Alitronika DVS AT88IP, Руководство пользователя

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Appedix1
DVB-IPI (ETSI TS 102 034) specifies protocols at the IP networking layer (IP Infrastructure) that must
be supported on key system interfaces to deliver DVB services over IP networks.
Below are presented the main points on MPEG2 TS over IP Encapsulation set by DVB-IPI.
All MPEG-2 transport streams shall be encapsulated in RTP (Real-time Transport Protocol) according to
RFC 1889 in conjunction with RFC 2250. Transport service is provided jointly by UDP (checksum and
multiplexing) and RTP (sequencing and time stamping / jitter removing).
RTP always uses an even UDP port number.
- V - Version, 2 bits. This field identifies the version of RTP.
- P - Padding, 1 bit. If the padding bit is set, the packet contains one or more additional padding bytes.
- X - Extension, 1 bit. If set, the fixed header is followed by exactly one header extension.
- CC - CSRC count. The CSRC count contains the number of CSRC identifiers that follow the fixed header.
- M - Marker, 1 bit. The interpretation of the marker is defined by a profile.
- PT - Payload type, 7 bits. Identifies the format of the RTP payload and determines its interpretation by the application.
- Sequence number Sequence number, 16 bits. The sequence number increments by one for each RTP data packet
sent and may be used by the receiver to detect packet loss and to restore packet sequence.
- Timestamp Timestamp, 32 bits. The timestamps reflects the sampling instant of the first byte in the RTP data packet.
- SSRC Synchronization source, 32 bits. Identifies the synchronization source.
- CSRC Contributing source, 0 to 15 items, 32 bits each. An array of 0 to 15
- CSRC elements identifying the contributing sources for the payload contained in this packet.
For most streams, the RTP/UDP/IP overhead of 40 bytes per RTP packet is relatively low (for example 3% with a 1 316
byte payload).
IP packets can carry from 1 to 7 TS packets, knowing that:
– overall size of RTP payload must not exceed the MTU (Maximum Transfer Unit) in order to prevent RTP packets
fragmentation around the network,
– short packets cause a high overhead.
There is no requirement for every RTP packet in a stream to contain the same number of transport
stream packets. The receiver should use the length field in the UDP header to determine the number of
transport stream packets contained in each RTP packet. The time stamp field in RTP header is based on the PCR
values from MPEG-2 with a resolution of 90 KHz.
Streams must include PAT (Program Association Table) & PMT (Program Map Table) – other tables are optional. SI
(Service Information) is intended to be delivered via separate IP streams, in e.g. XML format. RTCP (Real-time
Transport Control Protocol) can be included to periodically inform the sending side about network quality (e.g. lost
packets, delay, jitter, etc.).
The fact is that, at the output of the IP network, delivered Transport Stream must be fully ISO/IEC 13818-1 compliant
(40ms maximum jitter, 1 artifact every hour...).