Catherine Morlet catherine.morlet@esa.int Erling Kristiansen - - PowerPoint PPT Presentation

Catherine Morlet – catherine.morlet@esa.int Erling Kristiansen – erling.kristiansen@esa.int Technical Directorate ESA/ESTEC ICAO WG-I meeting, 25-29 August 2008

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Iris Architecture options : use cases Iris Architecture options : use cases Continental airspace + oceanic Continental airspace + oceanic

European ATCC

Airport network

Airport TMA / ENR (continental area: dual link ) ORP FCI terrestrial network

System Wide Information Management (SWIM)

Satcom European ATCC

Login (no traffic)

ICAO WG-I meeting 25-29 August 2008 3

Iris Phases are aligned with SESAR schedule

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Objectives of Iris Phase 1 Objectives of Iris Phase 1

Emphasis on key areas:

• Preparation work to support the SESAR Master Plan

– Initiate development of the communication standard – Initiate identification of the most efficient satcom system architecture

• Consider non-technical issues from the start:

– Business case – Service provision and governance model – ESA hand-over after development/deployment – Validation and qualification with SESAR

• Support frequencies allocation

– Contribute to estimations of spectrum requirements (e.g. ICAO WG-F and ITU WP4C)

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Outcome of Iris Phase 1 studies Outcome of Iris Phase 1 studies

Satellite Communication System studies Analysis & Design of Satellite System studies

Avionics Preliminary Design

Ant Diplexer LNA HPA Up/Dwn Cvtr MODEM Base-Band Unit

• Com. System

Iris Phase A studies 1Q08 to 3Q08

ICOS + Phoenix Satellite System Service provision + Business case model AVISAT + Samara Preliminary Design Specifications

...011010 1001010..

Which types of protocols? Are COCR performances achievable? Link budget?... What is new/COTS? Antennas: where? how many? Target cost?...

Preliminary Design Specifications

Outcome expected

Which type of architecture? Dependability? Target cost?... Who operates what? Who procures what? Financing scheme?...

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Communication System Design activity Communication System Design activity

The activity includes:

• Trade-off of existing and possible new standards
• Analysis and consolidation of user requirement as documented by SESAR
• Communication architecture design
• Interfaces with SESAR terrestrial network
• Definition of aircraft avionics
• Design of associated hardware and software, development, validation and

qualification procedures.

• Software simulator for validation
• Preparation of ICAO standardisation and certification activities

Preparation of ICAO standard (technical basis to support update of performance-based SARPS and preparation of a Technical Manual)

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Analysis and Definition of Satellite System Analysis and Definition of Satellite System

The activity includes:

• Analysis and consolidation of user requirements as documented by SESAR,

and needs for possible ancillary services (to support business case)

• All trade-offs on architecture, payload, platform, ground segment,

deployment and operations schemes and provision of possible ancillary services, starting from existing/projected satcom infrastructure: Iris is NOT designing new satellites

• Assessment of the business case for the provision of services
• Preliminary analysis of integration of Satcom into SESAR’s overall

infrastructure (SESAR task in the future).

• Certification issues regarding the operators and operation of the satellite

system

• Definition of the operational responsibilities and liabilities in the service

provision scheme. Prepare all elements to define the satellite service provision

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Iris programme Phase 1 funding has been secured by 10 Participating States (among ESA 17 Member States) Austria France Germany Ireland Italy Norway Portugal Spain Switzerland UK

Iris Programme: Iris Programme: Participating States and users Participating States and users’ ’ involvement involvement

Iris Expert Group: users’ voice in the Programme Core members: DSNA, DFS, NATS, LFV, Nav Portugal, Avinor, HCAA, Iberia, IATA, Eurocontrol, EASA + invitees on specific topics (e.g. SITA, ARINC, Business Airlines, Low Fare Airlines, Helicopter Association, Inmarsat...)

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Next steps Next steps

• A dynamic has been created, fostering collaboration between

industry, experts and R&D entities of both space and aeronautics background

• Results of Phase A studies will be available from 3Q08
• The next milestone is ESA Ministerial Conference in November

2008 which will fund Phase 2 activities

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Conclusions on the Iris programme Conclusions on the Iris programme

• Following WG-T recommended actions, ESA is developing a set of technical options

for a new Satcom open standard that will be proposed for discussion with international parties and brought to ICAO once the technical design is sufficiently mature

• Definition of this new communication system starts from the aircraft avionics side (i.e.

the cost multiplier) rather than from the satellite side.

• An effort is being made to minimise the operational and equipment costs, and

especially to make sure that the equipment can be supplied and operated worldwide.

• ESA mandate is to support the adoption of new satellite solutions where justified for

the benefit of society. ‘Justified’ means a solid cost/benefits case, not technology for the sake of technology. The Iris programme is not about developing new satellites.

• Through cooperation with Eurocontrol and SESAR, ESA will aim to obtain wide

international support for standardisation of this new satellite communication system.

Communication System Design Architecture

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Iris scope Iris scope

Iris

ATN/IPS ATN/IPS

ATN/OSI

ATN/IPS

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Objectives Phase A Objectives Phase A

• Analysis of options and initial trade-offs for the architecture of

the communication system and the communication protocols in the frame of SESAR technical and operation concept – Characterise user requirements (technical and commercial) – Characterise the most efficient solution to meet

» Safety, » Seamless integration, » Interoperability requirements

– Characterise the new avionics and its cost – Characterise the ground equipments and cost – Define developments required

• In Phase B, the design of a new or adapted satellite-based

communication standard is targeted

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Key features Phase A Key features Phase A

• Evaluation of capacity needs for 2025
• Communication design suitable for all type of space segment

architecture (satellite orbit, number of beams, frequency of the fixed link…) and supporting voice and data communication using limited spectrum

• Architecture flexible and scalable to be extended to any region
• f the world with any ground segment constraint (e.g. number
• f ground stations)
• Avionics cost to be kept low. Key parameters are the operation

mode (mono or multi-frequency) and the technology to limit power consumption and heating

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Analysis ATM traffic profile Analysis ATM traffic profile

• The design shall be optimised for the traffic profile expected

– Message length – Quality of service – Safety criticality – Security – Bandwidth occupancy

• Initial work is performed to characterise the ATM traffic

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Analysis ATM traffic profile (2) Analysis ATM traffic profile (2)

• Example message sizes on the forward link:

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Analysis ATM traffic profile (3) Analysis ATM traffic profile (3)

• Example message sizes on the return link:

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Analysis ATM traffic profile (4) Analysis ATM traffic profile (4)

• Aircraft traffic profile (without transport layer acknowledgments)

for the forward link (similar on the return link)

200 400 600 800 1000 1200 1400 53552.812 53878.057 54504.129 54812.877 55084.868 5 Time in s econds Length message

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SESAR dual link concept SESAR dual link concept

• SESAR prescribes the simultaneous availability of two independent A/G

links

• It is not yet clear how this is to be operated. The following general options

seem to present themselves: 1. Logged on to one link. The other in stand-by, not logged on 2. Logged on to both links, but using one only at a given time. The backup is ready for immediate use 3. Logged on to both links, sending some traffic on one link, some on the

• ther according to some rule

4. Logged on to both links, sending all traffic in parallel on both

• Option 4 is clearly the most reliable, but also the most resource-hungry

Selected Subjects on Iris Satcom

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Satellite Satellite as as an an ATN subnet ATN subnet

• The Iris satcom system should be an ATN subnet

– Transparently transports networks layer packets of ATN/OSI

• r ATN/IPS
• However, some performance issues that deserve attention

– Limited bandwidth and spectrum available – Relatively expensive medium

» Necessitates efficient use of available bandwidth

– Inherent delay larger than terrestrial systems

» Difficult to meet some performance targets

• In the following slides, highlight several issues that were

encountered when analyzing the requirements for the Iris system

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ATM ATM traffic traffic profile profile

• The ATM traffic profile is unusual:

– Relatively infrequent, mainly short data messages – Very short voice exchanges

» Push-to-talk style communication

• The traffic is inelastic

– Traffic is created by events (or clock “ticks”) – Stringent real time requirements on communication

• This profile is different from most Internet-style communication

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Example Example message message sizes sizes FWD FWD

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Example Example message message sizes sizes RL RL

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The The satellite satellite link link

• The satellite link is the bottleneck link in the overall path
• The satellite link is shared by many aircraft
• Flow and congestion control resides in end systems

– End systems have no direct knowledge of the bottleneck load – They must rely on transport protocols like TCP and OSI TP4 – Or UDP (if applicable to a future system)

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Reliable Reliable transport transport protocols protocols

• TCP (and TP4) was designed for transfer of large files
• TCP is an elastic protocol

– The speed of information exchange is determined by the TP – The TP can slow down the source in case of congestion

• There is a fundamental incompatibility between

– Inelastic traffic sources And – Elastic transport protocols

• Real time performance requirements can only be met reliably

by over-provisioning of communication resources

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TCP TCP properties properties

• TCP is a very complex protocol

– Optimized for relatively long, continuous data flows – Not well adjusted to the ATM traffic profile of very thin traffic with large gaps of silence

» Congestion control, using ACKs, assumes more or less continuous traffic » Timers and protocol windows may behave in unusual ways

– Many different flavours of TCP – Behaviour sensitive to parameter settings

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UDP UDP

• UDP is an unreliable datagram transport protocol

– In case of congestion, packets will simply be dropped by routers – No recovery of lost data

• UDP is compatible with inelastic traffic sources

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UDP (2) UDP (2)

• But

– UDP packets will be dropped at random (possibly respecting priority) – If one packet of a multi-packet message is lost, the message is lost – Queues are mostly drop-tail

» New data arriving will be dropped in favour of old (possibly even expired) data waiting in the queue » Short queues needed, otherwise packets may massively exceed deadline while in queue

– Packets may arrive out of sequence

» Applications must be capable of dealing with this

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Congestion Congestion control control

• One has to assume that congestion will happen
• And it will happen when least wanted

– Extra communication traffic load due to abnormal air traffic situations, accidents, incidents

• The incidence rate can be reduced by providing more

bandwidth – But it cannot affordably be reduced to zero

• Due to the inelastic nature of the traffic, there is only one way to

handle congestion: Discard traffic – At random – Or intelligently

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Congestion Congestion control control (2) (2)

• End-to-end TCP

– There is nothing the satcom system can do about TCP congestion – Any packet dropped will be retransmitted by TCP in the end systems – TCP congestion collapse is a realistic possibility

• UDP

– UDP will drop packets if queues overflow

» Packets are dropped at random

– It is left to the end systems to detect packet loss and take recovery action

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Protocol overhead Protocol overhead

• For many messages, protocol overhead is (much) larger that

the data payload – E.g. ACL message payload is ~16 bytes data, ~75 bytes of headers (Assuming ATN/OSI application, transport, network layer - numbers are similar for ATN/IPS) – If network layer security (e.g. IPsec) is used, large additional header – TCP will typically respond with one ACK for each packet sent (one or more ACKs per message, depending on length)

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Multicast Multicast

• Multicast currently not foreseen for terrestrial systems
• Not foreseen in ATN/OSI
• Satcom is ideally suited for multicast

– Large coverage per beam, even for spot beams – Huge bandwidth saving w.r.t. point-to-point delivery of the same content to a community of aircraft

• Is multicast foreseen in ATN/IPS?

– Should it be?

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Voice service Voice service

• Relatively little is known about voice service requirements
• Our understanding is

– Push-to-talk style service – Uni-directional voice “messages” with a typical duration of 3

• 10 seconds

– Occasional longer exchanges

• Voice may arrive to the satellite gateways as VoIP, or in other

formats – VoIP has a large overhead, but transcoding may be feasible – VoIPsec would have even larger overhead, and may not be transcodeable

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Security Security issues issues

• Which protocol layer?

– Application layer (as in ATN/OSI) – Transport layer (SSL) – Network layer (IPsec) – Link layer (link specific)

• What is the scope?

– True end-to-end – Per sub-network

• Which security functions?

– Authentication?

» Exactly who/what is authenticated?

– Encryption?

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Security Security issues (2) issues (2)

• End-to-end IPsec can be carried transparently
• But

– Very expensive in overhead – Prevents any kind of performance enhancing gateways to the satcom system that

» Need to look into higher layer headers » May or may not be capable of preserving end-to-end authentication

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Performance Performance enhancing enhancing gateways gateways

• Gateways at the ground and airborne interface points to the

satcom system can – Reduce overhead by using shorter local headers, re-creating

• riginal headers at destination gateway

– Perform optimized scheduling independently of the end system transport protocol – In case of congestion, discard traffic intelligently according to set rules, rather than at random

• A gateway can be a proxy or an explicit network element

– Are proxies permissible in safety critical systems?

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Performance Performance Enhancing Enhancing gateways gateways (2) (2)

• Transport layer gateway (“PEP”)

– Can optimize headers, scheduling, transport over satellite link – Cannot relieve congestion, since it must preserve the end-to- end reliable transport

• Application layer gateway (“AGW”)

– Can optimize headers, scheduling, transport over satellite link – Can relieve congestion by selectively discarding messages – Drawback: Must be aware of message formats

» Must be updated if message formats change

• Both PEP and AGW are incompatible with end-to-end encryption

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Mobility Mobility issues issues

• Two types of mobility

– External to the satcom system

» Changes in interworking point between satcom and terrestrial (or airborne) network » This is largely transparent to the satcom system, except maybe some signalling across the interworking interface » Who initiates handover and based on which criteria?

– Internal to the satcom system

» Moving traffic between different carriers or beams, e.g for load balancing » Does not change the interworking point » This is an internal satcom issue that is invisible outside the satcom system.

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QoS QoS/ /CoS CoS issues issues

• ATM QoS is different from Internet QoS like Diffserv/Intsrv

– CoS classes are defined in COCR in terms of

» Delay » Availability » Continuity

– For a satcom system, this translates into

» Dimensioning (to meet delay requirements for a specified load) » Queue management (to deal with priorities) » Redundancy management (to meet availability requirements) » Dealing with corrupted and lost packets (to meet continuity requirements » Prevent congestion collapse in case of traffic peaks (we haven’t quite come to grips with this yet)

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QoS QoS/ /CoS CoS issues (2) issues (2)

• In contrast, Internet QoS is intended for

– Providing some guarantees of throughput, delay and jitter etc for more or less continuous flows

» But ATM traffic is highly bursty

– Preventing a flow from taking more than the agreed BW by policing, traffic shaping

» In ATM, if an aircraft uses more than usual BW, it is likely because it is dealing with a problem. So the last thing one wants is to restrict its communication

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Conclusions Conclusions

• ATM traffic profile is unusual
• Performance and QoS requirements are unusual
• Satellite bandwidth is limited and expensive
• Therefore, concepts from OSI and Internet protocols have to be

applied with caution

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ESA Iris Programme ESA Iris System Design Studies Franco.Ongaro@esa.int Catherine.Morlet@esa.int Nathalie.Ricard@esa.int Domenico.Mignolo@esa.int www.telecom.esa.int/iris Directorate of Telecommunication & Integrated Applications European Space Agency

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