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Next Generation ICAO 9896 VoIP interfaces
for ATS Ground Voice Network - Part 1
Training Documentation
2 Copyright 2014 JSP-Teleconsultancy
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.
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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
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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
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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
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Next Generation ICAO 9896 VoIP interfaces for ATS Ground Voice Network
- Part 1
Introduction
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1. Introduction
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Introduction to course
Course Objectives - Part 1
Introduction
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10 Copyright 2014 JSP-Teleconsultancy
Course Objectives - Part 2
Course Objectives - Part 3
Introduction
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Next Generation ICAO 9896 VoIP interfaces for ATS Ground Voice Network
- Part 1
ATS Ground Voice Network overview
Produced by JSP Teleconsultancy 13
2. ATS Ground Voice Network overview
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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.
ATS Ground Voice Network overview
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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
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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.
ATS Ground Voice Network overview
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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.
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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%.
ATS Ground Voice Network overview
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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%
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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.
ATS Ground Voice Network overview
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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
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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
Interoperability Standard for Voice over IP ATM components: Part 2: Telephone and its functionality also
complies with the ICAO document 9804 description.
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.
ATS Ground Voice Network overview
Produced by JSP Teleconsultancy 23
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
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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
Interoperability Standard for Voice over IP ATM components: Part 2: Telephone and its functionality also
complies with the ICAO document 9804 description.
ATS Ground Voice Network overview
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Priority DA Call operation
Call Priority Interrupt - Allowed by ANSP
Call Intrusion Allowed by ANSP
Routine call (Priority Level 3)
Routine call (Priority Level 2)
Routine call (Priority Level 2)
Some ANSPs isolate unwanted user during
Call intrusion, others
allow 3-party conference
Emergency !
Priority Call
Priority Level 1 call
Hears interrupt
warning tone
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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.
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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
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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.
ATS Ground Voice Network overview
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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
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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.
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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
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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):
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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
ATS Ground Voice Network overview
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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
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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
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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.
38 Copyright 2014 JSP-Teleconsultancy
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.
ATS Ground Voice Network overview
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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
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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
ATS Ground Voice Network overview
Produced by JSP Teleconsultancy 41
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
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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.
ATS Ground Voice Network overview
Produced by JSP Teleconsultancy 43
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
44 Copyright 2014 JSP-Teleconsultancy
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