1 Introduction:
In telecommunication, LTE (Long Term Evaluation) is a standard, which is based on GSM/EDGE and UMTS/HSPA technologies, for high speed wireless communication for mobile phones and data terminals. The increasing in the capacity and speed are related to the using different radio interface in addition to core network enhancements. This standard is developed by 3GPP (3rd Generation Partnership Project) which is specified in the document series release 8. It supports packet switching only with its all-IP network, voice call are circuit switching, for this LTE need to redirect the voice call. To do this, there are different approaches: VoLTE (Voice over LTE), CSFB (Circuit Switched Fallback), and etc. The main Technique that is used in this paper is CSFB, which switches the user equipment (UE) or the mobile device who need to make or receive a voice call from LTE to 3G or 2G legacy system. Actually, to make CS fallback works the user need to be attached to LTE and 3G or 4G at the same time. Combined attach will used into achieve that, which is used normal attach to LTE and notified 3G and 2G voice call network that the user attached through SGs interface (link between MME and MSC server of voice call).
2 Network Architecture for CS Fallback
Figure 1: Network Architecture for CS Fallback
Figure 1. Shows the architecture of LTE for CS fallback in EPS (Evolved Packet System) to UTRAN or GERAN. It supports CS fallback through SGs interface, which is performed a UE location update over this interface so the core of UTRAN or GERAN learn about the UE location. In LTE architecture, CS fallback in EPS supports the providing of voice and traditional CS domain facilities. A CS fallback allow the terminal which is connected to E-UTRAN (Evolved UMTS (Universal Mobile Telecommunication System) Terrestrial Radio Access Network) to use UTRAN or GERAN to connect to CS domain. SGs interface is a new interface that is added in LTE Architecture, which is supported CS fallback in EPS. it is used for the management of mobility and the procedures of paging between EPS and CS domain, which is based on the Gs interface procedures. It is the reference point between MSC server and MME. It is used for delivery of both originating call and terminating SMS.
3 Network Scenario and Functionality Description
MS PowerPoint has been used to represent the signaling scenario of outgoing call which moves the user from E-UTRAN to either UTRAN or GERAN to make the voice call. CSFB message flows when a UE make a call in the LTE network. The UE initiates a combined EPS/IMSI attach procedure after it is attached to E-UTRAN network, which allows for the UE simultaneously location update both in the core of the legacy network and in the MME.
Step1, 2, 3, 4, and 5: random access, attach procedures and RRC connection setup messages.
Step6: Combined ESP/IMSI Attach Procedure will be requested by the UE by sending Attach request message to the MME. This message as additional shows that if the CS fallback over SGs function is required or not.
Step7, 8, 9 and 10: The MME assigns an LAI to the UE, then the MME chooses a CS PLMN based on the selected PLMN information that is reported by the eNodeB. Then, the MME sends to the MSC a Location Update Request message over the SGs interface, so the core of legacy network learns about the location of the UE, which includes IMSI, location update type, MME name, and the new LAI. Then the MSC in CS domain will perform the location update procedure and response with accepted message.
Step11, 12, 13, 14, 15, and 16: the UE is informed that the combined attach procedure is successful by sending a RRC connection configuration messages. Also, EPS bearer is in active mode which is a data pipe that transmits information between the UE and the PDN gateway as specified in TS36.300 3GPP specification.
Step17a: the UE send a NAS which is Extended Service Request message to the MME to initiates a CSFB procedure that identify the M-TMSI of the UE. It is encapsulated in S1AP and RRC messages.
Step17b: the MME sends S1AP UE context modification request message to eNodeB which is indicated the eNodeB that UE should move from E-UTRAN to with GERAN or UTRAN. This message has the indicator to CSFB and LAI that is included in PLMN ID which is allocated by the MME, which is identified the register PLMN of the CS domain.
Step17c: the eNodeB should send S1AP UE context modification response message. Step17a, 17b and 17c take 0.3 seconds.
Step17d: MME sends MO CSFB indication to MSC via SGs interface so that indicate the CSFB is due to MO (mobile originating) call, the CM service request message sent, if configured to support returning to last PLMN after CS fallback.
Step18: the UE optionally solicit a measurement report to determine the target legacy cell to which packet switching handover will be implemented.
Step19: the eNodeB send Handover required message to MME which triggers PS HO to legacy neighbor cell to request the CN to create resources in the target SGSN, target RNC, and the Serving GW.
Step20: the source MME sends forward relocation request message to the target SGSN for indicating HO resource allocation procedure.
Step21: the target SGSN determines if the serving gateway is allocated and sends create session request message to each PDN connection to target S-GW. In this message the EPS barer context(s) is established by the target SGSN.
Step21a: the create session response message from target S-GW is allocate its local resources to target SGSN.
Step22: the target SGSN sends relocation request message to the target RNC for establishing the RABs (radio network resources).
Step23: the target RNC allocates the resources and send a relocation request acknowledge message to the target SGSN.
Step24: the target SGSN sends create indicator data forwarding tunnel request message to the serving gateway. This message is included the address of the target RNC and TEID(s) for DL data forwarding if the indirect forwarding and relocation S-GW apply and the direct tunnel is used. However, it contains the address of the SGSN and TEID(s) for DL data forwarding if the indirect forwarding and relocation serving gateway applied and the direct tunnel in not used.
Step24a: the serving gateway sends create indicator data forwarding tunnel response message to the target SGSN.
Step25: the target SGSN sends forward relocation response message to the source MME.
Step26 and 26a: the source MME and the S-GW for indirect forwarding exchange create indictor data forwarding tunnel request and response messages id indirect forwarding applies.
Step27: the source MME sends handover command to source eNodeB which is the completion of preparation phase of HO.
Step28: the eNodeB sends the command to UE to handover from E-UTRAN to UTRAN and this message contains a transparent container containing the radio aspects parameter. It may include a CS Fallback pointer which indicates to UE that the handover is initiated due to a CS fallback request.
Step29: the UE starts suspend procedure if the target RAT is GERAN and the UE in detective mode except both the target cell and the UE support DTM (Dual Transfer mode) in which case TBF reestablishment may be implemented.
Step30: SGSN that receive the suspended message from the UE, they will follow the suspended procedure. The S4-SGSN disables GBR bearers and starts the protection and suspension of non-GBR bearers by sending suspending notification messages to the serving gateway which releases all RNC associated information for the UE if direct tunnel is created and send suspended notification message to PDN gateway.
Step31a: a Location Area Update procedure should initiate for the UE if the LA of the new cell is different from the one stored in the UE. The UE should set the “follow-on request” flag in the LAU Request when the UE initiates a Location Area Update so that indicate the MSC not to release the Iu/A association after the completion of the LAU procedure.
Step31b: the UE sends CM service request to the MME. As a result of CSFB, the UE need to indicate to the target MSC that there is an outgoing call establishment by containing the “CSMO” flag.
Step32 and 33: the MSC sends a CM service reject to the UE as a result of either the UE not registered in the MSC that serving the target legacy cell or the UE is not allowed in LA. And this message shall indicate the UE to perform a combined RA/LA or location area update procedure
Step34: the circuit switching mobile originating flag should be included in CM service request to MSC by the UE which initiates the CS call establishment procedure.
Step35: the UE performs the remaining steps of execution phase of the handover. And sends HO to UTRAN complete message to the RNC.
Step36: The target RNC sends relocation complete message to the target SGSN to indicate the relocation completion from the source E-UTRAN to the RNC by the target RNC. As additional, the target RNC need to contain the Local Home Network ID of the target cell in this message.
Step37 and 38: when the target SGSN knows that the UE has reached to the target side, it sends forward relocation complete notification message which including ISR activate and serving gateway change to the source MME. Also, the source MME sends acknowledge about that information and set a timer to manage the resources when they shall be released in source eNodeB, and source S-GW.
Step39: the target SGSN sends modify bearer request message to target S-GW to inform it that, the target SGSN is responsible for all the EPS bearer contexts the UE has established. This message might include user location information if the target SGSN support it.
Step40, 41: the serving gateway might inform the PDN gateway about the change by sending modify bearer request message per PDN connection and the PDN GW should send modify bearer response message to the serving GW as an acknowledge the request.
Step42: The Serving gateway sends Modify Bearer Response message as acknowledge of the user plane
switch to the target SGSN.
Step43: the UE initiates a Routing Area Update procedure with the target SGSN by notifying that the
UE is located in a new routing area, when the UE identifies the UE’s TIN indicates “GUTI” or when the current Routing Area of the UE is not registered with the network.
Step44, 45, and 46: When the timer expires, the source MME sends a Release Resources message to the Source eNodeB. The Source eNodeB should releases its resources that is related to the UE. By sending Delete Session Request messages to the Source S-GW, it deletes the EPS bearer resources. The Source Serving GW acknowledges with Delete Session Response messages to the MME.
Step47,48,49, and 50: If indirect forwarding was used then the expiry of the timer at source MME causes the triggers of the source MME and S-GW to exchange a Delete Indirect Data Forwarding Tunnel Request/response messages to release the temporary resources used for indirect forwarding. Also, if the Serving GW is relocated and indirect forwarding was used, then the expiry of the timer at target SGSN causes the triggers of the target SGSN and the target Serving GW to exchange a Delete Indirect Data Forwarding Tunnel Request/response messages to release temporary resources used for indirect forwarding.