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Broadcast Systems, DAB, DVB Name: CHEN, Hanliu email@example.com March 13, 2006 Overview Introduction to broadcast system DAB and DVB introduction Broadcast data scheduling technique Indexing technique Summary Broadcast system Wireless broadcast data delivery: to disseminate information to a large user community in a wireless asymmetric communication environment. Asymmetric communication environment: Systems are designed to deliver data from a few servers to a large number of mobile clients, but there is significantly more “downlink” bandwidth from servers to clients than in the opposite or “uplink” direction. Also, clients have low power reserves, while servers have plenty of power. Example: news feeds and traffic information systems cable TV with set-top box Special case: unidirectional distribution systems high bandwidth from server to client “downlink”, but no bandwidth vice versa “uplink”. Broadcast or point-to-point access Two fundamental information delivery methods for wireless data applications: Point-to-point access: A logical channel is established between the client and the server. Broadcast: Data are sent simultaneously to all users in the broadcast area. It is up to the client to select the data it wants. Why broadcast? Broadcast satisfies arbitrary number of users: A single broadcast of a data item can satisfy all the outstanding requests for that item at the same time. Broadcast satisfies the wireless environment: Mobile wireless environments are characterized by asymmetric communication. Data broadcast can take advantage of the large downlink capacity when delivering data to clients. Implemented without any additional cost: A wireless communication system essentially employs a broadcast component to deliver information. e.g. Server sends broadcast information to find the cell-phone user. DAB and DVB Digital Audio Broadcasting – In Digital Video Broadcasting – In service 1996 operation in Jan 2001 Australia and Europe Specifications: Transmits 6-10 CD quality Specifications: audio channels (data-rate: Transmits MPEG-2 2.304 Mbit/s) compressed video (same 1.5 MHz bandwidth format as used for DVD players) 192 - 1536 carriers depending on frequency Data rates up to 31 Mbps Transmit with OFDM signal will replace the current analog system by around 2008 sufficient to support High Definition Television. Transmit OFDM signal Physical layer of DAB and DVB - OFDM 5 subcarriers sinc (sin(x)/x) frequency response → overlap in the frequency domain The thick black line: the combined response Circles: signal seen by the receiver Receiver (FFT): Discrete frequency samples → the peaks Advantages of OFDM: Orthogonality → no ICI Strong multipath tolerance: no ISI Spectrally Efficient Each sub-carrier is independent: no ICI DAB Transmitter Service Information FIC Multiplex Information Trans- mission Multi- Trans- OFDM plexer mitter Audio Audio Channel Services Encoder Coder MSC Multi- Radio Frequency plexer Data Packet Channel FIC: Fast Information Channel Services Mux Coder MSC: Main Service Channel OFDM: Orthogonal Frequency Division Multiplexing DAB Receiver (partial) MSC OFDM Channel Audio Audio Tuner Service Demodulator Decoder Decoder FIC Independent Data Packet Service Demux Control Bus Controller User Interface DVB 1991 foundation of the ELG (European Launching Group) goal: development of digital television in Europe 1993 renaming into DVB (Digital Video Broadcasting) goal: introduction of digital television based on satellite transmission cable network technology later also terrestrial transmission DVB-S Satellites SDTV EDTV Multipoint Integrated HDTV Distribution Receiver-Decoder System DVB-C Cable Multimedia PC Terrestrial Receiver DVB-T B-ISDN, ADSL,etc. DVD, etc. Three Types of broadcast delivery systems Push-based broadcast: The users cannot place requests directly to the server. The broadcast schedule should be determined based solely on the access probabilities. The push-based model has already played an important role in our daily lives (e.g., the television programs) but, scheduling problem On-Demand (Pull-based) broadcast: The user can send a request to the server to tell what it is waiting for through a uplink channel. Hybrid broadcast: A combination of Push-based and Pull-based broadcast. Two Critical Issues in Wireless Broadcast System Access efficiency: It concerns how fast a request is satisfied. Solution: Scheduling Technique Power conservation: It concerns how to reduce a mobile client’s power consumption when it is accessing the data it wants. Solution: Indexing Technique Two basic performance metrics used to measure access efficiency and power conservation for a broadcast system: Access time: The time elapsed between the moment when a query is issued and the moment when it is satisfied. Tune-in time: The time a mobile client stays active to receive the requested data items. Data Scheduling Key issue: how the server organize the data in the broadcast in order that the clients can access the information efficiently in terms of the latency (access time) and the tune in time. Three categories: Push-based scheduling (e.g. flat broadcast, probabilistic-based broadcast, broadcast disks, optimal scheduling, etc.) Pull-based scheduling (e.g. for equal-size items, for variable-size items, energy-efficient scheduling, etc.) Hybrid scheduling (adaptive scheduling) Push-based Data Scheduling Probabilistic-Based Broadcast Flat Broadcast: all data items are broadcast in a round- selects an item i for inclusion in robin manner. The access time the broadcast program with for every data item is the same. probability fi. qi fi N qj The simplest broadcast scheme. j 1 Poor performance in terms of Improved performance for average access time when data skewed data access. access probabilities are skewed. it may have an arbitrarily large access time for a data item. Furthermore, this scheme shows inferior performance to other algorithms for skewed broadcast Push-based Data Scheduling Broadcast Disks: 1) Order the pages from the hottest to coldest. 2) Partition the list of pages into multiple ranges. 3) Choose the relative frequency of broadcast for each of the disks. 4) Split each disk into a number of chunks. 5) Create the broadcast program by interleaving the chunks of each disk. rel_feq(1)=4, rel_feq(2)=2, rel_feq(3)=1 LCM(4,2,1)=4 num_chunk(1)=1, num_chunk(2)=2, num_chunk(3)=4 Push-based Data Scheduling Empty slot problem: when it is not possible to evenly divide the number of pages (in a disk) into the required number of chunks. Complementary approach: After complementary to move some pages which are located approach near the end of a broadcast cycle to those empty slots which occur before those pages. Pull-based Data Scheduling For equal-size items: For variable-size items: A. First-Come-First-Served (FCFS) A. Preemptive-Longest-Wait First (PLWF): preemptive version of the LWF B. Most-Requests-First (MRF) algorithm. C. Most-Requests-First-Low B. Shortest-Remaining-Time-First (MRFL): essentially the same as MRF, (SRTF) but it breaks ties in favor of the item with C. Longest-Total-Stretch-First the lowest request probability. (LTSF): the stretch of a pending D. Longest-Wait-First (LWF) request = the time the request has been in the system so far / its service time. D. MAX algorithm: A deadline is assigned to each arriving request, and it schedules for the next broadcast the item with the earliest deadline. deadline=arrival time + service time * Smax Hybrid Data Scheduling Motivation: 1. Push-based data broadcast cannot adapt well to a large database and a dynamic environment. 2. Pull-based data broadcast consumes more uplink bandwidth and energy of the mobile clients. When the uplink channel is congested, the latency will be extremely high. Thus, adaptive data broadcast scheduling combined with both push and pull based scheduling is needed! Hybrid Data Scheduling Two issues of adaptive scheduling: 1. Where and how to get the request from the clients. 2. Balance the bandwidth allocation between push-based and pull based deliveries. Mechanism: • Feedback manager: monitors the incoming feedback and interest patterns of the documents in the current broadcast program. estimates the number of feedback and client requests that should be collected for the server to summarize interest patterns with a desired precision. • Request manager: deals with the sampled clients’ requests incrementally based on the approximate response mechanism. Adaptive scheduling architecture • Broadcast scheduler: generates new broadcast programs with the adaptive feedback. Indexing Technique Key issue: how to reduce the power consumption of mobile clients in broadcast environment? Basic idea: allowing the mobile users to predict the arrival time of a requested data item, so that a mobile user may stay with power saving mode and only selectively tune in the broadcast channels when the data of interest actually arrives. Indexing techniques: • The hash technique • The index tree technique • The signature technique • The hybrid index approach: a combination of index tree and signature technique • The unbalanced index tree technique • Multiattribute indexing Indexing technique NP-hard problem Indexing with scheduling technique: e.g. 1. The Index Tree Technique with Flat Broadcast 2. The Index Tree Technique with the Broadcast Disks 3. The Signature Technique with Flat Broadcast 4. The Signature Technique with the Broadcast Disks Summary Access efficiency and power consumption of the mobile clients are the main problems in wireless broadcast environment. Thus, scheduling and indexing techniques are important to deal with the two problems. The conclusion is that, by current technologies, no specific scheduling or indexing technique is the best solution for all broadcast systems. The scheduling and indexing technique should be carefully chosen according to various system requirements. Question and Answer Question 1: What are the three types of broadcast models? Answer: Push based broadcast; On-demand (or pull based) broadcast; Hybrid broadcast. Question 2: Scheduling and indexing techniques are very important in wireless broadcast system, why? Answer: In wireless data system, access efficiency and power conservation are two critical issues. Scheduling technique provides more access efficiency, which concerns how fast a request is satisfied. Indexing technique helps reduce a mobile client’s power consumption when it is accessing the data it wants. Question and Answer Question 3: The broadcast disk contains a multiplex of data from different disks onto the same broadcast channel. The multiplex can be weighted in order to separate between fast disks and slow disks. A cycle of broadcast disks is shown in the following figure. The disks with the same frequencies are shown in the same color chunks, e.g. disk 1 in black, disk 2, 3 in grey, and disk 4, 5, 6, 7, 8, 9, 10, 11 in white. What is the frequency relation between the black, grey and white? Answer: F (black): F (grey): F (white) = 4: 2: 1. Thank you!
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