An On-demand and Dynamic Slot Assignment Protocol for Ad Hoc Networks

Mingxia Xu, Minjian Zhao, Zhengwei Song, Shiju LiDepartment ofInformation Science and Electronic Engineering

Zhejiang UniversityHangzhou, 310027, Chinamilia2002@,163. com

Abstract assigns slots for a node basing on its two-hopneighborhood information. To our knowledge, most of

An on-demand and dynamic slot assignment (ODSA) current slot assignment schemes for ad hoc networksprotocolfor ad hoc networks is proposed. The protocol are inefficient in adapting to variations of applicationassigns slots for nodes basing on information in their and network topology, such as TSMA and USAP etc,two-hop neighborhood, which can be collected by except FPRP. In FPRP, the TDMA frame is partitionedlistening before slot assignment. In ODSA, a slot is into reservation subframe (RF) and informationdivided into several mini-slots, and the reservation subframe (IF). The two subframes are interleavedprocedure is realized in the selected idle slot by periodically. RF is divided into M reservation slotsmessage exchanging. ODSA can adapt to variations of (RS). Every RS is divided into K reservation cyclesapplication and network topology. Furthermore, it can (RC) and each RC is divided into five phases. Eachensure the time delays of nodes owning slots. ODSA node uses these five phases to compete for a slot andhas been studied in different network situations, and also inform neighbors of the competition results. Thethe results show that ODSA has smaller average winners are then allowed to use the correspondingaccess delay and higher channel utilization compared channels in the IF.with FPRP. Although FPRP can adapt to variations, the smallest

time delay cannot be ensured in the duration of an

1. Introduction application as one node can't ensure that it owns slotsin every time frame. Our proposed on-demand and

In recent years, ad hoc networks have received a lot dynamic slot assignment (ODSA) protocol for ad hocof attention. For their unique characteristics, it is a networks can adapt to variations and ensure the time

challenging problem to design an efficient Medium delay, which is very important to ad hoc networks withAccess Control (MAC) for ad hoc networks. TDMA- real-time applications. ODSA assigns slots for nodesbased MAC protocols are widely used in wireless basing on information in their two-hop neighborhood,networks. Nodes in TDMA share the wireless channel with which slot reservation collisions can be decreased.

by slots to avoid collisions and ensure real-time In ODSA, when a node has a transmission requirement,transmission. Slot synchronization and assignment are it randomly selects an idle slot from its local slottwo challenging problems when applying TDMA to ad assignment table, in which reservation procedure canhoc networks. Applying GPS or time analysis schemes be completed. As nodes can reserve slots when they[1] can realize slot synchronization. But it is difficult need and release slots no more occupied quickly,to realize dynamic slot assignment as ad hoc networks ODSA can adapt to variations of application andare distributed and their topologies are variable, topology to improve the channel utilization. It also can

Recently, several slot assignment protocols for ad ensure the time delay, because a node will keep thehoc networks have been proposed [2-8], which can be assigned slot until application or topology changes.classified in three classes according to the dependence The rest of the paper is organized as follows:on topology. The first class is absolute topology Section 2 introduces some basic concepts. Section 3transparent such as TSMA [2]. The second class is presents the details of ODS. Section 4 analyzes theinsensitive to topology variation [3,4] such as FPRP theoretical performance. Simulation results are given[3]. The third class of protocols such as USAP [6,7] in section 5 and section 6 iS a conclusion ofthis paper.

I1-4244-0574-2/06/$20.OO (0)2006 IEEE

2. Basic concepts (1) Collecting the information on slot assignment inits two-hop neighborhood.

2.1. TDMA time frame structure When a new node joins the network, it canconstruct the slot assignment table about nodes in its

N-2 IN-i N N±I N+2 . . . .

two-hop neighborhood by listening the INF packets fora period. In this period, when a node receives INF_Ain the first mini-slot of a slot, the node needs to send

1 frame=M slots INF_I in the last mini-slot of that slot with some

o 1 2 3 .... M2M, probability.(2) Reserving a slot when a node has a transmission

Reservation slot Assigned slot requirement.1 slot=P mini-slots 1 slotPmini-slots \When a node has a transmission requirement, it will

0 .14 Q P.2 P-1 0o lP-2 P1i select an idle slot randomly from the local slot

REQ REP ACK INF_A DAT INF_I assignment table. In ODSA slot reservation is realizedin the selected idle slot. When a slot is used for

Figure 1. TDMA time frame structure reserving, the first Q mini-slots are used for theThe frame structure of ODSA is shown in Figurel. reservation node to send REQ, the next P-Q-1 mini-

Time is divided into frames. Frames are then divided slots are used for nodes having detected collisions tointo slots and slots are divided into mini-slots. When a send REP and the last mini-slot is used for the nodeslot is used for reservation, the first Q mini-slots are which has reserved successfully sending ACK toused for sending requests, the slots from Q to P-2 are confirm its reservation.used for informing request collisions, and the last Q 0mini-slot is used for acknowledge when the request is 0 (accepted. In ODSA, a node sends local slot assignmentinformation in the first mini-slot and the data during 1 (a) (b)to P-2 mini-slots in its assigned slot. When a node Figure 2. Reservation collisions

wtotstinformation in In the reservation period, there are two types ofwithout slot receives the slot assignment collisions as shown in Figure2. The first is that two

informationinthefi st mini-slot,sn loa slot directly neighbored nodes reserve the same slot asshown in Figure2 (a). Selecting different REQ mini-

2.2. Packet format slots can alleviate this kind of collision. In ODSA, if anode receives REQ before its selected REQ mini-slot,it means that there are neighboring nodes applying the

There are five types of packets in ODSA: Data slot at the same time. So the node cancels its REQ

Packet (DAT), IfRma Packet (IF), Rqe sending to avoid collisions. The second is that two-Packet (REQ), Reply Packet (REP) and hop-away nodes reserve the same slot shown asAcknowledgment Packet (ACK). DAT contains data Fgr2() nwihtend rcie oetafor sending. INF contains the information of the slots

b

one REQ. Once a node receives more than one REQ, itassigned to the INF sender as well as its neighbors. will sl ct aR nio randoml tand se REPThere are two types of INF, INF A transmitted by wil seec a RE iiso admyadsn E

There aretw-tyesofNFIN_Arasi with the first-received REQ node ID in that mini-slotactive nodes and INF-I transmitted by idle nodes. later. If a reservation node sends REQ successfully andREQ is used to reserve a slot in the selected REQ mini- doesn't receive REP or receives the first-arrived REPslot. REP is used to inform REQ collisions. ACK is with the local node ID, it reserves the selected slotused to confirm slot reservation results. successfully and sends ACK in the last mini-slot. The

3. ODSA: detailmore complicated situation is that more than one node

-JL. :d.uetail select the same REQ or REP mini-slot, so collisionsare unavoidable. In ODSA, nodes are supposed to have

The procedure of ODSA includes: reserving slots, the ability to detect collisions, which can be realizedreleasing slots and resolving collisions, by power-detection algorithms. If a reservation node

detects collisions before receiving a REQ or sending3.1. Reserving slots its REQ, it will cancel its REQ transmission and send a

REP later to inform collisions. If the node detectsA node reserves a slot by the following steps. collisions in the REP period or receives a REP without

its node ID at first, the reservation is failure and it will

select an idle slot over again. By message exchanging If the node i selects an idle slot Bi, the probability ofand collision detecting, ODSA can avoid most of A M -Acollisions and ensure a successful slot reservation. B located in [0,A ]is and 0 in (A,MO -1] .

1 1 ~~~~MOMO3.2. Releasing slots As the reservation is processed at the selected idle slot,

ODSA has to wait for the slot B. in the next time frameIn ODSA, when a node has no data to send or when the selected slot B. is located before the current

change states because of moving or being shut down, slot; otherwise, the reservation can be processed laterits assigned slot will be released. Timers are used to in the current time frame. If T, 0 is defined as thedetect such situations. In the first situation, applicationtimers are used. To a node, when its application timer length of a slot in ODSA, we can get the access delaytimes out, it will send INF_A with a slot releasing flag ti as followsin its slot. The neighboring nodes receiving such A Mo -AiINF_A will release the corresponding slot. In the 1(M A±Bi) .Ts±+Msecond situation, slot-releasing notice can't betransmitted. To a node, timers are set for all its =(Ei +MO) Ts0 (1)neighbors. If the node hasn't received INF packets The last item of (1) is induced because the slot can befrom a neighboring node for a certain period, the used from the next time frame once reservationcorresponding timer would time out. Then the node succeeds. Let's suppose that there are N nodes in thewill release the slots assigned to that neighboring node network, the average access delay t isas well as two-hop-away nodes that only connected by 1 N I Nthat neighboring node. By releasing slots no more used, t Et = (Mo +-ZB ).Ts o (2)ODSA can adapt to variations of application and N i=1 N 1topology. As the location of Bi are uniformly distributed over

each frame, we can get the approximate average access3.3. Resolving collisions delay E(t) as

-3In ODSA, nodes assign slots according to their two- E(t)=-MO Ts0 (3)2

hop neighborhood information at that time. With time All results obtained above exclude the impact ofpassing by, network topology will change and the old unsuccessful reservation conditions, which are relatedassignment will not be suitable for the new topology, to the topology and the reservation scheme.so slot assignment collisions occur. When a node In ODSA, there are P-2 mini-slots really used fordetects DAT collisions in a slot, it will record the slot transmitting data in a slot. Let's suppose that all Nnumber and broadcast this information in its INF. The* nodes in the network have a slot to transmit in eachreceivers assigned the slot in collision would release time frame, so we can get the saturation channeltheir slots and reserve slots again. Nearly all collisions utilization as followscan be resolved within a short period by this way. N P-2 ~~~~~~~(4)4. Performance analysis M0 P

The overheads of ODSA are made up of two parts:In this section, we analyze the theoretical the first part is used for maintaining the slots

performance of ODSA on access delay and channel assignment information of two-hop neighborhood andutilization. The access delay is the time period begins the second is used for reserving slots. In (4), the firstwhen a node has a transmission requirement and ends part has been included. The second part doesn't existwhen the node really can send data. Channel utilization when every node owns a slot and the state is stable.is the percentage of channel in the network that really To study the efficiency of ODSA, we compare theused for transmitting data packets. performance of ODSA with FPRP. In FPRP, we

In the time frame structure of ODSA, we suppose suppose that every RF is followed by S IFs, and therethat there are M0 slots in each time frame and P mini- are MF slots in one IF. To a node i, the transmissionslots in each slot. Transmission attempts are uniformly attempt occurs at slot A, ( A E [0, (S+1)MF -1]). Ifdistributed over each frame. To a node i, its TSf is defined as the length of a slot in FPRP, we cantransmission attempt occurs at slot A (A E [0, M - 1])ge)h cesdlyta olw

ti = [(S + l)MF - A]. TS F + MF TSF distributed in a square of 3 x 3 kilometers around the[(S + 2)MF - Ai] TS F* (5) last position. The time interval of topology changing is

- [(S 20s and the total simulation time is 90s.Let's suppose that there are N nodes in the network, In the simulation, we compare the performance of

the average access delay t is ODSA with FPRP from two aspects, average accessI-N IN2)M- 1 N delay and channel utilization. Figure3 shows theN i= [ FN F ( average access delay of different network situations

As A is distributed uniformly, we can get the versus different node communication distance whennodes are static. It's clear that the average access delay

approximate average access delay E(t) as of FPRP is larger than ODSA. When there are enough

E(t) = MFM S_F (7) idle slots, slots reservation can be realized immediately,E(t)2 F -TS-F (7)so the average access delay is small. In ODSA, with

Obviously, when all parameters have the same the communication distance increasing, the nodesvalues, ODSA is better than FPRP on the average number in two-hop neighborhood increases too, soaccess delay. some slot reservation requests will be blocked. When

If we suppose that every node in each time frame the application changes, some slots will be releasedperiod has slots for transmitting, the channel utilization and can be assigned to nodes whose requests cannot beofFPRP is accepted at first. Access delay of those nodes is much

N S larger. So with the change of the application durationM S±* (8) and communication distance, the average access delay

F increases too. The results in figure4 show the averageThe main overhead ofFPRP is RF, which is used for access delay when nodes are mobile.

nodes to reserve slots in IF. As we can see from (4) Figure5 shows the percentage of time period thatand (8), the theoretical performance of channel channel really used for transmitting data when nodesutilization of ODSA and FPRP is related to the are static. The results are the average outputs of 20.48parameter P and S respectively, seconds after all nodes have transmission requirements.

In FPRP, Nodes reserves slots without the information5. Simulation results of their two-hop neighborhood, the reservation

collisions occur more frequently. Some slots willIn this section, we compare the performance of remain unreserved, so the achieved channel utilization

ODSA with FPRP in different network scenarios by of FPRP is lower than ODSA. Figure6 shows theNS-2. In the simulation, nodes are deployed randomly channel utilization when nodes are mobile. With thein an area of square lOx 10 kilometers. In ODSA, the overhead of slot assignment increasing, the channelmini-slot length is 4 milliseconds. A slot has 16 mini- utilization in mobile situation is lower than theslots, 6 for REQ, 9 for REP and 1 for ACK when the corresponding static situation.slot is used for reservation. So the slot length is 64 Table 1. Number of nodes cannot accessmilliseconds. Every time frame has 32 slots. In FPRP, the channelthe minimal slot unit is 1 millisecond. When RC is set Distance (km)to 8, the slot length is 40 milliseconds. The length of Algorithm 2 3 4 5IF is the same as RF, and every RF is followed by 7 ODSA 0 3 14 24IFs. In FPRP, the slot number in a frame is set to 32too. The time period of 8 time frames in FPRP is the FPRP 0 6 15 26same as 5 time frames in ODSA, so the channel can be Tablel shows the number of nodes that cannotused for data is the same in ODSA and FPRP. The data access the channel when nodes are static and thesource uses the ON-OFF model, where the data application duration is infinite. ODSA and FPRPgeneration interval and the packet size can be set in almost have the same results. Although some nodessimulations. The nodes number in the network and the cannot access the channel totally in ODSA, the timecommunication range of nodes can be configured as delay of nodes owning slots can be ensured. Howeverparameters. The channel impact is omitted in the FPRP can't offer this, as nodes cannot ensure that theysimulation. access channel in each time frame.

The simulation of node mobility is realized by Although ODSA is a little unfair on slot reservations,changing the position of nodes randomly and the unfairness can be alleviated with the variations ofperiodically. The position changing range is randomly application and topology.

. ODSA infODSAinf -+- FPRP inf

12 --*- FPRP inf 12 - ODSA 10-70s-e-ODSA 10-70s -6- FPRP 10-70s

--o- FPRP 10-70s

10 1 0 X '----

X e--- --6 -6

6 66

4 - 4-

2 3 4 5 2 3 4 5communication distance(km) communication distance(km)

Figure 3. Average access delay versus Figure 4. Average access delay versuscommunication distance (static) communication distance (mobile)

200 200ODSAinf ODSA inf

180-

FPR -n 18 FRPin

e-ODSA 10-70s 166DSA 10nf 0FPRP 10-70s~~~~~~~~~~~~~~~~~~~-o-FR 1-O160 --S S + \ 160

140 140

O120 -x5SO6120

100 6 100

80--X X800= ,

60 602 3 4 5 2 3 4 5

communication distance(km) communication distance(km)

Figure 5. Channel utilization versus Figure 6. Channel utilization versuscommunication distance (static) communication distance (mobile)

6. Conclusion analysis," Journal of Zhejiang University (EngineeringScience), v 39, n 6, Jun 2005, pp. 901-905.

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The simulationresults shw thatODSAhasgood access scheduling in ad hoc networks," in Proc. IEEE ICNPThe simulation results show that ODSA has good 2002. Paris, France, Nov 2002.performnance and can be applied to ad hoc networks. [6] C.D. Young, "USAP: a unifying dynamic distributed

In ODSA, once the time frame length is determined, multichannel TDMA slot assignment protocol," in Proc.it can't change with the situation. Future work includes IEEE MILCOM'96, vol.1, Oct 1996.designing a protocol that the frame length can be [7] C.D. Young, "USAP multiple access: dynamic resource

adjusted as required to improve the performance of slot allocation for mobile multihop multichannel wirelessassignment algorithm. Moreover, complicated channel networking," in Proc. IEEE MILCOM'99, Nov 1999.environment should also be included in study. [8] A. Kanzaki, T.Uemukai, T. Hara et cl, "Dynamic TDMA

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[1] Xu Ming-Xia, Zhao Min-Jian, Dong Fang et al, "Slot 330-335.'synchronization in Ad Hoc network based on parameter