Presented at SatNav 2003 th The 6 International Symposium on Satellite Navigation Technology Including Mobile Positioning & Location Serivces Melbourne, Australia 22–25 July 2003
Incorporating GPS into Wireless Networks: Issues and Challenges S. Omar, C. Rizos School of Surveying and Spatial Information Systems The University of New South Wales, Sydney NSW 2052 Tel: 02-9385 4205 Fax: 02-93137493 Email:
[email protected]
Presenter: S. Omar ABSTRACT The use of wireless LANs is expected to increase dramatically in the foreseeable future as businesses discover the enhanced productivity and increased mobility that wireless communications can provide in a society that is moving towards more ‘connectionless’ connections. WLANs require specific support and tools for maintaining network integrity. GPS has also become increasingly popular, and low-cost devices are readily available to civilian users. To improve the performance of the standard GPS receivers, groups of them may be linked together into networks. The Internet Protocol will provide the means for integration of services across different access networks. This study highlights the principles of GPS networking, focussing in particular on the influence of GPS and Wireless Networks on each other. Issues arise in relation to routing protocols, scalability, power and network management, and quality of service. The benefits of using GPS in WLANs are also addressed. Nevertheless, there are challenging problems arising from this stepwise integration. Solutions to address these problems, and to maximise network efficiency, by means of appropriate system design and data format are proposed. KEYWORDS: GPS networks, wireless telecommunications, WLAN ad hoc
networks
1. INTRODUCTION A wireless local area network (WLAN) is a flexible data communication system or configuration that has been developed to provide high-performance network connectivity in a limited geographical area, with all nodes operating in a common frequency band. Competition for the single channel is inevitable when multiple nodes wish to ‘talk’ simultaneously.
WLANs offer many attractive advantages over traditional wired networks, such as mobility, flexibility, scalability, fast and easy installation, and reduced long-term cost in rapidly changing environments that require frequent moves, additions and reconfigurations. However, compared with their wired computer parts, the design and implementation of WLANs must overcome certain challenges associated with interference, limits on bandwidth, throughput, power and coping with mixed traffic. GPS has become widely accepted in the telecommunications community as a vital component technology, playing several different roles. As the need for more accurate positioning increases, there is a discernable trend for the establishment of continuously operating reference station (CORS) networks. Network-based positioning techniques offer several advantages over standard single-base station techniques. In this paper the relationship between GPS networks and communications will be highlighted. In section 2 the direct connection between GPS and telecommunications, including the Internet and its protocols, will be introduced. Section 3 focuses on the wireless network. Section 4 addresses the routing protocols relevant to GPS network applications. Finally, section 5 discusses some important issues to be considered when planning GPS networks, while concluding remarks are made in section 6. 2. GPS AND TELECOMMUNICATIONS Wireless (mobile) Internet connectivity is highlighted in a Japanese project linking automobiles and the Internet, via fixed nodes (Hada H et al., 2000). An example of an application of such connectivity is where each automobile is equipped with sensors to collect information concerning outside temperature data, switching status of headlights, operating status of the wipers, and so on. This information is digitised and sent via the Internet to a Traffic Centre, where it is used to derive traffic and weather information, which can then be broadcast to all road users. The Internet as basis for Real-Time Kinematic (RTK) and Differential GPS (DGPS) positioning services is another obvious application where the Internet provides many advantages over conventional radio data links. Among these advantages are service unification, open architecture, bi-directional communications, and scalability. Scalability means the ability to ‘grow’ relatively effortlessly without changing the provider’s hardware and software. The main advantage of a radio broadcasting service is its scalability as a reference station can broadcast RTK/DGPS corrections to an unlimited number of users. However a disadvantage is that there is no means of communicating with the reference station, as in the case of the Internet as the data channel. In addition, the volume of information that can be broadcast is limited by the available bandwidth. The user can connect to the Internet through wired or wireless computer networks using mobile computers such as Personal Digital Assistant (PDA), or even laptop computers, at the measurement site and access RTK/DGPS correction information. Mobile users can select one of several data links types, including mobile phone, packet radio, telephone, WiFi, and even via cable modem, or combinations of these. An Internet-based RTK/DGPS service may have low cost infrastructure (if existing telecommunications infrastructure is used), and in addition there are no spectrum licence requirements, which might constrain the growth of the system.
2.1 Internet as a Means of Communication
The Internet is a global net of computers and networks. All data are converted to packets, which are forwarded to their destination via computer networks. The Internet Protocol (IP) assigns the basic packet and forwards it to a neighbouring computer that is on the ‘right’ route to its ultimate destination (Koufopavlou et al., 1992). If a problem occurs in the IP packet, the packet will be destroyed. Therefore, the arrival of the packet is not guaranteed under the IP. Wireless communication devices are indispensable when connecting automobiles or persons to the Internet. There are many wireless communication infrastructures available. However, they may be difficult to use in the Mobile Internet environment because the wireless communication link could be unstable or have a high usage cost. Current wireless communication devices have an error correction mechanism, which transforms this inherent instability into unexpected delays in delivering the packets of information. Transmission Control Protocol (TCP)
A stream-type protocol with reliable connection over the IP is the Transmission Control Protocol (TCP). TCP organises data streams between hosts and provides for reliable data transfer. TCP has congestion control and a data retransmit mechanism, which is the basis for a reliable network service. At the starting point of the TCP data stream a connection is established by 3-way handshake. It means that the packet crosses a network three times to establish the connection. It may take a long time to forward data over a line or link that is in bad condition because an acknowledgement of the packet’s arrival is required, and lost data must be sent again. The changing position of a mobile user can cause frequent changes of the network configuration, which may in turn cause TCP errors or delays. In the case of a bad line or host, the TCP connection could be released and will require re-establishment, all of which is time consuming. On the other hand, when TCP is used there is no need for an error detection mechanism at the server and/or client sites. User Datagram Protocol (UDP)
Another protocol used under the IP is the User Datagram Protocol (UDP). UDP does not guarantee data arrival, as UDP does not have a retransmission mechanism, error detection, or congestion control. Therefore, network congestion can cause UDP data loss. UDP does not need to confirm a connection before data transmission, which allows it to save time (Partridge & Pink, 1993). UDP provides a quicker data packet delivery than TCP, but as the data arrival is not guaranteed, it is necessary to detect the data arrival within the application software itself. There is also a limit to the size of a packet that can be sent in UDP. If the data chunk is larger than on packet, the application needs a re-assembly mechanism. 2.2 Timing and Synchronisation
Precision timing is very important in today’s public and private digital networks. Every telecommunications service depends on continuous, error-free transmission of information, which can only be accomplished with the use of precision synchronisation. Today, the GPS is the most frequently used time synchronisation technology. The American National Standards Institute (ANSI) has specified the Synchronization Interfaces Standard, which prescribes the minimum clock performance of a primary reference source (PRS) as 1x10-11. It also states that all clocks at the nodes must be traceable to the PRS. The telecommunications infrastructure is now heavily dependant upon GPS. Fortunately, the
cost of GPS timing receivers and companion PRS oscillators continues to come down in price as well as increase in quality. A large network may have a thousand or so GPS-based PRSs in use. If one or several GPS satellite signals were not available, telecommunications networks would not be affected. (The probability of a GPS constellation total failure is virtually zero.) However, should there be interference or jamming (either accidental or intentional) of the GPS signals in a locale, telecommunications network performance degradation will occur. This degradation will affect various telecommunications services differently, as indicated in Table 1. Type of Service Voice Facsimile Data Video Encryption
How Effected Noise Loss of picture contact Reduced throughput due to retransmission Freeze Frames Retransmission of encryption
Table 1. Effect of loss synchronisation on various types of services
3. GPS AND WIRELESS NETWORKS Wireless networks have experienced explosive growth in the past few years. Different types of wireless networks are emerging. Second generation cellular networks, which provide mainly voice services, are evolving into third generation systems, which also support packetbased data services. In addition Wireless Local Area Networks (WLAN) are becoming increasingly prevalent in buildings, shopping centres, etc. There is also significant interest in using multi-hop wireless networks for providing Internet access to residences. Such multi-hop ad hoc wireless networks are also being used for networking distributed control systems. Another type of ad hoc wireless networks is a sensor network where tiny sensors with radios are used to collect information about their environment. The utility of position information provided by GPS to wireless networks has been investigated by Egen et al. (2002). Currently most work concerning wireless networking is done without recourse to position information, that is most protocols and algorithms used in wireless networks do not use position information, despite the fact that GPS receivers are cheap and becoming increasingly ubiquitous (embedded within mobile phones, base stations and other wireless access devices). There are several advantages in using position information in the protocols and algorithms within wireless networks. One of the prospective uses of wireless networks is within vehicles. For example, mobile users want an Internet service without interruption, so as the vehicle moves it needs to perform handoff in order to ensure an uninterrupted service. This handoff should be as smooth as possible, in order not to disrupt the Internet service through possible packet losses. Small, dispersed sensor networks may collect data sent to mobile users. A network structure has been proposed by Egen et al. (2002) that consists of a sensor network, an ad hoc network and cellular network is illustrated in Figure 1. GPS-equipped mobile users (vehicles) that roam across a sensor area, in addition to fixed GPS-equipped base stations, has been considered. Sensor data will multi-hop wireless networks to the fixed base stations by ad hoc routing. Mobile IP is implemented for Internet connectivity. Mobiles
connect to a fixed base (Foreign Agent) and registers to its Home Agent (as indicated in Figure 1). In this way connection-to-base is not restricted to one-hop, and it may be multihop.
4. GPS AND AD HOC ROUTING The Geographical Routing Algorithm (GRA) (Jain et al., 2001) is an asynchronous, real-time distributed and scalable algorithm for ad hoc routing with incomplete knowledge of network topology. One assumes that each node gets its geographical position from GPS, and has the means to find the position of the destination node. When a node has a packet for a destination, it selects from the nodes (it knows about) the one which is closest to the destination, and sends the packet on its way. Along the path, a node may know of even a closer node to the destination, and the packet then gets redirected to that node. On its way to that node, it may get redirected again, and so on, until it reaches its destination. 4.1 Routing Protocols
With the advantage of wireless communication technologies, small-sized but highperformance computing/communication devices are increasingly used in daily life. A large population of such devices will increasingly wish to communicate (Hong et al., 2002). While the fixed (expensive) infrastructure of a cellular system is a traditional model for a mobile wireless network, the focus here is on a network that does not rely on a fixed infrastructure and works in a shared wireless media. Such a network, called a mobile ad hoc network (MANET), is a self-organising and self-configuring multi-hop wireless network, where the network structure changes dynamically due to member mobility. Ad hoc networks are very attractive for tactical communications in the military and law enforcement. They are also expected to play an important role in civilian life for connectivity within environments such as convention centres, shopping malls, electronic classrooms, and so on. Nodes in this network model share the same random access wireless channel.
4.2 Geographic Position Information Assisted Routing
Advances in the development of GPS make it possible to provide location information with an accuracy of only a few metres. GPS also provides universal timing. While location information can be used for directional routing in distributed ad hoc systems, the universal clock can provide global synchronizing among GPS-equipped nodes. Research has shown that geographical location information can improve routing performance in ad hoc networks. Additional care must be taken in a mobile environment, because locations may not be accurate by the time the information is used. All the protocols listed below assume that the nodes know their positions: -Geographic Addressing and Routing -Location-Aided Routing -Distance Routing Effect Algorithm for Mobility -Greedy Perimeter Stateless Routing -Comparisons of Geographic Position Assisted Routing 5. GPS AND POWER LINES The Power Line Carrier (PLC) is an important technique used in power lines for quick and reliable communication and telemetry. PLC uses low-medium frequency signals that are coupled to and propagate over power line conductors. Application of PLC includes protective relaying, telemetering, voice communication, supervisory control, etc. Many PLC systems use some form of discrete frequency shifting to transmit digital information in the 40-490Hzfrequency range. The PLC signal propagates along the line in three general modes (Kesheng et al., 1997). DGPS involves using ground stations that compare a GPS-derived position with its known location to compute a correction that can be used to mitigate systematic errors such as satellite clock, orbit errors, and atmospheric delay errors. The correction information can be broadcast to nearby users for real-time user computations or stored for post-processing the raw data. The DGPS accuracy is typically in the 1-3m range or better (Enge & Misra, 1999). The very narrow DGPS signal bandwidth can be an important consideration when concern arises about possible interference due to other radio signals at nearby frequencies. During normal operation the minimum specified field strength of the DGPS broadcast signal is usually 75 mv/m, or 37.5 dB referenced to 1 mv /m (Ruane et al., 1988). In the region close to the power lines the PLC signal may be strong enough to affect DGPS receiver performance. Interference to aeronautical and maritime non-directional beacons due to radiated emissions of High Voltage Direct Current (HVDC) converter stations or power transmission lines have been reported (Patterson, 1985). The potential of interference will depend on the emission within the beacon band and the signal-to-noise ratio. The simple solution to potential DGPS problems with PLC is frequency separation. If the DGPS signal is contained within a very narrow bandwidth about the centre frequency; 99% power containment is within a 117-234Hz bandwidth. The PLC bandwidth is variable but could be of the order of 300-2200Hz. For new PLC transmitters, engineers should determine DGPS beacon coverage in their area and avoid these regional DGPS frequencies in locating new PLC systems (Silva & Whitney, 2002).
6. DISCUSSION GPS is the technology most frequently selected for use in digital communication networks to meet the requirement for precision timing synchronisation. Increased timing accuracy provides overall improvements in system performance (quality and efficiency). The telecommunications infrastructure uses the GPS signal as an integral and basic part of the system. The stability of GPS, the ongoing health of the GPS constellation, and the GPS signal quality can impact telecommunication systems. Including GPS in ad hoc networks improves the Quality of Service as it reduces the overheads and increases the throughput. Powerlines can have a negative impact on the GPS signals, especially if they are close to the bandwidth of the power control lines; an issue, which is relevant when planning GPS reference station networks. REFERENCES Egen M, Coleri S, Dundar B, Rahul J, Puri A, Varaiya P (2002) Application of GPS to mobile IP and routing in wireless networks, Vehicular Technology Conference, Proceedings, VTC 2002-Fall, IEEE 56th, volume 2, 1115-1119. Enge P, Misra P (1999) special issueon on Global Positioning System, Proceedings of IEEE, 87(1), 315. Hada H, Sunahara H (2000) DGPS and RTK positioning using the Internet, GPS Solutions, 4(1), 3344. Hong X, Xu K, Gerla M (2002) Scalable routing protocols for mobile ad hoc networks, IEEE Network, 16(4), July/Aug, 11-21. Jain R, Puri A, Sengupta R (2001) Geographical routing using partial information for wireless ad hoc networks, IEEE Journal of Personal Communications, 8(1), February, 48-57. Kesheng Z, Linchang Z, Wei Y (1997) A study of second reflection effects of high voltage transmission line on HF antenna radiation characteristic by scaling model, Proceeding of International Symposium on Electromagnetic Compatibility, 21-23 May, 413-416. Koufopavlou OG, Zitterbart M (1992) Parallel TCP for high performance communication subsystems, GLOBECOM, IEEE press, 6-9 Dec, volume: 3, 1395-1399. Partridge C, Pink S (1993) A faster udp, IEEE/ACM Transaction on Networking, 1(4), 429-440. Patterson NA (1985) Carrier frequency interference from HVDC systems, IEEE Transactions Power Application Systems, Volume PAS-104, 3255. Ruane M, Enge P, Olson K (1988) Test experience using marine radiobeacons for DGPS communications, Position Location and Navigation symposium, Record. “Navigation into the 21st Century’, IEEE PLANS ’88, IEEE, 29 Nov-2 Dec, 303-309. Silva JM, Whitney B (2002) Evaluation of the potential for power line carrier (PLC) to interfere with use of the nationwide differential GPS network, IEEE Transactions on Power Delivery, 17(2), 348-352.
