English to Chinese Tech/Engineering Computers: Systems, Networks

In a VoWLAN system, the phones connect to the network via wireless access points. Some access points need vendor-specific software upgrades from the VoWLAN phone maker to enable VoWLAN operation. In addition, some products make use of voice servers that improve voice quality and management of the network.
At a higher level, the phone communicates directly with an IP PBX (private branch exchange), which understands IP (Internet Protocol) and connects via analog and digital circuit-switched connections to the public switched telephone network (PSTN). The IP PBX manages calls within the facility and interfaces with the PSTN for external calls, which travel across traditional telephone networks. A VoIP gateway can interface VoWLAN phones to a legacy PBX in cases where a company wants to make use of existing, older equipment.
在较高层面？什么意思？