Markku Kojo

Optimizing World-Wide Web for weakly connected mobile workstations: An indirect approach

Modern portable computers and wireless connections over cellular telephone networks have created a new platform for distributed information processing. We have designed a communication architecture that makes it possible to exploit the existing TCP/IP communication protocols but that also takes into account specific features of cellular links. Our communication architecture is based on the concept of indirect interaction. The mediating interceptor is the bridge between the worlds of wireless and wireline communication. It also provides enhanced functionality that improves fault-tolerance and performance. In this paper we demonstrate how the architecture is used to improve the performance of the WWW information browsing. Similar solution methods can be applied to other existing applications and protocols.<<ETX>>

F-RTO: an enhanced recovery algorithm for TCP retransmission timeouts

Spurious TCP retransmission timeouts (RTOs) have been reported to be a problem on network paths involving links that are prone to sudden delays due to various reasons. Especially many wireless network technologies contain such links. Spurious retransmission timeouts often cause unnecessary retransmission of several segments, which is harmful for TCP performance. Recent proposals for avoiding unnecessary retransmissions after a spurious RTO require use of TCP options which must be implemented and enabled at both ends of teh connection. We introduce a new TCP sender algorithm for recovery after a retransmission timeout and show that unnecessary retransmissions after a spurious retransmission timeout, improving the TCP performance considerably. The algorithm is friendly towards other TCP connections, because it follows the congestion control principles and injects packets to the network at same rate as a conventional TCP sender. We implemented the algorithm and compared its performance to conventional TCP and Eifel TCP when RTOs occurred either due to sudden delays or due to packet losses. The results show that our algorithm either improves performance or gives similar througput as the other TCP variants evaluated in different test cases.

Connecting Mobile Workstations to the Internet Over a Digital Cellular Telephone Network

Modern portable computers and wireless connections over a cellular telephone network have created a new platform for distributed information processing. We present a communication architecture framework which makes it possible to exploit the existing TCP/IP communication architecture but which also takes into account the specific features of wireless links. Our communication architecture is based on the principle of indirect interaction. The mediating interceptor, Mobile-Connection Host, is the bridge between the worlds of wireless and wireline communication. The interceptor also provides enhanced functionality that improves fault-tolerance and performance for applications aware of mobility. Prototypes of the architecture are implemented both for the Unix (Linux) and for the Windows (3.11) platform.

An efficient transport service for slow wireless telephone links

Modern digital cellular telephones and portable computers have created a new platform for distributed information processing. However, the characteristics of wireless telephone links are different from those of wireline links. With standard TCP/IP protocols, this can lead to severe performance problems; some are related to the control of the wireless link, some to the cooperation of the wireless link and the fixed network. One possible solution is to split the end-to-end communication path into two parts, and to establish a separate control for each part. The Mowgli communication architecture is a sophisticated elaboration of this basic idea covering several data communication layers. One of its main components is the Mowgli data channel service (MDCS), which transparently replaces the standard TCP/IP core protocols over the slow wireless link. We discuss how the Mowgli approach, using the MDCS, alleviates the problems encountered with TCP/IP protocols over slow wireless links. The results of our performance tests indicate the merits of the Mowgli approach. The transfer times and the response times become more stable, transfer times for multiple parallel bidirectional transfers are substantially reduced, and response times in interactive work can be kept at a low and predictable level, even when there is other traffic on the wireless link.

Measured performance of data transmission over cellular telephone networks

Recent developments in mobile communication and personal computer technology have laid a new foundation for mobile computing. Performance of the data communication system as seen by an application program is a fundamental factor when communication infrastructure at the application layer is designed. This paper provides results of performance measurements of data transmission over two different cellular telephone networks, a digital GSM-network and an analogue NMT-network. Since our emphasis is on performance as seen by application programs, we use the standard TCP/IP protocols in the measurements. The performance is measured using three basic operations: establishment of a wireless dial-up connection, exchange of request-reply messages, and bulk data transfer. The external conditions under which the measurements were carried out present a normal office environment when the field strength of the cellular link is good or fairly good.

An experimental study of home gateway characteristics

Many residential and small business users connect to the Internet via home gateways, such as DSL and cable modems. The characteristics of these devices heavily influence the quality and performance of the Internet service that these users receive. Anecdotal evidence suggests that an extremely diverse set of behaviors exists in the deployed base, forcing application developers to design for the lowest common denominator. This paper experimentally analyzes some characteristics of a substantial number of different home gateways: binding timeouts, queuing delays, throughput, protocol support and others.

Mowgli WWW software: improved usability of WWW in mobile WAN environments

Today the World-Wide Web is the most widely used distributed application. By utilising the data services of cellular telephone systems such as the digital GSM, WWW can be brought to nomadic users. However, the characteristics of cellular telephone links differ greatly from wire-line links. The narrow bandwidth, highly variable transmission delays, and sudden disconnections create problems for many Internet applications. WWW uses the HTTP and TCP/IP protocols, which exhibit a number of usability and performance problems in wide-area mobile networks. Mowgli WWW, a WWW middleware implementation, exploits several new techniques to solve these problems. Although Mowgli WWW was initially designed for a wireless WAN environment, the techniques are also profitable in fixed networks when slow or high-latency communication links are involved.

Mowgli: improvements for Internet applications using slow wireless links

Modern cellular telephone systems extend the usability of portable personal computers enormously. A nomadic user can be given ubiquitous access to remote information stores and computing services. However, the behavior of wireless links creates severe inconveniences within the traditional data communication paradigm. We give an overview of the problems related to wireless mobility. We also present a new software architecture for mastering the problems and discuss a new paradigm for designing mobile distributed applications. The key idea in the architecture is to place a mediator, a distributed intelligent agent, between the mobile node and the wireline network.

Using Quick-Start to Improve TCP Performance with Vertical Hand-offs

Vertical hand-offs between different wireless access technologies have become more relevant after the introduction of multi-access mobile terminals with wireless LAN (WLAN) and wireless WAN (WWAN) technologies. While the IP mobility mechanisms are rather well known, the performance of TCP still has problems when moving between WLAN and WWAN accesses. First, with a high-latency WWAN link technology such as GPRS it takes several seconds before the TCP congestion window has reached the path capacity. Second, when the notification of the first packet loss arrives at the TCP sender, several packets have already been lost due to the slow-start overshoot and the TCP sender needs to retransmit a large number of the packets from the last transmission window. Third, after a vertical hand-off the path characteristics might have changed dramatically in which case the TCP congestion control state is not valid any more. In this paper we investigate Quick-Start, a mechanism for avoiding the initial slow-start delay, in the context of wireless multi-access terminals. We also propose an enhancement to Quick-Start to alleviate the effects of slow-start overshoot and apply Quick-Start after a vertical hand-off to quickly learn the available capacity on the new end-to-end path. An explicit cross-layer hand-off notification is employed to trigger Quick-Start when the hand-off completes. We conduct simulations with different hand-off models, and our simulations yield promising results with Quick-Start

Toward network controlled IP traffic offloading

Operator controlled IP traffic offloading in cellular networks has been a lively topic in both product and standard development during recent years. Specifically, 3GPP has developed multiple IP traffic offloading solutions for their system architecture, typically requiring 3GPP-specific modifications for user equipment. We argue that an adequate lightweight access technology agnostic IP traffic offloading system is achievable with a generic internet layer solution. Such a solution can still utilize 3GPP system properties for operators to push offloading policies securely into mobile devices. We discuss existing 3GPP-specified offloading solutions and present three variations of our internet layer traffic offloading implementations, and compare them against 3GPP-specified solutions. Our hands-on experience in operator networks shows that such a direction is feasible and promising.