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Chapter 1: INTRODUCTION TO SATELLITE COMMUNICATIONS 27 Fig. 1.8: The four possible DVB-S2 constellations before physical layer scrambling. 1.4.5 Numerical details on the selected scenarios for performance evaluations This sub-Section provides some basic characteristics and numeric values for the parameters that have been used when evaluating the performance of the techniques proposed in the following Chapters of this book for the different scenarios. The details are provided below. Scenario 1: S-UMTS as well as S-HSDPA • GEO satellite • Multi-spot-beam satellite antenna • Bent-pipe satellite • Terrestrial gateway containing the scheduler (MAC layer) • Direct return link via satellite for channel quality measurements in case of point-to-point services • Mobile users (mean speed equal to 60 km/h) • GOOD-BAD Markov channel model (typically, 6 s mean GOOD duration and 2 s mean BAD duration) [29] • IP-based traffic flows with UMTS transport layer encapsulation • Traffic sources: video sources (sum of ON/OFF Markovian sources) [30] and Web sources (2-MMPP arrival process of Pareto-distributed data-grams) [31]. 28 Giovanni Giambene Scenario 2: DVB-S/DVB-RCS • GEO satellite • Single beam or multi-spot-beam satellite antenna • Bent-pipe satellite • Architecture involving an NCC and at least a GW • Fixed users • Direct return link for channel quality measurements; typically, Ka band is used (maximum capacity 2 Mbit/s) • Forward link in K band • Channel model: only troposphere effects (rain scintillation and gas) have to be considered. Basically an Additive White Gaussian Noise (AWGN) model has been adopted with a given packet error rate (uncorrelated losses) • IP-based traffic flows with MPE encapsulation and generation of packets according to the MPEG2-TS format • Traffic sources of the FTP type (elephant TCP connections). Scenario 3: LEO constellation • A Teledesic-like LEO system (the Boeing design with 288 satellites): altitude of 1375 km, and satellite capacity of 32 Mbit/s • Multi-spot-beam satellite antenna • End-users must switch from spot-beam to spot-beam and from satellite to satellite, resulting in frequent intra- and inter-satellite handovers • We assume a two-dimensional mobility model: users move in straight lines and at constant speed (satellite ground track speed composed with the Earth rotation speed) • All the spot-beam footprints are identical in shape and size (approximated by rectangles, 1790 km × 1790 km) • Traffic assumptions (study made in Chapter 8, Section 8.6): non-real-time traffic for email or FTP and real-time multimedia traffic, e.g., interactive voice and video applications. For each class: (i) new calls arrive in the foot-prints according to independent Poisson processes; (ii) call holding times are exponentially distributed. Within each traffic class, three different user types are considered that are differentiated depending on the call holding time and bit-rates. 1.5 Satellite networks A satellite network can play several roles [32]. In particular, it can be used as Access Network for final users or it can be part of the Core Network. Some examples are shown in Figure 1.9. The ETSI TC-SES/BSM (Satellite Earth Stations and Systems / Broad-band Satellite Multimedia) working group had the task to focus on IP layer Chapter 1: INTRODUCTION TO SATELLITE COMMUNICATIONS 29 Fig. 1.9: Examples for the use of satellite links in telecommunication networks. interworking, to define a new network architecture and to include alternative families of lower layer air interfaces [33]. A Broadband Satellite Multimedia (BSM) network is divided into 5 domains, as specified in ETSI TR 101 984 [32]: • User Domain, representing the group of end-users; • Access Domain that denotes the access network that is used to connect to the service provider (e.g., ADSL, UMTS, satellite); • Distribution Network: this is an intermediate network that is interposed between the access network and the core network; • Core Network, representing the backbone transport network that is used to connect the routers on a geographical area; • Content Domain that represents the area where contents and information are stored to be made available to users. The user requesting contents should access them feeling like as he/she was directly connected to the source of the information, the Content Domain; practically, many domains are traversed that are transparent to the user. Let us now consider the BSM network functions from the protocol stack standpoint (see Figure 1.10) that can involve different layers, as specified in ETSI TR 101 985 [34]: • The BSM network operates at layer 2, like a bridge. • The BSM network operates at layer 3, so that the satellite Earth stations are routers. • The BSM network operates at a layer above the 3rd one: the satellite Earth stations are gateways. In this case, these stations can perform a more accurate routing based not only on the IP datagram header, but also on information of higher layer headers. In such a case, the Earth station can implement special functions, like Performance Enhancing Proxies (PEPs) 30 Giovanni Giambene that are important in order to improve the TCP performance in satellite networks. Fig. 1.10: BSM general network architecture. Very Small Aperture Terminal (VSAT) networks are a special case of BSM networks where the user terminal employs a small antenna (i.e., VSAT) and simplified equipment so as to reduce costs. This small satellite terminal can be used for one way and/or interactive communications. VSATs can support several applications, such as: satellite news gathering, supervisory control and data acquisition, inquiry/response, TV and audio broadcasting, data distribution. VSAT networks are based on GEO satellites (typically of the bent-pipe type) according to a star topology: an Earth station acts as a hub (= gateway to the terrestrial network and master control station), receiving and transmitting all the data fluxes from/to VSATs. The forward link (from the hub to VSATs) is via GEO satellite. The return link (from VSAT to the hub) is typically via a terrestrial Public Switched Telephone Network (PSTN) link (to simplify the antenna design on the VSAT). Hence, forward and return links have an asymmetrical capacity; anyway recent advances in this field also allow the return link via satellite. Referring to the network architecture in Figure 1.10, the VSAT includes the client and the Earth station on the left; whereas, the hub coincides with the Earth station on the right. Different VSAT platforms use various technologies in order to access the satellite radio space segment and to share it among multiple users. One of the problems that VSAT networks have faced during their evolution has been the lack of compliance to any specific standards. In the last years, standardization bodies have established new standards to support satellite Internet [23]. The DVB standard has been the first one to be published, and ETSI adopted DVB-RCS for satellite return link transmissions. Another standard is IPoS (Internet Protocol over Satellite) developed by HNS (Hughes Network Systems) and Chapter 1: INTRODUCTION TO SATELLITE COMMUNICATIONS 31 standardized by ETSI. Finally, DOCSIS-S (Data Over Cable Service Interface Specification for Satellite), a modification to the DOCSIS cable-modem has been proposed for adapting it to the transmissions over satellite. Let us focus on satellite IP networks. The ETSI TC-SES/BSM working group has defined the protocol stack architecture shown in Figure 1.11 where lower layers depend on satellite system implementation (Satellite-Dependent, SD, layers) and higher layers are those typical of the Internet protocol stack (Satellite-Independent, SI, layers). These two blocks of stacked protocols are interconnected through the SI-SAP (Satellite-Independent - Service Access Point) interface. Only a small number of generic functions need to cross the SI-SAP; in particular: address resolution, resource management, traffic classes QoS. The SI-SAP interface is logically divided into three SAPs, each of them with a suitable function and security characteristics, as described in the ETSI TS 102 465 standard [35]. In particular, we have: • SI-U-SAP (User-SAP): transfer of IP packets between the users; • SI-C-SAP (Control-SAP): transfer of control data and of service signaling for SI-U-SAP; • SI-M-SAP (Management-SAP): transfer of management information. The protocol stack organization defined by TC-SES/BSM (see Figure 1.11) has been taken as the basis for the organization of the work in this book, where after a first part with introductory concepts, the second part deals with SD layers and the third part focuses on SI protocol layers. More details on the BSM protocol stack are provided in the following sub-Section. 1.5.1 SI-SAP interface overview SI-SAP defines an interface between SI upper layers and SD lower layers, that applies to all air interface families for satellite communication systems [32],[34]. SI-SAPs correspond to the endpoints of BSM bearer services. SI-SAP is used to define standard SI bearer services that are built upon lower layer transmission services. Point-to-point, point-to-multipoint, multipoint-to-multipoint and broadcast bearer services are defined as the edge-to-edge services provided by the BSM sub-network. SI-SAP provides an abstract interface allowing BSM protocols (BSM address resolution, BSM resource management, etc.) to perform over any BSM family (i.e., layer 1 and 2 technology) [33]. For traffic handling purposes, SI-SAP uses a BSM Identifier (BSM ID) and Queuing Identifiers (QIDs): • The BSM ID uniquely identifies a BSM network point of attachment and allows IP layer address resolution protocols (equivalent to Address Resolution Protocol, ARP for IPv4 and Neighbor Discovery, ND for IPv6) to be used over the BSM. ... - tailieumienphi.vn
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