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VDL Mode 3 Overview Briefing for Seminar on Implementation of Data Link and SATCOM Communications 17-19 November 2003 Bangkok, Thailand Rob Strain MITRE.

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Presentation on theme: "VDL Mode 3 Overview Briefing for Seminar on Implementation of Data Link and SATCOM Communications 17-19 November 2003 Bangkok, Thailand Rob Strain MITRE."— Presentation transcript:

1 VDL Mode 3 Overview Briefing for Seminar on Implementation of Data Link and SATCOM Communications November 2003 Bangkok, Thailand Rob Strain MITRE CAASD on behalf of Federal Aviation Administration

2 2 What is NEXCOM? FAA program to define and deploy the next generation air/ground communications for aviation in the U.S. –Alleviate the VHF spectrum problem Accommodate additional sectors and services –Reduce maintenance costs of radio systems FAA A/G radios nearing end of useful life –Provide new capabilities Data link Voice/data flexibility for future growth

3 3 What is NEXCOM? (contd) –Preserve capabilities of current analog voice system Dedicated channel per sector Party-line Air/air relay of ATC instructions –Address shortcomings of existing analog system Channel blockage Security Radio frequency interference –Thoughtful implementation strategy No required changes to airspace structure Phased implementation at acceptable cost and schedule –User community participation Coordination with ICAO states to achieve global interoperability

4 4 VDL Mode 3 System Overview

5 5 VDL Mode 3 System Objectives Support spectrum efficient voice operation to meet near term needs with minimal impact on existing ATC infrastructure Support natural time phased evolution toward a mixed voice and data environment with common airborne transceiver Maintain high spectrum efficiency with increasing levels of ATS data traffic

6 6 VDL Mode 3 Key Radio Characteristics Frequency range118–137 MHz Channelization25 kHz centers Channel structureSame frequency for uplink and downlink Radio range*200 nmi for 4-slot configurations 600 nmi for 3-slot configurations Symbol rate10.5 kbaud (3 bits /symbol) Modulation(D8PSK)Differential 8-ary Phase Shift Keying Access techniqueTime Division Multiple Access (TDMA) Voice encoding4.8 kbps (Normal Voice) 4.0 kbps encoding (Truncated Voice) DataFunctionally simultaneous with voice * Range takes into consideration of propagation delay and timing errors of the aircraft radios only

7 7 VDL Mode 3 Radio Implementation Perspective Approach intended to reduce number of airborne radios required Suitable for multimode radio implementation technology Data Only Simultaneous Voice and Data Analog Voice Only Analog Voice Only TDMA 25 kHz, D8PSK, 31.5 kbps 25 kHz DSB-AM 8.33 kHz DSB-AM Media Access Layer VDL Physical Layer Multi-Mode Transceiver CSMA VDL-2VDL-3

8 8 =120 ms TDMA frame is the fundamental timing framework =Each slot may contain two independent bursts =M bursts are used for channel management; while V/D bursts are used for voice or data transfers 4-slot Configurations TDMA Frame (120 ms) Time slot A Time slot B Time slot C Management Subchannel Voice/Data Subchannel 3-slot Configurations 40 ms slot VDL Mode 3 Channel Structure Time slot A B C D 30 ms slot Management Subchannel Voice/Data Subchannel TDMA Frame (120 ms)

9 9 VDL Mode 3 System Configurations ( 4 Slot ) ABC D System Configuration User Groups Supported Services to Each Group 4V4 Dedicated Voice Only Voice Only Discrete Addressed Voice and Data Voice Data Voice Data Voice/ Data * Voice/ Data Voice/ Data 3V1D 2V2D 3T 3 2 N/A Dedicated Voice Shared Data Slot Dedicated Voice and Data Demand Assigned Voice and Data (Trunked) * Slot devoted entirely as Management Subchannel TDMA Frame VoiceData 1V3D 1 Dedicated Voice and Data Addresses Supported

10 10 VDL Mode 3 Services Voice Communications Service –Basic Voice (requires no discrete addressing) Air-to-Ground and Air-to-Air –Enhanced Voice (requires discrete addressing) Basic Voice Functionally simultaneous voice and data services Enhanced features Air-to-Ground only Point-to-point Data Service –Ground-to-Air –Air-to-Ground –Require discrete addressing Data Broadcast Service –Ground-to-Air only

11 11 VDL Mode 3 Talk Group (Net) Login Process Time Beacon Net Entry Req Net Entry Resp Beacon Poll Resp NON-DISCRETE ADDESSED Basic Digital Voice & Broadcast DISCRETE ADDRESSED Enhanced Digital Voice & Data Link Step 1. Net InitializationStep 2. Net Entry (optional)

12 12 VDL Mode 3 Basic Digital Voice Radio mode providing two-way digital voice operation Available immediately upon net initialization Basic configuration for non-data link equipped aircraft Same operations and procedures that are in place today Enables efficient channel access and resolution of blockages –Antiblocking –Controller Override Has a basic feature set –Transmit Status Indicator Supports channel monitoring without using channel resources

13 13 VDL Mode 3 Enhanced Digital Voice Basic Digital Voice + Radio participates in net entry/exit process and obtains discrete address (default condition) Enhanced feature set –Next Channel Uplink –Urgent Downlink Request –Other provisional features available as need arises

14 14 * Additional equipment and networking procedures may be required for Data Link Operation VDL Mode 3 Data Link Operation Enhanced Digital Voice + Functionally simultaneous operation with digital voice Radio provides a air/ground subnetwork for (ATN) application data exchange –CPDLC –FIS Requires separate data processing functionality –Data link protocols and connection management –Interface to radio and user displays –Message routing –Application programs

15 15 VDL Mode 3 Antiblocking A means to reduce the incidence of step-on conditions –Active channel management –One user of the channel at a time –Small period when simultaneous access is possible (120 ms) Inherent in radio functionality Users provided aural indication if channel occupied and PTT activated (i.e., Transmit Status Indicator)

16 16 VDL Mode 3 Controller Override Capability to enable a controller to obtain access to the communication channel when necessary When activated, all aircraft radios are placed in receive mode Enhances safety and efficiency –Reset aircraft radio after stuck microphone (pilot unaware) –Pre-empt aircraft transmissions for urgent controller message Pre-empted users provided aural indication (i.e., Transmit Status Indicator)

17 17 VDL Mode 3 Transmit Status Indicator Indication to user that an attempt to transmit has failed –Simultaneous transmissions –Overridden transmission –Transmit time-out –Radio in special operating state Avionics implementation –Aural tone (busy signal) –Receipt of incoming audio mixed with indicator –Pilot re-keys PTT to re-access channel

18 18 VDL Mode 3 Next Channel Uplink An uplink of the next control channel during transfer of communication (TOC) procedure Supplemental information only, standard voiced or CPDLC TOC remain primary means Reduces errors in transmission, hearing and entering new channel data Reduces pilot workload tuning radio Dependent on peer capability in ground system Avionics implementation –Uplinked channel loaded into standby tuning window with indication –Pilot activates channel by transferring to primary tuning window upon receipt of TOC

19 19 VDL Mode 3 Urgent Downlink Request A pilot request to access a congested communication channel Supports channels access in urgent (non-emergency) situations Dependent on peer capability in ground system Avionics implementation –Activation button and visual status indicator on Radio Tuning Panel –Communication system manages technical acknowledgements –Controller provides operational acknowledgment to pilot –Deactivation by channel access, radio tuning or pilot cancellation

20 20 VDL Mode 3 Vocoder

21 21 VDL Mode 3 Vocoder Characteristics Speech Encoding Algorithm –Advanced Multi-band Excitation (AMBE)-ATC-10 Developed by Digital Voice Systems, Incorporated (DVSI) Built-in FEC –Dual rates 4.8 kbps (normal mode encoding) 4.0 kbps (truncated mode encoding) –By slowing down the clock rate to 5/6 of the normal rate –20 ms voice frame (96 bits/frame) 6 voice frames per V/D (voice) burst for 4.8 kbps rate 5 voice frames per V/D (voice) burst for 4.0 kbps

22 22 System Management

23 23 VDL Mode 3 Management Bursts Management bursts are used to convey VDL Mode 3 system management information between ground and aircraft radios and between aircraft radios –Signaling Beacon Ground to air voice signaling –Channel access control Voice Channel Data Channel Downlink M channel –Link management

24 24 VDL Mode 3 Management Messages Net Entry Request message/Net Entry Response message (no previous link)/initial Poll Response/Supported Options message Net Entry Request message/Net Entry Response message (previous link preserved)/initial Poll Response/Supported Options message Normal message (Poll)/Poll Response message/Normal message Next Net Command message/Next Net ACK message Reservation Request message/Normal message Recovery message Handoff Check message Terminate Net message Acknowledgement message Leaving net message

25 25 VDL Mode 3 Link Establishment Net Initialization –Required for basic voice operation –Establishes system timing and essential configuration parameters Net Entry –Required for enhanced voice and data operation –Establishes point to point addressing Initial Link Negotiation –Required for data operation –Establishes data link management configuration parameters

26 26 Channel Tuning Aspects

27 27 VDL Mode 3 Channel Tuning Aspects Channel Labeling for VDL Mode 3 –Pseudo-frequency vs. logical channel numbering –Coexistence with all other VHF modes

28 28 VDL Mode 3 Channel Labeling Scheme ( Examples ) Frequency (MHz) Time Slot Channel Spacing (kHz) Channel ABCDABCDABCDABCD

29 29 Backup Slides

30 30 TDMA Frame Structure (System Configuration 2V2D) D/L U/L D/L MMMM Poll Response/ Contention Channel Acknowledgment/ Contention Channel Acknowledgment/ Contention Channel Poll Request Reservation grant M M MM Even TDMA Frame (120 ms) MAC Cycle (240 ms) V/D (Voice) V/D (Data) V/D (Voice) V/D (Data) LBACs Odd TDMA Frame (120 ms) Note: Contention Channel is used for downlink M burst transmission of Net Entry, Reservation Request, Urgent Downlink Request, Leaving Net Message, based on slotted aloha protocol

31 31 Typical TDMA Frame Format (4-slot Configurations) Downlink Transmissions Uplink Transmissions Note: 30 ms slot at 31.5 kbps (10.5 kilo-symbols/sec) 1 symbol period = usec M Burst 5*1632 V/D Burst guard = 2.71 ms 0 30 ms User Information header sync System Data sync * - symbol 0 guard = 2.66 ms 30 ms M Burst 5*16 V/D Burst guard = 2.76 ms User Information header Ramp Up sync System Data Ramp Dn Ramp Up

32 32 Uplink Management Bursts Transfers most of the management information Uplink M-bursts –Transmitted from the ground station –Dedicated Logical Burst Access Channel (LBAC) for ground No contention with aircraft transmissions –All include beacons System Configuration information Voice Signaling for voice channel access control Squelch window Ground station code Aircraft ID and Slot ID –Must be coordinated among all Ground sites supporting the same user group

33 33 Downlink M bursts Transmitted from the aircraft stations Dedicated LBACs for User Group –aircraft in same user group share access Usages –Enhanced Voice features –Data Reservation requests –Poll Response –Data Acknowledgement –Link Establishment –Leaving Net Access to downlink M channel is dynamically controlled based on message types –Dedicated access for Poll Response and uplink data ACK – No contention –Slotted Aloha random access for all other messages – with contention

34 34 Management Burst Characteristics Management Burst consists of three segments –Training Sequence Transmitter ramp up and power stabilization –To ensure reaching full power quickly –Provide spectrum containment –To provide time for receiver AGC circuit to settle Synchronization and ambiguity resolution –4 unique words used to achieve receiver synchronization for different messages –The middle of the first symbol of the unique word is the TRP –System Data Actual M burst messages per format defined in DO-224A –Ramp Down Controls the rate the transmitter power should be reduced after burst to control potential interference in the reception of the following V/D burst Provide spectrum containment

35 35 Guard Time Guard time between bursts ensures no burst overlap for intra-user group and inter-user group burst transmissions –Guard time in VDL Mode 3 TDMA frame takes into account +/- 1 symbol period timing error relative to A/C radio TRP Propagation path difference among radios relative to the ground station for a maximum range of 200 nmi for 4-slot and 600 nmi for 3-slot configurations Increase guard time by reducing vocoder rate from 4.8 to 4.0 kbps (truncated voice) to compensate for less accurate timing (TS2) Operate in Free-running Voice in the absence of ground system timing

36 36 Header Characteristics for V/D Bursts V/D (Voice) Burst Header precedes each V/D (Voice) burst –Message ID indicates uplink voice, downlink TS1 voice, downlink TS2 voice, or downlink TS3 voice –Local user ID uniquely identifies the transmitting A/C –EOM (End of Message) field 0 indicates more bursts to follow 1 indicates the burst is the last burst (end of voice access) V/D (data) Burst Header precedes each V/D (Data) burst –Message ID indicates uplink or downlink and acknowledged or unacknowledged data frames –Ground station Code –Segment Number identifies the segment number of the frame group –EOM (End of Message) field 0 indicates more bursts to follow 1 indicates the burst is the last burst (end of data access)

37 37 Aircraft Radio Timing States

38 38 VDL Mode 3 System Timing Ground system timing synchronized to timing reference traceable to UTC Aircraft timing synchronized to ground timing –ensures interference-free operation (no burst overlap) A/C radio timing: –Ground system distributes timing to A/C in normal operation –Degraded time derived from ALT timing sources (e.g. A/C radios) –Free-running timing mode provided in loss of ground system timing oceanic operation Guard time provided in the TDMA frame structure to allow for A/C timing errors, timing offsets between ground stations, and signal propagation

39 39 VDL Mode 3 Aircraft Radio Timing States A/C radio uses two types of timing signals to update its System Timing and control its Timing State every MAC cycle –primary timing signal (PTS) from desired ground stations M bursts –alternate timing signal (ALT) Uplink M bursts from another time slot Poll Responses from aircraft radios of the same or another user group Timing States indicate the estimated timing accuracy of the A/C radio relative to ground system timing 4 Timing States defined for A/C radios –TS0: TDMA system time not yet acquired –TS1: Slaved to PTS (error 1 symbol period) –TS2: Slaved to ALT (error 17 symbol period) –TS3: TDMA system time not available Timing state transition will be delayed until ongoing PTT and data access are completed

40 40 Data Link Operation

41 41 Overview of Data Structure ATN ISO Stack VDL Mode 3 Stacks –Connectionless –Connection-oriented (ISO/IEC 8208 / X.25) High Level Description of Layer Entities VDL Mode 3 Protocol Processes

42 42 A/G Voice & Data Communications System Architecture DSR HOSTHID Display Router DLAP CMU/ATSU Primary GNI VDR Transceiver VDL-2 Ground Stations A/G ATN Router CMA NADIN II NAS LAN G/G ATN Router A/G ATN Router VDL-3 Ground Stations Display ARTCC FAA Firewall Aircraft Service Provider VDL-3 Ground Stations Secondary GNIs Router TRACON Firewall CLNP IP* CLNP Firewall Audio Management Unit Data V/D Voice Switch Voice Switch Service Provider Network * IP Tunnel used to connect Primary GNI to Secondary GNIs to exchange data

43 43 Physical Data Link (HDLC) Air/Ground SNAcP Air/Ground SNDCF Physical Data Link (HDLC) Air/Ground SNAcP Air/Ground SNDCF Physical Data Link (HDLC) Air/Ground SNAcP Air/Ground SNDCF Physical Data Link (HDLC) Air/Ground SNAcP Air/Ground SNDCF Network Layer ATN ISO Stack Application Process Application Entity Physical Data Link Ground SNAcP Ground SNDCF CLNP/RP Transport Upper Layer(s) Application Process Application Entity Physical Data Link Avionics SNAcP Avionics SNDCF CLNP/RP Transport Upper Layer(s) Physical Data Link Avionics SNAcP Avionics SNDCF CLNP/RP Physical Data Link Ground SNAcP Ground SNDCF CLNP/RP Physical Data Link Air/Ground SNAcP Air/Ground SNDCF CLNP/RP Physical Data Link Air/Ground SNAcP Air/Ground SNDCF CLNP/RP Ground Subnetwork Avionics Subnetwork Air/Ground Subnetwork ATN Router ATN Host Computer Relaying/Routing Subnetwork Points of Attachment Network Service Access Points LEGEND: Connectionless Network Protocol (CLNP) Routeing Information Exchange Protocol (RP) Subnetwork Dependent Convergence Function (SNDCF) Subnetwork Access Protocol (SNAcP) End System Intermediate System GroundAirborne

44 44 Compressor / IW VDL3-ATN Protocol Stack with Connectionless Subnetwork Interface Ground Subnetwork Avionics Bus VHF Subnetwork Subnetwork Points of Attachment LEGEND: Connectionless Network Protocol (CLNP) Subnetwork Dependent Convergence Function (SNDCF) Interworking (IW) Acknowledged Connectionless Datalink (A-CLDL) Intermediate System(s) Ground Airborne Physical Data Link Physical A-CL Data Link Physical CLNP A-CLDL TDMA D8PSK Ground Network Interface Local Data Link Compressor / IW CLNP MAC A-CL Data Link MAC Physical Data Link (HDLC) Ground SNAcP Ground SNDCF CLNP/RP Physical Data Link (HDLC) Air/Ground SNAcP Air/Ground SNDCF CLNP/RP Physical Data Link Ground SNAcP Ground SNDCF CLNP/RP Physical Data Link NULL CLNP SNDCF CLNP/RP Ground ATN Router(s) Relaying/Routing Physical Data Link Avionics SNAcP Avionics SNDCF Physical Data Link CLNP SNDCF Aircraft ATN Router(s) NULL Local Aircraft VDL3 Radio Relaying/Routing CLNP/RP

45 45 VDL3-ATN Protocol Stack with Connection- Oriented Subnetwork Interface Avionics Bus Subnetwork Points of Attachment LEGEND: Connectionless Network Protocol (CLNP) Subnetwork Dependent Convergence Function (SNDCF) Interworking (IW) Packet Layer Protocol (PLP) Acknowledged Connectionless Datalink (A-CLDL) Intermediate System(s) GroundAirborne Physical Data Link Physical A-CL Data Link Physical ISO 8208 A-CLDL TDMA D8PSK Ground Network Interface Local Data Link ISO 8208 MAC A-CL Data Link MAC Physical Data Link (HDLC) Ground SNAcP Ground SNDCF CLNP/RP Physical Data Link (HDLC) Air/Ground SNAcP Air/Ground SNDCF CLNP/RP Physical Data Link Ground SNAcP Ground SNDCF CLNP/RP Physical Data Link ISO8208 DTE ISO8208 SNDCF CLNP/RP Ground ATN Router(s) Relaying/Routing Physical Data Link Avionics SNAcP Avionics SNDCF CLNP/RP Physical Data Link ISO8208 SNDCF CLNP/RP Relaying/Routing Aircraft ATN Router(s) ISO8208 DTE Local ISO 8208 DCE ISO 8208 Compressor IW ISO 8208 Compressor ISO 8208 DCE IW VDL3 PLP Aircraft VDL3 Radio Ground Subnetwork VHF Subnetwork

46 46 Functional Descriptions of Layer Entities Transport Layer (End System) Network Layer –Internetworking CLNP (ATN Router) –Subnetwork Dependent Convergence Function (SNDCF) (ATN Router) –Subnetwork –Interworking (IW) Sublayer Data Link Layer –Link Management Entity (LME) –Data Link Service (DLS) Sublayer –Media Access Control (MAC) Sublayer Physical Layer NOTE: Italicized text denotes entities NOT resident in the VDL Mode 3 subnetwork

47 47 Subnetwork Architecture Aircraft ATN Router Air/Ground ATN Router Ground Subnetwork VDL Mode 3 Subnetwork LME CE CMP DLS MAC CE Aircraft Radio GS1 MAC GS2 MAC GS3 MAC GS4 LME DLS CE CMP CE GNI LME LME DLS CE CMP CE GNI LME Air/Ground ATN Router

48 48 Subnetwork Compression Provide a subnetwork layer above the DLS service that performs protocol specific compression Provides flexibility to support access to different subnetwork interfaces within VDL 3 –Allow industry to decide on best subnetwork protocol for desired application(s) Subnetwork Type and Compression is defined within first Octet in DLS user data – Defines subnetwork payload type – Defines compression performed (if any)

49 49 Subnetwork Compression 3 different ATN approaches: –CLNP Header Compression (Connectionless Service) –ISO 8208 Compression (Connection-Oriented Service) –ATN Frame Mode Ground system supports all options to provide aircraft with maximum flexibility Aircraft only needs to support one option –minimize avionics complexity –use of multiple is allowed but only 1 at a time

50 50 CLNP Interface (Connectionless) Provide a direct CLNP Interface to ATN Router Compression is performed on CLNP header within VDL Mode 3 subnetwork Broadcast compression supported

51 51 ISO 8208 Interface (Connection- Oriented) Follows traditionally defined ATN interface –CLNP Header Compression (LREF) performed prior to entering the VDL Mode 3 Subnetwork Additional compression performed on ISO 8208 headers and management packets within the subnetwork

52 52 ISO 8208 Interface Provides subnetwork flow control on a per- connection basis Employs full DCE state machine in aircraft station. Subnetwork compressor will incorporate –ISO 8208 header compression –Packet re-sequencing –Duplicate suppression

53 53 Link Management Entity Link establishment and release Link Maintenance (Poll/Poll Response) Handoffs between links Recovery processing Exchange Identity (XIDs) parameter handling (ISO 8885)

54 54 DLS based on an A-CLDL Protocol DLS based on Acknowledged-Connectionless Data Link (A-CLDL) protocol –Simplifies protocol –MAC is already ensuring sequencing within each priority stream via stop-and-wait protocol Error detection and recovery Address identification Frame sequencing –Priority handling Frame-based messages –Frame consists of up to 15 V/D (data) bursts

55 55 A-CLDL Operation: Frame Grouping Frames may be grouped into a single Media Access event to improve system efficiency –Frames that require acknowledgement must all be of the same priority and for the same destination, since only one peer can send an ACK at a time –Frames that dont require acknowledgement can be grouped as the space allows

56 56 A-CLDL Operation: Acknowledgement DLS acknowledges correct receipt of a media access event instead of specific frames Any frame requiring acknowledgement in group in error requires retransmission of all frames requiring acknowledgement within group Frames not requiring acknowledgement are not retransmitted Frames not requiring acknowledgement can be processed if its CRC passes, even if other frames in group are in error

57 57 A-CLDL Implications Reliance on MAC sublayer –Provides connection-oriented service to deal with link failures –Enforces sequencing within each priority queue –MAC controls retransmission and ACK timing DLS and MAC need to be tightly coupled for optimal performance Reliance on Transport Protocol or Subnetwork Protocol –Retransmission for any lost packets (highly unlikely)

58 58 Media Access Control Sublayer Specifies slot timing and media access Processes Burst-based messages –Formatting –M burst management messages –V/D (data) burst consists of 62 information bytes Schedules data access to V/D burst Manages M burst communications

59 59 Physical Layer Converts bit stream to/from RF waveform –Differential 8-ary Phase Shift Keying –10.5 kbaud Error detection and correction coding –(24, 12) Golay for M bursts and V/D headers –(72, 62) Reed-Solomon for V/D (data) –FEC built in the Vocoder Bit synchronization –S1 : for Downlink M bursts other than Net Entry and Poll Response –S2 : for V/D bursts –S1*: for Net Entry Requests –S2*: for Poll Responses, Uplink M bursts, and Handoff Check (H) Uplink


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