Overview


The goal of the MCML is to investigate issues in mobile computing and multimedia to devise innovative solutions to the technical challenges in these areas. Both areas face performance limitations and require optimizations for the unique characteristics of these media. Furthermore, solutions to problems that arise when these two areas are integrated are being investigated.


Mobile Computing

Emerging trends in technology indicate that future generation communication systems will consist of a high speed wired backbone and wireless Local Area Networks attached to the periphery of the network. Wireless LANs extend the coverage of broadband services and provide ubiquitous network access to mobile users. There are, however, many technical challenges to overcome before the vision of ubiquitous computing can be realized.

Host mobility and wireless access are two emerging design considerations which pose challenging problems at all layers of the networking protocol stack. In the MCML, research is being conducted using simulation, measurement, implementation, and anaylsis to develop feasible solutions to the operational and performance challenges faced by mobile computing.

Routing implementations demonstrate feasiblity of theoretical solutions and provide a test bed for performance measurements, implementations of migration between wired and wireless networks, and disconnected file access. Mobile computing simulations enhance the experimental data and enable research about large scale and highly dynamic mobile networking topologies, such as ad-hoc networks.

Routing mechanisms

We have implemented a wireless subnet, on which we conducted performance experiments and implement new solutions. This subnet is connected to the Internet via a Mobile IP routing solution. This solution exploits IP's Loose Source Route option to enable the current set of Internet standards to support routing to mobile hosts. The scheme requires the addition of two new entities, Mobile Routers and Mobile Access Stations, which perform all required mobility-aware functions, such as address translation, user tracking and location management. No modifications to existing host or router software are required.

Ad-hoc networks

We have also analyzed routing issues in ad-hoc networks and have developed solutions which allow a network of mobile hosts to communicate without centralized control. The mobile hosts must dynamically detect and adapt routes to the other hosts in the ad-hoc network. Each host advertises the shortest path from themselves to all other hosts. Each station then chooses amongst its neighbors for the best path to its destination. Due to the highly dynamic nature of these network, timestamps are required to avoid deadlock and looping.

Host migration between wired and wireless networks

Protocol enhancements have been added to Mobile IP to enable a mobile host to move from a wireless connection to a wired connection. This type of movement is desirable for a host which has multiple types of network connections, so that the highest available bandwidth can be utilized. In order for this type of migration to function without disrupting any current transport sessions, the packets must be routed to the original connection address and then redirected to the new address. The challenge here exists due to the two different addresses, wired and wireless, that the mobile host maintains.

Integration of wireless and multimedia

Video over wireless networks presents additional challenges due to the limited bandwidth available, the higher loss rates, and the temporary periods of disconnectivity from the network. In order to make video displays feasible on mobile hosts, new methods of cell encoding and compression must be developed which allow reconstruction of a video frame when losses do occur. Retransmission is not possible due to the expensive bandwidth and the real time demands. This requires altering the current compression methods and implementing protocols to control the flow of video packets over the wireless link.

Performance of wireless networks

The mobile routing solution which has been implemented provides seamless connectivity between the wireless subnet and the backbone wired systems. However, current mobile routing solutions do not provide performance transparency. Performance over wireless links is limited by low bandwidth and high error rates.

The performance of transport protocols, such as TCP and NFS, is suboptimal on wireless systems since these protocols were adopted from use in wired networks. In the wireless environment, for example, TCP incorrectly interprets packet losses as a sign of congestion in the network and immediately throttles packet transmissions. This causes unnecessary performance degradation and loss of throughput.

NFS responds to any losses with idle times of at least several seconds. These type of recovery mechanisms limit the performance of wireless systems. Modifications to these protocols are being developed which maintain compatability while improving performance. By studying these characteristics, performance of current wireless systems can be improved while maintaining interoperability with existing wired systems, and important principles can be extracted for use in the design of the next generation of wireless systems.

Disconnected file access

Continuous file system access is required by mobile computers even while disconnected from the host subnet. File systems such as NFS and AFS do not provide disconnected file service. We are evaluating extensions to these protocols which enable disconnected file service. A study to determine the applicability of standard database techniques in logging and crash recovery to the mobile environment in also underway.

Simulation Development

Interest in mobile computing is growing rapidly and future networks can expect to support numerous mobile users. Simulation is essential to test the scalability of mobile computing solutions. The characteristics of mobile computers differ dramatically from the fixed network hosts. The mobility, lower bandwidth, and periods of burst errors or temporary network disconnection greatly impact the performance of mobile networks.

A mobile routing simulator module is under development. It will be integrated into the existing routing simulator, MaRS. This will enable simulation of topology changes, high degrees of mobility, and error behaviors which are unique to wireless networking. Simulations with large numbers of mobile hosts moving through a wide area network can be studied as well as ad hoc networking topologies. This will enable simulation and analysis of routing algorithms for mobile networks. The goal of this project is to study the performance of large scale and highly dynamic mobile networks.


Multimedia

Multi-media communication is an integral part of modern communication. Video is a key component of this communication. The real time and high bandwidth demands of video challenge current technology.


Channel Dependent Packet Scheduling

One proposal improves the retransmission and loss recovery mechanisms that are suboptimal on wireless systems. Since the proposed solution requires enhancements only at the wireless link layer, it is applicable to all transport protocols. We use a channel dependent packet scheduler (CDPS) which takes wireless channel characteristics into consideration in making packet dispatching decisions. If a certain mobile host is currently moving through a fading zone and can not receive a clear signal, an optimal shedule would wait for the channel to return to a good state prior to retransmitting the lost packets. Our results show that the CDPS technique provides improved throughput, better channel utilization, and fairness among multiple TCP streams.


fwmiller@cs.umd.edu