Performance analysis of CFDAMA-PB protocol for packet satellite communications

1206 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 46, NO. 9, SEPTEMBER 1998

Performance Analysis of CFDAMA-PB Protocolfor Packet Satellite Communications

Tho Le-Ngoc,Fellow, IEEE, and I. Mohammed Jahangir

Abstract—Combined free/demand-assignment multiple-access(CFDAMA) schemes are suitable for broad-band packet satel-lite communications systems serving a finite number of burstydata sources [1]. The performance analysis of the CFDAMAusing piggy-backed (PB) reservation is presented in this paper.The probability generating function (pgf) of the packet delayis developed. The performance is evaluated in terms of threeperformance measures: average packet delay, variance of packetdelay, and cumulative probability distribution of packet delay.Performance comparison with other pertinent schemes showsCFDAMA-PB to be superior for a wide range of user populationsizes.

Index Terms—Demand-assignment multiple-access, multiple-access, packet satellite communications, satellite communications.

I. INTRODUCTION

BY COMBINING free assignment with demand assign-ment, combined free/demand-assignment multiple-access

(CFDAMA) schemes [1] offer a much shorter delay at low andmedium traffic loads while maintaining the high channel utilityof the demand-assignment multiple-access (DAMA) techniquein a packet satellite communications environment.

In a CFDAMA scheme the scheduler first allocates channelsto user stations on a demand basis similar to a DAMA scheme.However, whenever there is no demand, itfreely assignsremaining channels to user stations according to some strategy.A simple strategy to assignfree channels would be basedon a round-robin manner. At low traffic loads the chancethat a user station obtains free-assigned channel(s) is high.A data packet arriving at the user station can be immediatelytransmitted over a free-assigned channel. Hence, a minimumtransmission equivalent to one round-trip propagation can beachieved. The probability of obtaining a free-assigned channelis related to the population of user stations. Consequently, thedelay–throughput performance of CFDAMA schemes dependson the population size .

In a CFADAMA scheme there are three possible requestingstrategies. A user station can send its capacity reservationin a preassigned (PA) or random-access (RA) request slot,or piggy-backed (PB) in its data packet. The PA requesting

Paper approved by D. P. Taylor, the Editor for Signal Design, Modulation,and Detection of the IEEE Communications Society. Manuscript receivedApril 9, 1993; revised December 4, 1995 and October 6, 1997.

T. Le-Ngoc is with the Department of Electrical and Computer Engi-neering, Concordia University, Montreal, P.Q. H3G 1M8 Canada (e-mail:[email protected]).

I. M. Jahangir was with the Department of Electrical and ComputerEngineering, Concordia University, Montreal, P.Q., Canada. He is now withPhilips Consumer Communications, Freemont, CA 94538 USA.

Publisher Item Identifier S 0090-6778(98)06655-0.

strategy assumes that each userstation has a PA request slot.The RA requesting strategy assumes a separate area of requestslots which can be randomly accessed by all user stations. Inthe PB strategy the user stations sends its capacity requestembedded in the header of its data packet. Simulation resultspresented in [1] show piggy-backing to be the most efficientway of making reservation. In this paper we present an analysisof the performance of the CFDAMA-PB scheme.

The CFDAMA-PB scheme is modeled as a queue with anintermittently available server and the probability generatingfunction (pgf) of the packet delay is developed. In additionto the average delay, we also obtain the variance and cu-mulative probability of the packet delay. The variance is anindication of how widely the packet delay could vary from itsaverage value. The cumulative probability distribution givesinformation on how reliably a certain delay constraint couldbe met in delivering a packet to its destination. These threemeasures together give more useful information to the systemdesigner in evaluating a multiple-access protocol. Performanceof four multiple-access schemes: CFDAMA-PB, combinedrandom and reservation multiple-access (CRRMA) [2], timedivision multiple-access (TDMA), and TDMA-reservation [3]is evaluated for various user population sizes and comparedon the basis of these three measures.

II. CFDAMA-PB PROTOCOL

This section briefly describes the CFDAMA-PB scheme.Consider a packet satellite communications system servinghomogeneous user stations. The communications channel issimply divided into equal-size time slots; each time slot canaccommodate one data packet. A user station sends its datapacket in its assigned time slot. A data packet has two parts:header and payload. The header contains routing informationand a field for capacity request.

A straightforward reservation strategy would be to make acapacity request forall newly arrived packets. However, sucha reservation strategy puts the requesting user station in anunfair advantage and reduces the amount offree channels.Consequently, potential user stations that are waiting forfree-assigned time slots have to wait longer. A more efficientreservation strategy is as follows. Each user station keepstrack of thedue amount of requested time slots that are yetto be assigned and the number of data packets in itsqueue . is incremented by the amount of time slotsrequested and decremented by the number of assigned slots.If , the user station requests fortime slots.

0090–6778/98$10.00 1998 IEEE

LE-NGOC AND JAHANGIR: PERFORMANCE ANALYSIS OF CFDAMA-PB 1207

Fig. 1. Slot assignment by on-board scheduler.

The scheduler has two tables: one for reservation and theother for free assignment. The reservation table contains theidentification (ID) of the requesting user stations and theircorresponding amount of requested slots. The free-assignmenttable is a list of all user stations in the system. This list isinitially arranged in an arbitrary order.

Whenever the reservation table is empty, the schedulerassigns the upcoming time slot to the user station indicatedby the top entry of the free-assignment table. This entry isthen moved to the bottom of the list. The scheduler serves therequests on a first-come–first-serve basis, i.e., it assigns theupcoming time slot(s) to the requesting user station indicatedby the top entry of the reservation table. This entry is thenremoved from the reservation table. In order to give otheruser stations that do not have any reservation a better chanceto receive theirfree-assigned slots, the ID of this requestinguser station is also moved to the bottom of thefree-assignmenttable.

The CFDAMA-PB protocol can be applied to both bent-pipe(nonregenerative) and on-board processing (OBP) satellitesystems. For new-generation satellites with OBP capability,a centralized scheduler can be located in the satellite so that ittakes the requesting user station only one round-trip delay plusthe scheduler queueing/processing time to receive the reply toits reservation (Fig. 1). For bent-pipe satellites, the scheduler ison ground. We could have a centralized on-ground scheduler.In this case, the time interval from the requesting to receivinginstants is on the order of two round-trip delays. We could

also use adistributed scheduling scheme in which all userstations perform anidentical scheduling procedure. This ispossible in a global-beam satellite system. Using adistributedscheduling scheme, the time interval from the requesting toreceiving instants becomesone round-trip delay.

III. PERFORMANCE ANALYSIS

Consider a packet satellite communications system servinghomogeneous user stations. In the following analysis we

consider , where denotes the time slot size andis the time interval from the instant a user station sends itsrequest to the instant it receives a reply. For convenience, thetime unit is represented by the time slot size. Each userstation generates data packets under a Poisson regime with anintensity , where (smaller than 1 packet/time slot)denotes the overall system arrival rate. We tag a data packetarriving at a user station and evaluate the delay it incurs untilit reaches its destination. This delay is the sum ( ),where is the fixed transmission time equal to one round-trip propagation and indicates therandom time that thetagged packet waits for service in the user station queue. Theavailability of assigned time slot(s) to a user station is modeledas an intermittently attending server. The server is designatedascalled serverif it is available to the user station as a resultof demand assignment. It is calledfree serverfor the caseof a free-assigned time slot. The waiting time of the taggedpacket depends on whether the forthcoming server is a


called server (event ) or a free server (event ). Hence,the probability generating function of the packet waiting time

can be expressed as

(1)

where and denote the conditional pgf’s offor events and , respectively.

A. Derivation of and

The random time interval between the departure instants ofthe precedent and forthcoming servers is called the interserverdeparture interval (isdi). The conditional pgf’s of isdi forevents and are denoted by and , respec-tively, i.e., , where

, and , whereand denotes the request-to-reply transmission time.

The number of newly arrived packets in an isdiobeysthe Poisson distribution, i.e.,

(2)

The type of the forthcoming server depends on whether thequeue of the user station was empty or nonempty when theprecedentserver departed from the user station.

If the user station queue wasemptyat the departure instantof the precedentserver, then the user station did not requestfor capacity allocation. Therefore, theforthcomingserver willbe a free server and used by at mostone packet at the frontof the queue. In this case the number of packets remaining inthe queue at the departure instant of theforthcomingserverwill be . From (2) and the definitionof , we can write the pgf of as follows:

(3a)

If the user station queue wasnonemptyat the departureinstant of theprecedentserver, then the user station sentcapacity request for packets remaining in the queue at thatinstant. Therefore, theforthcoming server will be a calledserver and used by only those packets, i.e., the number ofpackets remaining in the queue at the departure instant of theforthcomingserver will be exactly . From (2) and thedefinition of , we can write the pgf of as follows:

(3b)

From (3a) and (3b), the pgf of the number of packets remainingin the queue at the server departure instant can be expressedin terms of and as

(3c)

where denotes the probability ofemptyqueue atthe server departure instant. From (3a)–(3c), we obtain

The probabilities of events and are [4]

(4a)

and

(4b)

B. Derivation of and

If the user station sent a PB request at the departure instantof the precedent server, the request will experience a delayequal to the sum of the queueing delay at the scheduler andthe fixed request-to-reply transmission time. This request-to-reply transmission time is (round-trip propagation time inunits of ) for the cases of on-board scheduler and distributedscheduler, or when a centralized on-ground scheduler isused. Therefore, can be written as

(5)

where is the pgf of the queueing delay at the scheduler.In (5) we omit the number of time slots assigned by the

called server, as it would be negligible in comparison toand the queueing delay at the scheduler. The requests

arrive at the scheduler queue in bulks with a highly complexinterarrival distribution. As an approximation, we assume aPoisson distribution of request arrivals, from which it followsthat [5]

(6a)

where the average request arrival rate is estimated fromthe overall system arrival rate as follows:

user station will send request

user station will send request

In event , the user station always sends a request. In event, it will receives onefree-assigned time slot. Therefore, it

will send a request only if there are more than one packetsarriving during the current isdi. It follows that

(6b)

Now consider the advancement of the ID of a given userstation in the free-assignment table. As previously discussedin Section II, when the scheduler assigns time slot(s) to thisuser station it also moves the tagged ID to the bottom of thefree-assignment table. The next free-assignment to this userstation will occur when the reservation table becomes emptyand the tagged ID reaches the top of the free-assignment table.


Whenever the reservation table is empty, the tagged ID movesup one step every time slot. However, when reservation tableis nonempty, the tagged ID can advance by one step only if theID of the other user station that currently obtains ademandassignment locates in a higher location. It is observed that,during reservation time, when the tagged ID is near the bottomof the free-assignment table, the chance to advance is high. Asit moves in the upper part of the free-assignment table close tothe top, the chance to advance becomes low. In average, thereare approximately half of the requests that effectively impedethe advancement of the tagged ID in the free-assignment table.Consequently, we can assume the arrivals of requests thateffectively stop the advancement of the tagged ID to be aPoisson process with an average rate . We call thetime in which the tagged ID cannot advance abusy period(BP). The time in which the tagged ID moves one step iscalled anidle period. An idle period lasts only one time slotwhile a BP can last one or more time slots and ends with anidle period. The tagged ID needs a total of idle periods tomove from the bottom to the top of the free-assignment table.Therefore, if there are BP’s in an isdi for an eventwith length then there are other idle periodsand the total duration of BP’s is time slots. In otherwords, we can write

BP’s BP’s last time slots

(7)

where denotes the probability that theBP’s last a totalof time slots. It can be shown that is the coefficientof in , where is the pgf of the busy period.Taking transform of (7), we obtain

(8)

Consider a busy period of time slots as a result ofpackets arriving at the system where . This busyperiod can be viewed as a concatenation of busy“sub”periods directly generated by of these packets. Thebusy “sub”period has the same distribution as the busy perioditself. Consequently, the probability that a BP laststimeslots can be written as

packet arrivals resulting a BP

BP’s with a total length of

(9)

It follows that

(10)

C. Derivation of and

We first consider the tagged packet arriving in an isdi oflength in event . Fig. 2 illustrates the waiting process. Thearrival instant of the tagged packetis uniformly distributedfrom one to with probability of . If it is the first packetarriving in this isdi, then it will be served by the forthcomingfree server and its waiting delay is . Otherwise,it has to send a PB request in the forthcomingfree-assignedtime slot and has to wait for an additional isdi of lengthto be served by the nextcalled server. is the pgf of .In this case the waiting delay becomes . Inother words, the probability that the tagged packet waits for

time slots in event is

delay

delay

where

delay

delay freeserver

delay called server

delay freeserver

no arrival in slots

delay called server

some arrival(s) in slots

It follows that

(11)

where is the coefficient of.

Now, we consider the tagged packet arriving in an isdiof length in event . In this case the tagged packet willnot be served by the forthcoming called server. It will makereservation and has to wait for the subsequent called server.If it arrives in the th time slot and the next isdi is , then its


Fig. 2. Arrival and departure of tagged packet.

waiting delay is . Therefore

delay

next isdi

tagged packet arrives atth slot

It follows that

(12)

where is the coefficient of.

From (1), (11), and (12), the average packet delay andthe variance of packet delay are

(13a)

(13b)

where

IV. I LLUSTRATIVE RESULTS AND COMPARISON

As an illustrative example, we consider a packet satellitecommunications system with the following parameters:

• transmission capacity 2.048 Mb/s;• time slot ms;

(a)

(b)

Fig. 3. Comparison of analytical and simulation results. (a) Average delay.(b) Delay variance.

• round-trip delay 270 ms, or, equivalently, ;• on-board scheduler, or, equivalently, .

Fig. 3 shows the analytical and simulation results of theaverage delay and delay variance versus utilization for

( ). Utilization indicates the traffic load normalizedto the uplink capacity, e.g., a utilization of 0.6 represents atraffic load equal to 60% of the uplink capacity. The analyticalresults are obtained from (13a) and (13b) for the average delayand delay variance, respectively. The analytical and simulationresults are in a very good agreement.

Simulations are also performed to evaluate the performanceof the CFDAMA-PB, TDMA, TDMA-reservation, and CR-RMA schemes for various values of . The average delay,delay variance, and cumulative probability distribution areconsidered as performance measures. For a fair comparison,we simulated the CRRMA with reservation overhead takeninto account. The length of a reservation mini-slot is assumedto be 6% of a data slot.


(a) (b)

(c)

Fig. 4. Average packet delay for various values ofN . (a) N = 300. (b) N = 500. (c) N = 700.

Fig. 4 shows the average delay performance of theseschemes for , , and . The TDMA and TDMA-reservation schemes are described in [3]. In TDMA a timeslot is periodically PA to each user station. Therefore, for apopulation size of , each user station obtains one PA timeslot every time slots. At low load, when the number ofpackets arriving in an interval of slots is often less thantwo, the packet delay equals the sum of the transmission time( ) and the waiting time uniformly distributed from one to

, i.e., the average packet delay at low load is approximately. However, when the load increases, the

probability that the number of packets arriving in an intervalof slots exceeds one, also increases, and the average packetdelay becomes longer.

TDMA-reservation is a DAMA scheme. Time slot(s) areassigned to a user station on a demand basis. For a TDMA-reservation system using an on-board scheduler, the timeinterval from the request instant to the assignment instant is

plus the request queueing delay at the scheduler. Therefore,the packet delay is the sum of , the time to wait for therequest slot (uniformly distributed from one to), and therequest queueing delay at the scheduler for the entire range ofchannel utilization.

By combining thefreeassignment withdemandassignment,the CFDAMA scheme achieves the same (short) averagepacket delay as the TDMA as low load and offers the same(high) channel utilization as the TDMA-reservation. As shownin Fig. 4, the CFDAMA-PB has a better average delayperformance than TDMA and TDMA-reservation over theentire utilization range for all .

Details of the CRRMA scheme are given in [2]. In gen-eral, this scheme combines the random-access and TDMA-

reservation techniques. The scheduler assigns time slot(s) tothe requesting user stations on a demand basis. In the absenceof capacity request, unreserved time slots are open for random-access. At very low load, there are more unreserved time slotsand the probability that more than one user stations sendingtheir data packets in a given unreserved time slot is verysmall. As a consequence, the collision probability in a givenunreserved time slot is very small and the average packet delayis approximately equal to the transmission delay of. How-ever, as the load increases, the collision probability increasesand the number of unreserved time slots decreases. Hence,the average packet delay of the CRRMA quickly increases.Compared to CRRMA, the CFDAMA-PB has shorter delayat medium and high utilization (Fig. 4). The average-delay-versus-utilization curve of CRRMA is almost insensitive to

, and hence suitable for infinite population. The CFDAMA-PB average delay increases with. However, even for apopulation as high as 700, it outperforms the CRRMA fora utilization higher than 0.45.

Fig. 5 shows the delay variance of CFDAMA-PB, TDMA,TDMA-reservation, and CRRMA. For TDMA and CRRMA,the delay variance increases rapidly with the utilization.CFDAMA-PB and TDMA-reservation have a comparabledelay variance. Their delay variance is small and almostconstant over the entire utilization range. Figs. 6 and 7show the cumulative probability distribution for the fourabovementioned schemes at channel utilizations of 0.4 and0.8, respectively.

As previously discussed, the packet delay in TDMA-reservation is the sum of , the request waiting timeuniformly distributed from one to , and the (negligible)request queueing delay at the scheduler. Therefore, the


(a) (b)

(c)

Fig. 5. Variance of packet delay for various values ofN . (a) N = 300. (b) N = 500. (c) N = 700.

(a) (b)

(c)

Fig. 6. Cumulative probability distribution of delay for 40% channel utilization. (a)N = 300. (b) N = 500. (c) N = 700.

cumulative probability distribution for TDMA-reservation isproportional to the packet delay.

In CRRMA the packet delay occurs in discrete steps. Ifa packet can be successfully transmitted in a unreserved

time slot as soon as it arrives, then the packet delay is justthe transmission delay of . If the packet does not find aunreserved time slot, then it makes a reservation and will useits reserved time slot. In this case the packet delay isif a


(a) (b)

(c)

Fig. 7. Cumulative probability distribution of delay for 80% channel utilization. (a)N = 300. (b) N = 500. (c) N = 700.

TABLE ICOMPARISON OF DELAY CONTRAINTS FOR 90% RELIABILITY

system uses an on-board scheduler and the request queueingdelay at the scheduler is assumed to be negligible. If collisionoccurs in the reservation slot, the user station has to resendits request. If the retransmission of its request is successful,the packet delay will be . Otherwise, the packet delay canbe . Hence, the cumulative probability distributionfor CRRMA is a staircase function (Figs. 6 and 7).

The cumulative probability distribution indicates how reli-ably a packet could meet a delay constraint at a given channelutilization. From the plots in Figs. 6 and 7, the delay constraintthat could be met with a reliability of 90% is derived andgiven in Table I. The CFDAMA-PB scheme can meet a muchlower delay constraint than others, especially at high channelutilization (0.8).

The CFDAMA-PB and TDMA schemes have a simplechannel structure with only one type of time slot for datapacket. The TDMA-reservation and CRRMA schemes havetwo types of time slots: mini-slots for reservation and dataslots, and hence require the user station to generate two types

of packets. Furthermore, the CRRMA needs a reliable collisiondetection of reservation packets.

V. CONCLUSION

The performance of the CFDAMA-PB scheme has been an-alyzed, evaluated, and compared with that of TDMA, TDMA-reservation, and CRRMA in terms of average delay, delayvariance, and cumulative probability of delay. By combiningfree assignment and demand assignment, the CFDAMA-PBscheme achieves a better average delay than that of TDMA andTDMA-reservation, while maintaining a small delay variancecomparable to that of TDMA-reservation over the entireutilization range. The CFDAMA-PB outperforms the CRRMAat utilization higher than 0.45 and has a much lower delayvariance. Compared to TDMA, TDMA-reservation (DAMA),and CRRMA, it can meet a much lower delay constraint,especially at high channel utilization.

REFERENCES

[1] Le-Ngoc and S. V. Krishnamurthy, “Performance of combinedfree/demand assignment multiple-access schemes in satellite communi-cations,” Int. J. Satellite Commun., vol. 14, no. 1, pp. 11–21, Jan./Feb.1996.

[2] H. W. Lee and J. W. Mark, “Combined random/reservation access forpacket switched transmission over a satellite with on-board processing:Part I—Global beam satellite,”IEEE Trans. Commun., vol. COM-31,pp. 1161–1171, Oct. 1983.

[3] S. Tasaka, “Multiple-access protocols for satellite packet communica-tion networks: A performance comparison,”Proc. IEEE, vol. 72, pp.1573–1582, Nov. 1984.

[4] D. R. Cox, Renewal Theory. London, U.K.: Methuen, 1962.[5] J. F. Hayes,Modeling and Analysis of Computer Communication Net-

works. New York: Plenum, 1984.


Tho Le-Ngoc (F’97) received the B.Eng. (withdistinction) degree in electrical engineering in 1976,the M.Eng. degree in microprocessor applicationsin 1978, both from McGill University, Montreal,P.Q., Candada, and the Ph.D. degree in digitalcommunications from the University of Ottawa,Ottawa, Ont., Canada, in 1983.

From 1977 to 1982 he was with Spar AerospaceLimited as a Design Engineer and then as a SeniorEngineer, involved in the development and design ofthe Canadarm, SCPC/FM, SCPC/PSK, and TDMA

satellite communications systems. From 1982 to 1985 he was an EngineeringSupervisor of the Radio Group, Department of Development Engineering,SRTelecom, Inc., where he developed the new point-to-multipoint DA-TDMA/TDM subscriber radio system SR500. He was the architect of thisfirst digital microwave TDMA system. In 1985 he joined the Departmentof Electrical and Computer Engineering, Concordia University, Montreal,P.Q., Canada, where he is currently a Professor. His research interest isin the area of wireless digital communications with a special emphasis onmodulation, coding, and multiple-access techniques. He is currently Leaderof the Major Project “Broad-Band Satellite Communications” of the CanadianInstitute of Telecommunications Research (CITR), a National Network Centreof Excellence (NCE). Since 1985, he has been a consultant to several differentcompanies in wireless communications.

Dr. Le-Ngoc is a Senior Member of the Ordre des Ingenieur du Quebec.

I. Mohammed Jahangir was born in Ammapat-tinam, India. He received the B.Eng. degree fromCIT, Coimbatore, India, in 1991, and the M.A.Sc.degree from Concordia University, Montreal, P.Q.,Canada, in 1993, under the supervision of Dr. ThoLe-Ngoc.

He is currently with Philips Consumer Communi-cations, Freemont, CA, and is managing the CellularSoftware Group. His interests include cellular com-munications and multiple access protocols.

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Performance analysis of CFDAMA-PB protocol for packet satellite communications