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A new DLS protocol for the VDL Mode 4 Data Link

A new DLS protocol for the VDL Mode 4 Data Link. Mike Shorthose, Helios Technology Ltd AMCP WG-M meeting, Brussels, 11 March 2002. Overview . Background Process Progress Way ahead Technical details. VDL Mode 4: status in WGM.

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A new DLS protocol for the VDL Mode 4 Data Link

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  1. A new DLS protocol for the VDL Mode 4 Data Link Mike Shorthose, Helios Technology Ltd AMCP WG-M meeting, Brussels, 11 March 2002 P196D006

  2. Overview • Background • Process • Progress • Way ahead • Technical details

  3. VDL Mode 4: status in WGM • AMCP/7 meeting approved SARPs and TM for VDL Mode 4 for surveillance applications only • WGM is considering the use of VM4 for point to point communications • Some deficiencies exist with the currently specified VM4 DLS protocol (AVLC)

  4. AVLC • AVLC is the current protocol specified within VDL Mode 4 • Closely based on the protocol used for VDL Mode 2 • Allows transfer of 4 frames before an acknowledgement is required • BUT: • AVLC does not provide support for message priority • AVLC does not provide facilities to segment longer messages • Problem with ‘leave event’ generation • AVLC protocol complexity, plus errors, inconsistencies and ambiguities in VDL Mode 2 references

  5. Need for new Data Link Service (DLS) protocol • A new DLS protocol for VDL4 would: • remove errors, inconsistencies and ambiguities • handle priority messages in an efficient way • provide fragmentation and performance benefits • simplify the specification by reducing protocol complexity

  6. What has happened • The basic principles of a new DLS proposal were presented to WGM/2 by Eurocontrol • WGM agreed that this is a good direction and requested to advance the work and make a complete proposal for a new DLS protocol. • Eurocontrol is funding the work and has established (Nov. 2001) a team to complete DLS proposal and validate the new DLS. • Team members: Mike Shorthose and Tony Whyman • Co-ordinated proposal with radio manufacturers (S. Friedman from ADSI and M. Gustafsson from SAAB) • Inputs from other WG-M members • Initial validation carried out by independent validation team at Helios • A process was agreed by the developing team to produce the new DLS proposal

  7. Process – define proposal for new DLS • Define a complete proposal for a new DLS • Resolution of issues – review of initial DLS outline proposal with analysis of outstanding issues • Produce outline design specification • Design of burst types • Produce first draft for technical manual • Review • Produce second draft for technical manual

  8. Process – validation of new DLS • Validation of the new DLS protocol • Independent validation team • Identification of protocol states • Identification of state diagrams and associated events • Protocol walk through: Step through of data transfer protocol including special transfer protocols • Assessment of link management • Assessment of performance issues • Production of validation report • Review • Update following review

  9. Progress • Following co-ordination of the developing team with VM4 radio manufactures (ADSI, SAAB), and LFV, FAA and DFS, a draft proposal was presented in the WGM/3 meeting • Since WGM/3, review carried out by validation team and drafting team • clarification of interface to LME • modification to support 16 priority levels • review of “T-bit” initialisation • broadcast support made explicit • Current status: Initial validation performed – further evaluation remains to be done • Update to DLS change proposal (v0.4) has been produced to reflect early conclusions of validation team

  10. Validation • Validation • carried out by independent team • Initial process • inspection • derivation of state diagrams • Connection Establishment • Data Transmission • Data Reception • report of initial findings

  11. Results of the initial DLS validation process • No fundamental problems found • A number of logical errors discovered • Corrective action suggested by the validation team

  12. Tasks remaining • Next stage of validation: • inspection and analysis of v0.4 • simulation of protocols using simple test harness • inspection of LME interface • production of validation report • Complete TM text • response to issues raised by validation team • review and re-draft LME text at interface with DLS • aim to keep functionally identical to Mode 2 • validation team will inspect and verify interface

  13. Tasks Remaining – change proposal • Change proposal for AMCP/WGM • Production of red-lined text • Production of implementation manual text • protocol description • use cases • Production of submission to WG-M (WGM/5 meeting)

  14. The Data Link Service • Provides services to external users to support point-to-point communications • Provides services to the LME to support link management • Supports services between peer DLSs • Supports broadcast services • Uses the services of the VSS in order to send and receive messages

  15. General DLS features • To support information exchange, DLS provides: • simple transmit/acknowledge protocol • duplicate suppression via “toggle bit” • FRMR frame used to reject INFO frame if, eg T bit synchronisation fails • message fragmentation via “M” bit • 15 message priority levels • pre-emption of lower priority messages • special short and long transmission protocols to make best use of reservation protocols provided by VSS • minimises use of random access protocol • re-transmission based on knowledge of when acknowledgements are expected • removes dependence on timers

  16. New DLS Burst • Burst id + one or two DLS frames • DATA frames for transfer of user information • CTRL (LME packets), INFO (point to point user data) and UDATA (broadcast user data or broadcast LME data) • ACK frames to acknowledge transfer of user information • CTRL_ACK and INFO_ACK • RTS frames to request reservation of slots for DATA transfer • CTRL_RTS, INFO_RTS, UDATA_ACK • CTS frames to provide reserved slots for DATA transfer • CTRL_CTS, INFO_CTS, UDATA_CTS • Other link control frames • FRMR, FRMR_ACK for link reset • DM/DISC for air/ground link disconnection • START for air/air link initialisation

  17. Fields used in DLS frames • Toggle bit (T)–alternately set to 1 and 0 on each successive ‘data’ transmission

  18. Fields used in DLS frames • More Bit (M)–set to zero to indicate end of message. • Message Fragmentation/Concatenation • If the length of the burst is longer than N4 slots the sending station willfragment the message. • For single fragmentmessages: • M bit is zero • For multiple fragmentmessages: • The M bit is set to 1 except the last segment, which is set to zero

  19. Fields used in DLS frames • Initialise bit (I) - used to initialise T bit • Priority (p)– absolute priority • Length (lg)–length of the data • Negotiation subfield (neg) – indicates link parameters for air/air link

  20. Short Transmission Procedures

  21. Long Transmission Procedures

  22. Burst combinations • Up to two DLS frames can be contained in a single DLS burst • enables protocols to be linked so as to avoid use of random access protocol • The following combinations are possible: • DATA/RTS and ACK/CTS • links a stream of messages or fragments sent from one peer to another • ACK/DATA • allows receiving station to send back a single slot response • ACK/RTS • allows a receiving station to reserved slots for multi-slot response • eg subsequent transport ACK

  23. Burst combinations

  24. Timers and counters • TD1 and TD2: control the air/air link • ND1: Maximum number of bits in any frame • ND2: The maximum length of the message to be sent as a short protocol • ND3: The maximum length of a segment

  25. Services offered to LME • The services offered to the LME will be: • transfer of LME packets encoded within CTRL frames using the T bit protocol (duplicate suppression) at the highest priority • T-bit initialisation via flag setting within CTRL frame • fragmentation for long LME packets • provision of short and long transmission protocols for CTRL frames • pre-emption of lower priority INFO frames • transfer of broadcast LME packets using modified short and long transmission protocols • ability to disconnect link via DM/DISC frame

  26. Air/ground INFO transfer • The services offered for air/ground INFO transfer are: • disconnect mode indication if INFO transfer is attempted before a link is established • operation on a full 4 bit priority basis • duplicate suppression • pre-emption • fragmentation • provision of short and long transmission protocols for INFO frames • transfer of broadcast INFO packets using modified short and long transmission protocols

  27. Air/air INFO transfer • The services offered for air/air INFO exchange are: • T-bit initialisation via a START frame which is combined with the first INFO frame • Link parameter negotiation via “version number” • Link maintenance via send and receive timers • Operation on a full 4 bit priority basis • Transfer of information encoded within INFO frames using the T-bit protocol • pre-emption • fragmentation • provision of short and long transmission protocols

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