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We present an autoregressive model for modeling samples autocorrelation from the same access point in WLAN location determination systems. Our work is in the context of the Horus system, which is a probabilistic WLAN location determination system. We show that the autocorrelation between consecutive samples from the same access point can be as high as 0.9. Using our model, we describe a technique to use multiple signal strength samples from each access point, taking the high autocorrelation into account, to achieve better accuracy. Implementation of the technique in the Horus system shows that the average system accuracy is increased by more than 50%. Our results show that assuming independence of samples from the same access point can lead to degraded performance as the number of samples used in the estimation algorithm is increased, due to the wrong independence assumption. We also discuss how to incorporate the new technique with other algorithms for enhancing the performance of WLAN location determination systems. (UMIACS-TR-2003-75)

This paper presents a general analysis for the performance of WLAN location determination systems. In particular, we present an analytical method for calculating the average distance error and probability of error of WLAN location determination systems. These expressions are obtained with no assumptions regarding the distribution of signal strength or the probability of the user being at a specific location, which is usually taken to be a uniform distribution over all the possible locations in current WLAN location determination systems. We use these expressions to find the optimal strategy to estimate the user location and to prove formally that probabilistic techniques give more accuracy than deterministic techniques, which has been taken for granted without proof for a long time. The analytical results are validated through simulation experiments. We also study the effect of the assumption that the user position follows a uniform distribution over the set of possible locations on the accuracy of WLAN location determination systems. The results show that knowing the probability distribution of the user position can reduce the number of access points required to obtain a given accuracy. However, with a high density of access points, the performance of a WLAN location determination system is consistent under different probability distributions for the user position. UMIACS-TR-2003-29

This paper presents a general analysis for the performance of WLAN location determination systems. In particular, we present an analytical method for calculating the average distance error and probability of error of WLAN location determination systems. These expressions are obtained with no assumptions regarding the distribution of signal strength or the probability of the user being at a specific location, which is usually taken to be a uniform distribution over all the possible locations in current WLAN location determination systems. We use these expressions to find the optimal strategy to estimate the user location and to prove formally that probabilistic techniques give more accuracy than deterministic techniques, which has been taken for granted without proof for a long time. The analytical results are validated through simulation experiments. We also study the effect of the assumption that the user position follows a uniform distribution over the set of possible locations on the accuracy of WLAN location determination systems. The results show that knowing the probability distribution of the user position can reduce the number of access points required to obtain a given accuracy. However, with a high density of access points, the performance of a WLAN location determination system is consistent under different probability distributions for the user position. UMIACS-TR-2003-29

We present an autoregressive model for modeling samples autocorrelation from the same access point in WLAN location determination systems. Our work is in the context of the Horus system, which is a probabilistic WLAN location determination system. We show that the autocorrelation between consecutive samples from the same access point can be as high as 0.9. Using our model, we describe a technique to use multiple signal strength samples from each access point, taking the high autocorrelation into account, to achieve better accuracy. Implementation of the technique in the Horus system shows that the average system accuracy is increased by more than 50%. Our results show that assuming independence of samples from the same access point can lead to degraded performance as the number of samples used in the estimation algorithm is increased, due to the wrong independence assumption. We also discuss how to incorporate the new technique with other algorithms for enhancing the performance of WLAN location determination systems. (UMIACS-TR-2003-75)

This paper presents a general analysis for the performance of WLAN location determination systems. In particular, we present an analytical method for calculating the average distance error and probability of error of WLAN location determination systems. These expressions are obtained with no assumptions regarding the distribution of signal strength or the probability of the user being at a specific location, which is usually taken to be a uniform distribution over all the possible locations in current WLAN location determination systems. We use these expressions to find the optimal strategy to estimate the user location and to prove formally that probabilistic techniques give more accuracy than deterministic techniques, which has been taken for granted without proof for a long time. The analytical results are validated through simulation experiments. We also study the effect of the assumption that the user position follows a uniform distribution over the set of possible locations on the accuracy of WLAN location determination systems. The results show that knowing the probability distribution of the user position can reduce the number of access points required to obtain a given accuracy. However, with a high density of access points, the performance of a WLAN location determination system is consistent under different probability distributions for the user position. UMIACS-TR-2003-29

We present an autoregressive model for modeling samples autocorrelation from the same access point in WLAN location determination systems. Our work is in the context of the Horus system, which is a probabilistic WLAN location determination system. We show that the autocorrelation between consecutive samples from the same access point can be as high as 0.9. Using our model, we describe a technique to use multiple signal strength samples from each access point, taking the high autocorrelation into account, to achieve better accuracy. Implementation of the technique in the Horus system shows that the average system accuracy is increased by more than 50%. Our results show that assuming independence of samples from the same access point can lead to degraded performance as the number of samples used in the estimation algorithm is increased, due to the wrong independence assumption. We also discuss how to incorporate the new technique with other algorithms for enhancing the performance of WLAN location determination systems. (UMIACS-TR-2003-75)

We present an indoor location determination system based on signal strength probability distributions for tackling the noisy wireless channel and clustering to reduce computation requirements. We provide two implementation techniques, namely, Joint Clustering and Incremental Triangulation and describe their tradeoffs in terms of location determination accuracy and computation requirement. Both techniques have been incorporated in two implemented context-aware systems: User Positioning System and the Rover System, both running on Compaq iPAQ Pocket PC's with Familiar distribution of Linux for PDA's. The results obtained show that both techniques give the user location with over 90% accuracy to within 7 feet with very low computation requirements, hence enabling a set of context-aware applications. Also UMIACS-TR-2002-30

A model for the specification and analysis of communication protocols called Systems of Communicating Machines is used to specify an IEEE 802.11 Point Coordination Function protocol, and to analyze it for safety and certain restricted liveness properties. The model uses a combination of finite state machines and variables in the specification of each machine, and the communication between machines is accomplished through shared variables. Enabling predicates and actions are associated with each transition; the enabling predicates determine when a transition may be taken, and the actions alter the variable values as the network progresses. One of the advantages which this model has over most other formal description techniques is that simultaneous transitions are allowed. Another advantage is the use of shared variables rather than FIFO queues for communication between machines. This allows the modeling of the shared medium as a single shared variable variable between all communicating processes. Unlike the SDL language which is used in the specification of the 802.11 standard, the Systems of Communicating Machines model is more compact, easier to understand, and easier to analyze for safety and liveness. (Also UMIACS-TR-2002-36)

We present an indoor location determination system based on signal strength probability distributions for tackling the noisy wireless channel and clustering to reduce computation requirements. We provide two implementation techniques, namely, Joint Clustering and Incremental Triangulation and describe their tradeoffs in terms of location determination accuracy and computation requirement. Both techniques have been incorporated in two implemented context-aware systems: User Positioning System and the Rover System, both running on Compaq iPAQ Pocket PC's with Familiar distribution of Linux for PDA's. The results obtained show that both techniques give the user location with over 90% accuracy to within 7 feet with very low computation requirements, hence enabling a set of context-aware applications. Also UMIACS-TR-2002-30

A model for the specification and analysis of communication protocols called Systems of Communicating Machines is used to specify an IEEE 802.11 Point Coordination Function protocol, and to analyze it for safety and certain restricted liveness properties. The model uses a combination of finite state machines and variables in the specification of each machine, and the communication between machines is accomplished through shared variables. Enabling predicates and actions are associated with each transition; the enabling predicates determine when a transition may be taken, and the actions alter the variable values as the network progresses. One of the advantages which this model has over most other formal description techniques is that simultaneous transitions are allowed. Another advantage is the use of shared variables rather than FIFO queues for communication between machines. This allows the modeling of the shared medium as a single shared variable variable between all communicating processes. Unlike the SDL language which is used in the specification of the 802.11 standard, the Systems of Communicating Machines model is more compact, easier to understand, and easier to analyze for safety and liveness. (Also UMIACS-TR-2002-36)

We present an indoor location determination system based on signal strength probability distributions for tackling the noisy wireless channel and clustering to reduce computation requirements. We provide two implementation techniques, namely, Joint Clustering and Incremental Triangulation and describe their tradeoffs in terms of location determination accuracy and computation requirement. Both techniques have been incorporated in two implemented context-aware systems: User Positioning System and the Rover System, both running on Compaq iPAQ Pocket PC's with Familiar distribution of Linux for PDA's. The results obtained show that both techniques give the user location with over 90% accuracy to within 7 feet with very low computation requirements, hence enabling a set of context-aware applications. Also UMIACS-TR-2002-30

A model for the specification and analysis of communication protocols called Systems of Communicating Machines is used to specify an IEEE 802.11 Point Coordination Function protocol, and to analyze it for safety and certain restricted liveness properties. The model uses a combination of finite state machines and variables in the specification of each machine, and the communication between machines is accomplished through shared variables. Enabling predicates and actions are associated with each transition; the enabling predicates determine when a transition may be taken, and the actions alter the variable values as the network progresses. One of the advantages which this model has over most other formal description techniques is that simultaneous transitions are allowed. Another advantage is the use of shared variables rather than FIFO queues for communication between machines. This allows the modeling of the shared medium as a single shared variable variable between all communicating processes. Unlike the SDL language which is used in the specification of the 802.11 standard, the Systems of Communicating Machines model is more compact, easier to understand, and easier to analyze for safety and liveness. (Also UMIACS-TR-2002-36)

Many studies on measurement and characterization of wireless LANs have been performed recently. Most of these measurements have been conducted from the wired portion of the network based on wired monitoring or SNMP statistics. In this paper we argue that traffic measurements from a wireless vantage point in the network are more appropriate than wired measurements or SNMP statistics, to expose the wireless medium characteristics and their impact on the traffic patterns. While it is easier to make consistent measurements in the wired part of a network, such measurements can not observe the significant vagaries present in the wireless medium itself. As a consequence constructing an accurate measurement system from a wireless vantage point is important but usually quite difficult due to the noisy wireless channel. In our work we have explored the various issues in implementing such a system to monitor traffic in an IEEE 802.11 based wireless network. We show the effectiveness of the wireless monitoring by quantitatively comparing it with SNMP and measurements at wired vantage points. We also show the analysis of a typical computer science department network traffic using the wireless monitoring technique. Our analysis reveals rich information about the PHY/MAC layers of the IEEE 802.11 protocol such as the typical traffic mix of different frame types, their temporal characteristics, correlation with the user activities and the error characteristics of the wireless medium. Moreover, we identify anomalies in the operation of the IEEE 802.11 MAC protocol. Our results show excessive retransmissions of some management frame types reducing the useful throughput of the wireless network. We also find that some features of the protocol, which were designed to reduce the retransmission errors, are not used. In addition, most of the clients fail to adapt the data rate according to the signal condition between them and the access point, which further reduce the useful throughput. (

Many studies on measurement and characterization of wireless LANs have been performed recently. Most of these measurements have been conducted from the wired portion of the network based on wired monitoring or SNMP statistics. In this paper we argue that traffic measurements from a wireless vantage point in the network are more appropriate than wired measurements or SNMP statistics, to expose the wireless medium characteristics and their impact on the traffic patterns. While it is easier to make consistent measurements in the wired part of a network, such measurements can not observe the significant vagaries present in the wireless medium itself. As a consequence constructing an accurate measurement system from a wireless vantage point is important but usually quite difficult due to the noisy wireless channel. In our work we have explored the various issues in implementing such a system to monitor traffic in an IEEE 802.11 based wireless network. We show the effectiveness of the wireless monitoring by quantitatively comparing it with SNMP and measurements at wired vantage points. We also show the analysis of a typical computer science department network traffic using the wireless monitoring technique. Our analysis reveals rich information about the PHY/MAC layers of the IEEE 802.11 protocol such as the typical traffic mix of different frame types, their temporal characteristics, correlation with the user activities and the error characteristics of the wireless medium. Moreover, we identify anomalies in the operation of the IEEE 802.11 MAC protocol. Our results show excessive retransmissions of some management frame types reducing the useful throughput of the wireless network. We also find that some features of the protocol, which were designed to reduce the retransmission errors, are not used. In addition, most of the clients fail to adapt the data rate according to the signal condition between them and the access point, which further reduce the useful throughput. (

Many studies on measurement and characterization of wireless LANs have been performed recently. Most of these measurements have been conducted from the wired portion of the network based on wired monitoring or SNMP statistics. In this paper we argue that traffic measurements from a wireless vantage point in the network are more appropriate than wired measurements or SNMP statistics, to expose the wireless medium characteristics and their impact on the traffic patterns. While it is easier to make consistent measurements in the wired part of a network, such measurements can not observe the significant vagaries present in the wireless medium itself. As a consequence constructing an accurate measurement system from a wireless vantage point is important but usually quite difficult due to the noisy wireless channel. In our work we have explored the various issues in implementing such a system to monitor traffic in an IEEE 802.11 based wireless network. We show the effectiveness of the wireless monitoring by quantitatively comparing it with SNMP and measurements at wired vantage points. We also show the analysis of a typical computer science department network traffic using the wireless monitoring technique. Our analysis reveals rich information about the PHY/MAC layers of the IEEE 802.11 protocol such as the typical traffic mix of different frame types, their temporal characteristics, correlation with the user activities and the error characteristics of the wireless medium. Moreover, we identify anomalies in the operation of the IEEE 802.11 MAC protocol. Our results show excessive retransmissions of some management frame types reducing the useful throughput of the wireless network. We also find that some features of the protocol, which were designed to reduce the retransmission errors, are not used. In addition, most of the clients fail to adapt the data rate according to the signal condition between them and the access point, which further reduce the useful throughput. (

In this report we present the design of the Autonomous Transport Protocol (ATP). The basic service provided by ATP is a reliable transport connection between two endpoints (identified by content identifiers) independent of their physical location. Autonomy allows dynamic endpoints relocation on different end hosts without disrupting the transport connection between them. ATP depends on the existence of an underlying Instance-Based Network (IBN) to achieve its goals. An IBN provides the flexibility of having different instances of the same content. It is up to the user of the IBN network to define the relation between these instances. An IBN allows its user to to map a content to a particular node. Application endpoints can send messages to other content-identified endpoints. Routing in the IBN is instance-based; the IBN can route a message to a specific content instance or to the nearest instance, if no exact match is found for the destination content instance. Moreover, the IBN replicates the stored contents in order to provide fault tolerance and IBN nodes along the query path can cache a content to provide fast answers to future queries. The ATP layer in the intermediate nodes between the source and destination endpoints can actively participate in the connection, for example, to buffer data for the destination endpoints during periods of unavailability. Data is transferred by a combination of active and passive operations, where the ATP layer of a node can decide whether to actively push the data to the destination or to passively wait for the destination endpoint to pull the data. The decision to whether to use the active or passive modes can be taken by a local policy on the node running the ATP protocol. (UMIACS-TR-2003-52)

In this report we present the design of the Autonomous Transport Protocol (ATP). The basic service provided by ATP is a reliable transport connection between two endpoints (identified by content identifiers) independent of their physical location. Autonomy allows dynamic endpoints relocation on different end hosts without disrupting the transport connection between them. ATP depends on the existence of an underlying Instance-Based Network (IBN) to achieve its goals. An IBN provides the flexibility of having different instances of the same content. It is up to the user of the IBN network to define the relation between these instances. An IBN allows its user to to map a content to a particular node. Application endpoints can send messages to other content-identified endpoints. Routing in the IBN is instance-based; the IBN can route a message to a specific content instance or to the nearest instance, if no exact match is found for the destination content instance. Moreover, the IBN replicates the stored contents in order to provide fault tolerance and IBN nodes along the query path can cache a content to provide fast answers to future queries. The ATP layer in the intermediate nodes between the source and destination endpoints can actively participate in the connection, for example, to buffer data for the destination endpoints during periods of unavailability. Data is transferred by a combination of active and passive operations, where the ATP layer of a node can decide whether to actively push the data to the destination or to passively wait for the destination endpoint to pull the data. The decision to whether to use the active or passive modes can be taken by a local policy on the node running the ATP protocol. (UMIACS-TR-2003-52)

In this report we present the design of the Autonomous Transport Protocol (ATP). The basic service provided by ATP is a reliable transport connection between two endpoints (identified by content identifiers) independent of their physical location. Autonomy allows dynamic endpoints relocation on different end hosts without disrupting the transport connection between them. ATP depends on the existence of an underlying Instance-Based Network (IBN) to achieve its goals. An IBN provides the flexibility of having different instances of the same content. It is up to the user of the IBN network to define the relation between these instances. An IBN allows its user to to map a content to a particular node. Application endpoints can send messages to other content-identified endpoints. Routing in the IBN is instance-based; the IBN can route a message to a specific content instance or to the nearest instance, if no exact match is found for the destination content instance. Moreover, the IBN replicates the stored contents in order to provide fault tolerance and IBN nodes along the query path can cache a content to provide fast answers to future queries. The ATP layer in the intermediate nodes between the source and destination endpoints can actively participate in the connection, for example, to buffer data for the destination endpoints during periods of unavailability. Data is transferred by a combination of active and passive operations, where the ATP layer of a node can decide whether to actively push the data to the destination or to passively wait for the destination endpoint to pull the data. The decision to whether to use the active or passive modes can be taken by a local policy on the node running the ATP protocol. (UMIACS-TR-2003-52)