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Voice and SMS over LTE 375 days ago 1 comments Categories: Blogger Tags:
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.
http://telecomyou.com/blogs/entry/Voice-and-SMS-over-LTE