Tuesday, August 26, 2008


The genetic material in retroviruses is in the form of RNA molecules, while the genetic material of their hosts is in the form of DNA. When a retrovirus infects a host cell, it will introduce its RNA together with some enzymes, namely reverse transcriptase and integrase, into the cell. This RNA molecule from the retrovirus must produce a DNA copy from its RNA molecule before it can be integrated into the genetic material of the host cell. The process of producing a DNA copy from an RNA molecule is termed reverse transcription. It is carried out by one of the enzymes carried in the virus, called reverse transcriptase. After this DNA copy is produced and is free in the nucleus of the host cell, it must be incorporated into the genome of the host cell. That is, it must be inserted into the large DNA molecules in the cell (the chromosomes). This process is done by another enzyme carried in the virus called integrase.

Now that the genetic material of the virus is incorporated and has become part of the genetic material of the host cell, it can be said that the host cell is now modified to contain a new gene. If this host cell divides later, its descendants will all contain the new genes. Sometimes the genes of the retrovirus do not express their information immediately.

Friday, August 22, 2008

Run-time system

In computer science, the runtime system is software that provides services for a running program but is itself not considered to be part of the operating system.[citation needed]

Examples include:

* the code that is generated by the compiler to manage the runtime stack.

* library code for handling memory management (for example, malloc).

* code that handles dynamic loading and linking.

* debugger code that is generated at compile time or run time.

* application-level thread management code.

Byte-code interpreters and virtual machines can also be considered runtime systems. Services that run in concurrent processes are more likely to be considered middleware.

Tuesday, August 12, 2008

Network telescope

A network telescope (also known as a darknet, internet motion sensor or black hole) is an internet system that allows one to observe different large-scale events taking place on the Internet. The basic idea is to observe traffic targeting the dark (unused) address-space of the network. Since all traffic to these addresses is suspicious, one can gain information about possible network attacks (random scanning worms, and DDoS backscatter) as well as other misconfigurations by observing it.

The resolution of the Internet telescope is dependent on the number of dark addresses it monitors. For example, a large Internet telescope that monitors traffic to 16,777,216 addresses (a /8 Internet telescope in IPv4), has a higher probability of observing a relatively small event than a smaller telescope that monitors 65,536 addresses (a /16 Internet telescope).

A variant of a network telescope is a sparse darknet, or greynet, consisting of a region of IP address space that is sparsely populated with 'darknet' addresses interspersed with active (or 'lit') IP addresses.

Wednesday, August 06, 2008

Wide Area Augmentation System

The Wide Area Augmentation System (WAAS) is an air navigation aid developed by the Federal Aviation Administration to augment the Global Positioning System (GPS), with the goal of improving its accuracy, integrity, and availability. Essentially, WAAS is intended to enable aircraft to rely on GPS for all phases of flight, including precision approaches to any airport within its coverage area.

WAAS uses a network of ground-based reference stations (Benchmark DGPSRs transmitting differential corrections (DCs, located within spaces protected from the public inside airportsin North America and Hawaii, to measure small variations in the GPS satellites' signals in the western hemisphere. Measurements from the reference stations are routed to master stations, which queue the received DCs and send the correction messages to geostationary WAAS satellites in a timely manner (at least every 5 seconds or better). Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receiver uses the corrections while computing its position to improve accuracy. The longer any given DC has been delayed, the less benefit it will produce.

The International Civil Aviation Organization (ICAO) calls this type of system a Satellite Based Augmentation System (SBAS). Europe and Asia are developing their own SBASs, the Indian Gagan, the European Geostationary Navigation Overlay Service (EGNOS) and the Japanese Multi-functional Satellite Augmentation System (MSAS), respectively. Commercial systems include StarFire and OmniSTAR.