Express YourSelf !

1. One can refer wikipedia for a long article on Valentine's day.

2. There is a funny story about Valentine's day, which claims Valentine's day celebration started in Gujarati Patel cast. :) VelanTime-Day

3. This year, I read an inspiring story on Valentine's day by Dr. Sharad Thakar. Here, the author narrates a story of a career oriented young boy, who focused on his study and after getting admission at IIM-A, he proposed the most beautiful girl of his college.

• The first one is like an "open source" love-latter. :) It is in Hindi. "MERA UNSE YAHI SAWAAL HAI" I want to ask her that only.

• "That Rose". The first half of poem is about description of a beautiful flower rose. Second half is about struggle by rose to compete with her. That rose is nothing in front of her beauty.

• "Mukt-Keshi" A description about a beautiful girl.

• What happened when such beautiful girls cook? A humorous picture, of girl-friend who removes the outer skin of hot potatoes. "BATEKU CHHOLATI PRIYA"

• Sometime back, I posted a poem about "communication evolution" among the lovers! "Ashaadhsya Prathame Divase"

• And let me conclude this list with a last poem. It is also about communication. Lovers follow "Heart Protocol". Please do NOT mask me, at which OSI layers, this protocol works !!??!! Earlier blog, I addressed: "GTP is on Layer 3 or Layer 7 ? " For Hear Protocol, find out yourself. click here.

Waiting for YOUR comments about MY poems.

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”

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.

So far we have explore, EMM and ECM aspects of NAS. EMM and ECM apply to per UE. So UE has single entity for EMM and ECM. This is not the case for ESM. At least theoretically... I mean to say, practically the earlier implementation may restrict one bearer per UE and connectivity with single PDN. However theoretically a UE can have multiple bearers per PDN and also connectivity with multiple PDNs. ESM is per EPS bearer. Each bearer has its own ESM FSM.

PDN is some network external to operator’s LTE infrastructure. LTE infrastructure includes eUTRAN and EPC. Ultimately LTE or any other wireless network provides layer 2 connectivity for a mobile UE. So that end-user can transfer its layer-3 user packets (most likely IP packets) to some external network (most likely Internet). UE has layer 3 user data IP packets, that go (and come also from) beyond LTE infrastructure to PDN. The ‘name of PDN’ is ‘APN value’. Internet, corporate intranet, MMS, IMS etc are popular example of such PDN. P-GW is at boundary between EPC and PDN.

UE needs an EPS bearer starting from UE to exit gateway (P-GW) of LTE network. Generally UE has one default bearer per PDN. Default bearer does not have neither QoS treatment nor TFT filters for user data. It just provides a basic connectivity between UE and P-GW for a single PDN. Some mobile application like VoIP call needs QoS treatment for user data. Dedicated bearers are meant for that. Here UE can demand QoS and TFT, but not new UE IP address. UE has single IP address per PDN. If UE and PDN both support both IPv4 and IPv6 dual stack, then only UE can have two default bearers and two IP addresses (IPv4 address and IPv6 address) per PDN.

If UE and network both capable to provide connectivity to multiple PDNs, then UE can have multiple default bearers and multiple IP addresses. It is like your desktop PC has multiple Ethernet cards, so you can connect your PC to multiple networks and configure it with multiple IP address. UE has single invisible OFDMA based LTE air interface towards network. However still it can emulate like two different Ethernet cards. At user plane UE can have multiple Layer 3 IP layers, with common Layer 2 LTE network. Each layer 3 connected to different PDNs. So UE have multiple bearers from UE to across LTE infrastructure till P-GW. Within single layer 3, UE can have default bearer for best effort treatment and possible dedicated bearer(s) for QoS treatment.

All EPS bearers (i.e. default and dedicated) have EPS Bearer ID (EBI). Network assigns its value. Earlier in 2.5G GPRS 3G UMTS network we had NSAPI, for EBI. So at UE few EBI values are used for default bearer(s) and rest are for dedicated bearer(s). UE can have one default bearer and zero or more dedicated bearer(s) per PDN. With EBI value, one can not discriminate whether it’s for default bearer or dedicated bearer. LBI plays important role to link them and bundle them together. All dedicated bearer related messages contain LBI IE. Its value is EBI of default bearer for that PDN.

UE may have static IP address configures with OTA for per APN/PDN. Most of the case, P-GW acts as DHCP server and assign dynamic IP address to UE at the time of default bearer creation. Yes in some P-GW implementations, it consult radius server to allocate IP address for UE. Dynamic IP address allocation happens during default bearer allocation. This IP address remain same and valid for all other subsequent new dedicated bearer(s) for that PDN.

The following tree makes this concept clearer.

1. LTE layer 2 connectivity with eUTRAN+EPC
   1. Layer 3 IP (IPv4 or IPv6)connectivity to PDN1, UE IP = IP1
      1. Default bearer. No QoS, EBI = EBI1
      2. Dedicated bearer 1 with QoS1, EBI = EBI2, LBI = EBI1
      3. Dedicated bearer 2 with QoS2, EBI = EBI3, LBI = EBI1
   2. Layer 3 IP (IPv4 or IPv6)connectivity to PDN2, UE IP = IP2
      1. Only single default bearer. No QoS, EBI = EBI4
   3. Layer 3 IP (IPv4)connectivity to PDN3, UE IP = IP3
      1. Default bearer. No QoS, EBI = EBI5
      2. Dedicated bearer 1 with QoS3, EBI = EBI6, LBI = EBI5
   4. Layer 3 IP (IPv6)connectivity to PDN3, UE IP = IP4
      1. Default bearer. No QoS EBI = EBI7
      2. Dedicated bearer 1 with QoS4, EBI = EBI8, LBI = EBI7

Here PDN1, PDN2 and PDN3 all are different, having different APN values. IP1, IP2, IP3 and IP4 may or may not be different. QoS1, QoS2, QoS3 and QoS4 may or may not different. EBI1 to EBI8 all are different values, not necessary they are in sequence.

Practically the minimum implementation without QoS lookes like:

2. LTE layer 2 connectivity with eUTRAN+EPC
   1. Layer 3 IP (IPv4)connectivity to only single PDN 1, UE IP = IP1
      1. Only single Default bearer. No QoS, EBI = EBI1

It looks like this article about NAS ESM is becoming lengthy. So let’s have break after some details about PCO IE.

PCO IE is used to carry UE address. It also carries Primary and Secondary DNS addresses for that particular PDN. So the application can query and resolve any domain name to IP address by contacting the DNS server, within that PDN. All these three IP addresses can be IPv4 address or IPv6 address or both. It depends. If UE already knows their values, it mentions them in PCO, to confirm. Else, UE mention value as 0.0.0.0 (for IPv4 case) to request network for new assignment. PCO IE contains, PPP. PPP contains IPCP for all these IP addresses. PPP can also contain PAP and/or CHAP protocol(s) for user authentication. PCO and TFT are IEs, that are initiated at UE, transparently carried by intermediate nodes (eNB, MME and S-GW) and reached ‘as they are’ at P-GW.

Recap: This article covers, some important concepts about bearer and few important IEs like EBI, LBI, APN, PCO. I will continue ESM in next article and we will explore various ESM procedures.