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Andrea G. Forte

Publications

Enhanced Mobility Support for Roaming Users: Extending the IEEE 802.21 Information Service (pdf) (WWIC 2010)
Many cell-phones and Personal Digital Assistants (PDAs) are equipped with multiple radio interfaces. Because of this, devices need to have ways of efficiently selecting the most suitable access network across multiple technologies based on the physical location of the device as well as user-defined parameters such as cost, bandwidth, and battery consumption. The IEEE has standardized the 802.21 framework for media-independent handovers where dynamic selection of network interfaces is an important feature. This paper describes and evaluates a novel architecture which extends the IEEE 802.21 information service. The architecture is based on a three-layer structure with Location-to-Service Translation (LoST) servers, service provider servers and independent evaluator servers. Evaluator servers are populated with information on coverage and quality of service as provided by trusted users. The proposed architecture allows for competition at all levels and scales well due to its distributed nature. A prototype has been developed and is presented in the paper.

Performance of Video-Chat Applications Under Congestion (pdf) (ISM 2009)
We study the performance of four popular IM clients focusing our attention on video-chat. In particular, we analyze how Skype, Windows Live Messenger, Eyebeam and X-Lite react to changes in available bandwidth, presence of HTTP and bit-torrent traffic and random packet losses.

vDelay: A Tool to Measure Capture-to-Display Latency and Frame Rate (pdf) (ISM 2009)
We present vDelay, a tool for measuring the capture-to-display latency (CDL) and frame-rate of real-time video applications such as video chat and conferencing. vDelay allows measuring CDL and frame-rate without modifying the source code of these applications. Further, it does not require any specialized hardware. We have used vDelay to measure the CDL and frame-rate of popular video chat applications such as Skype, Windows Live Messenger, and GMail video chat. vDelay can also be used to measure the CDL and frame-rate of these applications in the presence of bandwidth variations.

Distributed Delay Estimation and Call Admission Control in IEEE 802.11 WLANs (ICC 2009)
Voice over WiFi (VoWiFi) will soon be the main alternative to cellular phones. Providing a satisfactory user experience remains difficult, however. We focus on Call Admission Control (CAC) for both Constant Bit Rate (CBR) and Variable Bit Rate (VBR) VoIP traffic. Our approach is based on measuring the time between idle times. It requires no infrastructure changes, adds no probing traffic and has low complexity. We demonstrate through extended simulations that our approach achieves very good accuracy for both delay estimation and CAC when only VoIP sources are present and when both VoIP sources and data sources are present. Furthermore, we confirm TBIT performance through experiments.

Deployment Guidelines for Highly Congested IEEE 802.11b/g Networks (pdf) (LANMAN 2008)
Over the years, IEEE 802.11b/g wireless networks have been deployed in various locations such as hotels, airports and enterprises. Although IEEE 802.11b/g can be considered a mature technology, its deployment still presents challenges due to the limited number of non-overlapping channels available. This is particularly true in scenarios with a high density of users where a large number of APs covering roughly the same area is required. Through measurements we investigate different deployment scenarios, trying to provide a set of guidelines for the deployment of IEEE 802.11b/g networks so to minimize co-channel interference and maximize throughput. This, when the number of APs required to cover an area is larger than the number of non-overlapping channels available. In particular, we show how using partially overlapping channels causes lower retry rate and higher throughput than if deploying multiple APs on each of the non-overlapping channels.

Template-based Compression for Push-to-talk over Cellular (PoC) (IPTComm 2008)
The Session Initiation Protocol (SIP) has been chosen as the standard signaling protocol for the IP Multimedia Subsystem (IMS). SIP is a text-based protocol with messages often exceeding 1000 bytes in size, thus causing high call set-up delays on low bit-rate links.
Signaling Compression (SigComp) is currently the only option cellular operators have for the compression of signaling messages. We study the performance of SigComp, showing that SigComp cannot achieve the level of compression required by Push-To-Talk over Cellular (PoC) services in the IMS. Furthermore, we propose an alternative compression mechanism, namely Template Based Compression (TBC), and show through measurements how we can achieve higher compression ratios than SigComp, satisfying the requirements for PoC on low bit-rate links.

Cooperation Between Stations in Wireless Networks (ICNP 2007)
Many approaches have been proposed over the years for achieving fast handoffs in wireless networks. All of these approaches, however, introduce significant changes in the infrastructure and/or the protocol and almost none of them deals with the most recent authentication mechanisms such as IEEE 802.11i. IEEE 802.11r (work in progress) does address authentication and QoS for fast handoff, however, this too requires changes to both the infrastructure and the protocol.
We propose a novel approach, namely Cooperative Roaming, for fast Layer 2 and Layer 3 handoffs, fast application layer handoff and load balancing that potentially works in any wireless environment, with ANY kind of authentication mechanism. With the proposed approach we can achieve, on average, a L2+L3 handoff time of about 16 ms in an open network. When authentication is used (e.g.,IEEE 802.11i), the L2+L3 handoff time is about 21 ms. These handoff delays are less than half of the 50 ms requirement for achieving seamless handoffs for real-time traffic such as VoIP.

IEEE 802.11 in the Large: Observations at an IETF Meeting
IEEE 802.11 wireless networks have proven to be not reliable in highly congested environments. A lot of studies have shown many shortcomings of the current standards. We wanted to see how things really worked in highly congested environments, so we decided to analyze the wireless IEEE 802.11 network deployed at the 65th IETF meeting in Dallas. We found many problems in the network and in the clients in terms of throughput, handoff policies and interference.

Passive Duplicate Address Detection (DAD) for Fast Address Acquisition using Dynamic Host Configuration Protocol (DHCP) (GLOBECOM 2006)
The current DAD procedure has two major problems: one is the significant delay it introduces in the IP address assignment process; the other is that with the deployment of Windows XP SP2 (which disables by default answers to incoming ICMP echo requests), the DAD procedure is not much reliable any longer. We suggest a passive DAD (pDAD) procedure which solves both of the aforementioned problems and introduces other benefits.

Fast Layer 3 Handoff (WICON 2006)
We considered two different approaches. The first one involves no protocol changes, focusing only on the client side. This gives us a sub-optimal solution. The second approach involves the introduction of new network elements in the infrastructure which permit a seamless Layer 3 handoff for VoIP over 802.11 networks.

Improving the VoIP channel capacity in IEEE 802.11 networks (WCNC 2005)
In IEEE 802.11b networks the channel capacity is very limited due to inefficiencies of the MAC layer protocol. We propose a new approach based on the Point Coordination Function (PCF) achieving an improvement of up to 20% in capacity. We used the QualNet network simulator to implement our new algorithm.

Fast Layer 2 Handoff in IEEE 802.11a/b/g networks (MobiWAC'04)
We implemented our fast handoff algorithm in the HostAP wireless driver and used the Columbia University SIPz client (SIP client for handheld devices) to test it on the Zaurus PDA. Our new algorithm was patented by Columbia University and the related paper "Reducing MAC Layer Handoff Latency in IEEE 802.11 Wireless LANs" was acceppted at ACM MobiWac 2004.
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