Sunday, March 15, 2009

LTE Attach Procedure

At last I found what I needed. It was always there since beginning but I never found time to read it. So I went through the spec TS 23.401 (GPRS enhancements for EUTRAN) and I found all the call flows required. I started with the initial on, EUTRAN Initial Attach procedure. Now my interests lie in GTP based interfaces (I am avoiding PMIP here) and only on IP side. So I eliminate all the radio signals, S1- MME request response and what I arrive at is the below. A simple diagram depicting initial attach procedure of UE.

LTE_Initial_Attach.jpg  

A UE needs to register with the network to receive services that require registration. This registration is described as network attachment. The always on IP connectivity for UE of the EPS is enabled by establishing a default EPS bearer during Network attachment. The attach procedure may trigger one or multiple dedicated bearer establishment procedures to establish dedicated EPS bearer for that UE. During the attach procedure UE may request for an IP address allocation.

Now lets look at the request/responses and the IE;s that are passed in them. As I said, I am not much of radio guy I will directly jump into

3. Create Default Bearer Request: MME ----> S-GW

MME selects a S-GW and allocate an EPS bearer Identity for default bearer associated with UE. Then it sends Create Default Bearer Request with following IE;s.

  • IMSI
  • MSISDN
  • MME TEID for Control Plane
  • PDN GW address
  • PDN Address
  • APN
  • RAT Type
  • Default EPS bearer QoS
  • PDN Type
  • APN-AMBR
  • EPS Bearer Identity (EBI)
  • Protocol COnfiguration Options
  • Handover Indication
  • ME identity
  • User Location Information (ECGI)
  • MS Info Change Reporting Support Indication
  • Selection Mode
  • Charging Characteristics
  • Trace Reference
  • Trace Type
  • Trigger ID
  • OMC identity
  • Maximum APN Restriction
  • Dual Address Bearer Flag
  • Protocol over S5/S8.

Subscribed APN AMBR for the APN is provided in this message. MSISDN is included if the attach type indicates handover. Selection Mode indicates that a subscribed APN was selected. Charging characteristics indicates which kind of charging the bearer context is liable for.

4. Create Default Bearer Request : S-GW -------->PDN-GW

The S-GW creates a new entry in its EPS bearer table and sends a create default bearer request message to PDN GW indicated by the PDN GW address received in the previous step. This message contains

  • IMSI
  • MSISDN
  • APN
  • S-GW address for User Plane
  • S-GW TEID of the User Plane
  • S-GW TEID of the Control plane
  • RAT Type
  • Default EPS bearer QoS
  • PDN Type
  • PDN Address
  • Subscribed APN-AMBR
  • EPS bearer identity
  • Protocol Configuration Options
  • Handover Indication
  • ME Identity
  • USer Location Information (ECGI)
  • MS Info change Reporting SUpport Indication
  • Selection Mode
  • Charging Characteristics
  • Trace Reference
  • Trace Type
  • Trigger ID
  • OMC Identity
  • Max APN Restriction
  • Dual Address Bearer Flag.

5. Create Default Bearer Response : PDN-GW ------> S-GW

The P-GW creates a new entry in its EPS bearer context table and generates a charging ID. THe new entry allows the P-GW to route user plane PDU's between the S-GW and the packet data network and to start charging. The PDN-GW returns a create default bearer response with following IE's

  • PDN-GW address for the user plane
  • PDN GW TEID of the user plane
  • PDN GW TEID of the control plane
  • PDN Type
  • PDN address
  • EPS Bearer Identity
  • EPS bearer QoS
  • Protocol Configuration Options
  • Charging ID
  • Prohibit payload compression
  • APN Restriction
  • Cause
  • MS Info Change Reporting Action
  • APN AMBR

6. Create Default Bearer Response : S-GW --------> MME

S-GW returns a create default bearer response to MME with following IE's.

  • PDN Type
  • PDN Address
  • S-GW address for User Plane
  • S-GW TEID for User Plane
  • S-GW TEID for control plane
  • EPS bearer Identity
  • EPS bearer QoS
  • PDN GW addresses and TEID's at the PDN GW for uplink traffic
  • Protocol configuration options
  • Charging ID
  • Prohibit payload compression
  • APN restriction
  • Cause
  • MS Info change Reporting Action
  • APN-AMBR

These are the Default bearer request and response messages. Watch out more messages.




32 comments:

Anonymous said...

this's a nice post. i'm reading 23.401 too and i'm still wondering what this "attach request" message is on the radio interface...could you explain a little more?

Santosh said...

Hi

I am not much of a radio of guy so I will avoid the RRC parameters. From 23.401 the UE attach request has following IE’s along with some RRC messages.

IMSI or old GUTI, last visited TAI (if available), UE Core Network Capability, UE Specific DRX parameters, PDN Type, Protocol Configuration Options, Ciphered Options Transfer Flag, Attach Type, KSIASME, NAS sequence number, NAS-MAC, additional GUTI

This message is sent to eNB. Now eNB will forward the message MME (MME is dervied from the RRC message). Note that this attach request from eNB to MME is sent on S1-MME (not GTPv2). S1-MME has S1-AP protocol running over SCTP transport. So read my post(http://wired-n-wireless.blogspot.com/2009/01/lte-handoff.html) and go over TS 36.413 v 8.4.0 (http://www.3gpp.org/ftp/Specs/archive/36_series/36.413/). Look at Initial UE message there.

This helps?

Santosh

Li said...

I am a little puzzled by the EUTRAN Initial Attach procedure. According to 23.401 V9.0.0, "EUTRAN Initial Attach procedure" is for UEs in limited service states, and if UEs are not in limited service states, they should use "normal" initial attachment. So, what is the "normal" initial attachment? Is it UE Requested PDN Connectivity?

Thanks,

Li

Santosh said...

Hi Li

First, the spec you are referring to is from Release 9 and I am still on Release 8. But after I looked the Rel 9, I could see the a new paragraph was added here (to Initial attach section) which is not present in Rel 8. I am guessing that you must have mis-interpreted the the statement. I think what it means is UE can also perform EUTRAN Initial Attach Procedure even in limited service state. This point was missing in Rel 8. Its like in GSM if you roam from one network to other where your operator doesnt have coverage you will be shown an emergency only in the mobile.

I think in LTE, they have defined a process for Initial Attach procedure in emergency state too.

Makes sense??

Santosh.

Li said...

Santosh,

The added paragraph in Rel. 9 is really confusing, and you might be right.
My another question is what the "UE Requested PDN Connectivity procedure" is for? In other words, when this procedure will happen? My first guess is that this procedure is to allow UE to have multiple PDN connections. My understanding of the whole picture is:
1. When UE want to have its first PDN connection, it uses "EUTRAN Initial Attach Procedure"
2. If the same UE wants to set up the second (or more) PDN connection, it uses "UE Requested PDN Connectivity procedure".

What do you think?

Thanks,

Li

Santosh said...

Very correct Li. This request is similar to establishing secondary PDP context in GTP V1, yet it is little different. In GERAN/UTRAN UE may request for secondary PDP context for a new traffic activity. In LTE we call it as dedicated bearer. But the difference here is dedicated bearers are network initiated, that is this is initiated by PDN. So a UE or network can request for dedicated bearer based on the traffic activity QoS

I blogged on dedicated bearer some time back. You can have a look. (http://wired-n-wireless.blogspot.com/2009/03/dedicated-bearers-in-lte.html)

Santosh

Anonymous said...

These are really nice posts....helping to understand the scenarios..I have a doubt regarding multiple PDN connections in UE.......
Consider a scenario where UE supports multiple PDN connections. How these multiple PDN addresses can be used in UE appliaction level? Since normally in Windows system has one gateway....so how does UE application know which PDN address to choose?

Santosh said...

UE can have multiple PDN connections. The whole idea is that UE has one PDN gateway which it has to reach no matter how many applications it is running. Having said that, there can be a scenario where a UE can be attached to multiple PDN's, not sure though need to research more.

Considering the applications running on UE, PDN connections really doesnt matter. All applications can use one PDN connection say, default bearers, or one dedicated bearer per application can be created. It depends on UE and network. A max of 11 bearers per UE is supported.

Anonymous said...

hi Santosh, thanks for the quick reply.
Suppose there are 2 applications running app1 and app2 in UE and UE supports some PDN addresses. Suppose app1 wants to change PDN address (say to APN1). UE is using one PDN gateway address. Now here how app1 chooses the right PDN address?
If anybody here knows some scenario of multiple PDN support in UE, then he can throw some light on "how multiple PDN addresses can be handled by UE application"?

Santosh said...

I understand your question. Give me sometime, I will come up with a a new post on PDN gateway selection procedure. :)

Santosh said...

New Post is out. I hope I answer your questions. If not feel free to comment/question back.

http://wired-n-wireless.blogspot.com/2009/04/pdn-gw-selection-function.html

Thanks --Santosh

rakeshbitsindri said...

Nice blog santu! I was searching for LTE, 3GPP, UE,, MME etc etc and found your blog.

Coolest stuff of today!

Rakesh_

fahim said...

hi , can you tell me about configuring PPP link using Protocol configuration options

Santosh said...

Will this help?

http://wired-n-wireless.blogspot.com/2009/04/3gpp-radius.html

http://wired-n-wireless.blogspot.com/2009/08/3gpp-radius-part2.html

purnendu said...

Hi Santosh,
I was not able to find the "Create Default Bearer Request" in 29.274. Request you a clarification on this. Also as per the same specification the "Charging ID" is always generated for Dedicated Bearer and not for default bearer. So another clarification on this also

Santosh said...

Hi Purnendu

You will not find Create Default bearer in the spec, infact create default bearer was just a way to present the article.

Create Session Request is what you should be looking at in 29.274.

Charging ID is sent in Create session response over S5/S8 interface.I am not sure if the same exist in S11 interface.

santosh said...

Hi..I had a doubt... Can we send multiple qci values in a create bearer request sent from sgw. What should the mme reaction be..Also when can such a situation arise...

Thanks in advance,
Santosh.M

Santosh said...

Well, technical it is possible. Each dedicated bearer can be attached to a QCI and with one create bearer request we can establish multiple dedicated bearers. So I think it is possible. MME can go ahead and create multiple dedicated bearers and assign EBI's to them.

papu said...

HI Santhosh,

I am having a query regarding bearers, Can a default bearer can be used to run the multiple applications at the same time or there will be separate bearers for separate apps?

Santosh said...

We can use default bearer for any traffic. Thats why its called "default" :)

Cheers, Santosh

Prem said...

Hi,

I have a question related to Multiple PDN connection. In UMTS generally primary PDP contexts with Interactive/Background traffic class will have dedicated logical channels multiplexed(Muxed) onto one transport channel. In LTE will there be same logic as it is in UMTS. Please give me ur inputs.
Thanks in advance
Prem

Santosh said...

Hi Prem

The QoS has changed in LTE. In LTE there is always a default bearer established. I am not sure how this will translate to radio network, but yes there is always a bearer created.

Wei said...

Hi, Santosh,

Thanks for sharing this. I'm a radio guy and now I'm studying your area. I have a stupid question: what's the different of default EPS bearer and dedicated EPS bearer? Can you give me some examples about these two concepts? Thanks.

Santosh said...

Hi

I would recommend you to go through 3GPP TS 23.401 section 4.7.2 to understand the details. Also there is some info in the blog, please feel free to search.

Satish said...

As per 23.401 , it is stated as"An eNodeB can contain cells from more than one Tracking Area and intra-eNodeB cell changes are not normally notified to the MME. However, the MME needs to know the UE's current TAI in order to correctly produce a TAU accept message."

This means when UE changes its TAC within a TAI list..TAU doest happen?
What is ECGI ? i guess it is used to notify the MME when the above TAU happens ...kindly brief me regarding this.

Santosh said...

Tai can be a group of cells. When there is a cell change, which is ECGI, MME may NOT be notified. However TAI change should be notified to MME if it falls out of the TAI list that was sent to UE

Anonymous said...

Hi ,

Can you pls let me know in Create Session Request , in the IE APN, what would be the structure of the APN that should it be normal

APN-Name
or
APN-NAME ,APN OI, APN NI .gprs

Santosh said...

Apn NI + Apn OI + . Gprs

Prithvi said...

Hi Santhosh,

Thanks for your nice blog. It has hepled me a lot in understanding the Technology.

I have few doubts.

1. During attach procedure how MME will selects the SGW. i.e there can be 1 to N(n SGW) connection. Out of N, how MME will selects the particular SGW??

2. How many IP address an UE can have?? I was reading 5.3.1 IP address allocation of 23401-a50 specification.

3. Can there be more than one Default bearer?

Santosh said...

Hi Prithvi

1. MME will select SGW based on Tracking area. MME uses its own optimization techniques to select SGW
2. Depends on the number of default bearers
3. Yes, the max limit is 11 , but there may not be more than 3 default bearers acrtive, as we have 3 pdn types( ipv4 , ipv6, ipv4v6)

Cheers, Santosh

Anonymous said...

I have a doubt with respect to the allocation of TEID between SGW and PGW. According to 29.274 section 4.1 (GTP Tunnel)
“The TEID-C shall be unique per PDN-Connection on GTP based S5 and S8 interfaces. The same tunnel shall be shared for the control messages related to all bearers associated to the PDN-Connection. A TEID-C on the S5/S8 interface shall be released after all its associated EPS bearers are deleted.”

Our understanding of this statement is that the PGW generates the TEID for S5/S8 interface only once and is used for all sessions between the SGW-PGW (irrespective of the UE) for a specific PDN connection. But when we analyse in detail on this, we could see this could create problems between SGW and PGW to uniquely identify the UE for which a message is received from SGW to PGW. For example, In Delete Session Request, we don’t see any unique parameter, which can be used to identify a particular UE (or the received message).

Can you please clarify whether the understanding is correct and in that case how is each message identified with a particular UE.

Santosh said...

TEID over S5 is per UE per PDN based. SGW/PGW TEID pair is always unique