OK, so I am back with LTE NAS next episode. After EMM and ECM, we discussed some basic concepts about ESM. So now, we can have a look on ESM procedures.
ESM procedures also have two categories. However these categories are quite different from EMM procedures category. In case of EMM, the specific procedures are optionally made up of common procedure. So first, an EMM specific procedure starts. Then it optionally invokes one more EMM common procedure(s). Then EMM common procedure(s) get completed and finally the EMM specific procedure also get completed. ESM case is different. ESM also has two categories. (1) “Procedures related to EPS Bearer Context”. These are ESM procedures used for EPS bearer. So this category makes sense. (2) “Procedures related to transaction”. Generally the former one is invoked by network and later one by UE. What if, UE want to manipulate EPS bearer context. No. UE cannot do it directly. UE can invoke specific “Procedure relate to transaction”. In the response to that network invokes “Procedure related to EPS Bearer Context”. When network invoke corresponding “Procedure related to EPS Bearer Context”, then at UE end, the “Procedure relate to transaction” is declared/assumed as completed. These both categories of procedures are in sequence. So indirectly UE can also invoke EPS procedure !!!
Ok whenever network starts any “Procedure related to EPS Bearer Context” in the response to UE initiated “Procedure related to transaction”, it uses PTI in first message. Its value is same as UE have used in its earlier “Procedure related to transaction”. So that UE can co-relate about incoming ESM message to its ongoing transaction procedure. Till that time, PTI is used to co-relate the procedures for a particular (default or dedicated) bearer. Later on EBI is allocated to that bearer (default or dedicated), by network. So both start using EBI to mention about that bearer. UE and network both use LBI in “Procedure related to dedicated EPS Bearer Context” to point to default bearer for that particular PDN.
This table provides a relationship among UE initiated “Procedures related to transaction” and network initiated “Procedures related to EPS Bearer Context”
|Procedures related to EPS Bearer Cxt -->||Default EPS bearer context activation||Dedicated EPS bearer context activation||EPS bearer context modification||EPS bearer context deactivation|
|1||Procedures related to Transaction||PDN connectivity||x|
|3||Bearer resource allocation||x||x|
|4||Bearer resource modification||x||x||X|
|5||ESM information request||x|
|6||ESM status message|
Yes almost all transaction related procedures are invoked by UE with two exceptions. (1) ESM Status message can be sent by anyone UE or network. (2) “ESM information request” is always sent from network to UE. UE responds with “ESM information response”.
Almost all “Procedures related to EPS Bearer Context” can also be invoked by network, in response to UE initiated transaction related procedure, with two exceptions. (1) EPS bearer context modification (2) EPS bearer context deactivation. For such cases, obviously network skips PTI IE in messages and EBI IE becomes mandatory.
So now, we have enough building blocks about EMM, ECM and ESM. So next article will be some real scenarios. We will see, bundled EMM+ESM procedures. Yes, in practice, EMM and ESM messages goes as a single message. It’s implemented first time in LTE. It has its own challenges. We will see that in next post.