Editor’s Note: This post was originally published in March 2017 and has been updated for accuracy and comprehensiveness.
Voice over Long-Term Evolution, or just simply VoLTE, is a GSMA profile of the standards definition. It was created to deliver over the Packet-Switching only network of LTE services currently provided via Circuit-Switching networks – mainly voice and SMS -, leveraging the core network IP Multimedia SubSystem (IMS). The following chart shows a simplified VoLTE architecture.
What does it mean? First of all, that the voice service must be provisioned as any other IP Service over the LTE data bearer. Secondly, as it is based on IMS, the Session Initiation Protocol (SIP), used in registrations and call control is the same used in DOCSIS and GPON deployment. Let’s review how to provision VoLTE services a little further.
Provisioning VoLTE Services
Under an LTE coverage, a VoLTE device (mobile, tablet, etc.) will automatically perform an LTE Attach followed by an IMS registration for VoLTE if supported by the network. The LTE Attach phase will occur even when VoLTE services are not available. If the operator has already deployed LTE and it’s adding VoLTE services, such as incoming calls, outgoing calls, and supplementary services the only real difference lies in IMS registration for the new services. So let’s check which is new stuff.
As mentioned earlier, the IMS registration protocol (and the call initiation session as well) is SIP. Due to this, after IMS APN information is returned by the HSS, the UE sends the following registration information:
- “+sip.instance” containing an IMEI URN
- The IMS Public User Identity e.g: “tel:+447700900123”.
- The IMS Private User Identity as an NAI: e.g. username@realm
In the most common scenario, the UE will retrieve the Private User Identity and the Public User Identity and the shared secret (necessary to the IMS-AKA authentication) from the UICC, which is integrated into the SIM card. All information needs to be provisioned in the HSS so subscribers can successfully register the device in the IMS.
From the IMS point of view, the three distinctive parameters to be provisioned in the HSS (apart from the subscriber profile) that distinguish what makes a subscriber unique are retrieved from the UICC. They are also used in the IMS-AKA authentication as part of any other SIP-based voice service using IMS as the voice network (e.g DOCSIS, GPON, etc).
Fallback to 2G/3G and Roaming
Until there is a full coverage of LTE, there will be several areas where only 2G or 3G will be available. New subscribers with voice and text services will have to be provisioned as they are right now. VoLTE can be seen then as additional service running on top of the current services.
Leaving aside what we just said, the first VoLTE deployments will be self-contained in the mobile operator’s network, serving its own subscribers only (no inter-operator VoLTE interconnect or roaming capability). So if user roams from its operator’s network to a visited network, the subscriber will still expect to have service, so the current roaming services should be still provisioned.
The first challenge is to maintain the QoS. As bandwidth limitations imposed by the circuits are no longer there, more advanced codecs, such as the AMR-WB (which supports 16kHz sampling rate) is used to guarantee an excellent quality. In fact, most VoLTE calls have excellent audio quality, but if something doesn’t go as planned, they have a negative impact on the call. Even the most minimal temporary packet loss in a VoLTE call can cause an intolerable experience for the user.
The second challenge is the return of investment of deploying IMS. The adoption of IMS has been somewhat slow and a little bit disappointing. Network operators waited for the technology to become stable and the standard fully ratified, reducing the chances of launching advanced new services.
Overall voice profit is on demise due to the popularity of OTT Service services such as Skype, Messenger, WhatsApp, and the effects of competition and regulatory intervention on termination rates (e.g. roaming in Europe). It will be difficult for CSPs to charge more for VoLTE services and introducing expensive video plans is not a viable option when subscribers are getting this for free with OTT players.
Another challenge is the operators will have to make modifications to their current infrastructure in order to pass the certification process prior to VoLTE deployment. Once this is done, operators will have to connect the newly IMS infrastructure to multiple older networks, enterprise networks, and other service providers. Adding another layer to the OSS Full stack.
5G as a Driver for VoLTE Deployment
VoLTE in 4G networks is great, but the truth is that almost no one will pay an additional fee for High Quality voice service. So what may be driving the VoLTE deployment?
In a 5G circuit-switched fallback is also not supported, so if voice over 5G is not available, a fallback to VoLTE is needed. Now if VoLTE is not yet deployed, another fallback is needed to 3G/2G networks. All that leads to around six seconds (6s) to properly route the call, and that is too long.
The fallback to VoLTE is not great. Around two seconds are needed in that scenario, but that is quite acceptable for human beings.
Operators looking to offer voice services over 5G networks MUST have VoLTE deployed by the time the 5G network is deployed.
VoLTE Deployment status
Despite all these challenges, VoLTE is surely making its way. As of 2019, 256 operators are investing in VoLTE in 116 countries including 116 operators with commercially launched VoLTE-HD voice service in 89 countries worldwide.
In the US market, AT&T, T-Mobile, Verizon and Sprint (the four major operators) have already launched VoLTE services.
In LATAM, the main operators in Mexico (Telcel, Movistar), Brazil (TIM, ViVO), Argentina (Claro, Movistar, Personal), Brazil (TIM, ViVO), Peru (Movistar, Claro), Colombia (Tigo, Claro) and in other countries are already deployed or on the way of doing it.
The same scenario can be found in EMEA and APAC.
From the provisioning point of view, VoLTE adds a new layer to the OSS stack, adding an extra motivation to accelerate the OSS transformation and move to full stack.
The OSS activation, orchestration, and management platform at least has to be able to:
- Support handling the 2G/3Gmobile provisioning in the HSS with the IMSI, MME Identity, ME Identity, and MME capabilities to support the current voice and text services in the EPC; GPRS or any other 3GPP network. Fallback and roaming are a must.
- In a 5G scenario, VoLTE is a must-have service.
- Seamlessly support voice service provisioning convergence in the IMS (LTE, GPON, DOCSIS etc). Even though SIP is the common protocol for all packets networks, the provisioning details may differ a little. Having proven experience in DOCSIS and GPON IMS deployment is a plus.
- Be effective in supporting multiple provisioning flows and order decomposition as the subscriber’s services are not attached to a single network technology and the number of micro-services will grow exponentially
Learn more about how to deploy new services without coding.