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, an ongoing voice call over LTE should be seamlessly handed over to GSM or UMTS if the user leaves the LTE coverage area. In other words, the IP-based voice call must be converted to a circuit-switched voice call on the fly as otherwise the overall user experience will be unsatisfactory. Several solutions to bring circuit-switched services to LTE have been specified in the meantime as follows.
SMS over SGs
One of the most popular services besides voice telephony in wireless networks is the SMS. In GSM, SMS uses the signaling channels of the circuit-switched side of the network. In addition, SMS is important to deliver information on international roaming prices and bill shock warning messages to customers. While at first it was envisaged to bring voice service and SMS as a single function to LTE, it was later decided to speed up the deployment of SMS. The result of this is the SMS over SGs functionality as specified in 3GPP TS 23.272.
As shown in the illustration above, the new SGs interface has been specified to forward SMS messages between a GSM/UMTS circuit-switched MSC and the MME of the LTE core network. It is similar to the Gs interface that connects the circuit-switched MSC to the packet-switched SGSN in a GSM/GPRS network to exchange paging notifications and SMS messages. From the MME, the SMS message is delivered in an NAS signaling message to the mobile device. Mobile-originated messages take the reverse path. As in GSM and UMTS, the SMS service remains a non-IP-based service as it continues to be transmitted over signaling channels. On the LTE side of the network, however, the signaling channel is transported over the S1 link, which is based on IP. From an end-to-end point of view, however, SMS remains a non-IP service as the message over the air interface is not embedded in an IP packet but in an RRC signaling message. As a consequence, no IP-based higher layer application is required to send and receive SMS messages.
To send and receive SMS messages while in the LTE network, a mobile device has to inform the MME during the attach procedure of its SMS capabilities. Instead of performing a standard attach procedure, the mobile device sets a flag that the MME should also register the mobile device with the circuit switched GSM or UMTS core network. This is also referred to as a ‘combined attach SMS only’ in the specification documents.
CS Fallback
In addition to SMS messages, the SGs interface can also be used to deliver paging messages that inform the mobile device of an incoming call. The call itself, however, is not delivered over the LTE interface and the mobile device has to fall back to a GSM or UMTS network where a circuit-switched connection is then established for the call. This method of delivering voice calls is therefore referred to as CS (circuit-switched) fallback.
VoLGA
An alternative to CS fallback is the VoLGA solution that has been developed by the VoLGA forum. VoLGA reuses the 3GPP Generic Access Network (GAN) specifications, which add Wi-Fi as an access technology to 3GPP-based networks such as GSM and UMTS. GAN requires dual-mode mobile devices that have both a GSM/UMTS radio interface and a Wi-Fi radio interface. When these dual-mode devices detect the availability of a suitable Wi-Fi network, for example, at home or at a public hotspot, they connect to the Wi-Fi access point and register with the GSM/UMTS core network over the Wi-Fi link and the Internet. A GAN gateway securely connects a subscriber to the infrastructure of a network operator, and voice calls and other circuit-switched services such as SMS are then securely transported between the mobile device and the gateway over the intermediate Wi-Fi link and Internet access network. VoLGA reuses this principle by replacing the Wi-Fi access with LTE. From a mobile device point of view, there are only a few differences between the two access methods because both networks are based on IP. The illustration below gives an overview of the basic network setup for VoLGA as described in the VoLGA Stage 2 specification.
The only new network element introduced is the Voice over Long-Term Evolution via Generic Access Network Controller (VANC). On the LTE side, the VANC connects to the PDN-GW via the standard SGi interface. Both signaling and user data traffic (i.e. the voice packets) are transported over this interface. From an LTE core network point of view, the VANC looks like any other IP-based external node and IP packets exchanged between a wireless device and the VANC are transparently forwarded through the core network. On the circuit-switched network side, the GSM A-interface or the UMTS Iu-interface is used to connect the VANC to a GSM MSC.
IMS and the One Voice Profile
Another option for voice, SMS and multimedia communication in general is to use the IMS. The IMS is based on the SIP, which has become popular in fixed-line networks for establishing voice calls over IP, either in companies or over the Internet. The core of the IMS is formed by a number of logically separate entities that together form the Call Session Control Function (CSCF) as shown in the illustration below;
As the IMS is a network-operator-based platform for voice and multimedia applications, it has direct access to the HLR/HSS and can ensure the QoS for media flows established over it by directly communicating with the underlying transport network infrastructure. With the introduction of LTE, the industry realized that voice calls are still the predominant revenue generator and an initiative was started to use IMS for this purpose with LTE. To speed up IMS deployment, a number of network operators, infrastructure and device vendors started the ‘One Voice’ initiative in which a profile was developed that defines in detail as to which IMS functionalities and optional functionalities are to be used for the deployment of voice in LTE networks.
Internet-Based Alternatives
To a certain degree, Internet-based VoIP services or applications in which real-time voice call functionalities are included are also an alternative to operator-based voice services. Skype and various instant messengers with voice capabilities such as the Yahoo messenger are also available on 3G mobile devices today. With LTE, this trend is likely to continue. The main disadvantage over a network operator-based voice solution is that ongoing voice calls cannot be handed over to a circuit-switched bearer when the user leaves the coverage area where fast IP connectivity via LTE, HSPA or EvDO is available. In addition, Internet-based voice service cannot directly interact with the transport network and it is hence difficult to prefer IP packets that contain voice, especially on the air interface in loaded cells.
