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Overview Of The Medium Access Sublayer

ashish — Mon, 03 Aug 2009 04:27:57 +0000

Some networks have a single channel that is used for all communication. In these networks, the key design issue is the allocation of this channel among the competing stations wishing to use it. Numerous channel allocation algorithms have been devised like :

- FDM : Dedicate a frequency band to each station.
- TDM : Dedicate a time slot to each station.
- Pure ALOHA : Unsynchronized transmission at any instant.
- Slotted ALOHA : Random transmission in well defined time slots.
- 1-persistent CSMA : Standard carrier sense multiple access.
- Nonpersistent CSMA : Random delay when channel is sensed busy.
- P-persistent CSMA : CSMA, but with probability of p of persisting.
- CSMA/CD : CSMA. but abort on detecting a collision.
- Bit Map : Round robin scheduling using a bit map.
- Binary countdown : Highest numbered ready station goes next.
- Tree walk : Reduced contention by selective enabling.
- Wavelength division : A dynamic FDM scheme for fiber.
- MACA, MACAW : Wireless LAN protocols.
- GSM : FDM plus TDM for cellular radio.
- CDPD : Packet radio within an AMPS channel.
- CDMA : Everybody speak at once but in different language.
- Ethernet : CSMA/CD with binary exponential backoff.
- Token bus : Logical ring on a physical bus.
- Token Ring : Capture the token to send a frame.
- DQDB : Distributed queuing on a two-bus MAN.
- FDDI : Fiber-optic token ring.
- HIPPI : Crossbar using 50-100 twisted pairs.
- Fibre channel : Crossbar using fiber optics.
- SPADE : FDM with dynamic channel allocation.
- ACTS : TDM with centralized slot allocation.
- Binder : TDM with ALOHA when slot owner is not interested.
- Crowther : ALOHA with slot owner getting to keep it.
- Roberts : Channel time reserved in advance by ALOHA.

FDM and TDM are efficient when the number of stations is small and the traffic is continous.
ALOHA protocol, with and without slotting and control, has been proposed when the number of stations is large and variable.
BINARY COUNTDOWN completely eliminates contention.
TREE WALK reduces contention by dynamically dividing the stations into two disjoint groups, one of which is permitted to transmit and one of which is not.
WIRELESS LANs have their own problems and solutions. The biggest problem is caused by hidden stations, so CSMA does not work. MACA attempts to stimulate transmissions around the destination, to make CSMA work better.
GSM, CDPD and CDMA are widely used for mobile computers and telephones.
The IEEE 802 LANs are : CSMA/CD, TOKEN BUS, and TOKEN RING. Each of these has its unique advantages and disadvantages, and each has found its own user community.
FDDI, FAST ETHERNET, HIPPI, and FIBER CHANNEL offer bandwidth in the 100 Mbps range and up.

What are Web Applications ? (WebApps)

ashish — Mon, 09 Mar 2009 19:12:39 +0000

You must have heard of Web Applications for a long time now. Nowadays, you even hear of Web 2.0 Apps. But what exactly are Web Applications and how do they impact you ?

In the early days of the web, web sites consisted of static pages, which severely limited interaction with the user since there was no interactivity or very limited interactivity. In the early 1990’s, this limitation was removed when web servers were enhanced to allow communication with server-side custom scripts. As a result, applications were no longer just static brochure-ware, edited only by those who knew the arcane mysteries of HTML; with this single change, normal users could interact with the application for the first time. The trend towards increased interactivity has continued apace, with the advent of “Web 2.0”, a term that encompasses many existing technologies, but heavily features highly interactive, user centric, web-aware applications.
Web-based applications are computer programs that execute in a web browser environment (the overall environment could be a closed group intranet, or a public network such as the internet). An example of such an application would be an online store such as Amazon.com accessed via Firefox or Internet Explorer. Web applications are popular due to the ubiquity of web browsers, and the convenience of using a web browser as a client, sometimes called a thin client. The ability to update and maintain web applications without distributing and installing software on potentially thousands of client computers is a key reason for their popularity.
To put it even more simply, A Web application is just an application that is deployed on the Web. It is a Web page, or series of Web pages, allowing users to accomplish a task like obtaining information and forms, shopping, applying for a job, listening to Internet radio, or any of the many activities possible on the Web. To use a Web application, a user needs to know a URL for the application, and possibly a name and password. Another way to think of a Web application is a Web site offering a great deal of functionality. A web application can provide any functions that may historically be found on a desktop computer. There are web applications to provide weather information for your locale, to track sales calls for a sales force, or sales expenses, or on any topic at all.

Testing of Web Applications

ashish — Sun, 01 Mar 2009 17:35:38 +0000

The quality of a web application can be pretty evident right from the onset (from the beginning of testing). Some of the key things to check for, and that are visible right in the beginning are:
- Slow response time,
- Problems with the accuracy of information,
- Bad design / workflow problems or not having ease of use will compel the user
to click to a competitor’s site
Problems such as these that are easily visible directly translate into loss of users, declining or stagnant sales and a very poor image of the company.

These are outcomes that companies seek to avoid at any costs, and that is why there is the need for a strong QE effort. As a part of this, the following techniques can be used to do a more thorough checking:
1. Good Functionality testing – This sort of testing makes sure that the features that are visible to a user and affect interactions are working properly and as desired.
Some of these objects in a WebApp include: User enterable forms, Searches and their results, Pop-up windows (most users hate them because of their heritage), shopping carts.

2. Usability testing – Many users have a low tolerance for anything that is difficult to use, making having a usability testing program a critical part of the testing of any WebApp. You need to ensure that a user’s first impression is very important; what makes this more complex is that now-a-days applications have become complicated and cluttered with an increasing number of features.

The main steps involved in usability testing are:
– Identify the purpose of WebApp.
– Identify the intended users.
– Define the tests, review them for thoroughness and conduct usability testing.
– Collect information through various mechanisms.
– Carry out an analysis of the acquired information.
– Make the necessary changes based on the acquired information.

3. Navigation testing – Navigation testing makes sure that all navigation syntax and semantics are exercised to uncover any navigation errors. It should not happen that a user clicks on a navigation aid and then either reaches a dead end or goes off into a wrong direction.

4. Forms testing – WebApps that use forms requires tests to ensure that each field is working properly (including the validations such as not allowing users to enter more than a certain amount of text, fields not being left blank, etc) and that the form posts all the data as intended by the designer.

5. Content testing – Content is evaluated at both a syntactic and semantic level.At the syntactic level, spelling, punctuation, and grammar are assessed. At semantic level, correctness, consistency, and lack of ambiguity are all assessed. It creates a very bad impression if the user finds spelling mistakes (user assumes that there must be something wrong if the company put wrong spellings or wrong grammar and did not find it till now, or the company does not care that there are problems on the site)

6. Compatibility testing – This testing is done by executing the webApp under every browser/ platform combination to ensure that the web applications are working properly under different environments. This sort of testing is easier said than done, since the number of browsers and operating systems in the market are huge.

7. Performance testing – This testing evaluates the system performance under normal and heavy usage. An application that takes long to respond may frustrate the user which could result to move to a competitor’s site. This testing ensures that the website server responds to browser requests within defined parameters. The system should work perfectly and speedily under normal expected usage, and if possible, should be handle some amount of extra load.

8. Load testing – The purpose of this testing is to present real world experiences, typically by generating many users simultaneously accessing the web application. Typically, companies use automated test tools to increase the ability to conduct
a valid load test as it emulates thousand of users by sending requests simultaneously to the application. Critical, as failure under heavy loads does not convey a good impression, and may make the system susceptible to attacks by recreating heavy loads.

9. Security testing – Security is one of the primary concerns when communicating and conducting businesses over internet. One break-in can spoil the reputation of a company and lead to loss of business, stealing of user data, and other such cases with horrible consequences. Finding the vulnerabilities in an application that could grant an unauthorized user to access the system is important. Equally important is being able to track all access to the system, and do a frequent scan of these accesses.

WebApp testing – a short summary

ashish — Sun, 01 Mar 2009 05:39:00 +0000

A WebApp or Web Application is a type of application / software that is accessed via a web browser (such as Internet Explorer, Firefox, Opera, Safari) over a network such as the Internet or an intranet. It is typically coded in a browser-supported language (such as HTML, JavaScript, Java, etc.) with the browser environment making the application executable.
With such applications becoming more widespread, testing of such applications is much bigger than it was earlier. Here is a short summary about WebApp testing, and future posts will explore this area in more detail.
WebApp testing is a collection of related activities that has the goal of uncovering the errors in Web Applications related to content, function, usability, navigability, performance, capacity, and security. To accomplish this a testing
strategy that encompasses both reviews and executable testing is applied throughout the Web engineering process.
Generally testing of Web Applications is done by the same set of people who would be involved if the application was a normal client-server application, so webApp testing is done by web engineers and managers, customers, end-users and other stakeholders. This is generic advice for testing, and very relevant for Web Applications. Testing should not wait until the project is finished. It should start before you start thinking of writing a single line of code.
Testing is the process of finding errors and correcting them. The same philosophy goes with Web Applications also. In fact, WebApp testing becomes a more challenging
task for the web engineers as these applications reside on a different network and accessible by varied environments encompassing different operating systems, browsers, platforms. With such varied environments, the possibility of finding errors increases.
Web based applications present new challenges, with some of these challenges are:
1. Short release cycles
2. Constantly changing technology
3. Possible huge number of users during initial website launch
4. Inability to control the user’s running environment
5. 24-hour availability of the web site.