VOICE COMMUNICATION SYSTEM VCS9808

Transcription

1 VOICE COMMUNICATION SYSTEM VCS9808 TECHNICAL DESCRIPTION Troy Systems Limited 264 Water Road Wembley, Middlesex HA0 1HX Copyright Troy Systems Limited

2 Troy Systems VCS9808 Voice Communication Systems Introduction The Troy VCS9808 is a relatively low cost digital voice communication system designed for Control Room and Air traffic Control environments. The earliest systems of this type were delivered in 1999 and since that time it has benefited from continuous development. As a result, current systems are state-of-the-art, but with an impressive pedigree. The VCS9808 is suitable for any application where a voice communications system is required to combine a programmable intercom system with the ability to connect to external devices, such as telephone lines, radio transmitter/receivers, etc. A further area of application lies in the provision of voice communications in simulators where it is necessary to simulate a broad range of communications facilities. Over the last 10 years many VCS9808 systems, ranging in size from two to over 100 positions, have been supplied in various forms to customers around the world. These customers include Civil, Military, Air Traffic and other types of Control centres. Training Colleges have been supplied with simplified versions for use in basic training and large simulator systems based on the VCS 9808 have been produced to accurately simulate several major production Voice Switches for training purposes. Critical aspects of the VCS9808 architecture and strategic design are fixed for reliability, performance and regulatory reasons. However, several different implementations of the core design exist in the form of operational switches and simulators. A VCS9808 system consists of PCs linked together by Ethernet connections in a star formation. A PC may be a fully configured unit for an operating position with specialised peripherals such as headsets, loudspeakers and footswitches or may be a minimal unit such as an SBC (Single Board Computer) used for controlling a link to a radio transmitter/receiver or other communications equipment. The Ethernet components are standard commercial switches and Cat 5 cable. This document sets out technical aspects of the VCS9808. For information on system features and types please see the following sections on our web site troy.co.uk. Voice Switches ATC and Control Room Systems Simulator Systems Training College Systems Back Up Systems Design Features The 9808 accepts and delivers either analogue or digital voice but internally the switch is all-digital. At the simplest level, it operates as a position-to-position intercom. It also allows the connection or simulation of any required communications devices, as for example, direct or indirect connection to radios, intercom to external systems, PABX or PSTN (Public Switched Telephone Network) connections, attachment of directly wired on-site telephones, connection to external VOIP systems, etc. The basic building block of the VCS9808 is a COTS Pentium-class motherboard housed either in a special enclosure or built into the touchscreen assembly at each operator/controller position. Additional cards are added to the basic building block to Copyright Troy Systems Limited Page 1 of pages Issue 2 September 2009

3 customise the unit to perform a particular function and for the purpose of interfacing to the peripherals required for that system. In real-world applications the interfaces to radio transmitters and receivers, PABX lines, private wire telephones and direct-access telephones are conventional in that each interface uses a separate card (in order to minimise the impact of a failure) and the cards are mounted in one or more rack mounting card cages with dual redundant power supplies. Operator positions are connected together by either a single (for simulators) or dual (for operational switches) Ethernet network, as shown in the diagram below. There is no central control unit since the operator positions form a distributed system. In real world switches the communications line cards are also connected into the same network. The Administrative PCs shown in the diagram do not participate in switching decisions and are used to store and display BIT (Built In Test) information and used by a Supervisor to assign system facilities to positions. In operational systems, which use a small number of pre-set configurations, fixed program code is installed permanently in each PCU. On the first system start-up, the Administrative PC sends a dataset giving the complete system configuration to every PCU. This dataset is stored in non-volatile storage and is not sent on subsequent startups, unless authorised configuration changes are to be made. Once the PCUs have the system configuration information, they cooperate to build up a picture of the system status and thereafter each PCU holds and updates that picture as the system operates. The Administrative PC takes no further part in the operation of the VCS, except to handle BIT information, unless the Supervisor wishes to make some permitted change, as, for example, to change the facilities assigned to a particular PCU. Because simulators used in training environments necessarily require frequent changes of configuration, including program code, a somewhat different approach is taken to getting code and configuration information to the PCUs. However, once the information is there, the PCUs operate in the same cooperative manner as described above. Copyright Troy Systems Limited Page 2 of pages Issue 2 September 2009

4 In both cases a failure of the Administrative PC has no impact on the running system, which continues to provide voice switching facilities without interruption. The switching process The core of the 9808 is a packet-based messaging system that uses either a single or dual Ethernet as the communications medium both for digital control messages and for digital voice. The proprietary protocol used on the 100 Mbits/sec Ethernet is a variant of the popular VOIP protocol and currently allows for a maximum of 192 devices (either positions or communications line cards) to be connected without contention or compression. Packets on the Ethernet are received by every station, and hence it relatively easy to provide digital recording, broadcast facilities, conferencing, etc. Although every station is physically able to receive voice and data from any other station, the determination of who can use what facilities is under the control of the. In training applications a person at the Administrative PC can establish the connections and permissions that a particular task involves. This includes dividing the system into independent parts in order to run exercises in parallel. In real-world applications these permissions are restricted by regulatory considerations. In current Simulator and Training systems the Administrative PC (called the Download PC in the case of simulators) is used for the purpose of holding system code and data for download to the individual stations. Those stations receive their programs and data via the Ethernet using a custom boot process when they are turned on or, during a training operation, when an exercise change is made. One Download PC must be present for full functionality, but a configuration already established continues to operate without a Download PC being available. The supervisory control and BIT functions for simulators are carried out in a second PC (called the System PC) attached to the Download PC by a private Ethernet link. The Download and System PCs of simulators do not participate in switching decisions and, indeed, may be absent altogether without affecting operational running, although dynamic control is not then possible. These PCs can be duplicated for critical applications, but failure of one or both PCs will have no effect on communications in a running system. In brief, for both operational and simulator systems, communications switching is carried out by a combination of the processor at any given position and the processor(s) in the other station(s). The former checks its system status tables and, if the connection requested by the operator appears to be available, it sends appropriate request messages through the Ethernet digital communications system to the other station(s). If nearsimultaneous requests are made by different operators, the processor in the relevant station arbitrates on the basis of first come, first served. Digital synchronisation In order to be able to merge multiple sources of digital voice it is necessary for the sources to be synchronised. Synchronisation could be achieved by having a single oscillator for the internal system plus a way of synchronising that oscillator to the outside world. However, this would introduce a single point of failure and hence the VCS9808 uses digital signal processing techniques at each PCU to synchronise the digital voice streams that it has to process. Ethernet protocol A standard 100Mbit/sec Ethernet using a CDMA (collision detect multiple access) protocol and with many attached stations is limited to a relatively low real data rate (less than Copyright Troy Systems Limited Page 3 of pages Issue 2 September 2009

5 10Mbit/sec). After that the number of collisions becomes excessive for any real time application, such as good quality voice transmission. The use of switching hubs alleviates, but does not eliminate this problem, so that, for an Ethernet-based system (such as the VCS 9808 or a VOIP system) to provide good bandwidth and a guaranteed high level of service, the basic protocol must be augmented by additional procedures. Hence the VCS9808 uses a proprietary protocol that ensures that each position has a dedicated channel in a time division multiplexed scheme. In this case, each position can transmit one to four voice outputs (microphone outputs) that are carried in packets sent at five millisecond intervals. Each voice output is 40 bytes long and eight bytes of control data plus eight bytes of housekeeping information are carried in the same packet. Thus, with address information, standard Ethernet overhead, and the mandatory Ethernet spacing between packets, a full packet is some 220 bytes long and occupies some 1.6 microseconds on the network. This would allow some 250 packets to be carried in each five millisecond period. For a typical small VCS with four positions and 20 radio/telephone connections, the utilisation is about 10% and the safety margins are correspondingly higher. Every voice channel operates at approximately 64 kilobits/sec and the digital control channel at 1,600 bytes/sec. This architecture ensures that the delay in transferring and switching digital voice is some 9 milliseconds, a digital voice delay which is imperceptible in practice. Note that this protocol architecture involves no voice compression and delivers good quality speech at fixed (and low) latency. Facilities This is list of typical facilities. Others may be added to order. 1. Minimum key presses needed to perform any operation. 2. Any mix of main operators, radios, PABX lines, private wires and special interfaces. 3. Any mix of headset, telephone handset and loudspeaker/microphone interfaces. 4. Split headset working. 5. Any form of position-to-position intercom (immediate break-in, call/answer, dial-up, etc.). 6. Ability to control auxiliary equipment (traffic lights, door entry-phone, etc.) via isolating relays.. PTT switching on panels and/or in-line switches and/or footswitches. 8. Dual headset interfaces for trainee/instructor operation. 9. Separate mentor override switches for radio and telephone. 10. Dual headset interfaces for live handover operation. 11. Various radio cross-coupling options (simplex, duplex, etc.) millisecond PTT/mute response time Copyright Troy Systems Limited Page 4 of pages Issue 2 September 2009

6 13. Analogue interface to external recorder (on a per channel and per position basis): 600 ohm source with variable output. Normally set at -6dB relative to line levels. 14. Full digital record/replay/archive system. (Not suitable for regulatory purposes.) 15. Broadcast to all desks. 16. Break-in to any non-radio conversation. 1. Every desk may listen to any conversation (with permission). 18. Using a touch screen for a controller interface, a number of text pages plus an index page may be stored for instant (<100ms) retrieval. 19. Loudspeaker presentation of all current RT conversations. 20. Multiple loudspeakers per position, if required. (Alternative to mixed signals on a single loudspeaker. 21. All standard telephone facilities: multi-line hold, retrieved in any order, call transfer, multi-user conferencing, shared (system-wide) stored numbers, with auto-dial recall, personal stored numbers, with auto-dial recall last number redial, timed break recall, etc. 22. PABX or other telephone lines optionally operated as "hunt groups". 23. *, #, etc. passed from VCCS to PABX as required. 24. Radio signalling interfaces supported: clean contacts, phantom earth, phantom power, automatic incoming audio envelope detection, RS232, RS422/485 and Ethernet. 25. Telephone signalling interfaces supported as standard (others added as necessary): Pulse, DTMF, AC15 and 6 wire E & M. 26. Continuous BIT (Built-In Test) with statistics. 2. Per-position access to communication lines is controlled by the Administrative PC. Technical specifications All audio levels are in db relative to a level of 0.5 volts RMS, measured using a high impedance measuring instrument and with all inputs and outputs correctly terminated. All measurements are made at a test tone level of 0dBm and with system gain settings suitable for the headset in use. Gain/Frequency Response: The gain of all speech paths within the system is within +/-3 db of the gain at 1000 Hz over the frequency range 300Hz to 3.4 KHz. (Greater bandwidths are available to order.) Harmonic Distortion: The total harmonic distortion at any single frequency between 300Hz and 3.4 KHz does not exceed 3% at all signal levels, with the input gain control adjusted to give the maximum permitted safe (BABT) audio level at the controller headset. Crosstalk: The crosstalk level on any one speech circuit within the system does not exceed -50dBm with a 1000Hz tone injected into any other circuit at a level of 10dB above test tone level. Copyright Troy Systems Limited Page 5 of pages Issue 2 September 2009

7 Noise and hum: The combined noise and hum level introduced into any connected, but inactive, speech path with inputs and outputs correctly terminated does not exceed -50 dbm when the system gain is set to match the headsets in use. Other Clean contact signalling is provided for customer-specified functions, such as remote control of signals or doors or for Overshoot alarm control and Supervisor alert control. Copyright Troy Systems Limited Page 6 of pages Issue 2 September 2009