The service allows users to communicate with peers by voice, video, and instant messaging over the Internet. Phone calls may be placed to recipients on the traditional telephone networks. Calls to other users within the Skype service are free of charge, while calls to landline telephones and mobile phones are charged via a debit-based user account system. Skype has also become popular for its additional features, including file transfer, and videoconferencing. Competitors include SIP and H.323-based services, such as Empathy, Linphone, Ekiga as well as the Google Talk service.Skype has 663 million registered users as of September 2011. The network is operated by Microsoft, which has its Skype division headquarters in Luxembourg. Most of the development team and 44% of the overall employees of the division are situated in the offices of Tallinn and Tartu, Estonia.Unlike most VoIP services, Skype is a hybrid peer-to-peer and client–server system, and makes use of background processing on computers running Skype software; the original name proposed – Sky peer-to-peer – reflects this.Some network administrators have banned Skype on corporate, government, home, and education networks, citing reasons such as inappropriate usage of resources, excessive bandwidth usage, and security concerns.
Features
Registered users of Skype are identified by a unique Skype Name, and may be listed in the Skype directory. Skype allows these registered users to communicate through both instant messaging and voice chat. Voice chat allows telephone calls between pairs of users and conference calling, and uses a proprietary audio codec. Skype's text chat client allows group chats, emoticons, storing chat history and editing of previous messages. Offline messages were implemented in version 5, but removed after a few weeks without notification. The usual features familiar to instant messaging users — user profiles, online status indicators, and so on — are also included.
The Online Number, a.k.a. SkypeIn, service allows Skype users to receive calls on their computers dialed by conventional phone subscribers to a local Skype phone number; local numbers are available for Australia, Belgium, Chile, Colombia, Denmark, the Dominican Republic, Estonia, Finland, France, Germany, Hong Kong, Hungary, Ireland, Italy, Japan, Mexico, New Zealand, Poland, Romania, South Africa, South Korea, Sweden, Switzerland, Turkey, the Netherlands, the United Kingdom, and the United States. A Skype user can have local numbers in any of these countries, with calls to the number charged at the same rate as calls to fixed lines in the country.
Video conferencing between two users was introduced in January 2006 for the Windows and Mac OS X platform clients. Skype 2.0 forLinux, released on 13 March 2008, also features support for video conferencing. Version 5 beta 1 for Windows, released 13 May 2010, offers free video conferencing with up to five people.
Skype for Windows, starting with version 3.6.0.216, supports "High Quality Video" with quality and features, e.g., full-screen and screen-in-screen modes, similar to those of mid-range videoconferencing systems. Skype audio conferences currently support up to 25 people at a time, including the host.
Skype does not provide the ability to call emergency numbers such as 911 in the United States and Canada, 999 in the United Kingdom and many other countries, 111 in New Zealand, 000 in Australia, or 112 in Europe. The U.S. Federal Communications Commission (FCC) has ruled that, for the purposes of section 255 of the Telecommunications Act, Skype is not an "interconnected VoIP provider". As a result, the U.S. National Emergency Number Association recommends that all VoIP users have an analog line available as a backup.
In 2011, Skype partnered with Comcast to bring its video chat service to Comcast subscribers via their HDTV sets
Protocol detection and control
Many networking and security companies claim to detect and control Skype's protocol for enterprise and carrier applications. While the specific detection methods used by these companies are often private, Pearson's chi-squared test and Naive Bayes classification are two approaches that were published in 2007. Combining statistical measurements of payload properties (such as byte frequencies and initial byte sequences) as well as flow properties (like packet sizes and packet directions) has also shown to be an effective method for identifying Skype's TCP- and UDP-based protocols
Audio codecs
G.729 and SVOPC. Skype added a Skype-created codec called SILK to Skype 4.0 for Windows and other Skype clients. SILK is intended to be "lightweight and embeddable"
Video codecs
VP7 is used for versions prior to Skype 5.5
As of version 5.7 VP8 is used for both group and one on one standard definition video chat and H264 is used for 720p and 1080p high definition group and one on one video chat.
Client
Clients are available for Microsoft Windows (2000, XP, Vista, 7, Mobile), iOS (iPad, iPhone, iPod Touch), Mac OS X (Intel, PPC),Symbian S60, Sony PlayStation Portable (PSP), Linux (includes Android, Maemo), and for 2010 TV models from LG, Panasonic, and Samsung, but these TVs need a special webcam
Security and privacy
Skype is claimed to be a secure communication; encryption cannot be disabled, and is invisible to the user. Skype reportedly uses publicly documented, widely trusted encryption techniques: RSA for key negotiation and the Advanced Encryption Standard to encrypt conversations. However, it is impossible to verify that these algorithms are used correctly, completely and at all times as there is no public review possible without a protocol specification and/or the program source code. Skype provides an uncontrolled registration system for users with no proof of identity. Instead, a free choice of nicknames permits users to use the system without revealing their identity to other users. It is trivial to set up an account using any name; the displayed caller's name is no guarantee of authenticity. A third party paper analyzing the security and methodology of Skype was presented at Black Hat Europe 2006. It analyzed Skype and found a number of security issues with the current security model.
Skype incorporates some features which tend to hide its traffic, but it is not specifically designed to thwart traffic analysis and therefore does not provide anonymous communication. Some researchers have been able to watermark the traffic so that it is identifiable even after passing through an anonymizing network.
In an interview Kurt Sauer, the Chief Security Officer of Skype, said, "We provide a safe communication option. I will not tell you whether we can listen or not."Skype's client uses an undocumented and proprietary protocol. The Free Software Foundation (FSF) is concerned by user privacy issues arising from using proprietary software and protocols and has made a replacement for Skype one of their high priority projects. Security researchers Biondi and Desclaux have speculated that Skype may have a back door, since Skype sends traffic even when it is turned off and because Skype has taken extreme measures to obfuscate their traffic and functioning of their program. Several media sources reported that at a meeting about the "Lawful interception of IP based services" held on 25 June 2008, high-ranking unnamed officials at the Austrian interior ministry said that they could listen in on Skype conversations without problems. Austrian public broadcasting service ORF, citing minutes from the meeting, reported that "the Austrian police are able to listen in on Skype connections". Skype declined to comment on the reports.
The United States Federal Communications Commission (FCC) has interpreted the Communications Assistance for Law Enforcement Act (CALEA) as requiring digital phone networks to allow wiretapping if authorized by an FBI warrant, in the same way as other phone services. In February 2009 Skype said that, not being a telephone company owning phone lines, it is exempt from CALEA and similar laws which regulate US phone companies, and in fact it is not clear whether Skype could support wiretapping even if it wanted to.According to the ACLU, the Act is inconsistent with the original intent of the Fourth Amendment to the U.S. Constitution; more recently, the ACLU has expressed the concern that the FCC interpretation of the Act is incorrect.
On 20 February 2009 the European Union's Eurojust agency announced that the Italian Desk at Eurojust would "play a key role in the coordination and cooperation of the investigations on the use of internet telephony systems (VoIP), such as 'Skype'. ... The purpose of Eurojust’s coordination role is to overcome the technical and judicial obstacles to the interception of internet telephony systems, taking into account the various data protection rules and civil rights"
Another security risk are automatic updates, which cannot be disabled from version 5.6 on, both on Mac OS and Windows branches, although in the latter, and only from version 5.9 on, automatic updating can be turned off in certain cases
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