Sensor networks are another form of infrastructureless network, with many similarities to ad-hock
Fundamental concepts in wireless networks Sharing Resources Cellular concepts (reuse resources) WLAN (shared space) Adhock (shared resources) Sensor (shared resources, large space)
What is a Cell? Cell is the Basic Union in The System defined as the area where radio coverage is given by one base station. A cell has one or several frequencies, depending on traffic load. Fundamental idea: Frequencies are reused, but not in neighboring cells due to interference.
Cell characteristics Implements space division multiplex: base station covers a certain transmission area (cell) Mobile stations communicate only via the base station Advantages of cell structures: higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally Problems: fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies
Capacity & Spectrum Utilization Network capacity at required QoS with conventional frequency plan Subscriber growth Time Out of Capacity!!! The need: Optimum spectrum usage More capacity High quality of service Low cost I wish I could increase capacity without adding NEW BTS! What can I do?
Cell Planning (1/3) The K factor and Frequency Re-Use Distance K = i 2 + ij + j 2 K = 2 2 + 2*1 + 1 2 K = 4 + 2 + 1 K = 7 i j 1 2 3 4 5 6 7 Frequency re-use distance is based on the cluster size K The cluster size is specified in terms of the offset of the center of a cluster from the center of the adjacent cluster D = 3K * R D = 4.58R 1 2 3 5 6 7 D R
Cell Planning (3/3) Cell sectoring Directional antennas subdivide cell into 3 or 6 sectors Might also increase cell capacity by factor of 3 or 6 Cell splitting Decrease transmission power in base and mobile Results in more and smaller cells Reuse frequencies in non-contiguous cell groups Example: ½ cell radius leads 4 fold capacity increase
Hierarchical Cell Structures (HCS) (1/2) HCS allows traffic to be directed to a preferred cell Each cell is defined in a particular layer The lower the layer, the higher the priority Mobiles will select a cell on the lowest layer as long as it has “sufficient” signal strength, even if higher layer cell are stronger
WLAN: Definition A fast-growing market introducing the flexibility of wireless access into office, home, or production environments. Typically restricted in their diameter to buildings, a campus, single rooms etc. replace office cabling and, additionally, to introduce a higher flexibility for ad hoc communication in, e.g., group meetings. The global goal of WLANs is to replace office cabling and, additionally, to introduce a higher flexibility for ad hoc communication in, e.g., group meetings.
WLAN: Characteristics Advantages: very flexible within radio coverage ad-hoc networks without previous planning possible wireless networks allow for the design of small, independent devices more robust against disasters (e.g., earthquakes, fire) Disadvantages: typically very low bandwidth compared to wired networks (~11 – 54 Mbit/s) due to limitations in radio transmission, higher error rates due to interference, and higher delay/delay variation due to extensive error correction and error detection mechanisms offer lower QoS many proprietary solutions offered by companies, especially for higher bit- rates, standards take their time (e.g., IEEE 802.11) – slow standardization procedures standardized functionality plus many enhanced features these additional features only work in a homogeneous environment (i.e., when adapters from the same vendors are used for all wireless nodes) products have to follow many national restrictions if working wireless, it takes a very long time to establish global solutions
WLAN: Design goals global, seamless operation of WLAN products low power for battery use (special power saving modes and power management functions) no special permissions or licenses needed (license-free band) robust transmission technology simplified spontaneous cooperation at meetings easy to use for everyone, simple management protection of investment in wired networks (support the same data types and services) security – no one should be able to read other’s data, privacy – no one should be able to collect user profiles, safety – low radiation transparency concerning applications and higher layer protocols, but also location awareness if necessary
WLAN: Technology Overview Core technologies (IEEE 802.1x family) IEEE 802.11 (Wireless LAN) IEEE 802.15 (Wireless PAN – Bluetooth) IEEE 802.16 (Wireless M(etropolitan) AN) – Under development Facilitating technologies RF-Id IrDA Home-RF PAN LAN MAN
WLAN: Technology Can be categorized according to the transmission technique being used Infrared (IR) LANs: Very limited coverage area (IR can ’ t penetrate walls!) Spread Spectrum LANs: Operate in Industrial, Scientific, and Medical (ISM) bands Narrowband Microwave LANS: Operate at microwave frequencies but not using spread spectrum (in licensing or ISM bands)
WLAN: infrared vs. radio transmission Infrared uses IR diodes, diffuse light, multiple reflections (walls, furniture etc.) Advantages simple, cheap, available in many mobile devices no licenses needed simple shielding possible Disadvantages interference by sunlight, heat sources etc. many things shield or absorb IR light low bandwidth Example IrDA (Infrared Data Association) interface available everywhere Radio typically using the license free ISM band at 2.4 GHz Advantages experience from wireless WAN and mobile phones can be used coverage of larger areas possible (radio can penetrate walls, furniture etc.) Disadvantages very limited license free frequency bands shielding more difficult, interference with other electrical devices Example: WaveLAN, HIPERLAN, Bluetooth
WLAN: Spread Spectrum Most popular category! Spread Spectrum Communications Developed initially for military and intelligence requirements Essential idea: Spread the information signal over a wider bandwidth to make jamming and interception more difficult Frequency hopping Direct sequence spread spectrum
WLAN: infrastructure vs. ad-hoc networks infrastructure network ad-hoc network AP wired network AP: Access Point
WLAN: Infrastructure-based networks Infrastructure networks provide access to other networks. Communication typically takes place only between the wireless nodes and the access point, but not directly between the wireless nodes. The access point does not just control medium access, but also acts as a bridge to other wireless or wired networks. Several wireless networks may form one logical wireless network: The access points together with the fixed network in between can connect several wireless networks to form a larger network beyond actual radio coverage. Network functionality lies within the access point (controls network flow), whereas the wireless clients can remain quite simple. Use different access schemes with or without collision. Collisions may occur if medium access of the wireless nodes and the access point is not coordinated. If only the access point controls medium access, no collisions are possible. Useful for quality of service guarantees (e.g., minimum bandwidth for certain nodes) The access point may poll the single wireless nodes to ensure the data rate. Infrastructure-based wireless networks lose some of the flexibility wireless networks can offer in general: They cannot be used for disaster relief in cases where no infrastructure is left.
WLAN: ad-hoc networks No need of any infrastructure to work greatest possible flexibility Each node communicate with other nodes, so no access point controlling medium access is necessary. The complexity of each node is higher implement medium access mechanisms, forwarding data Nodes within an ad-hoc network can only communicate if they can reach each other physically if they are within each other’s radio range if other nodes can forward the message
WLAN: Standards (ii) IEEE 802.11 and HiperLAN2 are typically infrastructure-based networks, which additionally support ad-hoc networking Bluetooth is a typical wireless ad-hoc network IEEE 802.11b offering 11 Mbit/s at 2.4 GHz The same radio spectrum is used by Bluetooth A short-range technology to set-up wireless personal area networks with gross data rates less than 1 Mbit/s IEEE released a new WLAN standard, 802.11a, operating at 5 GHz and offering gross data rates of 54 Mbit/s Shading is much more severe compared to 2.4 GHz Depending on the SNR, propagation conditions and the distance between sender and receiver, data rates may drop fast uses the same physical layer as HiperLAN2 does HiperLAN2 tries to give QoS guarantees IEEE 802.11g offering up to 54 Mbit/s at 2.4 GHz. Benefits from the better propagation characteristics at 2.4 GHz compared to 5 GHz Backward compatible to 802.11b IEEE 802.11e: MAC enhancements for providing some QoS
Ad Hoc Networks: Definition A network made up exclusively of wireless nodes without any access points operating in peer-to-peer configuration, grouped together in a temporary manner.
Ad Hoc Networks: Some Features Lack of a centralized entity All the communication is carried over the wireless medium Rapid mobile host movements Limited wireless bandwidth Limited battery power Multi-hop routing
Ad Hoc Networks: Operation Assumption Unidirectional link Adjustable power level Directional antenna GPS Operation Broadcasting Routing Multicasting
Ad Hoc Networks: Challenges (i) Hidden terminal problem A transmits to B C wants transmits to B C does not hear A’s transmission Collision Exposed terminal problem B transmits to A C wants to transmit to D C hear B’s transmission Unnecessarily deferred A BC A BC D
Ad Hoc Networks: Challenges (ii) Challenges Mobility Scalability Power Minimizing power consumption during the idle time Minimizing power consumption during communication QoS End to End delay Bandwidth management Probability of packet loss
Ad Hoc Networks: Broadcast (i) Objective: paging a particular host sending an alarm signal finding a route to a particular host Two types: Be notified -> topology change Be shortest -> finding route A simple mechanism: Flooding Suffer from broadcast storm
Ad Hoc Networks: Broadcast (ii) source Be notifiedBe shortest 5 forwarding nodes 4 hop time source 6 forwarding nodes 3 hop time
Ad Hoc Networks: Routing Table Driven vs. On Demand DSDV, TORA, DSR, AODV Hierarchical and Hybrid ZONE Specific assumption Unidirectional link, Directional antenna, GPS QoS-aware Power, Delay, Bandwidth
Ad Hoc Networks: Multicast Parameter: The delay to send a packet to each destination The number of nodes that is concerned in multicast The number of forwarding nodes s D D D s D D D s D D D
Ad Hoc Networks: Recommended Introductory Reading M. Frodigh, et al, "Wireless Ad Hoc Networking: The Art of Networking without a Network," Ericsson Review, No. 4, 2000. F. Baker, "An outsider's view of MANET," Internet Engineering Task Force document, 17 March 2002. IEEE tutorial
Sensor Networks: Definition A sensor network is a collection of collaborating sensor nodes (ad hoc tiny nodes with sensor capabilities) forming a temporary network without the aid of any central administration or support services. Sensor nodes can collect, process, analyze and disseminate data in order to provide access to information anytime and anywhere.
Sensor Networks: Some Features Large number of sensors Low energy use Efficient use of the small memory Data aggregation Network self-organization Collaborative signal processing Querying ability
Sensor Networks: Operation Sensors work in clusters Each cluster assigns a cluster head to manage its sensors Three layers Services layer Data layer Physical Layer To compensate for hardware limitations (e.g. memory, battery, computational power): Applications deploy a large number of sensor nodes in the targeted region.
Sensor Networks: Challenges (i) Hardware design Communication protocols Applications design Extending the lifetime of a sensor network Building an intelligent data collecting system Topology changes very frequently Sensors are very limited in power Sensors are very prone to failures
Sensor Networks: Challenges (ii) Sensors use a broadcast paradigm Most networks are based on point to point communication Sensors may not have a global identification (ID) Very large overhead Dynamic environmental conditions require the system to adapt over time to changing connectivity and system stimuli
Sensor Networks: Aggregation Some sensor nodes are designed to aggregate data received from their neighbors. Aggregator nodes cache, process and filter data to more meaningful information. Aggregation is useful because: Increased circle of knowledge Increased accuracy level Data redundancy To compensate for sensor nodes’ failing
Sensor Networks: Dissemination Two ways for data dissemination: Query driven: sink broadcasts one query and sensor nodes send back a report in response Continuous update: sink node broadcasts one query and receives continuous updates in response (more energy consuming but more accurate) Problems: Intermediate nodes failing to forward a message Finding the shortest path (a routing protocol) Redundancy: a sensor may receive the same data packet more than once.
Sensor Networks: Advantages Coverage of a very large area through the scattering of thousands of sensors. Failure of individual sensors has no major impact on the overall network. Minimize human intervention and management. Work in hostile and unattended environments. Dynamically react to changing network conditions. E.g. Maintain connectivity in case of unexpected movement of the sensor nodes.
Sensor Networks: Recommended Introductory Reading I. F. Akyildiz, W. Su, Y. Sankarasubramaniam, E. Cyirci, “ A survey on Sensor Networks”, Computer Networks, 38(4):393-422, March 2002. Chee-Yong Chong, S. P. Kumar, “Sensor networks: evolution, opportunities, and challenges”, Proceedings of IEEE, pp 1247-1256, August 2003. IEEE tutorial