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Last article, I started a series on LTE NAS. I wrote some basics about NAS and its root to 3G. I focused more on EMM. This article is about ECM. ECM is used to establish connection. The connection is established between UE and MME either for session management or for SMS transfer. Let’s start with ECM specific procedure named.

So, what is paging? Yes it is also memory management technique, that we learnt in computer science. However in cellular wireless context paging is different. If someone dial your mobile number, your mobile start ringing. How it happens? First, network start paging procedure to intimate mobile about incoming call. Then mobile and network performs security procedures, radio resource allocation and finally it starts ringing. The call can be CS or PS and so paging also. We can foresee that initial LTE network will co-exist with 3G or even 2G network. Voice call will not move to PS VoIP over night. If valid NAS context exist at MME, LTE has “CS Service notification” ECM specific procedure use for CS paging. In the absence of NAS context, “Paging” ECM specific procedure is used establish NAS for incoming CS or PS call. NAS context consists of security parameters between UE and MME for NAS message exchanges. Generally LTE network address the UE with its S-TMSI in “Paging” procedure. However if MME restart or somehow, MME lost S-TMSI of UE, then it uses IMSI. It is rare case, abnormal condition and used to recover the error.

UE initiates “Service Request” ECM specific procedure, in response to paging. As part of this ECM specific procedure, the network may initiate optional common procedure(s): (1) Authentication and/or (2) Security Mode Control. We saw in last article, that specific procedures are made up of zero or more common procedure(s). The successful Service Request procedure result into ECM connected (EMM connected) state. NAS ECM has one more variant for Service Request procedure, that is Extended Service Request. It is used for CS fallback for voice call and handoff with non-3GPP networks. Non-3GPP networks are CDMA network, EVDO (HRPD) network, WiMAX network, etc.

The last NAS ECM specific procedure is “Transport of NAS”. It is used for SMS.

Paging always initiated by network. Detach and “Transport of NAS” procedures can be invoked by either network or UE. All other ECM and EMM NAS procedures are UE initiated. Service Request ECM specific procedure and all EMM specific procedures (i.e. Attach, Detach and TAU) are starting Initial NAS messages. NAS context may not present during this procedure. So the first message is not encrypted. These procedures may invoke other common EMM procedure for security purpose.

One can consider ECM as separate category of NAS procedure, or they are just part of EMM. Let me recap. So far we have seen EMM and ECM specific NAS procedure. Next is ESM. Here are various procedures that we cover in a tree format. Generally each procedure contains a pair of message exchange. Request and Response. A message contains IE.

EMM Elementary procedure.

1. EMM specific procedure
   1. Attach
      1. Attach
      2. Combined Attach
   2. Detach
      1. Detach
      2. Combined Detach
   3. TAU
      1. Normal TAU
      2. Periodic TAU.
2. ECM specific procedure
   1. Service Request
      1. Service Request
      2. Extended Service Request
   2. Paging
      1. With S-TMSI
      2. With IMSI
   3. CS Service Notification
   4. Transport of NAS