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Urban facility interconnection networks require a robustness and reliability usually found in public networks or private cable and radio networks in licensed bands. Both factors mean an increase in costs, which also increase as the size of the network increases. The recent rise in use of wireless technologies in open wavebands has attracted the int...

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... water networks in metropolitan areas operate over a network of distributed stations which fulfill a dou- ble function of monitoring and actions, sending environ- mental and water quality data to the central station, and re- ceiving orders to be executed for sluices and pumps. The traffic generated by these stations can be grouped into: control and telemetry traffic, voice traffic (between opera- tors and central) and video traffic (coming from cameras that monitor water levels, obstructions or collectors sta- tus). In general, the communication from the central to the remote stations is carried out using heterogeneous networks, which combine the coverage and performance of public networks (both wired and wireless) and private networks. The use of private networks is interesting in those segments where they have spread enough to be technically and economically viable, which will mean a considerable saving in costs of use and greater control of the network. Industrial networks giving support to these types of ins- tallations are characterized by high reliability, adaptabil- ity and scalability. However, the use of fieldbuses and other wired technology are limited to local use, which has sparked interest in the use of communication technology with wider coverage for interconnection in urban areas. It is usual in this context to see the use of wireless technology over licensed bands, which allows the use of an interference-free channel as well as high transmission power, which in turn means greater range and a higher level of signal/noise relationship in the receivers, giving greater reliability. However, the current trend is to use open bands, such as 868 MHz, 2.4 GHz or 5 GHz. The use of these bands offers significant benefits in terms of installation and using costs, as there is an extremely active market which is generating a huge quantity of new technology and products. However, there are certain disad- vantages which must be considered, such as the reduction in transmission power limits, which translates into a lower range and weaker signal/noise relationship. Another dis- advantage is that the band is shared with other users so on occasions may be saturated, as 2.4 GHz band. Industrial networks require high reliability and availability in communications, which means it is necessary to study mechanisms which can provide better guarantees for the traffic in these types of technologies. In this paper, the behavior of this three types of traffic necessary for the interconnection of remote stations of a real metropolitan waste water system using 802.11n [2] radio technology and the SIP [4] and RTP [5] session and transport protocols, is studied and evaluated. The experiments were carried out on a laboratory testbed, in order to obtain greater control over the tests, and further experiments will be conducted in a real exterior facility. The need for new services in Valencia’s waste water system, such as voice and video, has led to a gradual increase of means of access to public networks, installed by tele-operators to give service to the stations, which involves a monthly cost as well as the externalization of control of the network. With the objective of using a private network and reducing costs, we have used a laboratory testbed to recreate 1 and 2 hop links with 802.11n, and carried out an initial evaluation of the services offered over this real network. Fig. 1 shows an aerial photograph of the Central Control Station (CCS) and the nearby installations with which it must maintain fixed links. In this image, we can see 1 hop links (stations 1 and 2) and 2 hop links (stations 3, 4 or 5). The use of the 5 GHz band means there are no costs involved, and there is less saturation than with the 2.4 GHz band, as well as a greater quantity of available channels, making WiMax and 802.11n good candidates for these applications [6]. The testbed is made up of 3 Linksys WRT602N routers, with channels of 40 MHz bandwidth and a physical distance of 5 meters between each. Using virtual card confi- guration and bridge mode, they were connected by 2 hops at the data link layer level, obtaining a high capacity trunk with throughputs of up to 80 Mbps from end to end. In order to create a situation as close to real as possible, the SIP server and a client are connected to one end of the trunk, corresponding to the central control station, and the second client is located in the intermediate router or at the other end of the trunk, in order to carry out tests of 1 and 2 hops via remote station (Fig. 2). The server uses Open- SIPS 1.6.2 and Asterisk 1.2.21.2 to offer these services, and clients use free softphone X-Lite 2.0 for Linux environments and 4.0 for Windows. The 802.11n standard was ratified by the IEEE orga- nization on 09/11/2009, with a speed of up to 600 Mbps in the physical layer. Its range and quality are improv- ing thanks to new MIMO (Multiple Input Output) technology, which allows it to use several channels simultane- ously to send and receive data, thanks to the incorporation of several antennas. The use of OFDM modulation offers greater resistance to interference in the channel. 802.11n can work in two frequency bands: 2.4 GHz and 5 GHz, and at 20 or 40 MHz. Thanks to techniques to improve interference resistance, transmission speed and bandwidth, this new standard is becoming a real alternative, both economically and in terms of performance, to WiMaX for links of medium distance between urban installations. Session Initiation Protocol, developed by the working group MMUSIC (Multiparty MUltimedia SessIon Con- trol) of the IETF (Internet Engineering Task Force), is a signaling protocol for interactive multimedia sessions. SIP is a protocol oriented toward the internet, and which can deal with mobile users. It has some extremely interesting characteristics, such as low processing needs, authen- tication and encryption systems, modularity, extendibil- ity and simplicity of code and network scalability. SIP is the protocol that deals with establishing and finishing the communication, as well as negotiating the necessary parameters for this communication, such as the compression algorithms used, sampling frequencies and multimedia characteristics, pricing and information security. SDP (Session Description Protocol) is used to describe the content of the session, and RTP as the carrier for the voice and video of the session. The choice of SIP + RTP over UDP for the transfer of multimedia content offers a po- werful call routing and control system between clients, and ensures a constant flow and organized delivery thanks to temporal marking techniques for the synchronization of the connection and reorganization of packets received using RTCP (RTP Control Protocol). Control traffic is made up of short sequences which carry readings from sensors or initiate actions over the PLCs (Programmable Logic Controller) of the station. In our case the station’s largest frame would be 592 bytes of payload, this can be transmitted in a single Ethernet frame. Evaluation must consider the delay between transmission and reception (latency) and the percentage of losses. This can be done using the pingSIP tool [1] which sends a SIP message of type OPTION which circulates at the session level between origin and destination and diagnoses the proper functioning of the end to end link and across all levels involved. In contrast to ping, pingSIP does not suf- fer from blocks in routers with NAT and reaches directly the destination device and requires greater processing in the level of layer in which it is operating. Fig. 3 shows the results from 300 pingSIP tests for one and two hops, where a loss percentage of 0% was obtained in both cases. For one hop, 77% of the values are concentrated at around 2.88 ms, with an arithmetic average of 5.79 ms (marked in the figure with a dotted line) and a typical deviation of 5.73 ms. For two hops, the latency was concentrated at around 3.05 ms, the arithmetic average was 6.52 ms and the typical deviation was 7.49 ms. These latency values, in the order of a few milliseconds, are negligible for the ...

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... They also addressed the challenges of establishing communication reservation periods for exchanging real-time multimedia traffic between two stations in the presence of interference. The use of 802.11 standards in real-time multimedia applications has been studied in [69] and [70]. Seno et al. [27] proposed a centrally-controlled 802.11 network to decide the admission of nodes that require real-time communication. ...
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... They also addressed the challenges of establishing communication reservation periods for exchanging real-time multimedia traffic between two stations in the presence of interference. The use of 802.11 standards in real-time multimedia applications has been studied in [87] and [88]. Seno et al. [37] proposed a centrally-controlled 802.11 network to decide about the admission of nodes that require real-time communication. ...
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... Some papers, however, are worth to be mentioned. In both [2] and [3] some examples related to the adoption of IEEE 802.11n for real-time industrial multimedia traffic are discussed. Another typical issue of industrial communication, namely real-time frame scheduling, is addressed in both [4] and [5]. ...
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