LTE/IMS reference

                                        S1AP  Protocol

Functions of S1AP-

                                                           Control Plane

1) E-RAB management function-

E-RAB is having a formula, E-RAB=Radio bearer+ S1 bearer

Radio bearer is b/w UE to E-NB, S1 bearer is b/w E-NB to SGW. That together is actually called as E-RAB. So, main functionality of S1AP, MME is to establish the E-RAB by talking to SGW and E-NB. B/W E-NB and SGW there is a GTPU tunnel, actually one end of the GTPU tunnel is established by E-NB  and the other end of the tunnel is established by SGW. They can’t talk directly to each other because there is no control interface existing b/w E-NB and SGW. So, MME is the mediator. So, MME talk to SGW by GTPC protocol and MME should talk to E-NB by S1AP Protocol. That is why S1AP is playing important role in this E-RAB creation. E-RAB management means there will be three functions involved on that. That is RAB setup normally done at the time when fresh application is coming up, if modify the RAB and RAB delete means just delete.

2) Initial Context transfer function-

Initial context setup creation should be done in E-NB. MME is activating the initial context setup request by using S1AP protocol. Initial context setup means if mobile equipment ask for the default PDN connection, so MME is going to be part of important role of packet data connection because MME received the NAS message called as ESM message. Actually PDN connectivity request is part of ESM message. So mobile equipment asked for ESM message to MME that he want to connect so and so packet data network, so MME goes to HSS and HSS accepts and reply for the default bearer. So go out SGW and PGW and establish the required things for tunnel and that is done, MME will tell to E-NB that it is time to create the context for the user for a default bearer . As part of the context the information will be present like IP address of the user. The initial context creation contains how much RABs are getting created, what is the property of each and every radio bearer all these informations given to E-NB for first time . How many RABs are getting created at the starting time those thing are called as initial context setup but later onwards if any RABs are needed to establish, new dedicated bearer establish the command which is called as E-RAB setup. So, first message initial context setup when default bearer is getting created at the time of attached but later if new bearer need to be establish then it is called as E-RAB.

3) UE capability information indication function –

This functionality is used to provide the UE capability information when received from the UE to the MME.

UE Capability Information is an RRC message that UE sent to Network (in most the case during initial registration process). It informs on all the details of its capabilities. As LTE release goes higher and more features are added, UE Capability Information has become the longest and most complicated Radio Message.

4) Mobility function for UE’s in LTE active state-

Mobile equipment is moving from one E-NB to other E-NB (inter MME /serving SAE GW handover) via S1interface (with EPC involvement).

Mobile equipment is moving from one RAT to another RAT(Inter 3GPP RAT handover) via the S1 interface (with EPC involvement) like UMTS and GSM. So, MME should be playing the role.

5) Paging – ( Incoming call, any notification of system information change, ETWS notification-tsunami )

Paging means incoming call, when incoming call comes the data would come in SGW. SGW will tell to MME that there is some downlink data in EGTP_DOWNLINK DATA NOTIFICATION message but SGW don’t know under which E-NB mobile equipment will present.  So MME goes and search everywhere for the mobile paging message. MME send paging message to all the E-NB. E-NB send the paging message to the UE to paging  record in paging channel and mobile equipment response back to paging message. The RRC connection will established then the mobile equipment will get the data.

6) Reset:-

When either E-NB or MME goes down and come up, then reset take place on S1 interface.

The purpose of the reset procedure is to align the resources in E-NB and MME  in the event of an abnormal failure. The procedure reset the S1AP interface. This procedure does not affect the application level configuration data exchange during the S1AP setup procedure. This procedure is called a Global procedure or non UE associated signalling.

Who will send a reset request? The E-NB, who will go for an abnormal failure, he will send the reset request. So, they will start the relationship fresh.

7) Error Indication:-

If protocol message contains an error information.

8) Overload  Function:-

This function indicates the load situation in the control plane of the S1AP interface. MME load balancing will be done by talking to MME and E-NB.

Load balancing function to ensure equally loaded MMEs within an MME pool area.

Assume that in that MME pool area there are 10 MMEs . Now all the 10 MMEs can serve in same  PLMN. If mobile equipment is actually trying to connect to that PLMN, he send a attach request (NAS) message to E-NB. E-NB must forward the NAS message to one of the 10  MMEs because E-NB connected to all the 10  MMEs in that pool.      

That is why we called it as S1flux where any E-NB can be connected to any no. of MMEs . Now E-NB  have to pick up one MME out of 10 MMEs which is having least load.

9) S1- SETUP –

At the time of setup we will exchange the configuration information between E-NB and MME

(at first time ) but after that any update is happening either in E-NB and MME that will be used by using E-NB and MME configuration update.

10) Transfer of NAS message  is the duty of S1AP

11) UE Context Modification Function-

Existing context is getting modified.

12) Status Transfer-

 Explain in next page.

13) Trace Function-

Explain in next page.

14) Location Reporting -

Explain in next page.

15) S1 UE Context Release Function-

 UE context will be release at the time when mobile equipment is going in to idle mode.

16) S1 CDMA 2000 Tunneling Function-

This functionality is to carry CDMA 2000 signaling between UE and CDMA 2000RAT over the S1 interface

17) Warning messages transmission function-

Warning message means tsunami warning (ETWS message) is sent by MME only i.e SIB10 and SIB11.

18) RAN information management(RIM) function-

This functionality allows the request and transfer of RAN System information( e.g GERAN system information) between two RAN nodes via the core network.

19) Configuration transfer function –

This functionality allows the request and transfer of RAN configuration information( e.g SON information ) between two RAN nodes via the core network. SON means self optimization network  it is automatic configuration information without manual intervention.   

List of elementary procedure-

All the EPS are divided into class1 and class2.

Class 1 means after the message is initiated at least one response message will be there .

The one which don’t have any response, only sends message and forgets it, actually called as Class 2.This messages are reliable because SCTP transport protocol is there.

(UE associated signalling/ UE related function)

E-RAB Setup- 

E-RAB setup is used to setup the additional RABs after initial context is setup. So, MME will send E-RAB Setup to E-NB and E-NB will send the E-RAB Setup Response.

E-RAB = Radio Bearer+ S1Bearer

So, E-NB  now have responsibility of two things, one is Radio Bearer. How E-NB create the Radio Bearer for the mobile Equipment? By RRC CONNECTION RECONFIGURATION message. So, E-NB will be sending mobile equipment that RRC CONNECTION RECONFIGURATION as part of that E-NB will tell to create the  new radio bearer, radio bearer means each radio bearer will have special properties/configuration like PDCP configuration, RLC configuration, MAC configuration, logical channel configuration, PHY configuration will be different of each radio bearer. So MME send to E-NB  the E-RAB Setup Request and E-NB will send E-RAB Setup Response. So if successful response is coming it means E-NB has create a corresponding radio bearer + E-NB might have create the tunnel id towards SGW. In E-RAB setup request message MME will tell to E-NB that what is the tunnel id that is created by the SGW towards E-NB for this particular  S1 bearer because between E-NB and SGW there is a GTPU tunnel. So, once we got to know both tunnel id, the S1 bearer is successfully established.

Mobile equipment can contain many tunnels depending on how many bearers that we will create.

Normally when dedicated EPS bearer is getting created , MME will send E-RAB SETUP REQUEST to E-NB.

So, in E-RAB Setup request E-RAB ID will present and the UE aggregate maximum bit rate and various other information like pre-emption capability, pre-emption Vulnerability. pre-emption capability, pre-emption vulnerability these are depend on priority of each RABs Pre-emption means throwing somebody out.

Capability means your capability to throw the others out and your Vulnerability is you are vulnerability by others, they can come throw you out. When E-RAB is getting setup we have to give what is the pre-emption capability such that if resource is not available this E-RAB actually pre-empt other RABs which are existing in the system he can actually grab the resource from them  because his pre-emption capability is more than their pre-emption vulnerability. But once you are in and somebody pushes you out it is possible that his capability is more than your vulnerability, then he can push you out and he can get the resource in place of yours.

But in the response message – A list of RABs which are successfully established, and list of E-RABs which are failed  to be established shall be include because as a part E-RAB setup request multiple RABs can be setup. If 5 RABs are need to be create but at a time 3 RABs are created and 2 RABs are failure because priority is less. 

 Example-

E-RAB Modify-

If any one of the existing bearer need to be modify like QOS parameter are changing , accordingly to there data rate  priority Sometimes then E-RAB modify will issue from MME to E-NB and E-NB will do the corresponding modification in the radio bearer.

But in the response message – A list of RABs which are successfully modified, and list of E-RABs which are failed to be modified shall be included.

E-RAB Release-

The MME initiates the procedure by sending R-RAB Release command message.

The E-RAB release command message shall contain the information required by the least one E-RAB in the list of E-RABs. If a NAS PDU IE is contain in this message then E-NB shall pass it to UE.

The E-RAB release command message may contain the UE Aggregate Maximum Bit Rate IE.

Why normally AMBR is present in every RAB ? 

Aggregate means summation. If RAB is getting established at the first time freshly overall data rate is changing  Assume that already we have got 2 RABs and existing AMBR is 2 mbps but another 2 RAB is getting created another 2 mbps data rate . So total AMBR is 4 mbps. So that is why at the time of RAB creation and modification of the RABs and deletion of the RABs AMBR factor will be changing.

 But in the response message – A list of RABs which are release successfully and list of E-RABs which are failed to be release.  

E-RAB Release Indication-

The E-NB initiate the procedure by sending an E-RAB release indication  message towards the MME. If E-NB facing a resource problem, E-NB can actually release non-GBR RABs which are having higher pre-emption vulnerability, low priority RABs because other priority are getting established, E-NB can not survive with low Priority data. So, it happened actually for two reasons. One is inactivity if user is not uploading or downloading anything for particular RAB, so E-NB can release of that particular RAB that is one reason. The second reason is if resource is not available if one E-RAB is inactive, then  E-NB only release that E-RAB. But if  there is no activity at all, then E-NB detected that all the E-RABs that means UE CONTEXT RELEASE REQUEST PROCEDURE can be started.

Context release means everything. That means mobile equipment go back to idle state . So,E-NB propose to MME that UE Context Release request.  

Initial Context Setup –

In initial context setup request that is in first time default bearer will be created and in setup response message E-NB will send tunnel id of E-NB towards of the SGW for Default bearer. If 2 default bearer will create then E-NB Will send 2 tunnel id to MME and this same information will forward to SGW.

Failure means E-RAB setup creation is failed.

UE Context Release Request-

If E-NB detected that the E-RAB , there is no activity at all means in inactive case E-NB will send to MME that UE CONTEXT RELEASE REQUEST when ‘user inactivity’ or ‘Radio Connection with UE lost’.    

UE Context Release –

Normally UE context release done by MME when final E-RAB is getting removed at the time detach.

•In handover scenario when mobile equipment moved from old E-NB to new E-NB, that MME will

  send to old E-NB that  UE context release command

•When UE goes detach/flight mode. 

UE Context Modification-

The UE context modification request message may contain-

-Security key IE changes

-UE Aggregate maximum bit rate IE changes

-UE security capability IE changes.  

Handover Preparation-

When mobile equipment is going from one E-NB to other E-NB through S1 interface source E-NB will send a handover require to MME, after that MME will send handover request to target E-NB and target E-NB will send handover response. 

If response is successful then MME will send handover command message to source E-NB.

Handover Notification:

When handover is successful then target E-NB will send to MME Handover Notify message.

Once the handover command is coming from MME to source E-NB, source E-NB will send a mobile equipment as RRC CONNECTION RECONFIGURATION. Mobile equipment will get preamble(dedicated) information in all that in reconfiguration message, he leaves these frequency and come to new frequency, synchronize with the new E-NB cell and do the RACH procedure. After resource acquired successfully, mobile equipment will send out a message to new E-NB  called as RRC CONNECTION RECONFIGURATION COMPLETE. So, new E-NB understands that handover is completed. So mobile equipment is coming in new E-NB. So, new E-NB will go and report to MME that handover is successful that is Handover Notify and then MME will go to old E-NB, tell please release the context and old E-NB will report to MME for successful release the context.

Path Switch Request – 

Path switch request is needed is only X2 handover but path switch request is not needed for S1 handover.

Path switch request is work in X2 handover because in X2 handover source E-NB will tell to target E-NB that my user is going to your network, Target E-NB accept it that is handover request from source E-NB. When successful response message is received target E-NB, then source E-NB initiates the RRC CONNECTION RECONFIGURATION message to the Mobile equipment. Mobile equipment will get preamble(dedicated) information in all that in reconfiguration message, he leaves these frequency  and come to new frequency, synchronize with the new E-NB cell and do the RACH procedure. After resource is acquired successfully, mobile equipment will send out a message to new E-NB  called as RRC CONNECTION RECONFIGURATION COMPLETE. So, new E-NB understand that handover complete. So mobile equipment is coming in new E-NB . So, new E-NB will go and report to MME that handover is successful.

So target E-NB will tell source E-NB that release the context but SGW knows about source E-NB only. SGW does not know about target E-NB , not even MME. MME also does not know this two E-NB does the handover by each other. So, we definitely need to report to MME for that. So the information need to update to MME and SGW.

So, handover is successful new E-NB go and tell to MME that PATH SWITCH REQUEST. So new E-NB telling to MME that go and tell to SGW for changing the GTPU tunnel path between SGW and E-NB. So the tunneling information will be exchanging between SGW and E-NB through the MME.

So, from now onwards the data should come to new GTP tunneling path.

Once path switch is over the target E-NB tell to source E-NB release the context because SGW never sending the data to SGW. That time source E-NB will check his buffer to see any there any pending data still there in final time. If no data is pending then source E-NB delete the context.

If  path switch is failed then the handover must be redone that means SGW has rejected new path.

Handover Cancellation- 

z

Source E-NB will send to MME  that Handover required. Then MME will sent to target E-NB that handover request. But MME before sending the handover request to target  E-NB, if source E-NB has to decide the handover cancel, it  reports to E-NB, then MME sends the handover cancel acknowledge to E-NB.   

E-NB Status Transfer Procedure –

In X2 handover PDCP sequence number will exchange between each other if any pending

data left out in old E-NB at the time of handover but when MME is involved that is S1 handover the PDCP SN will transfer to old E-NB to MME and MME will forward to Target E-NB.

So, in E-NB status transfer message the IE are UL count Value and DL count value.

But there is a direct forwarding path also available that is source E-NB direct send to PDCP SN to target E-NB.

Paging-

Paging is incoming call for particular mobile phone for searching in a particular Tracking Area or list of Tracking Areas. A list of Tracking Area means multiple Tracking Area registration. Mobile equipment may able to register one Tracking area or may be multiple Tracking Areas. MME will give to E-NB  list of Tracking Areas in this paging message. So, E-NB will search in all these Tracking Areas.

Paging will be triggered for any Incoming call, any notification of system information change or notification of ETWS (Tsunami).

Initial UE message-

This is the first NAS message (EMM-Attach request, ESM- PDN Connectivity request) where E-NB will send it to MME after RRC Connection is made. 

Downlink NAS Transport/ Uplink NAS Transport-

Downlink NAS Transport-

Uplink NAS Transport-

Downlink NAS Transport-

Uplink NAS Transport-

NAS Non delivery indication-

E-NB will send a NAS Non delivery indication to MME because of radio link failure with a cause set to ‘X2 Handover triggered’ or ‘S1 Intra system handover triggered’ or ‘S1 Inter system handover triggered’.

If E-NB decide its not sure that UE specific NAS message what it received from MME will be delivered, so in that case E-NB will send back to MME .

When MME receives "NAS non delivery indication" from E-NB ,MME shall retransmits the PDU or shall be discarded based on NAS messages
For example :
TRACKING AREA UPDATE ACCEPT
AUTHENTICATION REQUEST
SECURITY MODE COMMAND etc. may take following action,
Upon successful completion of the handover the MME shall retransmit the NAS PDU.
If handover fails and if NAS signalling exists MME shall retransmit .

And for some NAS messages like DOWNLINK NAS TRANSPORT etc. MME will discard even though handover is successful.

MME should at-least wait for the handover procedure to complete before retransmitting any NAS PDU to the E-NB because MME should know which E-NB the UE is connected to & this is determined only after a successful/unsuccessful handover.

Management procedure or Global procedure/ Non UE associated signaling-

Reset-

Error indication-

S1 Setup-

The purpose of the S1 Setup procedure is to exchange application level data needed for the eNB and MME to interoperate correctly on the S1 interface. This procedure shall be the first S1AP procedure triggered after the SCTP association has become operational. The procedure uses non-UE associated signalling. 

S1 Setup can also be done in both direction.


E-NB Configuration update

Normally when configuration update is require ? 

If supported T.A(S) needs to be changed, or supported CSG ID(S) needs to be changed, or default paging DRX is changed. If some changes are happening then E-NB will trigger E-NB configuration update to MME. So based on that configuration MME will send paging message.

MME Configuration update-     

As part of the MME Configuration update serve GUMMEIs or MME name or Relative MME capacity(overload situation) are need to be changed.

Overload start –

If MME facing a overload situation then MME will send overload start message to the E-NB.

AS part of the message the IE’S are-

- Reject all RRC Connection establishments for  non emergency mobile originated data transfer or, reject all RRC Connection establishments for signaling .

- Only permit all RRC Connection establishments for emergency sessions.

Overload stop –

It means MME becomes free.

UE associated signaling-

UE Capability information-

Trace Procedure-

The purpose of the Trace Start procedure is to allow the MME to request the E-NB to initiate a trace function for a UE. The procedure uses UE-associated signalling.

Trace start means MME tell to E-NB that to start the trace . The E-NB will note down everything in which cell mobile is and moved in to which cell. All this information will be store at the end of the call. E-NB will forward all the trace information to MME.  

Location Reporting Procedure-

If reporting upon change of serving cell is requested, the E-NB shall report whenever the UE changes its serving cell to another cell belonging to the E-NB.

The Location Report Failure Indication procedure is initiated by an E-NB in order to inform the MME that a Location Reporting Control procedure has failed due to an interaction with a handover procedure.

Tracing will be triggered at the end of the call but Location reporting control will be triggered at any time.

Warning Message Transmission Procedures -

For ETWS warning message.

KILL request/ KILL response-

KILL the ETWS message. If the E-NB receives a KILL REQUEST message, the E-NB shall stop broadcasting the warning message within that area and discard the warning message for that area.