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Computers are networked together in order to share the information they store and process. The internet connects many of these networks together, offering a multitude of options for communication, productivity and entertainment. It also offers the opportunity for unscrupulous individuals to contact these networked computers and attempt to appropriate or destroy the data on them, the computing resources they provide, and the identity or reputation of the computer user. Measures to secure networks need to be implemented by network administrators and users to protect their computing assets.Firewalls filter information as it flows through a network. This filter can be implemented in hardware or software and can be used to protect computers from unwanted access. While software firewalls are considered easier to set up and use, hardware firewalls are often considered faster and more secure. Absent from the marketplace is an embedded hardware solution applicable to desktop systems.Traditional software firewalls use the processor of the computer to filter packets; this is disadvantageous because the computer can become unusable during a network attack when the processor is swamped by the firewall process. Traditional hardware firewalls are usually implemented in a single location, between a private network and the internet. Depending on the size of the private network, a hardware firewall may be responsible for filtering the network traffic of hundreds of clients. This not only makes the required hardware firewall quite expensive, but dedicates those financial resources to a single point that may fail.The dynamic silicon firewall project implements a hardware firewall using a softcore processor with a custom peripheral designed using a hardware description language. Embedding this hardware firewall on each network interface card in a network would offer many benefits. It would avoid the aforementioned denial of service problem that software firewalls are susceptible to since the custom peripheral handles the ii filtering of packets. It could also reduce the complexity required to secure a large private network, and eliminate the problem of a single point of failure. Also, the dynamic silicon firewall requires little to no administration since the filtering rules change with the users network activity. The design of the dynamic silicon firewall incorporates the best features from traditional hardware and software firewalls, while minimizing or avoiding the negative aspects of each.iii

This paper presents a segmented matched filter (SMF) for codephase acquisition in direct sequence spread spectrum systems. While conventional matched filters provide fast acquisition in the presence of high co-user noise, they are unable to handle significant carrier frequency offset (Doppler). This problem is alleviated by segmentation with non-coherent summation. The paper develops expressions to approximately relate the matched filter partitioning to the pre-detection filter and dwell time integrator of the conventional non-coherent correlator. It also investigates 1-bit versus 2-bit quantization. A mixed-signal application specific integrated circuit (ASIC) has been fabricated to implement a 512 chip SMF with half chip codephase resolution. The paper presents calculated and measured probability density functions (pdfs) for the filter output decision variable for 10, 25, and 50 co-users with 0 to 20 kHz Doppler shift. For the example of a GPS receiver, expected acquisition time is shown as a function of multiple access interference and carrier Doppler.

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