Ulf Körner

An enhancement to the IEEE 802.11e EDCA providing QoS guarantees

One of the challenges that must be overcome to realize the practical benefits of ad hoc networks is quality of service (QoS). However, the IEEE 802.11 standard, which undeniably is the most widespread wireless technology of choice for WLANs and ad hoc networks, does not address this issue. In order to support applications with QoS requirements, the upcoming IEEE 802.11e standard enhances the original IEEE 802.11 MAC protocol by introducing a new coordination function which has both contention-based and contention-free medium access methods. In this paper, we consider the contention-based medium access method, the EDCA, and propose an extension to it such that it can be used to provide QoS guarantees in WLANs operating in ad hoc mode. Our solution is fully distributed, uses admission control to regulate the usage of resources and gives stations with high-priority traffic streams an opportunity to reserve time for collision-free access to the medium.

A Distributed Scheduling Algorithm for a Bluetooth Scatternet

This paper deals with bandwidth allocation in a Bluetooth scatternet. the proposed assignment of transmission rights provides a controlled traffic transfer between bluetooth piconets and represents a key component in an ad-hoc wireless Bluetooth network in order to provide with efficient data packet passing within and across piconets. This assignment is obtained by executing a distributed algorithm in each node. The algorithm works for unrestricted topologies of both stationary and mobile networks and also deals with assignment changes caused by network re-configurations without the use of a centralized factor. The effect of this re-assignment can be limited to the locality of that topology change. It is also shown that the construction of a link schedule of minimum length in a Bluetooth scatternet is NP-complete.

Performance of internet access solutions in mobile ad hoc networks

Although an autonomous mobile ad hoc network (MANET) is useful in many scenarios, a MANET connected to the Internet is more desirable. This interconnection is achieved by using gateways, which act as bridges between a MANET and the Internet. Before a mobile node can communicate with an Internet host it needs to find a route to a gateway. Thus, a gateway discovery mechanism is required. In this paper the MANET routing protocol Ad hoc On-Demand Distance Vector (AODV) is extended to achieve the interconnection between a MANET and the Internet. Moreover, the paper investigates and compares three approaches for gateway discovery. The question of whether the configuration phase with the gateway should be initiated by the gateway, by the mobile node or by mixing these two approaches is being discussed. We have implemented and simulated these three methods and we discuss the advantages and disadvantages of the three alternatives.

TCP/IP over the Bluetooth Wireless Ad-hoc Network

Bluetooth is a wireless ad-hoc network concept that was presented in February 1998 by its five original promoters Ericsson, Nokia, IBM, Toshiba and Intel. With Bluetooth, mobile terminals within range of each other can set up ad-hoc connections for both synchronous traffic, e.g. voice, and asynchronous traffic, e.g. IP-based data traffic. In this paper we analyse how well Bluetooth can carry TCP/IP traffic and in particular we show that though the radio channel is very disturbed the TCP Vegas protocol with its flow control mechanism can be carried very well. With ARQ handled at the Bluetooth level, retransmissions are made immediately after a packet error and thus the delays, normally introduced are kept acceptably short. In our model important mechanisms in TCP Vegas as well as Bluetooth mechanisms are modelled in detail and we show that TCP throughput is quite high and delays are kept short for packet error probabilities up to 50% and moderate loads.

Traffic engineering problems in multiservice circuit switched networks

This paper overviews traffic engineering problems encountered in the design of multiple networks. We mainly inspect the problem of introducing an n x 64kbit/s service into a non-hierarchial telephone network and focus on network dimensioning.

End to end delays in a catenet environment

This paper presents a hierarchical model for a catenet environment. The model consists of three levels of models, and it reflects the end to end delay between two host computers each connected to a different LAN. The two LANs are connected via gateways by a WAN. The model incorporates a basic flow control mechanism, standardized local area network behaviour, as well as gateway functions in terms of packet fragmentation and reassembly. The model can be used to obtain performance measures such as the mean end to end delay and the systems throughput as a function of parameters such as arrival rate of packets, maximum window size, and traffic mix.

Providing QoS in ad hoc networks with distributed resource reservation

As the use of WLANs based on IEEE 802.11 increases, the need for QoS becomes more obvious. The new IEEE 802.11e standard aims at providing QoS, but its contention-based medium access mechanism, EDCA, provides only service differentiation, i.e. soft QoS. In order to provide hard QoS, earlier we have proposed an extension called EDCA with resource reservation (EDCA/RR), which enhances EDCA by offering also hard QoS through resource reservation. In this paper, we extend EDCA/RR to cope with the hidden terminal problem, outline a solution for multi-hop scenarios, and compare the proposed scheme with EDCA.

Improving Wi-Fi based indoor positioning using Particle Filter based on signal strength

Indoor positioning is recognized as one of the upcoming major applications which can be used in wide variety of applications such as indoor navigation and enterprise asset tracking. The significance of localization in indoor environments have made the use of Wi-Fi based indoor positioning so that it can utilize available current wireless infrastructure and perform positioning very easily. In this paper we introduced a user friendly prototype for Wi-Fi based indoor positioning system where a user can identify its own position in indoor. Wi-Fi received signal strength (RSS) fluctuations over time introduce incorrect positioning. To minimize the fluctuation of RSS, we developed Particle Filters with the prototype. A comparison between with and without Particle Filter for error performance is presented and at the same time it is also noticed that variation in number of particles could change the positioning accuracy. Moreover comparison between calibration data in all directions and in one direction while constructing a radio map is presented.

Urban mesh and ad hoc mesh networks

Mesh networking is currently gaining much attention, within both academia and industry. Mesh networking allows cheap and fast deployment of wireless services. It is regarded as a very promising solution for urban deployment scenarios as well as for temporary emergency response situations. Another related promising field is that of ad hoc wireless networking, which consists of mobile nodes that dynamically create and maintain a network without the need for any infrastructure. We propose a solution and architecture for urban mesh ad hoc networks, a network that combines mesh networking with ad hoc networks for urban environments. We present four types of ad hoc mesh and ad hoc mesh networks. The most general one consists of mesh nodes, called mesh points (MP), that act as a type of access point for user nodes (UN). The MPs have at least two interfaces: one which is used to communicate with UNs, and one which is used to maintain the mesh access network and transport data. These two interfaces can basically use any type of technology (IEEE 802.11 a/b/g, WiMax, etc.), and for capacity reasons it is generally regarded that the best solution is to let the mesh interface operate on a separate high-capacity channel or channels. An intricate part of these types of networks are routing and location services. In our solution, UN devices operate in ad hoc mode running an ad hoc routing protocol. This allows UNs that wish to communicate to connect directly in an ad hoc manner, or through an MP. An important question is therefore whether two UNs that wish to communicate should connect through the mesh or connect directly. We show that from a capacity point of view whether a UN should route its packets to the closest available MP, or through a ad hoc network, depends on the environment the network is located, the amount traffic and the type of protocols used. Since MPs need to know where to route packets within the mesh, i.e., locating the MP closest to the destination UN, each UN run a small application that registers the UN to the mesh network. In addition to the above features we have developed a new MAC that quickly queries two candidate nodes, which picks the candidate with the currently best radio conditions. This enable nodes to cope with deep dips in signal strength due to fast fading, a well-known problem in urban environments. We show that this new protocol achieves significantly lower delays. We also show that in dense urban environments performance and battery lifetime can be improved if ad hoc technologies are used.

Performance of information and communication systems

Reliability and maintenance of telecommunications equipment is evolving and continues to evolve. Improved hardware, development of software engineering principles, and better understanding of procedures have reduced system downtime. This is reflected in more stringent downtime specifications in the telecommunications industry. The makeup of failures leading to downtime has also changed. Advances in digital equipment have dramatically reduced hardware problems. Although software has also improved, more is demanded of it, and it has not improved at the same rate as hardware. Procedural techniques have also improved--better user interfaces, and improvements in process have led to fewer failures. However, maintenance personnel maintain more equipment than before, and in consequence, procedural failure rates have not improved as fast as those for hardware. Software and procedural problems--not hardware--are now the dominant reasons for outages. Not only do they cause most outages, but the public perceives these outages to be worse. Yet the current in-service auditing of telecommunications equipment may still be based on a paradigm of preventing outages caused by relatively unreliable hardware. Auditing--the inspection and testing of communications equipment--is performed on a regular basis. The purpose of auditing telecommunications switching equipment is to improve system reliability. For duplex equipment (equipment consisting of two identical units, one of which takes over if the other breaks down) auditing takes place on both the active and inactive units. If problems are found, repairs can be made before service is impacted. Despite this clear benefit, audits also incur costs. Auditing invokes software which itself has a failure rate. Audit testing can lead to two types of incorrect conclusion, both of which can lead to unnecessary repairs, or to a dangerous misperception that the equipment is working properly. The first type of incorrect conclusion is a false positive--the audit shows there is a problem Performance of Information and Communication Systems U. Komer & A. Nilsson (Eds.) @ 1998 IFIP. Published by Chapman & Hall 4 Part One ArM Switch Performance when none exists. The second is a false negative--the audit shows there is no problem when one exists. Reducing the incorrect audit conclusions will reduce the dominant failures of today--software and procedural. Since audits incur both costs and benefits, the natural conclusion is that audits should be run neither too frequently nor too infrequently. Yet there is a lack of guidelines on how often to perform audits. Our hypothesis is that audits are run too frequently for todays mix of failures. We develop a detailed reliability model of a generic duplex module for switching equipment. Our intent is to provide a useful methodology and some results that are independent of any manufacturers equipment. The model shows that selecting the correct audit frequency saves significantly on downtime and maintenance costs. The results of this modeling were then applied to live equipment in the field. The reported results are very encouraging--81 % less outages, 79% less software traps, and 39% fewer hardware failure reports.