The air interface used in PCS is structured in different layers, each with specific purposes. This conceptual split makes it easier to understand the interactions between the base station and the phone across the air interface. There are four layers:

• physical layer (Layer-1)—deals with the radio interface, bursts, slots, frames, and superframes
• data-link layer (Layer-2)—handles the data packaging, error correction, and message transport
• message layer (Layer-3)—reates and handles messages sent and received across the air
• upper application layers—represent the teleservice currently being used, such as voice and messaging transactions, or future services like on-air programming

The Air-Interface Model

Figure 3 shows the air-interface model. This structure simplifies the introduction of current and future services using the IS–136 DCCH platform because the lower layers in the air-interface protocol (the radio interface, data management, messages, and so on) remain unchanged.

Figure 3. The Air-Interface Model

Operating Principle

Figure 4 shows how one Layer-3 message is mapped into several Layer-2 frames and how a Layer-2 time frame is mapped onto a time slot. The time slot is further mapped onto a DCCH channel. The figure shows how information is passed from layer to layer down through the stack until a burst is created, ready for transmission. At the receiving end, information is stripped off as needed as the message is passed up to the application.

The Layer-3 message shown in Figure 4 can be an uplink registration, a downlink PCS message, a page response, or a broadcast message. The Layer-3 message is packaged into a Layer-2 frame where header and error-correction fields are added. The packet is then coded and the individual bits interleaved (mixed and distributed) to counteract errors introduced in the radio environment.

Figure 4. Layered 3-2-1 Mapping