vlbi standard interface – electronic (vsi-e) protocol fundamentals chet ruszczyk mit haystack...
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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 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.
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 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
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– 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
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) – 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
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
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