LTE & VoLTE

SRVCC Vs. eSRVCC

Posted by Prerak

In case of SRVCC, the home network application server viz., SCC-AS anchors the call.  This anchoring by home network tends to increase the handover delay in case a home subscriber roams to a distant network and latches on to a roaming partner.  The roaming network now awaits signalling from home network before the call can be handed from LTE to 2G/3G and this could result in handover delay exceeding minimum acceptable limits – causing degraded quality of experience for the end subscriber.

In order to avoid this problem, eSRVCC is proposed, wherein when a subscriber roams to a roaming partner network, the call is anchored at the roaming network itself by a new entity called ATCF/ATGW – which results in substantial reduction in delay.  Roaming partner’s ATCF/ATGW coordinates with the home network SCC-AS to complete necessary SIP updates without impacting handover time.

Original Source: http://lteuniversity.com/ask_the_expert/f/59/t/5672.aspx

GTP Cause Values Explained – TS 29.274

Posted by Prerak

“Request accepted” is returned when the GTPv2 entity has accepted a control plane request.

“Context Not Found” is used in the response message by a GTP entity when it receives a message for which it does not
have context, e.g. TEID-C or EBI is not known.

“Context Not Found” may also be used by the PGW during UE requested PDN connectivity procedure while non-3GPP
to 3GPP handover, if the request corresponds to the handover of the PDN connection which does not exist with the
PGW.

“Service not supported” is used by the GTP entity when it receives a message, which corresponds to a feature or a
service which is not supported by the node.

“Service denied” is used when the requested service cannot be granted.

“System failure” is used by the GTP entity to indicate a generic error condition.

“No resources available” is used by the GTP entity to indicate the temporary unavailability of the resource(s) to process
the received request.

“Semantic error in the TFT operation”, “Syntactic error in the TFT operation”, “Semantic errors in packet filter(s)”,

“Syntactic errors in packet filters(s)”, “UE context without TFT already activated”, “Semantic error in the TAD
operation” and “Syntactic error in the TAD operation” are indications of error cases involving TFT(s)/TAD(s) as
specified in subclause 7.7.11 in this specification.

“Missing or unknown APN” is used by the PGW when it does not support the Access Point Name, received in Create
Session Request message.

“Relocation failure” is used by the target MME/S4-SGSN to indicate the source MME/S4-SGSN that the relocation has
failed.

“Denied in RAT” is used by the GTP entity to indicate that the requested service is not accepted in the RAT.

“Preferred PDN type not supported” is used by the PGW to indicate that the PDN type received in the Create Session
Request message is not supported by the PGW for the PDN corresponding to the received Access Point Name.

“Protocol type not supported” is used by the SGW to indicate that the S5/S8 protocol type requested by the MME/S4-
SGSN is not supported by it.

“UE not responding” is used by the MME/S4-SGSN to indicate that the UE is not responding to the request initiated by
the network, e.g. Paging.

“UE refuses” is used by the GTP entity to indicate that the UE, without specifying further detail, rejected the request
from the network.

“Unable to page UE” is used by the MME/S4-SGSN to indicate its inability to page the UE, temporarily.

“User authentication failed” is used by the GTP entity to indicate that the request is rejected due to failure in
authentication/security procedure.

“APN access denied – no subscription” is used to indicate that the PGW has denied the user access to an APN because a
subscription is required, but the subscriber does not have the necessary subscription.

“Remote peer not responding” is used by the SGW for the messages spanning through two interfaces. This cause value
is returned by the SGW to the MME/S4-SGSN or PGW in a response message where no response message is received
from the PGW or MME/S4-SGSN.

“Collision with network initiated request” is used by the PGW to indicate that the UE-initiated bearer resource
allocation/modification request is rejected since the PGW has requested a bearer resource allocation/modification for
the same service using a network-initiated procedure.

“Unable to page UE due to Suspension” is used by the MME/S4-SGSN to indicate that the UE has not been paged
because the bearers of the UE are in a suspended state.

“APN Restriction type Incompatible with currently active PDN connection” is used by the PGW to indicate that the
newly requested PDN connection has APN restriction value that is not compatible with the currently active PDN
connection(s)’s APN restriction value(s).

“Context Not Found” is used in the response message by a GTP entity when it receives a message for which it does not
have context, e.g. TEID-C or EBI is not known.

“Context Not Found” may also be used by the PGW during UE requested PDN connectivity procedure while non-3GPP
to 3GPP handover, if the request corresponds to the handover of the PDN connection which does not exist with the
PGW.

“Service not supported” is used by the GTP entity when it receives a message, which corresponds to a feature or a
service which is not supported by the node.

“Service denied” is used when the requested service cannot be granted.

“System failure” is used by the GTP entity to indicate a generic error condition.

“No resources available” is used by the GTP entity to indicate the temporary unavailability of the resource(s) to process
the received request.

“Semantic error in the TFT operation”, “Syntactic error in the TFT operation”, “Semantic errors in packet filter(s)”,

“Syntactic errors in packet filters(s)”, “UE context without TFT already activated”, “Semantic error in the TAD
operation” and “Syntactic error in the TAD operation” are indications of error cases involving TFT(s)/TAD(s) as
specified in subclause 7.7.11 in this specification.

“Missing or unknown APN” is used by the PGW when it does not support the Access Point Name, received in Create
Session Request message.

“Relocation failure” is used by the target MME/S4-SGSN to indicate the source MME/S4-SGSN that the relocation has
failed.

“Denied in RAT” is used by the GTP entity to indicate that the requested service is not accepted in the RAT.

“Preferred PDN type not supported” is used by the PGW to indicate that the PDN type received in the Create Session
Request message is not supported by the PGW for the PDN corresponding to the received Access Point Name.

“Protocol type not supported” is used by the SGW to indicate that the S5/S8 protocol type requested by the MME/S4-
SGSN is not supported by it.

“UE not responding” is used by the MME/S4-SGSN to indicate that the UE is not responding to the request initiated by
the network, e.g. Paging.

“UE refuses” is used by the GTP entity to indicate that the UE, without specifying further detail, rejected the request
from the network.

“Unable to page UE” is used by the MME/S4-SGSN to indicate its inability to page the UE, temporarily.

“User authentication failed” is used by the GTP entity to indicate that the request is rejected due to failure in
authentication/security procedure.

“APN access denied – no subscription” is used to indicate that the PGW has denied the user access to an APN because a
subscription is required, but the subscriber does not have the necessary subscription.

“Remote peer not responding” is used by the SGW for the messages spanning through two interfaces. This cause value
is returned by the SGW to the MME/S4-SGSN or PGW in a response message where no response message is received
from the PGW or MME/S4-SGSN.

“Collision with network initiated request” is used by the PGW to indicate that the UE-initiated bearer resource
allocation/modification request is rejected since the PGW has requested a bearer resource allocation/modification for
the same service using a network-initiated procedure.

“Unable to page UE due to Suspension” is used by the MME/S4-SGSN to indicate that the UE has not been paged
because the bearers of the UE are in a suspended state.

“APN Restriction type Incompatible with currently active PDN connection” is used by the PGW to indicate that the
newly requested PDN connection has APN restriction value that is not compatible with the currently active PDN
connection(s)’s APN restriction value(s).

As specified in sub-clause 5.3.1.1 in 3GPP TS 23.401 [3] and sub-clause 9.2.1 in 3GPP TS 23.060 [35], the cause value
“New PDN type due to single address bearer only” indicates that the MS has requested PDN type IPv4v6 and both IPv4
and IPv6 addressing is possible in the PDN but the Dual Address Bearer Flag of the Indication IE is set to 0 or the
Indication IE is absent, or only single IP version addressing is possible in the PDN.

“PGW not responding” is used by the SGW in PGW Restart Notification to indicate that the peer PGW has failed and
not restarted as specified in subclause 7.9.5.

“UE context without TFT already activated” is used by the PGW in the Bearer Resource Failure Indication message to
indicate that the PGW has received the Bearer Resource Command message without TAD IE in the secondary PDP
Context Activation procedure.

“Target access restricted for the subscriber” is used by the MME/SGSN in the Context Response message to indicate
that the target access is prohibited for the subscriber, based on the subscription profile.

“P-TMSI Signature mismatch” is used by the SGSN or MME in the Identification Response and Context Response
message if the P-TMSI Signature stored in the old SGSN or MME does not match the value sent by the UE via the new
SGSN or MME.

Service request procedure

Posted by Prerak

The purpose of the service request procedure is to transfer the EMM mode from EMM-IDLE to EMM-CONNECTED
mode and establish the radio and S1 bearers when uplink user data or signalling is to be sent. Another purpose of this
procedure is to invoke MO/MT CS fallback or 1xCS fallback procedures.
This procedure is used when:
– the network has downlink signalling pending;
– the UE has uplink signalling pending;
– the UE or the network has user data pending and the UE is in EMM-IDLE mode;
– the UE in EMM-IDLE or EMM-CONNECTED mode has requested to perform mobile originating/terminating
CS fallback or 1xCS fallback;

The service request procedure is initiated by the UE, however, for the downlink transfer of signalling, cdma2000®
signalling or user data in EMM-IDLE mode, the trigger is given by the network by means of the paging procedure

The UE shall invoke the service request procedure when:
a) the UE in EMM-IDLE mode receives a paging request with CN domain indicator set to “PS” from the network;
b) the UE, in EMM-IDLE mode, has pending user data to be sent;
c) the UE, in EMM-IDLE mode, has uplink signalling pending;
d) the UE in EMM-IDLE or EMM-CONNECTED mode is configured to use CS fallback and has a mobile
originating CS fallback request from the upper layer;
e) the UE in EMM-IDLE mode is configured to use CS fallback and receives a paging request with CN domain
indicator set to “CS”, or the UE in EMM-CONNECTED mode is configured to use CS fallback and receives a
CS SERVICE NOTIFICATION message;

f) the UE performs an inter-system change from S101 mode to S1 mode and has user data pending.

 

Types of EMM procedures

Posted by Prerak

Depending on how they can be initiated, three types of EMM procedures can be distinguished:

1) EMM common procedures:

An EMM common procedure can always be initiated whilst a NAS signalling connection exists. The procedures
belonging to this type are:
Initiated by the network:
– GUTI reallocation;
– authentication;
– security mode control;
– identification;
– EMM information.

2) EMM specific procedures:
At any time only one UE initiated EMM specific procedure can be running. The procedures belonging to this
type are:
Initiated by the UE and used to attach the IMSI in the network for EPS services and/or non-EPS services, and
to establish an EMM context and a default bearer:
– attach and combined attach.
Initiated by the UE and used to attach the IMSI or IMEI for emergency bearer services, and to establish an
EMM context and a default bearer to a PDN that provides emergency bearer services:
– attach.
Initiated by the UE or the network and used to detach the IMSI in the network for EPS services and/or non-
EPS services and to release an EMM context and all bearers:
– detach and combined detach.
Initiated by the UE when an EMM context has been established:
– normal tracking area updating and combined tracking area updating (S1 mode only);
– periodic tracking area updating (S1 mode only).
The tracking area updating procedure can be used to request also the resource reservation for sending data.

3) EMM connection management procedures (S1 mode only):
Initiated by the UE and used to establish a secure connection to the network or to request the resource
reservation for sending data, or both:
– service request.
The service request procedure can only be initiated if no UE initiated EMM specific procedure is ongoing.
Initiated by the network and used to request the establishment of a NAS signalling connection or to prompt
the UE to re-attach if necessary as a result of a network failure:
– paging procedure.
Initiated by the UE or the network and used to transport NAS messages:
– transport of NAS messages;
– generic transport of NAS messages.
The transport of NAS messages procedure and the generic transport of NAS messages procedure cannot be
initiated while an EMM specific procedure or a service request procedure is ongoing.

Voice domain selection in VoLTE

Posted by Prerak

From TS 24.301 :
8.2.1.10:
EPS network feature support:
The network may include this IE in attach accept or TAU accept to inform the UE of the support of certain features. If this IE is not included then the UE shall interpret this as a receipt of an information element with all bits of the value part coded as zero. Note that this is an optional IE.

9.9.3.12A:
The EPS network feature support is a type 4 information element with a length of 3 octets and bits of third octet contains VoLTE feature support indicators as follows:

Bit 1  – IMS VoPS (IMS voice over PS session indicator)

0 –  IMS voice over PS session in S1 mode not supported (VoLTE not supported)
1  –  IMS voice over PS session in S1 mode supported (VoLTE supported)

Bit 2 – EMC BS (Emergency bearer services indicator)
0 – emergency bearer services in S1 mode not supported
1  – emergency bearer services in S1 mode supported

If IMS voice over PS session in S1 mode is not supported, MS may elect to utilize CS voice.

UE handling of this IE From TS 23.221:
If the UE is configured for Voice over IMS, the service domain selection functionality should take the “IMS voice over PS session supported indication” into account and should only initiate IMS voice calls (with the voice bearer in the PS domain) using the RAT where the “IMS voice over PS session supported indication” applies and indicates support.