Certain implementations suitable for Digital Video Broadcasting (DVB) broadcasting systems are supported by CATV infrastructures. Specifically, implementations of the Return Channel for interactive services are supported by CATV.
DVB involves a standard link.

Mpeg Header
4 byte Mpeg-2 transport stream header as defined in ISO 13818-1 with a specific PID designated for MAC messages. The value of this PID is 0 x 1C. The transport scrambling control field of the MPEG header is set to 00.

Upstream Marker
24 bit field, 3 byte marker that provides upstream QPSK synchronization information. At least one packet with synchronization information must be sent in every period of 3 msec. The definition of the field is as follows:

Bit 0: Upstream Marker Enable - When this field has the value ‘1,’ the slot marker pointer is valid. When this field has the value ‘0,’ the slot marker pointer is not valid.
Bits 1 - 3: MAC Message Framing - Bit 1 relates to the first MAC message slot within the MPEG frame, bit 2 to the second MAC message within the MPEG frame, and bit 3 to the last MAC message within the MPEG frame. Possible values:
0 - A MAC message terminates in this slot.
1 - A MAC message continues from this slot into the next, or the slot is unused. If the slot is unused, the first two bytes of the slot are
0 x 0000.
Bits 4 - 7: Reserved
Bits 8 - 23: Upstream Slot Marker Pointer - A 16 bit unsigned integer which indicates the number of downstream “symbol” clocks between the next Sync byte and the next 3 msec time marker. Bit 23 is considered the most significant bit of this field.

Slot Number4
A 16 bit field which is defined as follows:

Bit 0: Slot Position Register Enable (msb) - When this field has the value ‘1,’ the slot position register is valid. When this field has the value ‘0,’ the slot position register is not valid.
Bits 1-3: Reserved
Bit 4: Set to the value ‘1.’ This bit is equivalent to M12 in the case of OOB downstream.
Bit 5: Odd Parity - This bit provides odd parity for upstream slot position register. It is equivalent to M11 in the case of OOB downstream.
Bits 6 - 15: Upstream Slot Position Register - 10 bit counter which counts from 0 to n with bit 6 the msb. These bits are equivalent to M1 - M10 in the case of OOB downstream.

MAC Flag Control
24 bit field (b0 (msb), b1, b2 . . . b23) that provides control information used in conjunction with the ‘MAC Flags’ and ‘Extension Flags’ fields. The definition of the MAC Flag Control field is as follows:

* b0 - b2 - Channel 0 control field.
* b3 - b5 - Channel 1 control field.
* b6 - b8 - Channel 2 control field.
* b9 - b11 - Channel 3 control field.
* b12 - b14 - Channel 4 control field.
* b15 - b17 - Channel 5 control field.
* b18 - b20 - Channel 6 control field.
* b21 - b23 - Channel 7 control field.

MAC Flags
26 byte field containing 8 slot configuration fields (24 bits each) which contain slot configuration information for the related upstream channels followed by two reserved bytes. The first 3 bytes correspond to MAC Flag Set 1, the second 3 bytes to MAC Flag Set 2, etc.

Ext. Flags
A 26 byte field used when one or more 3.088 Mbit/s or 6.176 Mbit/s upstream QPSK links are used. The definition of the Extension Flags field is identical to the definition of the MAC Flags field (above). The Extension Flags field contains the MAC Flags from 9 to 16.

MAC Message
The MAC Message field contains a 40 byte message in hexadecimal code.