1. The Voice Dilemma: A Consequence of the All-IP Network

The architecture of LTE (Long Term Evolution) was a revolutionary step in mobile communications. Its designers made a bold and forward-thinking decision to create an All-IP network. This means that unlike previous generations (2G and 3G), the LTE network does not have a separate, dedicated infrastructure for traditional phone calls. In the world of LTE, everything is data.

Previous generations relied on a technology called Circuit Switching for voice calls. When you made a call on a 2G or 3G network, a dedicated, end-to-end circuit was established between you and the person you were calling. This circuit was reserved exclusively for your conversation for its entire duration. This was reliable, but also inefficient. The LTE network, being entirely packet-switched like the internet, has no concept of a dedicated circuit.

This created a major challenge: How do you make a normal phone call on a network that was built only to handle data packets? In the early days of 4G, this problem was solved with a temporary and rather clumsy workaround known as Circuit-Switched Fallback (CSFB).

2. The Temporary Fix: Circuit-Switched Fallback (CSFB)

Circuit-Switched Fallback was a clever but ultimately stopgap solution that allowed LTE-capable phones to make and receive traditional voice calls by temporarily "falling back" to an older network technology.

How CSFB Works

Imagine you are using your phone on the 4G network, browsing the web at high speed. A phone call comes in. The following sequence of events takes place with CSFB:

1. The LTE network (specifically the MME) detects an incoming voice call for your device.
2. The MME sends a command to your phone, instructing it to temporarily suspend its LTE data connection.
3. Your phone's radio then "falls back" and connects to the operator's older 3G (UMTS) or 2G (GSM) network, which has the necessary circuit-switched infrastructure for voice calls.
4. Once connected to the 2G/3G network, the call is established and you can talk.
5. During this entire call, your phone remains on the 2G/3G network. Any internet services you were using are either completely suspended or operate at the much slower speeds of the older network.
6. When the call ends, your phone disconnects from the 2G/3G network and re-attaches to the 4G LTE network to resume high-speed data services.

The Downsides of CSFB

While functional, CSFB provided a poor user experience and had several significant technical drawbacks:

• Longer Call Setup Times: The entire process of falling back to another network and then establishing the call takes several seconds, resulting in a noticeable delay between dialing and hearing the first ring.
• No Simultaneous Voice and Data: This was a major step backward. While on a CSFB call, your high-speed 4G data connection is inactive. You cannot browse a map to find directions while talking to someone or look up an email during a conference call.
• Inefficient Use of Network Resources: Maintaining both a modern 4G network and legacy 2G/3G networks purely for fallback is expensive and complex for operators. It also requires constant signaling between the different network cores.

It was clear that a native, long-term solution for voice was needed for the All-IP LTE network. That solution is VoLTE.

3. VoLTE: The True Voice of 4G

VoLTE stands for Voice over Long Term Evolution. It is the standardized technology for delivering high-quality voice communication as just another data service over the efficient LTE network. Instead of falling back to an old network, a VoLTE call is treated as a stream of IP packets, just like video from YouTube or a webpage from a server.

The Key Benefits of VoLTE

By handling voice natively within the LTE architecture, VoLTE provides a superior experience and numerous technical advantages:

• Superior Call Quality (HD Voice): VoLTE calls offer significantly clearer and more natural-sounding audio than traditional 2G/3G calls. This is achieved by using advanced, wideband audio codecs like AMR-WB (Adaptive Multi-Rate Wideband). While traditional calls are limited to a narrow frequency range of $300$ Hz to $3.4$ kHz, which makes voices sound "flat," HD Voice captures a much wider range, typically from $50$ Hz to $7$ kHz, preserving more of the nuances of the human voice.
• Faster Call Setup Times: Because the phone does not need to switch networks, VoLTE call setup times are dramatically reduced, often taking less than half the time of a CSFB call. The connection is established almost instantly after dialing.
• Simultaneous Voice and High-Speed Data: This is one of the most noticeable benefits for users. With VoLTE, your phone remains on the 4G network during the call. This means you can continue to use high-speed data services without interruption while you are on a call. You can talk on speakerphone while navigating with Google Maps, browse the web for information during a conversation, or download an email attachment.
• Improved Battery Life: The CSFB process of searching for and connecting to another network consumes a significant amount of battery power. By staying on the LTE network, VoLTE can lead to improved battery life compared to making frequent calls in a CSFB environment.
• Enabling Rich Communication Services (RCS): VoLTE's IP-based nature is the foundation for advanced communication services, known as RCS. This can include features like video calling integrated into the native dialer, real-time file sharing, and advanced messaging, turning the basic phone call into a richer multimedia experience.

4. How VoLTE Works: A Look Under the Hood

Delivering reliable, high-quality voice as data packets is not as simple as running an application like Skype over 4G. VoLTE requires a dedicated and sophisticated architecture to ensure that voice calls are prioritized and handled perfectly every time. This architecture is called the IP Multimedia Subsystem (IMS).

IMS: The Control Center for VoLTE

The IMS is a layer of network components that sits on top of the standard LTE data network (the EPC). Its job is to manage multimedia sessions, which includes VoLTE calls. The "language" used within the IMS to set up, manage, and tear down these sessions is a protocol called SIP (Session Initiation Protocol). When you make a VoLTE call, your phone uses SIP to send an "INVITE" message to the IMS, which then routes the call to the destination.

Quality of Service (QoS): The Secret to VoLTE's Success

This is the single most important concept that differentiates VoLTE from standard over-the-top (OTT) VoIP apps. When you use an app like WhatsApp or Skype for a call, your voice packets are sent over the internet on a "best-effort" basis. They are treated just like any other data packet, competing for bandwidth with emails, video streams, and downloads. If the network is congested, your call quality will suffer.

VoLTE works differently. When a VoLTE call is initiated, the IMS system instructs the LTE network to establish a dedicated EPS bearer with a specific Quality of Service Class Identifier (QCI).

• For VoLTE signaling (SIP messages), a dedicated bearer with QCI = 5 is established.
• For the actual voice media (the RTP packets carrying your speech), a dedicated bearer with the highest priority, QCI = 1, is established.

A bearer with QCI 1 is a special VIP lane on the data highway. It has a guaranteed low latency and a prioritized traffic flow. Packets on this bearer are given absolute priority over all other types of data traffic. This ensures that your voice packets always get through quickly and without interruption, even if the cell is heavily congested with other data users. This guaranteed Quality of Service is what gives VoLTE its carrier-grade reliability and clarity, making it a true replacement for traditional circuit-switched voice.

The VoLTE Call Flow: A Step-by-Step Guide

Here is a simplified sequence of events for making a VoLTE call:

1. IMS Registration: When your VoLTE-capable phone first connects to the LTE network, it performs a registration procedure with the IMS core. This establishes its identity and readiness to make and receive VoLTE calls. This involves setting up a default EPS bearer for signaling.
2. Call Initiation: You dial a number and press the call button. Your phone's VoLTE application creates a SIP INVITE message. This message contains information about the call, such as the number you are calling and the audio codecs your phone supports.
3. Signaling Path: The SIP INVITE is sent over the QCI 5 signaling bearer to the IMS core.
4. Bearer Setup: The IMS core receives the INVITE and signals to the core data network (EPC) and the radio network (eNodeB) to set up a new dedicated bearer with QCI 1 specifically for the upcoming voice media.
5. Call Alerting: The IMS network routes the SIP INVITE to the person you are calling. Their phone receives the message and starts to ring. Their phone sends back a SIP "Ringing" message.
6. Call Answered: The other person answers the call. Their phone sends a SIP 200 OK message back to the IMS network, which forwards it to your phone.
7. Media Flow: Both phones are now ready. The voice conversation begins. Your speech is digitized, compressed by the AMR-WB codec, placed into RTP (Real-time Transport Protocol) packets, and sent over the highly-prioritized QCI 1 dedicated bearer.
8. Call Termination: When one person hangs up, their phone sends a SIP BYE message. The network then tears down the QCI 1 dedicated bearer, freeing up the resources, and the call is terminated.