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NOAAPORT Data Format

NOAAPORT Broadcast System (NBS) uses the concept of encapsulation and the International Standards Organization (ISO) Open Systems Interconnection (OSI) reference model to define its data transmission format. The OSI reference model uses a seven-layer model to abstract the necessary functionality for communications between computer systems. The seven-layer model as shown contains the Physical Layer, Data-Link Layer, Network Layer, Transport Layer, Session Layer, Presentation Layer, and Application Layer.

The OSI reference model does not dictate the use of all layers for every application. Rather, it encourages application developers to use the functionality that is appropriate for their specific application. The NBS uses only four layers: the physical layer, the data link layer, transport layer and presentation layer.

OSI Reference Model vs. NOAAPORT


The diagram illustrates the NBS architecture using the classic OSI layers. This architecture uses a simplex broadcast scheme in which the communications between layers is in one direction only, broadcast to receiver. As a result, there is no flow control. The NBS uses high-level data-link control (HDLC) framing for the Data-Link Layer and the PRC-developed SBN protocol for the Transport Layer. The Presentation Layer incorporates the World Meteorological Association (WMO)/NESDIS headers for NESDIS products and the Communications Control Block (CCB)/WMO headers for NWSTG products.

As the individual products are packaged for transmission, they are enveloped by the headers of the Presentation, Transportation, and Link protocol layers used by the NBS. As shown the products are first wrapped in their corresponding Presentation Layer headers according to product type. The NESDIS products use the WMO and NESDIS headers, and the NWSTG products use the CCB/WMO headers. The first frame of a NESDIS product contains only header information, while the succeeding frames contain only the image data. Each frame of information contains one or more scan lines of image data, which are approximately 5,120 bytes. Summing the header data and the image data results in a NESDIS frame that can contain up to 5,152 bytes. The first frame of an NWSTG product contains the CCB/WMO header and product data. Succeeding frames contain only the product data, with each NWSTG frame containing up to 4,000 bytes of product data.

The next layer is the Transport Layer, which places the SBN protocol headers around the product/image data and Presentation Layer headers. The first frame of each distinct product of this layer contains the frame-level header (FH), a product-definition header (PDH), an AWIPS product-specific header (PSH), and the appropriate source data. Subsequent frames of the same product contain the FH, the PDH, and the appropriate source data.

The Transport Layer also contains several layers of sequence numbers. The first level sequences the frames, allowing the protocol to recognize lost frames. The next level sequences the products, which allows the protocol to recognize lost or damaged products. The final level of sequencing is used in reassembling the product. The sequence numbers do not provide flow control. Their purpose is strictly error control.

The final layer before the data are converted to a radio frequency signal and uplinked to the satellite is the Data-Link Layer. In this layer, the "data" created by the Transport and Presentation Layers are enveloped by an HDLC frame and stamped for error control with a frame check sequence Cyclical Redundancy Check (CRC)-16.

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