Joint Topology-Transparent Scheduling and QoS Routing in Ad Hoc Networks

372 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 63, NO. 1, JANUARY 2014

Joint Topology-Transparent Scheduling and QoSRouting in Ad Hoc Networks

Yi-Sheng Su, Szu-Lin Su, and Jung-Shian Li

Abstract—This paper considers the problem of joint topology-transparent scheduling (TTS) and quality-of-service (QoS) routingin ad hoc networks and presents a joint scheme for the problem.Due to its ability to guarantee single-hop QoS support, TTS ischosen as the underlying medium-access-control (MAC) protocol.By being built on top of TTS, this paper first designs meth-ods for bandwidth estimation and allocation (BWE and BWA,respectively) to provide QoS support without knowledge of slotstatus information, and then, estimates and allocates nonassignedeligible bandwidth for best effort (BE) flows. With these band-width management methods, this paper proposes a QoS routingprotocol for a mixture of QoS and BE flows. Idealized simulationresults based on the standard radio model, which ignores externalsources of radio interference and protocol inefficiencies, revealthat the proposed joint scheme can provide a reduction of atleast 93% in QoS violation rates and a reduction of 78%–89% incontrol overhead compared with the conventional dynamic sourcerouting (DSR)/IEEE 802.11 technique. A comparison with anotherconventional technique, i.e., DSR/carrier sense multiple access(CSMA), also reveals that the proposed joint scheme can reduceQoS violation rates by at least 93%. In addition, the proposedjoint scheme can provide an increase of 31%–104% in aggregatethroughput over two representative QoS routing protocols whileachieving a reduction of approximately 93% in QoS violationrates. The performance improvement to be achieved under arealistic radio model is yet to be determined.

Index Terms—Ad hoc networks, quality-of-service (QoS) rout-ing, time-division multiple-access (TDMA), topology-transparentscheduling (TTS).

I. INTRODUCTION

A. Motivation

AD HOC networks consist of wireless devices (called nodeshereafter) that can communicate with each other without

the help of a fixed infrastructure. As such, they are well suitedto create radio connectivity at any time and any place. Adhoc networks are used in many applications. Among thoseapplications, military or emergency operations may demand

Manuscript received December 17, 2012; revised May 18, 2013; acceptedJuly 13, 2013. Date of publication July 25, 2013; date of current versionJanuary 13, 2014. This work was supported by the National Science Council ofTaiwan under Contract NSC-99-2221-E-309-004 and Contract NSC-102-2221-E-309-003. This paper was presented in part at the 2008 IEEE Conference onNetworking, Architecture, and Storage, Chongqing, China, June 12–14, 2008and in part at the International Computer Symposium, Tainan, Taiwan, December16–18, 2010. The review of this paper was coordinated by Prof. Y. Cheng.

Y.-S. Su is with the Department of Computer Science and InformationEngineering, Chang Jung Christian University, Tainan 71101, Taiwan (e-mail:[email protected]).

S.-L. Su and J.-S. Li are with the Department of Electrical Engineering,National Cheng Kung University, Tainan 701, Taiwan (e-mail: [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TVT.2013.2274806

quality-of-service (QoS) support for effective communication.Therefore, flows in ad hoc networks can be generally cate-gorized into two types: QoS flow and best effort (BE) flow.For QoS flows, the traffic can be delivered to the destinationin a timely manner by QoS routing. For BE flows, however,there is no guarantee about whether and when a packet willbe delivered. Although there exist many QoS routing metrics,such as those described in [1], this paper considers bandwidthas the QoS routing metric because a bandwidth guarantee isgenerally one of the most critical requirements for effectivecommunication.

The ability of a QoS routing protocol to provide QoS supportis heavily dependent on how well channel resources are man-aged by a medium-access-control (MAC) protocol. MAC proto-cols can be categorized into two different categories accordingto the scheduling of their transmissions. The first category in-cludes contention-based MAC protocols, which allow nodes tocontend for transmission. Corrupted transmissions (collisions)are possible in this category, and the IEEE 802.11 standard [2]is a well-known example. Although the 802.11 MAC protocolis widely used, its contention-based nature makes it difficult toreserve bandwidth, which is often desirable for real-time multi-media traffic such as streamed voice or video. In [3], the IEEE802.11 Task Group E (802.11e) has defined enhancements tothe original 802.11 MAC protocol [2] to provide QoS support.However, it was not designed for multihop networks that areaddressed in this paper. The second category includes schedule-based MAC protocols, where each node has a certain set of slotsin which it is allowed to transmit. Researchers have developedmany schedule-based MAC protocols for ad hoc networks[4]–[16]. Unlike contention-based MAC protocols, schedule-based MAC protocols are potentially better suited to meet QoSrequirements by reserving bandwidth and by following theresulting transmission schedule.

Schedule-based MAC protocols can be classified into twocategories: topology-dependent scheduling (TDS) [4]–[10] andtopology-transparent scheduling (TTS) [11]–[16]. TDS con-centrates on finding conflict-free scheduling and maximizingsystem performance using network topology information. Thistype of protocol has been adopted in IEEE 802.16 Meshmode: coordinated distributed scheduling (CDS) [8], [9] andthe enhanced CDS, i.e., collision-free CDS (CF-CDS) [10].Although these TDS algorithms can obtain good performance,their obvious deficiency is that, when the network topologychanges, the previous transmission schedules expire, and newschedules must be generated. In contrast, TTS does not requireany topology information to perform transmission schedulingand can guarantee that each node has at least one collision-free

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SU et al.: JOINT TTS AND QoS ROUTING IN AD HOC NETWORKS 373

transmission slot for any (one-hop) neighbor in each schedule(i.e., single-hop QoS support) without the overhead due to therecomputation of transmission schedules.

Motivated by the advantages of TTS, this paper designsa QoS routing protocol using TTS as the underlying MACprotocol for ad hoc networks. To the best of our knowledge,this paper is the first to design a QoS routing protocol usingTTS as the underlying MAC protocol.

B. Challenges and Contributions

Integrating TTS into higher level routing protocols for QoSprovisioning in ad hoc networks is not a straightforward task.This is due to the following challenges.

• Although TTS does not require detailed topology infor-mation to schedule transmissions, it depends on the maxi-mum number of interference nodes (i.e., the nodes locatedwithin the interference range). Therefore, the guaranteeof single-hop QoS support may fail to hold at individualnodes because the maximum number of interference nodesis not easily estimated and may change with time due tothe inherent mobility of ad hoc networks.

• Even if the maximum number of interference nodes canbe sufficiently obtained to guarantee single-hop QoS sup-port at each node, resource decisions at individual nodesrequire information from nodes outside the transmissionrange. However, information from nodes outside the trans-mission range cannot be obtained as easily as in wirednetworks.

• It is unclear what information from nodes outside thetransmission range is needed for QoS provisioning if TTSis the underlying MAC protocol. Intuitively, slot statusinformation should be used for efficient admission control.However, any change in slot usage induces information ex-changes between nodes, possibly wasting many resources.It may be possible to provide QoS support without knowl-edge of slot status information.

• It is unclear whether a node can determine informationon available data channel resources provided by TTS,i.e., bandwidth information, after obtaining informationfrom nodes outside the transmission range. Moreover, afterdetermining bandwidth information, a node may not beable to efficiently utilize/reserve scarce bandwidth for QoStransmission.

• Due to the shared nature of the wireless medium in ad hocnetworks, each multihop flow encounters contentions fordata channel resources not only from other flows that passby the neighborhood, i.e., the interflow contention, butalso from the flow itself, i.e., the intraflow contention [17],[18]. It remains unclear how to cope with these two kindsof flow contentions when using TTS as the underlyingMAC protocol.

This paper presents solutions to address the issues raisedabove for joint TTS and QoS routing in ad hoc networks. Thispaper makes the following contributions.

• By being built on top of TTS, this paper designs a generalmethod for bandwidth estimation (BWE) to provide QoSsupport without knowledge of slot status information.

• This paper designs an algorithm for bandwidth allocation(BWA) to efficiently utilize the data channel resourcesprovided by TTS.

• This paper estimates and allocates nonassigned eligiblebandwidth for BE flows, i.e., nonassigned eligible BWEand BWA (BWEA).

• With the given bandwidth management methods, this pa-per proposes a QoS routing protocol for a mixture ofQoS and BE flows by giving routing access to bandwidthinformation.

C. Related Work

Many studies address the subject of QoS routing in ad hocnetworks in different environments using different models andapproaches [19]. These QoS routing protocols can be classifiedinto two categories: MAC-unaware QoS routing and MAC-aware QoS routing. In the following, we briefly describe someprior works in both categories.

MAC-unaware QoS routing focuses on QoS support at thenetwork layer and is not tuned to a particular MAC layer.For instance, assuming the existence of a MAC protocol thatresolves the contention and supports resource reservation, Chenand Nahrstedt [20] proposed a distributed QoS routing schemethat uses ticket-based probing to search for a low-cost pathsatisfying QoS requirements. Another example is the work in[21], in which a network-layer QoS support mechanism uses anin-band signaling system and the Temporally Ordered RoutingAlgorithm (TORA) [22] without taking into account the MAClayer. However, the routing itself generally has a significanteffect on the MAC-layer transmission. The transmission at theMAC layer also determines how many resources are availableat individual nodes, which in turn affects the decisions of QoSrouting. Therefore, QoS routing should be considered with bothMAC and network layers.

Unlike MAC-unaware QoS routing, MAC-aware QoS rout-ing deals with QoS provisioning by adding MAC awareness torouting decisions. Researchers have attempted to incorporateMAC in the design of QoS routing. For instance, on onehand, due to the unifying IEEE 802.11 standard, there hasbeen a bunch of QoS routing protocols designed for the IEEE802.11 MAC [17], [18], [23]–[26]. However, the contention-based nature of IEEE 802.11 MAC makes providing QoSassurances difficult. On the other hand, some [27], [28] usecode-division multiple access (CDMA) over time-division mul-tiple access (TDMA) to eliminate the interference betweendifferent transmissions, whereas others [29]–[35] address theproblem of MAC-aware QoS routing in simpler and less costlyTDMA environments. This paper focuses on the problem ofMAC-aware QoS routing in TDMA networks and reports thefollowing examples.

• Ho and Liu [29] proposed an on-demand QoS routingprotocol that only makes path BWE after discoveringpaths to the destination. Because this routing scheme onlymakes BWE at the destination, it has several potentialdrawbacks, such as poor tolerance to topology changes,low scalability, control packet flooding, etc. To cope with


these problems, path finding should be fully aware ofbandwidth resource availability [30]–[35].

• Given the knowledge of neighboring nodes’ slot usage,Liao et al. [30] considered the bandwidth reservation prob-lem to support QoS routing. The QoS routing algorithmpresented in [30] is an extension of the dynamic sourcerouting (DSR) [36]. The emphasis of their work is toaddress both the hidden-terminal and exposed-terminalproblems in TDMA environments.

• Zhu and Corson [31] considered the same problem asLiao et al. [30] and developed an algorithm for estimatingpath bandwidth. They used this algorithm, together withthe route discovery mechanism of the well-known AdHoc On-Demand Distance Vector (AODV) routing [37],to setup QoS routes.

• Gerasimov and Simon [32], [33] modified two well-knownon-demand ad hoc routing protocols, i.e., AODV andTORA, to achieve resource reservation in TDMA systems.The protocols in [32] and [33] borrow some of the mech-anisms presented in [27]. These routing algorithms addseveral messages and procedures to AODV and TORA tosupport QoS path reservation and release.

• Because simultaneous QoS route request (RREQ) mes-sages reserve slots independently, multiple path reserva-tions may be processed at intermediate nodes simultane-ously. These race conditions can reduce the throughput andefficiency of communications in TDMA environments.Based on [30], Jawhar and Wu [34] presented a bandwidthreservation protocol that can remedy the race conditionand provide a solution to the parallel reservation problem.

• Shih et al. [35] presented a distributed slot reservation pro-tocol for QoS routing. The main concept of this protocol isslot reuse. The primary differences between [35] and [30]are slot decision policies and a slot adjustment mechanism.

The proposed joint TTS and QoS routing scheme falls underthe category of MAC-aware QoS routing. This approach isfundamentally different from the given methods in the follow-ing aspects. First, the methods in [29]–[35] use a traditionalinterference model, in which no interference can arise out ofa receiving node’s transmission range. On the contrary, thispaper considers an interference model in which transmissionarising out of a receiving node’s transmission range can stillcause packet reception failure. Second, in [29]–[35], nodesexchange slot status information for BWE, potentially wastingmany resources. In contrast, the proposed joint scheme attemptsto provide QoS support without knowledge of slot status infor-mation, being capable of reducing the overhead added by QoSrouting. Third, the methods in [29]–[35] use TDS, whereas theproposed joint scheme is built on top of TTS.

D. Paper Organization and Notations

The remainder of this paper is organized as follows.Section II describes background information, including TTS.Section III presents in detail BWE, BWA, and nonassignedeligible BWEA at individual nodes. Section IV presents a QoSrouting protocol. Section V presents simulation results. A fewconcluding remarks are drawn in Section VI.

TABLE IMAIN NOTATIONS

Notations: Table I lists the main notations used in this paper.

II. BACKGROUND INFORMATION

A. System Model

Consider a multihop TDMA ad hoc network consisting ofN mobile nodes, each of which is equipped with an omnidi-rectional antenna. Randomly assign a unique ID to each nodebefore it enters the network, and let N = {0, 1, . . . , N − 1}denote the set of nodes. Each node operates in a half-duplexmode, which allows either transmission or reception at a giventime, but not both simultaneously. Each node has transmissionrange rT and interference range rI , which can be larger thanrT . Node j is called a neighbor of node i, i �= j, or vice versaif the distance dij between these two nodes satisfies dij < rT .Let N 1

i denote the set of neighbors of node i. Similarly, node jis called an interference node of node i, i �= j, if dij < rI . LetN 2

i denote the set of interference nodes of node i. Moreover, letD be the maximum number of interference nodes of any nodein the system, i.e., D = maxi∈N |N 2

i |, where | · | denotes thenumber of elements in a set. A direct transmission from nodei to a set J of nodes (abbreviated as “i → J ”) is successfulif: 1) each node in J , e.g., j, is a neighbor of i and also inreceive mode; and 2) any interference node of j, other than i,is not transmitting. This model is generally referred to as theprotocol model in the literature [38]. Based on this model, the


Fig. 1. Example network consisting of six nodes, i.e., N = {0, 1, 3, 4, 6, 9},where the distances between any two adjacent nodes are equal, rT is justsufficient to reach its adjacent nodes, and rI = 2rT .

Fig. 2. Frame structure with slots numbered starting from zero.

maximum distance between any two nodes that will potentiallyinterfere with each other’s transmission is equal to rT + rI ,which is henceforth called the unsafe range. Let N 3

i denotethe set of nodes located within the unsafe range of node i, i.e.,other than i. Fig. 1 shows a simple 1-D network consisting ofsix nodes. This figure shows that N 1

3 = {0, 6}, N 20 = {3, 6},

N 26 = {0, 1, 3, 9}, and D = 4. In addition, it is clear that N 3

3 ={0, 1, 6, 9}.

All nodes communicate over one channel. The channel inthe system is assumed to be time-slotted. As shown in Fig. 2,the transmission time scale is organized in fixed-length frames,each of which consists of two subframes, i.e., a control sub-frame (C-subframe) and a data subframe (D-subframe).1 AC-subframe contains nC fixed-size control slots (C-slots) and isused by each node to transmit control packets such that networkcontrol functions [e.g., neighbor detection, route discovery,and route reply (RREP)] can be performed distributively. AD-subframe contains nD fixed-length data slots (D-slots), eachof which is immediately followed by a fixed-size acknowledg-ment (ACK) slot (A-slot). D-slots are used to transmit datapackets for a mixture of QoS and BE flows. Since a trans-mitting node should have knowledge about the transmissionresult in a D-slot, this paper assumes that an ACK packetmust be immediately returned by the receiving node in theA-slot after successful reception of data packet transmission.Assuming that all nodes have the same interference ranges andthat he transmission of an ACK packet is required only aftersuccessful reception of a data packet, the mechanism for ACKof a successful data packet transmission is valid.

1A frame/slot synchronization mechanism can generally be achieved by thefollowing two approaches: using commercial GPS receivers [39] or runninga synchronization algorithm (which does not need a GPS receiver) [40]. GPSreceivers are typically more accurate than running a synchronization algorithm.However, using GPS receivers increases costs and energy consumption. Asa result, researchers have proposed many synchronization algorithms, fromwhich we can choose one to achieve synchronization among the nodes.

B. MAC Protocol: TTS

TTS was first introduced by Chlamtac and Farago [11] for adhoc networks and can be classified into two types: trivial TTSand nontrivial TTS. Trivial TTS is a pure TDMA working modefor the whole system without any transmission collision, i.e.,each node is assigned an exclusive slot for transmission in eachschedule. With this TTS, the number of slots in each scheduleshould be equal to the total number of nodes in the system. Thisscheduling algorithm is easy to implement but may have poorsystem performance because it does not make use of the spec-trum reuse capability of multihop networks. Another way toimplement TTS (i.e., nontrivial TTS) is to guarantee minimumperformance (i.e., collision-free slots) for each node in eachschedule while allowing occasional occurrence of transmissioncollisions.

To achieve transmission scheduling in a shared channel, eachnode in the network should be assigned a unique node activationtime (NAT), which is defined as follows.

Definition 1: The NAT of a node is a set of nA slots in aschedule, in which it can transmit.

Given N and D, the TTS problem can be formalized asseeking a set of NATs, i.e., {S0,S1, . . .}, which satisfies thefollowing requirements.

1) The total number of NATs is at least N because, in ashared channel, each node must have a distinct NAT.

2) Every pair of NATs has at most K slots in common.3) To guarantee that each node has at least γ collision-free

transmit slots for any neighbor in a schedule, it is requiredthat nA ≥ K ·D + γ.

The rationale behind Requirement 3 is explained as follows.For transmission i → {j}, j ∈ N 1

i , node j and its interferencenodes excluding i are those who may interfere with i’s transmis-sion to j. Since the number of these interfering nodes is equalto D and each interfering node interferes with i’s transmissionin at most K slots, the maximum number of slots in which theinterfering nodes can interfere with i’s transmission is equal toK ·D. Therefore, as long as the number nA of transmit slotsassigned to i in a schedule is not less than K ·D + γ, node iis guaranteed to have at least γ collision-free transmit slots forj in a schedule (for a more thorough discussion of TTS, see[11], [12], and [41]). The example below demonstrates simpleyet efficient nontrivial TTS based on affine planes (APs).

Example 1: Fig. 3 shows an example set of NATs. ThisNAT set has 12 NATs, and each NAT contains nA = 3 slots. Inaddition, each pair of NATs has at most K = 1 common slot,and each schedule consists of nine slots. This NAT set can beused to schedule transmissions in a network with N = 12 nodesand D = 2, leading to a guarantee of at least γ = 1 collision-free transmit slot in each schedule for each node. The NAT setis constructed from an AP of order 3. In general, an AP of ordern [denoted AP(n)] yields schedules of n2 slots and n2 + nNATs with nA = n and K = 1, where n is a prime power. Anexpanded description of APs and the way of mapping APs tofulfill TTS can be found in [42]. �

Although it is possible to construct nontrivial TTS using thealgorithms in [11]–[16], the remainder of this paper assumesthat nontrivial TTS is constructed from AP(n). The main


Fig. 3. Example set of NATs, where the filled squares in a column indicatethe slot indexes contained in an NAT. For example, it follows from the columnlabeled S0 that S0 = {0, 1, 2}.

reasons for selecting AP(n) are its simplicity in choosing thedesign parameter n and easy construction.

Since TTS serves the MAC in both the C-subframes andD-subframes, there should be two sets of NATs: one for theC-subframes and another for the D-subframes. Therefore, eachnode should be assigned two NATs that are separately chosenfrom the two sets of NATs. Let AC

i and ADi denote the NATs

assigned to node i for channel access in the C-subframes andD-subframes, respectively. Note that the two sets of NATscan be the same. With trivial TTS as the MAC in both theC-subframes and D-subframes, nC = nD = N , whereas withnontrivial TTS from AP(n), nC = nD = n2, which must notbe less than N . Intuitively, using an adequate ratio of nC andnD can help in improving system performance. However, bothnC and nD depend on N . Therefore, nC and nD are not veryflexible for performance optimization, although it is possible toprovide an analytical method or an algorithm to estimate therequired control and data bandwidth. To achieve BWE withoutknowledge of slot status information, this paper assumes a one-to-one mapping between node IDs and ADs. The one-to-onemapping can be designed offline based on random permutationand is known a priori to all nodes in the network.

III. BANDWIDTH ESTIMATION, BANDWIDTH ALLOCATION,AND NON-ASSIGNED ELIGIBLE BANDWIDTH

ESTIMATION AND ALLOCATION

Here, the essential components of a QoS routing protocolthat is built on top of TTS for a mixture of QoS and BE flowsare described. These components include BWE, BWA, andnonassigned eligible BWEA, and they are fulfilled with respectto ADs assigned to nodes. To facilitate these components,nodes should store and exchange information.

A. Information Storage and Exchange

To estimate the bandwidth for i → J , J ⊂ N 1i , node i is

required to maintain three tables (i.e., a neighbor list table, apriority table, and a receive table) and two one-bit flags (i.e.,Pi and Ri), which are described as follows.

• The neighbor list table of i contains an entry for each two-hop neighbor of i, e.g., j.2 This entry consists of j’s IDand neighbor list.

2j is called a two-hop neighbor of i if j can be reached from i within twohops. Therefore, a neighbor is also a two-hop neighbor, but the converse is notalways true. A similar definition can be made for a three-hop neighbor.

• i activates Pi if it needs to have priority over other nodesto exclusively use common D-slots (i.e., D-slots that arecommonly assigned); otherwise, i inactivates Pi.

• The priority table of i contains an entry for each three-hop neighbor of i, e.g., j. This entry consists of j’s Pflag and an extra indicator (a simple one-bit tag). The extraindicator is used for specifying whether j has priority overi to exclusively use common D-slots between AD

i and ADj .

• i activates Ri if it receives packets (i.e., acting as a relayor destination) for QoS flows; otherwise, i inactivates Ri.

• The receive table of i contains an entry for each two-hopneighbor of i, e.g., j. This entry keeps track of j’s R flag.

As previously mentioned, each node collects informationfrom nodes that are within three-hop distance from itself. Usinga three-hop limit for this collection is based on the followingtwo observations: 1) The unsafe range is rT + rI , and 2) theinterference range is replaced with the area that can be reachedby a two-hop limit. Although using a three-hop limit is nota sufficient condition to collect the required information fromnodes located within the unsafe range, the same hop countlimit appeared in [43]. A collection of the given informationis achieved by the following three-hop relay.

To construct a neighbor list table, each node must first knowwho its neighbors are, which can be facilitated by periodicbroadcasting of HELLO beacons. To maintain the given threetables, each node periodically broadcasts its neighbor list andR flag via control packets N_LIST and R_FLAG, respectively,for a depth of two hops (controlled by a hop-count-limit/time-to-live field in the control packets), whereas the P flag isperiodically broadcast via control packet P_FLAG for a depthof three hops.3

Upon receiving control information, each node copies thereceived information into its own three tables. However, to dis-tributively allocate common D-slots among nodes, the indicatorfield in the priority table must be updated using the followingrules (see details in Section III-B).

• If a node inactivates its P flag, it sets and clears theindicator for each priority table entry that has an activeand inactive P flag, respectively.

• Else, it cannot update the indicator field except when apriority table entry’s P flag is inactivated; in which case,the associated indicator is cleared.

B. BWE

For BWE, each node should keep track of the D-slots thathave been reserved for transmission, which are defined asfollows.

Definition 2: The set of D-slots that have been reserved fortransmission at node i is called the node reservation time of i,which is denoted Ri.

3Under the traditional assumption of rI = rT , a three-hop relay naturallyconverts to a two-hop relay. Although a three-hop relay might incur moreoverhead than a two-hop relay, it can help avoid many false admissions. Thetotal dominant pruning algorithm [44] can reduce the overhead produced byperiodic broadcasting of P and R flags in a three-hop relay.


Let Bi→J denote the set of available D-slots in a frame overwhich i → J can be successful. The bandwidth for i → J isthen estimated as the number of D-slots in Bi→J , i.e., |Bi→J |.The following discussion presents a way to determine Bi→J .

When calculating Bi→J , J ⊂ N 1i , it is necessary to consider

two requirements: 1) i → J is received successfully by thenodes in J ; and 2) i → J does not interfere with packetreception at i’s interference nodes. Requirement 1 can behandled by taking into account the potential interference fromthe nodes in J and their interference nodes other than i.This potential interference is mainly due to common D-slotsassigned by TTS to the nodes that are located within the unsaferange of i. To utilize common D-slots while not significantlyincreasing too much overhead due to exchanging slot statusinformation, the first node that needs to use common D-slotscan have priority over other competing nodes. Therefore, itis necessary to inform the nodes located within the unsaferange of the demand. To achieve this, when a node must use aD-slot that is commonly assigned to its three-hop neighbors, itactivates and disseminates its P flag (as stated in Section III-A)to announce that it needs to have priority over other nodes toexclusively use common D-slots. Upon receiving informationP , each node updates the indicator field in its priority table bythe rules described in Section III-A to allocate common D-slotsdistributively. On the other hand, it is not clear if Requirement2 can be met without using slot status information. However,since a node can only receive transmissions from its neighbors,Requirement 2 can be satisfied by simply not using the D-slotscommonly assigned to those nodes in N 3,P

i , which are alsoneighbors of i’s receiving interference nodes. To achieve this,in addition to P , each node must disseminate another one-bitflag R to announce whether it has received packets for QoSflows (as stated in Section III-A). For this reason, R only needsto be broadcast for a depth of two hops, as compared withP . The rationale behind maintaining neighbor list table alsofollows. Node i can then determine Bi→J , J ⊂ N 1

i , using thefollowing:

Bi→J =(AD

i −Ri

)︸︷︷︸set of available

D−slot for i →J

⋂⎛⎜⎝ ⋃

k∈(⋃

j∈JN 2

j∪{j}−{i}

)∩N 3,P

i

ADk

⎞⎟⎠

︸︷︷︸Requirement 1:

set of D−slots over whichno transmission from nodes in(⋃

j∈JN2

j∪{j}−{i}

)∩N3,P

itakes place

⋂⎛⎜⎜⎝ ⋃

k′∈⋃

k′′∈N2,Ri

(N 1k′′∩N

3,Pi )

ADk′

⎞⎟⎟⎠

︸︷︷︸Requirement 2:

set of D−slots over which transmissionfrom i can take place without interfering

with packet reception at i′s interference nodes

(1)

and the one-to-one mapping between node IDs and ADs.

Remark 1: The BWE in (1) is a general result that can beused to determine different types of bandwidth by varying J .For example, by substituting J with a set consisting of onlyone neighbor of node i, e.g., j, Bi→J determines the unicastbandwidth from i to j. By substituting J with N 1

i , Bi→Jdetermines the broadcast bandwidth from i to N 1

i , and aftersome algebraic manipulation, it becomes

Bi→N 1i=

(AD

i −Ri

)⋂⎛⎜⎝ ⋃

k∈N 3,Pi

ADk

⎞⎟⎠. (2)

Although (2) can be also used to determine unicast bandwidthfrom i to j, it is more conservative than substituting J with{j}. Using (2), however, has the advantage of no overhead ofbroadcasting information R.

Remark 2: In trivial TTS, all NATs are disjoint. Thus, iftrivial TTS is used instead of a nontrivial one, (1) will naturallyreduce to Bi→J = AD

i −Ri for each i ∈ N and J ⊂ N 1i ,

which is clearly the BWE method for the use of trivial TTSin the D-subframes.

Example 2: This example shows BWE and demonstratesthat the rules for updating the indicator field in the prioritytable can allocate common D-slots distributively among nodes.Consider the example topology shown in Fig. 1 and the exampleset of NATs in Example 1. For illustration, the assignmentof NATs to nodes is specially designed as follows: AD

i = Si,i ∈ N = {0, 1, 3, 4, 6, 9}. Suppose that Ri = ∅ ∀i ∈ N .

a) It follows from (1) that, initially, Bi→{j} = ADi for each

i ∈ N and j ∈ N 1i as none of the nodes in the network

have priority to exclusively use common D-slots. Sup-pose that a QoS flow f1 whose destination is node 0arrives at node 3 and requires a bandwidth of oneD-slot in a frame. Then, suppose node 3 admits flow f1 tothe network by reserving D-slot 3 for transmission 3 →{0}. Thus, B3→{0} = B3→{6} = B3→N 1

3= AD

3 −R3 =

{0, 6}. After noticing D-slot 3 is also assigned to itsthree-hop neighbor 1, node 3 activates and disseminatesP3. Finally, all the other nodes except node 4 updatetheir individual priority tables by setting the indicatorassociated with the table entry for node 3. In addition,node 0 activates and disseminates R0 because it receivespackets for flow f1. Then, it follows from (1) that

B9→{6} =(AD9 −R9)

⋂⎛⎝ ⋃

k∈{3}AD

k

⎞⎠⋂(⋃

k′∈∅AD

k′

)

= {5, 7}

B9→{1} =(AD9 −R9)

⋂(⋃k∈∅

ADk

)⋂(⋃k′∈∅

ADk′

)

= {0, 5, 7}.

In addition, it follows from (2) that

B9→N 19= (AD

9 −R9)⋂⎛

⎝ ⋃k∈{3}

ADk

⎞⎠ = {5, 7}.


Note that, according to B9→N 19

, node 9 cannot possiblyuse D-slot 0, although node 3 reserves only D-slot 3 forflow f1.

b) Suppose that another QoS flow f2 whose destination isnode 1 arrives at node 9, requires bandwidth of oneD-slot in a frame, and is admitted to the network byreserving D-slot 5 at node 9. This leads to B9→{6} = {7},B9→{1} = {0, 7}, and B9→N 1

9= {7}. After exchanging

control information, all the other nodes, except node 3,set the indicator associated with the entry for node 9 intheir individual priority tables. Therefore, the priority ofusing D-slot 5 is now fully owned by node 9, whereas thatof using D-slot 0 remains fully owned by node 3.

c) Now, suppose that another QoS flow f3 whose destinationis node 6 arrives at node 9 and requires bandwidth of twoD-slots in a frame. The current nodal statuses indicate thatflow f3 should be rejected due to the lack of bandwidthfor 9 → {6}.

d) Suppose that flow f1 terminates, and the reserved D-slot 3is released. In addition to inactivating and disseminatingP3, node 3 sets the indicator associated with the entry fornode 9 in its priority table, leading to B3→{0} = {0, 3, 6},B3→{6} = {3, 6}, and B3→N 1

3= {3, 6}. After receiving

the information sent by node 3, each node clears theindicator associated with the entry for node 3 in its pri-ority table. Then, B9→{6} = B9→{1} = B9→N 1

9= {0, 7},

indicating that D-slot 0 is now available to node 9. �Remark 3: In BWE, if a node supports only one QoS flow,

it will attempt to hold onto all common D-slots in its NAT evenwhen the flow uses only a small portion of common D-slots,as shown in Example 2a. This is because only a one-bit flagP is used to indicate which node has priority to use commonD-slots. To enhance bandwidth utilization, optimization canbe performed as follows. When a node attempts to use anycommon D-slot, it announces which nodes have lower prioritythan itself to exclusively use common D-slots, rather thansimply using a single one-bit flag.

According to (1), each node can estimate available band-width if all the required information is available. Once availablebandwidth is determined, the BWA algorithm should be devel-oped to properly utilize scarce bandwidth and is detailed in thefollowing.

C. BWA

Before formally describing the BWA algorithm, this paperuses Example 2 to illustrate how a node can efficiently allocateD-slots for transmission. In Example 2c, the lack of bandwidthfor 9 → {6} leads to the rejection of flow f3. Is this unavoid-able? Note that, in Example 2b, the reservation of D-slot 5 bynode 9 for 9 → {1} blocks 9 → {6} in D-slot 5. If node 9reserves D-slot 0 rather than D-slot 5 for 9 → {1}, which leadsto R9 = {0} and B9→{6} = B9→{1} = {5, 7}, node 9 can thenadmit flow f3 by reserving D-slots 5 and 7.

The given discussion indicates that, when performing BWAfor i → {j}, j ∈ N 1

i , node i should evaluate the blocking index(BI) of each D-slot t, t ∈ Bi→{j} �= ∅, denoted BIi→{j}(t),which is defined as the number of i → {k}, k ∈ N 1

i − {j},

with Bi→{k} containing D-slot t. In other words, BIi→{j}(t), t ∈Bi→{j} �= ∅, represents the number of i → {k}, k ∈ N 1

i − {j},which are blocked if i reserves D-slot t for i → {j}. Notethat BIi→{j}(t) ≥ 0, j ∈ N 1

i , ∀t ∈ Bi→{j} �= ∅. In Example 2a,for B9→{1} = {0, 5, 7}, it follows that BI9→{1}(0) = 0 andBI9→{1}(5) = BI9→{1}(7) = 1. The basic idea of allocatingbandwidth is to choose the set of D-slots with BIs as lowas possible. The details of the BWA algorithm are shown inAlgorithm 1.

Algorithm 1 BWA

Input: Node i wants to allocate b D-slots in a frame fori→{j}, j ∈ N 1

i . Let Bi→{j}={t0, t1, . . . , tm−1} �= ∅,where m = |Bi→{j}| ≥ b ≥ 0 and tl, 0 ≤ l ≤ m− 1,denotes a D-slot index.

Output: T � D-slot allocation set1: Compute BIi→{j}(tl) for l = 0, . . . ,m− 1 and sort

them in a nondecreasing order so that BIi→{j}(t(0)) ≤BIi→{j}(t(1)) ≤ · · · ≤ BIi→{j}(t(m−1))

2: T ← {t(0), t(1), . . . , t(b−1)}3: Ri ← Ri ∪ T4: Use (1) to update Bi→{j} for each j ∈ N 1

i

5: return T

D. Nonassigned Eligible BWEA

For illustration, consider the network setup in Example 2again. In this example, D-slot 2 has not been assigned to anynode except node 0, and D-slot 6 is assigned only to node 3. Withthe goal of enhancing bandwidth utilization while avoiding tointerfere with any packet transmission from the unsafe range,nodes 1 and 4 should be allowed to transmit in D-slot 2, whereasonly node 4 is allowed to transmit in D-slot 6. Similar to theNAT, the set of these D-slots is called the eligible NAT (ENAT),which is formally defined as follows.

Definition 3: The ENAT of node i, which is denoted Ei,is a set of D-slots in AD

i , in which i can transmit withoutinterfering with packet transmission from its unsafe range, i.e.,Ei = (

⋃j∈N 3

i∪{i} AD

j ).

According to this definition, Ei ∩ ADi = ∅. Clearly, if a node,

e.g., i, always transmits in each D-slot in its Ei, many suchtransmissions are expected to be unsuccessful. A simple wayto reduce the collision possibility while preserving topologytransparency is to let i transmit in a D-slot in Ei accordingto permission (transmission) probability pi. For simplicity, thepermission probability for i is chosen as pi = 1/|N 3

i ∪ {i}|.The rationale behind the choice for pi is based on the fact thatthe permission probability maximizing per-D-slot throughputof a fully connected ad hoc network consisting of k fully loadednodes is 1/k. Since channel access in ENAT is contentionbased, ENAT should not be used to carry the traffic of QoSflows. Instead, ENAT should be used exclusively to carry thetraffic of BE flows because BE flows are delay tolerant andshould receive services as well.


Fig. 4. Analytical results for different schedule-based MAC protocols. (a) Average normalized availability. (b) Overhead. (c) Average eligibility.

E. Analytical Study

The given BWE method and BWA algorithm are built ontop of TTS and are purely based on heuristics. To assesstheir performance, this paper provides an analytical evaluation.For tractability, suppose that each node has an empty R andconsider an arbitrarily chosen node, e.g., i, which is goingto reserve a D-slot in AD

i , e.g., t. This paper evaluates theaverage availability At

i of t for each of the remaining nodes(i.e., average normalized availability) and overhead Ot

i in thenetwork after i reserves t. In addition, this paper evaluates theaverage eligibility Et′

i of a D-slot in Ei, e.g., t′.Let us define the following notations.

• V is the number of nodes in N , each having an NATcontaining D-slot t.

• S is the number of nodes in N 3i , each having an NAT

containing D-slot t.• S ′ is the number of nodes in N 3

i , each having an NATcontaining D-slot t′.

• L is the number of control bits to be disseminated.

It is clear that

Pr{S = s} =P(V − 1, s) · P

(N − V,

∣∣N 3i

∣∣− s)

P (N − 1, |N 3i |)

(3)

Pr{S ′ = s′} =P(V, s′) · P

(N − 1 − V,

∣∣N 3i

∣∣− s′)

P (N − 1, |N 3i |)

(4)

where P(x, y) denotes the number of ordered arrangements of yof x distinct objects, i.e., P(x, y) = x!/(x− y)! for x ≥ y ≥ 0;otherwise, P(x, y) = 0. Because i has priority over other nodesto exclusively use D-slot t after reserving t and NATs arerandomly assigned to nodes, it follows that

Ati =

min(|N 3i |,V −1)∑

s=0

Pr{S = s} · V − (s+ 1)N − 1

. (5)

After reserving D-slot t, i needs to exchange control informa-tion. In a three-hop relay, because the overhead is dominatedby the control information that needs three-hop propagation,the overhead in the network after i reserves t can be approx-imated by

Oti = (1 − Pr{S = 0}) ·

(1 +

∣∣N 2i

∣∣) · L. (6)

According to Definition 3, it follows that

Et′

i = Pr{S ′ = 0}. (7)

Observe that Ati, O

ti , and Et′

i are independent of t and t′ and ofnode i; therefore, the upper and lower indices may be dropped.

Fig. 4 shows the analytical results for different schedule-based MAC protocols. For demonstration, this analytical studyconsiders a simple network with N = 12. To schedule trans-missions in the network using nontrivial TTS, AP(3) inExample 1 is employed so that V = 4 and L = 1. It is clearthat using trivial TTS to schedule transmissions leads to V = 1and L = 0. To compare with the nontrivial TTS constructedfrom AP(3), suppose that V = 12 and L = 9 for TDS becauseTDS assigns each D-slot to all nodes and needs to exchangeslot status information. In addition, to evaluate overhead, sup-pose nodes are uniformly distributed in space and rI = 2rT ,thereby leading to |N 2| = �(2/3)2|N 3|�. Fig. 4(a) shows thatTDS has the largest average normalized availability, whereaastrivial TTS has zero availability. This is because, in TDS, everyD-slot is shared by all nodes, whereas in trivial TTS, everyD-slot is assigned exclusively to a node. As such, trivial TTShas the highest average eligibility, whereas TDS has zero eli-gibility, as shown in Fig. 4(c). Fig. 4(b) shows that TDS hasthe highest overhead, whereas trivial TTS has zero overhead.This is because TDS needs to exchange slot status information,whereas trivial TTS does not disseminate any information. Insummary, nontrivial TTS is a compromise between TDS andtrivial TTS.

IV. QUALITY-OF-SERVICE ROUTING

Depending on the flow requirements, the goal of a QoSrouting protocol is to find a path with sufficient bandwidth and,if needed, to determine/reserve the set of D-slots that the nodesalong the path will use to carry traffic. Let {v0 ⇒ v1 ⇒ · · · ⇒vh−1 ⇒ vh} denote a path where: 1) h ≥ 1 is the path lengthin terms of hop count; 2) v0 is the source; 3) v1, . . . , vh−1 arerelays; and 4) vh is the destination. For a mixture of QoS andBE flows, bandwidth priority is usually given to QoS flows,and all BE flows then share the residual bandwidth. Thus, onone hand, for a QoS flow with a bandwidth requirement ofb > 0 free D-slots in each frame, nodes v0, v1, . . . , vh−1 alongthe path must find and reserve b free D-slots in each frame


to transmit to their respective downstream neighbors withoutcollision (i.e., to allocate b free D-slots in each frame forsuccessful vi → {vi+1}, i ∈ {0, 1, . . . , h− 1}). On the otherhand, since BE flows have lower priority of bandwidth usageand are delay tolerant, no bandwidth reservation is required atthe nodes along the path.

Similar to DSR [36], the proposed QoS routing protocol isbased on source routing and works on disseminating route-searching packets if necessary. This protocol uses a source-routing-based approach because it can directly specify whichroute a flow will use. This ensures that the packets of the flowonly go through the specified route established by admissioncontrol. Admission control is performed alongside route dis-covery and route reservation, both of which are flow based.

A. Route Discovery

The aim of route discovery is to find a route between thesource and the destination that should have enough resourcesfor a flow. Route discovery uses the flooding method to broad-cast a RREQ control packet if required.4 Each RREQ shouldcontain the following information:

〈S_ID,D_ID,R_L,B_BW,R_BW,NH_L〉.

The given information in the RREQ is defined as follows.• S_ID is the source’s ID.• D_ID is the destination’s ID.• R_L is a list of nodes discovered so far that can act as

relays for a flow. It has the format (v1, . . . , vh−1), wherevi, 1 ≤ i ≤ h− 1, is a node ID, such that {S_ID ⇒ v1 ⇒· · · ⇒ vh−1} represents the current partial path.

• B_BW is the bottleneck bandwidth along the currentpartial path (i.e., minimum per-hop bandwidth along thecurrent partial path).

• R_BW is the required bandwidth (i.e., the number ofD-slots in a frame that will be reserved for the flow).Note that the R_BW of a QoS flow is greater than zero,whereas that of a BE flow should be set equal to zero.

• NH_L is a list of next-hop nodes in the format(vh0

, b0)|(vh1, b1)| · · · |(vhm−1

, bm−1), where | denotesconcatenation, and m is the number of vh−1 → {j}’s,j ∈ N 1

vh−1, with bandwidth not less than R_BW. Each

node vhi, 0 ≤ i ≤ m− 1, can serve as a next hop of

node vh−1 to extend the current partial path, such that thenew path (i.e., {S_ID ⇒ v1 ⇒ · · · ⇒ vh−1 ⇒ vhi

}) canstill satisfy the bandwidth requirement. The correspondingparameter bi represents the number of available D-slots ina frame that vh−1 can use to transmit to vhi

successfully.To make better use of RREQs, each node is required to

maintain one route-request cache. The route-request cache ofa node contains an entry for each RREQ forwarded by itself,which stores the unique route-request identifier, the partial path,B_BW, and R_BW.

4Although the flooding method can carry much overhead, it is still employedin route discovery because some QoS paths cannot be discovered withoutthorough route searching, resulting in the rejection of any QoS flows whosebandwidth requirements do not exceed the actual available bandwidth.

Algorithm 2 Selection of next-hop candidates

Input: Node i wants to find an NH_L for a flow with abandwidth requirement of R_BW ≥ 0 D-slots in aframe.

Output: NH_L1: NH_L←∅ � initialize next-hop list as an empty set2: for each j ∈ N 1

i do3: If |Bi→{j}| ≥ R_BW > 0̂ then4: NH_L ← NH_L | (j, |Bi→{j}|)5: else if R_BW = 0 then6: NH_L ← NH_L | (j, |Bi→{j}|+ |Ei|)7: end if8: end for9: return NH_L

Now, suppose that a node (i.e., the source) is requesting toestablish a flow connection with a bandwidth requirement ofR_BW ≥ 0 D-slots in a frame to another node (the destina-tion). First, the source executes Algorithm 2 to obtain NH_L.

• If Algorithm 2 returns an empty NH_L, the source simplyrejects the flow.

• Else

– If the destination is listed in NH_L, a satisfactoryone-hop path has been found, and the source cancommence route reservation.

– Else, the source prepares an RREQ with 1) an emptyR_L, 2) B_BW = ∞, and 3) the NH_L returned byAlgorithm 2. The source then sends the RREQ to allof its neighbors.

When an intermediate node, e.g., i, receives an RREQ,it must determine if i is a node ID listed in the NH_L ofthe received RREQ.

• If so, it appends its node ID to the R_L and updatesB_BW of the received RREQ as follows:

B_BW = min{bk,B_BW} (8)

where bk is the second element of pair (vhk, bk) with

vhk= i in the NH_L.

– If this RREQ was not forwarded before (to discoverloop-free routes), i, just like the source, performsAlgorithm 2 with the input parameter R_BW of thereceived RREQ to find a new NH_L.∗ If the new NH_L is an empty set, then i simply

discards the received RREQ.∗ Else

· If the destination is listed in the new NH_L, apotentially satisfactory path has been formed,and RREP should be initiated.

· Else, i stores the partial path information on thereceived RREQ in its route-request cache, re-places the old NH_L of the received RREQ


with the new one, and sends the RREQ to allof its neighbors.

– Else∗ If the partial path of the received RREQ has a

better B_BW than that stored in its route-requestcache, i updates the partial path information in theroute-request cache.

∗ Else, discard the received RREQ.

• Else, i ignores this request.

B. Route Reply

Route reply is mainly used to notify the source of a poten-tially satisfactory path. To initiate RREP, the intermediate nodethat notices the formation of a potentially satisfactory path usesthe pair in the new NH_L that corresponds to the destinationto update B_BW according to (8). The intermediate node thensends a RREP control packet to the source, which should con-tain the following information: 〈S_ID,R_L,B_BW〉. Theinformation in the RREP is directly copied from the corre-sponding fields in the received RREQ. The RREP then travels inthe reverse direction of R_L through unicasting. Each time anintermediate node receives an RREP, it selects a more favorablepartial path from its route-request cache if available. The returnRREP is then updated and forwarded to the source accordingly.

C. Route Reservation

Each RREP reaching the source carries a potentially satisfac-tory path. After receiving RREPs, the source can use the pathreturned by the first RREP or choose based on other policies(e.g., choose the path with maximum B_BW) to forward datapackets through source routing. Before starting data transmis-sion, the source reserves D-slots according to Algorithm 1 inSection III-C or refreshes any reservations that have been made.Upon receiving a flow’s data packets, intermediate nodes thatact as relays for the flow also perform the same proceduresas the source, because, in the proposed QoS routing protocol,each data packet also plays the role of route reservation orreservation refreshing.

• If these procedures are successful, at the end of the flow,all nodes along the path, except the destination, automat-ically release their reservations for the flow and performbandwidth reallocation (BWR). BWR at individual nodesis achieved by performing Algorithm 3.

• Else,5 the intermediate node that cannot reserve/refreshD-slots repairs the route by performing the same routediscovery as the source.

– If route repair fails, a route failure (RF) control packetis sent back to the source by unicasting. Each nodethat relays the RF releases its reserved D-slots for theflow and drops the packets belonging to the flow. When

5This may result from the following: 1) The D-slots being reserved arealready occupied by another flow; 2) the D-slots desired are now used by othernodes due to priority to exclusively use common D-slots; and 3) transmissionson the path are impossible because of the movement of nodes.

the source receives the RF, it deletes this unavailablepath from candidates and picks the next available oneto forward data packets again.∗ If no more paths are available, the source initiates

another route discovery to discover a new route.

Algorithm 3 BWR

Input: Given a set F �= ∅ of free D-slots, node i wants toreallocate Ri �= ∅.

Output: Ri

1: T ← Ri

2: Ri ← ∅3: while T �= ∅ do4: Find D-slot t ∈ T with the largest BI, and suppose

that t is reserved for i → {j}, j ∈ N 1i

5: if there exists a D-slot t′ ∈ F with the lowestBIi→{j}(t

′) < BIi→{j}(t) then6: Ri ← Ri ∪ {t′}7: F ← F − {t′}8: else9: Ri ← Ri ∪ {t}

10: end if11: T ← T − {t}12: end while13: return Ri

To deal with the dynamics in ad hoc networks, each transmit-ting node should monitor the transmission results in A-slots. Ifa node notices that the packets of its served QoS flows cannotbe sent to the next-hop node due to increased interference levelsat the next-hop node,6 BWR should be performed for the QoSflows. When BWR fails and all data packet transmissions forthe QoS flows cannot be successful, perform the procedures forroute reservation failure.

D. Example

To better illustrate the proposed QoS routing protocol, thefollowing discussion presents an example including route dis-covery, RREP, and route reservation. Consider the initial net-work setup in Example 2. In this illustration, node 6 initiates aQoS flow with a bandwidth requirement of R_BW = 1 D-slotin a frame to node 0. Because no traffic is carried in the network,all nodal statuses before route discovery are set to their respec-tive default values. To initiate route discovery, node 6 preparesan RREQ in the format as 〈6, 0, (∅),∞, 1, (3, 3)|(9, 3)〉. Node6 then sends this RREQ to all of its neighbors [see Fig. 5(a)].Upon receiving the RREQ, nodes 3 and 9 need to processthe RREQ because they are in the NH_L, and they have not

6There are three reasons for increased interference levels at the next-hopnode. First, this paper does not take into consideration the effect of aggregatedsignals from multiple nodes when defining the interference range. Second,using a hop relay to collect information may not work in some topologies.Finally, nodes that can interfere with packet reception at the next-hop nodemove into the interference range of the next-hop node. These three cases arethe result of false assignment of D-slots.


Fig. 5. Example of the proposed QoS routing. (a) Route discovery. (b) RREP. (c) Route reservation performed by node 6. (d) Route reservation performed bynode 3.

forwarded it before. Because the RREQ traveling toward node 4is eventually discarded, this paper focuses on the RREQ travel-ing toward node 0. Node 3 updates R_L and B_BW, executesAlgorithm 2 to find a new NH_L, and then creates an RREPin the format as 〈6, (3), 2〉. Node 3 then unicasts the RREP tonode 6 [see Fig. 5(b)]. After receiving the RREP from node 3,node 6 performs route reservation such that D-slot 0 is reservedfor the flow [see Fig. 5(c)]. Node 6 then starts data transmissionto node 3. Upon receiving the first data packet of the flow fromnode 6, node 3 reserves D-slot 3 for the flow [see Fig. 5(d)].The first data packet is eventually forwarded by node 3 tonode 0.

V. PERFORMANCE EVALUATION

Here, we present a performance evaluation using simulationsfor two objectives: 1) to investigate the behavior of the proposedjoint TTS and QoS routing scheme under different scenarios;and 2) to determine how the proposed joint scheme performscompared with existing joint MAC and routing protocols. Allthe results presented in the following are obtained using theGloMoSim network simulator [45].

A. Simulation Setup

The simulations in the following consider several variants ofthe proposed joint TTS and QoS routing scheme, dependingon the type of TTS in the D-subframes and on the type ofBWE. For convenience, the joint scheme using nontrivial TTSin the D-subframes and broadcast BWE is referred to as QoS 1,whereas the one using nontrivial TTS in the D-subframes andunicast BWE is referred to as QoS 2. As noted in Remark 3, QoS 1and QoS 2 can be optimized. Therefore, denote the optimizedQoS 1 and QoS 2 as O-QoS 1 and O-QoS 2, respectively.To reveal the benefits of QoS support, this paper simulatesthe performance of DSR along with the widely used IEEE802.11 MAC protocol in request-to-send (RTS)/clear-to-send

(CTS)/data/ACK mode [2] (provided by GloMoSim), whichis referred to as DSR/802.11. Since it is well known that theRTS/CTS mechanism may lead to congestion [46], this paperalso simulates the performance of DSR running on top of thecarrier-sense multiple-access (CSMA) protocol (also providedby GloMoSim), which is referred to as DSR/CSMA. In additionto DSR/802.11 and DSR/CSMA, the given variants of the jointscheme are also compared with two other representative QoSrouting works in TDMA environments, including the one pro-posed by Liao et al. [30], which is called the L-T-S scheme, andthe one by Shih et al. [35], which is called the S-C-C-C scheme.

Unless otherwise mentioned, all the simulations below usethe following setup. The radio frequency is 2.4 GHz. Nodesare randomly placed in the simulation area of 1000 × 1000 m2.The transmission range is set as rT = 250 m. This paper mainlyuses the standard radio model provided by GloMoSim, i.e.,RADIO-ACCNOISE, to test the practicability of the presentedjoint scheme. With RADIO-ACCNOISE, signals arising fromthe simulation area other than the desired one are all consideredas interference. However, it is worth noting that interferencedoes not include that generated by other devices that alsooperate in the 2.4-GHz band, such as microwave ovens. For fur-ther comparison, this paper also uses the abstract radio modelprovided by GloMoSim, i.e., RADIO-NONOISE, to evaluateperformance. With RADIO-NONOISE, both transmission andinterference ranges are modeled as the same circle, i.e., signalsare assumed to cause interference only when they are receivedwithin the transmission range. The two-ray ground model isemployed to predict the signal power received by the receiver. Apacket is received without error if: 1) the signal power is at leastthe minimum power for received packets; and 2) the SNR doesnot fall below a threshold. Otherwise, the packet is dropped.The results presented in the following are the statistical aver-ages of 100 simulation runs, and each simulation run is 400 s inlength. Constant bit rate/User Datagram Protocol (CBR/UDP)and variable bit rate (VBR)/UDP connections are used to modelcommunication. For comparison, the packet interarrival time of


TABLE IISIMULATION PARAMETERS

VBR connections is exponentially distributed with mean equalto that of the corresponding CBR connections. Data traffic isgenerated with random source–destination pairs. The first QoSflow arrives at 10 s, and the interarrival time of QoS flowsis exponentially distributed with mean 10 s. To activate BWRmechanism, the first ten QoS flows each have a connection timeof 100 s, whereas the remaining QoS flows each have a connec-tion time that is long enough to remain in the network until theend of a simulation run. When incorporating mobility into thesimulations, a random waypoint model [47] is employed. Eachnode moves at a speed that is uniformly distributed from 0 to15 m/s, and the pause time interval is equal to 50 s. In addition,to take into account both intraflow and interflow contentions,two pairs of nodes are randomly chosen to establish QoS flowconnections at 10 s, and flows continuously attempt to gainadmission if they have not established connections, i.e., no flowis rejected. The results presented in the following include 95%confidence intervals. Table II lists the parameters used in thesimulations.

B. Performance Metrics

This paper compares routing schemes based on the followingperformance metrics.

• Aggregate throughput is the total throughput of all admit-ted flows in the network.

• The QoS violation rate is the summation of the actualaggregate throughput of admitted QoS flows minus thesummation of the data traffic generation rate of their CBRor VBR sources. (The rate of QoS violations of admittedQoS flows indicates the accuracy of admission control.Ideally, admission control should keep the QoS violationrate at zero, and a negative value of QoS violation rateindicates false admission.)

• Jain’s fairness index [48] is ((∑k

i=1 xi)2)/(k ·

∑ki=1 x

2i ),

where k is the number of admitted flows, and xi is thenormalized throughput of admitted flow fi, i.e., the ratioof the actual throughput of fi to the data traffic generationrate of its source.

• The flow rejection rate is the percentage of QoS flows thatare rejected.

• Control overhead is the number of control bytes per secondsent in the network.

• The average end-to-end (E2E) delay is defined as the aver-age time interval from the time a data packet is generatedat the source until the time the data packet reaches itsdestination.

C. Simulation Results and Discussion

This paper discusses the simulation results in two parts. Thefirst part evaluates the performance of the routing schemesusing trivial TTS in the C-subframes, whereas the sec-ond part evaluates performance using nontrivial TTS in theC-subframes.

1) Using Trivial TTS in the C-Subframes: Here, the underly-ing MAC protocol in the C-subframes is trivial TTS. Therefore,there is no penalty on performance caused by collision/loss ofcontrol packets because transmissions scheduled by trivial TTSare guaranteed to be collision-free/successful.

a) Effect of the average refresh interval of BWE informa-tion: Given a fixed number nD = 49 of D-slots in a frame,Table III shows the performance results for different values ofaverage refresh interval T of BWE information in static scenar-ios consisting of N = 56 randomly positioned nodes. BecauseDSR/802.11 and DSR/CSMA do not exchange information forBWE, their performances are independent of the value of T , asshown in Table III. In addition, Table III shows the followingresults.

• Aggregate throughput: The aggregate throughput does notsignificantly change as T varies. The variant schemesperform better than L-T-S and S-C-C-C schemes in termsof aggregate throughput, and more specifically, there are31%–104% increases in aggregate throughput. A some-what surprising result is that DSR/CSMA has higher ag-gregate throughput than DSR/802.11. This is because theRTS/CTS handshake of IEEE 802.11 MAC protocol cancause congestion.

• QoS violation rate: The QoS violation rates increasewith increasing T . This is because information for BWEmay become stale if T is too large. The QoS violationrates of the variant schemes are closer to zero than therates of the other schemes, although the standard radiomodel is adopted in the simulations. On the other hand,


TABLE IIIPERFORMANCE RESULTS VERSUS THE AVERAGE REFRESH INTERVAL T OF BWE INFORMATION IN STATIC SCENARIOS: STANDARD RADIO MODEL

DSR/802.11, DSR/CSMA, L-T-S, and S-C-C-C schemesall have much larger QoS violation rates, indicating thatthey cause more false admissions. False admissions areeither due to a lack of QoS support or to the assumptionthat no interference can arise out of a receiving node’stransmission range. The variant schemes provide at leasta 93% reduction in QoS violation rates.

• Jain’s fairness index: The Jain’s fairness indexes decreaseas T increases, indicating that a smaller value of Tachieves better fairness. This is because the more up-to-date the BWE information, the more accurate the admis-sion control. In addition, the Jain’s fairness indexes of thevariant schemes are very close to 1. This indicates thatthe variant schemes can provide good fairness among theadmitted QoS flows.

• Flow rejection rate: For the variant schemes, the lower theflow rejection rate is, the higher the aggregate throughputwill be. However, this relationship does not apply to L-T-Sand S-C-C-C schemes. This is because both of them maketoo many false admissions. Note that the S-C-C-C schemehas a slightly higher flow rejection rate than the L-T-Sscheme. This is because the S-C-C-C scheme uses a largerC-slot size than the L-T-S scheme.7 This also explains whythe S-C-C-C scheme has lower aggregate throughput thanthe L-T-S scheme.

• Control overhead: The control overhead of the S-C-C-Cscheme remains somewhat fixed, whereas that of the otherQoS routing schemes significantly decreases with a largeT . This is because the S-C-C-C scheme exploits only one-hop information for BWE, whereas the others need eithertwo- or three-hop information. Although DSR/802.11 doesnot exchange information for QoS provisioning, it has thelargest control overhead. The reason behind the largest

7According to [35], the S-C-C-C scheme requires an RREQ to carry moreinformation than the L-T-S scheme, indicating that the S-C-C-C scheme requiresa larger C-slot size than the L-T-S scheme. Therefore, different C-slot sizesshould be used to ensure a fair comparison, which differs from [35].

control overhead of DSR/802.11 is due to the fact that,in DSR/802.11, each time a data packet is going to betransmitted, the RTS/CTS handshake must first get trig-gered to reserve the channel for the entire packet trans-mission duration. Compared with DSR/802.11, the variantschemes provide 78%–89% decreases in control overhead.By disabling the RTS/CTS handshake, DSR/CSMA has thelowest control overhead. As expected, O-QoS 1 and O-QoS 2 provide better aggregate throughput than QoS 1and QoS 2, respectively. This is achieved at the expenseof slightly higher control overhead.

• Average E2E delay: The variant schemes achieve muchlower average E2E delay than DSR/802.11, L-T-S, andS-C-C-C schemes, although they are mainly designed tomanage bandwidth. This is because the variant schemescan quickly forward data packets to the destination onceroute reservation has been adequately made along thechosen path. To have even lower average E2E delay,the concept of E2E scheduling, which takes into accountthe added delay caused by the subsequent forwarding,can be employed [7]. For the purpose of E2E scheduling,knowledge of slot status information, however, is need-ed, which goes beyond the scope of this paper.

As noted in Section I-C, L-T-S and S-C-C-C schemes aredesigned based on the traditional interference model that nointerference can arise out of a receiving node’s transmissionrange. Therefore, this paper also evaluates performance un-der the traditional interference model. Note that, under thetraditional interference model, it is enough for the proposedjoint scheme to use two-hop information rather than three-hopinformation. Table IV shows the performance results under thesame settings as Table III, except the radio model. ComparingTable IV with Table III reveals the following results.

• Aggregate throughput: Clearly, the aggregate throughputof all schemes, except DSR/CSMA, improves. The ag-gregate throughput of DSR/CSMA decreases due to the


TABLE IVPERFORMANCE RESULTS VERSUS THE AVERAGE REFRESH INTERVAL T OF BWE INFORMATION IN STATIC SCENARIOS: ABSTRACT RADIO MODEL

TABLE VPERFORMANCE RESULTS IN STATIC SCENARIOS WITH MIXED TRAFFIC

lack of transmission reliability provided by the RTS/CTShandshake.

• QoS violation rate: Although the QoS violation rates ofL-T-S and S-C-C-C schemes decrease, they are still higherthan those of the variant schemes. This indicates that L-T-Sand S-C-C-C still make more false admissions than thevariant schemes. The variant schemes provide at least a92% reduction in QoS violation rates.

• Jain’s fairness index: The Jain’s fairness indexes of allschemes, except DSR/CSMA, become higher because theirrespective aggregate throughput increases.

• Flow rejection rate: The flow rejection rates of the variantschemes become lower because the variant schemes useonly two-hop information. The flow rejection rates of L-T-Sand S-C-C-C schemes also become lower. This is mainlybecause, under the abstract radio model, the network be-comes less congested, thereby shortening slot reservationtime.

• Control overhead: Under the abstract radio model, al-though the variant schemes cannot always provide aggre-gate throughput gains over L-T-S and S-C-C-C schemes,the control overhead of the variant schemes is greatlyreduced and can be lower than that of L-T-S and S-C-C-Cschemes, indicating the benefits of QoS support withoutknowledge of slot status information. This is because thevariant schemes use only two-hop information. Due to theRTS/CTS handshake, the control overhead of DSR/802.11increases with increasing aggregate throughput. The vari-ant schemes provide at least a 26% reduction in control

overhead compared with DSR/802.11, L-T-S, and S-C-C-Cschemes.

• Average E2E delay: The average E2E delay of all schemesdecreases, particularly for DSR/802.11.

According to the given performance results, the followingdiscussion uses T = 20 as the default average refresh intervalbecause it gives a good tradeoff between efficiency and over-head.

b) Network performance with mixed traffic: Table Vshows the performance results under the same settings asTable III, except that the traffic includes a mixture of QoSand BE flows. For illustration, in each simulation run, five BEflows of CBR sources start to establish connections at 10 s andremain in the network until the end of each simulation run.As shown in Table V, the aggregate throughput of the variantschemes increases, as compared with Table III. Moreover, theQoS violation rates of QoS 1 and QoS 2 remain closer tozero than the rates of the others. This indicates that, in QoS 1and QoS 2, the QoS support for admitted QoS flows is relativelyless affected when nodes can transmit in the D-slots that are notoriginally assigned to them (i.e., ENAT). Larger QoS violationrates of O-QoS 1 and O-QoS 2 appear mainly because thenetwork has become more saturated, and the simulations use thestandard radio model. Comparing Table V with Table III alsoshows that the performance of DSR/802.11 and DSR/CSMA de-grades. This is because, in DSR/802.11 and DSR/CSMA, moreflows admitted to the network create more congestion. Further,although the aggregate throughput of all schemes increases,


Fig. 6. Performance results in mobile scenarios with N = 56 and QoS traffic. (a) Aggregate throughput. (b) QoS violation rate. (c) Control overhead. (d) AverageE2E delay.

Fig. 7. Performance results in mobile scenarios with QoS traffic for fixed nD = 81. (a) Aggregate throughput. (b) QoS violation rate. (c) Control overhead.(d) Average E2E delay.

their Jain’s fairness indexes decrease. This is mainly due to theadmission of BE flows.

c) Effect of the number of D-slots in a frame: Fig. 6shows the performance results versus the number nD ofD-slots in a frame in mobile scenarios consisting of N = 56nodes. First, recall that, with the nontrivial TTS from AP(n)in the D-subframes, the total number of D-slots in a frame isequal to nD = n2, whereas the number of D-slots assigned toeach node in a frame is equal to n. In other words, the totalnumber of D-slots in a frame is proportional to the square of n,whereas the number of D-slots assigned to each node in a framelinearly increases with n. Therefore, the variant schemes assignmore D-slots to each node in each frame with increasing nD,and aggregate throughput should increase. However, the resultshown in Fig. 6(a) is somewhat different than expected. This ismainly because, for the variant schemes, as nD increases, thenumber of D-slots required by a QoS flow in a frame increasesfaster than the number of D-slots assigned to each node in aframe, resulting in decreased aggregate throughput. Because theperformance of DSR/802.11 is independent of the input designparameter nD, it exhibits a flat line. Fig. 6(b) shows that theQoS violation rates of the variant schemes do not significantlydepend on the value of nD. Fig. 6(b) also shows that, in mobilescenarios, the variant schemes have QoS violation rates closerto zero than others. Fig. 6(c) shows that the control overheadof the QoS routing schemes, excluding S-C-C-C, is somewhatindependent of nD. This is mainly because N is fixed, and con-trol packets are periodically broadcast. The control overheadof the S-C-C-C scheme increases with increasing nD becauseits slot adjustment mechanism uses slot status information toadjust slot allocation. Fig. 6(c) also shows that, in mobile sce-narios, L-T-S and S-C-C-C schemes have much larger controloverhead than the variant schemes. This is because their RREQscontain slot status information, and flows continuously attempt

to gain admission. Fig. 6(d) shows that, as nD increases, theaverage E2E delay of the variant schemes increases slightly.This is because, as nD increases, the D-subframe lengthincreases.

d) Effect of the number of nodes: Fig. 7 shows the per-formance results versus the number N of nodes in mobilescenarios for fixed number nD of D-slots in a frame. Thisfigure chooses nD = 81 for illustration, i.e., AP(9) is used togenerate a set of 90 ADs, thus being capable of scheduling90 node transmissions in the D-subframes. For the variantschemes, although a network with more nodes can providehigher route diversity, Fig. 7(a) shows that, as N increases,the aggregate throughput does not improve accordingly. Thisis because the C-subframe length increases with increasing N ,yielding the increase in the number of D-slots required by eachQoS flow in a frame. Fig. 7(b) shows that the QoS violationrates of the variant schemes do not significantly vary with N .Fig. 7(c) shows that the control overhead of the variant schemesincreases with increasing N , whereas that of DSR/802.11 andDSR/CSMA is somewhat independent of N . This is becausethe variant schemes periodically broadcast control packets,whereas DSR/802.11 and DSR/CSMA do not. Although thecontrol overhead of the variant schemes cannot always be lowerthan that of DSR/802.11 and DSR/CSMA, it is still much lessthan that of L-T-S and S-C-C-C schemes. Fig. 7(d) shows that,due to the increased D-subframe length, the average E2E delayof the variant schemes increases slightly with increasing N .

e) Effect of mobility: Fig. 8 shows the performance re-sults versus the pause time interval for fixed nD = 49 in mobilescenarios consisting of N = 56 nodes. Fig. 8 reveals that, asthe pause time interval increases, the performance roughlyimproves.

Fig. 9 shows the performance results under the same settingsas in Fig. 8, except that the maximum node speed decreases


Fig. 8. Performance results in mobile scenarios with N = 56, nD = 49, and QoS traffic. (a) Aggregate throughput. (b) QoS violation rate. (c) Control overhead.(d) Average E2E delay.

Fig. 9. Performance results in mobile scenarios where the maximum node speed decreases to 5 m/s. (a) Aggregate throughput. (b) QoS violation rate. (c) Controloverhead. (d) Average E2E delay.

TABLE VIPERFORMANCE RESULTS WITH DIFFERENT TRAFFIC TYPES IN MOBILE SCENARIOS

to 5 m/s. Comparing Fig. 9 with Fig. 8 reveals that betterperformance can be obtained at a lower maximum node speed.

f) Effect of traffic type: Table VI shows the performanceresults with different traffic types, including CBR or VBRsources. In addition, it is assumed in Table VI that the con-nections are either to or from the default network gateway, i.e.,node 0. Table VI shows that, for all schemes, VBR connectionscan receive better services than the corresponding CBR connec-tions, particularly for DSR/802.11 and DSR/CSMA.

2) Using Nontrivial TTS in the C-Subframes: Here, thefeasibility of using nontrivial TTS as the underlying MACprotocol in the C-subframes is investigated. For comparisonwith Table III, the settings are the same as those in Table III,except the type of TTS in the C-subframes. To schedule 56 nodetransmissions in the C-subframes, AP(7) is chosen to generatea set of 56 ACs. Therefore, the number of C-slots in a frameis nC = 49 < N = 56. For ease of comparison, the controloverhead of a routing scheme using nontrivial TTS in theC-subframes is normalized by dividing it by that of the samescheme using trivial TTS in the C-subframes.

Table VII shows the performance results. In random-topology scenarios, when the MAC in the C-subframes isthe nontrivial TTS constructed from AP(7), the guarantee ofcollision-free C-slots for every neighbor in each C-subframemay not hold at individual nodes. Therefore, control packetscarrying information relevant to QoS provisioning may be lost.To investigate the effect of loss of control packets, this paperconsiders two different numbers of packet repetitions in theC-subframes: no repetition and three repetitions. ComparingTable VII with Table III reveals one main effect of loss ofcontrol packets on performance. The aggregate throughput ofthe variant schemes decreases, whereas the flow rejection ratesincrease. This is primarily due to the loss of RREQs/RREPs,which causes some QoS flows whose bandwidth consumptionis not beyond the capacity of the network to get rejected. Mean-while, because of loss of control packets, the QoS violationrates of the variant schemes increase compared with Table IIIbut still remain less than those of other schemes. Comparing“three repetitions” with “no repetition” shows that using packetrepetition improves the aggregate throughput, QoS violation


TABLE VIIPERFORMANCE RESULTS USING NONTRIVIAL TTS IN THE C-SUBFRAMES IN STATIC SCENARIOS

rates, and Jain’s fairness indexes. This is because packet rep-etition can ensure transmission reliability as MAC-layer ACKmechanism is not applied in the C-subframes. However, packetrepetition also increases control overhead.

D. Further Discussion

Since the given simulations assume that all transmissions areover the 2.4-GHz band and the given comparisons have beenmade with respect to DSR/802.11, it is worth providing furtherdiscussion on practical issues with IEEE 802.11-based ad hocnetworks.

• In real IEEE 802.11-based ad hoc networks, devices thatoperate in the 2.4-GHz band, such as microwave ovens,can generate interference, and clearly, such interferencecannot be controlled. These sources of interference are notconsidered in the system model described in Section II-Aand in the analytic study provided in Section III-E. More-over, they are neither considered in the GloMoSim net-work simulator. For the proposed joint scheme, thesesources of interference will undoubtedly decrease aggre-gate throughput and increase QoS violation rates andcontrol overhead.

• According to real measurements of IEEE 802.11-based adhoc networks under ideal conditions (high SNR, little in-terference from external sources, high transmission power,short links, long data transmission, and highly directionalantennas), link efficiency is generally low, and packet-dropping rates are relatively high because of protocolinefficiency [49]. The proposed joint scheme attempts touse TDMA with short C-slots and D-slots in IEEE 802.11-based ad hoc networks. As such, the proposed joint schemewill also have similar inefficiencies due to the guard timenecessary for the use of TDMA.

VI. CONCLUSION

This paper has presented a joint TTS and QoS routingscheme for ad hoc networks. BWE and BWA are the essentialcomponents of the proposed joint scheme for a mixture ofQoS and BE flows. By giving routing access to bandwidthinformation, the routing and congestion control are coupled to

achieve effective admission control and to provide QoS support.The idealized simulations in this paper, which are based on thestandard and abstract radio models, have demonstrated that, bybeing built on top of TTS, the joint scheme not only provideseffective QoS support without knowledge of slot status infor-mation for QoS flows but also serves BE flows well. However,real improvements achieved in practical IEEE 802.11-based adhoc networks have yet to be established.

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Yi-Sheng Su received the B.S. and M.S. de-grees from National Tsing Hua University, Hsinchu,Taiwan, and the Ph.D. degree from National ChengKung University, Tainan, Taiwan, all in electricalengineering.

From 2011 to 2012, he was a Visiting Scholar withCarnegie Mellon University, Pittsburgh, PA, USA.He is currently with Chang Jung Christian Univer-sity, Tainan. His research interests include wirelesscommunications and ad hoc networking.

Szu-Lin Su received the B.S. and M.S. degrees fromNational Taiwan University, Taipei, Taiwan, in 1977and 1979, respectively, and the Ph.D. degree from theUniversity of Southern California, Los Angeles, CA,USA, in 1985, all in electrical engineering.

From 1979 to 1989, he was a Research Mem-ber with the Chung Shan Institute of Science andTechnology, Taoyuan, Taiwan. Since 1989, he hasbeen with National Cheng Kung University, Tainan,Taiwan, where he is currently a Professor with theDepartment of Electrical Engineering and the Insti-

tute of Computer and Communication Engineering. His research interests in-clude wireless communication technologies, mobile communication networks,cross-layer design, and cooperative communications.

Jung-Shian Li received the B.S. and M.S. degrees inelectrical engineering from National Taiwan Univer-sity, Taipei, Taiwan, and the Ph.D. degree from theTechnical University of Berlin, Berlin, Germany.

He is currently a Full Professor with the De-partment of Electrical Engineering, National ChengKung University, Tainan, Taiwan, where he teachescommunication courses. His research interests in-clude wired and wireless network protocol design,network security, and network management. He iscurrently involved in funded research projects deal-

ing with information security, radio-frequency identification, and cloud com-puting systems.

Dr. Li serves as a member of the Editorial Board of the International Journalof Communication Systems and the Journal of Security and CommunicationNetworks , and he has been a Guest Editor of the IEEE Wireless Communica-tions Magazine.

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Joint Topology-Transparent Scheduling and QoS Routing in Ad Hoc Networks