(ROUTINE)  is  the  first  element.  The  prosign  Y (YANKEE)  is  an  emergency  command  precedence (ECP) and is assigned to emergency action messages (EAMs). The prosign Y indicates that a message has FLASH preemption capability. EAMs are processed ahead of all other traffic and interrupt lower precedence traffic already in processing within the AUTODIN system. Positions   2   and   3   (Language   and   Media Format)  —The  language  and  media  format  (LMF) consists  of  two  alphabetical  characters.  The  LMF  is  the mode used to insert a message into the AUTODIN system.  The  LMF  of  the  originating  station  is  placed  in position 2, and the LMF of the preferred output device of the addressee is placed in position 3. For example, in figure 1-2, positions 2 and 3 have the character T. The character Tin position 2 indicates that the originator’s transmitting  mode  is  paper  tape  (TTY/teleprinter)  (five- level  ITA2  code).  The  character  T  in  position  3 indicates that the output device at the receiving end will be paper tape (TTY) (five-level ITA2 code).  If the character C was used in position 3, this would indicate that the message was prepared and transmitted on paper tape and the output device at the receiving message center would be magnetic tape. Automated Digital Network  (AUTODIN)  Operating  Procedures,  JANAP 128, lists the LMFs used in the AUTODIN system. Position   4   (Classification)   —The   letters authorized  to  indicate  the  message  classification  or special handling in this position are: Positions 5 through 8 (Content Indicator Code [CIC]/Communication    Action    Identifier [CAI])  —These   positions   of   the   header   are   a combination of either four letters or three letters and one number. These combinations are used to indicate message content and to provide identification for communications handling. For example, in figure 1-2, the CAI in positions 5 through 8 is ZYUW. This identifies the message as a narrative message. A CAI of ZFH2  would  mean  that  the  message  is  being  forwarded to the addressee for information only. A CAI of ZYVW would indicate that the message is a service message. A complete listing of these codes is found in JANAP 128. Position 9 (Separator)  —At this point in the header, the operator must press the space bar to insert the TTYcode equivalent for space on the message tape. Positions 10 through 16 (Originator)  —The appropriate routing indicator of the originating station is placed in these positions. Positions   17   through   20   (Station   Serial Number) —The station serial number (SSN) of the sending station is inserted here. The SSN serves two specific purposes. First, when used in combination with the originator’s routing indicator, it provides positive identification for each transmission. Second, in the end of message (EOM) validation (discussed later in this section), the SSN appearing in format line 15 provides a means by which the ASCs can check for the existence of straggler messages. The SSN is expressed in four numeric characters, beginning  with  0001  and  continuing  consecutively through 9999.  A new series begins when the number 9999 is reached.  Operating stations may use SSNs to identify local activities, channels, or positions within a station by assigning each activity a specific block of numbers.  For example, one station may be assigned numbers 0001 to 2000; the next station 2001 to 4000, and so on. Position 21 (Separator)  —This position requires the same information as that for position 9. Positions 22 through 24 (Julian Date) —The Julian date is the date that the message was received from   the   originator   for   transmission   by   the communications center. The first day of the calendar year  is  Julian  001,  and  each  day  is  numbered consecutively   thereafter. Positions 25 through 28 (Time Filed)  —The time filed is the time that the message was received from the originator   by   the   communications   center   for transmission.   Each   filing   time   is   expressed   in Greenwich mean time (GMT) and must contain four numerical characters. Positions   29   through   33   (Classification Redundancy)  —For  security  reasons,  the  classification designator used in position 4 is repeated here. Position 29   is   filled   with   a   hyphen   as   a   sentinel.   The classification  designator  in  position  4  is  repeated  in positions 30 through 33. 1-8