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2Way-SatA DVB-RCS Satellite Access Network

by Dirk Breynaert, NewtecBrussels - 23 September 2003

Abstract

The article describes a DVB-RCS compliant Satellite Access Network-concept called"2Way-Sat". After describing the different players in the Business Model, an overviewof the total Network Architecture is given, with more details on the heart of the HUBstation : "The Carrier Blade". Several examples of services offered to end customersare provided. With Satellite Bandwidth being quite expensive, adequate bandwidthshaping and QoS-prioritisation is realised. Bench marking is foreseen in order toverify the end performance.Finally, some implementation aspects are described, for all elements of the Network.

Introduction

Newtec is active in DVB-RCS since 1998 and

delivered its first fully commercial network in2002. During 2003, more networks are beingdelivered worldwide.Today, many end-customers have a daily realbroadband experience on our DVB-RCSSatellite Access Networks.

DVB-RCS : "The Open Standard"

In 1999, the DVB-RCS was standardised [1],[2] and complemented in 2003 by a new

standard for the Forward Link called DVB-S2[3]. The DVB standardisation process is nowadopted by a large number of manufacturers,operators, users and research institutesassembled in the Satlabs Group [4] and theDVB-RCS interoperability process is ongoing.Satlabs certification procedures are beingdeveloped. The standard has been fieldproven in a commercial environment [5].

2 Way-Sat Business Model

The main players in the business model are :

(i) SAPThe Satellite Access Provider usually

sells transponder capacity and operatesthe HUB. The SAP will guarantee acertain performance / price for multipleISPs like :

- a set of services, which are offered(e.g. Internet Access, Pre-fetching,

Web Caching, Software Download,etc)

- traffic volume (average, maximum, )- quality of service (priority, bandwidth,

availability)- number of terminals.

(ii) ISPsThe Internet Service Providers (orequivalent distributors) will sell InternetAccess and / or other Value AddedServices to End Customers.The ISP will guarantee performance /price for these End Customers and act asa single point of contact to manage theirSIT (Satellite Interactive terminal).

(iii) End-customersThese are usually SMEs (Small andMedium-sized Enterprises), InternetCafs and SOHOs which are in need of abroadband return link (upload).DVB-RCS will be most attractive for thoseupload users who need a combination ofa high peak and a low-to-medium averagereturn rate, since in that case thestatistical multiplexing of the satellite ismost effective.

(iv) SIT-installersInstall SITs at the End-Customer onbehalf of the ISPs.

(v) Network Supplier

Delivers the HUB to the SAP.

(vi) SIT SupplierSupplies SITs to the ISP. These SITs areapproved by the Network Supplier beforeintroduction into the Network.

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The Network Architecture

A typical 2 Way-Sat Network consists of thefollowing elements (see Figure 1) :

(i) SatelliteFor non-mass-market networks, Ku-bandhas the advantage that existing satellitescan be used.

For the Forward-Link (FW), often the 13GHz band is used in single or dual carrierper transponder operation (low gain - ALCactivated).For the Return-Link (RT), mainly the14.0 - 14.25 GHz VSAT band is used inmulti-carrier, high-gain operation.For mass-market networks (for example 1Million users) Ka-band satellites withmulti-spot beams are recommended.This will certainly require the realisation ofadaptive modulation and coding the FW-link, as has been standardised in DVB-S2.

(ii) SIT (Satellite Interactive Terminal)

A generic SIT consists of the followingelements :- Outdoor Unit

Consisting of a reflector (antenna), amasthead and a front-end (amplifier,LNA, up- and down-converter). Inabsence of interference from adjacentsatellites at 2 Degree offset , reflectordiameters as small as 75 cm can be

used ; else 90, 120 or 180 cm arerequired.High peak RT rates (e.g. 600 to 1000kbps) can already be achieved with 2Watt front-ends.

- Indoor Unit

Consisting of a Demodulator (DVB-S

or DVB-S2), a Burst Modulator and aDVB-RCS processor, which maps IPpackets (over Ethernet) to and fromthe DVB-RCS air interface.The Indoor Unit is managed via thePOP Router.

- POP Router

Allowing remote software updates viathe HUB and a series of services,such as :

DNS Caching

HTTP Services (with Pre-fetchingand TCP acceleration)

Web Caching

E-mail relay

NAT (Network Address Translation)

And Value Added Services, such as :

DHCP (to automatically assignan IP address to a host on thecustomers LAN)

Firewall

- Optionally, a VPN Router can beprovided by the End-customer.

- CPE LAN

(iii) HUB-station

A SIT will be assigned to one FW carrier,and will return data to the HUB over apool of RT-carriers using MF-TDMA.

The HUB is realised in a modular way andconsists of the following parts :

- an RF Subsystem : which usuallyconsists of a large Antenna, aTracking SS (Sub-System), an HPASS and LNB SS.

- a Carrier Blade SS, which realises inthe FW-Link and corresponding RT -Links :

- statistical multiplexing in FW & RT- bandwidth shaping & prioritisation

of value added services specific toSatellite Access, like HTTP Pre-fetching & Acceleration and WebCaching).

Note that during the HUB & SITlifetime, there will be constant

evolution in features (e.g. DVB-S2).The Carrier Blades will supportupcoming features over the years tocome.

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- a Common SS with functionalities forall Carrier Blades : such as Time &Frequency Generation, SatellitePosition Tracking, CustomerConfiguration Management, POPManagement, etc .

- an Internet Edge Router, performingterrestrial interconnection to theInternet Backbone, is usually

provided by the SAP.

Carrier Blade Architecture

The general block diagram of a Carrier BladeSS is shown in Figure 2.

(i) The FW-Link is consisting of followingelements :

- the SLR (Satellite Link Router), whichperforms bandwidth shaping and QoSprioritisation (DiffServ) in combinationwith access services like HPS (HTTPPre-fetching Server) and WCS (WebCaching Server).

- The FLE ( Forward Link Combiner &

IP Encapsulator) will combine the FWtraffic from the SLR and signallinginformation from the RLP-Schedulerand encapsulate both into a DVB-Sstream.

- the FW_MOD (Forward Modulator),will combine the encapsulated datawith NCR (Network Clock Reference)information and modulate the FWcarrier according DVB-S(2). Accuratetime information is also sent to theBDMs (Burst Demodulators) in theReturn Link via an ASI-signal.

(ii) The RT-Link is consisting of :- the BDM-bank (Burst Demodulator) in

an N+1 redundancy configuration. A

pool of RT carriers (in MF-TDMA) aresent to the SLR via ATM-AAL5-OC3.An ATM_MUX is required to combineseveral BDMs. The capacity requestsfrom the SITs are sent by the BDM-bank to an RLP-bank (Return LinkProcessor) via Ethernet. Note that theBDMs are outperforming competitionin link budget performance.

- the RLP-bank will calculate theoptimum RT-burst time everysuperframe (around 140 msec), andsends this to all SITs via the FLE. TheRLP-Statistical Multiplexing algorithmsare very performing and suitable for amix of bursty traffic (like HTTP) andreal-time constant traffic (like VoIP

and Video Conference).The RLP will realise bandwidthShaping & QoS prioritisation in theReturn Link.

- the SLR will realise ATM-AAL5termination for the Return Link).

(iii) IP Value Added ServicesTo minimise the effect of satellite delay,which is very annoying when browsingmulti-object pages, a combination of HPS(HTTP Pre-fetching Server), TCPenhancement and WCS (Web CachingServer) is realised. For reasons ofscalability, this is done in each CarrierBlade and not in the Common SS.

(iv) Management ServicesInclude following parts :

- The SEMS (Satellite Earth StationManagement) handles possible faultmanagement of each element of eachCarrier Blade and configures the RFlayer equipment, as well as theAccess Equipment (except FLE andScheduler).

- SDR (SIT Drive)The SDR collects performance datafrom the BDM-bank and generates acorrelation with the scheduled bursttime plan it receives from the RLP.This quasi-real time SIT & RLPperformance data is sent to the PMT

for further management.- PMT (Performance Monitor Tool)The PMT collects performance datafrom several elements (SDR, PMS,etc..) in the Carrier Blade (theCommon part of the PMT functionalitywill be hosted in the CCMT).

- PMS (POP Management Server)The PMS performs fault &configuration management of the SITsand sends performance data to thePMT for further processing.

- CCMT (Customer ConfigurationManagement Tool)This unit is located in theCommon_SS only.The CCMT performs configuration,

service and resource managementtasks like e.g. enabling and disablingSITs, FW & RT bandwidth selection,etc; in interaction with a CustomerConfiguration Centre (CCC).

User Access Service Examples

(i) DHCP servicePCs connected to the Customers LANcan automatically obtain an IP address, aDNS server address and a defaultGateway address (of the POP Router).

(ii) DNS Cache service

The POP Router has an integrated DNSProxy Server, which communicates withthe DNS Server at the HUB (located atPMS).

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(iii) E-mail transfer serviceThe End-Customer's outgoing mails areforwarded to the Mail Transfer Serverlocated at the Internet. Incoming mailsare forwarded to the Mail Server of theEnd-Customer.

(iv) Firewall serviceDefault Firewall rules can be enabled.

(v) VPN transparent serviceCreation of virtual private networks byopening the ports and protocols of thePOP Router (PPTP or IPsec) is allowed.This can be used in combination with acustomer provided VPN Router.

(vi) BW reservation serviceThe SAP can reserve a bandwidth on FW& RT link for real-time traffic. (This typeof traffic has highest priority).

(vii) Static Multicast serviceThis allows End-Customers to join anactive multicast session via their web-browser.

(viii) Video Conference serviceThis allows End-Customers to connect aVideo Conference client box to their LANand get access to the Video ConferenceServers connected to the HUB (futureservice).

(ix) VoIP serviceBy providing one or more connectionpoints to the local PSTN network via aSIT and a Media Gateway, cost effectiveinternational phone calls can be realized(future service).

Bandwidth Shaping & QoS prioritisation

Each end-user will experience the followingQoS performance, based on :

(i) The FW-pool to which he has beenassigned. The FW capacity is split intoseveral pools. In each pool, a certainpeak and average bandwidth isguaranteed, with fair sharing by all usersin the same pool.

(ii) The HPS (HTTPPre-fetching Server)class, to which it has been assigned. Pre-fetching will use push technology atcertain rates.

(iii) The FW-SIT bandwidth is configurable for

each SIT/POP Router.

(iv) The RT bandwidth, configurable for eachSIT in the RLP.Prioritisation in RT is possible via DiffServmechanism in the POP Router, which ismapped to DVB-RCS allocation classes inthe RLP, like VBDC (Volume BasedDynamic Capacity) and CRA (ConstantRate Assignment).

(v) To increase the RT-availability in regionswith large fading, ACM (Adaptive Coding& Modulation) is realised ; whereby theSIT will dynamically change betweencarrier pools to maintain a quasi-error-freeRT-link.

Bench Marking of Satellite AccessPerformance

(i) End-Customer ExperienceOne of the success factors of 2Way-Sat isthe real Broadband experience thecustomers must feel. It is well known thatthe satellite delay (250 msec one-way)results in a throughput limit up to around300 kbps for typical TCP connections.

TCP accelerators help to overcome thiseffect and increase throughput speed,however a non-Broadband responseexperience (e.g. 40 sec) is normally feltwhen multi-object pages have to bedownloaded. A typical example of such apage is " www.nytimes.com", with appr.50 objects and around 200 Kbytes ofcontents to transfer. To solve thisproblem, an HPS (HTTP Pre-fetchingServer) in installed in the HUB.When a user downloads this page, theHPS will fetch it over the IP Backbone(very wideband and low delay) and whenit is complete, it will push it over theAccess Network to the end customer.The typical result is that in 50% of the

cases, the download time is below 6 sec,in 90% below 10 sec, in 99% below 15sec. For this typical page a cumulativehistogram of the HTTP Download test caneasily be made over several accessnetworks. Another important BroadbandExperience is how fast files can beuploaded. A typical example is an FTPupload of a 1 MByte file. In case of amaximum RT-speed of 400 kbps, thisupload will be minimum around 20 sec.

Performance in case of low load is shownin Table 1 hereafter.

Probability HTTPDownload

"nytimes.com"

FTPUpload

(1 MByte )50% 7 sec 20 sec

90% 11 sec 24 sec

99% 13 sec 28 sec

Table 1 : Bench marking performance of2Way-Sat (under low load condition)

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When the load is increasing, the responsetime will increase also ; for example whenthe load increases from 0 to full forwardcarrier capacity, the mean download timenytimes.com increases from 7 secondsto 9 seconds.When at this load, the number of userswould be doubled, the response timewould also be doubled to 18 seconds.

So, by monitoring the mean downloadtime, the virtual requesting load can bemeasured.The response time increase limits are tobe decided by each ISP.

(ii) ISP ExperienceThe ISP of course is also interested in theEnd Customer Broadband Experience,but he also has to pay the SAP for HUBuse and satellite bandwidth consumption.So price experience will be veryimportant. The SAP cost is mainlyrelative to the traffic volume and not thatmuch to bandwidth experience (oncondition that enough clients are presentto allow the use of an efficient statistical

multiplexing).

For example :

SAP cost =

X1 x (GByte-FW) + X2 x (Gbyte -RT)e.g. X1 = 60 Euro/GByte - FW

X2 = 90 Euro/GByte - RT

The challenge of the 2Way-Sat designhas been to realise a good compromisebetween efficient transponder usage andstill good End-Customer BroadbandExperience.

The transmission efficiency can bemeasured by logging the total FW & RT

traffic every day and compare it to thetransponder limits. In general, it isexpected that the transmission limits willbe updated every month in order to followthe measured growing traffic.An example is shown hereafter in Fig. 3.

2Way-Sat Implementation Aspects

(i) HUB ImplementationsThe basic implementation is a 2Way-Sat.Plus version, which supports max. 8Carrier Blades and up to max. 32000SITs.Being a scalable system, it means thatthe satellite bandwidth and number ofterminals can grow by simply adding more

equipment, without interrupting the normaloperation of the network. Note that forsuch operational systems, full automaticredundancy is foreseen, resulting into anavailability better than 99.99%.A simpler implementation is the 2Way-Sat.Light version, which supports only 1

Carrier Blade and a max. of 1000 SITs.2Way-Sat Light cannot grow over 1000SITs without rewiring and reconfigurationof the HUB.

A third implementation will be a 2Way-Sat.Consumer version. It will requirefurther development of adaptivemodulation and coding in the FW link, as

presently foreseen in the DVB-S2standard. It is expected to becommercially deployed from 2005onwards.

(ii) SIT Implementation- For professional End-Customers (cfr.

2Way-Sat.Plus), the most successfulnetworks [5] are using Ku-band, sincemany satellites are available andantenna sizes can be as small as 75cm. However, depending on thesatellite contours and potentialinterference from and to adjacentsatellites, 90 cm - 120 cm and even 180cm could be required.

- Presently, 2 front-ends are available :one non-integrated front-end, whereFeed, OMT, Block Up Converter andLNB are separate units ; and anintegrated front-end, where OMT, BUCand LNB are mechanically integrated.The integrated version is suitable fornetworks using a large volume ofidentical outdoor units.The IDU and POP Router areseparately housed units. In specificcases (higher volumes, sameapplications), it will be possible to haveboth integrated.

- It is possible to install SITs on ships, byusing conventionally 3-axis stabilised

antennas, or on transportable platforms.This however requires the permanentupdate of the SIT position (via localGPS), and calculation of the changingtransmit time.

- Interoperability with other SITmanufacturers is progressing quite welland reporting of this activity is given viathe SATLABS organisation.

(iii) SatellitesIt is not required that the HUB receives itsown FW link signal or accurate SatellitePosition Tables from the SatelliteProvider, since it will track the SatellitePosition itself.

In general, satellites stay within a 0.1degree variation box. Larger variationscan be allowed as long as the satellitestays within the beam-width of the SITantennas.

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Security Aspects

The system has built-in security like "StaticRouting & Firewalls". However for those End-Customers, accessing the www.security is nota too big issue.When business clients are communicating witheach other, often VPNs are created. 2Way-Sat

has the possibility to realise a solution wherebysecurity (VPN & IPsec, controlled by ISP) andbroadband experience (acceleration) arecombined. However, those clients, who wantto have full control over the security part, cando this outside the 2Way-Sat domain. Due tothe satellite delay, they should combinesecurity and acceleration (or even Pre-fetching)themselves.

List of Acronyms

ACM : Adaptive Coding & ModulationALC : Automatic Level ControlBDM : Burst DemodulatorBUC : Block Up Converter

CCC : Customer Care CentreCCMT : Customer ConfigurationManagement Tool

CPE : Customer Premises EquipmentCRA : Constant Rate Assignment

DHCP : Dynamic Host ConfigurationProtocol

DNS : Domain Name ServerDVB : Digital Video BroadcastFLE : FW Combiner & IP EncapsulatorFW : Forward (Link)GPS : General Positioning SystemHPS : HTTP Pre-fetching ServerHTTP : Hyper Text Transfer protocolIP : Internet ProtocolISP : Internet Service Provider

LNA : Low Noise AmplifierLNB : Low Noise Block ConverterMF-TDMA : Multi Frequency-Time Division

Multiple AccessNAT : Network Address TranslationNCR : Network Clock ReferenceOMT : Ortho Mode TransducerPMS : POP Management ServerPMT : Performance Monitor ToolQoS : Quality of ServiceRCS : Return Channel by SatelliteRLP : Return Link ProcessorRT : Return (Link)SAP : Satellite Access ProviderSDR : Sit DriveSIT : Satellite Interactive TerminalSLR : Satellite LInk Router

SME : Small-Medium-Sized EnterpriseSOHO : Small Office Home OfficeTCP : Transmission Control ProtocolVBDC : Volume Based Dynamic CapacityVPN : Virtual Private NetworkVSAT : Very Small Aperture TerminalWCS : Web Caching Server

References

[1] DVB-RCS Standard : EN 301790 V131Mar-03

[2] DVB-RCS Guideline : TR 101790 v121Jan-03

[3] DVB-S2 DRAFT Next Generation SatelliteModulation / Coding Standard TM2860

[4] Satlabs Grouphttp://telecom.esa.int/telecom

Special Internet groups Satlabs

[5] www.aramiska.com

About Newtec and the author

Newtec is a leading SATCOM supplier forBroadband Access Networks, for ProfessionalSATCOM systems and for iTV Solutions.Dirk Breynaert is one of the founders and CEO

of Newtec Cy.

Contact information :Tel : +32 3 780 65 00Fax : +32 3 780 65 49web : www.newtec.be

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Figure 3 : Typical example of yearly traffic growth

Transponder Loading- FW.CAR1 & RT.RLP1.1 (Yearly)

0

2000

4000

6000

8000

10000

12000

14000

1 31 61 91 121 151 181 211 241 271 301 331

Day of the Year

Averageover

1day(kbps)

ForwardCarrier1(kbps)

Return

RLP1.1(kbps)

FWlimitWeekly(kbps)

January February March April May June July August September October November December

WEEKLY LIMIT IN SLR1