Distributed admission control protocol for end-to-end QoS assurance in ad hoc wireless networks
© Youn et al; licensee Springer. 2011
Received: 1 February 2011
Accepted: 10 November 2011
Published: 10 November 2011
To reserve end-to-end bandwidth in quality of service (QoS) supported wireless ad hoc networks, local bandwidth requirement should be carefully determined by considering the number of contending nodes in an interference range. In this article, we propose a novel admission control protocol, called DACP (distributed admission control protocol), which is implemented over a reactive ad hoc routing protocol with minimal overhead. DACP computes the required bandwidth for end-to-end band-width provision at each node and estimates the available bandwidth at the medium access control layer. After that, DACP makes a decision for admitting a flow in a per-hop basis. Extensive simulations are carried out via the OPNET simulator. The simulation results demonstrate that DACP not only provides guaranteed end-to-end resource but also reduces the control overhead to provide QoS support, compared with the existing admission control schemes.
Over the last few years, research on quality of service (QoS) provisioning in wireless ad hoc networks has increased significantly. These networks can be adopted in commercial environments in which there are multimedia systems that enable users to access multimedia data, such as IP television and voice over IP (VoIP). Moreover, these multimedia systems need better service quality than best-effort service. To this end, an admission control scheme including resource reservation in wireless ad hoc networks should be devised to support the end-to-end bandwidth demanded by wireless multimedia applications.
However, they are not appropriate solutions for providing users with QoS because of system complexity and implementation overhead. Instead, simple admission control with low complexity can be an alternative approach.
In this article, we propose a distributed admission control protocol (DACP). DACP is implemented over an ad hoc on-demand distance vector (AODV) routing protocol and uses a route request (RREQ) packet during the route discovery procedure for admission control. DACP utilizes Hello messages to calculate the number of contending nodes within the sender's interference range, which can significantly reduce the network overhead. In addition, DACP achieves more accurate estimation of available local bandwidth by exploiting the interaction between IEEE 802.11 MAC and AODV routing protocol. Also, in point of the complexity of the proposed algorithm for admission control, DACP only use RREQ message of AODV protocol. This means DACP can reduce the complexity for establishing QoS session and be sample admission control scheme with low complexity. To demonstrate the effectiveness of DACP, we conduct extensive simulations via the OPNET simulator . Simulation results indicate that DACP can support accurate resource reservation for QoS provision and alleviate network saturation and achieve higher throughput and lower end-to-end delay with low signaling overhead and low complexity.
The remainder of the article is organized as follows. Section 2 summarizes the previous works on QoS in wireless ad hoc networks. In Section 3, the bandwidth requirement for the end-to-end bandwidth reservation is discussed, and an accurate estimation method for the local available bandwidth is proposed in Section 4. Section 5 describes the DACP, and Section 6 demonstrates the simulation results. Finally, Section 7 concludes this article.
2 Previous QoS works in wireless ad hoc networks
Several QoS provisioning schemes for resource reservation have been proposed in [1, 3, 4]. These mechanisms, for resource discovery and admission decisions, send probe packets on preselected routes. Each node predicts the achievable QoS based on available resources and admits the QoS session if the QoS requirement of end-to-end path delivered by the probe packet is sufficient. Then, these mechanisms using the probe packets have signaling overhead to provide QoS assurances. In , another alternative is to probe routes end-to-end and use the interval between packet arrivals to calculate the route capacity. Differentiated scheduling and medium access algorithms have been proposed in  to provide a prioritized service model to guarantee real-time traffic over best-effort traffic. These solutions still face the issue of reducing the overhead for QoS guarantee. In , the softMAC architecture is addressed. The softMAC scheme resides at layer 2.5 between the MAC layer and the network layer. It takes the autorate feature of 802.11 into account. Then, to establish link capacities, the experienced delay between transmitting back-to-back probe packets of various sizes is used. This scheme also has the signaling overhead of probe packet to provide QoS assurances. In , the authors highlight the necessity of local data control and admission control to guarantee QoS for real-time traffic under high traffic load conditions. Further, in this model, each node maps the measured traffic load condition into backoff parameters locally and dynamically. However, this model does not consider bandwidth reduction in multi-hop ad hoc environments. On the other hand, admission control schemes for wireless multi-hop environments have been also proposed in [10–15]. Contention-aware admission control protocol (CACP)  considers the contention among flows within a node's interference range and uses on-demand resource discovery-based scheme to provide QoS assurances. In CACP, three methods are proposed. First, an admission request packet is flooded to a distance of two hops to test the node's carrier sensing (CS) neighbors' residual capacities. Second, CACP uses a higher power to transmit an admission request packet to ensure it reaches all the nodes within the CS range with a single transmission. The third method employs passive resource discovery-based approach. These methods' overhead depends on the node density. In addition, while the admission request packet is transmitted, a high level of interference is produced at neighbor nodes. Furthermore, CACP is based on inaccurate estimation required bandwidth at each node along the established end-to-end route for making the admission decision. In , the perceptive admission control (PAC) protocol is introduced. This protocol operates on a similar to CACP. It uses passive monitoring to estimate the available bandwidth at the node and its neighbor node. However, PAC's monitoring threshold is set such that the average CS range is less than that used by CACP. PAC also has the problem about a high level of interference like CACP. Admission control and bandwidth reservation (ACBR)  is compatible with the existing AODV routing protocol. A shortcoming of ACBR is that it only tests the available capacity of the neighbor nodes of a route, and only considers intra-route contention in 1-hop node. In addition, it also uses inaccurate calculation of the required bandwidth at each node along the established end-to-end route, because it does not take the contenting nodes in the interference range into account. In other words, this scheme only considers the contention of nodes within a node's transmission range.
3 Revisit: bandwidth requirement for end-to-end bandwidth reservation
3.1 The network model
We consider wireless ad hoc networks consisting of mobile devices, such as laptop and Smartphone. In the networks, each node communicates over a shared medium. Each node has a fixed radio range and exchanges messages only with nodes with this range. For medium access, the distributed coordination function (DCF) in IEEE 802.11  is assumed, as the access method used in ad hoc mode. IEEE 802.11 MAC uses a fourway handshake scheme (RTS/CTS/Data/ACK exchange).
3.2 End-to-end QoS assurance
In the networks with the system for QoS support, applications of each node with end-to-end flows require specific end-to-end bandwidth from the network. To enable end-to-end bandwidth reservation, the required bandwidth of a flow at each node should be carefully determined. Specifically, the amount of the required bandwidth is affected the location of the node, i.e., source, intermediate, and destination nodes require different local bandwidth for end-to-end bandwidth reservation. Therefore, the required local bandwidth should be determined in a per-hop basis.
Existing schemes in [10, 11] estimate local bandwidth requirement based on the number of contenting nodes on the route in the interference or transmission ranges. However, they do not consider the relation between the end-to-end throughput and the hop number over the end-to-end route. Therefore, we revisit the required local bandwidth for end-to-end bandwidth reservation in this section.
Basically, in IEEE 802.11 ad hoc networks, a node cannot transmit and receive data simultaneously. In other words, to guarantee a packet transmission on a single-hop path, the same amount of bandwidth is needed at the sender and the receiver. If the same packet was transmitted over a multi-hop path in terms of an intra-flow, the bandwidth requirement is cumulative. And the accumulative bandwidth requirement is different according to whether or not the receiver transmits the same packet toward a destination node and the number of contention links in the interference range. The following subsection describes the analysis in detail.
3.3 Local bandwidth requirement on end-to-end route
In Figure 1, when N3 wants to send the packet to N4 through link 3, all the nodes in the networks should be deferred because they are included in N3's and N4's interference range. The existing work [10, 11, 17] analyzes this case in terms of the contending nodes that are all the nodes within CS range of the transmission path. Therefore, it is shown that 3R[10, 11] or 5R is required at the intermediate node as the local bandwidth. However, both values are inaccurate. This is because links 1 and 5 are used simultaneously to transmit a packet of intra-flow at the point of link 3. Therefore, if intra-flow wants to be transmitted at N3 using link 3, links 1 and 5 affect the transmission of intra-flow of N3 simultaneously. In other works, if end-to-end hop number is more than 4, four contending links are affected at an intermediate node, such as N2, N3, and N4. As a result, 4R is only required at N2, N3, and N4. This result is based on the analysis in .
4 Available local bandwidth estimation
In our work, we estimate the available local bandwidth at a node in terms of MAC throughput. In IEEE 802.11 networks, a packet generated by an application layer is handled through a reliable transmission service including a fourway handshake scheme in the MAC layer. Thus, we have to continuously observe the throughput achieved by the MAC layer. To get accurate available MAC throughput, two parameters, such as the available channel time and the average MAC forwarding delay, are used.
4.1 Available channel time (Tava_chann_time)
To estimate the available bandwidth, intuitively, each node has to determine how much free channel is available by listening to the channel every measurement time. Free channel time is available channel time of a node. It chooses the measurement time (Tmeas_time) that is the same as the default broadcast interval of a Hello message in the AODV routing protocol.
Busy state: the value of the network allocation vector (NAV) is set, receiver state is any other state except for idle, and the transmitter state is not idle.
Free state: the value of the NAV is less than the current time, receiver state is idle, and transmitter state is idle.
4.2 Available local bandwidth
where PL is any MAC layer payload length transmitted in the current measurement time.
5 A new admission control based on AODV protocol
Basically, in our admission control, each node receiving the RREQ packet first determines which of the destination nodes of the RREQ packet is in its interference range, and then with the above result, it predicts its hop number on end-to-end route through hop number in the RREQ packet. Thus, our protocol performs admission control during the route discovery procedure. To predict an end-to-end hop number, our protocol needs the information of the first neighbor nodes and second neighbor nodes. To this end, we can utilize the Hello message specified in the AODV protocol. This overall procedure reduces the number of a RREQ packet during the route discovery for a QoS session. Moreover, since the number of routing packet can be reduced, the overall network performance can be improved.
In this section, AODV protocol-based distributed admission control (DAC) including resource reservation is elaborated. The reason to choose AODV as the platform for our QoS model is that AODV uses "Hello" messages for keeping track of its continued connectivity to its next active nodes. In our model, through the "Hello" message, each node makes up the information of its first neighbor nodes and its second neighbor nodes.
5.1 The connectivity tables
5.2 A DAC and resource reservation algorithm
This subsection details admission control and resource reservation schemes. As mentioned previously, our QoS solution utilizes a cross-layer design. With the available local bandwidth and the connectivity table defined in the subsection above, the whole procedure is progressed when disseminating a RREQ packet and a RREP packet during the route discovery.
To initiate the route discovery, the application at a source node indicates, in the request message, the bandwidth requirement, Breq, that must be guaranteed, and then a source node disseminates a RREQ packet. At this time, the source node first checks whether there is a destination IP in the first neighbor table and the second neighbor table. Through this procedure, it can determine whether the end-to-end hop number is 1-hop, 2-hop, or more than 3-hop. The local bandwidth requirement of the source node is determined with the end-to-end hop number obtained by the above procedure. At an intermediate node and a destination node, the local bandwidth requirement is also determined with this procedure and the hop number achieved by the received RREQ packet.
In Figure 9, when a destination node forwards a RREP packet to a source node, the algorithm for resource reservation is shown, where EteHopCount is the end-to-end hop number achieved by a RREQ packet, and BackHopCount is the hop number from a destination node. First of all, a destination node checks EteHopCount. If EteHopCount is 1, Breq is reserved for the QoS session. If EteHopCount is more than 2, 2Breq is reserved. Then, one forwards a RREP packet to the source node. Figure 10 shows the pseudo code for resource reservation at an intermediate node. In this case, one first checks which one is the last intermediate node through EteHopCount and BackHopCount, and then reserves bandwidth. If the node is the last intermediate (BackHopCount == 1) and EteHopCount is 2, it reserves 2Breq. If EteHopCount is more than 2, 3Breq is reserved. But, if this node is not the last intermediate, 3Breq or 4Breq is reserved according to EteHopCount. Figure 11 shows the pseudo code in a source node. According to EteHopCount, Breq, 2Breq, or 3Breq is reserved. After the source node reserves the local bandwidth, the QoS session of the end-to-end route is finally accepted. In spite of the admission control, an end-to-end route may still be broken from time to time due to various reasons, such as node mobility and topology changes when nodes die. In this case, we adopt the explicit ICMP QoS-LOST used by AODV-QoS  to inform the source nodes of all the unmaintainable sessions. Thus, the corresponding source nodes have to reinitiate session requests for new ones. The old broken routes will expire after the lifetime.
6 Simulation studies
To test the performance of our QoS solution, DACP, with comprehensive simulations, is evaluated and compared with the non-service model, which is the standard AODV routing protocol without admission control (the non-admission control model)  and existing service models with admission control, such as power scheme in CACP  and ACBR , in three scenarios; simple topology on chain environment, grid topology, and random topology on static environment. In the simulations, we use the IEEE 802.11 MAC protocol with a channel data rate of 2 Mb/s. Nodes have a 250 m radio transmission range and 550 m CS range. Simulations are conducted using the OPNET v11.5 simulator .
6.1 The performance on simple topology on chain environment
Throughput achieved by a 6-hop chain topology with different sending rate
The transmission rate of flow (Mbps)
Average end-to-end throughput
Delivery ratio (%)
Based on the simulation results, when considering the retransmission and time of AIFS and DIFS, the maximum throughput is achieved at 0.4 Mbps. When compared with the maximum throughput of 0.44 Mbps achieved using the weight factor of 1.128, 0.04 Mbps is different. In other words, 0.04 Mbps is used in the retransmission period, AIFS and DIFS. Therefore, we choose the real weight factor as 1.1 × 1.128 = 1.24. In the case of the transmission rate, such as 0.5 and 0.45 Mbps, since the collision occurs easily in the saturation network, these cases achieve lower throughput than 0.4 Mbps. In order to consider non-collision of intra-flow in the network and the transmission for routing packets in the simulations shown in Figure 12, we select 0.35 Mbps as the transmission rate of flow 1. In this case, the transmission rate in the MAC layer is 0.35 × 1.24 = 0.4342 Mbps. Therefore, the bandwidth used by flow 1 at node 3 is 4 × 0.4342 Mbps = 1.74 Mbps. Thus, in theory, 0.26 Mbps as available channel bandwidth is allowed.
The performance of flow 1 according to the sending rate of flow 2
The transmission rate of flow 2 (Mbps)
Average end-to-end throughput of flow 1
Delivery ratio of flow 1 (%)
Consequently, through these results, the inaccurate calculation of bandwidth requirement at an intermediate node in CACP and ACBR is investigated. In addition, we prove that calculation of the bandwidth requirement using the number of contending link in our work is correct. In our work, the weigh factor is considered in our scheme for estimating available local bandwidth that takes the MAC's overhead and retransmission into account is proposed.
6.2 The performance in grid topology-based ad hoc environments
The information of each flow in the grid topology shown in Figure 13
Packet inter-arrival time (Pkts/s)
Transmission rate (Mbps)
Stating time (s)
The end-to-end service stability in the grid topology shown in Figure 13
Figure 14 shows the throughput of each flow in AODV-based networks without admission control. As expected, all the flows become active, and the channel becomes congested. Thus, the service stability, S, of all the flows looks like significant instability. Figure 15 shows the throughput of each flow achieved by the admission control in CACP. In the result, flow 2 is not admitted during the simulation time. The end-to-end service of flow 3 is also unstable. For the simulation time from 200 s to ending time, QoS session for flow 3 is disconnected. This is because the available local bandwidth will decrease at the intermediate nodes, since the network becomes overloaded. Figure 16 shows the throughput of each flow obtained by ACBR. In the result, at the beginning simulation time, all the flows are admitted by admission control. However, traffic of flow 2 is dropped from 200 s. Also, after transmitting flows 2 and 3, the service quality of flows 1, 4, and 5 becomes unstable. This is because flows 2 and 3 are admitted even if the local bandwidth is not sufficient. This indicates the inaccurate calculation of the required local bandwidth at each node.
The overhead of a signaling packet in the grid topology shown in Figure 13
AODV.routing traffic sent (pkts/s)
AODV.routing traffic received (pkts/s)
6.3 The performance in static multi-hop ad hoc environments
The overhead of signaling packets in the static random topology
AODV.routing traffic sent (pkts/s)
AODV.routing traffic received (pkts/s)
In this article, we propose a novel admission control scheme, called the DACP, which is designed for guaranteeing end-to-end bandwidth in wireless ad hoc networks. We first exploit the problem of the bandwidth requirement for end-to-end bandwidth assurance. DACP makes admission control decisions only using RREQ messages during route discovery, and thus it can reduce routing traffic overhead significantly. In addition, an accurate estimation scheme for available resources of each node in the MAC is introduced. Simulation results demonstrate that DACP can significantly improve end-to-end QoS in terms of end-to-end throughput and service quality.
This research was supported by the Korea Creative Content Agency (KOCCA) in the Culture Technology (CT) Joint Research Center Project 2010 (No. 1-10-7602-001-10002-00-001).
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