PERFORMANCE STUDY OF PACKET RESERVATION MULTIPLE ACCESS PROTOCOL FOR DIGITAL MOBILE COMMUNICATION

A.B.M. Siddique Hossain, Member IEEE, Professor and Chairman

IEEE Bangladesh Section. Department of Electrical & Electronic Engineering, Bangladesh University of Engineerig & Technology, Dhaka- 1000, Bangladesh.

Abstract

Digital cc I 111 I iir mob i I c radio communication tcc h n iq ues is an :irca o f wide public acceptance. This paper looks onto tlic performance of packet reservation multiple iicccss (PRMA) protocol h r the transmission of digital speech packets from mobile speech sources. Packets li.om activc spccch sources contend for access to a channel timc slot. Computer simulations were done to study thc perforinancc of the protocol and to determine thc copaciry 01‘ channels having different bandwidth and I’ramc s i x . Computer simulations were done to study PRMA protocol and its performance when applied in clifl’ercnt second generation networks like GSM, USDC/IS-54. PDC. DECT etc. It is observed that PRMA iricrcases systcni capacity significantly if mobile tcrininal’s data transmission rate can he reduced to a I‘igurc ;IS low as 8Kbps or lower than that.

I. Introduction

Ccllular niobilc radio ancl rcsiclential cordless telephones arc two new coiiimunication techniques with rapidly growing public acceptance. Present products arid devices usc lirst and second generation technology. First gcnera(ion technology is bascd o n analog ( l.requency modulation) voice transmission and liniited flexibility I I I. Second generation products are characterized by digital wicc transmission and uses Time Division MulLiple Acccss (TDMA) and code Division Multiple Access (CDMA) a s nctwork access methods. Proposals ant1 rcscarcli works on third generation wireless nctworking is i n progress now a days [ I ] . The vision of t h i r d gcneration is ;I singlc set 01‘ standards that can meet ;I wide range o f wircless access applications. It will criiploy switching architecture referred to as cellular pckct switch and ;I packct transmission technique rclcrrccl to a s Packet Reservation Multiple Access (PRMA).

2. Performance Study of PRMA protocol

Earlier studies on perforniance o f PRMA protocal were carried out for arbitrary set of system parameters 13.51. Our study on PRMA performancc was done by considering existing second generation system parameters. The vision of this work is to demonstrate how the system capacity increases if PRMA protocol is implemented in stead of TDMA i n presently available digital mobile communication systems. Digital mobile communication systems that were studied are GSM, USDC/IS-54, PDC and digital European Cordless telecommunication (DECT) system for different terminal/source rate.

Performance of PRMA protocol has hcen examined hy computer simulations. In PRMA protocol. packcts held in the transmit queue for a duration longer than maximum allowable delay are dropped. Thus criteria for PRMA performance mcasurement is Pdrop , thc packet dropping- - probability. Since PRMA is statistical multiplexer of speech packets transmitted from terminals, Pdrop increases as number of activc terminals increases and vice versa. To deterininc per channel capacity in terms o f number of active terminals of a mobile communication system, we set a packct dropping probability limit Pdrop < 0.01. That ineans, speech quality is acceptable as long as packct dropping probability is less than or equal to I %.

Parameter Value

27 1,576,48.6,42 Packet size. bits 552

14. I Speech hits I 4 7 2 5. I Header hits H I 80 6. I Delay limit, Dmax ins, 1 32

I 7. I Permission Probabilitv. P I 0.75. 0.5.0.33. 0.25 I 8. I Mean Talk spurt, tt sec I I .OO 9. I Mean Silent gap, ts sec I 1.35

509 1997 IEEE TENCON - Speech and Image Technologies for Computing and Telecommunications

For some 01' this parameters. several values have been used. For example we examined performance for three values of source rates. namely, 32kbps, 16 kbps, Skbps. But capacity calculation of all the systems were not pcrformcd for all three source rates. Reason for using dil'ferent S O L I ~ C C rates is, in case of GSM, bit rate that is generated by each terminal is (270.8/8)=33.85kbps. but recent research works suggest that this bit rate can be rcduccd to hall' 01' its value by using half rate codecs which is under development [2]. It was also rccommended that speech coding and cliannel coding can be perl'oi-ined i n such a way that speech quality can be inaintained cvcn tliougli it is coded at the rate of S kbps or lower. Considering time points and rounding up the I'igures 32,16 and 8 kbps so~irce rates were selected. Channel rates that were used are those of existing digital n i ob i 1 e coni mu 11 icat i on systems.

Packet sizc used is according to the proposed packet size l.or PRMA protocol implementation, [1,4 1. Delay limit was chosen as 32 ins as i t was used in earlier studies [3]. Dil'lkrcnt permission probabilities 0.75, 0.5, 0.33., 0.25 were selected i'or different run of the simulation program tlcpencling upon the requirements for better collision resolution. Based on mcasuretnent of recorded speech, it was I'ound that the mean duration of a talkspurt is 1 sec iincl mean duration ol' a silent gap is I .35 sec [ I ,3].

4. Simulation of PRMA protocol

We selected process interaction approach or continuous time advance mechanism i n modelling simulation program. Wc treated conversation as time-ordered scquencc o f talkspurt and silent gap. Duyation of talkspurt and silent gap is cxponentially distributed with ii incan valuc of talkspurt and silent gap respectively. Durations ol' silent gap and talkspurt were converted into fraincs and slots. as these were generated as simulation time advances. Total simulation time is converted into l.rames. As the program advances simulation time, it scanned every slot i n a frame. Duration & each slot, status 01' cvery terminal was monitored and updated accoring to the requirement. As talkspurt was generated, i t was packetized into proposed PRMA packet format which has 472 speech bits and SO header bits. PRMA frame duration is the time required for generating 472 bit by a terminal or source. If source rate is Rs bitskec,

channel rate Rc bitskec and header bit per packet H, the number o f slot per PRMA fi-ame is N=int[RcT/(RsT+H)I, where i n t [ x] is the greatest integer 5 x.

PRMA I;:amc duration is, T=472/Rs second . With

The program was run for a sufficiently long period of time to include a large number of talkspurts and silent intervals from each terminal. The output of this simulation program is the percentage of packet drop for different number of active terminals or conversations. 5. Results and Discussion

Output data of the simulation are plotted for different channel rate, source rate and permission probability. In some cases it was observed that when number of terminals were large and permission probability was high, the system suffered repeated collision. That means, once a collision had started i t continued in successive frames. All the terminals in contention had high permission probability but none of thein got i-eservation in the next available frames. Hence percentage of packet drop that we got was abruptly high. In those cases permission probability was reduced to get better collision resolution. Hence permission probability was chosen, for different set of data, so as to get reasonable values of packet dropping probability with incrcinent of number of conversation.

A design spread sheet for all different types of digital network , if PRMA incorporated with them. is given in the attached table 1 . The entries of row 1 1 were obtained from the plots of output data of simulation. Terminal per channel is calculated in row 13. Here the number of terminal per channel is greater than bne ( exccpt USDC and PDG -system) because the speech activity detectors allow PRMA to operate as a statistical multiplexer i n which the terminals in the silent state give up their channel resources (reserved timc slots) to terminals in the talking state. As it is given mean duration of talkspurt is I .OO sec. and mean silent duration is I .35 sec. Therefore, speech activity factor is I / ( 1+1.35)=0.43. So, the maximum number of terminals per channel is 1/0.43=2.35. the difference between 2.35 terminals per channel and entries in row 13 is due to -

'E statistical fluctuations i n the number of terminals with simultaneous talkspurts; d: collisins caused by terminals contending for the same time slot;

an empty slot produced by PRMA at the end of each talkspurt; and *' nonspeech material that occupies I0/69= 14% of each packet.

But in case of USDC and PDC systems, where channel rate is quite sinall figure, the number of tcrminal per channel is less than one. That means, in those systems

510 1997 IEEE TENCON - Speech and Image Technologies for Computing and Telecommunications

I O I I I 2 1.7 I4

I5

Tahlc I : Dcsigii sprcx l slicet Ihr digital mobilc nctworks with PRMA.

I~criiiissioii probability 0.5 0.5 0.5 0.33 0.33 0.5 0.75 0.75 0.75 0.75

Equivolciit climicls 33.85 16.Y2 8.46 72 36 I8 6.08 3.04 5.25 2.63 Tcrriiillnl per clianncl I .3 1.36 1.18 1.47 1.53 1.5 0.66 0.66 0.76 0.76 ' l ~ l l o ~ ~ ~ l l ~ l l ~ ~ t l ~ . khps 177.0 185.0 160.9 426.4 442.5 434.4 16.09 16.1 16.09 16.1

Norinalizeil llirouplipul 0.65 0.68 0.59 0.74 0.77 0.75 0.33 0.34 0.38 0.38

Capacity. tcl-iiiiiinls 44 23 10 106 55 27 4 2 4 2

I 6 2 4 3 9

- p - 0.5 _ _ _ _ p = 0.25

2 : D

0 - x -

0 - a -

- d

1-

o - ~ l n - m n , , l , a I h , l , , , , , l h , ~ , , l , , , ~ r , , , , , , , " , , ~ 2.0 z3 30 U 4 45 !Y?

Active Terminals No. of active terminal vs. percentage of pocket drop for channel rate = 270.8 kbps. source rote = 8 kbps permission probobility p = 0.5. 0.25.

' . e 8 10 12 14 16

Active Terminals FigureA.@,. No. of active terminol vs. percentage of pocket drop

lor channel rate = 270.8 kbps. source rate = 32 kbps permission probobility p = 0.5, 0.25.

1997 IEEE TENCON - Speech and Image Technologies for Computing and Telecommunications 51 1

6. Teletraffic Comparison

8

To compare teletraffic capacity interms of number of cnlls per hour ol‘a network with TDMA and with PRMA, Ict LIS considcr GSM network. We have selected GSM 1xx;iitse GSM with TDMA supports 8 simultaneous conversation with equivalent data transmission rate of 33.XSkbps while i n one of our study of GSM with PRMA. wc liad cliosen 32Kbps as source rate. For the purpose 01‘ approximate comparison we can consider liicsc two systcm as cquivalent, one of those uses TDMA wliilc oh.x LISCS PRMA. If we consider a GSM network t h a t operates in Dhaka city being allocated a l0Mhz tluplcx band. number of‘ carriers ( of 200 Khz) that will bcavailable is SO. These SO carrier frequencies’ will be availablc for use per cluster of cells, i.e., per reuse pattern. If the network is designed such a way that it cmploys 7-ccll reusc pattern then each cell will have approximtely 7 carriers. If radius of each cell is 2km, area per ccll will be rcx2’ = 12.56 kin’. Say area of Dhaka city is 1200km’ . So. the city will be divided into 95 cells. Now let us calculate calls per hour for GSM network with TDMA and equivalent GSM network with PRMA, I‘or tlic above mentioned situation.

10 Channels per carrier/Number 01’ s i mu1 taneous conversation that can bc carried O L I ~ per carrier Total channels per cell. o r total number ol‘ simultaiieous conversation tha t can

‘11 I ied out pcr ccil Ol‘l‘ered leletraf‘fic load, A. per cell i n tcrms ol‘ crlang Calls per hour per ccll TOlUl calls per IlOLIr I.or the WllOIC

network

be c. ..

9 1 8 x Y S = 872 I O

GSM with GSM with Li-LL-

1182x95= 112290

60 x

Tab I e 2 : Te I e i r a ffi c co in pari son s LI i n in ar y .

From above calculations we get about 28.7% increment of teletraflic capacity, i f TDMA is rcplaccd by PRMA in presently available GSM system. From tlic plots of simulation we can also say that a significant improvcnient in teletraffic capacity is achievable if sourcc of data rate l6kbps can be introduced into GSM with PRMA system.

References

[ I ] David J. Goodman, ’* Cellular Packet Communication” IEEE trans. on comm. Vo1.38, No.8, August, 1990.

[2] R.J. Horrocks, R.W.A. Scarr, ”Future Trends in Telecommunications”, John Willey & Sons. 1993.

[3] S. Nanda, D.J. Goodman, U. Timer, ” Performance of PRMA: A packet voice protocol for cellular systems”, IEEE trans. on vehicular technology, Vo1.40, No.3, Aug. 199 1 .

[4] G.H. Clapp, M. Singh, and S. Karr. ” Metropolitan area network architecture and services,” in proc. IEEE Global comm. conf. GLOBECOM.88, Hollywood, FLA, Dec., 1988, pp. 1246- 1254.

[5] Molianimad J. Abedin, ”Packet Reservation Multiple Access Protocol of digital cellular mobile communications: A performance study.” J. King Saud, Univ., Vol.6, Computer & Informhtion Sci.. pp. 17-30. (A.H. 14-1 4/ 1994)

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