Table PID and Program Number. The PAT should end with a 32-bit CRC PMT (Program map specific data) PMTs contain information about programs. For each program, there is one PMT. While the MPEG-2 standard permits more than one PMT section to be transmitted on a single PID (Single Transport stream PID contains PMT information of more than one program), most MPEG-2 "users" such as ATSC and SCTE require each PMT to be transmitted on a separate PID that is not used for any other packets. The PMTs provide information on each program present in the transport stream, including the program_number, and list the elementary streams that comprise the described MPEG-2 program. There are also locations for optional descriptors that describe the entire MPEG-2 program, as well as an optional descriptor for each elementary stream. Each elementary stream is labeled with a stream_type value. This table contains PID numbers of elementary streams associated with the program and it has information about the type of these elementary streams (video, audio, etc.). In addition it may also contain an ECM (entitlement control messages) stream for any other stream that is encrypted. These messages provide the information used in the cipher key selection stage. CAT (Conditional access specific data) Table ID value is 0x01. This table is used for conditional access management of the cypher keys used for decryption of restricted streams. This table contains privately defined descriptors of the system used and the associated EMM PID. It is used by a network provider to maintain regular key updates. NIT (Network information specific data) This optional table may group transport stream identifiers into a network, providing access parameters and other details. ITU-T Rec. H.222 and ISO/IEC 13818-1 do not define the NIT structure. The European Broadcasting Union DVB specification ETSI EN 300 468 (DVB-SI) does. The purpose of mentioning but not defining this table in H.222 is that it has reserved program number 0 (zero) in the PAT. The Table ID extension is used to identify the local network together with a directory listing of transport streams. Descriptors are used to list the modulation, source of those streams and programs. The original network identifier is meant to allow transport streams and programs on foreign networks to be included in the local network which allows no remapping of transport and program IDs that may be duplicated between networks. The DVB specification defines the transport packet