PDP context during Attach procedure

Question

Why in LTE we establish a PDP context during Attach procedure but in UMTS it is not. Any pointers, it will also be good if someone can describe the Attach procedure in detail (though there are so many blogs but don't know which one is to follow)

Answer 1

LTE provides "The always-on IP connectivity" feature to the UEs. This means the UE needs to have an ip address to connected to the IP data network.So the attach procedure(which is initiated when a UE is switched ON) includes the PDN connectivity Req. As a part of this procedure there will be a default bearer established, which provides connectivity to the IP network through PGW. UMTS can have CS and PS connections.Only when a PS connection is established ,PDN connectivity Request is initiated to the network.

You can refer 3GPP specification 23.401 section 5.3.2 for a detail description of attach procedure of LTE