1). CS Fallback OV:
§ Both CS Fallback (CSFB) and VoLGA rely on the existing circuit voice network, but
§ CSFB allows LTE mobile devices to fall back to
§ The Circuit Switched Fallback (CSFB). Clearly this is *not* a voice over LTE solution as the voice isn’t being carried over LTE; it’s still carried over the GSM/UMTS network.
§ In a CSFB solution, the idea is that the handset ‘falls back’ to GSM/UMTS when it needs to make (or receive) a call.
§ To do this, the operator’s entire circuit voice network must be upgraded to add a new feature to ‘page’ the handset when it’s on LTE and tell it to ‘fall back’.
§ This function is only available in case E‑UTRAN coverage is overlapped by either GERAN coverage or UTRAN coverage.
§ The idea behind this solution is to use a
§ A drawback to the circuit-switched GSM fallback mechanism is that data services will be lost or suspended if the GSM network does not support dual-transfer mode (DTM). Moreover, even where DTM is available, the supported data rates will be very low compared with those of LTE.
§ The main problems with this approach are longer call setup times which result in a significant degradation of the user experience
§ Changing to another network takes time, which has an adverse effect on call setup times. It is estimated that even in the best case scenario, both mobile originating and mobile terminating call establishment times would increase by at least 1.5 seconds
§ To support CSFB, an operator’s existing network of mobile switching centers (MSCs) must be upgraded with a new capability to page devices over LTE through the mobile management entity (MME).
§ The necessity for software upgrades on circuit switched network nodes such as the Mobile Switching Centers (MSCs) is required.
§ Many network operators have bought MSCs and SGSNs from different vendors, further increasing cost and interoperability testing.
§ No fallback to a
§ CS Fallback and IMS‑based services shall be able to co-exist in the same operator’s network
§ CSFB significantly increases the amount of network signaling. Switching networks to make or receive a phone call adds signaling load on the mobile core network.
§ CSFB adds complexity for unique operator situations. For an operator with an LTE license and an MVNO relationship for
§ For those operators planning to deploy LTE femtocells along with their LTE macro network, CSFB adds significant cost complexity. If a user is attached to an LTE femtocell at home and needs to make or receive a voice call, the handset will need to drop the LTE femtocell connection and fall back to a
§ There currently seem to be a number of open issues, especially around roaming availability and standardization gaps concerning concatenated SMS delivery
§ TS 23.272 - Circuit Switched (CS) fallback in Evolved Packet System (EPS); Stage 2
2). CS Fallback – Pros & Cons:
Pros:
§ Proven technology
§ Supports migration from CDMA as well as GSM/UMTS
Cons:
§ Call setup time delay - Minimum 30% increase* in call setup times (requires a ‘blind fallback’ to achieve 30%, delay is higher if handset must do network scan before fallback)
§ Requires MSC upgrade - Dependent on MSC vendors to implement (home and visited networks)
§ Impact on coverage and handoff for
§ Standard is ‘barely adequate’
§ No support for simultaneous voice/data over LTE - No LTE during voice call (Dropped data connections)
§ CSFB Device for LTE voice (Poor fit with Multi-tasking devices)
§ CSFB fits very poorly with LTE femtocells - “Falling back” to GSM network negates value of LTE femtocell
§ No support for combinational IMS/RCS + voice over LTE
§ No validating LTE network QoS capabilities
§ No verification of IMS telephony plumbing features
3). Architecture for CS Fallback:
§ In LTE architecture, the circuit switched (CS) fallback in EPS enables the provisioning of voice and traditional CS-domain services (e.g. CS UDI video/ SMS/ LCS/ USSD). To provide these services LTE reuses CS infrastructure when the UE is served by E-UTRAN.
§ A CS fallback enabled terminal, connected to E-UTRAN may use GERAN or UTRAN to connect to the CS domain.
§ This CSFB function is only available in case E-UTRAN coverage is overlapped by either GERAN coverage or UTRAN coverage.
§ CS Fallback and IMS based services can co-exist in the same operator’s network. Although its not very straight forward to support CS fallback, all participating elements i.e UE, MME, E-UTRAN and MSC/SGSN needs to support additional functionalities.
§ CSFB Architecture: http://docs.google.com/View?id=ddh56dhg_291cttkthf8
§ The figure above provides architecture for CS fallback in EPS.
§ The approach favored by many 3GPP members as an initial solution for delivering voice and SMS services over LTE is 'circuit switched fallback', which is specified in TS 23.272.
4). Interfaces:
§ From the network point of view a new MSC (SGs) and SGSN (S3) interface is required to signal incoming calls and SMS messages to the MME.
§ The interface is based on IP and therefore requires new software on network nodes (MSCs and SGSNs) that are delivering the main services today.
§ SGs interface between the MME and MSC server is used for the mobility management and paging procedures between EPS and CS domain, and is based on the Gs interface procedures.
§ The SGs reference point is also used for the delivery of both mobile originating and mobile terminating SMS.
4-1). SGs:
§ It is the reference point between the MME and MSC server. The SGs reference point is used for the mobility management and paging procedures between EPS and CS domain, and is based on the Gs interface procedures. The SGs reference point is also used for the delivery of both mobile originating and mobile terminating SMS.
§ At MME - MSC Server interface a new protcol SGsAP is being added to support CS fallback. SGsAP protocol is based on the BSSAP+.
§ SGsAP: This protocol is used to connect an MME to an MSC Server based on the BSSAP+.
§ Stream Control Transmission Protocol (SCTP): This protocol transfers signalling messages and is used to transport SGsAP signaling messages.
4-2). S3: It is defined in TS 23.40 with the additional functionality to support CS fallback with ISR as defined in this specification, and enables user and bearer information exchange for inter-3GPP access system mobility in idle and/or active state. S3 enables user and bearer information exchange for inter-3GPP access system mobility in idle and/or active state between SGSN and MME.
§ GPRS Tunnelling Protocol for the control plane (GTP‑C): This protocol tunnels signalling messages between SGSN and MME (for S3).
§ User Datagram Protocol (UDP): This protocol transfers signalling messages. UDP is defined in RFC 768.
4-3). Gs: Interface between an SGSN and an MSC/VLR.
4-4). Gn: Interface between a SGSN within the same or different PLMNs or between an SGSN and a GGSN within the same PLMN.
5). Functional Entity:
§ CS Fallback and IMS based services can co-exist in the same operator’s network. Although its not very straight forward to support CS fallback, all participating elements i.e UE, MME, MSC & E-UTRAN needs to support additional functionalities.
5-1). UE
§ supports access to E-UTRAN/EPC as well as access to the CS domain over GERAN and/or UTRAN.
§ Combined procedures for EPS/IMSI attach, update and detach.
§ CS fallback and SMS procedures for using CS domain services.
5-2). MME
§ Deriving a VLR number and LAI from the GUTI received from the UE or from a default LAI.
§ Maintaining of SGs association towards MSC/VLR for EPS/IMSI attached UE.
§ Initiating IMSI detach at EPS detach.
§ Initiating paging procedure towards eNodeB when MSC pages the UE for CS services.
§ Supporting SMS procedures.
§ Rejecting CS Fallback call request (e.g. due to O&M reasons)
§ An MME that supports CS Fallback uses the LAI and a hash value from the IMSI to determine the VLR number when multiple MSC/VLRs serve the same LAI.
§ The same hash value/function is used by SGSN to determine the VLR number
§ When UE is performing CS fallback procedure for Mobile Originating Call for the purpose of emergency call, it needs to indicate to the MME that this CS fallback request is for emergency purpose. MME also indicates to the E-UTRAN via the appropriate S1-AP message that this CS fallback procedure is for emergency purpose.
5-3). MSC
§ Maintaining SGs association towards MME for EPS/IMSI attached UE.
§ Support of SMS procedures as provided in 3GPP specification
5-4). SGSN
§ SGSN supports ISR, SGSN shall follow the rules and procedures described in TS 23.401 and TS 23.060 with the following additions and clarifications:
§ The SGSN shall not send the ISR activated indication at combined RAU/LAU procedure.
§ An SGSN that supports Gs uses LAI and a hash value from the IMSI to determine the VLR number when multiple MSC/VLRs serve the same LAI.
§ The same hash value/function is used by MME to determine the VLR number
5-5). E-UTRAN
§ Forwarding paging request and SMS to the UE.
§ Directing the UE to the target CS capable cell.
6). SMS Service
§ To use “SMS over SGs” without full-scale CSFB deployment - This procedure also does not require dropping the LTE connection and falling back to
§ SMS support (both the mobile originating and mobile terminating SMS) is based on the connectionless SGs reference point between the MME and the MSC Server and use of NAS signalling between the UE and the MME.
§ The SMS protocol entities are reused from the existing MS/UE and MSC implementations.
§ Problems relate to provision of information for charging purposes, “message waiting” signals and delayed delivery when a mobile is out of coverage, sending multiple (“concatenated”) SMS messages, multiple-addressing of sent messages, location data (important for lawful interception) and so on.
§ A particular scenario – what happens to SMS received during the fallback procedure itself – is a particular issue which might affect voicemail notifications which are often received while a user dials in response to a missed call.
7). Emergency Service
§ When UE is performing CS fallback procedure for
§ MME also indicates to the E-UTRAN via the appropriate S1-AP message that this CS fallback procedure is for emergency purpose.
§ E-UTRAN may use the emergency indication for selecting a particular radio access network (
8). Message Flow: (N/A Here)
