OSI Layers

The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

* Application layer supports application and end-user processes. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

* Presentation layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa.

* Session layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end.

* Transport layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

* Network layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node.

* Data Link layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization.

* Physical layer conveys the bit stream - electrical impulse, light or radio signal through the network at the electrical and mechanical level.

Aortic Aneurysm

Most aneurysms -- abnormal bulges or "ballooning" in the wall of an artery -- occur in the aorta. The aorta is the main artery that carries blood from the heart to the rest of the body.

There are two types of aortic aneurysm:

* Thoracic aortic aneurysms occur in the part of the aorta running through the chest.
* Abdominal aortic aneurysms occur in the part of the aorta running through the abdomen.

Because aneurysms can develop and become large before causing any symptoms, it is important to look for them in people who are at high risk. Ultrasound screening for abdominal aortic aneurysms is recommended for men who have ever smoked and are between the ages of 65 and 75. Medicines and surgery are the two main treatments for aneurysms.

An aneurysm is a bulge or "ballooning" in the wall of an artery. Arteries are blood vessels that carry oxygen-rich blood from the heart to other parts of the body. If an aneurysm grows large, it can burst and cause dangerous bleeding or even death.

Most aneurysms occur in the aorta, the main artery traveling from the heart through the chest and abdomen. Aneurysms also can happen in arteries in the brain, heart and other parts of the body. If an aneurysm in the brain bursts, it causes a stroke.

Herniated Disk

Also called: Bulging disk, Compressed disk, Herniated intervertebral disk, Herniated nucleus pulposus, Prolapsed disk, Ruptured disk, Slipped disk

Your backbone, or spine, is made up of 26 bones called vertebrae. In between them are soft disks filled with a jelly-like substance. These disks cushion the vertebrae and keep them in place. A herniated disk is a disk that slips out of place or ruptures. If it presses on a nerve, it can cause back pain or sciatica.

Symptoms include

* Back pain that spreads to the buttocks and legs, when the herniated disk is in your lower back
* Neck pain that spreads to the shoulders and upper arms, when the herniated disk is in your upper back
* Tingling or numbness
* Muscle spasms or weakness

With treatment, most people recover. It can take a long time. Treatments include rest, pain and anti-inflammatory medicines, physical therapy and sometimes surgery. Losing weight can help, too.

Web page

A web page or webpage is a resource of information that is suitable for the World Wide Web and can be accessed through a web browser. This information is usually in HTML or XHTML format, and may provide navigation to other web pages via hypertext links.

Web pages may be retrieved from a local computer or from a remote web server. The web server may restrict access only to a private network, e.g. a corporate intranet, or it may publish pages on the World Wide Web. Web pages are requested and served from web servers using Hypertext Transfer Protocol (HTTP).

Web pages may consist of files of static text stored within the web server's file system (static web pages), or the web server may construct the (X)HTML for each web page when it is requested by a browser (dynamic web pages). Client-side scripting can make web pages more responsive to user input once in the client browser

Reye Syndrome

Reye syndrome is a rare illness that can affect the blood, liver and brain of someone who recently had a viral infection. It always follows another illness. Although it mostly affects children and teens, anyone can get it. It can develop quickly and without warning. It is most common during flu season. Symptoms include

* Nausea and vomiting
* Listlessness
* Personality change - such as irritability, combativeness or confusion
* Delirium
* Convulsions
* Loss of consciousness

If these symptoms occur soon after a viral illness, seek medical attention immediately. Reye syndrome can lead to a coma and brain death, so quick diagnosis and treatment are critical. Treatment focuses on preventing brain damage. There is no cure.

The cause of Reye syndrome is unknown. However, studies have shown that taking aspirin increases the risk of getting it. Because of that, health care professionals now recommend other pain relievers for young patients.

Syndrome

Rett syndrome is a rare inherited disease that causes developmental and nervous system problems, mostly in girls. It's related to autism. Babies with Rett syndrome seem to grow and develop normally at first. Between 3 months and 3 years of age, though, they stop developing and even lose some skills. Symptoms include

* Loss of speech
* Loss of hand movements such as grasping
* Compulsive movements such as hand wringing
* Balance problems
* Breathing problems
* Behavior problems
* Learning problems or mental retardation

Rett syndrome has no cure. You can treat some of the symptoms with medicines, surgery, and physical and speech therapy. Most people with Rett syndrome live into middle age and beyond. They will usually need care throughout their lives.

Arthritis

If you feel pain and stiffness in your body or have trouble moving around, you might have arthritis. Most kinds of arthritis cause pain and swelling in your joints. Joints are places where two bones meet, such as your elbow or knee. Over time, a swollen joint can become severely damaged. Some kinds of arthritis can also cause problems in your organs, such as your eyes or skin.

One type of arthritis, osteoarthritis, is often related to aging or to an injury. Other types occur when your immune system, which normally protects your body from infection, attacks your body's own tissues. Rheumatoid arthritis is the most common form of this kind of arthritis. Juvenile rheumatoid arthritis is a form of the disease that happens in children.