Saturday, August 29, 2009

LTE : Tracking Area Update with SGW Change

After an hour of Metallica rock and few shots of Espresso I sat down on the saturday afternoon to understand the another handover scenario in LTE. Some time back I wrote about Tracking Area Update Procedure without SGW change. In this post I would like to explore Tracking Area Update procedure WITH SGW change. Ref : 3GPP TS 23.401

Before we dive into the post a quick note.

Tracking Area is the area/cell being tracked by an eNB. TAI change may indicate a change in eNB. Routing Area is the area/cells tracked by a SGW. So a SGW can track multiple tracking areas. The title of the post is little confusing. I say Tracking area update with SGW change and also say tracking area is taken care by eNB. Well if a eNB changes and the corresponding MME changes which may also lead to a SGW change. This is the scenario we are dealing below. The below picture should be of some help.

When UE detects that it has entered a new Cell then it begins a Tracking Area Update procedure by sending TAU request to the new eNB.
TAU Request : Tracking Area Request : TAU includes Active flags, EPS bearer status, Old GUTI, last visited TAI .. etc. EPS bearer status indicates each EPS bearer that is active in UE. Active flag is request by UE to activate radio and s1 bearer for all the active EPS bearer by the TAU procedure when UE is in ECM Idle mode. This message is sent along with other RRC parameters to eNB.
Once eNB receives this request it derives the MME from RRC parameters (which I have no idea) carrying the Old GUMMEI and indicated selected network. If this Old GUMMEI indicates the MME which eNB is not associated with, then eNB sends the TAU request to the new MME. This means that MME has changed here.
Before we dig more another very important note :
GUTI : Globally unique Temporary UE Identifier.
GUTI is an ID which uniquely identifies a UE in EPS without revealing the users permanent ID. GUTI is allocated by a MME which can be used to
  • uniquely identify the MME which allocated the GUTI
  • Uniquely identify the UE within the MME that allocated the GUTI


where GUMMEI = MCC+MNC+MME Identifier and MME Identifier = MME Group ID+MME Code

GUMMEI : Globally unique MME Identifier which is used to identify a MME uniquely.

M-TMSI : It is a ID of UE. The relation between IMSI and M-TMSI is known only to UE and MME.

Context Request : The new MME receives the TAU request from eNB. New MME uses GUTI received from UE to derive old MME and sends a context request to old MME. A context request includes old GUTI, complete TAU request, P-TMSI, MME address etc. Basically this message is sent by new MME to old MME to inquire about UE's authenticity, the bearers created if any etc.

Context Response: The old MME receives context request message and validates the UE. Upon successful verification of UE old MME responds with context response. Context response include IMSI, MEI, MSISDN, EPS bearers context, SGW address and TEID's etc. This response gives out all the UE contexts setup in old MME to new MME. The new MME receives this message and happily store the UE context in it and sends context ACK to old MME. Context ack is sent with "SGW Change indication" to indicate the old MME that SGW is going to change.

Create Session Request/Response: If there was no change in SGW there will not be this message. Now that we are dealing with a case where SGW has changed, MME sends a Create session request message to new SGW. Why? Because the new SGW has no information about UE and there is a no way that two SGW's can communicate directly (Correct ?) Create session request is sent with IMSI, bearer contexts, Protocol type on S5/S8 interface, RAT type etc. SGW reads the PGW address and TFT's from bearer context. Based on the S5/S8 interface the bearer is modified. If the S5 interface is based on PMIP we shall see a proxy binding update message and if GTP is the protocol then we shall see a modify bearer request to indicate the change in RAT type, SGW fteids etc. The PDN may contact PCRF for the other info which I have no clue. :)

Update Location : Upon successful reception of create session response, the new MME sends a location update message to HSS to indicate that UE has changed its location. HSS identifies the UE old location in its database and sends a cancel location to old MME. Old MME acknowledges the cancel location with an Ack.

Delete Session Request/Response: The old MME sends the delete session request to clear all the bearer contexts to old SGW. Old SGW acknowledges to it by sending response and deleting the UE contexts.

Finally the new MME sends a TAU accept to UE which is accepted by UE. Now UE is communicating with entirely new SGW.

There are just too many things in this procedure to cover. I just highlighted the important sections and hope I got it correct. Feel free to correct me if I am wrong and any comments are always welcome.

Note: Context Request and Response are GTPv2 messages. Two MME's communicate over S10 interface which is GTP based.

Tuesday, August 25, 2009

LTE Flat Architecture or Insane Architecture?

So every one is talking of how easy LTE is to work with all IP involved. Does making the whole network IP make LTE simpler than 3G networks? Now I am not an expert in 3G networks but I will still say LTE is no easy to deploy than finding a diamond in an ocean. And making it all IP doesnt make it simple too. Ok, so what do people mean by Flat? Is it IP everywhere which is making it flat? What about the protocols over IP? Lets try to analyze the below figure.

All that nice lady wants to do is make some calls and read some emails on her phone. What we have is LTE Uu interface which is an air interface. UE communicates to eNB via RRC protocol. Above RRC we have something called UE Layer 3. This is where all the attach request/response etc are created. RRC is way to communicate to with eNB. Below it is the complex OFDM network. OFDM along with MIMO makes it more complex for my little brain. When the sweet lady switches on here mobile phone UE triggers some messages to get attached to the network. This is where the control plane communication begins. Packet goes to eNB. S1-AP is the protocol which eNB uses to communicate with MME. This S1-AP is over SCTP transport which is yet another protocol in itself.
MME communicates with SGW using GTPc protocol which is over UDP. So MME here needs to understand S1-AP over SCTP on one side and GTPc on the other side. More over it should be a powerful router to run normal routing and switching protocols (MPLS?). So far so good. Lets make it more dramatic. Bring on the PMIP over S5 interface. PMIP in itself is completely another technology. Till now we have protocols specific to LTE domain, but PMIP is generic protocol which is made to fit into LTE network. Thank god that PMIP is an IP over IP tunnel which makes it little simple. Mobile options sit over IPv6 which are put in UDP and transported over IPv4 between SGW and PGW (PMIPv6 over IPv4).
What happened till now? Just the control plane is established. Wait a minute. Where is the security? Charging Policy? Where are the registers? Well they are lying around some where and using "IP" for communication. Again, what happened till now? We have UE context established in eNB. eNB will map the air interface to particular tunnel in S1-U interface. MME has the UE context setup and has told SGW and eNB what information is required to run the user plane (tunnel id's, qos etc). Then we have user plane info exchanged between SGW and PGW which is PMIP based. So we have a mapping between S5 user plane (GRE key), S1-U (TEID's) and LTE Uu interface (God knows what).
Sweet lady wants to make a call. She dials in number and starts talking. User plane begins. Somehow data reaches eNB. eNB tunnels the data over GTP-U header to SGW. SGW removes the GTP header place GRE header and tunnels the data to PGW. PGW removes the GRE header and forwards the data to internet cloud. So simple! Right. I pity SGW here. The poor thing is struggling hard to decapsulate one header and encapsulate another.
Its not over yet. What about the routing protocols here? Many are suggesting the whole network to be based on MPLS cloud, which makes it more complicated. Even with all IP if somebody wants to understand LTE from end to end it will take days. This is one heck of network which looks more scary than a internet core network which is of BGP, OSPF or whatever greek and latin based.
Its just enough to understand one interface in LTE. Say if you want to master S11 interface which is GTP based reading GTP specs will just not get you there. We also need to understand what is happening before and after GTP to actually figure why GTP is behaving the way it is behaving. So I say download the whole 3GPP specs, make you own database and get insanely lost in the wireless network which also has too many wires :).
Oh wait wait. Dont go away, its still not over. Sweet lady is not done for the day. At this moment sweet lady opens her laptop and connects her phone to the laptop using USB. Now she is accessing her notebook over LTE network. Well USB has underlying RS232 techniques (? no idea) over which they make PPP to run. Now the packet first hops from the note book to phone over PPP and phone moves the packet further. Huh! Now it's done. Atleast I cant think any more. (3G/CDMA integration ?)
Tidbits : If you understand what I wrote above, it proves that direct tunneling cannot be established between eNB and PGW. It also proves the we just cannot integrate PGW and SGW into a single entity (When PMIP is used, else we should be able to get both in one device). S5 is the interface which SGW uses to communicate with PGW. S8 is the interface which visiting SGW uses to communicate with home PGW.
Thats it folks, I am still working on below post. Let me know how the above looks and feel very very free to correct me if I get things wrong.

Wednesday, August 19, 2009

LTE Unleashed - (Work in Progress)

Folks! This is very much work in progress. I wanted to post everything in one single post, but I decided. The below work is still a draft, more like a rough notes, check back soon for more details. All your comments are greatly appreciated. Thanks!

Reference :

3GPP TS 23.401 : General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access

3GPP TS 24.301 : Universal Mobile Telecommunications System (UMTS);Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS);

3GPP TS 24.007 : Digital cellular telecommunications system (Phase 2+);Universal Mobile Telecommunications System (UMTS); LTE; Mobile radio interface signalling layer 3; General Aspects

3GPP TS 36.413 : LTE; Evolved Universal Terrestrial Radio Access (E-UTRA); S1 Application Protocol (S1AP)

3GPP TS 36.300 : LTE;Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2


During the Initial Attach procedure the Mobile Equipment Identity is obtained from the UE. The MME operator may check the ME Identity with an EIR. At least in roaming situations, the MME should pass the ME Identity to the HSS, and, if a PDN-GW outside of the VPLMN, should pass the ME Identity to the PDN-GW.

Initial Attach Procedure   

3GPP TS 24.301 : NAS signalling:

The non-access stratum (NAS) described in the present document forms the highest stratum of the control plane between UE and MME at the radio interface (reference point "LTE-Uu"; see 3GPP TS 23.401 [10]).

Main functions of the protocols that are part of the NAS are:

  • the support of mobility of the user equipment (UE); and
  • the support of session management procedures to establish and maintain IP connectivity between the UE and a packet data network gateway (PDN GW).

3GPP TS 24_007 v 8.2.0 : Max EBI is 11.

Link between EMM and ESM

  • EPS session management messages for the default EPS bearer context activation are transmitted in an information element in the EPS mobility management messages
  • The success of the attach procedure is dependent on the success of the default EPS bearer context activation procedure

Avoiding NAS security here

EPS Session Management : ESM

24_301 : EPS Session Management (Chapter 6)

  • The main function of the ESM sublayer is to support the EPS bearer context handling in the UE and in the MME
  • The ESM comprises procedures for:
    • the activation, deactivation and modification of EPS bearer contexts; and
    • the request for resources (IP connectivity to a PDN or dedicated bearer resources) by the UE.

EPS bearer contexts can remain activated even if the radio and S1 bearers constituting the corresponding EPS bearers between UE and MME are temporarily released.

Default and dedicated EPS bearer contexts can be modified. Dedicated EPS bearer contexts can be released without affecting the default EPS bearer context. When the default EPS bearer context is released, then all dedicated EPS bearer contexts linked to it are released, too

The UE can request the network to allocate, modify or release additional EPS bearer resources. The network decides whether to fulfil a request for additional resources by activating a new dedicated EPS bearer context or modifying an existing dedicated or default EPS bearer context.

Two types of ESM procedures can be distinguished:

1) Procedures related to EPS bearer contexts:

These procedures are initiated by the network and are used for the manipulation of EPS bearer contexts:

- default EPS bearer context activation;

- dedicated EPS bearer context activation;

- EPS bearer context modification;

- EPS bearer context deactivation.

2) Transaction related procedures:

These procedures are initiated by the UE to request for resources, i.e. a new PDN connection or dedicated bearer resources, or to release these resources:

- PDN connectivity procedure;

- PDN disconnect procedure;

- bearer resource allocation procedure;

- bearer resource modification procedure.

When combined with the attach procedure, the PDN connectivity procedure can trigger the network to execute the following transaction related procedure:

- ESM information request procedure.

A successful transaction related procedure initiated by the UE triggers the network to execute one of the procedures related to EPS bearer contexts. The UE treats the start of the procedure related to the EPS bearer context as completion of the transaction related procedure.

When the UE or the network initiates a transaction related procedure, it shall include a valid procedure transaction identity value in the message header and set the EPS bearer identity to "no EPS bearer identity assigned".

Attach request is an EMM message.

Attach request is at Layer 3 in UE. It is send together with RRC parameters indicating the selected network and old GUMMEI.

Few things :-

-- GUTI : Globally unique temporary identity.

-- GUMMEI : Globally unique MME identifier. This consists of PLMN id, MME group Id, and an MME code. MME code is used in the eNB by NAS node selection function to select MME.

Once Attach request reaches eNB, eNB creates enter Initial Context setup function. The communication between eNB and MME is over S1 interface and S1-AP is protocol used.

Initial Context Setup Procedure : 36.300

eNB will send S1-AP initial UE message along with NAS service request, which is nothing but Attach Request. (FFS)

This will trigger MME to send Create session request. SGW will send a session response. Once the session response is recieved MME will send a S1-AP Initial Context Setup Request along with NAS message, which is Attach Accept. This message also contains EPS bearer QoS, EBI and TEID for SGW for user plane.

eNB will send Attach accept message to UE. UE shall store QOs. APN will be provided to UE if it is not aware of.

The UE may provide EPS Bearer QoS parameters to the application handling the traffic flow(s). The application usage of the EPS Bearer QoS is implementation dependent. The UE shall not reject the RRC Connection Reconfiguration on the basis of the EPS Bearer QoS parameters contained in the Session Management Request. UE may recieve an IP address too.

UE sends RRC message to eNB, indicating the completion of procedure.

eNB sends Initial Context Response message which includes TEID of the eNB and address of eNB. In the mean time Ue sends Attach complete with EBI.

After the Attach Accept message and once the UE has obtained a PDN Address, the UE can then send uplink packets towards the eNodeB which will then be tunnelled to the Serving GW and PDN GW.

Upon reception of both, the Initial Context Response message and the Attach Complete message in, the new MME sends a Modify Bearer Request (EPS Bearer Identity, eNodeB address, eNodeB TEID, Handover Indication) message to the Serving GW

3GPP TS 23.401 Annex E : LTE Qos To Pre Rel 8 QoS mapping

Saturday, August 15, 2009

PMIP, LTE, CDMA and Mobility

Last night we played a game of Cricket. I was returning home on my wonderful bike cruising at just over 105 KMPH for a stretch of 5 KM. With all the adrenalin kicking in and air gushing through ears I suddenly realized something. Can I receive or make a call at this speed? I dint test it though, but I believe people do test these scenarios. What if I am moving from one base to station and other? How smooth will the handover be? I will try to do some tests. I am a sensible driver and care of safety. Will do tests under guidance of experts. So relax. :)


I read a report from Unstrung which says Verizon will start commercial deployments of LTE in 2010. Boston and Seattle are the selected cities. Now, Verizon 3G network is CDMA EVDO based, which means there is no GTP in packet core. PMIP is the the protocol used in CDMA and Wimax, as far as I know. So looking at LTE network, lets guess how Verizon might deploy the network. I would say the MME and SGW (s11 interface) will be a GTP based interface, which is obvious. But S5/8 interface might have PMIPv6 running. If S5/8 interface is GTP too then I there might be problem converging Verizon 3G network with LTE(?). This leads to more questions. Lets say Verizon is using ALU 7750 rotuer with SGW blade and PGW blade.

First question : Will both the blades sit in the same router? Is it a good idea?

Second question : WIll these blades be generic? One firmware for GTP and other for PMIP? I believe the technology will be software controlled, just like the cisco routers.

Third question : How many interfaces are we looking at in a single SGW blade? Two interfaces are minimum, one towards MME and other towards SGW?

Fourth question : Can I combine both SGW and PGW functionality in single blade? PMIP and GTP will be software controlled, so will combining both SGW and PGW make any sense or will that violate the specs?

Just few thoughts! Let me know what you think.

Ok, I am not a big twitter user or fan. But I sport a small twitter box here (right side), do keep checking it. I usually put something related to technology there, you really dont have to follow me, the box there gives all the info. Incase you want to follow me on twitter here is the link :

Saturday, August 8, 2009

Customer is God!

This is slightly off the track from usual stuff I write in the blog, but I feel it is very important and it is one of my interests too. Excuse me for doing so. Kindly go ahead and have a read, there might be some thing for you in the post. :)

With 2+ years of work experience I will not say that I have seen everything, but I will say that I have seen enough to quote that "Customer is God". You might be a guy sitting in lab and lost in your world trying to do something, but you should realize that you are working on something which will be sold to customers. You may be a entrepreneur or working for some organization, but the ultimate goal is to reach the customer.

In the present situation where every one is hit by recession, customer retention is the most important aspect of any organization. Holding to your customers by reducing the prices might work for few ,but I would say invention is important in retention. The customer expectations are growing day by day and we should be on our toes to deliver what customer wants rather forcing customer to use what we develop.

It take the whole organization to satisfy customer needs, not just bunch of marketing guys. Now that I am a tester I am the second point of contact to the customer. The first contact will be with support which might escalate the issue to the QA if required. So when I actually start writing my test plan and test cases I will keep in my mind the so called "Customer Use Cases" and make sure that piece of software or hardware works accordingly. The software or hardware should function the way the customer wants it to not the way we want it to. There can be a trade off. Development and R & D may come up with wonderful things but if the product is hard to use then effort is lost. My point here is customer satisfaction is not one mans job, but it takes the whole organization. Customer focus should be one of the key aspects of an organization. One unsatisfied customer might bring you huge losses.


I was going through my facebook messages some time back one guy said he recently bought an IPTV at his home. There was an immediate response to his message by one of his friends saying even he has an IPTV and they started discussing about the brand, support etc etc. Both of them were happy with the service and they exchanged good words for the company from which they purchased the IPTV. Now what happened here is visible to all their friends. So when I want an IPTV i would go for the brand which my friends bought because they have already used it and are pretty happy with it.

On the other hand I had bad experience with my mobile connection. I use a XYZ connection (I am a good guy so I am not mentioning the brand here :). I already did the damage I could do ) . I wanted them to enable Voice Mail service, unfortunately they were unable to do it. They dint even give a proper reason for it. Then I had problems calling my dad. When ever I dial my dad's number there is only 1 in 10 chance that I can make it through the first time. I am tired of their service. All my efforts in calling there customer care resulted in nothing but frustration. Unfortunately I cant move to another service provider. My number is spread across all my contacts and is printed in my business cards. Now if somebody asks me which service provider they should go for, when getting a new connection, my response would be "Don;t go for XYZ" . This is strange, instead of suggesting a service provider I am asking them not to go for XYZ. I will always give a bad response when somebody asks me about XYZ. Now I have few friends and they have few and the chain continues. Because of my response there is good chance the XYZ might loose many customers. This is what I am saying, one bad customer might screw the whole thing up for you. This is very much true in every market.

Now why did write this. I was browsing through and came across a website called "Customer experience board". There was a report from them with title "Service Invention to increase retention". The executive summary of the report actually made think in this direction. I recommend you to go ahead and download it. Read it, there might be something in for you to. If you are service provider then it is a must read. Take a look at the numbers.


The below numbers are from reports and they make so much sense.

"The biggest sources of customer pain are said to be unmet needs and expectations (63 percent), followed by product/service usability and complexity (42 percent), billing errors (44 percent), as well as quality or relevancy of service or product offerings (31 percent)."

Now I want to emphasis the point that I am not here to promoting things. I write things which make sense to me and the above summary really made some sense in a way that I myself have seen changes in my organization. The process for support escalation have been minimized and there is lot of customer emphasis going on. The processes have been simplified when there is customer involved. I my self have pushed the regular work aside to solve the problems of the customers. I am not in position to say big things but if you want to make some money treat your customers as Gods.

Hope I managed to say what I wanted to say. :) Let me know your thoughts, I will be glad.

Thursday, August 6, 2009


Phones! Yes, I love the technology.

My interests in mobile phones started two years ago. Since my child hood I was technology addict. We used to watch youtube videos of Surface Computing and think that is the next generation. One day my friend showed me a video of a gadget which was working based on touch inputs. Untill then I only knew that ATM's and few computers could be operated by touching the screen. This was two years ago and that video blew my mind. Off course I am talking of the great "iPhone" & "iPod". I was luckily in US when the first gen iPod was released in 2007. I remember the trips to Best Buy and Fry;s almost everyday and asking the sales guys when I can get my iPod. After 7 days I could lay my hands on one of them and it was brilliant. I got home with it and the whole college and office was running behind me to get a glimpse of it. I enjoyed every moment of it. :) That was my first contact with Apple and I started loving its technology. Then I got a MacBook (An award! wow!) and I loved it even more.

Having said so much about Apple, I should say I am not an Apple fanboy. I love Nokia when it comes to phones. Some how iPhone never impressed me. In January this year I decided to buy a smart phone. So I started my search. New phones were being launched and I was pretty excited with all the things going on in the mobile world. Nokia, Apple, Palm, HTC..all lauched their new phones. But ..but ...but.... there is still no phone which actually satisfies all my fantasies. So I framed up a picture of "MyPhone".

MyPhone is combination of several phones. MyPhone has the features which are handpicked by me. So how does this MyPhone look and behave, what does it have internally. Lets dig in.

MyPhone is combination of four phones. Nokia N97+HTC Touch Pro 2+Palm Pre+iPhone. Pick the best in these phones and make a new phone which becomes MyPhone.

MyPhone is designed like a HTC Touch Pro 2. It has a solid QWERTY Keyboard which slides out to the left. The Screen can be adjustable. The keyboard is nicely designed, feels robust and easy to use. The keys light up when pressed. The build of the phone is like N97. Hard and Robust! How ever I dont like the way key board is designed in N97. But the slide out mechanism in N97 rocks. It feels so solid, but I would still prefer a adjustable screen instead of just one angle. So the design and form factor of MyPhone will be like Nokia N97 with a HTC Touch Pro 2 key board and adjustable screen.

When it comes to hardware MyPhone will feature a powerful 800 Mhz processor with 256 MB Ram. I am not much into processor technology so I will leave the processor to be selected by experts, but I insist it to be a high speed one. 800 MHz sounds pretty good and 256 MB of Ram will just give the boost. MyPhone will feature a 3.5 MM headset jack with a Micro USB and memory card expansion slot. It will have 32 GB of internal memory expandable to 48 GB. Just like N97. MyPhone will also feature a removable battery.

When it comes to taking pictures and recording video, MyPhone will feature a 5 MP cam with auto focus and powerful flash. It will record a HD quality video. MyPhone will be a robust phone which makes its presence felt when in your hands.

MyPhone will also feature a touch screen. The touch will be a capacitive touch unlike N97. Touch screen would be like iPhone with an excellent response. A scroll of phone book or play list will be like magic. So MyPhone will have HTC Touch Pro 2 design with Nokia N97 form factor and iPhone as a touch screen. Movement from normal to landscape mode will be like magic..iPhone accelerometer?. MyPhone will also feature a virtual key board just for one those times. Zoom in and zoom out can done by pinch on the screen. Flicking through photos will be amazing.

MyPhone will run apps like Palm Pre. Each app will run as a card which will make the phone a true multitasking phone. Moving from app to other will be just like the way we do in desktop. Web browsing will be just like the one in iPhone and a pinch can zoom in or zoom out the page. MyPhone will also feature VPN so that I can connect to my office network.

Oh yeah, MyPhone will also feature Wifi with WPA 2, a GPS receiver and a FM transmitter. Maps and Photos can be tagged using GPS. MyPhone will also support flash which makes web browsing a great experience. Email will be fun, all the email clients can accessed at one place just like in iPhone.

All this will come at a good price, some where around 750$ including all the taxes.

If any body is manufacturing the kind of phone I mentioned above then let me know, I will be the perfect one to test it.

Final comments: In my the view the perfect phone has still not arrived. I love the N97 form factor but the resistive screen, application response time and processor makes it not a great choice. On the other hand iPhone does those like a magic but it doesnt have a external key board. Moreover iPhone just can do multitasking. Palm pre looks small. The screen isnt big enough and some how it doesnt suite me. HTC on the other hand seems great, but Windows on mobile... nope .. sorry. How about Android based phones? I never looked at them carefully, may be its the time now. So the "buying phone this year" decision is put on hold till next year. Moreover I am a victim of recession and just cant afford an expensive device now. I will be stuck with the below phone for a while and try to get the home button of my iPod fixed, the damn thing is not working for past 3 months :(

imgMotorola W2202.jpg

Let me know how MyPhone looks, I will be glad.

Sunday, August 2, 2009

3GPP Radius Part2

Some time back I wrote about 3GPP Radius. This is continuation of it.

What I said in the previous post was UE sends the authentication details in PCO field of Create PDP context/Create default bearer to GGSN/PGW. GGSN/PGW looks at that field and creates a Radius Access Request and sends it to a AAA server. AAA server can accept or reject the request. Its plain and simple. Lets look a little further.

IP address allocation: It would be interesting to see if Radius server can assign IP addresses to the UE. When a GGSN sends access request to AAA server, AAA server can respond to the request with access accept by placing an IP address in the Framed-IP field. This way we will not have to maintain a IP pool in GGSN, some processing reduced. GGSN can send the IP address received in Access accept to UE in PDP response message. So radius server will maintain a pool of ip addresses to be assigned to each UE and I believe it would be easy to maintain too.

The IP address can be returned to the radius server in Accounting Stop message. Once AAA server receives accounting stop it can free that ip address and assign it to other UE.

Accounting: I am quite not sure about it but I think accounting is done based on MSISDN. If so AAA server can maintain a table mapping MSISDN, IP address allocated, number of packets/octets in and out. Charging can be done based on this data. More over we can have static values of IP addresses to be assigned based on MSISDN. Assign this IP address to this MSISDN. Basically MSISDN becomes a kind of MAC address (?).

I believe most of the service providers do the way I mentioned above, if not I would be happy to know the other techniques.

FreeRadius : I am impressed. Simply superb is the word. I used Free Radius extensively in past few days and my god its just sweet. 100 requests at a time and still counting. What I did was create a local ip pool and when ever there is access request, reply with an IP address. It works superb. What I dint do was SQL Ip pool. I believe we can do Accounting scenario mentioned above pretty easily. (Log MSISDN, IP address, Accounting Packets/Octets in and out). The only problem with local IP pool was returning the IP addresses. IP addresses were not released when accounting stop was received. I tried every possible way but nope, it dint work. But Sql IP pool is supposed to work fine, I havent tested it but I believe it works fine. I dint try assigning IP addresses based on MSISDN too. Hope it works.

Thats pretty much it. Comments are always welcome.

Oh! Wish you all a very happy friendship day!