Wednesday, March 4, 2009

LTE queries


If you have any queries regarding LTE, post them as comments to this post. I have series of meetings with bunch of LTE experts soon. I will try to get the questions answered from them.

Iam looking for queries in core network side of LTE. Please excuse me with radio side as I have no clue about it.

All IP related queries are welcome. Thank you!


Santosh Kumar Dornal

Notes :

- S11 interface : MME and S-GW

- S5/S8 interface : S-GW and PDN

- Default and dedicated bearer

- Uplink TFT and Downlink TFT

- The decision to establish or modify a dedicated bearer can only be taken by the EPC and the bearer level QoS parameter values are always assigned by the EPC.

- No QoS negotiation between E-UTRAN and EPC during dedicated bearer establishment/modification is supported

Open Questions

- How is QoS initiated? Will we support two TFT's? Both UL and DL TFT?

- How are dedicated bearers established or torn down?

- I hope by default we have direct tunneling in LTE. Traffic will be flowing from eNB to S-GW directly without MME intervention.

- Will we supporting SGSN based interface too? (S4/S12)

- We will be implementing the GTP signaling in the call flows which are mentioned in 23.401 spec. (GPRS enhancements for EUTRAN)


Anonymous said...

Hi Santosh,

Can you tell me What is the main Difference between LTE NAS and UMTS NAS(Non Access Stratum).


nagaraju said...

LTE NAS has added responsibility of integrity and ciphering over UMTS NAS.
Other procedures similar to UMTS NAS.

Anonymous said...

How is QoS initiated? Will we support two TFT's? Both UL and DL TFT?

Did you get an answer for this one.
I am also looking for this.

Santosh said...

This should help.

Santosh said...

Still working on detailed QoS working in LTE. More to come, may be this weekend :)

suryalekha said...

In EUTRAN technology,UE in IDLE mode DRX is configured by upper layers.Which is the upper layer that configures it.If its NAS,what is the dedicated NAS signalling message to configure the DRX cycle?

Santosh said...

Sorry Suryalekha, I am unaware of that technology, trying to learn though.

nagaraju said...

DRX for dedicated is configured by network in RRC reconfig message in RadioResourceConfigDedicated IE

vkkstyle said...

What protocol will have to use for S3(MME to SGSN) and S4(SGW to SGSN) i.e either GTPv1 r GTPv2.if any one find this any where plz let us know that link .....

Santosh said...

Its GTPv2 on interfaces: S3, S4,S5/8,S11 (29.274 -- Scope)

fahim said...

can u tell me what is the maximum number of PDN to be supported ???

Santosh said...

Per APN there can be max of 11 PDN connections (default+dedicated)

3GPP TS 24_007 v 8.2.0 : Max EBI is 11

Ranjeeta Nath said...

Hi Santosh,

The S1-C interface instructs the S1-U interface to create bearers.So bearers are created on the S1-U interface. So how come default bearers are created on S-11 interface when Create Session Request is sent from MME to S-GW.

Santosh said...


All the bearers are created on control plane. The required bearer information is passed on to the user plane.

User plane is just used to run the traffic. The parameters used by user plane are negotiated in control plane.

vkkstyle said...


All the bearers are created on control plane using S1ap interface b/n eNB to MME ,correspondingly tunnels will be created for user plane b/n SGW to eNB i.e S1U tunnels.All the TEIDs tht are used in user plane will shared in control plane transactions in all the entities (eNB,MME,SGW,PGW) respectively.
With the available info. eNB-pdcp will take the traffic n farword to UE.

Gustav Holst said...


Can you provide me a flowchart (or even just link me to the standard) about how a data call is placed via LTE? What about an IMS-based voice call? I can find standards about attachment, detachment and the proposed means for CS fallback, but I don't see anything about how an LTE call is actually placed. Thanks! This is a great blog!

Santosh said...

Hello there!

Thanks for writing! I cannot get you the details for IMS based/CS fall back call procedures as I am not aware yet. But on a different note, analyzing the LTE bearer architecture, LTE UE states and RRC States what I believe is the call handling is upto the application running on the UE.

From the network side, it doesnt matter if it is a VoIP call or http or ftp, all are handled in the same way except the QoS for each service may be different.

Things might look little different when IMS is in place, but ultimately from the network it is again a bearer which is providing the service. Not aware of CS fall back though.

nagaraju said...

IMS call is no different than placing in LTE or 3G. It is uses SIP protocol for VoIP. If you are looking for IMS call you can refer 3GPP 23.228.

CS fallback has no relation to IMS call. CSFB is used when network does not support voice over IMS.

vkkstyle said...

HI all,

In LTE,people are looking for a service that mitigate the cost and runs for long run.In this view VoLGA is a new standered that is capable of taking voice and sms services in LTE.

Below link gives u some idea
try this...

And From LTE network(EPC) prospective there wont be any difference in handling any type of services, n/w uses only bearers to carry them...

Gustav Holst said...

Thanks for all your help! Part of my problem is that I don't understand IMS very well. But, I'm getting better.

My next question is, how do the EPC and IMS layers interface when you place a call? I understand the connection between the UE and eNB and MME, etc, and I understand the connection between the UE and the CSCFs, but I don't see how they all work together. Any insight would be greatly appreciated! Thanks!

I think it would be easier if we did all use VoLGA...but, will it work?

Santosh said...


VoLGA for sure is a pretty good solution.

Next, IMS is just a service running over the network. IMS messages will go in GTP U tunnel to PGW/SGW from eNB and PGW/SGW shall communicate to the IMS box with standard IMS messages.

If we are talking of SIP here then consider INVITE message. Before UE can send INVITE the network bearers are to be established. Once the bearers are ready INVITE is a just a message sent over a bearer to IMS box. I dont think there is anything else network can do than provide bearers to applications with qos.

Gustav Holst said...

I learn so much every time I read your blog. Thank you. :) I have some simple questions...I am not as smart as everybody else.

1) Is LTE backwards compatible with WCDMA, HSPA? I know it's a new air interface so I didn't think they were. But I have read reports saying that it is and that it isn't. Does it depend on the chipsets being employed?? I thought E-UTRAN was supposed to be backwards compatible with HSPA??

2) How does CS fallback change when you are using CDMA? Verizon's network makes me wonder.

Also I have been curious about mentions some interesting things about Band VI in Japan. E-UTRA Band VI will not be available in Japan and maybe not other places due to spurious emissions? It's interesting to read about.

Thank you :)

Santosh said...


Neither am I the smartest kid in the town, anyway I will try to answer your questions.

~ LTE air interface is OFDMA based, which is totally different from WCDMA. As far as I know, only packet core is compatalible with other 3GPP and no 3GPP networks. That is communication between SGSN and MME, MME and ASN gateway etc are defined in the spec. BUt from the airinterface they are not compatible (?)

~ I dont understand CS fall back much, but I believe we will have a seperate chip on UE to do that. That means UE can either operate in LTE mode or in CS fall back, but not both simultaneously. If this is the case then verizon should be able to use its existing network for handling CS calls .Communication between EUTRAN and CS network must be defined somewhere (?)

~ I never got a chance to look at 36.800, will try and find some time later.

Anybody anyother thoughts :-)

Thanks, Santosh

Gustav Holst said...

I continue to have questions! You do such a good job answering them. I will need to spend more time looking at 24.301.

1) Is every LTE chip going to have an IP address? It seems to me that all LTE devices will be "connected" even if they are not conventionally two-way devices like handsets.

2) Can we have LTE without IPv6?

3) Do different security measures impact QoS or any other part of a user's experience on the phone?

There's so much I'm trying to understand...thank you... :) Your blog is the best resource!

Santosh said...

Hello again

Iam glad that blog is helping.

~ I dont understand the term "LTE chip". If you are asking whether a UE will have an IP address then yes. UE will have an IP address all the time it is connected to the network.

~ LTE core and radio network(UE,eNB,MME,SGW,PGW etc..) supports both IPv4 and IPv6. Infact you can have combintaions of both.

~ Qos is something that is done on the core network side. If UE and network agrees on a particular QOS than that shall be granted. Infact no security measures should impact the QoS or QoE.

Cheers, Santosh

vkkstyle said...

This blog helped me too a lot
ANS(1):As per my understanding of LTE UE,each PDN connection i.e each default bearer needs to have one IP address(ipv4 and/or ipv6).At max one UE at a given time can have 3 PDN connections,with which we can say three ip's it needs to talk with different PDNs.

ANS(2):Yes,we can have direct answer.But as per verizion spec thy want ipv6 as mandatory feature.As per spec we can have only ipv4,ipv6 and ipv4ipv6 together.It will be always good that we have a solution tht fits for future needs.....IPv6

ANS(3):1st thing is from a user centric world nothing will effect users features if thy want more r less security.Security will be toways in LTE,one authentication and data ciphering.At the initial stage itself primary security will be resolved and allowed in to network.Based on the SLA secondary i.e data security will be used for data.
Effect of security on QOS will be like india MMS is not allowed which is part of operator n govt issues.It is due to lac of operators qos handling knowledge and problems with hacking n so..
I welcome corrections or additions to the above

all the best & enjoy the technology with its fullest......

Santosh said...

Regarding Security:- Why should it even effect the QoS? QoS is something that is done on the user plane according to the SLA's. Ciphering, authentication etc should really not effect QoE.

Santosh said...

One more thing - MMS not allowed in India? Since when? I remember using MMS service 3 years ago over Airtel. Things changed after that?