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 activationDedicated EPS bearer context activationEPS bearer context modificationEPS bearer context deactivation
1Procedures related to TransactionPDN connectivity x

2PDN disconnect

3Bearer resource allocation
4Bearer resource modification
5ESM information requestx

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.


I had one interesting e-mail conversation with a friend about GTP protocol. He asked me, GTP runs over UDP. UDP is layer 4. Then GTP should be application layer protocol. He was wondering, how come GTP itself is considered as layer 3 protocol!! Here is my answer.

  • Yes he is right, as per OSI layers, GTP is above L4 (UDP). So it is application layer L7 protocol. However, when one says GTP is at L7, we are only considering two nodes (MME and S-GW) of EPC. (SGSN and GGSN of 3G core network.)

  • Just expand the horizon and consider the whole network.

  • It is the GTP protocol, who helped for proper routing of user data packets.

  • For UL data, GTP routes user data packets to appropriate S-GW by establishing tunnel with it. (For UL data it route user data packets from SGSN to GGSN and from RNC to SGSN. )

  • For DL data, GTP route user data packets to appropriate MME . (For DL data it route user data packets from GGSN to SGSN and from SGSN to RNC. )

  • For mobility case, “Modify Session Request” changes the route to new MME and new eNB. (For Mobility case, “Update PDP context request” changes the route to new SGSN. )

  • Yet the exit gateway P-GW is the same, the common anchor point. (Yet the exit gateway GGSN is the same, the common anchor point. )

  • So thus, GTP helps to route the user data, to appropriate node, and it also change the route for mobility case.

  • Routing is done by network layer So GTP is L3 within PLMN.

  • Again, if you expand the horizon further, end to end from mobile till content server in Internet cloud, then The whole wireless network (GPRS, UMTS or LTE) is just L2. It just carries the user IP packet towards ISP (Internet cloud).

Now let me recap, this L2, means the wireless network has its own stack (OSI layer), where GTP is used to route, within different nodes of core network or EPC. So GTP is L3 within L2
Again this L3 GTP is between 2 nodes. It has its own stack (OSI layer). GTP over UDP over IP over L2 over L1. So when we consider only these 2 nodes, within core networks, then yes, GTP is L7 application layer protocol.

So one line summary of long answer "Just perform: Zoom in and zoom out" :)


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:

  1. 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 (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.


An interesting article on LTE EPC market.

Here is the summary.

Alcatel-Lucent: Starting from scratch with 4G, designing its LTE evolved packet core using its IP division 7750 Service RouterCisco: Cisco has built P-gateways and S-gateways using its router platforms, but with the acquisition of Starent Networks it will likely focus on its purpose-built ST40 platform for both 3G and 4G

Ericsson: It's partnership with Juniper ends at 3G. Instead it's using Redback SmartEdge technology to build its gateways.

NSN: While evolving its 3G technology for the LTE core, it is redesigning on the Advanced Telecom Computing Architecture to give it extra kick on the control plane

Tellabs: Bought WiMax gateway specialist WiChorus and plans to use its technology to enter LTE market as well as use augment its backhaul products.

2010 - A year for inner peace

7 Ways to Achieve Inner Peace and prosperity in the New Year - 2010

By H.H. Sri Sri Ravi Shankar

Happiness, peace and prosperity are wished for by every lip on New Year, but do we really know what peace means? Peace lies within us and we know it too. As we move beyond a year and enter new year, lets all take a resolution to become aware of this peace inside and allow our smiles to reflect the true sign of the prosperity that lies inside us. Here are seven ways to do it.

1. Give divine a chance, be grateful

This year, let the devotion flower to its fullest and give it a chance to work. Our love, faith & belief should be deep rooted, and then everything else moves on its own. The feeling that “I am blessed” can help you overcome any failure. Once you realise that you are blessed, then all the complaints & grumbling disappear, all the insecurities disappear & you become grateful, contented and peaceful.

2. Take out time for yourself

Everyday you are engaged in only gathering information and you do not take out time for yourself to think and reflect. Then you feel dull & tired. A few quiet moments are sources for creativity. Silence heals and rejuvenates and gives you depth and stability. Some time during the day, sit for a few minutes, get into the cave of your heart, eyes closed, and kick the world away like a ball. Taking out some time for yourself improves the quality of your life.

3. Know life’s impermanence

See the impermanence in this life. Millions of years have passed & millions will come and pass. Nothing is permanent. What is your life? It is not even a drop in the ocean.

Just open your eyes & ask; who am I? How am I on this planet? What is my lifetime? Awareness dawns & you will stop worrying about little things. All
smallness will simply drop & you will be able to live every moment of your
life. If you review the context of your life, the quality of your life improves.

4. Do random acts of kindness

Make a commitment to make this world a better place to live. Do some acts of kindness without expecting anything in return. Service alone can bring contentment in life. It creates a sense of belongingness. When you bring some relief to someone through selfless service, good vibrations come to you. When you show kindness, your true nature which is love & peace
comes to play.

5. Make your smile cheaper

Every day, every morning, look at the mirror and give a good smile to yourself. Don't let your smile be snatched away by anybody! Usually, you give your anger freely and smile rarely as though a smile is costly. Make your smile cheaper and anger expensive! When you smile all the muscles in your face get relaxed. The nerves in your brain get relaxed and you become peaceful within. It gives you the confidence, courage & energy to move on in life.

6. Make meditation a part of life

When we have higher goals in life it leads to stress & restlessness which can only be released through a few minutes of meditation & introspection. Meditation gives you deep rest. Deeper you are able to rest, dynamic you will be in activity. What is meditation?

Mind without agitation is meditation.

Mind in the present moment is meditation.

Mind that has no hesitation, no anticipation is meditation.

Mind that comes back home to the source which is peace & joy is meditation.

7. Be a Student always

Know that you are a student forever. Do not underestimate anybody. Knowledge may come to you from any corner. Each occasion teaches you & each person teaches you. The world is your teacher. When you are always looking to learn, you will stop underestimating others. Humility will dawn in your life.

Wish you all a very Happy & peaceful New Year.