Chapter 1: INTRODUCTION TO SATELLITE COMMUNICATIONS 17
MPEG2-TS are of the TDM type. In the channel adaptation section, packets are processed in several steps, such as: channel encoding (outer Reed-Solomon coding, convolutional interleaver, inner convolutional encoding, puncturing), base-band shaping of impulses, and QPSK modulation. The resulting DVB-S transmissions via satellite are very robust, considering a minimum BER of about 10−11. As an example, a typical data rate of about 38 Mbit/s is obtained with modern satellite transponders that have a bandwidth of about 33 MHz .
1.4.3 DVB-RCS standard
One of the reasons for the deﬁnition of a DVB standard with satellite return channel (DVB - Return Channel via Satellite, DVB-RCS) has been the increasing request of interactive applications and services with major informative volumes (1) that could not be achieved with a DVB-S-based system, where the return channel (realized through a terrestrial link via modem) cannot permit an adequate bit-rate capacity (maximum 64 kbit/s).
The speciﬁcations of DVB-RCS use and modify the DVB-S ones ,; moreover, they are independent of frequency, making easier to realize network and security mechanisms with an eﬃcient transport layer. The DVB-S channel has been named Forward Channel, while the Return Channel is related to the link from the end-user back to the content network (see Figure 1.4). The return channel has a variable bit-rate up to a maximum of 2 Mbit/s and can dynamically assign its time-frequency resources (according to an MF-TDMA air interface) to the requesting terminals. The Return Channel Satellite Terminal (RCST) transmission capacity is constrained. According to the standardization, RCSTs can be single-user (144-384 kbit/s) or corporate (2 Mbit/s).
The standard , deﬁnes a reference model for the Interactive Satellite Network (ISN) architecture, composed of a certain number of RCSTs, a GEO bent-pipe satellite, and the following elements:
• Network Control Center (NCC): it provides Control and Monitoring Functions (CMF); moreover, it produces timing & control signals that one or several Feeder Stations transmit for the ISN operations.
• Traﬃc Gateway (GW): it is a router that sends/receives data to/from the RCSTs, managing the exchange of data with public, proprietary and private providers.
• Feeder: it is the Earth station that transmits Forward Link (DVB-S) signal, where user data and ISN timing & control signals are multiplexed together.
1 Recently, also other systems have been standardized for broadband satellite access such as DOCSIS-S and IPoS .
18 Giovanni Giambene
Figure 1.4 shows a simpliﬁed version of the DVB-S/DVB-RCS system architecture where NCC, GW and Feeder are ‘collapsed’ into the NCC, i.e., in a single Earth station.
Fig. 1.4: Example of DVB-S/DVB-RCS system architecture.
Air interface characteristics of DVB-RCS
In order to operate successfully an ISN, it is important to use the satellite resources as eﬃciently as possible. Therefore, Bandwidth on Demand (BoD) schemes (also known with the name of Demand Assignment Multiple Access, DAMA, techniques) have been introduced in the DVB-RCS standardization in order to improve the utilization of satellite resources in the presence of distinct traﬃc classes.
The DVB-RCS standard speciﬁes a MAC layer in which the NCC controls the allocation of the uplink capacity for RCST transmissions. BoD is deﬁned as a set of MAC protocols and algorithms that allow an RCST to request resources to the NCC, when the RCST has traﬃc to pass to GW.
Return link transmissions are based on an MF-TDMA air interface, where RCSTs transmit their data using a range of carrier frequencies (with potentially diﬀerent bandwidth size), each of them organized in super-frames, frames and time-slots. The NCC assigns to each active RCST a set of bursts, each of them is deﬁned by frequency, bandwidth, starting time and duration. Diﬀerent carriers can have the same or diﬀerent timeslots characteristics, thus having a ﬁxed or a dynamic timeslot structure. In the former case, timeslots have ﬁxed characteristics, in terms of bandwidth and duration. Whereas, in the latter case, besides bandwidth and time-slot duration, both transmission rate and code rate can be changed in consecutive slots. Such ﬂexibility allows a better RCST adaptivity to the variable requirements of
Chapter 1: INTRODUCTION TO SATELLITE COMMUNICATIONS 19
The return link time and frequency organization of the air interface is depicted in Figure 1.5. Each super-frame is characterized by a superframe id, and can be assigned to a group of RCSTs. In turn, each super-frame is divided in parts, characterized by a superframe counter that can be divided in frames, identiﬁed by a frame number (F nb) or by a frame ID (F id). Frames can have diﬀerent duration, bandwidth and composition of timeslots. Each frame is divided in timeslots characterized by a timeslot number (TS nb); also timeslots can be organized in slot groups with similar characteristics.
Fig. 1.5: Organization of the resources in the MF-TDMA air interface.
The RCST is responsible for analyzing, estimating and requesting the needed capacity for uplink transmissions (DAMA case), and for distributing the allocated capacity to the internal applications according to some rules. In particular, when an RCST has data to transmit, it ﬁrst explicitly requests the needed capacity to the NCC (Capacity Request, CR, message). The NCC allocates return channel time slots based on each requests and informs all RCSTs of allowable transmission slots by using Terminal Burst Time Plan (TBTP) messages, sent regularly (e.g., once per super-frame) over the forward channel. Each RCST looks at the received TBTP and transmits data during the allocated time slots.
20 Giovanni Giambene
Allocation methods and traﬃc classes in DVB-RCS
Five capacity allocation methods (layer 2) are deﬁned in the DVB-RCS standard ,:
• Continuous Rate Assignment (CRA),
• Rate Based Dynamic Capacity (RBDC),
• Volume Based Dynamic Capacity (VBDC),
• Absolute Volume Based Dynamic Capacity (AVBDC) and • Free Capacity Assignment (FCA).
Note that CRA is a ﬁxed capacity allocation, while RBDC, VBDC and AVBDC are DAMA schemes. Finally, with FCA the NCC assigns unutilized resources in a super-frame (after the fulﬁllment of the other request types), without any particular requests made by RCSTs. In allocating resources, the NCC adopts the following priority order:
CRA > RBDC > A(VBDC) > FCA.
Details on the capacity allocation methods are provided below.
Continuous Rate Assignment (CRA): CRA is a rate capacity that shall be provided in full for every super-frame while required. CRA is a ﬁxed (and static) allocation of resources after an initial set-up phase with a nego-tiation between the RCST and the NCC. With CRA, a given number of time slots (i.e., packets) are continuously assigned to that RCST every super-frame until that RCST sends the assignment release message. CRA would typically be subscription-based: the user subscribed to a certain constant rate, and the RCST has automatically assigned this constant rate at log-on. CRA should be used for traﬃc, which requires a ﬁxed guaranteed rate, with minimum delay and minimum delay jitter, such as the Constant Bit Rate (CBR) class of ATM networks. The CRA allocation method could also be used in conjunction with RBDC to manage a Variable Bit Rate (VBR) traﬃc that could not tolerate the request-allocation loop delay. In this case, CRA would guarantee a minimum bit-rate and RBDC should provide an additional dynamic capacity.
Rate Based Dynamic Capacity (RBDC): RBDC is a rate capacity that is dynamically requested by the RCST. RBDC capacity shall be provided in response to explicit CR messages from the RCST to the NCC, such requests being absolute (i.e., corresponding to the full rate currently being requested). Each request shall override all previous RBDC requests from the same RCST, and shall be subject to a maximum rate limit negotiated directly between the RCST and the NCC, RBDCmax. To prevent an RCST anomaly resulting in a hanging capacity assignment, the last RBDC request received by the NCC from a given RCST shall automatically expire after a time-out period, whose
Chapter 1: INTRODUCTION TO SATELLITE COMMUNICATIONS 21
default value is 2 super-frames, such expiry resulting in the RBDC being reset to zero rate. CRA and RBDC could be used in combination, as previously explained. A typical application for RBDC over a GEO satellite could be to support the Available Bit Rate (ABR) traﬃc class of ATM networks.
Volume Based Dynamic Capacity (VBDC): VBDC is a volume capacity, dynamically requested by the RCST. VBDC capacity shall be provided in response to explicit CR messages from the RCSTs to the NCC, such requests being cumulative (i.e., each request shall add to all previous requests from the same RCST). The request indicates a total number of needed traﬃc slots (i.e., packets) that can be shared between several super-frames; successive VBDC requests add up. VBDC should be used only for traﬃc that can tolerate delay jitter, such as the Unspeciﬁed Bit Rate (UBR) traﬃc class of ATM or standard IP traﬃc. VBDC and RBDC can also be used in combination for ABR traﬃc, with the VBDC component providing a low priority capacity extension above the guaranteed limit of the RBDC category. MAC parameters are the minimum (VBDCmin) and the maximum (VBDCmax) volume request.
Absolute Volume Based Dynamic Capacity (AVBDC): AVBDC is a volume capacity that is dynamically requested by the RCST. This AVBDC capacity shall be provided in response to explicit CR messages from the RCST to the NCC, such requests being absolute (i.e., a request replaces the previous ones from the same RCST). The request indicates a total number of traﬃc slots that can be shared between several super-frames; a new AVDBC allocation cancels the previous ones. AVBDC is similar to VBDC and should be used instead of VBDC for the initial request or when the RCST senses that the VBDC request might be lost (re-initialization of a previous request); this might happen when requests are sent on contention bursts (see the next description on related signaling methods) or when channel conditions (e.g., packet error rate, Eb/N0) are degraded. AVBDC is suitable to support the same traﬃc classes of VBDC.
Free Capacity Assignment (FCA): FCA is a volume capacity that shall be assigned to RCSTs from capacity, which would be otherwise unused. Such capacity assignment shall be automatic, not involving any requests from the RCSTs to the NCC. In particular, FCA should not be mapped to any traﬃc category since availability is highly variable. The assigned capacity is intended as a bonus, which can be used to reduce delays on any traﬃc type that can tolerate delay jitter. It should be noted that the term ‘free’ in FCA refers to ‘spare’ system capacity. CRA and FCA can also be viewed as two mechanisms to grant dynamically capacity to an RCST without explicit requests. FCA resources should be distributed to RCSTs according to the following criterions ranked by priority: