Impact of ROHC on IP Encapsulation Efficiency in a DVB-S2 GSE-Only Transmission System
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2010 5th Advanced Satellite Multimedia Systems Conference and the 11th Signal Processing for Space Communications Workshop Impact of ROHC on IP encapsulation efficiency in a DVB-S2 GSE-only transmission system N.G. Ninan, N.L. Ewald and G. Fairhurst Electronics Research Group University of Aberdeen Aberdeen, UK Abstract— The 2nd generation of Digital Video Broadcasting for Return Channel for Satellite (RCS) [12] also benefit from this Satellite (DVB-S2) standards allows the direct transmission of IP GSE-only approach, as discussed in section V. content using the Generic Stream Encapsulation (GSE) protocol, replacing the existing Transport Stream (MPEG-2 TS). This The remainder of the paper is divided as follows: a description represents an evolution from the two current encapsulation of the current DVB-S2 encapsulation methods is given in methods: the native use of the Packetized Elementary Stream section II. GSE and ROHC are briefly reviewed together with, (PES) over TS for MPEG-2 encoded applications and, the use of an envisaged GSE-only framework and its potential benefits adaptation protocols such as the Multi-Protocol Encapsulation are discussed in section III. The impact of ROHC on IP (MPE) and the Unidirectional Lightweight Encapsulation (ULE) encapsulation efficiency for the proposed GSE-only over TS for IP-based MPEG-4 encoded content. This paper architecture is shown in section IV. Then, the use of ROHC on reviews this evolution and analyses the overhead improvement DVB signalling metadata, the inclusion of security and GSE resulting from this IP-only architecture. This evaluation suitability for interactive systems (DVB-S2/RCS) are discussed considers the impact of using RObust Header Compression in section V. Finally, section VI provides the conclusions and (ROHC) on the efficiency for DVB-S2 IP/GSE-only transmission. future work. Keywords: GSE, MPEG-2, MPEG-4, ROHC, DVB-S2 II. CURRENT ENCAPSULATION METHODS I. INTRODUCTION The two current A/V encapsulation methods defined for DVB for satellite systems are described in this section. The 1st generation of Digital Video Broadcasting for Satellite (DVB-S) standards [1] adapts variable sized Audio-Visual (A/V) payload units and encapsulated Protocol Data Units A. MPEG-2/PES/TS. (PDUs) to fixed-size 188 B packets. Currently, two Fig. 1 shows the native protocol stack for A/V encapsulation st encapsulation methods are used in DVB-S and DVB-S2 for the 1 generation of DVB for satellite applications (DVB- transmission systems: MPEG encoded A/V is usually sent in S) [1]. This encapsulation may also be used in DVB-S2 Packetized Elementary Streams (PES) over TS but may also be systems. The shadowed layer in Figure 1 represents the Link sent as IP datagrams using the Real Time Protocol, Layer (LL) transmission system. encapsulated with Multi-Protocol Encapsulation (MPE) [6] or Unidirectional Lightweight Encapsulation (ULE) [7] over TS. The paper considers MPEG-2 Main Profile video using the PES format and compares this with IP-based transmission using MPEG-4. The resulting overhead is a trade-off with the benefits of MPEG-4 [8] and IP transmission. Over 100 million DVB-S receivers have now been deployed worldwide nd The 2 generation of DVB for Satellite standards (DVB-S2) Figure 1. A/V MPEG-2 TS protocol stack. [2] defines a Generic Stream Encapsulation (GSE) protocol [3]&[4] in addition to the native TS [5]. GSE allows the direct The MPEG-2 A/V specifications [5] describe the mechanisms, encapsulation of different sizes of PDU, saving the overhead syntax and semantics to encode elementary streams. Each from the adaptation protocol when using IP. The paper reviews stream is encapsulated in variable-length (up to 64 KB) this TS to GSE envisaged encapsulation evolution, considering Packetized Elementary Stream (PES) data units with a 6 B also the enhancements in capacity provided by MPEG-4 header. The PES is fragmented to fit the 184 B TS packet combined with DVB-S2 technologies. payload. MPEG-2 has been adopted as an industry standard for It is desirable that any IP encapsulation method for DVB-S2 TV (including 16:9 and HDTV). Current, bit rates for MPEG-2 achieves similar or higher efficiency than that of the native TS. encoded HDTV services are between 13 and 19 Mbps [13]. Therefore, header compression methods such as Robust Header In DVB-S [1], the MPEG TS packets are grouped into 8 packet Compression (ROHC) [9]-[11] are considered in this context. frames (1503 B). Reed Solomon (RS) coding is added to each The main focus of this paper is a unidirectional broadcast TS packet to provide Forward Error Correction (FEC) using a scenario. Although, DVB-S2 interactive systems using the RS (204,188,8) code. For satellite, the resultant bit stream is This work is partially funded by the European Space Agency (ESA) contract 22471/09/NL/AD. 978-1-4244-6833-1/10/$26.00 ©2010 IEEE 484 then interleaved and convolutional coding applied at a rate of MPEG-4 A/V frames are encapsulated using the Real-time 1/2 to 7/8, depending on the intended application and available Transport Protocol (RTP) and User Datagram Protocol (UDP) bandwidth. The digital bit stream is then modulated using into IP packets, adding 12, 8 and 20 B (for IPv4) headers, QPSK modulation. DVB-S is widely deployed. respectively. DVB-S2 systems typically present a 20-35% capacity increase MPEG-4 encoded services over IP are adapted to the TS using over DVB-S due to the powerful FEC scheme based on the MPE [6] or ULE [7]. MPE builds upon the MPEG-2 Part 6 concatenation of Bose-Chaudhuri-Hocquengham (BCH) with standard and is a widely deployed for IP encapsulation over Low Density Parity Check (LDPC) inner coding and the four DVB. Its minimum header is 12 B (plus a CRC-32) while its modulation modes available, QPSK, 8PSK, 16 PSK and 32 maximum payload size is typically 4 KB (1 KB in some APSK [2]. Future systems can use this to support Adaptive systems). However, it does not support IPv6 or ROHC without Coding Modulation (ACM) allowing transmission parameters an additional 8 B LLC/SNAP header. to be changed on a frame-by-frame basis for point-to-point ULE [7] is an IETF protocol that adds a minimum 4 B base interactive applications. The different modulation and coding modes results in 21 frame payload sizes, Data Frame Lengths header (plus a CRC-32), allows transport of IPv6 packets (in addition to IPv4 packets) without additional overhead and (DFL), in the BBFrame. supports a maximum payload of 32 KB. ULE has 3 fields in its Then, the number of TS packets per BBFrame depends on the base header in contrast to 18 of MPE and defines extension selected coding and modulation modes. Native TS headers to carry additional information; these two features encapsulation adds the PES and TS headers and, in the case of assist in improving network/receiver-processing efficiency. DVB-S2, padding, if the DFL length is not a multiple of the 188 B. The encapsulation efficiency is impacted by the MPE/ULE packets are fragmented and placed in TS packets, for transmission over DVB-S/S2 systems. The encapsulation fragmentation of PES into TS packets. overhead is respectively at least 56 B and 48 B per IP packet The impact of DVB-S2 systems on HDTV transmission can be for MPE and ULE (without a destination receiver address) in illustrated by considering a 19 Mbps HDTV MPEG-2 encoded addition to the TS headers and padding (if necessary in the service and a typical 36 MHz satellite channel with respective DVB-S2 frame). This overhead needs to be mitigated so the bit rates of 44.4 and 58.8 kbps for DVB-S and DVB-S2 using advantages of MPEG-4 and DVB-S2 are not overweighing by QPSK 7/8 and 8PSK 2/3 [14]. Two HDTV channels can be poor efficiency. allocated per transponder in DVB-S systems while up to three of these services can be transmitted by a DVB-S2 system. Several research papers have been published on encapsulation efficiencies offered by MPE & ULE [19]. B. MPEG-4/RTP/UDP/IP/MPE(ULE)/TS In line with the previous HDTV example, if we consider a Fig. 2 shows the protocol stack defined for DVB-S2 using current bit rate of 13 Mbps for MPEG-4 encoded HDTV MPEG-4 video and IP transmission. IP-based transmission is services, three and four HDTV channels can be allocated per attractive to allow convergence with Internet and also to allow transponder for DVB-S and DVB-S2 systems, respectively. extending the reach of TV over home/commercial packet Thus, the higher efficiencies achieved by MPEG-4 and DVB- networks. It also accommodates multiplexing of other IP-based S2 can double the transmission capacity for TV services, services. compared to MPEG-2/DVB-S transmission. This significant improvement has been used to justify commercial deployment of the new system. III. A DVB-S2 IP/GSE-ONLY TRANSMISSION SYSTEM In this section, the envisaged GSE-only system is reviewed and, GSE and ROHC are briefly described. The envisaged DVB-S2 GSE-only system is shown in Fig. 3. DVB-S2 specification foresees the possibility of converged IP- based transmission that supports both broadcast applications and broadband access service by adopting a common IP-based infrastructure. The use of IP allows common use of IP delivery Figure 2. MPEG-4 A/V over MPEG-2 TS protocol stack. techniques at the receiver, presenting new opportunities for integrating broadcast content with standard IP applications.