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Transmission Protocol

EMWIN is designed to be an open system. The format of the EMWIN datastream transmissions is public domain and presented here. This format is intentionally simple, for reception by a wide range of user hardware. This format, called the Quick Block Transfer (QBT) Protocol, is used across all of the EMWIN dissemination methods, whether Radio, Satellite, Internet or direct cable.

The EMWIN datastream consists of NWS weather products and other data files. Each product or file, whether ASCII text or binary data, is divided into 1 KB packets and sent as a series of async 8-bit bytes, N parity. For example, most current EMWIN Radio broadcasts are receivable, when demodulated, as async 1200,8,N,1 while the Satellite broadcasts are async 9600,8,N,1.

Each product or file is sent as one or more packets, which are numbered 1..N within the given product. Because the data is packetized, a particular product can be gracefully interrupted by a high priority warning or alert product and then resume. Note: the broadcast is receive-only -- hence the receiver has no means of notifying the transmitter of any block errors or of requesting retransmission of individual blocks. Instead each product is usually transmitted at least twice, to "fill in" any blocks received in error.

Each packet of data contains 1116 bytes, in the following fields:

  1. 6 bytes of ASCII 0 (null) to clear the receiver buffer.
  2. "/PF" followed by an 8-character filename, a period, and a 3- character filetype.
  3. "/PN" followed by the block number -- the number of this block (1..N) within this file.
  4. "/PT" followed by the total number of blocks (N) being sent for this file.
  5. "/CS" followed by a checksum number -- the sum of all bytes in the 1024-byte data portion of this packet, as a 16-bit unsigned decimal.
  6. "/FD" followed by the date/time stamp of this file -- in the format of: MM/DD/YY HH:MM:SS AM, without space padding.
  7. ASCII 32 (SP) fill -- to pad the total bytes in fields 2..8 to a full line of 80 bytes.
  8. ASCII 13 (CR) and ASCII 10 (LF) to enhance readability.
  9. the data, as a 1024-byte block; if the remaining data of the product is less than 1024 bytes, this block is null-filled so that each packet's data block is always 1024 bytes long.
  10. 6 bytes of ASCII 0 (nullNUL) to clear the receiver buffer.

An example of a typical packet header is:

     /PFZFPSFOCA.TXT/PN 3    /PT 5    /CS 63366 /FD2/10/95 5:24:26 PM

The content of the NWS weather products (in the 1024-byte blocks) may be plain ASCII text or graphics or imagery. The products are not encrypted, but will often be compressed. Interpretation of the content of the products is up to the receiver's software. Details of the particular text, graphics, imagery, and compression formats are given below.

Text :

Text products are transmitted in English and are usually public forecasts. However, some products may contain a variety of abbreviations or weather-specific acronyms, or may simply be "readable" tables of computer-summarized data. The content is generally 7-bit printable ASCII text, but often also contains hex bytes C5, 80, 03, or 83.

The first line of text of a product is the "WMO" heading, which includes a 4-6 character product identifier, a 4 character source site code, and a 6 digit origination date/time (UTC). The next line may contain a AWIPS identifier, of 4-6 characters portion. In some products the next line (or embedded lines) may be a "UGC" (Universal Generic Code) line, giving specific states/zones/counties to which this product applies, plus a product-purge date/time (UTC).

Graphics :

Graphic products are transmitted in NWS Universal Transmission Format (UTF), a display-independent format. The UTF format includes vectors, characters, and gridded data, but not bitmaps or rasters, and was originally designed to be closely compatible with our "legacy hardware" displays. The EMWIN UTF products are typically national or regional coarse radar images, that can be zoomed by the display software.

This format is described in AFOS Handbook 5, Volume 6, Part 2, Appendix B (29 pages), which you can download as a zipped file. This is in WP 5.1 format (90KB) as a text document plus 11 separate GIF diagrams. You can also download a small Postscript form of the Handbook, plus source code for displaying products on Unix, from Maitland Bottoms.

If writing your own code, be aware of the "byte stuffing" described on page B-27. The first line of a graphic product is text, with the NWS product identifier. This is followed immediately by UTF code bytes, C1 hex and 01 hex, signalling further graphics (in UTF format) or arbitrary binary contents.

Imagery :

Satellite weather images (products from GOES) and other interesting pictures are transmitted in standard formats as indicated by the filetype/extension. Currently, .GIF and .JPG are used.

Compression :

Local data and watches/warnings/advisories are transmitted as clear text and not compressed. Other national data products, such as all surface observations (METARs) for a given hour, are first packed into one "file" and then compressed and transmitted. The EMWIN "UNPACKER" software task will decompress these files upon receipt, and then unpack the constituent data products as if received normally.

The compression/decompression software used is the standard PKUNZIP Data Compression format.


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