APN 和 PDN 关系

PDN is packet data network which will give data transmission services to the mobile user through Access point name which is configured in mobile device .. In short APN identifies the PDN. APN is used in 3GPP data access networks, e.g. General Packet Radio Service (GPRS), evolved packet core (EPC).

What is PDN?

PDN is a network established for the specific purpose of providing data transmission services.
What is APN?
APN is nothing but link between two networks(i.e simple case, LTE and external world). 
Difference between them.
APN is one who identifies a PDN-GW. How?
APN structure is comprised into two parts: 1) Network identifier and 2) operator identifier.
- Network identifier which defines the PDN to which the UE requests connectivity, and Operator identifier which defines in which Public Land Mobile Network (PLMN) the PDN-GW is located.