Idle Mode Signaling Reduction

When ever a UE moves to a new tracking area or routing area it triggers a tracking area update (LTE) or Routing area update (3G) procedure to inform the network about its current location. Note that these procedures are triggered while UE is in Idle mode or Connected Mode. Read below considering UE in Idle mode.

With the LTE systems coming in, both the network antennas (3G and LTE) might remain close to each other. This means a UE can be in a tracking area of a MME at one point and it can immediately move to routing area of SGSN at another. If both coverage areas are pretty close and UE is lingering at the border of both networks, it could result in ping-pong effect. That is a UE may get into a loop where it sends RAU to 3G system de-registering itself from LTE and vice versa by continuously doing cell reselection.

Solution to this is proposed in Annex J of 3GPP TS 23.401, which is ISR.

The solution assumes that SGSN is S4 SGSN. Also it is a requirement that both UE and network should understand ISR. ISR support is mandatory for E-UTRAN UEs that support GERAN and/or UTRAN and optional for the network.

Assume that UE is registered to the network through EUTRAN. Now UE re-selects a GERAN/UTRAN network and initiates a Routing Area Update. SGSN will send context request to MME asking for the UE context. Here MME may reply with context response along with ISR activate indication. Network shall switch on the ISR once it realizes the UE context is being pulled from a different RAT. Once the context response is received, RAU accept along with ISR active indication is sent to UE. Once UE receives the RAU accept along with ISR active, it shall store the GUTI from EUTRAN and also the newly received P-TMSI from GERAN/UTRAN. MME will also keep the UE context. SGW is informed about ISR activation by SGSN in Modify bearer request (ISR flag set). At this point UE is registered in both EUTRAN and GERAN/UTRAN, has both the network identifiers (P-TMSI and GUTI) and UE may not initiate a TAU or RAU (until the UE has identified a new TA/RA that is not in its list).

When data for UE arrives from network, SGW will send downlink data notification to both SGSN and MME. Both MME and SGSN will initiate paging. UE may respond to which ever paging message it has received and initiate a normal service request procedure later.

Thus the ping pong effect of TAU and RAU is minimized at the expense of paging the both networks.

Spec Ambiguity

Could somebody kindly tell me if a SGW could be relocated by a TAU request in connected mode? I think not? Interested in a debate? 3GPP TS 23.401 Chapter:- 5.3.3 Cheers, Santosh

LTE Tidbits IV

Continuation to LTE Tidbits I, II, III


There have been several questions on how RRC and EMM/ECM states are in sync. To begin with, RRC and EMM/ECM states are two seperate layers, each layer provides service to other but doesnt have authority to modify each other. eNB, acts as a relay between UE and MME, i.e it will receives RRC messages from UE and puts them in S1AP messages towards MME. This means eNB will simple pass the NAS pdu's from UE to MME without modifying them. SO at UE, there are two layers to maintain, RRC state and NAS state. We cannot have a state where there no radio bearers for dedicated bearer, but the NAS layer has the EBI for dedicated bearer.

S1AP Initial UE message holds an extereme importance. It is sent only when UE first establishes an RRC connection with eNB. This message is what makes UE move from ECM idle to connected mode.

FAQ's:-

1. In a normal TAU procedure, when MME decides to change the SGW, how does the new SGW come to know about the IP and TEID of eNB for data downlink flow and how does the eNB come to know about SGW IP and TEID?

[S] When a SGW has changed during TAU, MME will send a create session request towards the new SGW. This CSReq is for establishing the control plane.Now in CSRes the new SGW will respond with its own user plane IP and TEID that is used by eNB for uplink. This info is propagated to eNB in Initial context setup message. Now eNB will create radio bearers and once the radio bearers are created it will send downlink user plane IP and TEID in intial context setup response message to MME. This info is send to SGW in modify bearer request.

2. In Initial Attach procedure, how does the eNB comes to know about the SGW IP and TEID?
[S] Same philosophy as above applies. The trick here is radio bearers will be created only once the network has setup the EPS bearers. Once EPS bearer is created using CSReq/Res, only then then eNB creates the radio bearers and this info is sent to SGW by MME in Modify bearer req/resp

3. If OI=0 in both CSR during initial attach and S1 based HO, why does the SGW not send CSR towards the PGW in S1 based HO?

[S]CSReq is used to create EPS bearers. In S1 HO case the EPS bearers are already created, so CSReq will not be sent to PGW. BUt since the SGW has changed, PGW should be informed of new SGW, so a modify bearer request is sent to PGW.

Summer Break! Wedding bells are ringing!

Hands are trembling, feet are cold. Yes, I am getting married. Will be off from blog and email untill June.

See you all then!

Cheers, Santosh

EMM, ECM, RRC States, TAU and Handovers

Its very interesting to observe the connection between 5 terms mentioned in the subject line.

Refer here and here for details on EMM, ECM and RRC states.

EMM has two states:- EMM Registered and EMM De-registered. When a UE is connected to the network it moves from EMM De-registered to EMM Registered. Also the ECM state moves to ECM connected from ECM idle. Before these two happening the RRC state in UE is moved from RRC idle to RRC connected. So when UE is actively connected to the network the states in UE/network are RRC Connected, ECM Connected and EMM Registered.

When UE moves to Idle mode first the RRC connection is released. That is RRC state moves from RRC connected to idle. Then ECM state moves to ECM idle from connected. This mean eNB has released the RRC connection and network has released the UE bearer context information but EMM state is Registered. This is termed as S1 release in 3GPP TS 23.401. It would be interesting to know that ECM idle state is valid only on S1 and S11 interface. The UE information is actively maintained in PGW but is released in eNB and MME. Also the behavior of network in ECM idle mode is different in different cases. (future posts). The basic idea of idle mode is UE is not know to the network and in connected mode UE is known to the network. The term "known" refers to UE location, bearer information etc.

TAU - Tracking Area Update. A tracking area is a group of cells that are being tracked by a SGW. When UE is connected to the network, a list of tracking areas are sent to it. When a UE moves to new tracking area it will trigger a tracking area update procedure as defined in 3GPP TS 23.401. A TAU can be triggered when UE is in idle mode or connected mode.

Now at any given point of time UE can be in idle or connected mode. If UE is in connected mode and moving from one place to other then handovers take place. Note that handovers are transparent to UE except that UE now camps on to a new eNB. Also handovers are network initiated. This means a handover can happen when the UE is known to the network, i.e when UE is in connected state. Also after the handover if UE finds that it is present in new tracking area then it can initiate a tracking area update.

But a UE need not be in a connected state all the time. What happens when a UE is in idle mode and is moving from one place to other? When UE is in idle mode, MME will have the UE's last know location. So if some data arrives to UE, while it is in idle mode, MME will initiate paging process to find UE. Now if UE has moved from the last know location how will the MME know the UE's new location. This is where TAU becomes extremely important. In this case it is the responsibility of UE to inform about its location to network. This is done by sending TAU. As I said when UE is attached to the network MME sends a list of tracking areas. So if MME wants to find UE i.e is in idle mode, it will simply send paging message to the eNB and eNB will page for the UE. But if UE is in new tracking area, that is not in the list received during initial attach, it will initiate a TAU. Once network receives TAU it will store the UE's location info.

To initiate a TAU UE should have a RRC connection. First thing it does is it camps on to a nearest eNB by doing cell re-selection process. After RRC connection is established UE may send TAU to network. Note that ECM state in network is still idle for that UE. But TAU may move the ECM state in network from idle to connected. (future post)

Thus if we look at overall picture a handover can happen when UE is in EMM Registered, ECM connected and RRC connected state. A TAU may be initiated by UE when it is in EMM Registered, ECM idle and RRC idle state. Both the procedures help UE in its movement from one place to other.

TAU and RAU

This is in continuation to what I wrote some time back. (here)
TAU corresponds to Tracking Area Update in LTE and RAU corresponds to Routing Area Update in GERAN/UTRAN. A TAU or RAU is initiated by UE when it detects a new tracking area or routing area. Below call flows show how a UE performs RAU in GERAN and TAU in LTE when both GERAN/UTRAN and LTE networks are converged using Gn interfaces.
LTE UE's are expected to operate in dual mode, i.e support both LTE and UTRAN and more over both the radios are expected to be turned on all the time. Below call flows are with reference to Annex D of 23.401. SGSN is considered to be Gn based and MME is expected to support GTPv1. This means no changes to the existing 3G network (SGSN) is required to support below.

LTE - Routing Area Update

Looking at the call flow we can see that GGSN is missing. It is believed that to implement above both GGSN and PGW must reside on single box. So PGW is acting as GGSN for 3G world and it remains itself for LTE. PGW shall again speak both GTPv1 and v2 and will be responsible to map the contexts and qos (of 3G and 4G).

Once UE detects that it is in 3G world it immediately triggers a Routing Area Update. We are assuming that UE is registered in LTE domain now. Once SGSN gets the RAU it looks for the old SGSN information to pull the UE context from (P-TMSI ?).To a Gn SGSN MME just acts as an another Gn SGSN. So the Gn SGSN sends SGSN context request to MME to which MME replies with the UE context. Later an Update PDP context is sent to GGSN(i.e PGW) to inform the change in the data path. At this point PGW shall map the LTE bearers to 3G PDP;s

LTE - Tracking Area Update

Similarly when UE moves from 3G to LTE it triggers a tracking area update.

Clearly the above are not handovers. Both TAU and RAU are triggered by UE which means UE is responsible for choosing the corresponding technology. Also this method might not be seamless too.

More thoughts on this?

Piggybacking in LTE

I observed an interesting feature in 3GPP TS 23.401. Feature is defined in Annex F. Now I wonder are Annexes really that important?

In LTE we always have a default bearer. Dedicated bearer may be created on need basis. Looking at the way industry is moving it is pretty evident that VOIP will be mainly used for voice over LTE. Operators would like to provide a good QoS for a voip call so they would want a dedicated bearer to handle the same. Also with LTE phones VOIP will be a very basic feature and would become a must on every LTE device. This means the network infrastructure should be capable enough to support dedicated bearer for every LTE phone and a dedicated bearer has to be activated as soon as the UE connects to the network to provision VOIP.

A dedicated bearer may be requested by UE or can be triggered by network based on rules and charging functions. Considering the above it would be nice to have both default and dedicated bearer established while UE is being attached to the network. Annex F exactly defines the same.

During the LTE UE initial attach, a create session request is sent to PGW. PGW responds with create session response. Note that dedicated bearer creation is initiated by PGW by sending create bearer request to SGW. Now 3GPP TS 29.274 provisions a method where a GTP message can be piggybacked to other. During initial attach, PGW can send a create bearer request piggybacked to the create session response indicating that it has initiated a dedicated bearer creation. Just a flag needs to be set in the GTP-C header of create session response (Octect 1 bit 5 from LSB) and create bearer request can be appended. There will be only one UDP header though. Once MME receives the create session response with piggybacked create bearer request it will initiate both default and dedicated bearer activation in single shot.

I find this mechanism very cool as lot of processing in PGW can be avoided with respect to PCRF interaction for dedicated bearer creation. This piggybacking can be enabled based on a IMSI or based on APN that is UE connecting to. Interesting!

Any thoughts?

LTE Tidbits III

Gn/GP handovers:

LTE will not be fully functional from day 1. We will need the legacy systems to support major number of customers. LTE specs describe the inter RAT handovers, but they do insist to upgrade the existing SGSNs and GGSNs. Now no operator wants to touch their existing fully functional system untill the new system is working without any problems. This means for some point of time both legacy and LTE systems should work together. This also means that operators would still want their legacy systems to talk to the LTE and vice versa.

To provision this 3GPP Ts 23.401 Annex D provides the call flows. According to the Annex existing SGSNs need not be touched.LTE functional elements such as MME and PGW will also support legacy protocols (GTPv1) so that both networks can communicate. To a SGSN, MME will just behave as another SGSN.

But these handovers are hard handovers which means there could be a connection loss. Also GGSNs from the existing networks should be replaced with LTE PGW, as the whole conecpt is based on LTE PGW as the comman anchor between the two networks. This shouldnt be much of a problem as PGWs are written keeping this mind and I personally believe that GGSN replacement will not be that much of a problem. The dissected call flows shall be posted soon.

ISR/TAU/RAU:-

When ever a UE moves to a new tracking area or routing area it triggers a tracking area update (LTE) or Routing area update(3G) procedures to inform the network about its current location. Note that these procedures can be triggered while UE is in Idle mode or Connected Mode. Read below considering UE in Idle mode.

With the LTE systems coming in, both the network antennas (3G and LTE) might remain close to each other. This means a UE can be in a tracking area of a MME at one point and it can immediately move to a routing area of SGSN at another. If both coverage areas are pretty close then it could result in ping-pong effect. That is a UE may get into a loop where it sends TAU to LTE network and next moment it sends RAU to 3G systems de-registering itself from LTE and vice versa.

Solution to this is proposed in Annex J of 3GPP TS 23.401. ISR - Idle-mode singalling Reduction.

The soultion suggests that UE be registered with both networks while it is in Idle mode. That is UE is registered to a tracking area of a MME and also to a routing area of a SGSN. This needs UE to explicitly support ISR and also both the networks should be capable enough to support the ISR. The common achor point between the networks is considered to be SGW, this also means that SGSNs and GGSNs should be upgraded to support Rel 8. This way ping pong effect is avoided. But this comes at an expense. As the UE is registered to both networks both the SGSN and MME will have to initiate a paging process to make UE move to connected state. However UE will initiate serivice request to network that it is currently camped on, i.e either LTE or 3G.

More to follow.

LTE, Wimax and WIFI

I observed this particular trend in India with regards to household. People typically have a PSTN phone at their home provided by the local govt operator (BSNL). Then they use mobile phones from various operators. Though mobile phones are extensively used for communication I observed that people prefer wired telephones to convey important information to the other party whenever possible. Many are very comfortable with using a PSTN line. Though mobile communication is exploding PSTNs are doing very good business too. I was thinking if we could offer more services over wired line it would be big hit in India. At-least my mom would be very happy to talk to her sister over a video phone. VOIP phones?

On a same note I still dont find the wireless internet in India to be great. I recently got a Wimax connection and its kinda bad. First, the connection is costly and next the speeds are bad. More over the webauth makes it worse when there are multiple computers at home. The basic connection setup takes forever and it is really frustrating sometimes. Though companies have been pushing for wimax, end users are really not happy with it, at-least me and my friends are not happy with it. Now I always think how my fixed line (DSL) used to work. Its been a downgrade for me. Similar is the experience with the 3G modems. Worst uplink speeds and during peak hours you hardly get anything. The point here is Wimax and 3G are still not proving to be perfect replacement for the wired connections.

Recently I came across a service provider providing wifi services. The adds display 4G enabled technology and gives away words like MIMO and OFDM. It caught my interest and was wondering for a while what the underlying technology could be. Guess what its our humble Wifi. I was always a fan of WIFI and with latest 802.11n its time to explore the wifi mesh networks. About 3 years back, when I was in college, I happened to listen to a lecture from a professor who came all the way from Israel. The lecture was on Wifi mesh networks and various routing algorithms involved in it. Typically a wifi mesh is formed of multiple adhoc networks. The lecture was quite interesting and we did some experiments with our laptops and could route packets through multiple laptops. It was fun but I never thought it would be commercially launched (college kid, lack of experience :-)) Now when I see the add, it reminds me of the lecture and experiments. 802.11n on the backhaul and supported 802.11a/b/g towards the customer end seems great. Backhaul is powered with MIMO and OFDM which provide high speeds between the nodes can help boost the network performance. But again the network has to be carefully designed and routes to the main node are to be optimally configured. I quite dont know how the network is really performing, should talk to someone who is using the mesh, but I am quite happy to see wifi growing. Imagine if these networks grow then wimax and mobile networks can get in trouble. More over this wifi mesh is seeming to be a lot cheaper than what other technologies are offering.

Just a thought!

3GPP Specs

As we know there are hundreds of 3GPP specs. But when coming to the implementation will all that is written in the spec will be implemented. Of-course not. I think the specs give an detailed view of the architecture and how things should work . But there are some many things that are outside the specifications scope and mind you they can happen in the real world.

Ambiguous scenarios, negative scenarios, collision scenarios and what not. When coming to LTE I pity eNB and MME. They are most vulnerable to take a beating from various handsets. More over the interpretation of specifications from one engineer to other may be different, so the product might seem different. Hence IOTs are needed.

The wireless world is so different from the regular networking at Layer 2 and Layer 3. The IEEE standards and RFCs are clearly written, so the implementation is very clear. The thing to notice here is there are not many end user to it. Who would be running BGP or OSPF or MPLS? Most of the traffic in the internet is at Layer 7 and hence I feel difficulty is a bit less in internet. But I do agree at there are very complex protocols involved in the internet. But in wireless, the whole network is at the customers stake. A fake UE can bring down the back-end devices. So the equipment manufactures do implement lot of things that are outside the scope of the 3GPP specs. Just a thought.