Peter Langendörfer

Vertical optimization of data transmission for mobile wireless terminals

A major problem for TCP connections over wireless links is that errors introduced by the wireless channel interfere with the TCP protocol, leading to reduced data rates and power wastage. Based on accurate simulations for the TCP and IEEE 802.11 MAC protocols, we discuss recipes to optimize transmission. It is argued that the best approach is to restrict modifications to the mobile device. While this requires separate solutions for the uplink and downlink, the results of optimization are then available when roaming into any WLAN obeying the relevant MAC protocol. Simulation results show that the combination of specific strategies with a vertical interaction between the protocol layers can lead to the required improvements, giving a promising approach to enhance the performance of wireless mobile terminals.

Design and optimization of high-performance protocols with the DO-IT toolbox

In the telecommunication industry, the Specification and Description Language (SDL) is a widely accepted technique to support the software development process. While several commercial SDL tools exist that focus on functional aspects, rather little research has been done concerning the integration of nonfunctional aspects in the development process.

Shielding TCP from Wireless Link Errors: Retransmission Effort and Fragmentation

A known problem for TCP connections over wireless links is that errors in the wireless channel interfere with the TCP protocol even for minor packet loss. In the first part of this paper we evaluate how the data rate reduction depends on the channel delay. For comparatively short delays in the order of 100 ms, the decrease of the throughout is noticeable but not dramatic. This indicates that the problem is not severe if the communication partners are located in the same WLAN or interact over a fast Internet connection. A significant throughput reduction arises in the case of a large network delay. Simulation results for the uplink transmission are presented as part of an overall strategy in which all improvements are made by optimizing the mobile end device only, an approach which allows performance improvements without any protocol modifications.

Automated protocol implementations based on activity threads

In this paper we present a new approach for the automated mapping of formal descriptions into activity thread implementations. The approach resolves semantic conflicts by reordering of statements at compile time. This simplifies the mapping process and considerably improves the efficiency of the generated code. The approach is implemented in the SDL compiler COCOS. We describe the approach as well as its implementation and prove how semantic conflicts are resolved. Finally we present measurements which show the achieved performance gain.

Deriving Activity Thread Implementations from Formal Descriptions Using Transition Reordering

In this paper we show how formal descriptions can efficiently be mapped into activity thread implementations using transition reordering. This technique prevents semantic conflicts by reordering of statements at compile time. It considerably simplifies the derivation process and improves the efficiency of the generated code. The technique is implemented in the SDL compiler COCOS. We describe the approach as well as its implementation and show how semantic conflicts are resolved. Finally we present measurements which prove a considerable performance gain up to 120 per cent.

Automated Derivation of ILP Implementations from SDL Specifications

In this paper we present a mapping strategy for an important manual protocol implementation technique: the Integrated Layer Processing (ILP) which is used for protocol stack implementations with complex data manipulation operations. The Integrated Layer Processing was implemented beside the server and the activity thread model as third code generation procedure in the configurable SDL compiler COCOS. We describe the chosen transformation principle and discuss in particular the semantic problems which have to be solved for this mapping. Finally we present first measurements which show the effect of the transformation. Performance gains up to 20 per cent compared to COCOS server implementations and of about 300 per cent compared to the SDT Cadvanced tool were measured.

Evaluation of Well-Known Protocol Implementation Techniques for Application in Wireless Networks

Minimizing power consumption is a key requirement for mobile terminals. Here we discuss how power consumption of mobile terminals is influenced by the implementation of the protocol stack in the mobile. We compare the integrated layer processing (ILP) approach with the server model and the activity thread model. ILP is expected to lead to the fastest protocol implementation, and thus to the one with the lowest power consumption. To investigate this, we have implemented the eXample Data Transfer (XDT) Protocol using all three approaches. Our measurements show that the ILP approach can be outperformed by both of the other techniques when the packet error rate of the channel exceeds a threshold of 7%. To cope with dynamic changes we propose an adaptable implementation strategy, concluding that the question of the response to errors on a wireless channel requires careful consideration.