OTT and VoLTE Calls

Over and over again I can hear:

Why we need VoLTE, when I can use Skype or Whatsapp (for free)?

Or also:

What is the difference between native (volte) and non-native client?

Of course, everyone likes applications for free. And it’s a great thing we can use our facebook, skype or google accounts practically on any device and from anywhere. Maybe it’s a bit annoying that in order to communicate using such an Over-The-Top (OTT) application, we have to install the same applications as all our buddies.

By OTT we understand an application for a real-time communication using audio, video, and other media over the Internet without the involvement of a mobile operators’ IMS network. But when it comes to mobility we can’t get along without the operators’ infrastructure completely, we still need them to provide us with the internet connectivity.



 Skype and other icons are used only for illustration.

There are many aspects and technical details which can’t be covered in this short article. Please, take this post as a brief introduction into the matter.


At the end we can use a same device for a voice/video calling using some OTT application and a native VoLTE client.



We know already some differences or limitations between OTT application and VoLTE service (or VoWifi).

VoLTE Call

  • Using sip uri or MSISDN (tel uri)
  • Able to call to other telco networks
  • Application of Supplementary services
  • Support of Emergency calls
  • Support of CS fallback and SRVCC
  • Roaming
  • SIM card needed, mostly calling possible only from mobile phone or tablet
  • Paying for bundle


  • Using some OTT identity (skype id, google id, whatsapp id,..)
  • OTT islands (skype-skype, google-google, whatsapp-whatsapp,..)
  • Roaming not needed, but we have to buy data connectivity in a foreign country
  • Using any device
  • Paying for connectivity (+ some advanced features)

But is there any difference when it comes to multimedia session itself? Yes, it is. As VoLTE is handled only by the (fully controlled) operator’s network, we can guarantee the quality of service. TS 23.203 standardizes a set of QoS Class Identifiers (QCI) for different services with the related QoS parameters.

QCI Resource Type Priority Level Packet Delay Budget

(NOTE 13)

Packet Error Loss

Rate (NOTE 2)

Example Services
(NOTE 3)
 GBR 2 100 ms
(NOTE 1, NOTE 11)
10-2 Conversational Voice
(NOTE 3)
 GBR 4 150 ms
(NOTE 1, NOTE 11)
10-3 Conversational Video (Live Streaming)
(NOTE 3)
 GBR 3 50 ms
(NOTE 1, NOTE 11)
10-3 Real Time Gaming
(NOTE 3)
 GBR 5 300 ms
(NOTE 1, NOTE 11)
10-6 Non-Conversational Video (Buffered Streaming)
(NOTE 3, NOTE 9, NOTE 12)
 GBR 0.7 75 ms
(NOTE 7,
10-2 Mission Critical user plane Push To Talk voice (e.g., MCPTT)
(NOTE 3, NOTE 12)
 GBR 2 100 ms
(NOTE 1,
NOTE 10)
10-2 Non-Mission-Critical user plane Push To Talk voice
(NOTE 3)
Non-GBR 1 100 ms
(NOTE 1, NOTE 10)
10-6 IMS Signalling
(NOTE 4)
Non-GBR 6 300 ms
(NOTE 1, NOTE 10)
10-6 Video (Buffered Streaming)
TCP-based (e.g., www, e-mail, chat, ftp, p2p file sharing, progressive video, etc.)
(NOTE 3)
Non-GBR 7 100 ms
(NOTE 1, NOTE 10)
10-3 Voice,
Video (Live Streaming)
Interactive Gaming
(NOTE 5)
Non-GBR 8 300 ms
(NOTE 1)
10-6 Video (Buffered Streaming)
TCP-based (e.g., www, e-mail, chat, ftp, p2p file
(NOTE 6)
Non-GBR 9 sharing, progressive video, etc.)
(NOTE 3, NOTE 9, NOTE 12)
Non-GBR 0.5 60 ms
(NOTE 7, NOTE 8)
10-6 Mission Critical delay sensitive signalling (e.g., MC-PTT signalling)
(NOTE 4, NOTE 12)
Non-GBR 5.5 200 ms
(NOTE 7, NOTE 10)
10-6 Mission Critical Data (e.g. example services are the same as QCI 6/8/9)

For more details (including notes and conversion between QCI and DiffServ) see the VoLTE Policy Control Summary. QCIs are associated with so-called bearers which define how to handle the data in LTE network. Thanks to bearers we can separate the traffic related to IMS (SIP, RTP) from everything else. In general we distinguish default and dedicated bearers. A default bearer is assigned to a particular Access Point Name (APN). It persists as long as the client is connected. A dedicated bearer is created on request and linked to a default bearer which initiated its creation.

QoS and VoLTE and OTT

QoS and VoLTE and OTT – simplified

QoS for VoLTE/VoWifi/ViLTE:

  • QCI=5 for IMS Signalling on the default bearer
  • QCI=1 for conversational voice on dedicated bearers
  • QCI=2 for conversational video on dedicated bearers (ViLTE bearer)

QoS for OTT

  • QCI value of a default bearer depends on a particular operator, in practice we can find QCI in the range 6 to 9
  • The default bearer is shared by Internet applications
  • Technically it’d be possible to create a dedicated bearer for some selected services (this is related also to Net Neutrality, read also 5G and Net Neutrality, or 5G and Telenor)

Within the IMS network we are not creating any bearers, however most of the Session Border Controllers (SBCs) – typically the only network elements which work with media – do support QoS handling and DSCP marking of packets. Moreover SBC should also support media transcoding.

This architecture allows us to achieve HD or even higher quality for VoLTE.

The real quality of a call depends on a particular operator and its QoS implementation. The QoS in IP networks is defined per-hop basis and to achieve end-to-end QoS handling is not an easy task.

Other related posts:


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