VLBI Standard Interface – Electronic (VSI-E) Protocol Fundamentals

1. VLBI Standard Interface – Electronic (VSI-E) Protocol Fundamentals Chet Ruszczyk MIT Haystack Observatory

2. Agenda • VLBI Standard Interface (VSI) – Why? • VSI’s Model • VSI-E’s Primary Objective • VSI-E’s Goals • RTP Summary • RTP Extensions for e-VLBI • Open Source Linux Libraries • SC05: Kashima – Haystack using VSI-E • Documentation • Conclusion

3. VLBI Standard Interface (VSI) • VSI defines • A standard interface to and from a VLBI Data Transmission System (DTS) • Allows heterogeneous DTS’s to be interfaced to both data-acquisition and correlator systems with a minimum of effort. • VSI is defined to be compatible with: • tradition recording/playback systems, • network data transmission, and • direct-connect systems.

4. VSI (Cont) • VSI is designed to: • Hides the detailed characteristics of the DTS • Allows the data to be transferred from acquisition to correlator • in transparent manner • Relieve existing incompatibilities • between various VLBI data systems.

5. VSI (cont) • Three VSI specifications developed • VSI-Hardware • VSI-Software • VSI-Electronic • VSI-H defines the electrical and interfaces • To / from a DTS • Also specifies a control philosophy.

6. VSI (cont) • VSI-S defines the software component of the VSI-H specification • Specifies communications protocol, • Control a VSI-H-compliant DTS. • VSI-H and VSI-S explicitly refrain from • specifying the format of data • from the Data Input Module (DIM) • to the Data Output Module (DOM).

7. VSI (cont) • VSI-E primary objective • A media independent data format • Transmitted “on the wire” • from source to destination • DIM to DOM • Is compatible between heterogeneous DTSs

8. VSI’s Model

9. VSI-E • Goals • Efficient transport mechanism • Standard protocols • Internet-friendly transport • Scalable Implementation • Ability to transport individual data-channel streams as individual packet streams • Ability to make use of multicasting to transport data and/or control information in an efficient manner • could be used in the future for support of distributed correlation

10. Network Topologies

11. VSI-E (cont) • The following assumptions were made in the development of the VSI-E specification: • The DTS is compliant with the VSI-H specification • All active bit streams, associated relevant parameters must be derivable from the information arriving at the DOM, in particular: • Primary data stream (i.e. active bit-stream data) • Active bit-stream mask • DOT time-tagging • Bit-stream information rate (BSIR) • Valid-data indicator • TVG-data indicator • PDATA messages • Underlying network structure is IP-based

12. VSI-E (cont) • Critical Definitions: • A channel is an exclusive subset of 2n of the active bit streams. The intent of the channel abstraction is that it carry the digitized data from a single analog data source. • A channel sample is 2n bits collected from a single ‘channel’ on a single DIM CLOCK cycle. The DIM collects channel samples at the Bit-Stream Information Rate (BSIR). • A channel stream is a contiguous set of channel samples collected over some period of time.

13. VSI-E Proposal • Real-time Transport Protocol (RTP) / RTP Control Protocol (RTCP) • Proposed as the basis for the VSI-E Standard • IETF Standards RFC3550, RFC3551, RFC3605

14. RTP Philosophy • Build a mechanism for robust, real-time media delivery above an unreliable and unpredictable transport layer • Without changing the transport layer

15. VSI-E Proposal (cont) • Why RTP/RTCP • RTP is the standard for real-time transport over IP • Transmission of sampled analog data • Dissemination of session information • Monitoring of network and end system performance (by participants and third parties) • Adaptation to varying network capability / performance • Appropriate reliability / repair model • Message Sequencing / un-reordering • Multi-cast distribution of statistics, control and data

16. RTP Summary • A wealth of implementation and operational experience • Seen as internet-friendly by the network community • RTP pays attention to: • efficiency • resource constraints, • scaling issues. • Framework for transporting real-time data • Transport layer independent • Timing and synchronization • Merging, bridging, and translation support • Application-specific control data • e.g. PDATA, time, data collection parameters, antenna pointing, system temperature

17. Protocol Components

18. RTP Extensions for e-VLBI • RTP Profile for e-VLBI • defines the structure and semantics of the RTP packets used to transport VLBI data. • Six packet types • RTP Data Packet • RTCP Sender Report Packet • RTCP Receiver Report Packet • RTCP Source DEScription Packet • RTCP BYE Packet • Application Defined RTCP Packet

19. RTP Data Packet • Used to encapsulate and transport e-VLBI data. • Payload type (PT) • # bits per channel sample • Sequence number • RTP timestamp • Source identifier • Data Payload • data samples

20. Channel-stream encapsulated into an integer number of 32-bit words in format. DIM input => 32 individual bit streams A subset of 2n is chosen to be ‘active bit streams’. The ‘active bit streams’ are further subdivided into some number of mutually exclusive channels each sample of which is a channel sample A sequential set of channel samples from a single channel is encapsulated into each RTP Data Payload

21. RTCP Sender Report Packet • Provides 3 functions: • Transmission statistics • Defines the relationship between UT and RTP packet sequence number. • Reception statistics for all of the sources that have sent packets to this source since the time of the last Sender Report

22. RTCP Receiver Report • Informs other session members of the quality of their reception • Statistics: • Fraction of packets lost • Cumulative number of packets lost • Approximation of the inter-arrival jitter for RTP data packets • received at the receiver from a particular source

23. RTCP Source DEScription Packet (SDES) • Describes the source of a particular packet stream • CNAME: Canonical endpoint Name Identifier. • NAME: User Name • EMAIL: contact person. • PHONE: contact person. • LOC: Geographical Location • TOOL: Application generating the stream. • NOTE: Notice/Status SDES item. Transient packets describing the state of the source during a session. • PRIV: A mechanism to enable users to define application specific SDES packets

24. RTCP-SDES Priv Extensions • Add VLBI specific extensions to the SDES packet. • Four additional message types are added, identified by their prefix string • Evlbi-abm: Active Bitstream Mask • indicates which bits in a channel stream are active. • Evlbi-cid: Channel Identifier • which channel was the source of this stream of samples. • Evlbi-sfr: Sampling FRequency • sampling frequency of the channel samples. • Evlbi-spp: Samples Per Packet • how many channel samples are contained in a single RTP data packet. • Evlbi-tsf: Timestamp Scaling Factor • Communicate the Timestamp Scaling Factor

25. RTCP Bye Packet • Indicates • A source is leaving a session and is no longer active. • It is distributed to all session participants • to allow them to update their internal tables appropriately. • Allows session participants to track the number of active sources • Important for the calculation of RTCP bandwidth.

26. RTCP Application Defined Packet • Communicates other VLBI control information between DIMs/DOMs • subtype of (1) • the PDATA packet.

28. RTP Open Source Libraries • Libraries (RTP / RTCP extensions for e-VLBI) • Vsocket • Application – VLBI Transport Protocol (vtp) • Libraries (RTP-only and H.323) • ccRTP • Bell Labs/Columbia/UMass library • EDM Media over IP libray • JVOIPLIB • Java Media Framework (JMF) • jrtplib • LIVE.COM Streaming Media • NetLab Java library • RADVision H.323 • WebCanal • UCL RTP library • Vovida

29. RTP Tools • Tools • MultiMON • a monitor that collects, organises and displays all the IP multicast traffic that is detected at the location of the MultiMON Server • Rtpdump • display, decode and generate RTP packet • Rtpmon • Monitors RTP transmissions by displaying RTCP • rtpplay • Play back RTP packet stream recorded with rtpdump. • rtpsend • Send RTP packet stream with configurable parameters. • RTP MIB • Real-Time Transport Protocol Management Information Base

30. Documentation • VSI-H: • VSI-S: • VSI-E: • RTP – RFC3550 • RTCP – RFC3605

31. SC05: VSI-E Experiment • During SC05 Issues: • Onsala, Jodrell Bank, Westerbork • Jumbo Frame Support • Kashima • Lack of jumbo frame support • RTT made TCP not feasible • UDP was the only option • Data format miss-match • K5 – M4 data format • Deployed VSI-E between Kashima – Haystack

32. SC05 - Kashima-Haystack • Local Network – TCP • Long haul network – VSI-E • Results • Sustained 540Mbps during show • 8% packet loss • Failed to incorporate the data in correlation process

33. Conclusion • VSI-E • A media independent data format • Transmitted “on the wire” • Is compatible between heterogeneous DTSs • Efficient transport mechanism • Using Standard protocols • Internet-friendly transport • Scalable Implementation • Ability to transport individual data-channel streams as individual packet streams • Multicasting to transport data and/or control information in an efficient manner