In the last post we understood TFT and QoS behavior in LTE. What's important to understand is bearers are all about achieving QoS. The whole concept of bearers is just to provide proper quality of service to user thus enhancing the quality of experience. So what we know at this point is a default bearer always exists once the UE is switched ON, until it is powered off. Any other bearer is a dedicated bearer which is associated with a TFT and QoS.
In this post lets try to understand few more terms like TAD and QCI (QoS Class Identifier). The concentration is on UE requested bearer creation/modification procedure.
Basically a UE may request for dedicated bearer. Lets say we are running a web browser on our mobile phone. Now UE wants to establish a dedicated bearer for that. So UE sends out a request for bearer creation procedure. MME here sends sends out a bearer resource command which includes LBI (Linked Bearer ID), PTI (Procedure Transaction ID), TAD (Traffic Aggregate Descriptor), QoS.
TAD is a partial TFT. It includes packet filters for a particular L7 activity. In our case as we are running HTTP, TAD will include packet filter for HTTP. Now MME will send this information to PGW. Now PGW understands that a dedicated bearer has to be created for running HTTP traffic. So it sends out a create bearer request with TFT (HTTP) and QoS. Here UE may request for particular QoS.
Lets look at QCI here. QCI field may be mapped to a diff serv. (Assured Forwarding?) . Say QCI value is set to 2 which indicates a particular class of assured forwarding, say AF12. We have MBR and GBR values in QoS which may be set to some value.
Once MME requests create bearer request it responds with a bearer response creating a dedicated bearer for HTTP activity with a QoS value (QCI: AF12). After this our mobiles should start displaying the web pages with a speed equal to Roger Federer's ace. :)
Now, you remember that your boss has asked something and you want to email him. So another dedicated bearer for this email? Ok... lets stop and understand this a bit clearly. If your http dedicated bearer QoS is fine for running email then we may just use it. But you need to indicate this to PGW. So we do a bearer modification (update?) procedure instead of bearer creation. MME here will send bearer resource command with TAD mapped to email traffic, say IMAP(?). But note here the QoS is same. (QCI still is AF12). PGW looks at this and thinks that may be I should update the dedicated bearer to include IMAP too. So it sends out Update bearer request with newly added TFT for IMAP. (We already have TFT for HTTP). MME responds to saying OK.
At this point we have two activities running on single dedicated bearer. That means when there is no QoS change a single bearer may be used to run various traffics. This brings us to square one where I said bearer is all about QoS. Once email is done, PGW may remove that TFT (IMAP) from dedicated bearer.
This was just a short post to look at things when no QoS is changed. I will try to analyze the whole QoS with various IE's in coming days.
I hope it was useful and as usual comments are greatly welcomed.
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