2Way-Sat A DVB-RCS Satellite Access Network

Transcription

1 2Way-Sat A DVB-RCS Satellite Access Network by Dirk Breynaert, Newtec Brussels - 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 overview of the total Network Architecture is given, with more details on the heart of the HUB station : "The Carrier Blade". Several examples of services offered to end customers are provided. With Satellite Bandwidth being quite expensive, adequate bandwidth shaping and QoS-prioritisation is realised. Bench marking is foreseen in order to verify 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 in During 2003, more networks are being delivered worldwide. Today, many end-customers have a daily real broadband experience on our DVB-RCS Satellite 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 now adopted by a large number of manufacturers, operators, users and research institutes assembled in the Satlabs Group [4] and the DVB-RCS interoperability process is ongoing. Satlabs certification procedures are being developed. The standard has been field proven in a commercial environment [5]. 2 Way-Sat Business Model The main players in the business model are : SAP The Satellite Access Provider usually sells transponder capacity and operates the HUB. The SAP will guarantee a certain performance / price for multiple ISPs 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. ISPs The Internet Service Providers (or equivalent distributors) will sell Internet Access and / or other Value Added Services to End Customers. The ISP will guarantee performance / price for these End Customers and act as a single point of contact to manage their SIT (Satellite Interactive terminal). (iii) End-customers These are usually SMEs (Small and Medium-sized Enterprises), Internet Cafés and SOHOs which are in need of a broadband return link (upload). DVB-RCS will be most attractive for those upload users who need a combination of a high peak and a low-to-medium average return rate, since in that case the statistical multiplexing of the satellite is most effective. (iv) SIT-installers Install SITs at the End-Customer on behalf of the ISPs. (v) Network Supplier Delivers the HUB to the SAP. (vi) SIT Supplier Supplies SITs to the ISP. These SITs are approved by the Network Supplier before introduction into the Network. 1

2 The Network Architecture - POP Router A typical 2 Way-Sat Network consists of the following elements (see Figure 1) : Satellite For non-mass-market networks, Ku-band has the advantage that existing satellites can be used. For the Forward-Link ( FW ), often the 13 GHz band is used in single or dual carrier per transponder operation (low gain - ALC activated). For the Return-Link ( RT ), mainly the GHz VSAT band is used in multi-carrier, high-gain operation. For mass-market networks (for example 1 Million users) Ka-band satellites with multi-spot beams are recommended. This will certainly require the realisation of adaptive modulation and coding the FWlink, as has been standardised in DVB- S2. SIT (Satellite Interactive Terminal) A generic SIT consists of the following elements : - Outdoor Allowing remote software updates via the HUB and a series of services, such as : DNS Caching HTTP Services (with Pre-fetching and TCP acceleration) Web Caching relay NAT (Network Address Translation) And Value Added Services, such as : DHCP (to automatically assign an IP address to a host on the customer s LAN) Firewall - Optionally, a VPN Router can be provided by the End-customer. - CPE LAN (iii) HUB-station Consisting of a reflector (antenna), a masthead and a front-end (amplifier, LNA, up- and down-converter). In absence of interference from adjacent satellites at 2 Degree offset, reflector diameters as small as 75 cm can be used ; else 90, 120 or 180 cm are required. High peak RT rates (e.g. 600 to 1000 kbps) can already be achieved with 2 Watt front-ends. - Indoor Consisting of a Demodulator (DVB-S or DVB-S2), a Burst Modulator and a DVB-RCS processor, which maps IP packets (over Ethernet) to and from the DVB-RCS air interface. The Indoor is managed via the POP Router. A SIT will be assigned to one FW carrier, and will return data to the HUB over a pool of RT-carriers using MF-TDMA. The HUB is realised in a modular way and consists of the following parts : - an RF Subsystem : which usually consists of a large Antenna, a Tracking SS (Sub-System), an HPA SS and LNB SS. - a Carrier Blade SS, which realises in the FW-Link and corresponding RT - Links : - statistical multiplexing in FW & RT - bandwidth shaping & prioritisation of value added services specific to Satellite Access, like HTTP Prefetching & Acceleration and Web Caching). Note that during the HUB & SIT lifetime, there will be constant evolution in features (e.g. DVB-S2). The Carrier Blades will support upcoming features over the years to come. 2

3 - a Common SS with functionalities for all Carrier Blades : such as Time & Frequency Generation, Satellite Position Tracking, Customer Configuration Management, POP Management, etc. - an Internet Edge Router, performing terrestrial interconnection to the Internet Backbone, is usually provided by the SAP. Carrier Blade Architecture The general block diagram of a Carrier Blade SS is shown in Figure 2. The FW-Link is consisting of following elements : - the SLR (Satellite Link Router), which performs bandwidth shaping and QoS prioritisation (DiffServ) in combination with access services like HPS (HTTP Pre-fetching Server) and WCS (Web Caching Server). - The FLE ( Forward Link Combiner & IP Encapsulator) will combine the FW traffic from the SLR and signalling information from the RLP-Scheduler and encapsulate both into a DVB-S stream. - the FW_MOD (Forward Modulator), will combine the encapsulated data with NCR (Network Clock Reference) information and modulate the FW carrier according DVB-S(2). Accurate time information is also sent to the BDMs (Burst Demodulators) in the Return Link via an ASI-signal. 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) are sent to the SLR via ATM-AAL5-OC3. An ATM_MUX is required to combine several BDMs. The capacity requests from the SITs are sent by the BDMbank to an RLP-bank (Return Link Processor) via Ethernet. Note that the BDMs are outperforming competition in link budget performance. - the RLP-bank will calculate the optimum RT-burst time every superframe (around 140 msec), and sends this to all SITs via the FLE. The RLP-Statistical Multiplexing algorithms are very performing and suitable for a mix of bursty traffic (like HTTP) and real-time constant traffic (like VoIP and Video Conference). The RLP will realise bandwidth Shaping & QoS prioritisation in the Return Link. - the SLR will realise ATM-AAL5 termination for the Return Link). (iii) IP Value Added Services To minimise the effect of satellite delay, which is very annoying when browsing multi-object pages, a combination of HPS (HTTP Pre-fetching Server), TCP enhancement and WCS (Web Caching Server) is realised. For reasons of scalability, this is done in each Carrier Blade and not in the Common SS. (iv) Management Services Include following parts : - The SEMS (Satellite Earth Station Management) handles possible fault management of each element of each Carrier Blade and configures the RF layer equipment, as well as the Access Equipment (except FLE and Scheduler). - SDR (SIT Drive) The SDR collects performance data from the BDM-bank and generates a correlation with the scheduled burst time plan it receives from the RLP. This quasi-real time SIT & RLP performance data is sent to the PMT for further management. - PMT (Performance Monitor Tool) The PMT collects performance data from several elements (SDR, PMS, etc..) in the Carrier Blade (the Common part of the PMT functionality will be hosted in the CCMT). - PMS (POP Management Server) The PMS performs fault & configuration management of the SITs and sends performance data to the PMT for further processing. - CCMT (Customer Configuration Management Tool) This unit is located in the Common_SS only. The CCMT performs configuration, service and resource management tasks like e.g. enabling and disabling SITs, FW & RT bandwidth selection, etc; in interaction with a Customer Configuration Centre (CCC). User Access Service Examples DHCP service PCs connected to the Customer s LAN can automatically obtain an IP address, a DNS server address and a default Gateway address (of the POP Router). DNS Cache service The POP Router has an integrated DNS Proxy Server, which communicates with the DNS Server at the HUB (located at PMS). 3

4 (iii) transfer service The End-Customer's outgoing mails are forwarded to the Mail Transfer Server located at the Internet. Incoming mails are forwarded to the Mail Server of the End-Customer. (iv) Firewall service Default Firewall rules can be enabled. (v) VPN transparent service Creation of virtual private networks by opening the ports and protocols of the POP Router (PPTP or IPsec) is allowed. This can be used in combination with a customer provided VPN Router. (vi) BW reservation service The SAP can reserve a bandwidth on FW & RT link for real-time traffic. (This type of traffic has highest priority). (vii) Static Multicast service This allows End-Customers to join an active multicast session via their webbrowser. (viii) Video Conference service This allows End-Customers to connect a Video Conference client box to their LAN and get access to the Video Conference Servers connected to the HUB (future service). (ix) VoIP service By providing one or more connection points to the local PSTN network via a SIT and a Media Gateway, cost effective international phone calls can be realized (future service). Bandwidth Shaping & QoS prioritisation Each end-user will experience the following QoS performance, based on : The FW-pool to which he has been assigned. The FW capacity is split into several pools. In each pool, a certain peak and average bandwidth is guaranteed, with fair sharing by all users in the same pool. The HPS (HTTP Pre-fetching Server) class, to which it has been assigned. Prefetching will use push technology at certain rates. (iii) The FW-SIT bandwidth is configurable for each SIT/POP Router. (iv) The RT bandwidth, configurable for each SIT in the RLP. Prioritisation in RT is possible via DiffServ mechanism in the POP Router, which is mapped to DVB-RCS allocation classes in the RLP, like VBDC (Volume Based Dynamic Capacity) and CRA (Constant Rate Assignment). (v) To increase the RT-availability in regions with large fading, ACM (Adaptive Coding & Modulation) is realised ; whereby the SIT will dynamically change between carrier pools to maintain a quasi-error-free RT-link. Bench Marking of Satellite Access Performance End-Customer Experience One of the success factors of 2Way-Sat is the real Broadband experience the customers must feel. It is well known that the satellite delay (250 msec one-way) results in a throughput limit up to around 300 kbps for typical TCP connections. TCP accelerators help to overcome this effect and increase throughput speed, however a non-broadband response experience (e.g. 40 sec) is normally felt when multi-object pages have to be downloaded. A typical example of such a page is " with appr. 50 objects and around 200 Kbytes of contents to transfer. To solve this problem, an HPS (HTTP Pre-fetching Server) in installed in the HUB. When a user downloads this page, the HPS will fetch it over the IP Backbone (very wideband and low delay) and when it is complete, it will push it over the Access 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 15 sec. For this typical page a cumulative histogram of the HTTP Download test can easily be made over several access networks. Another important Broadband Experience is how fast files can be uploaded. A typical example is an FTP upload of a 1 MByte file. In case of a maximum RT-speed of 400 kbps, this upload will be minimum around 20 sec. Performance in case of low load is shown in Table 1 hereafter. Probability HTTP Download "nytimes.com" FTP Upload (1 MByte ) 50% 7 sec 20 sec 90% 11 sec 24 sec 99% 13 sec 28 sec Table 1 : Bench marking performance of 2Way-Sat (under low load condition) 4

5 When the load is increasing, the response time will increase also ; for example when the load increases from 0 to full forward carrier capacity, the mean download time nytimes.com increases from 7 seconds to 9 seconds. When at this load, the number of users would be doubled, the response time would also be doubled to 18 seconds. So, by monitoring the mean download time, the virtual requesting load can be measured. The response time increase limits are to be decided by each ISP. ISP Experience The ISP of course is also interested in the End Customer Broadband Experience, but he also has to pay the SAP for HUB use and satellite bandwidth consumption. So price experience will be very important. The SAP cost is mainly relative to the traffic volume and not that much to bandwidth experience (on condition that enough clients are present to allow the use of an efficient statistical multiplexing). For example : SAP cost = X 1 x (GByte-FW) + X 2 x (Gbyte -RT) e.g. X 1 = 60 Euro/GByte - FW X 2 = 90 Euro/GByte - RT The challenge of the 2Way-Sat design has been to realise a good compromise between efficient transponder usage and still good End-Customer Broadband Experience. The transmission efficiency can be measured by logging the total FW & RT traffic every day and compare it to the transponder limits. In general, it is expected that the transmission limits will be updated every month in order to follow the measured growing traffic. An example is shown hereafter in Fig. 3. 2Way-Sat Implementation Aspects HUB Implementations The basic implementation is a 2Way- Sat.Plus version, which supports max. 8 Carrier Blades and up to max SITs. Being a scalable system, it means that the satellite bandwidth and number of terminals can grow by simply adding more equipment, without interrupting the normal operation of the network. Note that for such operational systems, full automatic redundancy is foreseen, resulting into an availability 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 1000 SITs without rewiring and reconfiguration of the HUB. A third implementation will be a 2Way- Sat.Consumer version. It will require further development of adaptive modulation and coding in the FW link, as presently foreseen in the DVB-S2 standard. It is expected to be commercially deployed from 2005 onwards. SIT Implementation - For professional End-Customers (cfr. 2Way-Sat.Plus), the most successful networks [5] are using Ku-band, since many satellites are available and antenna sizes can be as small as 75 cm. However, depending on the satellite contours and potential interference from and to adjacent satellites, 90 cm cm and even 180 cm could be required. - Presently, 2 front-ends are available : one non-integrated front-end, where Feed, OMT, Block Up Converter and LNB are separate units ; and an integrated front-end, where OMT, BUC and LNB are mechanically integrated. The integrated version is suitable for networks using a large volume of identical outdoor units. The IDU and POP Router are separately housed units. In specific cases (higher volumes, same applications), it will be possible to have both integrated. - It is possible to install SITs on ships, by using conventionally 3-axis stabilised antennas, or on transportable platforms. This however requires the permanent update of the SIT position (via local GPS), and calculation of the changing transmit time. - Interoperability with other SIT manufacturers is progressing quite well and reporting of this activity is given via the SATLABS organisation. (iii) Satellites It is not required that the HUB receives its own FW link signal or accurate Satellite Position Tables from the Satellite Provider, since it will track the Satellite Position itself. In general, satellites stay within a 0.1 degree variation box. Larger variations can be allowed as long as the satellite stays within the beam-width of the SIT antennas. 5

6 Security Aspects The system has built-in security like "Static Routing & Firewalls". However for those End- Customers, accessing the is not a too big issue. When business clients are communicating with each other, often VPNs are created. 2Way-Sat has the possibility to realise a solution whereby security (VPN & IPsec, controlled by ISP) and broadband experience (acceleration) are combined. However, those clients, who want to have full control over the security part, can do this outside the 2Way-Sat domain. Due to the satellite delay, they should combine security and acceleration (or even Pre-fetching) themselves. References [1] DVB-RCS Standard : EN V131 Mar-03 [2] DVB-RCS Guideline : TR v121 Jan-03 [3] DVB-S2 DRAFT Next Generation Satellite Modulation / Coding Standard TM2860 [4] Satlabs Group Special Internet groups Satlabs [5] List of Acronyms ACM : Adaptive Coding & Modulation ALC : Automatic Level Control BDM : Burst Demodulator BUC : Block Up Converter CCC : Customer Care Centre CCMT : Customer Configuration Management Tool CPE : Customer Premises Equipment CRA : Constant Rate Assignment DHCP : Dynamic Host Configuration Protocol DNS : Domain Name Server DVB : Digital Video Broadcast FLE : FW Combiner & IP Encapsulator FW : Forward (Link) GPS : General Positioning System HPS : HTTP Pre-fetching Server HTTP : Hyper Text Transfer protocol IP : Internet Protocol ISP : Internet Service Provider LNA : Low Noise Amplifier LNB : Low Noise Block Converter MF-TDMA : Multi Frequency-Time Division Multiple Access NAT : Network Address Translation NCR : Network Clock Reference OMT : Ortho Mode Transducer PMS : POP Management Server PMT : Performance Monitor Tool QoS : Quality of Service RCS : Return Channel by Satellite RLP : Return Link Processor RT : Return (Link) SAP : Satellite Access Provider SDR : Sit Drive SIT : Satellite Interactive Terminal SLR : Satellite LInk Router SME : Small-Medium-Sized Enterprise SOHO : Small Office Home Office TCP : Transmission Control Protocol VBDC : Volume Based Dynamic Capacity VPN : Virtual Private Network VSAT : Very Small Aperture Terminal WCS : Web Caching Server About Newtec and the author Newtec is a leading SATCOM supplier for Broadband Access Networks, for Professional SATCOM systems and for itv Solutions. Dirk Breynaert is one of the founders and CEO of Newtec Cy. Contact information : Tel : Fax : web : 6

7 Satellite Forward Return Common SS RF SS Outdoor Outdoor Outdoor Carrier Blade SS1 Carrier Blade SS2 Indoor POP Box Indoor POP Box Indoor POP Box Carrier Blade SS8 Optional VPN Router Optional VPN Router Optional VPN Router CPE LAN CPE LAN CPE LAN Edge Router PC1 PCn PC1 PCn PC1 PCn IP Backbone HUB SIT 1 SIT 2 SIT n Figure 1 : 2Way-Sat Network Architecture To/from Edge Router SLR 1 SLR 2 HPS WCS FLE SDR RLP ETH OC3 FW MOD Incl.NCR Inserter ASI BDM-Bank Incl.ATM MUX Tx to RF SS Rx from RF SS PMS PMT SEMS CCMT ( in Common SS) Figure 2 : Carrier Blade SS Architecture 7

8 Transponder Loading- FW.CAR1 & RT.RLP1.1 (Yearly) January February March April May June July August September October November December Average over 1 day (kbps) WEEKLY LIMIT IN SLR1 Forward Carrier1 (kbps) Return RLP1.1 (kbps) FWlimit Weekly (kbps) Day of the Year Figure 3 : Typical example of yearly traffic growth 8