AIE

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? (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
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 118137 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 3Radio Implementation Perspective
Multi-Mode
Transceiver
Simultaneous
Data
Analog
Analog
Voice and Data
Only
Voice
Voice
Only
Only
Media Access Layer
CSMA
TDMA
25 kHz, D8PSK, 31.5 kbps
8.33 kHz DSB-AM
VDL Physical Layer
25 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 3Channel Structure
4-slot Configurations
3-slot Configurations
TDMA Frame (120 ms)
TDMA Frame (120 ms)
Time slot C
Time slot A
Time slot B
Management Subchannel
Voice/Data Subchannel
Management
Voice/Data
Subchannel
Subchannel
40 ms slot
30 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
System
User Groups
Services to
Addresses
Supported
Supported
Configuration
Each Group
Voice Only
Dedicated Voice Only
4V
4
Voice
Voice
Voice
Voice
60
Dedicated Voice Shared Data Slot
Data
Voice
Voice
Voice
3
60
3V1D
Dedicated Voice and Data
Voice
2V2D
Voice
2
60
Discrete Addressed Voice
and Data
Dedicated Voice and Data
Data
Voice
Data
Data
1
240
1V3D
Demand Assigned Voice and Data (Trunked)
Voice/
Voice/
Voice/
3T
N/A
180

Data
Data
Data
Slot devoted entirely as Management Subchannel
10
VDL Mode 3Services

• 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 3Talk 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 3Basic 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 3Enhanced 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 3Data 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 3Antiblocking

• 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 3Controller 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 3Transmit 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 3Next 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 3Urgent 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 3Vocoder 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 3Management 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 3Management 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 3Link 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 3Channel Tuning Aspects

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

28
VDL Mode 3Channel Labeling Scheme (Examples)
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
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
V/D Burst
5
16
16
192
5
16
8
User Information
Ramp Up
Ramp Up
Ramp Dn
Ramp Dn
sync
sync
System
Data
header
0
Downlink Transmissions
30 ms
M Burst
V/D Burst
5
16
32
192
5
16
8
System
User Information
Ramp Dn
Ramp Dn
Ramp Up
sync
Ramp Up
header
sync
Data
0
30 ms
Uplink Transmissions
- symbol
Note 30 ms slot at 31.5 kbps (10.5
kilo-symbols/sec) 1 symbol period 95.24 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
• Provide spectrum containment

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
• 0 indicates more bursts to follow
• 1 indicates the burst is the last burst (end of
  data access)

37
Aircraft Radio Timing States
38
VDL Mode 3System 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 3Aircraft 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
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 DataCommunications System
Architecture
Display
CMU/ATSU
Display
VDR Transceiver
Audio Management Unit
Aircraft
V/D
V/D
Data
VDL-3 Ground Stations
VDL-3 Ground Stations
VDL-2 Ground Stations
TRACON
Secondary GNIs
CLNP
Primary GNI
Firewall
Firewall
ARTCC
Router
Voice Switch
IP
CLNP
DSR
A/G ATN Router
Service Provider Network
IP
Voice Switch
NADIN II
NAS LAN
HOST
HID
Router
CMA
DLAP
A/G ATN Router
G/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
End System
Intermediate System
Intermediate System
Ground
Airborne
Application Process
Network Service Access Points
Application Entity
Upper Layer(s)
Subnetwork Points of Attachment
Transport
CLNP/RP
CLNP/RP
CLNP/RP
CLNP/RP
CLNP/RP
Relaying/Routing
Relaying/Routing
Air/Ground SNDCF
Air/Ground SNDCF
Air/Ground SNDCF
Air/Ground SNDCF
Avionics SNDCF
Avionics SNDCF
Ground SNDCF
Air/Ground SNDCF
Air/Ground SNDCF
Network Layer
Air/Ground SNAcP
Air/Ground SNAcP
Air/Ground SNAcP
Air/Ground SNAcP
Avionics SNAcP
Avionics SNAcP
Ground SNAcP
Air/Ground SNAcP
Air/Ground 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
Avionics Subnetwork
Air/Ground Subnetwork
ATN Router
ATN Host Computer
ATN Host Computer
ATN Router
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
Aircraft ATN Router(s)
Ground 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
Avionics SNDCF
CLNP SNDCF
Aircraft VDL3 Radio
Ground Network Interface
Ground SNAcP
Air/Ground SNAcP
NULL
Compressor / IW
CLNP
Compressor / IW
Ground SNAcP
NULL
Avionics SNAcP
CLNP
A-CLDL
Data Link (HDLC)
Data Link (HDLC)
Local
Data Link
A-CL Data Link
Data Link
A-CL Data Link
Data Link
Data Link
Data Link
Data Link
Local
TDMA
MAC
MAC
Physical
Physical
Physical
Physical
Physical
Physical
Physical
Physical
Physical
Physical
D8PSK
Avionics Bus
Ground Subnetwork
VHF Subnetwork
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
Aircraft ATN Router(s)
Ground ATN Router(s)
Subnetwork Points of Attachment
CLNP/RP
CLNP/RP
CLNP/RP
CLNP/RP
CLNP/RP
CLNP/RP
Relaying/Routing
Relaying/Routing
Ground SNDCF
Air/Ground SNDCF
Ground SNDCF
ISO8208 SNDCF
Avionics SNDCF
ISO8208 SNDCF
Aircraft VDL3 Radio
Ground Network Interface
VDL3 PLP
Ground SNAcP
Air/Ground SNAcP
Ground SNAcP
ISO8208 DTE
Avionics SNAcP
ISO8208 DTE
ISO 8208 Compressor
ISO 8208 DCE
ISO 8208
ISO 8208 DCE
ISO 8208 Compressor
ISO 8208
IW
IW
A-CLDL
Data Link (HDLC)
Data Link (HDLC)
Local
Data Link
A-CL Data Link
Data Link
Data Link
Data Link
Data Link
Data Link
A-CL Data Link
Local
TDMA
MAC
MAC
Physical
Physical
Physical
Physical
Physical
Physical
Physical
Physical
Physical
Physical
D8PSK
VHF Subnetwork
Ground 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
CE
CE
CMP
VDL Mode 3 Subnetwork
Aircraft Radio
LME
DLS
MAC
Ground Subnetwork
Air/Ground ATN Router
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 dont 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