DPRMA(Distributed Packet Reservation Multiple

Access)

2007. 5. 23 ( 수 )김 희 준

icemichy@hufs.ac.kr

Contents

Abstract Introduction Principle of DPRMA Approximate Performance Analysis Numerical Examples and Discussion Conclusion

2/37

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 802.11

The results show that D-PRMA is much more suitable than IEEE 802.11 for voice application

3/37

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 QoSthe 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

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

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

PRINCIPLE OF D-PRMA

PRINCIPLE OF D-PRMA Slot Reservation Scheme Solution for the Hidden and Exposed Terminal

Problems

7/37

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

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

Slot Reservation Scheme

Reservation process is similar to the RTS/CTS used in IEEE 802.11▲ 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

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

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

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

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

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

Approximate Performance Analysis

Approximate Performance Analysis Voice Traffic Model Analysis of System State Distribution Calculation of pi,j

Calculation of pdrop

16/37

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 (pdrop )▲ voice packet will be dropped if it is queued beyond a threshold

Generally, should be less than 10-2 for an acceptable voice communication quality

Leftover bandwidth for data traffic(Lband)▲ left over by voice terminals can be used for data applications

17/37

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

Voice Traffic Model

Eqn. about periods

▲ Talkspurt periods ends within τ period

▲ t1 = length of talkspurt

▲ slience periods ends within τ period

▲ t2 = length of silence

Author said▲ Applying Two equations, can calculate p0 and p1 by setting τ

equal to F

19/37

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

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

Analysis of System State Distribution

Let pi,j the probability for a transition from system states to zi to zj

denote the one step transition probability▲ then, can have form with

▲ and

Expectation Value▲ and

22/37

Calculation of pi,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 sc,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 dc, dr, ds denote numbers of terminals that have

departed from states “contention” and “reservation” as well as “silence” at the frame’s boundary

▲ Thus, rj and cj for state Zj that system at the beginning of the next frame

23/37

Calculation of pi,j

pi,j and can be calculated from the distributions of Dr, Ds,Dc and Sc,r ▲ Where are random variable dr, ds, dc and sc,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

Calculation of pi,j

dd

▲ Where c=ci and e=O-ri for state Zi

d

The one-step state transmission probability

25/37

Calculation of pdrop

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

Calculation of Lband

A slot can be used▲ by data terminals if and only if no voice terminal has reserved

or contend for this slot

▲ E[Sa] is the average number of free slots Slots available for contention in a frame

▲ E[Svc] is the average number of voice terminals Terminals start their contentions per frame

27/37

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

▲ rs = source rate of voice traffic

▲ h = physical and MAC layer headers for each digitized packet

▲ rs x F = amount of source information per packet

▲ rs x F + h = total packet length

▲ To, Tg = durations of minislot 0 and a slot guardband

▲ rc = channel transmission rate

28/37

Numerical Examples and Discussion

▲ Rx/Tx and Processing overhead set to zero

▲ Nv,Nd = The number of voice terminals and data terminals are denoted

▲ Pv, Pd = the contending probability of voice and data terminals are denoted

29/37

Analytical Results Versus Simulation Results

30/37

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 Pf,0(Nv)

-> Calculate value is Pf,0(10)= 0.1013 and Pf,0(20)= 0.2019

* Nv is larger, then Pf,0(Nv) is low

31/37

Performance of D-PRMA

Pdrop generally increases with Nv

A little different results for Pv increases (0.005-0.5)

Pdrop generally increases with Nv

big different results for Pv increases (0.5-0.95)

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, Pv becomes important in

determining the success of contention in the m extra minislots

Figure 3 Figure 4

32/37

Performance of D-PRMA

Figure 5 Figure 6

Lband generally decreases with Nv

A little different results for Pv increases (0.005-0.5)

Lband generally decreases with Nv

big different results for Pv increases (0.5-0.95)

33/37

Performance of D-PRMA

Tendency of Pdrop and Lband versus Pv

Pv around 0.5 is suitable for most case of Nv

The following simulation, Pv is set to 0.6

Figure 8Figure 7

34/37

Comparison With IEEE 802.11

Simulations with OPNET (between D-PRMA and IEEE 802.11)

Data packets arrive at each data terminal according to a Poisson process with mean arrival rate

35/37

Ld (data traffic load)

Delay of voice packets▲ Nv =16 and Nv=24 (IEEE

802.11)

▲ Nv =24 (D-PRMA) Value over 10-2 Nv Should be controlled under

about 15 to have Pdrop < 10-2

Data traffic support▲ IEEE 802.11 performs better

▲ Nv =16 and Nv =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 802.11 especially in the case of high data traffic load

36/37

Conclusion

Pros ▲ D-PRMA more suitable than IEEE 802.11 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