Saturday, February 16, 2013

Subscriber Tracing in lte network

Subscriber Tracing in lte

Subscriber Tracing provides detailed information at the call level for the UE(user).

This is done in the lte network elements(MME,SGW,PGW) to measure the performance of network elements as well as the quality of services given to the user while using lte network.

Subscriber is also traced on demand of the user itself if it is not getting the good or appropriate quality of services from the lte network.

Operator can also trace some of the random users to check the quality of network.

Purpose of Subscriber Tracing in lte network:

-         Optimize the lte network performance.

-         Upon user demand if it is not getting the good quality of services from the network.

Operator applies the tracing on lte network elements for the specific users based on any one of the information provided by user i.e. IMSI or IMEI or MSISDN.


How Subscriber Tracing Works:


Two types of Subscriber Tracing can be done in network.

1.      Management based.

2.      Signaling based.

In management based tracing operator configure User id type (imsi(0) or imei(1) or msisdn(2)) and User id value (imsi value) on the lte node(pgw,sgw,mme) from which operator wants the traces for user.

Example: UE type – IMSI, UE value – 1234566778899, Node – PGW.

Now when the UE tries to attach with the lte network all the traces sent to TCE(external entity) by PGW.

Traces contains all the details related to the user like apn, plmn, imsi, rat type, qos parameters, user location etc.


In Signalling based tracing operator modify the configuration data in HSS with subscriber tracing enabled for the particular UE. Whenever UE attaches with the network, HSS sends Subscriber tracing enabled in the Subscription data in message Update Location answer and then MME forwards subscriber tracing parameter in Create Session request to SGW and then to PGW. Upon receving this parameter all the nodes starts tracing of that user and send all the trace records to TCE (Trace Collection entity).


Configuration for Signaling based Subscriber Tracing.

In HSS –

1.      Trace depth – Low, medium, high. (At what detailed level traces required).

2.      Nodes – SGW, MME, PGW. (On which of the node tracing is required).

3.      Interfaces – S5,S8,S11,Gx,Gn etc. (On which interface tracing is required).

4.      TCE Address – Ip address of TCE.  (Which ip address node should send traces).

Configuration for Management based Subscriber Tracing.


1.      Trace Depth – Low, medium, high.

2.      Type of Traces – XML , Syslog.

3.      TCE Address – Ip address of TCE.


Note: In operator based tracing two types of tracing is possible XML, Syslog.

but in Signaling based only xml tracing is possible.

Saturday, January 19, 2013

LTE call flow beyond PGW

                       Basic LTE Call Flow beyond PGW.

Here are the call flow from PGW to PCRF on Gx interface.

During Initial Attach procedure, PGW check for the APN/Charging Characterstics received in Create session request for whether dynamic Policy(configured in PCRF) or Static policy(configured on PGW) should be applied. Depending upon the configuration, PGW forwards the request to PCRF for providing PCC Rule for the UE attach request.

PCC rules consist of:
  • Rule name - This is to be used to reference a PCC rule in communication between PCEF and the PCRF . 
  • Service data flow filters - shall be used to select the type of traffic that passes through the bearer on which the rule applies.
  • precedence : Determines the precedence of the bearer on which the rule is applied when data is received uplink or downlink for a particular type UDP/TCP etc.  
  • QoS parameters - Includes QoS Class Identifier (QCI), also the Allocation and Retention Priority (ARP) and GBR,MBR for uplink and downlik.
  • Charging type - Defines whether online,offline or both or none charging is applied.
  PGW sends CCR(Init) to PCRF to get the PCC Rule from PCRF.
  PCRF has primarily two entities. PDT (Policy Derivation table) and PCT (Policy and
  Configuration table).
  In PCT, PCRF has number of PCC Rules configured.
  In PDT, PCRF has the selection criteria based upon which PCRF sends the selected PCC Rules in 
  Selection criteria- Can be IMSI,APN name, QCI, MBR/GBR etc. means if CCR request contains 
  the given value of IMSI or QCI and so then particular PDT is applied. 
  For Example: In CCR (Init), PCRF receive IMSI 122334455667001. Based upon this IMSI, PCRF
  has PDT configured in which PCRF applies two PCC Rules let suppose PCC1 and PCC2.
  Upon getting successfull PDT matched PCRF sends successfull CCA (Init) to PGW.
  Now, upon getting of successfull CCA (Init) PGW sends CCR (Update) to confirm the PCRF that  
  the Rule is applied  successfully on the particular bearer (in this case default bearer) and also
  inform the ip address of the UE.Then PCRF acknowlege with CCA(Update) success.

  Now PGW has already applied the Rule(Qos,filters,MBR,GBR etc) received from PCRF to
  the requested bearer, it sends Create Session Response to SGW and then the Default session created
Similar Procedure follows for dedicated bearer also.
  In dedicated bearer creation/updation/deletion, PGW sends CCR Update to PCRF.PCRF 
  acknowledge the request with CCA Update.

Note: CCR (Init) send by PGW only once for a UE session i.e on default bearer creation. 

Closed Subscriber Group in lte (CSG)

         LTE - Closed Subscriber Group (CSG)


Closed Subscriber Group identifies a group of subscribers who are permitted to access one or more CSG cells of the PLMN.


To make a CSG call UE should send CSG Id to which it belongs and its access type in Attach request.


MME then performs UE authentication with the HSS and then exchanges Update Location Request and Answer. In Update Location Answer HSS sends CSG Information(CSG Id, Subscription timer) in Subscription data.

MME then verifies the CSG Id with the CSG received in Attach request if it matches then UE proceed the CSG call and sends Create session request with CSG Information IE to SGW. Upon receiving of successful response from SGW, MME sends Attach Accept message with Member status as ‘member’. After expiry of the Subscription timer for which UE is subscribed to attach with CSG cell, MME initiate PDN connection deletion.


What if CSG Id receive in Attach Request is not same as received in Subscription Data from HSS?

MME still make a successful call but for the specific time(300 sec) and send Attach Accept message to UE with Member status as ‘non-member’.
After expiry of non-member timer MME initiates PDN Connection deletion.


Friday, January 18, 2013

lte Emergency Call Secrets

                                      LTE - Emergency Call Secrets:

          Dial Emergency numbers like 112,999,111 etc.
          Known Emergency numbers are already stored in the USIM/UE.
          MME can also provide the Emergency number List in Attach accept or TAU accept.
          If Emergency number list is provided by the MME, it is stored in the USIM and when the user initiate a call            to one of the number provided in the list then UE treat it as a Emergency number and initiate Attach procedure with Type Emergency_attach.

  1. Default Emergency APN should be configured in MME in case where UE does not send APN in attach request and also HSS does not provide it in Subscription data for Emergency attached UE or Authentication is optional. In that case MME provides its default APN and Qos and procced the emergency call.

  1. MME rejects the emergency call when APN received from UE is not configured in MME      Emergency APN list.

  1. MME rejects the emergency call only when HSS sends Authentication failure with cause ‘IMSI unknown’ in case of MME is configured to contact the HSS. For all other authentication failure cause MME proceed with the call successfully.
  1. When a UE attached normally(non-emergency) with the Lte network it can also calls for emergency services by sending PDN connectivity request with request type emergency.

  1. UE can not create two emergency default bearers. It can be one default and other is dedicated.
  1. UE can create any no. of dedicated bearer with emergency default bearer but the ARP(Qos) value should be same for all the bearers and the IMSI authentication is mandatory.

  1. Bearer modification is not possible for emergency bearers.

  1. If any one of the attach type or the request type in Attach Request is non-emergency then call is considered as normal call(non-emergency call).

  1. Once the call is completed UE need not to detach. As soon as UE goes into idle mode, MME automatically release the call.

To know about MME configuration for Emegency Click  MME EmergencyConfiguration Data

IMS Emergency Call in LTE Network

    LTE : IMS Emergency Call in LTE Network
Emergency service is provided to the Lte enabled UE in the following situations:
  1. User calls an emergency number.
  2. UE is in the state where network is not available to serve the UE.
  3. User can make IMS emergency call even if does not have a SIM card.
  4. UE is in limited service state i.e limited service state where UE is not allowed to call or roam.
-         To make an IMS Emergency call, UE sends Attach type as Emergency attach and request type as Emergency in Attach Request.
-         MME(Lte Network) should be configured with the Emergency support enabled otherwise MME rejects the emergency call.
-         Following are the various configurations in MME according to which MME proceeds the Emergency call.
Emergency Type      
             · IMSI Authentication mandatory: Emergency call is successful only when
               IMSI comes in Attach Request and Authentication from HSS
               should be successfully done for this IMSI.
             · IMSI Mandatory Authentication optional: Emergency call is successful
                only when IMSI comes in Attach Request and Authentication from HSS
                needs not to be successfully done.In case authentication fails, network keeps
                tracking of that IMSI's that are unauthenticated.
             · IMSI Optional IMEI mandatory: In this case UE is identified in the network
               by its IMEI only.UE may or may not send IMSI in Attach Request. If UE sends IMSI
               it is not neccassary be authenticated. 
             · Emergency not supported.
Emergency APN
         That is needed to identify the PGW for emergency call.
Emergency QoS
               That is required by the MME only when the UE authentication is optional.
               Otherwise HSS sends it in Subscription Data.
For Emergency call secrets Click on  Emergency_Secrets