Next Generation ICAO 9896 VoIP Interfaces for ATS Ground Voice Network - Part 1

Copyright 2014 JSP-Teleconsultancy 1

Next Generation ICAO 9896 VoIP interfaces

for ATS Ground Voice Network - Part 1

Training Documentation

Produced by JSP Teleconsultancy 1

Copyright Notice

2014 JSP Teleconsultancy . All rights reserved.

JSP Teleconsultancy has the exclusive rights to present Air Traffic Services Ground Voice Network Communication courses, including ATS-QSIG courses under a Licence Agreement (L/01/03-JSP/QSIG) with EUROCONTROL Institute of Air Navigation Services. This documentation has been produced by JSP Teleconsultancy and may only be used in the framework of the Licence Agreement and/or in relation with the pertinent Eurocontrol courses, workshops and seminars. The reproduction of this material is permitted for personal and non-commercial purposes only. This is subject to the material being reproduced accurately and not used in a misleading context. This document or CD-ROM may be copied to third parties under the same restrictions, provided the source is acknowledged.

Any other use is subject to prior written consent by JSP Teleconsultancy.

Requests shall be addressed to: JSP Teleconsultancy, Via Gorizia, 11 FOLIGNANO, 63040, Ascoli piceno, Italy.


Table of Contents

1. INTRODUCTION ............................................................................................................................. 7 Introduction to course ..................................................................................................................................... 8 Course Objectives - Part 1 ............................................................................................................................. 8 Course Objectives - Part 2 ........................................................................................................................... 10 Course Objectives - Part 3 ........................................................................................................................... 10

2. ATS GROUND VOICE NETWORK OVERVIEW .......................................................................... 13 Telephone network infrastructure overview .................................................................................................. 14 Circuit switched connections and TDM transport efficiency .......................................................................... 18 Direct Access Call operation......................................................................................................................... 20 Instantaneous Access Call operation ........................................................................................................... 22 Priority DA Call operation ............................................................................................................................. 24 Telephony features ....................................................................................................................................... 26 Radio network infrastructure overview .......................................................................................................... 28 Radio Access Modes of Operation ............................................................................................................... 32 Push-To-Talk (PTT) ...................................................................................................................................... 34 Incoming Aircraft Call detection/Squelch signal ............................................................................................ 36 Radio features - Cross Coupling (Retransmission) ....................................................................................... 38 Radio features - Best Signal Selection (Rx Voting) ...................................................................................... 40 Best Transmitter Selection (Transmitter Voting) ........................................................................................... 42 Radio features - Multicarrier Offset Transmission (Climax) .......................................................................... 44 Stepped-on radio transmissions (1) .............................................................................................................. 46 Stepped-on radio transmissions (2) .............................................................................................................. 48 Stepped-on radio transmissions (3) .............................................................................................................. 50 Main/Standby Radio switchover ................................................................................................................... 50 Sunset dates for analogue & digital leased lines .......................................................................................... 52

3. EUROCAE WORKING GROUP 67 .............................................................................................. 55 What is EUROCAE? ..................................................................................................................................... 56 EUROCAE Working Group 67 background .................................................................................................. 58 WG67 Mission Statement ............................................................................................................................. 60 The Vienna Agreement ................................................................................................................................. 62 ED deliverables ............................................................................................................................................ 64 ICAO ACP WG-I consideration of EUROCAE ED docs ................................................................................ 66

4. VOICE OVER INTERNET PROTOCOL FUNCTIONALITY ......................................................... 69 What is Voice over IP? ................................................................................................................................. 70 Packet switched connections........................................................................................................................ 72 Packet Transport Efficiency .......................................................................................................................... 74 IP Core Network ........................................................................................................................................... 76 Voice over IP deployed in Corporate Networks ............................................................................................ 80 Signalling in an IP ATS Ground Voice Network ............................................................................................ 82 How is Voice Transported over an IP network? ............................................................................................ 84 Example- Voice Transport over an IP Network ............................................................................................. 86 Why IP version 6? ........................................................................................................................................ 88 IPv6 Packet Header Fields ........................................................................................................................... 90 IPv6 Packet header field structure ................................................................................................................ 90 User Datagram Protocol (UDP) .................................................................................................................... 92 Real-time Transport of Voice using RTP ...................................................................................................... 94


RTP Media Streams ..................................................................................................................................... 98 What is the Session Initiation Protocol (SIP)? ............................................................................................ 100 SIP History.................................................................................................................................................. 102 Why SIP signalling? .................................................................................................................................... 104 What is Session Description Protocol (SDP)? ............................................................................................ 106

5. VOIP IN AERONAUTICAL COMMUNICATIONS ...................................................................... 111 Circuit to Packet switching interworking ..................................................................................................... 112 Working towards end-to-end VoIP .............................................................................................................. 116 Example of SIP Telephone technology today ............................................................................................. 120 Functional Airspace Blocks (FABs) ............................................................................................................ 122 Today no sector delegation between ACCs ............................................................................................... 124 Sector delegation between ACCs now possible ......................................................................................... 124 Today ACCs configured with adjacent ACCs addresses only ................................................................... 128 Sector Delegation between ACCs now possible......................................................................................... 128 Last Resort communications between ACCs ............................................................................................. 130 What are Roles / Missions? ........................................................................................................................ 132 What is Sector Delegation? ........................................................................................................................ 132 Mission applied to CWP=Role(s) + Sector(s) ............................................................................................. 134 Example of Role and Sector combination to form mission applied to a CWP ............................................. 134 Configuring addresses for sector delegation .............................................................................................. 136 Example of Sector delegation from ACC1 to ACC2 .................................................................................... 142 IP network impacts for Telephone calls ...................................................................................................... 146 Communication between VCS's ................................................................................................................. 148 FAA Override Call operation ....................................................................................................................... 150 FAA OVR call Audio loop detection ............................................................................................................ 152 Impacts of VoIP on Radios ......................................................................................................................... 154 IP network impacts for Radio calls .............................................................................................................. 156 Communication between VCS's and Radios .............................................................................................. 156 What is Remote Radio Control Equipment? ............................................................................................... 158 FAA Radio Gateway Overview ................................................................................................................... 160 Impacts of VoIP on Recorders .................................................................................................................... 162 Improved Access to Real Time Information (Subscriptions) ....................................................................... 164 Migration to IP through Telephone Gateways............................................................................................. 166 Migration to IP through Radio Gateways .................................................................................................... 166 CFMU Migration to PENS ........................................................................................................................... 168

6. EUROCONTROL & FAA VOIP INITIATIVES............................................................................ 171

EUROCONTROL VoIP in ATM test specifications ..................................................................................... 172 EUROCONTROL VoIP in ATM Test Suite development ............................................................................ 174 FAA Addendums ........................................................................................................................................ 176 European Single Sky Implementation (ESSI) Objective COM11 .............................................................. 178 VoIP in ATM implementation & Transition Expert Group ............................................................................ 184

7. NETWORK OVERVIEW ............................................................................................................. 187 Network Domain Concept ........................................................................................................................... 188 Pan European Network Services - Core network & services ...................................................................... 190 Guaranteeing IP network availability - Built-in redundancy ......................................................................... 190 PENS network architecture......................................................................................................................... 192 PENS - a shared infrastructure ................................................................................................................... 192 PENS User Status ...................................................................................................................................... 194 Virtual Private Networks available on PENS............................................................................................... 196 Multiprotocol Label Switching (MPLS) ........................................................................................................ 196


VPNs over SITA MPLS Backbone .............................................................................................................. 198 MPLS path establishment through network ................................................................................................ 198 Layer 2 Virtual Leased Line (VLL) over MPLS Core network ..................................................................... 200 Layer 2 Virtual Private LAN service (VPLS) over MPLS Core network ....................................................... 200 Guaranteeing Real Time Voice over network ............................................................................................. 202

8. SESAR WP 15.2.10 VOIP VALIDATION WORK ....................................................................... 205 SESAR Programme .................................................................................................................................... 206 SESAR Master Plan for CNS activities ....................................................................................................... 208 SESAR Work packages .............................................................................................................................. 210 Validation of VoIP in ATM part of SESAR JU 15.2.10 ................................................................................ 212 SESAR JU Work Programme 2009-2014 ................................................................................................... 214 SESAR JU Description of Work- WP 15.2.10- VoIP Validation .................................................................. 216 SESAR members & WP 15 members ........................................................................................................ 218 WAN test bed network topology ................................................................................................................. 220 SESAR 15.2.10 VoIP related deliverables .................................................................................................. 222 SESAR 15.2.10 D11 Tests ......................................................................................................................... 224 VoIP validation activities (SESAR 15.2.10) ................................................................................................ 226

GLOSSARY ........................................................................................................................................ 229


Next Generation ICAO 9896 VoIP interfaces for ATS Ground Voice Network

- Part 1

Introduction


1. Introduction


Introduction to course

Course Objectives - Part 1

Introduction



Next Generation ICAO 9896 VoIP interfaces for ATS Ground Voice Network

- Part 1

ATS Ground Voice Network overview


2. ATS Ground Voice Network overview


Telephone network infrastructure overview

Todays Telephone Network Infrastructure Overview

Todays ATS Ground Voice network (AGVN) is comprised of a mix of Local Battery 2W/4W circuits (using M.1030), analogue leased lines (M.1030) used to establish calls using either ATS_R2 (MFC-R2) or ATS-No.5

analogue signalling, 64kbps co-directional digital leased lines (EN 301 288/289) with octet integrity used to

establish calls using ATS-QSIG. Sometimes E1 leased lines are used to connect E1 CAS interfaces (digital

transport of ATS-R2 protocol) or to connect E1 Multiplexers at both endpoints in order that the E1 timeslots can

be used to transport different traffic types.

The Local Battery lines are dedicated point-to-point lines, while the analogue lines used for ATS-R2/ATS-No.5

are switched lines while the 64kbps and 2Mbps digital lines allow channel switching.

The current network is designed to have direct routes (between 2 VCSs nodes only) through which calls are

normally routed and indirect routes (going via one transit VCS node maximum) through which calls are routed

in case of direct route failure or line/channel congestion. The normal number of analogue circuits or digital

channels available between two VCS nodes is proportional to the voice traffic between them. There should be

sufficient lines/channels available in order to ensure that congestion is not experienced by controllers (even

during the busiest peak traffic hour of the peak season).

In case of an emergency situation arising concerning the safety of aircraft or in the case of network congestion

when calls cant be made, it is possible to make a priority (emergency call). This has the scope of reaching the desired end-user and has the capability of interrupting other less important calls in progress in order to free-up

circuits/channels and intruding into a call-in-progress if the desired end-user is busy with a call already in-

progress.

The ATS 6-digit numbering plan exists in the switched ATS network. Every CWP is identified by a unique 6-

digit number. It is therefore important to ensure that routing tables are correctly updated in order to make

contact with the desired CWP.

A mix of VCS systems from different VCS suppliers are in operational use across Europe in ATC centres,

Approaches and Towers. The main common interface in use today is still the ATS-R2 analogue signalling

interface. ATS-QSIG digital signalling interface has started to be adopted by some ANSPs in some European

States (13 States that have signed the EUROCONTROL ECIP objective COM06), but in reality there are a

minimal number of ATS-QSIG operational lines today across Europe.

Analogue interfaces to dedicated analogue leased lines

Local Battery (LB)

Interface: 2 and 4 wire versions are available. Connected to analogue leased lines compliant with ITU -T

M.1030 and ITU-T M.1040. The lines are dedicated point-to-point lines (i.e. they are not switched by

the VCS). This interface is also nominated Ring In/Ring Out.

ATS-R2 (MFC-R2)

Interface: ATS R2 analogue interface compliant with EUROCONTROL ATS R2 and ATS No.5 protocol specification, is frequently used in the analogue part of the European AGVN. This 4 -wire

interface is connected to an analogue leased line with characteristics as defined in ITU -T

recommendation M.1030. This interface allows the lines to be switched by the VCS.

ATS-No.5 (ITU.T No.5)

Interface: ATS No.5 analogue interface compliant with EUROCONTROL ATS R2 and ATS No.5 protocol specification, are also used in the analogue part of the European AGVN. This 4 -wire interface

is connected to an analogue leased line with characteristics as defined in ITU -T recommendation

M.1030.

Central Battery (CB)

Interface: 2 wire versions are available. Connected to analogue leased lines or to standard LD/DTMF

telephones. This interface is also nominated Dial In/Ring Out.



PSTN/PABX

Interface: 2 wire interface to connect to the PSTN, PABX or to another VCS. This interface is also

nominated Ring In/Dial Out. It emulates a standard telephone set.

Loop start

Interface: 2 wire interface to connect to a telephone without a dial pad. This interface is also nominated

Loop In/Ring Out.

Digital interfaces to digital leased lines

ATS-QSIG (64kbps: 3B+D)

Interface: Is defined by the ECMA 312 and ETSI EN 301 846 standards. ATS-QSIG uses an ITU-T

G.703 co-directional interface as defined by EN 300 290. It is connected to 64kbps digital unrestricted

leased line with octet integrity as defined Standards EN 300 288/289. It can send an octet timing signal

on its transmit path and requires an octet timing signal on its receive path. The 64kbps bearer is used to transport four 16kbps sub-channels as defined by the ECMA 253 standard. Three of these channels

are used to transport compressed voice (ITU-T G.728 LD-CELP) while one is used as a signalling

channel.

2Mbps E1 CAS

Interface: Some VCS suppliers have developed the standard E1 Interface for their VCS for connection

to the PSTN, PABX or another VCS. This uses a 64kbps signalling channel (D) and thirty 64kbps media

channels (30B). These use the unstructured ITU-T G.703 interface (D2048U) with the G.704 structured

frame format superimposed on the signal. This allows timeslot 0 to be used for synchronization

purposes. The channel 16 is used as the D signalling channel and channels 1 -15 and 17-31 for audio.

Using G.704 frame structure, the interface is normally synchronized to the leased line supplied by the

Telecom Operator.

Telephone network infrastructure overview

LB

ATS-R2

ATS-QSIG

Local Battery M.1030 (2w, 4w)

Switched analogue leased lines (M.1030)

64kbps (EN 300 288/289) codirectional

digital leased line

VCSCWP

Recorder

VCS

Maintenance/

Configuration

Dedicated point-to-point leased linesLB

ATS-R2

ATS-QSIG

VCS/

E1 CAS E1 CAS

E1 digital leased line (EN 300 418/419)

ATS-No5 ATS-No5Switched analogue leased lines (M.1030)

ATS-R2

ATS-QSIG

E1 CAS

ATS-No5

VCS configuration

DA, IDA to ATC centres (DA call Performance


2Mbps ISDN Primary Rate Access (30B+D)

Interface: Some VCS suppliers have developed the standard ISDN Primary Rate Interface for their

VCS, for connection to the PSTN or a PBX. This uses a 64kbps signalling channel (D) and up to thirty

64kbps media channels (30B). These use the unstructured ITU-T G.703 interface (D2048U) with the

G.704 structured frame format superimposed on the signal.

144kbps ISDN Basic Rate Interface (2B+D)

Interface: Some VCS suppliers have developed the standard ISDN Basic Rate Interface (U-interface)

for their VCS, for connection to the PSTN. This uses a 16kbps signalling channel (D) and two 64kbps

channels (2B) for the transport of voice or data. These interfaces are often used for back -up in case of

leased line failure enabling calls to be established via the PSTN.

64kbps X.21 digital interface

Interface: Some VCS suppliers offer a X.21 data communication digital interface for their VCS. This is

an interface for public data networks, but can be used to send 64kbps voice over a private data network

using a 2Mbps E1 leased line.

Instantaneous Access (IA) interfaces

This is Instantaneous Controller-to-Controller communication between controllers within same ACC or between

an ACC and Approach or between Approach and Tower, through dedicated Instantaneous (Hot-line or

Intercom) keys or selection of a HOT function key prior to DA key establishment; IA calls have an automatic

one-way voice path to the called user.

Interface: These have dedicated proprietary IA interfaces and lines.

Signalling: The signalling between the CWP and VCS for an Instantaneous Access call is proprietary.

VCS architectures

Most VCS architectures today are distributed, decentralised and fully redundant systems using PCM

switching technology. They have been developed to prevent any critical or centralised points of failure.

A distributed processing architecture design ensures that a fault occurring in one system module does

not effect the continued operation of other parts of the system and prevents the danger of a complete

system failure. VCSs tend to have a modular design, with the number of interfaces per module kept to a

minimum (i.e. normally a maximum of two).

A decentralised switching methodology is often used in todays VCSs. VCSs which use a duplicated Switch card philosophy have one switch card as a back-up for the other (in hot stand-by) which enables

a seamless changeover in the case that one develops a fault. The line interfaces and CWPs have access to

each of these switch cards for increased resilience.

Normally all primary equipment and devices within the VCS are duplicated. This can also include the

main core operating system of the VCS that is designed to work in parallel. Any non -duplicated

equipment (i.e. peripheral line interface modules) can easily be substituted in the case of failure. These

modules can be swapped while the VCS is in operation without any effects.

Most VCSs use a non-blocking switching architecture which permits all VCS internal and external po rts

(i.e. CWP and leased lines) to have simultaneous access through it. This implies that any external

interface and CWP in the system can be connected to all other CWPs at all times and never experience a

switch blocking condition as a result of call processing or control limitations. With all internals and

external VCS ports configured, the switch matrix should not experience blocking .

Todays VCSs have their Call control either centralised or decentralised. Centralised implies that there is one central unit responsible for call control, which is normally duplicated in case of software failures.

If call control is de-centralised implies that each peripheral interface has its own call control, reducing

the risk in case of software failures. The overall picture of which calls are currently in progress within a

VCS may not be possible however.



Three different architectures are used within todays VCSs, these have been nominated Star, Ring and Ethernet/LAN type architectures.

Access Methods

Within the ATS network, there are three types of Primary Telephone Facilities by which calls can be

made known as "Access Methods"; these are:

Direct Access;

Instantaneous Access;

Indirect Access.

The VCS systems in a European AGVN should be able to make and receive network calls using these

access methods.

VCS Ground Telephony supplementary services (features)

Todays VCSs offer a range of supplementary services (features) to its users. Many of these however have been developed using proprietary signalling methods or have been customized to satisfy customer

requirements. This makes interworking of supplementary services between different VCS suppliers

impossible. Many of the following supplementary services offered by a VCS however are today utilized

only within the VCS domain.

Common appearance / Ring group A call alerts at many CWPs simultaneously

Call Hold Places a call on hold

Call Transfer Transfer a call to another CWP or to an external CWP/terminal via an external line.

Conference - With a number of internal CWPs and an external line. Many VCS allow a conference of up to 5 users;

Call Pick-up Answer a call alerting at another CWP

Call Diversion (Forwarding) Divert calls to another CWP

Group hunting A call alerts at a single CWP defined in group. If not answered after a preset time, call is directed to next CWP.

Call completion/call back on busy Allows caller to complete call to a user found to be busy, by calling them when that user is available.

Some important supplementary services relating to Priority (Emergency) calls however have been

defined in recommendations and standards in order to guarantee interoperability between different

suppliers.

Call Intrusion Attempts intrusion into a call in progress if called user is busy. This is u sed by an emergency call in order to communicate with the called user in the fastest way possible.

Call Priority Interrupt Attempts to interrupt an existing lowest protected call when there is network congestion and the emergency call lacks the resources to arrive at its destination.

The Eurocontrol ATS-R2 and ATS-No.5 signalling protocol specification originally defined the

signalling required for these supplementary services and later the ECMA 312 standard for ATS -QSIG

went further by defining them at international level.


Circuit switched connections and TDM transport efficiency

Circuit Switched Connections

A circuit switched network is one that establishes a circuit (or channel) between nodes and terminals before the

users may communicate, as if the nodes were physically connected with an electrical circuit.

The bit delay is constant during a connection, as opposed to packet switching, where packet queues may cause

varying packet transfer delay. Each circuit cannot be used by other callers until the circuit is released and a new

connection is set up. Even if no actual communication is taking place in a dedicated circuit that channel remains

unavailable to other users. Channels that are available for new calls to be set up are said to be idle.

TDM transport efficiency

Time Division Multiplexing (TDM) is the means by which multiple digital signals (or analogue signals carrying

digital data) can be carried on a single transmission path by interleaving portions of each signal in time. This

enables digitally encoded speech signals to be transmitted and switched optimally within a circuit-switched

network. A circuit switched network establishes dedicated connection(s) between the two nodes for their

exclusive use for the duration of the communication.

The slide illustrates the use of TDM circuit switched technology to transport different applications (i.e. Voice,

Legacy, LAN and video) over a single WAN link. Each application is assigned one or more timeslots within the

TDM frame structure. TDM allocates a fixed dedicated bandwidth to establish an end-to-end connection for

each medium type and this bandwidth is reserved for the medium even when it is not being used.

If an application has no data to send during its pre-established timeslot(s), the bandwidth is wasted as no useful

data is transmitted. In the case of a voice for example, there are periods of silence between phrases when the

time slot is used to transmit just silence. Also a video signal is normally transmitted as bursts of data, with no

transmission between the bursts.

Due to each application having its own dedicated timeslots allocated for transmission, there is no congestion

experienced by any of the applications.

The utilization of the overall bandwidth of a TDM transmission system is inefficient and is calculated at

between 50 and 60%.



Access

NetworkRouters

Circuit switched connections and

TDM transport efficiency

A circuit (analogue) or channel (digital) dedicated to a call from end to end;

Signal passed digitally through the core network, analogue in the access network;

Access

Network

Core Network

Trunk Switches

Local

Switch

Local

Switch

Wasted Bandwidth

Single WAN Link

LAN

Voice

Video

Legacy

PBX

Types of Traffic

Time Slot Assignments

Utilization

5060%


Direct Access Call operation

Direct Access Basic Call Type

All Ground Telephone Basic call types comply with the Primary user Ground Telephone facilities as defined in

ICAO Doc 9804 Manual on Air Traffic Services (ATS) Ground-Ground Voice Switching and Signalling edition 2002. A detailed description of the Direct Access facility is defined in ICAO document 9804.

The Direct Access (DA) Facility is one of the most important features of a VCS. It is most commonly used

between ACCs, but can be used between ACCs and Approaches and between Approaches and Towers.

DA facility operation

The operation of the Direct Access facility is described as follows:

1. The operation of a single key by the A-party is all that is required to initiate a call to the B-party. The B-party address is assigned and fixed semi-permanently in the A-party VCS. It is, thus, uniquely associated

with a particular key and each key is labelled as such.

2. Dial tone and outgoing signalling tones are not given to the A-party. Ringing/ringback tone is optional by bilateral agreement between the A-party and B-party administrations. Busy tone shall be given, if

appropriate. However, due to either the exclusive, one-to-one assignments of the keys between the A- and B-parties or the reserved capacity in the B-party dynamic display, it is abnormal for the A-party to encounter the B-party busy. This is a fundamental attribute of the direct access facility.

3. A suitable mechanism (i.e. number unobtainable/ reorder tone) shall be provided to inform the A-party, should the call fail for any reason other than because the B-party is busy.

4. The B-party is alerted to the presence of the incoming call by audio and/or visual means, as determined by the B-party VCS. The A-party identity is indicated to the B-party either by association with a key assigned

and fixed semi-permanently in the B-party VCS or by means of a dynamic display. The B-party accepts the

incoming call by means of a single action associated with a key or dynamic display.

5. Once the call has been answered, speech can be exchanged between the A and B-parties.

6. Under normal conditions the B-party can receive one or more direct access calls simultaneously and by using the A-party identities, together with a defined operational procedure or experience, the B-party will

deal with each call appropriately.

7. At the end of a call, either the A-party or the B-party shall be required to deselect/clear the call. Some implementations may require both parties to deselect/clear the call.

Implementation of the DA facility across an IP network has been included by EUROCAE WG67 in

Interoperability Standard for Voice over IP ATM components: Part 2: Telephone and its functionality also

complies with the ICAO document 9804 description.

DA call routing

A DA call can be routed on a Direct Point-to-Point Route or Direct Network route or can be switched to a

Detour route in the case that the Direct Route is congested or is out-of-service.

DA calls can however be performed using Local Battery signalling method over dedicated point-to-point

circuits, ATS QSIG digital signalling protocol, ATS R2 and ATS No.5 analogue signalling protocols in Circuit

Switched Networks.

DA call performance

The Direct Access Call Performance Criteria in a Network as defined in the ICAO Doc 9804:

a) Direct Access should meet the requirements for Direct Controller-Controller Voice Communication (DCCVC) which stipulates that communication be established between radar controllers within

2 seconds in 99% of the time.

b) The interval of 2 seconds is the delay between the calling user initiating the call and the called user receiving the call alert/indication.

c) The interval of 2 seconds applies to DA calls on a Direct Route.



Direct Access Call operation

Outgoing scenario: Press DA key on DA panel to make call to labelled destination;

No Dial tone; Ringing tone through bi-lateral agreement; Busy tone and number unobtainable mandatory;

Incoming scenario: DA call indicated by audio or visual means; Calling Identity shown on key; Call accepted by pressing DA key;

Many DA calls can be received simultaneously;

Calling or called party can clear call;

Performance (ICAO 9804): DA calls should be established within 2 seconds or less for 99% of call attempts. This is delay between caller

initiating call and called party being alerted to its presence.

VCS A

DA Panel DA Panel

VCS B

DA call signalling exchange B

Two-way speechCRA

CR


Instantaneous Access Call operation

Instantaneous Access Basic Call Type.

All Ground Telephone Basic call types comply with the Primary user Ground Telephone facilities as defined in

ICAO Doc 9804 Manual on Air Traffic Services (ATS) Ground-Ground Voice Switching and Signalling edition 2002. A detailed description of the Instantaneous Access facility is defined in ICAO document 9804.

The IA facility is most commonly used between ACCs and Approaches and between Approaches and Towers.

Today it is not used over international circuits.

The operation of the Instantaneous Access facility is described as follows:

1. The operation of a single key by the A-party is all that is required to initiate a call to the B-party. The B-party address is assigned and fixed semi-permanently in the A-party VCS. It is thus uniquely associated

with a particular key and each key is labelled as such.

Note. Some administrations may prefer that it be necessary for the A-party to sustain the key operation for the duration of the call.

2. Dial tone and outgoing signalling tones are not given to the A-party. Ringing/ringback tone is not given to the A-party. Number unobtainable/reorder tone is given to the A-party if the call fails for any reason,

including any busy conditions encountered.

3. The arrival of the call from the A-party to the B-party causes, simultaneously, the events detailed in a) to d):

a) The A-party identity is indicated to the B-party either by association with a key assigned and fixed semi-

permanently in the B-party VCS or by means of a dynamic display. Due to the usually urgent nature of

instantaneous access calls, any visual (and/or audible) alerts should be distinctive from other types of calls.

b) An audible alert is generated at the B-party VCS in accordance with the following options:

1) no audible alert;

2) an alert of fixed duration; or

3) a continuous alert requiring a silencing action by the B-party.

c) The B-party VCS automatically accepts/answers the incoming call without any intervention required by

the user. This occurs regardless of the B-party being engaged on any other type of call. Thus, B-party busy

is an abnormal situation and should result in number unobtainable/reorder tone being given to the A-party.

At this stage the speech channel from the A-party to the B-party is established. How speech from the A-

party is managed (i.e. conferenced with other speech at the B-party working position, switched to a

loudspeaker or to a split-headset) is a matter for the B-party administration to decide.

d) By bilateral agreement, the establishment of the call as detailed in c) may also result in the A-party having some monitoring facilities of the B-partys working position, including ground-ground and air-to-ground radiotelephony. This enables the A-party to exercise discretion before passing the message.

4. The B-party may respond to the A-party by activation of a key associated with the incoming call. This action only enables the return speech path and is not a new instantaneous access call. The call is cleared by the A-party only

with no effect on other calls in progress at the B-party.

Implementation of the IA facility across an IP network has been included by EUROCAE WG67 in



IA call routing

It is recommended that an IA call within an AGVN should only be routed on a Direct Point-to-Point Route

dedicated to IA calls, in order to achieve the fastest possible call establishment time. Ideally a speech path

should be rapidly established after IA key depression in order to prevent speech clipping of the first syllable of a

message. Detour Routes should not be used for IA calls.



In todays VCSs based on TDM architectures, IA call signalling between VCSs has not been standardized to date, implying that IA call interoperability between VCSs from different suppliers is improbable. WG67 ED137

doc has defined IA signalling between VCSs for the first time, implying that in the future IA call interoperability

will be possible.

The cross-border use of the IA facility should be subject to a bilateral agreement between respective ANSPs.

IA call performance

Instantaneous Access Call Performance Criteria in a Network (as defined in the ICAO Doc 9804):

a) Instantaneous Access should meet the requirements of Instantaneous Controller-Controller Voice Communication (ICCVC) which stipulates that two-way direct communication be established between

non-physically adjacent controllers within 1 second or less in 99% of the time.

b) The interval of 1 second is the delay between the calling user initiating the call and the speech path being established.

c) In practice however a network should seek the fastest call establishment time possible. Connection times should be as fast as possible if speech clipping is not to be witnessed by the called user.

Instantaneous Access Call

operation

Outgoing scenario: Press IA key on IA panel to automatically establish speech path to loudspeaker, split headset; Option to monitor

ground and radio calls in progress at called CWP before speaking;

No Dial or Ringing tone; Number unobtainable tone mandatory;

Incoming scenario: IA call arrival can be indicated by audio or visual means; automatically answered regardless of ground or radio calls in

progress;

Called CWP can return speech by pressing its IA key;

Performance (ICAO 9804): IA calls should be established within 1 second or less for 99% of call attempts.

This is delay between caller initiating call and speech path being

established.

VCS A

IA Panel IA Panel

VCS B

IA call signalling exchange B

One-way speechA

Monitoring- Radio & Tel. Speech at B


Priority DA Call operation Operation of Priority (Emergency) calls

The priority facility is a means of attaching an indicator (or flag) to a telephone call to show that it is urgent as opposed to routine. It is intended for use when it is necessary to make an urgent call concerning the safety of aircraft (i.e. an emergency situation) and to enable, if necessary, the interruption of less urgent calls in progress

at the time. The use of priority is generally agreed by bilateral agreement between administrations. The ultimate

decision and responsibility as to whether a call has priority rests with the A-party in accordance with local

operational procedures.

A description of the Priority call facility is defined in ICAO document 9804.

There are two ways in which priority can be set:

a) manually, before the call is made: before making the call, a priority key is operated on the VCS to

set the priority of the call to urgent. This method is used when the call is already known to be urgent;

b) automatic setting of priority: the priority of all calls from a particular CWP or set of keys is

pre-programmed in the VCS to be emergency. This method can be used for operational reasons when calls made from a particular CWP or key are always to be treated as urgent. An example of this is to use the

priority facility to distinguish between instantaneous access and direct access calls.

Equally, the B-party VCS should react to an incoming priority call in the following manner:

a) provide some means of indicating that a priority call has been received (e.g. special visual and/or audible

indications); and

b) allow the priority call to intrude in a call already established preceded by a warning indication.

If a priority call cannot proceed due to congestion (all available circuits, links or channels are busy), the priority

call should interrupt an established routine call (should one exist), thus allowing the priority call to proceed.

Before the established routine call is interrupted, all parties engaged in that call should receive an interrupt

warning tone.

Implementation of the Priority call facility across an IP network has been included by EUROCAE WG67 in





Priority DA Call operation

Call Priority Interrupt - Allowed by ANSP

Call Intrusion Allowed by ANSP

Routine call (Priority Level 3)



Some ANSPs isolate unwanted user during

Call intrusion, others

allow 3-party conference

Emergency !

Priority Call

Priority Level 1 call

Hears interrupt

warning tone


Telephony features

VCS Ground Telephony supplementary services (features)

Todays VCSs offer a range of supplementary services (features) to its users. Many of these however have been developed using proprietary signalling methods or have been customized to satisfy customer

requirements. This makes interworking of supplementary services between different VCS suppliers

impossible. Many of the following supplementary services offered by a VCS however are today utilized

only within the VCS domain.

Common appearance / Ring group A call alerts at many CWPs simultaneously

Call Hold Places a call on hold

Call Transfer Transfer a call to another CWP or to an external CWP/terminal via an external line.

Conference - With a number of internal CWPs and an external line. Many VCS allow a conference of up to 5 users;

Call Pick-up Answer a call alerting at another CWP

Call Diversion (Forwarding) Divert calls to another CWP. Temporarily inhibited incoming telephone calls from arriving at a position. All incoming calls will be routed to the specified

destination.

Group hunting A call alerts at a single CWP defined in group. If not answered after a preset time, call is directed to next CWP.

Call completion/call back on busy Allows caller to complete call to a user found to be busy, by calling them when that user is available.

Position Monitor: Allows authorised supervisors to listen in to telephone calls and incoming and outgoing radio traffic at another position. The monitored position usually receives no indication that

it is being monitored and there is no degradation in audio level or voice quality.

Some important supplementary services relating to Priority (Emergency) calls however have been

defined in recommendations and standards in order to guarantee interoperability between different

suppliers.

Call Intrusion Attempts intrusion into a call in progress if called user is busy. This is used by an emergency call in order to communicate with the called user in the fastest way possible.

Call Priority Interrupt Attempts to interrupt an existing lowest protected call when there is network congestion and the emergency call lacks the resources to arrive at its destination.

The Eurocontrol ATS-R2 and ATS-No.5 signalling protocol specification originally defined the

signalling required for these supplementary services and later the ECMA 312 standard for ATS -QSIG

went further by defining them at international level.



Telephony features

Following proprietary features used in VCS domain:

Common appearance / Ring group (Call alerts many CWPs simultaneously)

Call Hold, Call Transfer, Conference, Call Pick-up,

Call Diversion (Forwarding), Call Completion,

Group hunting (Call alerts single CWP defined in group & if unanswered is directed to next CWP).

Following features standardized (interoperability possible in AGVN):

Call Intrusion (Attempts Intrusion into call in progress if called user is busy).

Call Priority Interrupt (Attempts to interrupt existing lowest protected call when there is network congestion).

A priority call uses both Call Intrusion & Call Priority Interrupt

supplementary services


Radio network infrastructure overview

Todays Radio Network Infrastructure Overview

Todays ATS Ground Voice Network (AGVN) is comprised of a mix of analogue and digital point -to-point leased lines that connect each VCS directly to Remote Control Equipment (RCE) located at a

series of Remote Radio Sites where its associated Radio Transceivers, Transmitters and Receivers are

positioned. Some Ground Radio Stations have the RCE built-in its architecture. The RCE therefore

bridges the gap between the VCS and the GRS equipment.

Each Ground Radio Station is configured to operate at a set frequency. GRS Transceivers able to

transmit and receive on the same frequency, GRS Transmitters are only able to transmit while GRS

Receivers are only able to receive. Separated Transmitters and Receivers can be located at the same or

different locations.

A VCS will have a pre-defined number of frequencies covering its sectors and will hence have access to

the same pre-defined number of Ground Radio Stations, which can be doubled in the case of a redundant

architecture with Main/Standby Radios.

In the case of analogue lines, one line is required per frequency. In the case of fully redundant solutions

with Main/Standby Radios configured for same frequency, two lines are required with the Main/Standby

switching performed at the VCS side.

In the case of 64kbps or 2Mbps digital lines with the capability of transporting multiple channels

between VCS and Remote Control Equipment at the Remote Radio sites is possible, with each Tx and or

Rx channel being associated with a GRS Transmitter and/ or GRS Receiver at a pre -set frequency. In

this case the RCE acts like a Digital Access Cross-connect switch (DACS) and switches the channels

through to the individual GRS at the Remote Radio Site. In the case of fully redundant solutions with

Main/Standby Radios configured for same frequency, two digital lines are required with the

Main/Standby switching performed at the VCS side.

For analogue and digital interfaces no international body has defined a common Radio Interface and

signalling standard to be used for communication between VCSs and Ground Radio Stations

(Transceivers/Transmitters/Receivers). The analogue E&M tie-line interface has been adopted as a

common radio interface due to its simplicity and due to most GRS equipment manufacturers also having

this as its common interface.

With the migration to digital radio interfaces, although common interfaces have been used by the VCS

suppliers (i.e. E1 CAS and G.703 64kbps) for their connection to a remote RCE, the signalling methods

they employ can generally be classed as proprietary.

Due to proprietary signalling methods in use over these links, it is likely that the Remote RCE

equipment at the Ground Radio site is also supplied by the same VCS supplier. Through the RCE, a

radio interface within a VCS not only has the capability of transmitting and receiving audio, but also has

the capability to monitor and control the Radios remotely through either serial interfaces, separate

signalling/channels in the 2Mbps frame.

It is however more common to monitor and control single or multiple remote radio sites from a VCS

side, using a serial data interface (i.e. RS485, RS422 or RS232). Some radios allow monitoring and

control via an Ethernet/LAN interface. Application programs running on PCs or workstations act as the

user interface for the monitoring and control of the single and multiple GRSs at remote radio sites. These can be used for example to control Power settings, Selected Channels, Frequency Tuning, handle

Main/standby changeover and monitor Status information etc.

Today analogue/digital radio interfaces and leased lines are usually configured on a point -to-point basis

for a permanent connection between VCSs and Remote Control Equipment (RCE) at the Remote Radio sites. It is generally not possible to switch the connection of a Radio interface within the VCS to another

Ground Radio Station (pre-set to a different frequency) at the same or a different Remote radio site.

Likewise it is not possible for more than one VCS (in a Multi -supplier network) to access the same

Remote Control Equipment/Ground Radio Station. The GRS today therefore do not have call control or

switching functionality and its pre-configured frequency cant be identified by an address. Without an address scheme in operation for GRSs, it is not possible to establish connections to GRS on demand.



The network topology in use today can lead to a VCS having many point -to-point connections to many

remote radio sites. Up until now there has never been a requirement for the VCS to gain access to GRSs at Remote Radio Sites that were not its own.

The point-to-point infrastructure in operation today within the AGVN can indirectly e xplain why there

has not been a need for a common interface and signalling system to be defined for Radio interfaces

between all the VCS and GRS suppliers. This point-to-point infrastructure however implies that a VCS

suppliers radio interface will work with the RCE from the same VCS supplier, but will probably not interoperate with RCE from different suppliers.

Independent of the fact that interfaces between the VCS and RCE are analogue or digital, the interface

between the RCE and GRS equipment is still nearly always implemented through E&M interfaces, the

most common being the 4-wire E&M interface.

Today many ANSPs using VCSs from different VCS suppliers are tied to adopting different solutions for each

of their VCSs when designing their radio network topologies, with interoperability between the solutions usually

being unachievable.

Current Radio Interfaces

The current methods of radio signalling interoperability between VCSs and GRS sites employed within todays AGVN, identifies only the analogue E&M tie-line interface and the digital E1 interface using CAS as common

radio interfaces. Three 64kbps (ATS-QSIG similar) interfaces being employed as Radio Telephony interfaces

were identified, but although identical at the physical and data link layers and employing the same voice

compression algorithm, the methods employed by the individual suppliers for PTT and Squelch transportation

all differ and hence interoperability between them is impossible.

Within the present ATS Ground Voice Network, each radio frequency can either be served by:

a single radio interface (i.e. E&M, E1 CAS or 64kbps (3B+D)), providing access to a single transmitter and a single receiver, via an RCE;

a single radio interface (i.e. E&M, E1 CAS or 64kbps (3B+D)), providing access to d uplicated transmitters and receivers, via an RCE. These can either be in a main/standby configuration or

Radio network infrastructure overview

VCS configuration

Radio Access Modes: Silent,

Rx-only, Tx/Rx and Coupling;

Best Signal Selection (Rx Voting)

Cross-coupling (local frequencies only), some

proprietary network solutions allow local/remote

frequency mix

Best Transmitter Selection or

Climax (Multi-carrier off-set transmission);

M&C performed through Serial or Ethernet I/F;

Main/Standby GRS switchover etc, Power Levels etc

Lines

Point-to-point dedicated

No Switched lines

Redundant lines to

Standby GRSs

RCE

Normally from VCS supplier

Common GRS i/f=E&M 4w

M&C methods proprietary

GRS (E&M)

E&M

G.703

64kps

E1 CAS

Analogue leased line (4w, 6w)

64kbps (3B+D) co-directional digital leased line

2Mbps (30B+D) co-directional digital leased line

VCSCWP

Recorder

VCS

Maintenance/

Configuration

f1

f1 f2

f3

f1 f2

f3 ..

f30

RCE

PTT/squelch Ch.16

PTT/squelch layer2, layer 3, inband

PTT/squelch Inband tones, Outband signals

GRS

64kps

RCE

RCEE1 CAS

RCE GRS

4 wire

E&M

GRS

Used only by 1 VCS (or multi-VCSs from same supplier)

No call control or address scheme

No on-demand connections

No knowledge of coupling state

Transmission cut when no PTT signal

Some have E1 CAS I/F

Transceivers or Transmitters/Receivers


in a Best Signal Selection (receiver voting) configuration. The main/standby switching or Best Signal Selection is performed at the remote radio si te, either controlled by the RCE itself

or initiated from the VCS;

a number of radio interfaces (i.e. E&M, E1 CAS or 64kbps (3B+D), each providing access to different radios operating on the same frequency, either in a main/standby configuration or a

Best signal selection (Receiver Voting) configuration. The later is used in order to obtain wider

radio coverage, with selection taking place at the VCS;

PTT & Squelch transportation methods

Also the method used to transport PTT and Squelch signals between VCS and GRS also differs between the

various solutions available. E&M using inband tones or out-of-band signals to transport PTT on Tx path and

Squelch on Rx path. 64kbps interfaces using bits in Layer 2 frames, Layer 3 messages or inband bits robbed

from the compressed audio stream. The 2Mbps CAS E1 interface using Channel associated signalling in channel

16 to periodically transport the PTT ON/OFF bit towards the GRS on the Tx path and to receive SQUELCH

ON/OFF bit from the GRS on the Rx path.



This page is intentionally blank.


Radio Access Modes of Operation

Radio access enables a user to transmit and receive voice communications on selected radio frequencies.

Radio access at a CWP is activated by the operation of a key associated with a particular freque ncy. The

key enables a particular radio frequency to be in one of the following five modes:

Off/Idle

The Frequency has not been allocated at the CWP, hence transmission or reception are not possible.

There is however usually an unmonitored channel alarm generated by the VCS, if no other CWP is

monitoring or transmitting on this frequency.

Silent/Deselected

A Frequency has been allocated at the CWP but has not been selected. There is a visual indication at the

key configured for that frequency, if it is selected for PTT transmission at another CWP. There is a

different visual indication at that key, if a Squelch signal arrives on that same frequency, but no audio is

heard however. Many CWPs are able to watch the same frequency while in Silent mode. A single CWP

can watch different frequencies while in silent mode.

Receive only (Rx) or Monitor

When Rx or Monitor mode has been selected the User can hear any transmissions that are made on the selected frequency. At the same time the presence of the carrier f requency, regardless of speech

modulation, will also cause the A/C call (Squelch) visual indication at the key configured for that

frequency. The CWP can select whether transmissions received from aircraft are directed to either a

headset, handset or loudspeaker at the CWP.

Simultaneous reception on more than one frequency or on all frequencies assigned to a CWP is normally

possible; It is also possible for more than one CWP to monitor the same frequency.

A CWP must have audio device (headset, handset or loudspeaker) selected and physically plugged-in,

prior to being able to select a Frequency. Any frequency that has been enabled at the VCS is normally

monitored (Rx selected) by at least one (typically a supervisors) CWP.

Transmit and Receive (Tx/Rx)

When both receive and transmit (Tx/Rx) mode has been selected the user can transmit on the frequency by operating a Push-To-Talk (PTT) key. It is not possible to transmit on a frequency without receive also being selected. When a CWP transmits on a part icular frequency, besides the CWP

indicating the frequency is in use, all other CWPs with that the same frequency allocated have an

indication that the frequency is in use. Simultaneous transmission on more than one frequency is

normally possible by firstly selecting the desired frequencies for transmission and then activating the

PTT key. A frequency being used for transmission normally cant be used by other CWPs.

Some VCSs allow definition of priority levels to CWPs for frequency access, in which case if two

CWPs select the same frequency, only the CWP with the higher priority will be able to transmit. A

Priority level hierarchy is sometimes also applied to the PTT switch, such that a supervisor can take over

transmission on a selected frequency, cutting off the previous user. The PTT delay quoted by many VCS

suppliers is



Radio Access Modes of Operation

Radio access activated by key assigned a particular frequency. Key has one of following five modes:

OFF / IDLE: Frequency unallocated (Tx & Rx not possible). Unmonitored channel alarm generated if no CWP monitors/transmits on this frequency.

Silent Deselected: Frequency allocated but unselected. Visual indication if PTT pressed at another CWP or if Squelch signal arrives on frequency. No

audio heard however.

Receive only/Monitor (Rx): User hears all transmissions on selected frequency. Presence of carrier frequency causes A/C call (Squelch) visual

indication. Audio device (headset, handset or loudspeaker) must be plugged-

in, prior to being able to select a Frequency;

Transmit and Receive (Tx/Rx): Allows user to transmit on frequency by pressing PTT key. Receive must also be selected. Other CWPs receive

frequency in use indication. Simultaneous transmission on multiple

frequencies is possible. A frequency used for Tx normally cant be used by other CWPs. Priority hierarchy can be applied to CWP to gain access to

frequency (e.g. Supervisor takes over transmission from controller)

Frequency Cross coupling (Retransmssion):


Push-To-Talk (PTT)

Push-To-Talk (PTT)

When both receive and transmit (Tx/Rx) mode has been selected the User can transmit on the frequency by operating a Push-To-Talk (PTT) key. It should not be possible to transmit on a frequency without receive also being selected. The PTT operation is performed by a mechanical key which can be

either:

integrated into hand microphones

desk-mounted

floor/foot mounted switches

free or clip-on lapel switches integrated into a Headset cable

As a means of preventing the Remote Radio Transmitter from being left in a permanently on condition,

caused by the failure of the Radio interface or leased line, the PTT ON signal is normally repeated

periodically during a transmission. On interface or line failure, the PTT ON signal would therefore no

longer be sent and a pre-set timer within the RCE would expire, causing transmission on the selected

frequency to be stopped.

Performance parameters for PTT

The PTT delay parameter defined by many VCS suppliers is



Push-To-Talk (PTT)

Press PTT key to transmit on selected frequency (Tx/Rx mode only). PTT signal sent from VCS to RCE-R to activate Ground Radio Transmitter.

PTT key integrated into hand microphones, desk-mounted, floor/foot mounted switches or integrated in Headset cable.

PTT delay (sum of GTx1 to GTx5) should be


Incoming Aircraft Call detection/Squelch signal

A/C call (Squelch) Detection

The term Aircraft call (A/C) or Squelch is used to signify that the ground radio receiver has detected the

presence of a carrier signal above the background noise level. Most Ground Radio receivers can provide

a signal indicating squelch. If available, a VCS will use this signal to indicate the presence of squelch at

the CWP and switch the received audio through to the audio device (i.e. handset, headset or

loudspeaker). Various types of squelch signals may be used (e.g. ground, DC voltage).

In some cases the A/C Call indication at the CWP may be extended deliberately, by some seconds, in

order to reduce the possibility that an A/C Call of short duration is not observed by the controller.

Performance parameters for Aircraft Call (Squelch)

The Squelch delay is the delay between the recognition of a Squelch signal by the RCE -R equipment

(output by the ground radio receiver) to the lifting of any muting signal on that frequency by the VCS,

such that audio can be heard at the CWPs that have the same frequency selected.

When Silent, Receiver (Monitor) or Cross Coupling modes are being used at a CWP, the audio channel

is always active and the CWP is able to receive on the selected frequency or frequencies, even if a

Squelch signal is not present. The Squelch signal is therefore used to:

Remove (Lift) any mute signal enabled at the CWP for that frequency;

Provide a visual indication at the CWP that an Air Craft call has been received on that frequency.

The delay is therefore is not as critical as the PTT delay.

The VCS radio interface activation within the VCS (i.e. time between detecting squelch signal generated

as a result of an Air Craft Call and lifting any mute on the frequency is defined in Eurocontro l guideline

documents as



Incoming Aircraft Call

detection/Squelch signal Incoming call from aircraft at Ground Radio receiver (i.e. presence of carrier

signal above background noise level) generates Squelch signal sent to VCS.

Squelch delay: Delay to recognise signal at RCE-R, send to VCS (where muting signal on frequency is lifted) and provide visual indication at CWP so that

audio can be heard at CWPs with frequency selected.

Squelch signal sent out-of-band (i.e. E wire or D-Channel) or sent Inband (Tones or channel Bit-robbing);

When Silent, Rx only or Cross Coupling modes

used- Squelch signal

not used as audio

channel is always active.

CWP able to receive on

selected frequency

or frequencies, even

if a Squelch signal

is not present.

The delay is therefore is not as critical as the PTT delay.


Radio features - Cross Coupling (Retransmission)

Frequency Cross-coupling (Retransmission)

The Cross-coupling function causes incoming audio received on any one of the frequencies in a defined

cross-coupled group (minimum 2 frequencies) to be retransmitted on the other frequencies in the group.

Listeners who are tuned-in to one frequency can hear what is being said on other frequencies. This

function can be very useful for pilots in busy airspace. With cross-coupling it is possible to merge a

number of physical radio frequencies into a kind of logical frequency.

The purpose of frequency coupling is to help the controller to work in merged roles. This makes it easier

to split and merge roles, and it improves the controllers efficiency, avoiding simultaneous calls of pilots using different frequencies.

A Cross coupling group can be set up directly from the CWP. Unless prohibited by a restriction set in

the VCS, any frequency that has been allocated to the CWP can be included in a Cross -coupling group

unless the frequency is already in use within another cross-coupling group. A CWP indicates if it

has selected a frequency for Cross-coupling. There is a different indication at the CWP, if the frequency

has been selected by another CWP.

Any transmission by a CWP on any frequency in the group will be transmitted on all frequencies in the

group. If the key of a frequency within the group is selected while audio is being received on another

frequency in the group, the transmission usually has the higher priority and will interrupt retransmission

of the audio being received at the receiver.

The ANSP decides how many frequencies can be cross-coupled in total, the number of cross-coupling

sessions at a single CWP and for the VCS as a whole. The means of selection and control of cross -

coupling is decided by the ANSP but the following are typical opt ions:

At any CWP

At a specified supervisor CWP

By means of the system management terminal

Cross-coupling may be applied to two or more frequencies but the principles may be illustrated with

reference to two cross-coupled frequencies A and B as follows. If an aircraft transmits on frequency A it is received on the ground and cross-coupled to be re-transmitted on frequency B. For the user on the ground at the CWP where coupling has been enabled, when he/she transmits on either frequency

A or B both transmitters will be activated at the same time.

The original aircraft transmission on frequency A only is fed to the User on the CWP where the cross -coupling has been enabled. The signal received by re-transmission on any other frequency in the cross-

coupling group is not fed to the user. The received transmission on frequency A is termed the incoming frequency.

In the event of several aircraft transmitting simultaneously on cross -coupled frequencies, only the voice

signal associated with the first squelch received by the system is heard by the controller and

retransmitted on the other frequencies. In the event that the system is unable to determine which

frequency was first received, the system normally selects one as the incoming frequency and this selection shall be maintained until the end of that reception.

Example of 3 frequencies F1, F2, F3 cross-coupled at a CWP:

In the event of an aircraft call on F1, retransmission is operated on F2 and F3. Should an aircraft call

also occur on F2 during the retransmission, the corresponding voice signal (F2) shall not be

retransmitted so long as a voice signal on incoming frequency F1 is present. However as soon as there is

no voice signal on F1 and after a delay, cross coupling shall operate with F2 as the incoming frequency.

The incoming voice signal F2 shall, however, always be transmitted to all CWPs with F2 selected.



Radio features - Cross Coupling

(Retransmission) Incoming audio received on any one of

frequencies in pre-defined cross-coupled

group (minimum 2 freq.) is retransmitted on

other freq. in group. Listeners tuned-in to one

freq.can also hear other frequencies. Function

useful for pilots in busy airspace.

Tx by CWP on any frequency in group is transmitted on all frequencies in group. This

interrupts retransmission of any audio being

received.

2nd Incoming radio call not retransmitted until first terminated;

Activated by each controller independent of other controllers;

Single frequencies can be disconnected from the group;

Duplex or Simplex retransmission can be defined;

118.100124.750

VCS


Radio features - Best Signal Selection (Rx Voting)

Best Signal Selection (Receiver Voting)

Within todays AGVN when a single frequency covers a large geographic area or where the terrain causes radio signal shadows, it may be necessary to deploy several radio receivers over the area in order

to obtain satisfactory signal strengths for all aircraft positions.

Most VCSs allow a Best Signal Selection facility to be enabled (also known as Receiver Voting). This

makes it possible to automatically select the best signal available at any time.

Each receiver in a Receiver Voting group is presented at the CWP with its own frequency key. A visual

indication displays the frequency key of the receiver voted as having the best signal when signals are

received on the frequency, based on signal/noise ratio comparison. Some VCS suppliers only present the

best signal at any time to a single CWP frequency key.

The Receiver Voting function can usually be toggled on and off locally at each CWP. Whether the

function is activated or deactivated causes no effect at the other CWPs. When Receiver Voting is

deactivated at a particular CWP, all selected receivers are monitored. Some VCS suppliers offer

customised methods of controlling and presenting the Receiver Voting function at the CWPs.

However the main parameters today that are employed for monitoring the quality of the received signal

are listed below:

RSSI: The Received Signal Strength Indication is a measure of the energy of carrier, it is normally

expressed in dBm and a dedicated circuit measures it. This circuit outputs an electrical signal or a

numerical value proportional to the Received Signal Strength.

The RSSI can be used also for providing indications about the presence of activity on a Radio Channel.

This functionality is used in CSMA/CA (802.11) algorithm in order to check if a channel is free for

starting a transmission.

The RSSI is normally evaluated at the IF stage within the RX equipment. When there is no IF stage the

measurement is made directly in base-band level, where the DC level is proportional at the carrier

amplitude.

With modern Receivers, that include microprocessors, DSP and Ethernet connectivity, the processing

power is powerful enough to calculate the RSSI in real Time.

Giving the RSSI directly, in dBm, no scale calibration is requested and consequently the provided value

is absolute and manufacturer-independent.

C/N Ratio: The C/N ratio is measured in a manner similar to the way the signal -to-noise ratio (S/N) is

measured, and both specifications give an indication of the quality of a communications channel.

However, the S/N ratio specification is more meaningful in practical situations. The C/N ratio is

commonly used in satellite communications systems to point or align the receiving dish; the best dish

alignment is indicated by the maximum C/N ratio.

The C/N is specified in dB, it is the ratio between the power of the car rier of received signal and the

total received noise power. If the incoming carrier strength is Pc and the noise level is Pn, then the

carrier-to-noise ratio, C/N, in dB is given by the formula

C/N = 10 Log10 (Pc/Pn)

If the RSSI value is available, estimating the noise power in IF band, allows the calculation of the C/N

ratio in a direct way. Like the RSSI measurement, the provided value is very accurate, normally within



S/N Ratio: The SNR is defined as the ratio between the amplitude of a signal and the amplitude of noise

in audio band. It is defined for a specific frequency (e.g., 1 kHz test tone) and for a well -specified noise

band.

The SINAD (Signal to Noise Ratio and Distortion) could be generally used instead of SNR. This is a

relationship in which the distortion of the in band signal is also considered with noise.

It is easier to perform a S/N calculation in a laboratory, where a reference signal is normally available.

In a Radio receiver or in a VCS is becomes more complicated to perform this calculation in real time.

AGC voltage: In the old AM receivers, with diode demodulation, the mean value of demodulator output

voltage was normally used to drive the Automatic Gain Control (AGC) circuit. This voltage is

proportional to the carrier amplitude.

The relationship between AGC voltage and the carrier amplitude usually isn't linear, and it depends on

the circuit employed.

Moreover, the AGC voltage trend isn't absolute and it depends on the manufacturer implementation and

on the receiver model. This behaviour can be a problem and it requires the definition of a reference

table.

Therefore, this parameter is considered the least reliable to be employed in order to implement BSS

schemes.

PSD: The Power Spectral Density estimation is another method for detecting the quality of received

signal. Using the Fast Fourier Transform algorithm, the PSD can be computed easily; this is possible by

the high processing power available in modern RTX and VCSs.

Partitioning the audio band in many sub-bands, and setting a weighting for each sub-band in function of

its importance to other bands, improves the quality information provided by PSD measure.

An example of weighting in audio band filtering is the psophometric filter (defined by ITU -T), where

the frequency response is proportional to the sensibility of the human ear.

Radio features - Best Signal

Selection (Rx Voting) Sometimes wide areas or areas with signal shadows

deploy several receivers to obtain good signal

strengths for all aircraft positions.

BSS group of channels using same frequency defined by Admin terminal; Contoller can sometimes

remove single channels from group;

Best signal -calculated on S/N ratio comparison or signal strength.

Manual: each receiver in BSS channel group shown at CWP with its own frequency key. Receiver with

best signal indicated on key.

Automatic: Only best signal shown on single freq. key at any time.

BSS can be toggled ON/OFF by each controller independent of others;

Customised methods of controlling & presenting Receiver Voting function at CWPs possible;

Strong

Signal Weak

Signal

VCS


Best Transmitter Selection (Transmitter Voting)

Automatic Transmitter Selection (Transmitter Voting)

Some VCSs allow a transmitter to be associated with each receiver in a Receiver Vot ing group. The

appropriate transmitter can therefore be automatically selected as the selected receiver changes. At any

time the frequency key on the CWP indicates which transmitter is currently selected.

With this functionality however an override function normally allows the transmitter to be manually

selected.

Some VCS suppliers offer customised methods of controlling and presenting the Transmitter voting

function.

This function is not used when Climax operation is employed.

Climax (off-set transmission)

It is possible to pre-configure several combinations of transmitter groups of a radio frequency channel

for parallel transmission. This feature can be used for a large area coverage with transmitters working

with frequency offset.

The CWP can manually select between transmitting on the climax transmitter groups or individual

transmitters working on the related radio frequency channel.

This function is not used when Automatic Transmitter selection is employed.



Best Transmitter Selection (Transmitter

Voting)

Transmitter associated with each receiver in a BSS (Receiver voting) group. Appropriate

transmitter is selected automatically as

selected receiver changes. Frequency key on

CWP indicates which transmitter currently

selected.

Group of channels configured for same frequency defined by Admin. Terminal - for both

Tx & Rx.

VCS selects the best transmitter paired with the voted best receiver.

BTS enabled by each controller independent of others;

Not necessary when using climax operation.

Override function allows transmitter to be manually selected.

Strong

SignalWeak

Signal

VCS

Tx

Tx

Rx

Rx


Radio features - Multicarrier Offset Transmission (Climax)

Multi-carrier operation (CLIMAX or off-set carrier transmission)

CLIMAX is also known as Multi-carrier operation or Off-set carrier transmission is currently the only

Air-Ground Voice Communication solution to provide extended coverage beyond the inherent line of

site horizon limitation of VHF communications.. It is possible to pre -configure several combinations of

transmitter groups of a radio frequency channel for parallel transmission. This feature can be used for a

large area coverage with transmitters working with frequency offset. The CWP can manually select

between transmitting on the climax transmitter groups or individual transmitters working on the related

radio frequency channel.

The main reason for using climax is to provide adequate coverage for large and often low sectors. In

addition, climax systems are used to overcome local terrain problems (e.g. mountains) and reliability

problems.

Many en-route control frequencies are transmitted from up to 4 remote locations to r ender the coverage

as wide as possible. To eliminate the characteristic screech, known as heterodyne, when more than one

station transmits simultaneously, an offset carrier system is used. The offsets are +/ - 5 kHz for a two-

carrier system, +/- 7.5 kHz and 0 kHz for a three-carrier, and +/- 7.5 kHz and +/- 2.5 kHz for a four-

carrier system.

Today, CLIMAX operation is limited to 25 kHz channel spaced environment and therefore is a limiting

factor for the deployment of 8.33 kHz channel spaced environment. The spectrum mask for an 8.33kHz

carrier has a reduced bandwidth, when compared to that of a 25 kHz carrier however. Consequently, the

classic CLIMAX operation cannot be used in an 8.33 kHz environment.

Current development work

Documents

Next Generation ICAO 9896 VoIP Interfaces for ATS Ground Voice Network - Part 1