Learn Software Development

Refactoring - Code improvement without changing behavior

ashish — Thu, 09 Jul 2009 13:01:54 +0000

Refactoring is the process of changing a software system in such a way that it does not alter the external behaviour of the code/design, yet improves its internal structure. When the software is refactored, the existing design is examined for redundancy, unused design elements, inefficient or unnecessary algorithms, poorly constructed or inappropriate data structures, or any other design failure; once this inspection is done, then the results are analysed for getting a more improved design.
If you were to ask for a simple explanation, then code refactoring means that you ‘clean up’ the code. Refactoring is not meant to be used for changes where you added new functionality, or fixed bugs, or something similar. It is in principle used for analysing code to remove dead code, to make the code more understandable, and modify the internal structure and design (maybe to make the code more elegant, as well as to follow some standard code processes and layouts).
Why do refactoring ? The process intends to make code easier to read and comprehend, and also to be able to make modifications to it. Given the amount of time that code remains in-use, and with the people handling the code not remaining constant, it is important that the code be easy to understand and maintain. Sometimes people take on the task of improving the code in order to fix defects or solve other design issues, but that is not refactoring.
A common method used within refactoring is to change the definition of variables, such as when a non-meaning variable such as ‘j’ is instead changed to actually be closer to the real use of the variable, say ‘ItemWeight’. This makes it easier for somebody else to quickly read the code and comprehend. This however, has its own set of complications since the same variable may be used in many different parts of the application, and the person making the modification may need to study each usage before making the change.

Quick Tech Tip - FDDI - Fiber distributed data interface

ashish — Thu, 09 Jul 2009 09:38:23 +0000

Fiber distributed data interface (FDDI) provides a standard for data transmission in a local area network that can extend in range up to 200 kilometers (124 miles). FDDI is a token-passing, fiber ring, network. The fiber optic media can be multimode fiber and can be as large as 100 kilometers - with no more than 2 kilometers between nodes.

Why FDDI?
- More Powerful Workstations and Servers.
- Resource Intensive Network Applications.
- Growing Distributed Client/Server Applications.
- Larger Spans of Distributed Networks.
- Increasing Numbers of Network Users.
- Bigger and More Powerful Software Applications.

FDDI Benefits:
- Higher Capacity and Performance than older LANs.
- More Simultaneous Transactions.
- Higher Availability (dual ring topology).
- Predetermined Performance (adding users have minimal impact on throughput).
- Longer Distance Loops (2 kilometers to 100 kilometer).

FDDI uses dual-ring architecture with traffic on each ring flowing in opposite directions (called counter-rotating). The dual rings consist of a primary and a secondary ring. During normal operation, the primary ring is used for data transmission, and the secondary ring remains idle.

FDDI specifies the physical and media-access portions of the OSI reference model. FDDI is not actually a single specification, but it is a collection of four separate specifications, each with a specific function. Combined, these specifications have the capability to provide high-speed connectivity between upper-layer protocols such as TCP/IP and IPX, and media such as fiber-optic cabling.

Quick Tip: URLs - Uniform Resource Locator

ashish — Thu, 09 Jul 2009 09:21:49 +0000

URLs, or Uniform Resource Locators, are the method by which documents or data are addressed in the World Wide Web. The URL contains the following information:

- the protocol.
- the DNS name of the machine on which the page is located.
- the local name uniquely indicating the specific page.
- the location of the resource in the directory structure of the server.

To make a piece of text clickable, the page writer must provide two items of information : the clickable text to be displayed and the URL of the page to go to if the text is selected. Once the text is selected, the browser looks up the host name using DNS. Now armed with the host’s IP address, the browser establish TCP connection to host. Over that connection, it sends the file name using the specified protocol.
The URL scheme is open to have protocols other than HTTP also. In short, URL’s have been designed to not only allow users to navigate the Web, but to deal with FTP, news, Gopher, email, and telnet as well, making all the specialized user interface programs for those other services unnecessary, and thus integrating nearly all Internet access into a single program, the Web Browser.
The growing use of the Web has turned up an weakness in URL scheme. A URL points to one specific host. For pages that are heavily referenced, it is desirable to have multiple copies far apart, to reduce network traffic. The advent of systems such as Akami are meant to meet that need, distributing content over multiple servers on a global level.

WWW - The Server Side

ashish — Mon, 06 Jul 2009 19:20:52 +0000

For all the incoming connectionts from different clients, every website is associated with a server process listening to TCP port 80. The client sends a request after the connection is made and the server sends the reply and then the connection is released. The protocol that is responsible for requests and replies is called HTTP.

The steps that occur between the user clicking and a page being displayed are:
- The browser determines the URL.
- The browser asks DNS for IP address.
- DNS replies.
- Browser makes a TCP connection to the port.
- It then sendsthe GET command.
- The server sends the file.
- The TCP connection is released.
- The browser displays the text of the file.
- The browser fetchesand displays all images of the file.

Not all servers speak HTTP, Old servers use FTP, Gopher or other protocols. Given the number of different protocols, it was thought impractical to make browser understand different protocols. However, since there is a need to make information available (where the server talks in protocols other than HTTP), a solution was required. This solution is something called a proxy server. A proxy server takes a HTTP request from the browser and translates these requests into the FTP/Gopher/other protocols. The proxy server is a separate logical server.
A proxy server also serves to provide an important function called caching. Through caching, a proxy server keeps a local copy of the pages that pass through it. If a user requests for a page, if the page is present on the cache of the proxy server, it serves the page to the user. this way it serves to reduce load on final server.

Introduction: WWW - The Client Side

ashish — Fri, 03 Jul 2009 15:04:44 +0000

The Internet / World Wide Web consists of pages and each page contains links or pointers to other pages. Users follow the link by clicking on them. This process can be repeated indefinitely, possibly traversing hundreds of linked pages. Pages that point to other pages are said to use “hypertext”.

Pages are viewed with a program called a browser. When a page is requested, the browser fetches, interprets the text and formats the commands that it contains and displays the page. Strings of text that are links to other pages are called hyperlinks.

Most browsers have numerous buttons and features to navigate the Web. In addition to having ordinary text and hypertext, web pages also contain icons, line drawings, maps and photographs. Some pages also consist of audio tracks, video clips, or both. When hypertext pages are combined with other media, the result is called hypermedia. Many Web pages consists of large images which take a long time to load.

Some browsers deal with slow loading of images by first fetching and displaying the text, then getting the images. Some Web pages contain forms that request the user to enter information. Some browsers use the local disk to cache pages that they have fetched. A check is made before a page is fetched to see if it is in the local cache. If so, check if it is up to date. If so,there is no need to load the page again.

To host a web browser, a machine must be directly connected to Internet or have a SLIP or PPP connection to a router or other machine that is directly on Internet.

Quick Introduction to the World Wide Web (WWW) / Internet

ashish — Fri, 03 Jul 2009 14:49:38 +0000

The World Wide Web is an architectural framework for accessing linked documents spread out over thousands of machines all over the Internet. The World Wide Web began as a networked information project at CERN, where Tim Berners-Lee, now Director of the World Wide Web Consortium [W3C], developed a vision of the project.

The Web has a body of software, and a set of protocols and conventions. Through the use of hypertext and multimedia techniques, the web is easy for anyone to roam, browse, and contribute to.

World Wide Web can also be defined as system of Internet servers that support specially formatted documents. The documents are formatted in a markup language called HTML (HyperText Markup Language) that supports links to other documents, as well as graphics, audio, and video files. You can jump from one document to another simply by clicking on hot spots. Not all Internet servers are part of the World Wide Web. Web is basically a client-server system. Web pages are written in HTML and java.

MIMO ROUTERS - A quick summary

ashish — Thu, 02 Jul 2009 19:26:33 +0000

MIMO routers build on the MIMO technology, a relatively new set of technology protocols that reinvent the way signals are transmitted and received. MIMO routers send out multiple data streams at the same time and then use multiple antennas to sift through the signals for the requisite pattern. All this is done while maintaining a fast connection and, hence, a fast data stream. One characteristic sign of a MIMO router is multiple antennae.

Benefits of MIMO Routers. These include:
• Faster speeds.
• Greater distances between your paired devices.
• More simultaneous users.
• Less signal fading and dead spots.
• Better resistance to interference.

Concerns of MIMO Routers:

1. MIMO routers have a premium associated with them. The prices can fall as the product gets matured.
2. Pending 802.11N standard.

MIMO (Multiple Input Multiple Output) technology

ashish — Thu, 02 Jul 2009 19:23:28 +0000

1. MIMO is a new wireless technology conceived in the mid 90’s
2. It is based on an entirely new paradigm for digital signal processing that multiplies the data rate throughput achievable in wireless communication products.
3. Greatly improves the reliability, range and robustness of the connection providing a much better user experience that is closer to “wired” Ethernet quality.
4. MIMO technology is now being introduced to the Wi-Fi market.

MIMO is a smart antenna technique that increases speed, range, reliability and spectral efficiency for wireless systems.
MIMO is one technology being considered for 802.11n, a standard for next-generation 802.11 that boosts throughput to 100M bit/sec.

• Smart antenna techniques use multiple antennas to improve wireless performance and reliability.
– Antennas themselves are “dumb” pieces of metal.
– “Smartness” comes from signal processing that is applied to the multiple antennas.
– There are differing degrees of smartness.
• Conventional, “single-dimension” (1D) smart antenna techniques transmit just one data stream per channel
– RF beamforming.
– Digital beamforming.
– Digital receive diversity combining.
• MIMO makes smart antennas “multi-dimension”.
– Multiple data streams in the same channel.
– 2-D signals.

BENEFITS OF MIMO TECHNOLOGY:
For Business
• Enables truly wireless office – replaces Ethernet
– Improves wireless reliability and robustness
– Reduces infrastructure cost - Doubles coverage area of each AP
– Rates to 108 Mbps in each channel – similar to wired Ethernet speed
• Improves VoIP performance
– Extends handset battery life
– Increases call capacity
For Consumers
• One AP covers your whole home with reliable service
– Penetrates more walls at higher rates
– No need to sit in the right place to use your laptop
• Supports new wireless multimedia applications
– Whole-home coverage for high-speed broadband access
– Reliable SDTV and HDTV video transport in home networks
– Multi-service applications – voice, video, data

What are Routers, Switches, Hubs — Is There a Difference between them ?

ashish — Wed, 01 Jul 2009 14:23:59 +0000

Each device has its own function in a network environment. Two reasons confusion exists are
(1) all three are simple boxes with several plugs that accept cables and
(2) at times the functions of each device are rolled into one single device.

A hub, also known as a repeater, is a simple device used for years to connect all nodes, or computers, on a network to a central location. Each node on a network has a unique hardware address called a MAC address. A hub is known as a repeater because when a packet of data, or frame, is sent through the hub, it is repeated to each and every computer on the network.

A switch-based network is one that utilizes switches instead of hubs. A switch is a major upgrade to a hub. Instead of sending all network data to each and every network node, the switch will analyze the MAC address and determine where to send the data. Network bandwidth is not wasted by sending every frame to every port.

Routers not only provide connections to the internet, they also protect the LAN from the Internet. The router could block any Packet that has a destination address outside of the LAN. In short, a router can perform many of the same functions as switches and hubs, but it has address translation and filtering capabilities.

Quick description - How does a Router work?

ashish — Wed, 01 Jul 2009 14:20:45 +0000

Here is a technical description of how a router works. This would make sense if you have already an understanding of the network protocols.
The layers 2 and 3 of the OSI model i.e. Data Link layer, is where the Ethernet protocol, MAC addresses and switches fit in.
Layer 3, the Network layer, is where the IP protocol, IP addresses and routers fit in. All traffic is sent from your computer, starting with Layer 7 (your network application) and going down to Layer 1 (physical). With the physical layer, the traffic is going across your network medium.
Traffic goes to a router only if it is not on your local LAN. Routers work primarily at Layer 3 but must understand Layers 1-3, at a minimum. Many routers understand traffic all the way up to Layers 4-7 in varying ways.
One of the tools a router uses to decide where a packet should go is a configuration table. A configuration table is a collection of information, including:
• Information on which connections lead to particular groups of addresses
• Priorities for connections to be used
• Rules for handling both routine and special cases of traffic
Internet data, whether in the form of a Web page, a downloaded file or an e-mail message, travels over a system known as a packet-switching network. In this system, the data in a message or file is broken up into packages about 1,500 bytes long which includes information on the sender’s address, the receiver’s address, the package’s place in the entire message, and how the receiving computer can be sure that the package arrived intact. Each data package, called a packet, is then sent off to its destination via the best available route — a route that might be taken by all the other packets in the message or by none of the other packets in the message.