+-------------+| Header_Size || (1 Octet) |+-------------+--------------+-----------+------------+-----------+| Msg_ID | Freg_ID | Start_Pos | End_pos | Flag || (2 Octets) | (2 Octets) | (1 Octet) | (1 Octet) | (1 Octet) |+-------------+--------------+-----------+------------+-----------+. .. .. .+------------+| Direction || (1 Octet) |+------------+-----------+-----------+-----------------+-------------+-----------+-----------+| Flag_Mux | Address | Control | Length | Information | FCS | Flag_Mux || (1 Octet) | (1 Octet) | (1 Octet) | (1 or 2 Octets) | (n Octets) | (1 Octet) | (1 Octet) |+------------+-----------+-----------+-----------------+-------------+-----------+-----------+. .. .. .
Parts of the packets (the Flag_Mux octets and the octets between them)of this link type frames are based on the 3GPP standardTS 27.010,but there are slight deviations to match the actual implementation ofCinterion and Siemens modules, e.g. noI frame support, but anadditionalUIH_E frame. In addition, the original MUX_Frame (theFlag_Mux octets and the octets between them) is extended by some extrafields for PPP chunks and direction indication (the Msg_ID through Flagoctets).
If there are PPP chunks surrounded by Flag_Mux octets, theywill be indicated by the {Msg_ID, Freg_ID, Start_Pos, End_Pos, Flag}quintuplets; there will be one quintuplet for every chunk. Header_Sizeand Direction are always present. The {Msg_ID, Freg_ID, Start_Pos,End_Pos, Flag} quintuplets are optional and not always present.The Header_Size field indicates whether the quintuplets are present ornot and how many of them are present—if a frame containsNPPP chunks, the Header_Size field has the value 7N; if noquintuplets are present,N, and thus the Header_Size field,is zero. If the Header_Size value is not a multiple of 7, the frame isinvalid.
Start_Pos is the 1-origin index (from the octet after the Directionoctet, so the index of the first Flag_Mux octet would be 1) of the firstoctet of the chunk, and End_Pos is the 1-origin index (from the octetafter the Direction octet) of the last octet of the chunk.
All the chunks of a given PPP packet have the same Msg_ID value. Freq_ID is a sequence number for the PPP chunks. The first chunkhas a Freq_ID of 0 and the Freq_ID of the next chunk will beincremented. The last chunk of a given PPP packet has a Flag valueof 1; the others have a Flag value of 0.
The Direction field indicates the direction of the Mux frame: "0"means from GSM Modem to the Host; "1" means from Host to GSM Modem.
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