The cellular air interface is arguably the defining networking interface for modern mobile devices. Initial cellular systems, such as second generation (2G) Global System for Mobile Communications (GSM) and third generation (3G) Universal Mobile Telecommunications System, were modeled after the traditional wireline circuit-switched telephone system. Each call was provided with a dedicated circuit providing a user making a telephone call with a baseline guarantee of service. In contrast, newer fourth generation (4G) Long Term Evolution (LTE) networks were designed to utilize a packet-switched model for both data and voice. An LTE network provides consistent IP connectivity between an end user's mobile device and IP-based services on the packet data network (PDN).
There are many cellular network types, each with its own air interface standards. The cellular air interface is the technical term for the radio connection between a mobile device and the cellular tower. This air interface can generally communicate with many types of base stations (e.g., cellular towers) which come in many sizes and types — cellular repeater/relay nodes, and even other handsets.