that the geographical distribution of nodes influences a lot the behavior of the network. The usage of directional antennas in this kind of networks improves the ...
Influence of directional antennas in STDMA ad hoc network schedule creation Imanol Martinez∗ , Jon Altuna∗ ∗ Signal
Theory and Communications Department Mondragon Unibertsitatea, Mondragon, Guipuzcoa, Spain Email: {imartinez, jaltuna}@eps.mondragon.edu
Abstract— The Spatial reuse Time Division Multiple Access (STDMA) ad hoc networks take advantage of the electromagnetic spectrum reuse to increase the global capacity of the network. The schedule creation takes into account different variables. One of them, the Signal-Interference plus Noise Ratio (SINR) generates that the geographical distribution of nodes influences a lot the behavior of the network. The usage of directional antennas in this kind of networks improves the spatial reuse and consequently the capacity of the network. In this article the schedule in a STDMA network is analyzed using antennas with different lobe angles. The created frame lengths for different connectivities are analyzed as well as the number of links/nodes per slot or the spatial reuse that it is achieved comparing to the one in a Time Division Multiple Access (TDMA) network. Results are presented for link-based and node-based assignment methods.
I. I NTRODUCTION Wireless ad hoc multihop networks (MANET) are created by nodes or terminals that beside working as source or data destination, they work as data repeaters. This characteristic offers the possibility to transmit packets using one or more of these repeaters as an intermediate platform between a sourcedestination pair. One of the main problems when using a MANET, due to its distributed nature, is the no efficient resource management that is done by the network participants. In a MANET the main resource to share, and therefore the bottleneck of the system that reduces the performance of the network, is the electromagnetic spectrum. The medium access control layer (MAC) is the one in charge of managing the electromagnetic spectrum inside the OSI layer stack. Another important variable for a MANET, and specially if ad hoc networks are to be commercially viable, is the quality of service (QoS) offered by the system. QoS can be defined, in a general way, as the effect of service performance which determines the satisfaction degree of a service user. If the MAC layer can not manage free resources in an efficient way, it is impossible to offer QoS connections inside the network. Therefore there is a clear relation between the QoS and the MAC layer [1]. The medium access mechanisms can be divided basically in two types, random access or deterministic access mechanisms [2]. There are some works where it is said that using random access protocols to obtain QoS levels in a connection is impossible due to the uncertainty of the connection delay [3]. The Spatial reuse Time Division Multiple Access (STDMA) [4] protocol is a deterministic mechanism, it takes advantage of
the spatial separation between terminals to reuse the transmission slots, increasing the network performance. However, its behavior varies considerably when using directional antennas [5], increasing the transmission capacity of the global system. This work is focused in the description and quantification of the effects that are produced when using directional antennas in a STDMA network. This article is divided in the following way. Section II presents the STDMA protocol and the node-based and link-based assignment variants. Section III shows the results obtained for different directional antennas when analyzing the frame length, the spatial reuse factor and the network capacity. The effects that are produced by topological aspects are analyzed in section IV. Finally in section V concluding remarks about these work are presented. II. S PATIAL T IME D IVISION M ULTIPLE ACCESS Spatial reuse TDMA allows more than one node to transmit simultaneously during a slot when the spatial distance between nodes is high enough so as to generate small interference. In particular the objective is to create a conflict free schedule, a schedule where the transmission rights have been assigned to guarantee that all transmitted packets are correctly detected by the destination nodes and not influenced by the interferences of other node transmissions. This section presents the two most common assignment strategies and their advantages: nodebased and link-based assignment mechanisms. However it is important to notice that in general, for most assignment algorithms both strategies can be indiscriminately supported. A. Transmission constraints In order to obtain a conflict free STDMA schedule two fundamental constraints need to be simultaneously guaranteed. On one side, a node can not receive more than one packet in a time slot and it can not transmit and receive simultaneously. On the other side, a set of nodes or links which satisfy the first constraint can transmit simultaneously only if the signal to interference and noise ratio (SINR) is greater than a selected communication threshold. The selection of this value depends on various factor, modulation, sensibility, transmission rate and Bit-Error rate (BER). B. Schedule creation The schedule creation in STDMA can be done using linkbased or node-based strategies. A node assignment schedule
allows some nodes to transmit simultaneously in a time slot. When a node is assigned to a certain time slot it can transmit to any of its neighboring nodes. If the schedule has to be conflict free, it is necessary to ensure that no conflict exists between any neighboring nodes. This condition can be interpreted saying that two neighbors can not transmit simultaneously. The principal advantage of node-based strategy is that when a node is allowed to transmit it can activate one of all of its outgoing links. Normally the one that improves the network performance. A link-based schedule allows links to transmit simultaneously during a slot. This strategy assigns transmission rights in a slot to specific links, so that node can transmit only to one specific neighboring. When transmitting the node can only activate the selected link although this selection does not increase the network performance. The advantage is that the spatial reuse factor is higher than when selecting the node-based assignment method. It can be seen that the use of directional antennas is going to reduce the interference level between transmitting nodes or links increasing this way the network performance.
Fig. 1.
Number of hops mean value for the simulated network.
In this section some results for 450 ad hoc networks, 50 for each connectivity, with 26 nodes are presented. The schedule is created with the algorithm proposed by [6].
the smooth effect of the curve can be explained. The difference depending on the type of antenna is minimal, anyway, it can be appreciated that in the central part of the curve, for low antenna angles, the length of the frame is shorter because the interference between nodes is lower. For high connectivity this effect disappears because the connectivity effect prevails over the interference one.
A. Number of hops evolution
C. Nodes per slot evolution with node-based strategy
As it can be seen in figure 1 the number of hops decreases from 5.5 to 1. The greater drop happens when the connectivity changes from 0.1 to 0.2 where the number of hops drop from 5.5 to 3. The rest of the drop is more moderated in the plot from 0.2 to 0.9. The reduction in the number of hops is a consequence of the network connectivity. Greater connectivity means more connections, therefore more and shorter routes are created between nodes. The limit will be at connectivity 0.9 where almost all nodes are connected to each other and the number of hops is reduced to approximately 1.
The evolution of node number per slot, as it can be seen in figure 2(b), is descending. The dropping happens between connectivities of 0.1 and 0.5. From 0.6 to 0.9, the number of nodes for each slot is 1. If networks with 0.9 connectivity are analyzed, it can be observed that if one of the nodes is activated the rest are blocked because all of them are inside the range of 2 hops from the transmission source. This is the reason of the plots tendency to 1. This tendency is regular until a 0.6 connectivity. When connectivity is smaller than 0.6 the range of 2 nodes around the source does not include so many nodes and therefore the spatial reuse increases. The deviation depending on the antenna is almost inexistent. As a conclusion it can be said that in node-based assignment strategies the antenna type has no influence because of the 2 hops blocking effect that is predominant over the effect of using directional antennas.
III. D IRECTIONAL ANTENNAS IN STDMA
B. Frame length evolution with node-based strategy The evolution of the frame length in figure 2(a) is descending. Its reduction is more obvious for low and high connectivities. The decrease in the central part is more moderated with a variation of approximately 100 units between 0.2 and 0.6. The descending tendency is produced by the connectivity increase, the number of slots that are assigned to each node is smaller because there are more direct connections and the number of required slots for each node is distributed in a more uniform way. Because of the smaller number of slots to be assigned the created frame where they have to fit is smaller. The central part does not have such a visible dropping for two reasons, a rising effect is produced due to a higher interference level when the connectivity is increased, a dropping effect is produced due to the more uniform distribution of the assigned slots. Combining these two effects, one of rising and the other of dropping in more or less level depending on the connectivity,
D. Frame length evolution with link-based strategy As it can be seen in figure 3(a), the evolution of the frame length is descending, but in the last stage of the curve a smoothing effect can be appreciated. The dropping effect can be justified because of the uniform distribution of necessary slots, and because of the lower necessity of slots due to connectivity. However it is supposed that increasing the connectivity, the interference level would increase as well and the reduction would happen in a more moderate way. This effect can be seen in the difference depending on the antenna type at high connectivity. The bigger the aperture angle of the
(a) Frame Length. Fig. 2.
(b) Nodes per Slot.
Frame length and nodes per slot for a node-based assignment strategy with different antenna lobes.
(a) Frame Length. Fig. 3.
(b) Links per Slot.
Frame length and links per slot for a link-based assignment strategy with different antenna lobes.
antenna is, the bigger the interference level is and the dropping of the frame length it is considerably attenuated. E. Links per slot evolution with link-based strategy The number of links per slot, figure 3(b) is directly related to the aperture angle of the antenna being used. If an ideal antenna is used, it is observed that the number of links per slot increases when the connectivity increases. This effect is produced by the greater number of connections that can be activated because there are no interferences between them. As the aperture angle increases the evolution of the number of links per slot is modified and starts to drop. This effect is
the consequence of a higher probability for an interference to occur when transmitting. The variation using different antennas is quite large. It can vary from practically 12 links per slot to 2 links per slot using omni directional antennas. F. Network capacity The results that have been obtained previously considering the frame length and the spatial re-usability are directly related to the network global capacity. If a link or node has quite a lot of assigned slots to transmit in each frame, the global data rate is going to be higher than with a link that is repeated only few times in each frame. The maximum throughput or
(a) Link assignment strategy. Fig. 4.
(b) Node assignment strategy.
Maximum bit rate for the application running in each terminal.
network capacity is going to limit the speed of the applications that are going to run in each terminal. In an ad hoc network there are some routes that are going to support more traffic than others depending on the selected routing algorithm. This paths or routes, with a high data traffic, are considered the network bottleneck in order to increase the system performance. In [6] the global network capacity, defined as the largest admissible traffic load yielding a finite network delay, in a STDMA ad hoc network with a link assignment strategy can be defined as: hij N (N − 1) (1) λ = min TL Λij and for a node assignment strategy: hi N (N − 1) λ = min TL Λi
(2)
As it can be observed the equation takes into account the frame length and the number of assigned slots for a certain link or node. h The obtained results can be seen in figure 4. The Λij value ij in the worst case is going to be 1. That is why TL , the frame length, is going to be the variable that affects the network capacity. In section III, the frame length decreases when the connectivity is increased, so the capacity of the network increases as well. The antenna influence can only be appreciated in the link assignment strategy. IV. T OPOLOGY INFLUENCE IN THE NETWORK
transmission is going to depend on the geographical situation of each one of the participant nodes. However, the SINR is not the only parameter that is going to affect the network performance. In figure 5 two different ad hoc networks can be seen. Both of them have connectivity 0.2 but the topology is quite different. One of them, 5(a), is divided in two sub-networks connected by two links. The other network, 5(b), is created as a solid block where sub-networks can not be appreciated. The first network creates a bottleneck in the connection between the sub-networks. This bottleneck needs quite a lot of transmission slots to transfer the information from one of the sub-networks to the other one. What it is the same, the number of paths or routes that are going to flow through the intermediate link is very high. This effect produces very long frames in this kind of networks, reducing the global system performance. The second network offers different paths so there is not a unique link or node that has to deal with a lot of traffic, the traffic is divided in the network in a more uniform way. The assigning algorithm generates in this network shorter frames increasing the capacity and efficiency of the network. When the connectivity is increased, the possibility to generate sub-networks connected by only a few links decreases, and the network efficiency is not affected in such a several way. In table I and table II the maximum, minimum values and the deviation for the frame length can be seen. This statistical values have been obtained with the analysis of 50 networks for each connectivity, nodes are using antennas with 30 grades lobes.
PERFORMANCE
V. C ONCLUSIONS
The algorithm to create the schedule in a STDMA network is based in the SINR. It tries to assign slots so as to generate a schedule where collisions are avoided. The SINR for each
In this article the frame length, the reuse factor and the capacity have been analyzed for a STDMA ad hoc network with directional antennas. Two assignment mechanisms
(a) Case I.
(b) Case II. Fig. 5.
Networks of 0.2 connectivity with different topologies
TABLE I F RAME L ENGTH S TATISTICS FOR L INK A SSIGNMENT S TRATEGY 0.1
0.3
0.5
0.7
0.9
Max
873
684
673
393
217
Min
537
290
159
149
79
Deviation
69
102
117
59
33
TABLE II F RAME L ENGTH S TATISTICS FOR N ODE A SSIGNMENT S TRATEGY 0.1
0.3
0.5
0.7
0.9
Max
1363
1228
980
891
727
Min
833
660
810
784
669
Deviation
126
110
30
22
14
have been used, the node-based and link-based assignment strategies. It has been probed that with the node-based assignment mechanism the influence of directional antennas is practically null. It can be seen also, that increasing the connectivity of the network does not increase the performance because of the flatten effect produced by the combination of the interference increase and the uniform distribution of slots. In the link-based assignment strategy the influence of the directional antennas is very high, with a perfect directional antenna the performance of the network is increased when working with high connectivity. This performance starts to decrease when the antenna works with wider angles approaching the performance of the node-based assignment mechanism.
Talking about the capacity it has been shown that the network performance is modified depending on the network topology, this effect can be seen better for the link-based assignment method. It can be concluded that directional antennas should only be used with a link-based assignment method because in the node-based mechanism the benefits that are obtained are not worthy. ACKNOWLEDGMENT This work has been supported by the University and Research Department of the Basque Country Government (BFI04.110). R EFERENCES [1] H. Zhai et al, “Medium Access Control in Mobile Ad Hoc Networks: Challenges and Solutions”, Wiley Wireless Communications and Mobile Computing, Sept 2004. [2] A. Chandra and V. Gummalla, “Wireless Medium Access Control Protocols”, IEEE Communications Surveys and Tutorials, 2002. [3] P. Garg et al, “Using IEEE 802.11e MAC for QoS over Wireless”, IEEE International Performance Computing and Communications Conference, April 2003. [4] R. Nelson and L. Kleinrock, “Spatial TDMA: A Collision-free Multihop Channel Access Protocol”, IEEE Transactions on Communications, vol. COM-33, pp.934-944. September 1985. [5] M. Sanchez, “Multiple Access Protocols with Smart Antennas in Multihop Ad Hoc Rural Area Networks”, Master Thesis, KTH, Radio Communications Systems Laboratory, June 2002. [6] J. Grnkvist, “Assignment Strategies for Spatial Reuse TDMA”, Technical Report, Radio Communications Systems Laboratory, March 2002.
