Three problems with internetworking in cellular networks
Abstract: Over the last decade, cell phones have gone from providing the traditional interactive voice service to also include support for Internet services. Cellular networks that originally were designed for cost-efficient voice services supply low bandwidth, high error rates and long round-trip delays, which poorly fit the generous header sizes, lack of error tolerance and reliance on retransmissions of the Internet. In order to enable cost-efficient Internet connectivity with sufficient service quality in cellular networks, problematic mechanisms in the Internet must be identified and redesigned. The size of the existing Internet calls for solutions that can be deployed only where needed without requiring a major update of the Internet. In this thesis, three specific problems are described together with proposed solutions. Focus is on IP-based voice services, since this is identified as a key application in future cellular networks. The first problem is related to the low bandwidth and high error rates of cellular networks. Real-time applications tend to encode their data with codecs that are in some degree error-tolerant. Interactive cellular voice services use this to a large extent to support usage of data that have been exposed to interference. Using data despite errors is however more problematic in the Internet, since the transport protocol used for this type of service, UDP, discards all datagrams with errors. To address this problem, we introduce a new variant of UDP known as UDP Lite, which provides increased flexibility for applications that prefer partially damaged datagrams over discarded ones. Due to the close relationship between UDP Lite and UDP, UDP Lite is simple to implement. In combination with header compression, UDP Lite enables cost-efficient IP-based voice services in a cellular phone network. The second problem is related to the introduction of new header compression algorithms, which suffers from long delay due to standardization and deployment. This prevents efficient compression of application-level headers that can be large. Even if headers can be compressed in theory, the delay until the algorithm is widespread is too high. We propose a framework in which header compression algorithms can be expressed with platform-independent descriptions. Descriptions of header compression algorithms are dynamically installed and removed when needed. These descriptions are retrieved either from the application, the link peer, or from a well-known server. The third problem addresses a specific problem in the design of the Internet protocol stack - the lack of communication between the link layer and upper layers. With knowledge about the properties of the data in a link layer frame, expensive wireless bandwidth in cellular systems could be used more efficiently. This problem has been addressed in many papers that focus on particular applications or protocols. We propose a generic system called HAN that enables cross-layer interaction to support intelligent decision-making where needed. By sending hints to, and getting notifications from, the link layer, better performance can be obtained in parts of the Internet where HAN is supported. The solutions proposed in this thesis all aim at being generic and backward compatible with existing Internet technology. The UDP Lite protocol is currently being standardized within the IETF. The combination of UDP Lite and header compression is likely to exist in upcoming 3G networks. The work with a header compression framework and cross-layer interaction is still in progress.
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