Tuesday, March 31, 2009

Linux Linux!!!

Today I had a requirement to change the kernel of a Red Hat Enterprise Linux 5, customer issue from Sweden. Huh!. I am linux freak since my college days, so I jumped into action immediately. I had some past experience changing/upgrading the linux kernels of Red hat in college on beautiful and brand new laptops. Wow! Sounds terrific though. Any way I started searching for a kernel. I found one src.rpm.

Here the drama begins, I run the rpm and start searching for the source code. Man, its such a pain. I found the source under /usr/src/redhat/BUILD/..

Next is to make the source code. "Make" looked as if it ran fine. But nope, it dint. I did make install and saw that some thing happend in the /boot folder. I saw that i have vmlinz image. Hurray, success. I just had to change the grub.conf to point to the new image and restart the box. Then what, KERNEL PANIC. He he he he. What a jerk you are, said the linux box. I was missing the initrd image. How should I get the image. Oh forget it, I am not working on this customer issue any more. I will say we are not supporting that kernel and that ends the chapter.

Wait a min says one of my bosses, and points me to a link. Life saver. I could find .rpm there. That means no source code, that mean no make, that mean no make install. Cool. I ran the rpm. Wala, i had the vmliuz and initrd image in the /boot folder along with updated grub.conf. Superb. Restart the linux with new kernel and it works excellent. Supercool. Customer will be happy.

Oh, by the way, here is the link for the rpm, just in case if you need. http://download.filesystems.org/linux/centos/updates/5-i386/RPMS/kernel-2.6.18-92.1.22.el5.i686.rpm

That was one hell of an experience. :)

Saturday, March 28, 2009

Mobile Switching Center

I read a very interesting post on MSC by Martin Sauter of Wireless moves. I recommend you to have a look at it. Post can be found here.

Apple "Genius" is a Genius

I am heavy music listener. I am crazy about music and I choose my play list very very carefully. My mood swings a lot too and I feel like listening to something according to my mood, off course everybody do. I love my ipod touch. So just couple of days back I looked at the Genius button in my itunes, though it was lying there for long. I should admit I am pretty much impressed with it. Its wonderful. Select one song based on your mood and let Genius find the songs in that genre. Not genre exactly but the tunes match the mood. So nice and fantastic. I love listening to Killers. So I choose the song "All these things that I have done" and press Genius. Wala, I have a list of songs in my playlist which sounds exactly like the one I chose.

Its a nice feature. Things like these a again making me to look for an IPhone. IPhone 3.0 OS is amazing. Watched the entire keynote presentation and was pretty much impressed. I am looking forward to the new IPhone to be launched in june. I think I might want to add it to my wish list.

Watching No Reservations now. Catherine is amazing. God I love her, she looks so fantastic even in her 40's. Wish you all a happy weekend.

Monday, March 23, 2009

LTE Testing : Help! Help!

All developers and testers working on various LTE projects :-

How are you testing your devices?

Is there any tool available to test conformance of protocols?

How is LTE device tested for performance?

Any inhouse tools being built?

Any tools available for corporates to buy?


If you have any clue about what I am trying to say please feel free to write to me. Iam desperately looking for some ideas for testing LTE protocols (GTPv2) .

Mail: santoshdornal@gmail.com

Thanks

Santosh Kumar Dornal

Sunday, March 22, 2009

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 APNAMBR 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.

Tuesday, March 17, 2009

Dedicated Bearers in LTE

After a long list of questions and debates I finally understand how dedicated bearers are established in LTE. The following is what TS 23.401 has to say about dedicated bearer establishment.


Here is how dedicated bearer is established.

  • The PDN GW uses this QoS policy to assign the EPS Bearer QoS, i.e., it assigns the values to the bearer level QoS parameters QCI, ARP, GBR and MBR. The PDN GW sends a Create Dedicated Bearer Request message (IMSI, PTI, EPS Bearer QoS, TFT, S5/S8 TEID, LBI, Protocol Configuration Options) to the Serving GW, the Linked EPS Bearer Identity (LBI) is the EPS Bearer Identity of the default bearer. Procedure Transaction Id (PTI) parameter is only used when the procedure was initiated by a UE Requested Bearer Resource Modification Procedure
  • The Serving GW sends the Create Dedicated Bearer Request (IMSI, PTI, EPS Bearer QoS, TFT, S1-TEID, LBI, Protocol Configuration Options) message to the MME. If the UE is in ECM-IDLE state the MME will trigger the Network Triggered Service Request from step 3 .In that case the following steps 4-7 may be combined into Network Triggered Service Request procedure or be performed standalone.
  • The MME selects an EPS Bearer Identity, which has not yet been assigned to the UE. The MME then builds a Session Management Request including the PTI, TFT, EPS Bearer QoS parameters (excluding ARP), Protocol Configuration Options, the EPS Bearer Identity and the Linked EPS Bearer Identity (LBI). If the UE has UTRAN or GERAN capabilities, the MME uses the EPS bearer QoS parameters to derive the corresponding PDP context parameters QoS Negotiated (R99 QoS profile), Radio Priority, Packet Flow Id and TI and includes them in the Session Management Request. MME then signals the Bearer Setup Request (EPS Bearer Identity, EPS Bearer QoS, Session Management Request, S1-TEID) message to the eNodeB.
  • I will leave the rest of the "Radio Level" Messages.
  • The eNodeB acknowledges the bearer activation to the MME with a Bearer Setup Response (EPS Bearer Identity, S1-TEID) message. The eNodeB indicates whether the requested EPS Bearer QoS could be allocated or not.
  • The eNodeB sends an Uplink NAS Transport (Session Management Response) message to the MME.
  • Upon reception of the Bearer Setup Response message in step 7 and the Session Management Response message in step 9, the MME acknowledges the bearer activation to the Serving GW by sending a Create Dedicated Bearer Response (EPS Bearer Identity, S1-TEID) message.
  • The Serving GW acknowledges the bearer activation to the PDN GW by sending a Create Dedicated Bearer Response (EPS Bearer Identity, S5/S8-TEID) message
Open Question: Can MME or S-GW initiate a dedicated bearer? Is so how?

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.




Wednesday, March 4, 2009

LTE queries

All

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)