看到一个关于LTE Bearer的好帖子，转过来。大概说两个问题：
1. Default bearer 是每个UE一个还是每个PDN一个？答案是每个PDN一个。所以一个UE如果连到多个PDN，就要给每个PDN建一个default bearer。然后每个default bearer上可以建多个dedicated bearer。当然了标准限定最多只能建11个。
2. P-GW可以给一个UE分配多少个TFT。极端的情况是一个UE只建了一个default bearer，剩下的10个都是dedicated bearer。这样的话P-GW需要给这个UE分配10个TFT，因为每个dedicated bearer需要不同的TFT。

There were a couple of good questions on the LTE University forum page recently. I thought it would be good to address those questions. The first question was: are default bearers created on a per UE or a per PDN (Packet Data Network) basis? Default bearers are created on a per PDN basis. So if a UE is connecting to two PDNs it will need to establish two default bearers. The best example that I can come up with for why a subscriber may want to connect to multiple PDNs is if subscriber wants to connect to an IP Multimedia Subsystem (IMS) network and the Internet. The first default bearer will be established during the Attach process when the UE first powers up and the second will be done using Activate default EPS bearer context request procedure. When the second default bearer is established will be dependent on the service and the UE.

The second question is, exactly how many TFTs can be used per UE in a P-GW? There are a number of ways to take this question. One way to interpret this question is to ask how many active bearers a UE can have? Another way to interpret this question is to ask what the the number of TFTs that a P-GW can support per UE? Let's look at both interpretations of the question. From the LTE specs, the maximum number of bearers that a UE can establish is 11 (as much as I would love to put a Spinal Tap reference in at this point, I will restrain myself). The maximum of 11 is based on a parameter called the EPS bearer identity size. It is four bits in length with five values reserved. The number of TFTs that will be required for a UE is based on the number of default and dedicated bearers. There will need to be a TFT for every dedicated bearer. That could be a maximum number of TFTs of 10 (assuming one default bearer and 10 dedicated bearers). That seems an unlikely scenario. More than likely there will be a few default bearers and each of those default bearers will also have a dedicated bearer or two, depending on the scenario.

I hope these help to understand how the EPS bearers are established. If you have a question, try the LTE University forum.