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From GSM to LTE: An Introduction to Mobile Networks and Mobile Broadband by Martin Sauter

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4.12 Voice and SMS over LTE

One of the major design choices of LTE was to focus on the development of a packet-based core and access network infrastructure. The circuit-switched core network and dedicated telephony features of GSM and UMTS radio access networks have not been adapted for LTE. This significantly reduces the overall complexity of the network and follows the direction that has been taken in fixed-line networks many years earlier. Here, a clear trend toward IP and voice services over IP is well underway. At the homes of customers or in offices, multifunctional gateways that include a DSL modem, a Wi-Fi access point, fixed-line Ethernet ports and also RJ-11 ports to connect ordinary telephones are now common. Inside the device, Session Initiation Protocol (SIP)-based IP telephony data streams and signaling are converted into the classic analog or ISDN format and the user can thus continue to use his legacy devices.

With LTE, a reuse of legacy equipment is not possible, and hence, other ways have to be found to offer traditional circuit-switched services over an IP connection. Another major complication that is not found in fixed-line networks is the necessity for voice and other previously circuit-switched services such as SMS to be backward compatible to the services offered in fixed-line networks. For a user, it should be invisible if the service is offered over the circuit-switched part of the GSM or UMTS network or the packet-switched IP-based LTE network. Also, ...

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