1. A customizable mobile broadband network system, comprising at least one processor and non-transitory, computer readable memory storing instructions for execution by the processor so as to configure the processor to provide a forwarding layer, a control layer, and a capability opening layer, wherein (a) the forwarding layer connects to the control layer through a first open interface, and provides a forwarding capability of the mobile broadband network system, wherein the first open interface opens the forwarding capability to the control layer; (b) the control layer connects to the capability opening layer through a second open interface, and provides a control capability of the mobile broadband network system, wherein the second open interface opens the control capability and the forwarding capability to the capability opening layer; and (c) the capability opening layer receives, through a third open interface, a network customization request from a user, and combine at least one of the control capability and the forwarding capability according to the network customization request, so as to manage a customized-network instance; and the processor is further configured to provide an on-demand mobile network enabler of the capability opening layer that (a) selects a required atomic-level control capability and forwarding capability according to the network customization request, (b) sends a resource allocation request to a capability abstraction sublayer of the control layer, (c) receives a result of network resource allocation that is performed by the capability abstraction sublayer according to the resource allocation request, and (d) combines the selected atomic-level control capability and forwarding capability based on the result of the network resource allocation, so as to create, modify, or delete the customized-network instance; wherein the capability abstraction sublayer is configured to abstract the forwarding capability of the forwarding layer and the control capability of the control layer to obtain an atomic-level forwarding capability and control capability, and open the atomic-level control capability and forwarding capability to the capability opening layer through the second open interface.