What is SKYPE?

(Source: www.wikipedia.org)

Skype (IPA pronunciation: /skayp/, rhymes with type) is a proprietary peer-to-peer Internet telephony (VoIP) network, founded by Niklas Zennstrom and Janus Friis, the creators of KaZaA and competing against established open VoIP protocols like SIP or H.323. The system has a reputation for working across different types of network connections (including firewalls and NAT) because voice packets are routed by the combined users of the free desktop software application. Skype users can speak to other Skype users for free, call traditional telephone numbers for a fee (SkypeOut), receive calls from traditional phones for a fee (SkypeIn), and receive voicemail messages for a fee.

Contacts & events Your home base gives your a handy overview over your contacts and if they're online or not. The little panels tell you if you missed a call or received a voicemail   Making a call To call someone on Skype, just find that person in your Contact list, and click the big, green button.   Talking Calls to other people on Skype are crystal-clear — some people says it feels like the person is right next to you! When you don’t feel like talking any more, just click the red button to hang up   Conference call Talking on Skype is more than just 1-on-1. You can make conference calls with up to four other people, and you can all talk to each other at the same time.   Call regular phones Skype, you can call other people With even if they don’t have a computer. It’s called SkypeOut, and lets you call any phone number in the world at pretty low rates   File transfer Need to send an important document to your colleague? Downloaded a cool picture from the web that you want your friends to see? Just send it over with Skype file transfer. It’s secure and encrypted like the rest of Skype, and handles files of any size   Chat You can type in addition to talking and you can invite up to 48 other friends to join in and make it a group chat. Chat is great for sharing gossip or organizing your next party   Video call Smile, wave or say hello to anyone, anywhere in the world with free one-to-one video conversations

Unlike international long-distance calls made from conventional telephones, which are traditionally rated according to the relative distance between countries, SkypeOut bills all calls according to the level of telecommunications liberalisation of the destination country, the volume of calls made from and to a given country, and access charges such as those to mobile (cell) phones. This rating method is used due to the fact that calls are rated the same regardless of place of origin.

This way, a SkypeOut user will be billed the same for a call placed to a telephone number in London, whether the user is calling from his/her computer in London itself, or from a different country. The Global Rate which is the one used for many countries, including nearly all First World countries, is currently 1.7 euro cents per minute. Credit, which is necessary to use SkypeOut, is usually purchased via credit card, personal check or money order, or online services such as PayPal or Moneybookers. The current deposit is either ˆ10 or ˆ25, which automatically expires after 180 days of inactivity — a timer is reset after each successful SkypeOut connect. In European Union countries, Luxembourg VAT of 15% is added to the charge, which makes the actual prices closer to ˆ0.02 per minute.

Cost is ˆ30 for a twelve-month subscription, or ˆ10 for a three-month subscription. Users can have multiple SkypeIn phone numbers bound to the same account (a separate subscription is required for each). For example, someone who does business in both San Francisco and Helsinki could create a local telephone number in each city, and callers from those locations would pay cheap or free local rates.

The main difference between Skype and other VoIP clients is that it operates on a peer-to-peer model rather than the more traditional server-client model. The Skype user directory is entirely decentralised and distributed among the nodes in the network, which means the network can scale very easily to large sizes (currently just over forty million users) without a complex and costly centralised infrastructure.

Skype also routes calls through other Skype peers on the network, which allows it to traverse Symmetric NATs and firewalls, unlike most other VoIP programs (The two most common VoIP protocols, SIP and H323 are usually UDP and point-to-point, making NAT traversal problematic; see article). This, however, puts an extra burden on those who connect to the Internet without NAT, as their computers and network bandwidth may be used to route the calls of other users. The selection of intermediary computers is fully automatic, with individual users having no option to disable such use of their resources. This fact is not clearly communicated, however, and seems to contradict the license agreement which would limit Skype's utilisation of the user's "processor and bandwidth [to the] purpose of facilitating the communication between [the user] and other Skype Software users" (section 4.1).

The Skype code is closed source and the protocol is proprietary which has raised suspicion and drawn broad criticism from software developers and the VoIP user communities.

The Skype client's application programming interface (API) exposes the network to software developers. The Skype API allows other programs to use the Skype network to get "white pages" information and manage calls.

The Windows user interface was developed in Pascal using Delphi, while the Linux version is written in C++ with Qt and the Mac OS X version is written in Objective-C with Cocoa.