An Access Protocol for Urgent Traffic in Wireless NetworksEnhanced with Ad Hoc Networking

Takahiro Fujiwara,1,2 Noboru Iida,3 and Takashi Watanabe4

1Enegene Co., Hamamatsu, 432-8511 Japan

2Graduate School of Science and Technology, Shizuoka University, Hamamatsu, 432-8011 Japan

3Department of International Economics, University of Hamamatsu, Hamamatsu, 431-2102 Japan

4Faculty of Information, Shizuoka University, Hamamatsu, 432-8011 Japan

SUMMARY

The communications environment is currently wellorganized. Still, it is not easy to handle requirements forurgent communications in a disaster. In a large-scale disas-ter, it is highly probable that communications functions willbe hampered, making access to communication links itselfdifficult. There is a need for studies directed at assuringrobustness and reliability in the emergency communica-tions system. This paper proposes an access protocol forwireless networks in which the centralized control hierar-chical network and the horizontal connection ad hoc net-work are combined. The routing method in which a terminalthat cannot directly communicate with the base station (BS)sends urgent data to the BS through neighboring terminals,and an access control protocol for multihopping, are de-scribed. In the proposed scheme, the terminal monitorscommunications in neighboring nodes and dynamicallygenerates a route based on the number of hops to the BS.Evaluations show that the percentage of terminals that canreach the BS by multihopping involving a few hops issignificantly improved. It is also shown that the terminalknows the number of hops to the BS in the proposedmethod, which makes it possible to predict the throughputand the delay time, which is a property suited to urgentcommunications. © 2005 Wiley Periodicals, Inc. ElectronComm Jpn Pt 1, 88(7): 52–64, 2005; Published online in

Wiley InterScience (www.interscience.wiley.com). DOI10.1002/ecja.20141

Key words: wireless network; ad hoc network;multihop; medium access control; routing protocol.

1. Introduction

With recent progress in communications and net-working technology, the communications environment hasbeen greatly improved and the environment allowing thenetwork to be utilized “anytime anywhere” is coming intobeing. It may be expected that in such a communicationsenvironment, the necessary communications facilities canalso be assured in an emergency, such as a natural disaster.However, the communications facilities can be assured onlyif the communications system works normally. Access be-comes difficult when communications channels are heavilyloaded or congested. Especially severe restrictions may beplaced on urgent communications.

In the Great Hanshin-Awaji Earthquake of 1995,most communications systems were damaged and severelyhampered [1]. In the wake of the experiences, many studiesof communications in a disaster have been carried out.Examples include multiple route design for major commu-

© 2005 Wiley Periodicals, Inc.

Electronics and Communications in Japan, Part 1, Vol. 88, No. 7, 2005Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J86-B, No. 11, November 2003, pp. 2345–2356

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nications networks, distributed placement of switching de-vices, and priority assignment of links to socially importantcommunications. The system presented in Ref. 2, for exam-ple, aims to control the shut-off valves at 3600 sites in thegas pipelines using prioritized telephone services for emer-gencies. Such an approach is useful in specific installations,but it is generally impractical to provide urgent communi-cations for ordinary citizens, if not impossible.

Systems using the Internet have been considered inorder to collect and provide safety information in emergen-cies [3, 4]. Safety information is an important factor forcitizens, as well as for rescue activities and damage assess-ment. Thereby, safety information has to be collectedquickly with a simple operation. The use of the Internet forthis purpose is a practical approach [3, 4], but the problemsof how to provide communications environment on theInternet and the means of access to the network must beaddressed.

There has been a study of the collection of safetyinformation using a cellular network [20]. When a basestation is damaged, the network is reconstructed autono-mously in a distributed fashion from the remaining basestations of neighboring cells, and the safety information onthe routes is collected via the neighboring base stations.Damage to the base station is a serious issue due to theimpact on the whole cell. In most cases, however, buildingscontaining the base station have been made robust to disas-ters, comparing the case of ordinary residences which arevulnerable to disasters as reported in Ref. 1. We mustconsider a means of collecting information from thoseresidences. In addition, the probability of communicationserrors increases with the number of hops. Consequently, anetwork in which information is collected with fewer hopsis required.

There has also been a study of the realization ofurgent communications by using voice communicationsservices (VoIP) over the Internet [5]. At the present stage,however, there are problems with verification of originatorlocations, reliability of urgent communications, and re-sponsibility for communications. There is also the key issuethat a means of accessing the Internet must be providedstably in an emergency.

The number of mobile phone sets has increased rap-idly, from approximately 4,300,000 at the end of 1994 toapproximately 72,000,000 at the end of October 2002 [6].In addition, traffic and frequency of use have increased dueto image transmission services and accessing of the Internetfrom mobile phones. Thus, the mobile phone system, whichwas useful in the disaster of 1995, now suffers seriouslyfrom congestion. It is likely to be difficult to use mobilephones in a large-scale disaster. In this regard, there hasbeen a study intended to improve access probability byrestricting the holding time in order to reduce the call lossprobability [7]. However, from the viewpoint of the urgent

communications it is necessary to assure access by somemeans, such as classification of communications, in addi-tion to reducing the call loss probability.

Whereas there are systems based on public networks,such as telephone and the Internet, a disaster informationcollection system using a dedicated wireless communica-tions network has been proposed [8, 9]. This system aimsto monitor the state of lifelines by using a dedicated wirelessnetwork. In this system, urgent information collected by aterminal is integrated in a small data packet (several tens ofbytes) and is delivered to the base station and the centralstation as an urgent message. Since a dedicated communi-cations system is used, communications congestion willrarely present a problem. However, this system consists ofa centralized control network and is not suited to dynamicactions such as construction of substitute routes in order todeal with an obstacle on a channel or damage to communi-cations installations such as antennas.

This paper discusses a network scheme for assuringurgent communications, and the associated access protocol.An emergency communications wireless network scheme(ECCA, emergency communications combining central-ized and ad hoc networking), which combines centralizedcontrol and ad hoc networking, is proposed. In the proposedsystem, the centralized hierarchical control network (CH-Net), in which terminals communicate directly with a basestation (BS), and the ad hoc network (AD-Net), in whichterminals communicate directly with each other and reachthe BS through multihops, are combined as a hybrid wire-less network [10]. This paper focuses on a routing protocol,LISNER (listen and search neighboring environment pro-tocol), in which the route is modified dynamically in orderto bypass nonfunctioning sites, and a time-division accesscontrol protocol for multihops, MICS (multihopping oninitial channel access scheme).

In the following, Section 2 outlines the proposednetwork and its configuration. Section 3 describes the ac-cess protocol. Section 4 describes an evaluation by simula-tion, Section 5 presents a discussion, and Section 6concludes the paper.

2. Emergency Communications CombiningCentralized Control and Ad Hoc Networking

2.1. Outline

It is a vital factor in emergency communications thata communications terminal is able to access the networkstably. For this purpose, terminals must obtain stable accessto an access station (such as BS) of the network and main-tain connections robustly and reliably.

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In a cellular network in which the terminals and BScommunicate directly, the BS controls the communications,which assures efficiency. However, since the network con-sists of single links, it is difficult to compose substitute linksonce a link is disconnected. In a network in which terminalscommunicate with each other by multihopping, on the otherhand, the links are dynamically created, which is a usefulproperty in an emergency such as a disaster. When thenumber of hops is increased, however, it becomes difficultto maintain the connection, degrading the reliability of thenetwork.

The proposed ECCA combines networking technolo-gies for direct connections between the BS and terminalsand for multihop communications between terminals. It isa network in which a terminal can reach a BS with fewerhops, and a substitute route can be built dynamically for alink in which a fault occurs.

2.2. Network model

As shown in Fig. 1, the ECCA emergency communi-cations wireless network which is proposed in this paperconsists of wireless terminals that monitor the state of thehome by means of various sensors or a home server com-puter (HS), access points in the wireless network, andwireless relay stations, in order to make possible commu-nication with the BS. Most of the terminals are fixedterminals, which usually communicate directly with the BS.The whole network is configured with multiple cells, withcell radii ranging from several hundreds of meters to 1 km.

When an adverse communications condition arisesfor a terminal due to a disaster or communication obstacles,direct communications to the BS may not be maintained, orthe transmitter power must be lowered in order to reduce

power consumption. Even in such conditions, communica-tions between neighboring terminals within several tens ofmeters may be available.

The proposed ECCA is a hybrid wireless networkwhich combines CH-Net, in which the BS and the terminalscommunicate directly, and AD-Net, in which the terminalscommunicate with each other by multihopping. Though acity is covered by a multicell configuration, we discussnetworking within a single cell in this paper.

In ECCA, a terminal that can communicate directlywith the BS is called a direct connection node (DCN), anda terminal that communicates with the BS by multihoppingthrough neighboring nodes is called an indirect connectionnode (ICN). In CH-Net, the DCN and BS send terminal dataand control data to each other by direct communications.The control data are transmitted periodically through acommon control channel (the shared channel CCCH de-scribed later).

When a fault arises in the link between a DCN andBS and the terminal decides that direct communication isnot available, it switches the communications mode fromCH-Net to AD-Net, and monitors the communications ofthe control channel in the neighboring nodes. If a node thatcan communicate with the BS is detected, that node is usedas the relay station and the route to the BS is formed. It thensends the control information and the urgent data.

2.3. Channel structure

In ECCA, the communications channel consists of adata channel and a control channel. Figure 2 shows the

Fig. 1. Overview of emergency communicationscombining centralized and ad hoc networking (ECCA). Fig. 2. Channel structure of ECCA.

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channel structure. The data channel is used for larger-sizedata traffic between terminals and the BS in CH-Net. Itconsists of an uplink data channel (UDCH) and a downlinkdata channel (DDCH). The control channel, on the otherhand, is a common communication channel (CCCH) sharedby CH-Net and AD-Net. Communications between the twokinds of networks are carried out in this channel.

This study assumes a network for data collection, inwhich a large amount of data flows in the uplink. TD-CDMA (time-division and code-division multiple access)combining CDMA and TDMA is used, where the numberof multiple accesses can be increased while maintaining theorthogonality of the CDMA code [9]. The channel structureof the uplink UDCH is shown in Fig. 2(a). By using the pCDMA channels and q time slots shown in the figure,multiple access is conducted for p × q terminals. The BSassigns time slots dynamically, based on requests from theterminals. In the downlink DDCH, all terminals share thechannel, as shown in Fig. 2(b), and the BS delivers data inTDM (Time Division Multiplexing) by specifying the timeslots with terminal address (T1, T2, . . . , Tk in this case).

The CCCH is shared by CH-Net and AD-Net for usein communications between the terminals and the BS, aswell as in communications between terminals. The controldata for CCCH are sent periodically. Consequently, urgentdata can be delivered regularly by sending them over thischannel. The communications frame of CCCH is dividedas shown in Fig. 2(c) into mini-slots corresponding to thenumber of terminals, and the terminals are assigned in afixed way to the mini-slots beforehand. The purpose of thispaper is to investigate urgent wireless communications,focusing on communications in CCCH.

3. Medium Access Control ConsideringRouting

This section discusses a routing protocol and a me-dium access control protocol considering multihopping inthe control channel CCCH.

3.1. Routing

In most of the routing protocols of ad hoc networks,a route request (RREQ) is handled by flooding, whichconsumes communications bandwidth to search and retainthe route [11, 12]. In on-demand routing protocols such asAODV [13] and DSR [14], an RREQ is handled by floodingto the neighboring nodes at packet transmission, and theroute is created on the basis of the response (RREP) fromintermediate nodes or the destination.

In the case of a table-driven routing protocol such asDSDV [15], nodes use a routing table to establish a corre-

spondence between the packet destination and the node ofthe next hop, which must be updated and maintained peri-odically using the control packet. Consequently, communi-cations bandwidth is also consumed in this protocol toupdate route information.

A cluster scheme such as the flooding gateway selec-tion (FGS) protocol [16] has been proposed in order toreduce flooding traffic, and hierarchical ad hoc networkingusing mobile backbone nodes (MBN) was employed [17]in order to extend the scale of ad hoc networks. Since theschemes using clusters are capable of restricting the flood-ing range, they are effective in reducing bandwidth con-sumption. However, it has been pointed out that overheadis induced to build and maintain the clusters, and that thetraffic is increased due to table updating accompanying thechange of the cluster head [11].

This paper proposes a routing protocol based on thelisten and search neighbor environment scheme (LISNER),due to reducing communications bandwidth consumption.A terminal that cannot communicate with the BS by CH-Net does not send an RREQ packet for routing, but insteadsearches a route by monitoring control data communica-tions in the CCCH with periodical transmission from theneighboring nodes. Thus, even when the condition of thenetwork changes suddenly and the network is rebuilt fre-quently, as in a disaster, the traffic is not increased. Thereby,it is a routing scheme which is suited to an environment withlimited communications resources.

Furthermore, by attaching the state of the terminaland the urgent data to the regularly sent control data, urgentdata can be delivered stably. The scenario for this processis as follows. When a situation arises in which a terminalcannot communicate with the BS, the terminal monitors thecommunications in the CCCH of the neighboring node. Asshown in Fig. 3, the header of the communications packeton the CCCH contains the number of hops to the BS(Hop-CNT), together with the address information. Theterminal can determine the number of hops to the BS fromHop-CNT of the packet header received from the neighbor-ing node. Based on the number of hops, the terminal deter-

Fig. 3. CCCH frame format.

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mines the relay node and requests packet transfer to the BS.The requested node transfers the packet over an alreadyconstructed route to the BS. If there are multiple relay nodesin the neighborhood of the terminal which can reach the BS,the terminal selects the relay node whose route has thesmallest number of hops. If there are multiple relay nodeswith the minimum number of hops, one of them is arbitrar-ily selected.

In order to manage the transfer route, the terminal orthe relay node builds and updates the routing table on thebasis of the communications header of the uplink. Accord-ing to the information in the table, the node selects thenext-hop node and the downlink route.

Table 1 shows, as an example, the routing table forT1 and T2 when the route is T4 → T3 → T2 → T1 → BS.“Out,” “In,” “Dest,” and “Src” in the table represents theneighbor output node, neighbor input node, final destina-tion, and source node, respectively. In the case of T1, forexample, when a packet sent from T4 arrives at T1 from T2,it is recorded in the table that the output is the BS, the inputis T2, the destination is the BS, and the source is T4, sinceT1 can directly communicate with the BS. In the case ofT2, the node has recognized that T1 can communicate withthe BS. Consequently, when a packet originating at T4arrives at T2 from T3, the node records that the output isT1, the input is T3, the destination is the BS, and the sourceis T4.

In the downlink, on the other hand, when the BSsends a packet to T4, the packet, at first, is sent to T1 inaccordance with the routing table. T1 recognizes from therouting table shown in Table 1 that destination T4 existsbeyond T2, and transfers the packet to T2. Similarly, T2recognizes from the table that T4 exists beyond T3, andsends the packet to T3. Then T3 transfers the packet to T4.The multihopping route is determined in this way in theuplink and downlink.

3.2. Access control

In the access control in ECCA using time slots, framesynchronization must be established between the BS andthe nodes. Several methods such as the GPS clock signal[18] or autonomous slave synchronization schemes [19, 21,22] have been proposed to establish the frame synchroniza-tion in communications. In this paper, we assume that theframe synchronization has already been established on thebasis of those results.

Provided that the arriving packet is transferred byusing a mini-slot assigned to the node, in the nodes with ahigh packet arrival frequency, the number of packets in thequeue may increase, making processing impossible. Wepropose an access protocol for multihopping, MICS (mul-tihopping on initial channel access scheme), to deliverurgent data. In this protocol, the relay node transfers thepacket by using a mini-slot assigned to the source node(Src). In other words, the packet is transferred at everyframe, resulting in high capability of packet transmission,even if the node has a large number of packets to relay.Although the operating load of the relay node is increasedunder this scheme, the time for the packet to arrive at theBS is determined by the number of hops. Consequently, thescheme allows a node to avoid the increase in the delay ofthe packet due to queuing.

3.3. Multihopping

Figure 4 illustrates multihop communications usingMICS. A packet is transferred in a mini-slot assigned to thesource node, in accordance with the address recorded in theheader. When a packet arrives at the BS, BS-ACK is re-turned from the BS to the source node as the response. Onreceiving this, the node finds that communication has beencorrectly achieved and that the mini-slot is in the idle state.

As a model, consider the multihopping T3 → T2 →T1 → BS. Suppose that terminals T1, T2, and T3 areassigned to mini-slots 1, 2, and 3, respectively. The arrowin each mini-slot in the figure indicates the flow of datapackets from terminals to the BS and the flow of BS-ACKfrom the BS to the terminals.

• Since T1 can communicate directly with the BS,the data are sent to the BS at every frame. Thecorresponding BS-ACK is returned within thesame mini-slot.

• T2 sends the packet to T1 in the second slot of thefirst frame, and T1 sends that packet to the BS inthe second slot of the second frame. BS-ACK isreturned to T2 via T1 in the second slot of thesecond and third frames.

Table 1. Sample of routing tables in T1 and T2

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• Similarly, the packet from T3 is relayed by T2 andT1 in the third slot, to be transferred to the BS.BS-ACK is returned to T3 via T1 and T2 in thethird slot.

Thus, the packets are transferred successively, andthen T1, T2, and T3 can send packets with 1, 3, and 5 frameperiods, respectively.

Figure 4 also shows the header information when apacket is relayed by multihopping. As an example, theheader when T3 sends a packet to T2 for relay is as follows.

• In a packet sent from T3 to T2, the header is Out= T2, In = T3, Dest = BS, Src = T3, as shown inthe third slot of the first frame. Since the packetcan reach the BS from T3 in three hops, Hop-CNT= 3.

• When T2 relays this packet to T1, the third slot ofthe second frame is used. The packet is sent withOut = T1, In = T2, Dest = BS, Src = T3, Hop-CNT= 2.

• Similarly, T1 sends the packet to BS in the thirdslot of the third frame. BS-ACK is returned in thesame slot.

• T1 sends the BS-ACK to T2 in the third slot in thefourth frame with Out = T2, In = T1, Dest = T3,

Src = BS, Hop-CNT = –1. Hop-CNT of the BS-ACK is set as –1 (meaning Don’t Care).

• Similarly, T2 relays the BS-ACK to T3 in the thirdslot in the fifth frame.

4. Performance Evaluation

This section shows the results of connectivity be-tween terminals and the BS, throughput and delay withregard to the proposed protocol in ECCA.

4.1. Simulation conditions

The terminal transfers packets to the BS in accord-ance with the routing protocol and medium access controlprotocol of the proposed system. We assume a network inwhich the data always exist in each terminal by collectingfrom sensors and the like. A circular cell of radius R isassumed at 250, 340, 500, and 1000 m, respectively. Theterminals are placed on a lattice model with spacing d. Thecorresponding number of terminals (N) is shown in Table 2(1).

In CH-Net, where direct communications to the BSis considered, the whole area of the cell is covered as therange of communications. In AD-Net, on the other hand,

Fig. 4. Access control for multihopping.

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the range of communications for the ICN is denoted by L(radius). Under conditions where the distance d to theneighboring terminal is set as L ≥ d, any terminal in therange of communications can communicate with eachother. The number of nodes in the range of communicationsis defined as the “node density.” The density when the rangeof communications is equal to the terminal distance (L = d)is called the “reference density.”

In this evaluation, the packet is used as the unit of dataand the frame period is used as the unit of time. Providedthat the conditions are assumed as shown in Table 2 (2), thatis, Payload (PL) = 32 bytes, Header (Hdr) = 16 bytes, ACK= 8 bytes, and the transmission rate is 10 Mbits/s, thetransmission time is calculated at 44.8 µs. Adding the guardtime, the mini-slot length is set as 50 µs. Provided that thenumber of nodes to be contained in a cell is 2000, the framelength is set as 100 ms. To evaluate the basic characteristicsof the access control protocol, the error rate in communica-tions is assumed to be negligible.

4.2. Simulation model

4.2.1. Evaluation model for routing

The simulation model for routing in ECCA is definedas follows for performance evaluation.

(1) Definition of model

• Let the set of all terminals be N and the set of DCNbe N1. The ratio of |N1| to |N| is denoted as DCNR:

DCNR = number of DCN

total number of terminals =

| N1|| N |

(1)

• For each DCNR, the terminals are placed at ran-dom in the cell so that the DCN are uniformlydistributed. The Hop-CNT is set as 1. Communi-cation with the BS is attempted in each frame.

• A terminal which is within the range of commu-nications (L) of a DCN but cannot communicate

with the BS monitors the communications of theneighboring nodes. When the terminal detects aDCN, it sets the Hop-CNT as 2 and sends thepacket to the DCN. The DCN relays the packet tothe BS. In this process, the packet is transferred ina slot of the source node according to the MICSprotocol. When the response from the BS (BS-ACK) is returned to the terminal, the terminal isregarded as a member of the set of Hop-CNT = 2.

• Similarly, when a terminal in the range of commu-nications of an ICN with Hop-CNT = 2 overhearsthe communications, it sets the Hop-CNT as 3, andsends the packet to the ICN. When the route isdecided by receiving BS-ACK, the terminal isregarded as a member of the set of Hop-CNT = 3.

• By iterating the above procedure, Hop-CNT ofeach terminal which is not DCN is determined.Provided that the set of terminals is denoted byN1, N2, . . . , Nn, respectively, for each value ofHop-CNT, we define Nn = {x|x ∈ N, hx = n},where hx is the number of hops of terminal x.

(2) Connectivity

Among all terminals, the percentage of terminalswhich can reach the BS within n hops (written as MR = n)is defined as connectivity. Using the set of terminals Ni foreach Hop-CNT, the connectivity is represented as in Eq. (2);in this evaluation, the communications error is assumed tobe negligible, and thus the connectivity is equal to thepacket delivery rate to the BS:

where N and Ni are the set of all terminals and the set ofterminals that can reach BS in i hops, respectively.

4.2.2. Performance evaluation model based oncalculation

Using Hop-CNT as the parameter, evaluation formu-las are derived for throughput and delay in the network.

(1) Throughput

The total number of packets that can be transmittedto the BS in unit time through CH-Net is defined as thechannel capacity of the whole network (C0). The ratio ofthe number of packets that can arrive at the BS in n or fewerhops to the channel capacity is defined as the averagethroughput (ηc(n)).

Table 2. Simulation conditions

(2)

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Let the unit of time be the frame length. In CH-Net,each terminal transmits a packet in a unit time, so thatC0 = | N |. In the proposed protocol, as shown in Fig. 4, afterBS-ACK returns to the source node, the new packet is sentin the next frame. Thus, the transmission interval in a nodefrom which the BS can be reached in i hops is 2i – 1frames. Let the set of packets arriving at the BS in a unittime by i hops be Pi. Then the number of packets |Pi | is givenas follows, since the communications error rate is assumedto be negligible:

Thus, the average throughput for n or fewer hops is givenby

(2) Delay

The time (number of frames) required for a packet toarrive at the BS from a source node and for the BS-ACK toreturn to the source is defined as the delay. In the case of aDCN, the BS-ACK is returned immediately after transmis-sion. Consequently, its communications time is ignored.Thus, the delay for i hops is given by 2i – 2. Using the setof packets Pi arriving at the BS in a unit time by i hops, theaverage delay for n or fewer hops is given by

4.2.3. Performance evaluation model forexperiment

(1) Definition of model

The simulation model for performance evaluation ofECCA is defined as follows.

• The DCNs are arranged according to the modelpresented in Section 4.2.1, and their Hop-CNT isdefined for each terminal.

• The terminal and the relay node send packets inthe specified slots as determined by the MICS

protocol. The BS returns BS-ACK for an arrivingpacket.

• It is assumed in this network that data are alwayspresent in the terminal. Consequently, when BS-ACK arrives at the terminal, the next packet isimmediately sent in the next frame.

(2) Throughput

The number of packets qi (T) arriving by each numberof hops is examined for the measurement period (T frames),and the average throughput (ηm(n)) for n or fewer hops isgiven by

(3) Delay

For all packets arriving at the BS by n or fewer hopsin the measurement period (T), the number of frames re-quired from the transmission by the terminal to the recep-tion of BS-ACK is examined, and the average delay(TD___

m(n)) for a packet is given by

where qi (T) is the number of packets arriving in T framesby i hops. fij is the number of frames, under conditionswhere the j-th packet is transferred at i hops.

4.3. Results of connectivity

The experiment was carried out by computer simula-tion under conditions where the cell radius is 340 m, thenumber of terminals (N) is 901, and the range of commu-nications from the terminal L is equal to the terminalinterval d (i.e., reference density). Provided that the sets ofterminals Ni for Hop-CNT = 1~4 are determined for DCNR= 0.01~1, connectivity for MR = 1 to 4 is derived by Eq. (2),and is shown with the solid line in Fig. 5.

The graph shows that when DCNR = 0.2, the connec-tivity of the terminals that can reach the BS in 1 hop (MR= 1) is 20%, and connectivity for MR = 2 is 67%. Thus, 47%of the terminals can be connected in 2 hops. For MR = 3 and4, the connectivity is improved to 93% and 99%, respec-tively. On the other hand, when the network composed onlyof AD-Net, corresponds to the condition DCNR ≈ 0, we

(3)

(4)

(5)

(6)

(7)

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easily find from the results in Fig. 5 that it is difficult toreach the BS by a small number of hops.

Next, consider the conditions in which terminals havebeen damaged in a disaster. The ratio of the failed terminalsto all terminals is defined as the broken rate (BR). Thedashed line in Fig. 5 shows the connectivity for a brokenrate of 20% and MR = 1 to 4. When BR = 0.2 and DCNR =0.2, the connectivity for MR = 2, 3, 4 is 54%, 72%, and 77%,respectively. This indicates that more than 50% of theterminals can communicate with the BS by 2 or fewer hops(MR = 2). In other words, even under conditions where 20%of the buildings are completely destroyed in a disaster[which is twice the number of seriously damaged buildings(approximately 10%) estimated in Ref. 23] and the termi-nals installed in the destroyed buildings cannot operate, andfurthermore, only 20% of all terminals can directly com-municate with the BS, more than 50% of the terminals canreach the BS within 2 hops. This means that 46% ofterminals cannot communicate with the BS in MR = 2, 20%have been damaged and cannot operate, and the rest (26%)cannot reach the BS in the specified number of hops. Hence,those terminals are called isolated terminals. In the case ofMR = 3, the connectivity is up to 72%. Therefore, since 28%of terminals cannot communicate with the BS, only 8% ofterminals are still in the isolated terminals. Thus, when thenumber of hops is increased from 2 to 3, communicationsis restored in 18%(= 26% – 8%) of the isolated terminals.

Assuming a reference density in which the commu-nication range of the terminal is equal to the distancebetween terminals, the ratio of the number of nodes (nodedensity) in the communication range to the reference den-sity is defined as the node density ratio (NDR). Figure 6shows the connectivity as a function of the node densityratio. The abscissa represents the node density ratio and theordinate represents the connectivity. The figure shows theconnectivity for DCNR = 0.05 and 0.1, and MR = 2 and 3.

When DCNR = 0.05 and MR = 3, NDR = 1, for example,the connectivity is 48%. When the node density ratio isincreased by a factor of 4, the connectivity is improved to85%. This indicates that in the high-density state in whichthe number of nodes within the range of communicationsis increased, the probability that a terminal can reach theBS within 2 hops is sufficiently high even if DCNR is low.

Figure 7 shows the connectivity for 4 cell radii (R)when MR = 2 and 3. We see that the connectivity is im-proved for any cell size by increasing MR. The improve-ment is nearly the same regardless of the cell size. In otherwords, the connectivity is greatly affected by the number ofhops, but not much by the cell size.

The above results indicate that the connectivity doesnot depend much on the cell size, and that connectivity islargely restored by a small number of hops such as MR = 3.When the density is high, the probability is high that termi-nals can be connected to the BS in a small number of hops,thus decreasing the number of isolated terminals. In an area

Fig. 5. Connectivity for DCNR in multihopping range(MR) and broken rate (BR).

Fig. 7. Connectivity for cell sizes.

Fig. 6. Connectivity for node density ratio (NDR).

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where the density is low, the probability that a neighboringnode exists is reduced, thus increasing the number of iso-lated terminals, which decreases the probability that com-munications can be restored by multihopping.

4.4. Result of throughput

The average throughput was examined based on theabove calculation model and was calculated with Eq. (4),under conditions where the cell radii (R) are 250 and 1000m, and MR = 1 to 3 and ∞. In addition, the value ofHop-CNT of terminals Ni (i = 1 ~ n) was determinedaccording to the results of connectivity. The results shownin Fig. 8 indicate that the throughputs for cell sizes of R =250 and 1000 m are nearly the same for each value of MR.Thus, the impact of the cell size is small for throughput.

Figure 9 shows the results of average throughputexamined by the number of arriving packets under themeasurement condition of R = 340 m, MR = 1 ~ 3, ∞ withinthe measuring time of 300 frames. Comparing the resultsin Figs. 8 and 9, we see that the throughput differs littlebetween MR = ∞ and MR = 3. In particular, the two arealmost identical when DCNR ≥ 0.2. The throughputreaches almost a maximum when MR = 3, and is not muchimproved even if MR is further increased. The results forthe calculation model and the experimental model shownin Figs. 8 and 9, respectively, correspond well for any valueof MR. Thereby, the average throughput can be derived byusing Eq. (4), based on the number of hops (i) and the setof terminals Ni. Thus, the throughput η of each terminal isexpressed, as a function of the number of hops (i) of theterminal, as η = 1 / (2i − 1). Consequently, a terminal thatknows the number of hops to the BS can estimate in advancethe throughput for the channel to be used from the numberof hops.

4.5. Result of delay

As in the evaluation of the throughput, the set ofterminals Ni is determined for each Hop-CNT, and theaverage delay for MR = 2 and 3 is derived by using Eq. (5)based on the calculation model. The results were shownwith the dashed line in Fig. 10. The solid line also showsthe average delay derived from the results of the experimen-tal model. We see that even if DCNR is 0.05 or less, theaverage delay is still small, for example, approximately 2frames for MR = 3.

In addition, Fig. 10 indicates that the results fromcalculation and experiment almost coincide, and then theaverage delay is derived from Eq. (5) by using the set ofterminals Ni. In the proposed protocol, there is no delaycaused by queuing in the relay node, and the total delay isestimated from the number of hops (i). Thus, the delay tdfor the terminal is expressed as a function of the number ofhops (i) of the terminal as td = 2i − 2. Consequently, thetime required for the packet to arrive at BS is estimated inadvance.

Fig. 8. Throughput in calculation method.

Fig. 9. Throughput in measurements.

Fig. 10. Delay time in measurement and calculation forMR = 2 and 3.

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5. Discussion

5.1. Synchronization

Technologies for frame synchronization such as theuse of GPS signals [18] and autonomous slave synchroni-zation [19, 21, 23] have been proposed. In the former, whena terminal is damaged and the GPS synchronization signalcannot be received, communications fails because theframe synchronization is no longer possible. In such casesthe BS can detect that some failure has occurred in theterminal, since communications from the terminal are sud-denly terminated. However, it cannot ascertain the detailsof the failure.

In autonomous slave synchronization, although it hasflexibility, the throughput may be degraded due to theoverhead for frame synchronization. In the case of synchro-nization in multihopping, cumulative error is a criticalmatter. However, in the proposed scheme, the network canbe configured with a smaller number of hops, and then therequirements regarding cumulative error may be reduced.Frame synchronization is a vital factor for the proposedprotocol, and further investigation should be carried out.

5.2. Routing

The proposed protocol discovers and builds a routeto the destination via multihopping by monitoring of neigh-boring node communications. The protocol has an advan-tage that the amount of communications is not increased forrouting. On the other hand, the delay is increased and thethroughput is decreased due to the access protocol formultihopping and channel assignment. Another issue is tospend time in routing when the number of hops is increased.As is evident from the evaluation results, however, most ofthe terminals can communicate with the BS by at most 3hops, resulting in short time routing.

5.3. Emergency communications

In the proposed access protocol, the throughput anddelay can be predicted from already known number of hops.It is an important factor to predict the delay in emergencycommunications, for which the time for the packet to arriveshould be reliably assured. This implies that a terminal cansend a packet for communications, considering thethroughput and the delay of the channel. For example, whenemergency data or disaster information are to be sent froma disaster site, the transmission can be arranged accordingto the priority of the information, considering the through-put and the delay.

6. Conclusions

This paper has proposed the ECCA wireless networkscheme for emergency communications enhanced with adhoc networking, and has discussed the routing protocol andthe medium access control protocol. In the routing protocol,the terminal monitors the communications of the neighbor-ing nodes and builds the route dynamically by discoveringa route to the BS on the basis of the number of hops recordedin the packet header. The protocol has the feature that excessbandwidth is not consumed.

It has been shown that connectivity is significantlyimproved by multihopping involving at most 3 hops. It hasalso been shown that the throughput of the network and thedelay are determined in the proposed scheme from thenumber of hops. Consequently, once it knows the numberof hops to the BS, a terminal can perform communicationsby considering the throughput and the delay. This is asuitable property for emergency communications, where ashort delay must be guaranteed.

In the future, the performance should be evaluated bymodeling the conditions from the viewpoint of a probabilityprocess, considering information sent at random from ter-minals. The retransmission control scheme should also beinvestigated, together with the throughput improvementand the communications stability. In addition, the perform-ance of the proposed scheme and of other ad hoc network-ing protocols should be compared quantitatively,considering the conditions of disaster. A protocol to buildmultihopping routes to neighboring cells should be studiedfor the case in which the BS is damaged, for a communica-tions system that can flexibly deal with disasters.

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AUTHORS

Takahiro Fujiwara (member) received his B.S. and M.S. degrees from the Department of Electronic Engineering,Shizuoka University, in 1978 and 1980, and Ph.D. degree in 2004. He joined Enegene in 1998. He became a researcher at TAOHamamatsu Lifeline Research Center in 2000. He has been engaged in the development of computer application systems andwireless communication systems. He is a member of IEEE, IEEJ, IEICE, and the Institute of Social Safety Science.

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AUTHORS (continued) (from left to right)

Noboru Iida (member) received his B.S. degree from the Department of Mathematics, Waseda University, in 1971 andjoined Mitsubishi Electric Corporation. He is now a professor on the Faculty of International Economics, Hamamatsu University.He has been engaged in the development of computer network systems. He holds a Ph.D. degree, and is a member of IEICEand the Information Processing Society of Japan.

Takashi Watanabe (member) received his B.S., M.S., and Ph.D. degrees from the Department of Communications, OsakaUniversity, in 1982, 1984, and 1987. He is now a professor in the Department of Computer Science, Shizuoka University. Hewas a Ministry of Education visiting researcher at the University of California Irvine in 1995. His research interests are computernetworks, distributed systems, and multiagent systems. In 1997, he was Secretary of the Mobile Computing Group, IPS. He isthe translator of Computer Design Techniques and other books. He holds a Ph.D. degree, and is a member of IEICE, IEEE, andthe Information Processing Society of Japan.

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