LTE Tidbits

Few more interesting things on LTE.

GTP & PMIPv6

So, looking at the market I came to know that PMIPv6 is the one that will be deployed over S5 interface, no matter wether the existing service provider network is GSM based or CDMA based. Even I have been PMIP supporter for a while as it avoids complexity over S5. Reduces complexity? I was thinking over bearers one day and realized something.

In LTE a UE can have 11 bearers altogether established per APN. Note that EPS bearers are GTP based bearers. Now on S5 interface there is no concept of default bearer or dedicated bearer. That is there is only one bearer (PMIP) per UE per APN. The question is how are multiple bearers on S1_U interface mapped to single bearer on S5 interface. Lets call each bearer as a pipe with different quality of service running different applications. Since each pipe is identified by a TEID we can enforce all the QoS on it over S1_U. But how will the same be mapped over S5 interface? Interesting? Any clues?

Hint: There is an entity called PCRF which is responsible for bearer establishments, QoS enforcements etc. So SGW contacts the PCRF or rather PCRF informs to SGW on how to handle each GTP pipe over S5 interface.

More later.

Handovers & Tracking Area Update

Have you noticed that there are no NAS messages sent for handovers? That means all the handover decisions are taken by eNB based on the power measurements etc and UE is informed to modify its RRC connections.

But if you take a look at 23.401, there is a call flow for Tracking Area Update which gives us an illusion of handover. This is special case I guess so it is separately dealt, not as handover, in the spec. The difference between handover and Tracking Area Update is the later is UE initiated. Each MME(or SGW?) has a list of tracking areas which it tracks. This list is sent to UE during default bearer establishment. If UE detects that it has entered a new tracking area that is not present in the list sent by MME, then it will trigger a Tracking Area Update procedure.

Radio Bearers and EPS Bearers

If you look at the default bearer and dedicated bearer establishment procedure there is an interesting fact hidden with respect to radio and eps bearers. For default bearer, EPS bearer is established first and then the corresponding radio bearer is established. But for dedicated bearer radio bearer is established first and then the EPS bearer. Before I prove it, this fact leads to another two interesting facts. They are MME is responsible for assigning EPS Bearer Identities and we need modify bearer request for default bearer establishment. Let prove all the three.

Looking at default bearer establishment, Create session request is sent from MME to SGW after it receives Initial UE message with PDN connectivity request+Attach request. Create session request includes the EBI and same is informed to SGW. SGW sends the response if the bearer is accepted along with its user plane information. With Create Session Response the EPS bearer is established (EBI is assigned & SGW user plane info is known). Later MME goes ahead with establishment of Radio bearers for the same using the EBI. Once the radio bearer is established (eNB user plane info is known) the same is indicated to SGW in modify bearer request. This proves the third fact.

Next, take a look at dedicated bearer establishment. Dedicated bearer is network initiated. That means Create Bearer request is coming from SGW and it contains LBI, SGW user plane info, TFT etc but no EBI (set to 0?). Once this message is received by MME, MME assigns an EBI and goes ahead with establishment of radio bearer. Once the radio bearer is established (eNB fteid is known), then MME informs the same to SGW in create bearer response along with the EBI. This proves another fact that MME has to assign EBI, but not SGW.

So the above two explanations prove the first fact.

Why EBI is of 4 bits?

EBI is 4 bits because NSAPI is of 4 bits. NSAPI is used to uniquely identify a PDP context in GSM/UMTS networks. To maintain the compatibility EBI is also set to 4 bits. This means there cannot be more than 16 values for EBI. Out of these 16 values 5 are reserved which explains why there cannot be more than 11 bearers. Dont ask me my NSAPI is of 4 bits :-)

Thats it folks, I have run out the ideas. More to follow. As usual any corrections or comments are are greatly welcome.

UE initiated Bearer Resource Modification : a.k.a Dedicated Bearer

Source : 3GPP TS 23.401 : Clause : 5.4.5 and 3GPP TS 29.274 Clause : 7.2.5

Some time back I wrote about Dedicated Bearer establishment in LTE. i was still under confusion about how dedicated bearers are established and how different are they from secondary PDP context. I closed that post with an open question can MME initiate a dedicated bearer. After much reading and lot of questions which readers asked me, I understand the behavior of Dedicated bearers to certain extent now. In this post I want to concentrate on UE requested Dedicated bearers, just like secondary PDP contexts yet so different. In GTP v1 UE initiates the secondary PDP context with a new TFT, but in LTE UE starts with Bearer Resource Modification procedure. (3GPP TS 29.274 Clause : 7.2.5)

Bearer resource modification procedure can do more, but lets just consider that we are trying to get a dedicated bearer established.

The UE sends a Request Bearer Resource Modification (LBI, PTI, EPS Bearer Identity, QoS, TAD, Protocol Configuration Options) message to the MME.

TAD : Traffic Aggregate Description indicate add packet filters operation. EPS Bearer Identity may not be sent here. Linked Bearer Identity is sent to indicate to which PDN connection additional bearer resource is linked to. PTI is dynamically allocated by UE.

The MME sends the Request Bearer Resource Modification (IMSI, LBI, PTI, EPS Bearer Identity, QoS, TAD, Protocol Configuration Options) message to the selected Serving GW. The MME validates the request using the Linked Bearer Id

The Serving GW sends the Request Bearer Resource Modification (IMSI, LBI, PTI, EPS Bearer Identity, QoS, TAD, Protocol Configuration Options) message to the PDN GW

If requested is accepted then dedicated bearer establishment is invoked. The PTI allocated by the UE is used as a parameter in the invoked Dedicated Bearer Activation Procedure to correlate it to the UE Requested Bearer Resource Modification Procedure. This provides the UE with the necessary linkage to what EPS Bearer to be used for the new traffic flow aggregate. The PDN GW shall not modify the QoS parameters requested by the UE  

Now go back to the dedicated bearer establishment post.

This takes one more step towards understanding bearers better. As usual comments are welcome.

Bearer Level QoS (TS 23.401, Clause 4.7.3)

The EPS bearer QoS profile includes the parameters QCI, ARP, GBR and MBR, described in this post. This post also describes QoS parameters which are applied to an aggregated set of EPS Bearers: APN‑AMBR and UE‑AMBR.

Each EPS bearer (GBR and Non-GBR) is associated with the following bearer level QoS parameters:

-   QoS Class Identifier (QCI);

-   Allocation and Retention Priority (ARP).

A QCI is a scalar that is used as a reference to access node-specific parameters that control bearer level packet forwarding treatment (e.g. scheduling weights, admission thresholds, queue management thresholds, link layer protocol configuration, etc.), and that have been pre-configured by the operator owning the access node (e.g. eNodeB).

The ARP shall contain information about the priority level (scalar), the pre-emption capability (flag) and the pre-emption vulnerability (flag). The primary purpose of ARP is to decide whether a bearer establishment / modification request can be accepted or needs to be rejected in case of resource limitations (typically available radio capacity in case of GBR bearers). The priority level information of the ARP is used for this decision to ensure that the request of the bearer with the higher priority level is preferred. In addition, the ARP can be used (e.g. by the eNodeB) to decide which bearer(s) to drop during exceptional resource limitations (e.g. at handover). The pre-emption capability information of the ARP defines whether a bearer with a lower ARP priority level should be dropped to free up the required resources. The pre-emption vulnerability information of the ARP defines whether a bearer is applicable for such dropping by a pre-emption capable bearer with a higher ARP priority value. Once successfully established, a bearer's ARP shall not have any impact on the bearer level packet forwarding treatment (e.g. scheduling and rate control). Such packet forwarding treatment should be solely determined by the other EPS bearer QoS parameters: QCI, GBR and MBR, and by the AMBR parameters. The ARP is not included within the EPS QoS Profile sent to the UE.

Each GBR bearer is additionally associated with the following bearer level QoS parameters:

-   Guaranteed Bit Rate (GBR);

-   Maximum Bit Rate (MBR).

The GBR denotes the bit rate that can be expected to be provided by a GBR bearer. The MBR limits the bit rate that can be expected to be provided by a GBR bearer (e.g. excess traffic may get discarded by a rate shaping function).

Each APN access, by a UE, is associated with the following QoS parameter:

-   per APN Aggregate Maximum Bit Rate (APN-AMBR).

The APN‑AMBR is a subscription parameter stored per APN in the HSS. It limits the aggregate bit rate that can be expected to be provided across all Non‑GBR bearers and across all PDN connections of the same APN (e.g. excess traffic may get discarded by a rate shaping function). Each of those Non‑GBR bearers could potentially utilize the entire APN‑AMBR, e.g. when the other Non‑GBR bearers do not carry any traffic. GBR bearers are outside the scope of APN‑AMBR. The P‑GW enforces the APN‑AMBR in downlink. Enforcement of APN‑AMBR in uplink is done in the UE and additionally in the P‑GW.

Each UE in state EMM-REGISTERED is associated with the following bearer aggregate level QoS parameter:

-   per UE Aggregate Maximum Bit Rate (UE-AMBR).

The UE‑AMBR is limited by a subscription parameter stored in the HSS. The MME shall set the UE‑AMBR to the sum of the APN‑AMBR of all active APNs up to the value of the subscribed UE‑AMBR. The UE‑AMBR limits the aggregate bit rate that can be expected to be provided across all Non‑GBR bearers of a UE (e.g. excess traffic may get discarded by a rate shaping function). Each of those Non‑GBR bearers could potentially utilize the entire UE‑AMBR, e.g. when the other Non‑GBR bearers do not carry any traffic. GBR bearers are outside the scope of UE AMBR. The E‑UTRAN enforces the UE‑AMBR in uplink and downlink.

The GBR and MBR denote bit rates of traffic per bearer while UE-AMBR/APN-AMBR denote bit rates of traffic per group of bearers. Each of those QoS parameters has an uplink and a downlink component. On S1_MME the values of the GBR, MBR, and AMBR refer to the bit stream excluding the GTP-U/IP header overhead of the tunnel on S1_U.

The HSS defines, for each PDN subscription context, the 'EPS subscribed QoS profile' which contains the bearer level QoS parameter values for the default bearer (QCI and ARP) and the subscribed APN-AMBR value. The subscribed ARP shall be used to set the priority level of the EPS bearer parameter ARP for the default bearer while the pre-emption capability and the pre-emption vulnerability information for the default bearer are set based on MME operator policy.

The ARP of the default bearer should be set appropriately to minimize the risk of unnecessary release of the default bearer.

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.

  

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.

EPS Bearers -- Short Notes

I havent read anything for while, work is keeping me busy. Today i got a chance to get back to 3GPP Spec TS 23401 and I started reading the QoS in EPS bearers. Now LTE being all IP with no circuit swtching, the operation is little different from UMTS. In UMTS/GSM networks the phone will first get an IP address to do packet data. But in LTE its always IP. Lest see what EPS bearer is.

EPS bearer uniquely identifies traffic flow that recieves a common QoS treatment between a UE and a PDN GW.
Packet filters signalled in the NAS procedure are associated with unique packet filter identifier on per PDN connection basis. The EPS bearer Traffic Flow Template (TFT) is set of all packet fileters associated with EPS bearer.

One EPS bearer is estabalished when UE connects to PDN and that remains lifetime of that PDN connection to provide UE with always on IP connnectivity to PDN. THis is default bearer. Any additional bearer that is established to the same PDN is referred as dedicated bearer.

Up Link Traffic Flow Template (UL TFT) : - Set of uplink packet filters in TFT
Downlink Traffic Flow Template (DL TFT) : - Set of downlink packet filters in TFT
Every dedicted EPS bearer is associated with TFT.

More to follow, please correct me if I am wrong and comments are always welcome

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.

Will we support two TFT's? Both UL and DL TFT?