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Skype is a Voice over IP (VoIP) protocol based packet voice phone system that uses a system of nodes akin to a peer-to-peer network to carry calls from one computer, through the network, to another computer. Skype uses a a few features found in the more well known and more often used Session Initiation Protocol (SIP) VoIP Systems to set up and tear down calls between users. This allows the user to maintain their identity and status via a user@ address rather than a traditional telephone call. When a user initiates a SIP call to someone on their buddy list, that buddy has already started the program and logged into the Skype network. That login creates a "where is" record for that specific user@ account and matches it with the current IP address of that computer. Incoming setup messages first check availability then begin a three-way negotiation between the sending computer and the receiving computer using a Skype server in the middle to handle user to network and network to user signaling.
Skype's main selling point is the robustness and relative inexpensive nature of it's peer-to-peer network where the duties of specific servers, such as in Q.931 and SIP networks, are handled by the consumer peers logged into the service. Skype offers better than land line voice quality because it does not filter the voice at 4Khz as in traditional analog telephony and samples at a fill 64 kilobits per second, a full 100% better than typical digital telephony.
Skype users can call outside the Skype network (for a fee). Customers of this service initiate a call to a landline user, the server does an address lookup based on dialed number, then routes the call information to the nearest Skype server connected to a tranditional central office. This server sets up a typical call with the central office switch in the receiving caller's area, and handles the analog/digital conversion at the edge of the network. What makes Skype's out of network calling is the same feature that makes all VoIP calling relative inexpensive. The access charge for the Skype servers can be as low as the access charges of a single local phone.
Cellular telephony resembles Skype only in that it is a separate network from the Public Switched Telephone Network (PSTN). However, cellular carriers rely on PSTN backbone infrastructure to move digitized voice signals from the tower into the regular phone system and back out of the tower. Price wise, digital telephone can be as low as a $.15 a minute for pre-paid and low usage plans lacking features such as text messaging, multimedia messaging, and other popular services that come at additional cost.
Cellular phones in the US operate on one of three possible digital access schemes, Code Division Multiple Access (CDMA), Global System for Mobile Telecommunications (GSM) and Time Division Multiple Access (TDMA). Of these three systems, CDMA offers the most effient use of nearly unlimited bandwidth, while GSM and TDMA offer finite usage limits with regard to call carrying capacity within any given cell.
I hope this helps get you started. Please ask further questions and I will try to help as the evening goes on.
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