Sunday, January 29, 2012

ATT and IPhone

Some time back I got hold of an IPhone from ATT and did a field test. It releived quite an interesting results and gave a blue print of ATT network. Field test application can be invoked using keypad and by typing *3001#12345#*. It shows a lot of information about cellular network including RRC, NAS and PDP context. I tried to analyze some part by taking a bus ride, from one place to another, which is apporximately 5 miles.

Routing Area and Cell id 

Cell id kept changing every quarter to half a mile. Routing area changed every mile to 2. A cell id change mean that there was a NodeB change and Routing area change indicates that RNC has changed. I hope location reporting for cell id is not enabled, otherwise there will be so many updates to the network. Imagine a bus loaded with 40 people out which atleast 20 are carrying phones by ATT network. This creates 20 cell updates at almost same point. Cell Id update is optional so I am sure the cell id update must has been turned off in the network.

But Routing area cannot be avoided. Which means the network was receiving 20 Routing Area Updates every 5 mins from the bus. So if you combine the traffic that is outside the bus, during a peak time, its is quite a load on SGSN/RNCs to process the routing area updates. Unfortunately every Routing Area needs to be reprted to SGSN, but with LTE and concept of Tracking Area List the updates can be significantly reduced. But again that will depend on how the network is designed. During attach in LTE, network may send a max of 16 tracking areas and If UE is moving across those tracking areas then there is no need for a Tracking Area Update.

PDP contexts 

The IPhone atleast opens 2 primary PDP contexts. One is for regular data and other is for visual voice mail. Now another interesting thing, for pulling an voice mail, phone always connects to a different APN, this means voice mail can be given free of charge without much of hassle. Because voice mail APN can be a plain APN withouth charging or DPI turned on.. On the other hand if voice mail is pulled from regular APN, then DPI needs to be turned on the APN and voice mail traffic should be zero rated. Its an absolute pain. So the work aroud is to make phone connect to different APN.

I was wondering, if a Phone is bought unlocked then how to make it connect to two different APNs. I know that I can trigger another primary pdp using a console connection and AT commands, but how do we do it from a phone. May be that is one of the reasons why ATT doenst unlock IPhones.

There are still some more interesting aspects to look at, but I will leave it you. Will grab an LTE phone and perform some more tests as and when time permits.

Friday, January 13, 2012

DPI, Policy and Charging

I am particularly impressed by the amount subscriber management that 3GPP has provided. Per packet flow treatment per subscriber is super cool. However the implementation on mobile gateways is quite complex and need a lot of cpu and memory cycles. In simple words we really need powerful hardware sitting in edge with a efficient software to correctly enforce the policy and perform corresponding charging actions.  
Just to point out, 3GPP allows service provider to treat each subscriber differently based on the subscription. A subscriber may be allowed to use certain traffic or block certain traffic, certain traffic may be treated with higher priority and bill him based on the application or location etc. The limit is endless. A very good business model is needed to make it happen though. So to allow or block certain traffic from a subscriber the gateways need to look deep into the packets, detect the flows, apply policies and report to the billing system. This is quite a lot of pressure on the gateways.
I was doing a study of various telecom plans available across the world. Interestingly some service providers were offering services like free Facebook or free sport channel at a flat monthly rate or free email etc. This makes me wonder why we even need these plans with LTE. These plans are efficient when there is sever crunch of radio resources and subscriber doesn't  have fancy handsets, but with LTE the spectrum has widened and fancy handsets are becoming cheaper and cheaper. Will these plans still hold well?
For e.g a plan like free facebook will need the mobile gateway to look at all the packets  flowing from subscriber, zero rate it and then inform the same to the billing system. This churns a lot of bandwidth in term of cpu and memory cycles on the gateway. More over the software on the gateways needs to be highly efficient.
But there is other side to it. A subscriber may be offered limited data over a period of time but as token of appreciation allow a service free. For e.g I would be very happy if “Google Maps” is offered free along with my monthly of say 200 meg. I mean truly free here, that is actually look at the Google Maps traffic and zero rating it. But service provider may not want to do, so he may allow another 50 meg of additional quota and call it token of appreciation.  Duh!
In a nutshell, 3GPP subscriber management is amazing and I believe the broadband forum agrees. DPI, Policy and Charging are cool features on mobile gateways from engineering stand point. There are several other issues with DPI as people really don’t want service provider to look at their data. Anyway the service is available and I will leave it to the marketing to decide :-) 
I would like to hear from you. Would you like certain services to be free along with your subscription or would you like service to be free for a low monthly or weekly rate or any other plans you want to see in the market?