VoLTE: End to end QoS control

The end to end QoS control is an important factor to be considered to provide a qualified service to users. It would become more so when the service is required to be provided in real time such as voice call or multimedia streaming.  If the voice call service is provided through the best-effort network where the QoS is not guaranteed, the users will experience frequent call attempt failure, call setup delay, call drop, voice cracking, etc. 

In the following are introduced several different level of QoS control methodologies for VoLTE service.

I. Separated APN for VoLTE

The UE can attach multiple PDNs at the same time and each PDN connection can have multiple EPS bearers. Each PDN connectivity has one default EPS bearer and can have multiple dedicated EPS bearers up to 11 in total. The UE is assigned with a different IP address for each APN by the P-GW during PDN connectivity procedure for each APN.

In general, the operator uses an IMS APN separately from the Internet APN. This dedicated APN for VoLTE service may have the intention to provide the quality of VoLTE service not to be affected by other non-real time service such as the internet service. The default APN is provided by the network based on subscriber profile fetched from the HSS during the initial attach procedure. 

The following diagram shows a call flow of initial attach procedure. During the initial attach procedure, the MME obtains the APN list from the HSS in the Location Update Answer message as a part of subscriber's profile. The MME determines the default APN based on the received APN list and parameters, creates the GTP-C session with the corresponding S-GW/P-GW. Once the GTP-C session is created successfully, the default APN information is delivered to the UE in the Attach Accept message. If the default APN is pre-configured in the UE, the same should also be the part of the subscriber profile stored in HSS in order to avoid the conflict which leads to the attach failure. If the default APN is not an IMS APN, the UE has to trigger additional PDN Connectivity with its APN parameter set to "IMS".

NOTE the EPS bearer creation procedure is not shown in this diagram. 

The following shows an example of the Location Update Answer message from the HSS which contains the APN list for the subscriber. Each APN-Configuration AVP under the APN-Configuration-Profile AVP contains the APN information. The Context-Identifier AVP which is one of sub AVPs of the APN-Configuration-Profile AVP is used to determine the defalut APN. Whereas, the same AVP under the Configuration-Profile AVP indicates the context identifier for respective APNs. Given this, the APN whose own Context-Identifier is matched with the upper level Context-Identifier will become the defualt APN. 

In this example, there are three APNs in total and the "IMS" APN will be selected as a default APN of which Context-Identifier(=10) has the same value as upper level Context-Identifier.

II. PCC(Policy and Charging Control) rules

When the PDN connection is established with the IMS APN, the EPC and the PCRF creates the EPS bearer of QCI=5 for IMS signaling. Since then, all the SIP signaling in VoLTE goes through this EPS bearer of QCI=5. Subsequently, when the user requests a VoLTE call setup and the media information is exchanged end to end, the EPC and the UE creates dedicated EPS bearer(e.g., QCI=1 for Voice, QCI=2 for Video, etc) to transfer media traffic. 

The PCC architecture consists of P-CSCF(i.e., AF), PCRF, P-GW(i.e., PCEF) and S-GW(i.e., BBERRF). All of these components are involved in delivering the service data flow to the right EPS bearer with the right QoS conditions. The PCC rules generated or assigned by the PCRF define the mapping relations between the QoS paramemters and the service data flows. Upon receiving a VoLTE request(i.e., SIP INVITE with SDP offer), the P-CSCF extracts the 5-tuples of media information(i.e., source ip address, destination ip address, source port, destination port, protocol), codec profile, etc. The P-CSCF interworks these service data flow information to the PCRF and the PCRF either dynamically generates the corresponding PCC rules or assigns the predefined PCC rule. The PCC rules are provisioned to the P-GW. As its logical name suggests(i.e., PCEF), the P-GW performs gating control for the uplink/downlink traffic based on the given PCC rules. After successful PCC provisioning, the P-GW initiates the dedicated EPS bearer creation via the S-GW towards the eNB. 



The following example shows the PCC rule information in the RAR(Re-Authenticate Request) over Gx. In this case, there are two Charging-Rule-Definition AVPs defined each for RTP and RTCP respectively. The Charging-Rule-Definition AVP contains various information that is required for service data control such as PCC rule name(i.e., Charging-Rule-Name AVP), 5-tuple for each direction(i.e., Flow-Information AVP), QoS parameters(i.e., QoS-Information AVP), etc. The QOS parameters includes the QCI, GBR(UL/DL), AMBR(UL/DL), ARP, etc.

• QCI(CoS Class Index): A level of QoS classified based on the required quality per service usages.
• GBR(Guaranteed Bit Rate) : The minimum bit rate to be guaranteed for the given bearer. A certain amount of bandwidth will be reserved for this bearer. The GBR bearer always takes up resources over the radio link, even if no traffic is sent.
• AMBR(Aggregated Maximum Bit Rate) : The total bit rate that is allowed to be used for all non-GBR bearers associated with a specific APN/UE.
• ARP(Allocation and Retention Priority) : Being used to indicate a priority for the allocation and retention of bearers. It’s typically used to decide whether a bearer establishment or modification can be accepted or needs to be rejected due to resource limitations..

III. DSCP(Differentiated Services Code Point) marking

The DSCP marking is an IP level QoS control for each IP packet to flow through the backhaul with a certain priority and the appropriate latency. As the IP packets from different PDN with different QCI values will be mixed inside the backhaul(e.g., VoLTE, internet), the DSCP marking at IP layer would be an important factor to be considered by switches and routers to guarantee the quality service for VoLTE. Without DSCP marking, IP packets can be delayed or dropped significantly when there is a burst of IP traffic at the same time within the IP network.

The DSCP marking shall be done by both end points of the backhaul for both uplink and downlink IP packets based on the QCI of the service traffic. The following shows a recommended DSCP mapping relations between the QCI and the DSCP values as specified in GSMA IR.34 "Inter-Service Provider IP backbone guidelines". For instance, the IP packets with QCI=1 and 2 for voice and video respectively is marked with the 2E, whereas the IP packets with QCI=5 for IMS signaling will be marked as 1A.

NOTE the DSCP-QCI mapping relation is a part of operator's policy. All the switches and routers aware of DSCP values needs to adjust itself to the correct behavior according to the agreed DSCP values as to handling the IP packets.

The following shows an example of DSCP marking in the IP packet. In this example, the IP packet has been used to deliver the SIP INVITE and the IPv4 packet is wrapped by the IPv6 header at P-GW/S-GW. The 6-bit DS field in this IPv6 header is set to "2e"(i.e.,the Expedited Forwarding). This is a different value from the one specified in the above case 

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