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