allow the resulting checksum bits to be filled up in a way that only valid symbols of the chosen channel code are in the telegram where each symbol has 11 bits. The payload data consists of a header followed by multiple packets defined in the ERTMS protocols. Typical packets are: Packet 5 - Linking Packet 12 - Movement Authority Packet 21 - Gradient Profile Packet 27 - International Static Speed Profile Packet 255 - End of information Many applications include optional packets like Packet 3 - National Values, Packet 41 - Level Transition Order, and Packet 136 - Infill Location Reference. If the telegram maximum of 830 bits is reached then more packets can be sent in the following balises of the same balise group - with up to 8 balises in a balise group the maximum ERTMS message per balise group can encompass 8 * 830 = 6640 bits (note that every telegram must contain a header and the trailer packet 255). A fixed balise transmits a stable message which typically can include the linking information, gradient profile, and speed profile. It may also contain track information such as route suitability data for different train types and axle load restrictions. Almost all packet types contain a parameter flagging whether its information is relevant for the "nominal" or "reverse" direction (or both). If a train sees balise 1 before balise 2 then it passes over the group in the nominal direction. Consequently, some packets may be dropped by the application software of the receiver if they are not designated for the relevant direction. The ERTMS header block of 50 bits contains the ETCS version, the current number and total count of balises within a balise group (up to 8 balises), a flag whether it is a copy (up to 4 copies) that increases chances for the receiver to see the telegram of the balise in a group, a serial number flagging whether the message has changed lately, a 10-bit country identifier along with the 14-bit balise group identifier allowing for a unique ID of every balise group. The linking information informs about the distance to the next balise group (one linking packet per direction) and the required train reaction if the next balise group is missed (e.g. train stop). The movement authority packet defines a maximum speed that may be used for a given maximum distance and maximum time -