2.5 G (generation) Phone Technology
2.5-generation (2.5G or 2G+) networks are an interim step toward third-generation (3G) networks. 2.5G networks integrate technology that extends 2G networks to include packet-switched data service. Packet-switched data service is faster than the circuit-switched data service offered on some 2G networks. GPRS is an example of a 2.5G technology. The following list discusses 2.5G technologies in detail:
General packet radio service¡ªGPRS provides packet-switched data service to GSM networks. Because it uses packet switching like the Internet, GPRS makes many Internet applications available via a mobile device. Using GPRS, carriers can provide services such as Web browsing, games, e-mail, or file transfers at speeds up to 40 Kbps (much higher speed is theoretically possible, but unlikely due to the nature of cellular networks). To move data on a GSM network, GPRS uses the same time slots used for voice with each time slot carrying data packets. This allows for efficient use of radio resources, because several users can send packets over the same channel, unlike a circuit-switched connection that requires a dedicated line. GPRS always allow Internet connections, with many carriers only charging for the amount of data transferred, not the length of time actually connected.
Multimedia messaging service¡ªMultimedia messaging service (MMS) is an enhanced service for cell phones that provides for transmission of multimedia messages between mobile phones, and uses the wireless application protocol (WAP). Multimedia messages can consist of pictures, audio, video, and formatted text. MMS users send messages through an intermediary server, or MMS center (MMSC), rather than sending them in real-time. The MMSC receives messages and holds them until the receiving phone retrieves them. This is also known as store-and-forward.
A popular application of MMS is photo messaging, which enables users to take a picture with their phone and send it to another user, accompanied by a voice message or text. Often, if a photo message is sent to a user with a phone that doesn't support the feature; the recipient receives an Internet address where he can view the picture.
MMS supports e-mail addressing and allowing users to send MMS messages to an e-mail address rather than a phone. MMS is usually backward-compatible with earlier messaging services, such as SMS and EMS, with users of those services receiving a modified message rather than a full MMS message.
Wireless application protocol¡ªWAP is a standard that enables users of mobile devices to access the Internet and view data on mobile devices such as phones. Comparable to HTTP on the World Wide Web, WAP is secure and most mobile networks and operating systems (OSs) support it.
WAP was specifically developed for handheld wireless devices and uses very little of a device's resources to operate. WAP uses the wireless markup language (WML) but supports both the extensible markup language (XML), and HTML. For page scripting, WAP uses WMLScript, which is efficient and developed with mobile devices in mind. The latest version of WAP integrates the features of I-mode.
I-mode¡ªDeveloped by NTT DoCoMo of Japan, I-mode is a proprietary service that allows browsing of Internet pages on mobile phones, as well as e-mail, games, and chat services. Unlike WAP, which uses WML, I-mode uses cHTML, a subset of HTML. The I-mode service is one of the largest Internet service providers in Japan, with more users accessing the Internet from their phones than from desktop PCs. The latest version of the I-mode service uses an early version of the 3G technology, known as wideband code division multiple access (W-CDMA). Because of this, I-mode users enjoy speeds of up to 384 Kbps.
High-speed circuit-switched data¡ªLike GPRS, high-speed circuit-switched data (HSCSD) is another extension of GSM networks, designed to add data transfer capabilities. HSCSD uses circuit switching, and unlike GPRS, it requires a dedicated time-slot for data transfer. HSCSD devices simultaneously use up to four time-slots, unlike GPRS, which can allow different users to send packets in the same time-slot. Because it requires the dedicated use of multiple time-slots, HSCSD does not use radio resources as efficiently as GPRS. However, the use of multiple, dedicated time-slots has greater quality of service than GPRS. At increased data speeds, this increase in quality of service makes HSCSD a better choice than GPRS for delivery of live video. Because HSCSD uses a dedicated connection, it isn't ideal for always on Internet service, as users would have to maintain a connection and as a result would be charged more.