SRVCC and CSFB: What You Need to Know

SRVCC and CSFB are two technical terms you might have heard of, but what exactly do they mean? SRVCC stands for Single Radio Voice Call Continuity, which allows seamless handover of voice calls between different networks, ensuring a smooth experience for users.

In simpler terms, SRVCC enables voice calls to continue uninterrupted even when a user switches from one network to another. This is particularly useful when traveling or in areas with weak network coverage.

CSFB, on the other hand, stands for Circuit Switched FallBack, which is a technique used to provide voice services over a cellular network. It allows users to make voice calls even when their device is connected to a data network.

CSFB is often used in conjunction with SRVCC to provide a more comprehensive voice experience for users.

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SRVCC is a feature that allows Voice over IP (VoIP) subscribers to continue a voice call even when they move out of an LTE coverage area.

It uses the IMS network to anchor the call, which is then transferred to the circuit-switched network to ensure continuity.

SRVCC works without requiring a dual-mode radio, unlike other methods like CSFB.

This means that users can enjoy seamless voice call continuity without the need for additional hardware or complex setup processes.

SRVCC is specifically designed for VoIP subscribers, who can benefit from this feature to keep their calls going even when they're out of LTE range.

By leveraging the IMS network and the circuit-switched network, SRVCC provides a reliable and efficient way to maintain voice call continuity.

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SRVCC is a game-changer for mobile devices and network operators alike. It allows mobile devices to continue voice calls even when they move out of range of the LTE network.

One of the key benefits of SRVCC is that it reduces the number of dropped calls. This is especially important for emergency calls, where continuity is critical.

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SRVCC also improves the quality of voice calls. By transferring VoLTE calls in progress from LTE to legacy voice networks, SRVCC ensures that voice calls remain uninterrupted.

Here are some of the specific benefits of SRVCC:

By using SRVCC, operators can accelerate time to market and realize benefits during the entire time span from today’s hybrid network environments. They can also get more ROI from Legacy 2G/3G Network by offloading some traffic.

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CSFB can cause a short interruption in the voice call during the handover process. This is one of the challenges of using CSFB.

CSFB can require more network resources than a traditional voice call. This is another challenge of using CSFB.

Here are some of the challenges of using CSFB:

CSFB offers several benefits that make it a valuable feature for mobile devices. It allows mobile devices to continue voice calls even when they move out of range of the LTE network.

One of the main advantages of CSFB is that it reduces the number of dropped calls. This is especially important for people who make a lot of calls or have poor network coverage in certain areas.

CSFB also improves the quality of voice calls. This means that users can enjoy clear and uninterrupted conversations, even when they're on the move.

Here are some of the key benefits of CSFB at a glance:

CSFB can cause a short interruption in the voice call during the handover process.

These interruptions can be frustrating for users, especially if they're in the middle of an important conversation.

CSFB can require more network resources than a traditional voice call.

This can put a strain on the network, potentially leading to slower speeds or dropped calls.

Here are some of the challenges of using CSFB:

The Oracle Communications Session Border Controller (OCSBC) supports SRVCC handover in the pre-alerting phase, ensuring seamless calls between PS and CS networks. This includes support for both outgoing and incoming calls.

To manage the handover, the OCSBC, acting as ATCF, selects any early dialogs as the session being transferred when certain conditions are met. These conditions include the absence of confirmed dialogs with inactive speech media components and the presence of dialogs with active speech media components.

The SRVCC network architecture has evolved continuously since 3GPP Release 8, with the GSMA providing guidelines for interoperability with legacy networks. The recommended architecture for SRVCC is 3GPP Release 10, which reduces voice interruption delay and dropped call rate.

Here are the key components of the Release 10 configuration:

SRVCC Network Architecture is designed to provide continuity for PS to CS handover between LTE and WCDMA/GSM networks.

Starting from 3GPP Release 8, SRVCC has evolved continuously to ensure interoperability with legacy networks. The GSMA has provided guidelines detailing the requirements for networks and user devices.

The Release 10 configuration is recommended by GSMA guidelines, as it reduces both voice interruption delay during handover and the dropped call rate compared with earlier configurations.

SRVCC provides a fully network-controlled handover mechanism, with calls remaining under the control of the IMS core network. This network control guides the user device from LTE to 2G/3G as the user moves out of LTE coverage.

The Release 10 configuration includes all components needed to manage time-critical signaling between the user's device and the network. This ensures that signaling follows the shortest possible path and is as robust as possible.

Signaling between network elements within the serving network, including visited networks during roaming, is also managed by the Release 10 configuration. This minimizes voice interruption time caused by switching from the PS core to the CS core.

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The Oracle Communications Session Border Controller (OCSBC) supports SRVCC handover support in the pre-alerting phase to manage handovers from 4G to 3G/2G of sessions.

To ensure calls in a pre-alerting phase are transferred between PS and CS networks, you need to set the sip-feature-caps value as follows: set state to enabled, set atcf-management-uri to management or psi, and set atcf-pre-alerting to enabled.

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The OCSBC, acting as Proxy Call Session Control Function (P-CSCF) or Active Transfer Control Function (ATCF), selects any of the early dialogs as the session being transferred when there are no confirmed dialogs supporting a session with an inactive speech media component.

Applicable dialogs include those for which a SIP 180 (Ringing) response to the SIP INVITE request has not been received yet in any of the existing dialogs, all dialogs are early dialogs created by the same SIP INVITE request, the Contact header field provided by the SC UE includes the g.3gpp.ps2cs-srvcc-orig-pre-alerting media feature tag, and the Feature-Caps header field provided by the SCC AS towards the SC UE includes the g.3gpp.ps2cs-srvcc-orig-pre-alerting feature-capability indicator.

The OCSBC SRVCC pre-alerting phase support includes SRVCC support for both outgoing and incoming calls, responding to media feature tag and fcaps indicator, SRVCC support of EATF emergency calls, handover (HO) cancelling scenarios, Rx interactions, and SRVCC Error scenarios.

Here are some key features of OCSBC SRVCC pre-alerting phase support:

SRVCC works by enabling the transfer of calls in both directions from 4G to 2G/3G and 2G/3G to 4G. This is done through two steps: Inter RAT Handover and Session transfer.

The handover process is initiated by a request for session transfer from the IMS CSCF. The IMS CSCF responds simultaneously with two commands, one to the LTE network, and the other to the legacy network.

The LTE network receives a radio Access Network handover execution command through the MME and LTE RAN, instructing the user device to prepare to move to a circuit-switched network for the voice call. This is done in a number of steps, which include:

The SRVCC handover process is similar when returning a call to the LTE network. The legacy RAN can implement two options to provide a swift and effective return: allowing LTE information to be broadcast on the legacy RAN, or simultaneously releasing the connection to the user device and redirecting it to the LTE RAN.

KPIs are a crucial part of evaluating SRVCC performance.

SRVCC Interruption Performance is a key issue with VoLTE and SRVCC, where the interruption time when handing over from an LTE RAN to a legacy RAN can be a problem. The key technique behind reducing this time is to simultaneously perform the IRAT Handover and session transfer.

To measure SRVCC success, we look at the SRVCC Success Rate (%), which tells us the percentage of calls successfully transferred with SRVCC from LTE to 2G/3G Network. This includes both CS and PS call transfers.

The Call Drop rate (%) is another important metric, which tells us the percentage of calls getting dropped abnormally. Requirements for Call Drop rate is less than 1%.

Here's a summary of the key SRVCC KPIs: