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