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ISO/IEC 26908:2009 — MAC-PHY Interface for ISO/IEC 26907
Information technology — Telecommunications and information exchange between systems
Purpose and Scope of the MAC-PHY Interface
ISO/IEC 26908:2009 defines the logical interface between the medium access control (MAC) sublayer and the physical layer (PHY) of high-rate ultra-wideband (UWB) systems conforming to ISO/IEC 26907. This interface specification is essential for ensuring interoperability between MAC and PHY implementations from different vendors, enabling a clean separation of concerns that facilitates modular system design and independent technology evolution.
A well-defined MAC-PHY interface allows silicon vendors to develop PHY and MAC independently, fostering a competitive ecosystem of interoperable components. This is analogous to the MII (Media Independent Interface) in Ethernet systems.
The interface encompasses two distinct service access points (SAPs): the PHY data service access point (PD-SAP) for user data transport and the PHY layer management entity service access point (PLME-SAP) for management and control functions. The standard specifies the primitives, data structures, and timing relationships for both SAPs, providing a complete functional boundary between the MAC and PHY layers.
| Interface | SAP Name | Primary Function | Key Primitives |
|---|---|---|---|
| Data Service | PD-SAP | Transmit/receive PHY protocol data units (PPDUs) | PD-DATA.request, PD-DATA.indication, PD-DATA.confirm |
| Management Service | PLME-SAP | PHY configuration, status reporting, measurement | PLME-GET/SET, PLME-RX/TX-START, PLME-CCA, PLME-ED |
The separation of data and management planes follows established networking design patterns, allowing the data path to be optimized for throughput and latency while the management path handles configuration, monitoring, and control functions with lower priority.
Interface Signals and Data Flow
The PD-SAP handles the transport of MAC frames encapsulated as PHY service data units (PSDUs). When the MAC sublayer requests a transmission, it passes a PSDU to the PHY via the PD-DATA.request primitive, along with transmission parameters including data rate, transmit power level, and time-frequency code. The PHY responds with PD-DATA.confirm to indicate the status of the transmission attempt (success, failure, or deferred). For reception, the PHY delivers received frames to the MAC via PD-DATA.indication, accompanied by receive quality metrics such as received signal strength indicator (RSSI) and link quality indicator (LQI).
The PLME-SAP provides a comprehensive set of management primitives. PLME-GET and PLME-SET allow the MAC to read and write PHY management information base (MIB) attributes, enabling configuration of parameters such as operating channel, data rate, and power-saving modes. PLME-RX-START and PLME-TX-START control the activation and deactivation of the PHY receiver and transmitter, respectively. PLME-CCA initiates clear channel assessment to support the MAC’s channel access decisions, and PLME-ED performs energy detection measurements for use in channel selection algorithms.
| PLME Primitive | Type | Purpose |
|---|---|---|
| PLME-GET | Management | Read PHY MIB attribute value |
| PLME-SET | Management | Write PHY MIB attribute value |
| PLME-RX-START | Control | Enable PHY receiver |
| PLME-RX-STOP | Control | Disable PHY receiver |
| PLME-TX-START | Control | Enable PHY transmitter |
| PLME-TX-STOP | Control | Disable PHY transmitter |
| PLME-CCA | Measurement | Perform clear channel assessment |
| PLME-ED | Measurement | Perform energy detection measurement |
| PLME-CCA-START | Measurement | Start continuous CCA monitoring |
The timing between a PLME-TX-START request and the actual start of transmission on the air interface is critical — the PHY must begin preamble transmission within 1 µs of receiving the request. Any additional latency directly reduces the available time for MAC-level frame aggregation and scheduling, potentially impacting overall system throughput.
Timing and Synchronization Considerations
Timing accuracy is one of the most critical aspects of the MAC-PHY interface for UWB systems. The PHY provides timing reference information to the MAC through the PLME-SAP, enabling the MAC to maintain superframe synchronization and execute DRP slot reservations with precision. The MAC relies on the PHY’s ability to detect and lock to the received beacon preamble within a defined time window, typically requiring the PHY to achieve frame synchronization within the first 12 OFDM symbols of the preamble sequence.
The interface specifies turnaround timing requirements: the time required for the PHY to switch from receive to transmit mode (RX-to-TX turnaround) and from transmit to receive mode (TX-to-RX turnaround). These turnaround times directly affect MAC protocol efficiency, as shorter turnaround times reduce interframe spacing and improve channel utilization. For the 26907/26908 system, the specified turnaround time is on the order of 10 µs, which is aggressive for wideband RF systems and requires careful transceiver architecture design.
Q2: What are the typical latency contributions of the MAC-PHY interface in a 26908-compliant design?
Interface latency typically comprises: (1) primitive processing delay at the MAC side (0.5–2 µs), (2) bus transaction time dependent on clock rate and bus width (0.5–1 µs for a parallel interface), (3) PHY-side processing including CRC computation and preamble insertion (2–5 µs), and (4) RF front-end settling time (1–3 µs). Total one-way latency is typically 5–10 µs, well within the superframe timing budget when properly designed.
Interface latency typically comprises: (1) primitive processing delay at the MAC side (0.5–2 µs), (2) bus transaction time dependent on clock rate and bus width (0.5–1 µs for a parallel interface), (3) PHY-side processing including CRC computation and preamble insertion (2–5 µs), and (4) RF front-end settling time (1–3 µs). Total one-way latency is typically 5–10 µs, well within the superframe timing budget when properly designed.
Q3: How does the 26908 interface handle PHY errors such as CRC failure or loss of carrier?
The interface defines specific error indication mechanisms through the PD-DATA.indication primitive, which includes a reception status parameter indicating success, CRC error, format error, or other PHY-level failures. The MAC is responsible for interpreting these indications and taking appropriate action, such as requesting retransmission through higher-layer protocols. Additionally, the PLME-SAP provides primitives for the PHY to asynchronously notify the MAC of status changes, including loss of synchronization or channel quality degradation.
The interface defines specific error indication mechanisms through the PD-DATA.indication primitive, which includes a reception status parameter indicating success, CRC error, format error, or other PHY-level failures. The MAC is responsible for interpreting these indications and taking appropriate action, such as requesting retransmission through higher-layer protocols. Additionally, the PLME-SAP provides primitives for the PHY to asynchronously notify the MAC of status changes, including loss of synchronization or channel quality degradation.
