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VDL Mode 3 Overview Briefing for

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1 VDL Mode 3 Overview Briefing for Seminar on Implementation of Data Link and SATCOM Communications 17-19 November 2003 Bangkok, Thailand Rob Strain MITRE CAASD on behalf of Federal Aviation Administration

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 What is NEXCOM? (cont’d)
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 VDL Mode 3 System Overview

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 VDL Mode 3 Key Radio Characteristics
Frequency range 118–137 MHz Channelization 25 kHz centers Channel structure Same frequency for uplink and downlink Radio range* 200 nmi for 4-slot configurations 600 nmi for 3-slot configurations Symbol rate 10.5 kbaud (3 bits /symbol) Modulation(D8PSK) Differential 8-ary Phase Shift Keying Access technique Time Division Multiple Access (TDMA) Voice encoding 4.8 kbps (Normal Voice) 4.0 kbps encoding (Truncated Voice) Data Functionally simultaneous with voice * Range takes into consideration of propagation delay and timing errors of the aircraft radios only

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

8 VDL Mode 3 Channel Structure
4-slot Configurations 3-slot Configurations TDMA Frame (120 ms) TDMA Frame (120 ms) Time slot A B C D Time slot A Time slot B Time slot C Management Management Subchannel Voice/Data Subchannel Voice/Data Subchannel Subchannel 30 ms slot 40 ms slot 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

9 VDL Mode 3 System Configurations (4 Slot)
TDMA Frame A B C D System User Groups Addresses Services to Configuration Supported Supported Each Group Voice Only 4V Voice Voice Voice Voice 4 60 Dedicated Voice Only Dedicated Voice Shared Data Slot 3V1D Voice Voice Voice Data 3 60 2V2D Voice Voice Data Data Dedicated Voice and Data A set of system configurations has been established to meet various operational requirements (i.e., functional capability and operating range) All system configurations operate within the 120 ms TDMA frame and standard M, and V/D burst definitions (except 3T) System configurations (except 3T) vary only in the time scheduling of bursts relative to the timing reference point Each ground station adopts a specific system configuration as established by the FAA and communicated to aircraft radios via uplink M bursts in a manner transparent to users Two classes of system configurations 4-slot configurations 3-slot configurations All support voice, some also support data 2 60 Discrete Addressed Voice and Data Dedicated Voice and Data 1V3D Voice Data Data Data 1 240 Demand Assigned Voice and Data (Trunked) 3T * Voice/ Voice/ Voice/ N/A 180 Data Data Data * Slot devoted entirely as Management Subchannel

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

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 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 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 * Additional equipment and networking procedures may be required for Data Link Operation

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 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 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 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 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 VDL Mode 3 Vocoder

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 System Management

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 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 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 Channel Tuning Aspects

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 VDL Mode 3 Channel Labeling Scheme (Examples)
Frequency (MHz) Time Slot Channel Spacing (kHz) Channel A B C D 25 8.33

29 Backup Slides

30 TDMA Frame Structure (System Configuration 2V2D)
MAC Cycle (240 ms) Even TDMA Frame (120 ms) Odd TDMA Frame (120 ms) LBACs 1 2 3 4 5 6 7 8 V/D (Voice) V/D (Data) V/D (Voice) V/D (Data) M M M M M M M M D/L D/L D/L D/L U/L Poll Response/ Contention Channel Acknowledgment/ Contention Channel Poll Request Reservation grant Acknowledgment/ Contention Channel 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 Typical TDMA Frame Format (4-slot Configurations)
M Burst guard = 2.66 ms V/D Burst guard = 2.76 ms 5* 16 16 5 16 8 192 Ramp Up sync System Data Ramp Dn Ramp Up sync User Information header Ramp Dn Downlink Transmissions 30 ms M Burst V/D Burst guard = 2.71 ms 5* 16 32 5 16 8 192 System Ramp Up sync Ramp Dn Ramp Up sync header User Information Ramp Dn Data 30 ms Uplink Transmissions * - symbol Note: 30 ms slot at 31.5 kbps (10.5 kilo-symbols/sec) 1 symbol period = usec

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 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 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

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 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 1 indicates the burst is the last burst (end of data access)

37 Aircraft Radio Timing States

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 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 station’s 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 Data Link Operation

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

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

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

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

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

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 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 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 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 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 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 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 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 don’t require acknowledgement can be grouped as the space allows

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 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 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 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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