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

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

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.

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.

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.

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).

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

VSI’s Model

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

Network Topologies

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

VSI-E (cont) Critical Definitions: –A channel is an exclusive subset of 2 n 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 2 n 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.

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

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

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

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

Protocol Components

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

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

Data Payload Channel-stream encapsulated into an integer number of 32- bit words in format. DIM input => 32 individual bit streams A subset of 2 n 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

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

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

RTCP Source DEScription Packet (SDES) Describes the source of a particular packet stream –CNAME: Canonical endpoint Name Identifier. – NAME: User Name – 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

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

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.

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

RTP Open Source Libraries Libraries (RTP / RTCP extensions for e-VLBI) –Vsocket –Application – VLBI Transport Protocol (vtp) Libraries (RTP-only and H.323) –ccRTPccRTP –Bell Labs/Columbia/UMass libraryBell Labs/Columbia/UMass library –EDM Media over IP librayEDM Media over IP libray –JVOIPLIBJVOIPLIB –Java Media Framework (JMF)Java Media Framework (JMF) –jrtplibjrtplib –LIVE.COM Streaming MediaLIVE.COM Streaming Media –NetLab Java libraryNetLab Java library –RADVision H.323RADVision H.323 –WebCanalWebCanal –UCL RTP libraryUCL RTP library –VovidaVovida

RTP Tools Tools –MultiMONMultiMON a monitor that collects, organises and displays all the IP multicast traffic that is detected at the location of the MultiMON Server –RtpdumpRtpdump display, decode and generate RTP packet –RtpmonRtpmon Monitors RTP transmissions by displaying RTCP –rtpplayrtpplay Play back RTP packet stream recorded with rtpdump. –rtpsendrtpsend Send RTP packet stream with configurable parameters. –RTP MIBRTP MIB Real-Time Transport Protocol Management Information Base

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

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

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

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