Monday, March 29, 2010


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?

Wednesday, March 24, 2010

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?

Thursday, March 18, 2010

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.


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.

Monday, March 8, 2010

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!