Skype uses wide band codec iLBC iSAC and iPCM developed by GlobalIPSound which

Skype uses wide band codec ilbc isac and ipcm

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Skype uses wide-band codec (iLBC, iSAC and iPCM developed by GlobalIPSound ) which allows it to maintain reasonable call quality at an available bandwidth of 32 kb/s (The Skype claimed bandwidth usage of 3-16 kilobytes/s) and the minimum and maximum audible frequency Skype codec allowed to pass-through are 50 Hz and 8,000 Hz respectively. The network contains three types of entities: supernodes, ordinary nodes, and the login server. Each client maintains a host cache with the IP address and port numbers of reachable supernodes. The Skype user directory is decentralized and distributed among the supernodes in the network.
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Skype Network 3.2 Key Components Skype Client (SC): Skype application which can be used to place calls, send messages and etc. The Skype network is an overlay network and thus each SC needs to build and refresh a table of reachable nodes. In Skype, this table is called host cache (HC) and it contains IP address and port number of super nodes. This host cache is stored in an XML file called "shared.xml". Also, NAT and firewall information is stored in "shared.xml". If this file is not present, SC
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tries to establish a TCP connection with each of the seven Skype maintained default SNs IP address on port 33033. Super Node (SN): Super nodes are the endpoints where Skype clients connect to. Any node with a public IP address having sufficient CPU, memory, and network bandwidth is a candidate to become a super node and a Skype client cannot prevent itself from becoming a super node. Also, if a SC cannot establish a TCP connection with a SN then it will report a login failure. Skype Authentication Server: This is the only centralized Skype server which is used to authenticate Skype users. An authenticated user is then announced to other peers and buddies. If the user saves his/her credentials, authentication will not be necessary. This server (IP address: [Buddy list] or also stores the buddy list for each user. Note that the buddy list is also stored locally in an unencrypted file called "config.xml". In addition, if two SCs have the same buddy, their corresponding config.xml files have a different four-byte number for the same buddy. Finally, it has been shown that Skype routes login messages through SNs if the authentication server is blocked. Start of Message (SoM) Structure: Skype uses the same port to communicate with the outside world. Therefore, it needs an unencrypted structure in the beginning of each UDP packet to analyze the sequence and the flows at the application layer. This structure is called SoM. 3.3 Skype Connections Skype to Skype (End to End) (E2E) Call signalling and media transfer 1. If both caller and receiver are on public IPs and receiver is in the buddy list of the caller, then they establish a call through a direct TCP connection with each other and transfer media using UDP. 2. If the caller or receiver is behind a port-restricted NAT then they establish a call through a few packets initially transferred between
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caller, receiver, SN and other hosts and a UDP connection is established between the caller and receiver which is used to transfer media as well.
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