VDL Mode 3VDL Mode 3

OverviewOverview

Briefing for

Seminar on Implementation of Data Link and SATCOM Communications

17-19 November 2003Bangkok, Thailand

Rob StrainMITRE CAASD

on behalf ofFederal 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 3Radio Implementation Perspective

• Approach intended to reduce number of airborne radios required

• Suitable for multimode radio implementation technology

DataOnly

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-2 VDL-3

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

ManagementSubchannel

Voice/DataSubchannel

3-slot Configurations

40 ms slot

VDL Mode 3Channel Structure

Time slot A

Time slot B

Time slot C

Time slot D

30 ms slot

Management Subchannel

Voice/Data Subchannel

TDMA Frame (120 ms)

9

VDL Mode 3 System Configurations (4 Slot)

A B C D

System Configuration

User Groups Supported

Services to Each Group

4V 4 Dedicated VoiceOnly

Voice Voice Voice Voice

Vo

ice

On

lyD

iscr

ete

Ad

dre

ssed

Vo

ice

and

Dat

a

Voice Voice Voice Data

Voice Voice Data Data

Voice/ Data

* Voice/ Data

Voice/ Data

3V1D

2V2D

3T

3

2

N/A

Dedicated VoiceShared Data Slot

Dedicated Voiceand Data

Demand Assigned Voice and Data

(Trunked)

* Slot devoted entirely as Management Subchannel

TDMA Frame

Voice Data Data Data1V3D 1Dedicated Voice

and Data

60

60

60

180

240

Addresses Supported

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

Time

Beacon

BeaconNet

EntryReq

NetEntryResp

Beacon

PollResp

NON-DISCRETE ADDESSEDBasic Digital Voice & Broadcast

DISCRETE ADDRESSEDEnhanced Digital Voice & Data Link

Step 1. Net Initialization Step 2. Net Entry (optional)

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

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

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)

Frequency (MHz)

Time Slot

Channel Spacing (kHz)

Channel

118.0000

118.0000118.0000118.0000118.0000

118.0000118.0083118.0167

118.0250118.0250118.0250118.0250

118.0250

ABCD

ABCD

25

25252525

8.338.338.33

25252525

25

118.000

118.001118.002118.003118.004

118.005118.010118.015

118.021118.022118.023118.024

118.025

29

Backup Slides

30

TDMA Frame Structure(System Configuration 2V2D)

D/L U/L D/LD/LD/L

MMMM

Poll Response/Contention Channel

Acknowledgment/Contention Channel

Acknowledgment/Contention Channel

Poll RequestReservation grant

M M M M

Even TDMA Frame(120 ms)

MAC Cycle(240 ms)

1 2 3 4 5 6 7 8

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

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 = 95.24 usec

M Burst

5* 16 32

V/D Burst

1925 16 8

guard = 2.71 ms

030 ms

User Information

hea

der

syncSystem

Data sync

* - symbol

0

guard = 2.66 ms

30 ms

M Burst

5* 16 16

V/D Burst

1925 16 8

guard

= 2.76 ms

User Information

hea

der

Ram

p U

p

sync sync

Sys

tem

D

ata

Ram

p D

n

Ram

p D

n

Ram

p D

nR

amp

Dn

Ram

p U

p

Ram

p U

pR

amp

Up

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 RadioTiming 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 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 & DataCommunications System Architecture

DSR

HOST HID

Display

Router

DLAP

CMU/ATSU

PrimaryGNI

VDRTransceiver

VDL-2 GroundStations

A/GATN Router

CMA

NADIN IINAS LAN

G/GATN Router

A/G ATNRouter

VDL-3 GroundStations

Display

ARTCC

FAA

Firew

all

Aircraft

Service Provider

VDL-3 GroundStations

SecondaryGNIs

Router

TRACONFirewall

CLNP

IP*

IP*

CLNP

Firewall

Audio Management Unit

DataV/DV/D

VoiceSwitch

VoiceSwitch

Service Provider Network

* IP “Tunnel” used to connect Primary GNI to Secondary GNIs to exchange data

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

NetworkLayer

ATN ISO Stack

Application Process

ApplicationEntity

Physical

Data Link

Ground SNAcP

Ground SNDCF

CLNP/RP

Transport

Upper Layer(s)

Application Process

ApplicationEntity

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 SubnetworkAir/Ground SubnetworkATN Router ATN Router ATN Host ComputerATN Host Computer

Relaying/Routing 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 End SystemIntermediate System Intermediate System

Ground Airborne

44

Compressor / IW

VDL3-ATN Protocol Stack with Connectionless Subnetwork Interface

Ground Subnetwork Avionics BusVHF 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) Intermediate System(s)

Ground Airborne

Physical

Data Link

Physical

A-CLData Link

Physical Physical

CLNP

A-CLDL

TDMA

D8PSK

Ground Network Interface

LocalData Link

Compressor / IWCLNP

MAC

A-CLData 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/RPCLNP/RP

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) Intermediate System(s)

Ground Airborne

Physical

Data Link

Physical

A-CLData Link

Physical Physical

ISO8208

A-CLDL

TDMA

D8PSK

Ground Network Interface

Local

Data Link

ISO8208

MAC

A-CLData 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

ISO8208DTE

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

ISO8208 DCE

ISO8208Compressor

IWISO

8208Compressor

ISO8208 DCE

IW

VDL3 PLP

Aircraft VDL3 Radio

Ground Subnetwork VHF Subnetwork

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 LayerNOTE: Italicized text denotes entities NOT resident in the VDL Mode 3 subnetwork

47

Subnetwork Architecture

Aircraft ATN Router

Air/Ground ATN Router

Ground Subnetwork

VDL Mode 3 SubnetworkLME

CECMP

DLS

MAC

CE

Aircraft Radio

GS1

MAC

GS2

MAC

GS3

MAC MAC

GS4

• • •

LME

DLS

CECMP CE

GNI LME• • •

• • •

LME

DLS

CECMP CE

GNI LME• • •

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