA average delay experienced by voice packets with D- PRMA ▲ All Delay is shorter than a frame duration of 16ms -> longer than 16 ms are dropped by the MAC layer ▲ Pv has almost no effect on delay but Nv -> lets voice terminals start contention from minislot 0 of a free slot and the probability of such a successful contention is high with the configuration given by Table I ▲ Author can get the probability of such unsuccessful contention P f,0 (Nv) -> Calculate value is P f,0 (10)= and P f,0 (20)= * Nv is larger, then P f,0 (Nv) is low 31/37

32 Performance of D-PRMA P drop generally increases with N v A little different results for P v increases ( ) P drop generally increases with N v big different results for P v increases ( ) The reason is probabilistic contention in the m extra minislot is not so important as that in minislot 0 with prabability 1 If collision in minislot 0 occurs, P v becomes important in determining the success of contention in the m extra minislots Figure 3Figure 4 32/37

33 Performance of D-PRMA Figure 5Figure 6 L band generally decreases with N v A little different results for P v increases ( ) L band generally decreases with N v big different results for P v increases ( ) 33/37

34 Performance of D-PRMA Tendency of P drop and L band versus P v P v around 0.5 is suitable for most case of N v The following simulation, P v is set to 0.6 Figure 8Figure 7 34/37

35 Comparison With IEEE Simulations with OPNET (between D-PRMA and IEEE ) Data packets arrive at each data terminal according to a Poisson process with mean arrival rate 35/37

36 Ld (data traffic load) Delay of voice packets ▲ N v =16 and N v =24 (IEEE ) ▲ N v =24 (D-PRMA)  Value over  N v Should be controlled under about 15 to have P drop < Data traffic support ▲ IEEE performs better ▲ N v =16 and N v =24 (D-PRMA)  Especially longer The channel efficiency ▲ the ratio of the time used to transmit user packets to a given time period ▲ D-PRMA is lower than that given by IEEE especially in the case of high data traffic load 36/37

37 Conclusion Pros ▲ D-PRMA more suitable than IEEE for voice traffic while the latter is better for data traffic Cons ▲ deficiency of D-PRMA for data traffic  can be resolved by introducing a piggyback reservation scheme for data traffic ▲ a proper call admission scheme to control the number of data and voice terminals  requires a voice terminal to contend for every talkspurt for high channel utilization ▲ The channel efficiency of D-PRMA  further improved by maximizing the use of minislots for packet transmission 37/37


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