Saturday, 30 July 2011

Wednesday, 27 July 2011

MRO: Handover failures signalling

Continuing on the Self-organising Network (SON) feature of Mobility Robust Optimisation, Handover failures.


Click on image to enlarge

One of the discussions I had with a colleague is how would the signalling happen in case of Handover failures I mentioned earlier.

After the handover failure, when the connection is successfully established again either as a normal Setup or Re-Establishment or RRC Reconfiguration then a new optional field is available:

rlf-InfoAvailable-r10 ENUMERATED {true} OPTIONAL,

This is used to indicate to the network that the UE has some information relating to the RL Failure that occurred.

The network will then use the UE Information Request I blogged about earlier to ask for this information. The UE will send the information back in the response.

It should be noted that this UEInformationRequest and Response messages were introduced part of Release-9 but there has been since some updates in Release-10. The Response message now looks as follows:

RLF-Report-r9 ::= SEQUENCE {
measResultLastServCell-r9 SEQUENCE {
rsrpResult-r9 RSRP-Range,
rsrqResult-r9 RSRQ-Range OPTIONAL
},
measResultNeighCells-r9 SEQUENCE {
measResultListEUTRA-r9 MeasResultList2EUTRA-r9 OPTIONAL,
measResultListUTRA-r9 MeasResultList2UTRA-r9 OPTIONAL,
measResultListGERAN-r9 MeasResultListGERAN OPTIONAL,
measResultsCDMA2000-r9 MeasResultList2CDMA2000-r9 OPTIONAL
} OPTIONAL,
...,
[[ locationInfo-r10 LocationInfo-r10 OPTIONAL,
failedPCellId-r10 CHOICE {
cellGlobalId-r10 CellGlobalIdEUTRA,
pci-arfcn-r10 SEQUENCE {
physCellId-r10 PhysCellId,
carrierFreq-r10 ARFCN-ValueEUTRA
}
} OPTIONAL,
reestablishmentCellId-r10 CellGlobalIdEUTRA OPTIONAL,
timeConnFailure-r10 INTEGER (0..1023) OPTIONAL,
connectionFailureType-r10 ENUMERATED {rlf, hof} OPTIONAL,
previousPCellId-r10 CellGlobalIdEUTRA OPTIONAL
]]
}

Everything after the extension marker ellipses (...) is added in release 10. More information in Release-10 RRC specs (36.331)

Tuesday, 26 July 2011

Outline of GCF Certification Process


Click on Image to enlarge


From a presentation by Colin Hamling, Vice Chair, GCF Steering Group in LTE World Summit, Amsterdam, 18 May 2011

Monday, 25 July 2011

Femto Hacking in UMTS and LTE

Couple of weeks back, The Hacker’s Choice (THC) made available some documents about how the Vodafone's (UK) Femtocell (a.k.a. SureSignal) is unsecure and can be hacked. Everyone seemed to jump on this bandwagon with some news articles even sounding like the whole Vodafone network has been hacked and hackers may be sending messages and making calls via your phone number.

In the end it came to light that the problem was fixed over a year back when Vodafone was made aware of this problem. THC is still arguing that there is an architecture fault and the Femto can be compromised.

As a result I decided to think about what could happen if the Femtocell is hacked.

Lets take case of UMTS Femtocell. A simple network architecture with femtocell (oficially known as Home NodeB) is as follows:

As you can see, the signalling over the air interface is encrypted and integrity protected. If a hacker is able to get into the Femto and able to listen to all the packets using some tool like WireShark, he would be able to get hold of the Ciphering and Integrity Keys as they come in cleartext in the RANAP Security Mode Command message.

It wouldnt be difficult to have a device that can listen to the conversations once provided with this keys. In fact if the hacker is able to listen to the messages, there is no reason he cannot stick his own messages at the right interval (when a voice call is ongoing) to send SMS and would appear that the message actually went from the phone number. Note that this message would be inserted in the Home NodeB and would be a NAS message. The end user would generally never find out that a message has been sent on behalf of his phone.

One thing that should be remembered though is that the phone would have to be in the range of the Femtocell and connected successfully to the network (via the Femto). One question someone may have is that can I not reverse engineer the key so that I can clone the SIM card. Fortunately for us, this is not easily possible. There are multiple levels of protection and generally it would be difficult to get the algorithms for generating the key. Also it should be noted that the authentication algorithms are confidential and only the operators know the algorithm.


Now lets look at the LTE Femtocell (a.k.a. Home eNodeB) as shown below:

One of the differences you may notice is that the signalling from Femto to the Core Network over S1 is encrypted and Integrity Protected. In case of the LTE Femto, there are multiple keys and only the required key (Kenb) is provided to the Femto. See the key hierarchy below:

Source: RedYoda

This would sound like an ideal protection from the end user perspective but some of the problems still remain. If the hacker can get hold of the Kenb which is sent in cleartext over the S1 interface via Initial Context Setup Request message then he could easily use it to listen to the packets. Since there is no voice support as of yet in LTE, it would only be the packets that the hacker can listen to.

As you may notice, there is now an Integrity and Ciphering on the S1 interface for the UE messages, the hacker cannot get hold of the Kasme or the master keys K, CK and IK. This means that he cannot insert rouge messages that would for example send unsolicited SMS on behalf of the user as he would be able to do in case of UMTS.

There is a small caveat though. There are multiple Ciphering and Integrity algorithms defined in the standard. No ciphering is defined as eea0 algorithm. In Release-8 of LTE, there was no possibility to have Integrity switched off as there was no eia0 algorithm defined. In Release-9 though, the new eia0 has been defined which means that the network can set the Integrity to NULL. I am sure that the network would not want to do so as it makes absolutely no sense but the hacker can force it to do so.

When the Network requests the UE to send the capability information, the hacker can force it to say that it only supports eia0 and eea0 which would mean that the integrity and ciphering in the call would be off. To be honest, this is quite a difficult thing to do in real time and also the network would not accept a UE that does not support other Integrity and Ciphering algorithms.


3GPP has already forseen these kind of threats that could be affecting the networks in the future when they roll out the Femtocells. As a result they have produced 3GPP TR 33.820 that lists all the possible threats and the best practices that can help to minimise the chances of the network being compromised. If that document is too big and technical, you can go though this presentation as it summarises some of the problems.

Feel free to comment or correct any mistakes that you think I have made.

Friday, 22 July 2011

Mobility Robustness Optimization to avoid Handover failures

The following is from 4G Americas Whitepaper on SON:


Mobility Robustness Optimization (MRO) encompasses the automated optimization of parameters affecting active mode and idle mode handovers to ensure good end-user quality and performance, while considering possible competing interactions with other SON features such as, automatic neighbor relation and load balancing.

There is also some potential for interaction with Cell Outage Compensation and Energy Savings as these could also potentially adjust the handover boundaries in a way that conflicts with MRO. While the goal of MRO is the same regardless of radio technology namely, the optimization of end-user performance and system capacity, the specific algorithms and parameters vary with technology.

The objective of MRO is to dynamically improve the network performance of HO (Handovers) in order to provide improved end-user experience as well as increased network capacity. This is done by automatically adapting cell parameters to adjust handover boundaries based on feedback of performance indicators. Typically, the objective is to eliminate Radio Link Failures and reduce unnecessary handovers. Automation of MRO minimizes human intervention in the network management and optimization tasks.

The scope of mobility robustness optimization as described here assumes a well-designed network with overlapping RF coverage of neighboring sites. The optimization of handover parameters by system operators typically involves either focused drive-testing, detailed system log collection and postprocessing, or a combination of these manual and intensive tasks. Incorrect HO parameter settings can negatively affect user experience and waste network resources by causing HO ping-pongs, HO failures and Radio Link Failures (RLF). While HO failures that do not lead to RLFs are often recoverable and invisible to the user, RLFs caused by incorrect HO parameter settings have a combined impact on user experience and network resources. Therefore, the main objective of mobility robustness optimization should be the reduction of the number of HO-related radio link failures. Additionally, sub-optimal configuration of HO parameters may lead to degradation of service performance, even if it does not result in RLFs. One example is the incorrect setting of HO hysteresis, which may results in ping-pongs or excessively delayed handovers to a target cell. Therefore, the secondary objective of MRO is the reduction of the inefficient use of network resources due to unnecessary or missed handovers.

Most problems associated with HO failures or sub-optimal system performance can ultimately be categorized, as either too-early or too-late triggering of the handover, provided that the required fundamental network RF coverage exists. Thus, poor HO-related performance can generally be categorized by the following events:

* Intra-RAT late HO triggering
* Intra-RAT early HO triggering
* Intra-RAT HO to an incorrect cell
* Inter-RAT too late HO
* Inter RAT unnecessary HO

Up to Release 9, a UE is required to send RLF report only in case of successful RRC re-establishment after a connection failure. Release 10 allows support for RLF reports to be sent even when the RRC reestablishment does not succeed. The UE is required to report additional information to assist the eNB in determining if the problem is coverage related (no strong neighbors) or handover problems (too early, too late or wrong cell). Furthermore, Release 10 allows for precise detection of too early / wrong cell HO.

Thursday, 21 July 2011

Smart Deployment with Smart Antennas and ORI

This is from a presentation by Dr. Peter Meissner, Operating Officer, NGMN Alliance.

Its very interesting the way the Antennas are evolving.


If you are interested in reading more about ORI, see the earlier post here.

Wednesday, 20 July 2011

NSN Celebrating 20 years of GSM

Its been 20 years since the first GSM call was made and GSM is still as relevant today as it was 10 years back.My earlier post today was about the technology deployment and adoption trends and my guess is that GSM/GPRS will be still relevant for long time to come especially its de-facto fallback for the roaming calls. Some Facts about GSM that would should know:* First network launched in 1991* There are 838 GSM Networks in 234 countries with 4.4 Billion subscribers* In 2010, 1.44 million GSM subscribers were added every day* 545 EDGE networks in 198 countries with 1.5 Billion subscribers* By 2015, 1.5billion GSM M2M subscribers will be present

Here is a presentation from NSN about 20 years of GSM and since they had the privilege of launching the first commercial network I am sure they have a good reason to celebrate.

20 Years of GSM: Past, Present & Future
View more presentations from Nokia Siemens Networks
A new section on 3G4G website on GSM has been added here.