Saturday, 31 August 2013

VoLTE Bearers

While going through Anritsu whitepaper on VoLTE, I found this picture that explains the concepts of bearers in a VoLTE call well. From the whitepaper:

All networks and mobile devices are required to utilize a common access point name (APN) for VoLTE, namely, “IMS”. Unlike many legacy networks, LTE networks employ the “always-on” conception of packet connectivity: Devices have PDN connectivity virtually from the moment they perform their initial attach to the core network. During the initial attach procedure, some devices choose to name the access point through which they prefer to connect. However, mobile devices are not permitted to name the VoLTE APN during initial attach, i.e., to utilize the IMS as their main PDN, but rather to establish a connection with the IMS AP separately. Thus, VoLTE devices must support multiple simultaneous default EPS bearers.

Note that because the VoLTE APN is universal, mobile devices will always connect through the visited PLMN’s IMS PDN-GW. This architecture also implies the non-optionality of the P-CSCF:

As stated, VoLTE sessions employ two or three DRBs. This, in turn, implies the use of one default EPS bearer plus one or two dedicated EPS bearers. The default EPS bearer is always used for SIP signaling and exactly one dedicated EPS bearer is used for voice packets (regardless of the number of active voice media streams.) XCAP signaling may be transported on its own dedicated EPS bearer – for a total of three active EPS bearers – or it may be multiplexed with the SIP signaling on the default EPS bearer, in which case only two EPS bearers are utilized.

My understanding is that initially when the UE is switched on, a default bearer with QCI 9 (see old posts on QoS/QCI here) is established that would be used for all the signalling. Later on, another default bearer with QCI 5 is established with the IMS CN. When a VoLTE call is being setup, a dedicated bearer with QCI 1 is setup for the voice call. As the article says, another dedicated bearer may be needed for XCAP signalling. If a Video call on top of VoLTE is being used than an additional dedicated bearer with QCI 2 will be setup. Note that the voice pat will still be carried by dedicated bearer with QCI 1.

Do you disagree or have more insight, please feel free to add the comment at the end of the post.

The whitepaper is embedded below and is available to download from slideshare.

Related posts:


Neil Wiffen (Via LTE Evolution group on Linkedin) said...

Hi Zahid,
there are several choices / options available to establish VoLTE connectivity, the one you have extracted from the Anritsu paper is totally valid, but is simply one option. The default bearer for a given handset could be a QCI 5 providing automatic connectivity to the IMS and then subsequent IMS/VoLTE signalling would use this bearer to establish a dedicated bearer with QCI 1 for voice media flow as and when required. If a user with this configuration active then wants to access a non VoLTE / non IMS service (e.g. internet access), then a second default bearer would be established to a different PDN (the PGW that provides internet access), which would possibly be QCI 9. But again this is just another option. The main point here is that the service provider / network operator can provision services in a way that they choose based on the subscription profiles of the users, device capabilities and network configuration. Therefore different devices / subscribers on the same network can have a different order of bearer setup.
Hope this helps

raghu said...

How about emergency call scenario. 1 default and 1 dedicated bearer.

Bem-Disposto said...

Hi Zahid,

As Neil Wiffen said there are some different approaches to VoLTE with different complexities. The Anritsu paper has a possible network architecture but they are missing an important node that policies the priorities of the data and can differenciates different types of users. This node is the PCRF. It is not mandatory but it has an extremely important role in the network.

By the way, the Neil's comment is right about the QCIs. In my opinion, a VoLTE compliant network should have 4 different QoS profiles:

- QCI=9 > General internet data traffic
- QCI=5 > IMS signalling (to be distinguished from the previous one)
- QCI=2 > Video media (if it is an available feature)
- QCI=1 > Audio media

With this configuration the network can differenciate the traffic types and proccess the data in the right order.

Carlos Miguel Pereira

Ivan Cavlina said...

Hi Zahid,
I completely agree with the comments above and just want to add that VoLTE subscriber need to use UE that is supporting VoLTE (VoLTE capable Smartphone). UE will indicate this to MME by setting Voice domain preference for E-UTRAN: IMS PS voice preferred, CS Voice as secondary.
For this kind of subscriber I would assume first default bearer will be toward IMS APN since UE will immediately after Attach perform SIP registration. If we assume you have Android smartphone and you want to access Internet via this UE you would have to configure APN in Android OS settings. Once you configure APN and activate data UE will trigger second Default bearer that will be used for Internet connection. There is also an option to use this UE as a modem for your PC and in this case you would probably use some kind of Connection Manager SW that have possibility to configure additional APN (this should correspond to the tethering APN if allowed by operator). This will additionally trigger third default bearer. On some smartphones there is also possibility to define tethering APN. This would then be used when you enable tethering on the UE in order to access internet from the PC.
I hope this is usefull addition to the previous comments.

Zahid Ghadialy said...

Thanks Ivan and Carlos.

Erwan Duval (Via LTE Evolution group on Linkedin) said...

Hi Zahid,

As mentioned Neil, there are several choices, but some recommendations are defined in the spec GSMA IR 92 (

Anonymous said...

There is a point in not having the IMS PDN connectivity as part of the initial attach. In my understanding not all operators allow roaming for the IMS APN so if a UE is roaming and tries to establish IMS PDN connection in the intial attach it will not get any connectivity at all and it will not be able to perform any subsequent PDN connectivity request for any other APN.

Zahid Ghadialy said...

@Anonymous, interesting information. Lets see if anybody else has an opinion on this.

Erwan Duval (Via 4G Evolution group on Linkedin) said...

The operator can restrict the use of APN at PLMN level. This restriction is managed in the LTE-HSS.

Therefore, the operator can authorize some subscribers to use APN for internet while roaming in particular PLMNs and inhibit APN for IMS in other PLMNs.

Neil Wiffen (Via 4G Evolution group on Linkedin) said...

In an initial attach, the UE doesn't specify a particular PDN to connect to, it indicates a 'PDN connectivity request'. The decision to which PDN the default bearer is established is made by the MME taking into account the UE / Subscription / supported services / allowed PDNs, current loading etc. etc.
Therefore if it is appropriate to create a default bearer to the IMS based on the above, then the MME can make that decision (not the UE)

Anonymous said...

@Erwan: So do the LTE-HSS have different profiles with different default APN per roaming network? And how do the UE know what pdn connectivity it do get in the initial attach. The PDN Connectivity Accept contains the serving APN but can the different layers in the UE use that information to determine what APN it got and what it have to further request? At least what I have heard the Android OS needs to have a UI configured APN for internet to be able to access internet services.

Erwan Duval (Via 4G Evolution group on Linkedin) said...

the HSS have a single profile for both LTE and IMS. In the LTE subscription, we can configure different APN per subscriber which one is the default one.
At the initial attach, the MME fetches from the HSS all the APNs authorized in the PLMN.
The one which is tag as default is used for the initial attach (so in our discussion either IMS apn or Internet apn).

vikas said...

@Erwan: So as it is clear from the discussion that there are several choices available with the operator if they want to use Internet or IMS APN for the first default bearer. I wanted to know in case the second default bearer is established due to data connectivity (Internet APN), will this default bearer remain as long as user is in registered mode or it will vanish once the data usage is over and will again appear when user initiates any data application?

Anonymous said...

Actually there is a mistake in this description. The XCAP signaling cannot use the IMS APN but must use a home routed APN. This signaling is from the Ue to the AP in the home network and as such it cannot use a locally terminated APN.

Richard Good said...


Excellent discussion - there are several options but I think IR-92 specifies a particular one:

"The IMS application must use the IMS well known APN as defined in PRD IR.88; any other application must not use this APN."

So for data you would use internet APN with default bearer QCI9 and for IMS you would use IMS APN.

IR-92 also says:
A default bearer must be created when the UE creates the PDN connection to the IMS well known APN, as defined in 3GPP specifications. A standardised QCI value of five (5) must be used for the default bearer. It is used for IMS SIP signalling."

So the IMS APN will have a default bearer with QCI 5 for IMS Signalling and will support dedicated bearers for voice (QCI 1) and video (QCI 2).

IR-92 also says:
"it is highly recommended that the default bearer is used for SIP and XCAP. This reduces the number of EPS bearers used. However, in multimedia operations, another configuration can be used."

It makes sense to use the IMS Signalling bearer for XCAP as well.

I have a question though - if we have an IMS APN with default bearer QCI 5 - how do we stop a cracked device from using this for prioritised (and potentially zero rated) data traffic? D we put a filter on the default bearer dropping all traffic that isn't IMS or XCAP signaling?


Mihai S said...

@Richard Good
As Anonymous explained, it's good to route the XCAP through the "internet" APN. As for the filters, I think is better to assign to the "ims" APN default bearer some traffic flow templates (TFTs) that allow only SIP (for IMS) and DNS traffic (for discovering the P-CSCF if it is the case). Anyway the ims default bearer doesn't have routing towards internet, so no traffic can be done through it. PGW protects the network against a cracked UE that might ignore the TFTs and flood the network with some garbage.

To have a clear picture of a UE connectivity when is powered on :

1. at attach the network is assigning the "internet" APN bearer to the UE. This is configured in HSS as “default APN” for the subscriber.
2. afterwards the UE initiates a "PDN connectivity request" asking for "ims" APN default bearer that will be used for SIP signaling. A this point the network can attach the above TFTs to the bearer. If the network denies the “ims” default bearer activation, the UE will stay still connected through “internet” APN and will be able to do data traffic.

Taro Shirai (via GSMA VoLTE Linkedin group) said...

operator can choice, but my conclusion is same as you. IMS PDN should not be the 1st PDN.
in case of LTE roaming, and VPLMN doesnt' support VoLTE... any PDN can not be activated properly.

Mircea said...

I have a question which might be related to qcis. If I turn off my data connection in my smartphone will I still be able to do a volte service? For sure I will, but what exactly is forbiden by closing the ue data connection?

Mihai S said...


Disabling the data connection in UE is done only locally, it does not send any messages to the network. Both internet (QCI 9) and ims (QCI 5) default bearers are kept in the UE, however the internet one is marked as inactive while ims one can be used any time and the VoLTE calls can be done through dedicated bearers (QCI 1 for voice and QCI2 for video).

Anonymous said...

@ Mihai S
You think that PDN activation request with internet APN will be accepted by the UE ?

Kim N

Mihai S said...

At attach request the UE does also a "PDN connectivity request" without specifying the APN.
At attach accept, the Network is sending a "Activate default bearer request" to the UE specifying the internet APN (the default APN Configured in HSS), IP, QCI, QOS and other parameters.
If these all these parameters are supported by UE it will accept the connection at attach accept phase through "Activate default bearer accept" message.

Anonymous said...


If the user desactivates internet flow, for me this forbide the use of "internet" APN, so UE will reject the default bearer acivation request for this APN, then


Mihai S said...

The 4G phones I worked with (Samsung, Sony) do not reject any internet APN when the data mode is disabled. They just don't use this APN at all; this is the easiest implementation in my opinion. Of course other implementations/behaviours can be possible ... it's up to the UE manufacturer.

Anonymous said...

Hi All, after read through this post, a simple question, if my mobile phone (LTE) installed the RCS linked to IMS, when i make a call, this is VoLTE? as RCS through IMS also means the QCI=5 right? so when i make a SIP Call, the PCRF together with MME will create Dedicated Bearer for Voice QCI=1 right? so this is called VoLTE also? Thanks

rudy said...

Hi mihai,so ues need to have 2 default bearer qci 9 and 5 to support internet and ims,right? If yes,in the attach request what will be voice-domain-preference-and-ues-setting value to support 2 default bearer internet and ims? And what will be the pco content value...

Anonymous said...

The UE will send first attach request with PDN connectivity request as Internet to establish the first default bearer for data and then another attach with PDN connectivity request as voice to establish a default bearer for IMS signalling.
If UE does not send any indication in the attach request then how does HSS return the default APN value for data and voice?
Please correct me if wrong.

Anonymous said...

Interesting discussion this.
After reading through all opinions, I feel it really depends on the operator which APN to be deemed as "default" for initial UE attach.
I believe it makes sense for the "internet APN" to be the default one. In such a scenario, considering home conditions, following should be the flow (please correct me if I'm wrong):

1. UE switched on (LTE network is available)
2. MME requests a PDN attach with Internet APN.
3. UE attaches itself with the LTE network and is ready for any data communication (Default Bearer, QCI-9)
4. The user decides to make a VoLTE call (or receives one).
5. Another default bearer (QCI-5) is established, device is ready for SIP signalling.
6. As a result of call being established through SIP signalling, a dedicated bearer (QCI-1) is established, voice communication can now begin.
7. After the user ends the call, both the dedicated bearer (QCI-1) and default IMS bearer (QCI-5) are removed.

Anonymous said...

What is going if you have ims apn set in UE and non capable SIM card. How should UE behave?

shatrunjai singh said...

Hi All,
Can anyone please let me know what are UE capabilty for VoLTE?

Anonymous said...

Hi, here is my question:

is it possible a pure 4G LTE phone (not a VoLTE phone) to register successfully in pure PS LTE network with IMS Core?

Anonymous said...

I noticed 'hos' as a apn for Telstra network but not much info on its function. Thanks for any reply

Anonymous said...

In one network I noticed QCI 9 and QCI 5 are always established and deactivated together for VoLTE provisioned subscribers for any Internet/data activity/inactivity. So QCI 5 attempts are almost same as QCI 9 attempts for a VoLTE subscriber (radio end counter). It seems to be unnecessary as we need QCI 5 only during VoLTE session. The benefit could be fast VoLTE setup if UE is already in RRC connected state due to data activity. I want to know if it is as per 3GPP standard? If yes then I would appreciate the reference document number.


Carlos P. said...


Actually the QCI5 is used for the default bearer of IMS signalling (IMS control plane) so it must be always activated together with the "internet" default bearer (in your case QCI9). If it was deactivated you cannot register towards IMS network neither do calls in VoLTE. The bearer that is activated only with calls is a dedicated bearer with QCI1 that is used to transport RTP packets (media packets) of the user. During the call you should have 3 bearers:
- default bearer with QCI9 (apn "internet") - used for internet data
- default bearer with QCI5 (apn "ims") - used for IMS signalling
- dedicated bearer with QCI1 (over the apn "ims") - used for media (audio)

You read GSMA IR.92 that explains these concepts.

Anonymous said...

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Thus that's why this piece of writing is great.