TOWARD A COMPLETE CONTROL FRAMEWORK FOR ADAPTIVE CAPACITY ALLOCATION IN MOBILE DVB-RCS

1. TOWARD A COMPLETE CONTROL FRAMEWORK FOR ADAPTIVE CAPACITY ALLOCATION IN MOBILE DVB-RCS Nedo Celandroni*, Franco Davoli°, Erina Ferro*, Alberto Gotta*, Raffaello Secchi* *ISTI-CNR, Area della Ricerca del C.N.R., Via Moruzzi 1, I-56124 Pisa, Italy °DIST-University of Genoa and CNIT (Italian National Consortium for Telecommunications), Italy 13th Ka-band conference

2. Objectives • We address the general problem of controlling and optimizing access and bandwidth sharing among different applications in mobile DVB-RCS • This work deals explicitly with the dynamic assignment of bandwidth to elastic (TCP) traffic on the uplink. Different access techniques are compared: • Random Access: Diversity Slotted Aloha (DSA) • Deterministic Access: Demand Assignment Multiple Access (DAMA) • The convenience of one technique with respect to the other as concerns TCP performance is determined by: • Bandwidth Cost per connection • Total number of slots allocated in the case of DAMA • Data successfully received in DSA case divided by DSA throughput • Average Completion Time 13th Ka-band conference

3. MUltimedia MObile Bandwidth ALlocation (MUMOBAL) architecture • Class-based Dynamic Bandwidth Allocation • One portion of bandwidth for each typology of traffic and moving boundaries between bandwidth portions • In each portion, the Network Control Centre (NCC) assigns bandwidth to the relevant traffic classes, by taking into account their QoS requirements and traffic characteristics • Collision free Optimal controller: • Optimal DBA Control Algorithmfor Service Level Agreement • Collision free DAMA: • Rate Based Dynamic Capacity • Volume Based Dynamic Capacity • FCA option • Contention access DSA: • Contention-based MAC • EF traffic: • Voice, video & Real Timeapplications with QoS requirements • Large volume AF & BE traffic: • web applications, video streaming, interactive and peer-to-peer, etc... • Small volume BE traffic: • web-browsing, e-mail, signaling, etc… Collision free DAMA Collision free Optimal Controller Contention access DSA Collision free Optimal Controller Collision free DAMA Contention access DSA 13th Ka-band conference

4. DAMA scheme • DAMA scheme adopted resembles the FIFO Ordered Demand Assignment (FODA) • N. Celandroni, E. Ferro, “The FODA-TDMA satellite access scheme: presentation, study of the system and results”, IEEE Transactions on Communications, December 1991 • The bandwidth requests, contained in each packet header, is done by each station taking into account the instantaneous queue size and a term proportional to the incoming traffic • The NCC performs the bandwidth assignment by cyclically scanning all requests and giving each station an amount of bandwidth proportional to the request • The assigned amount of bandwidth is subtracted from the request for the next assignment cycle; successive requests coming from the stations override the current residual values of the previous ones • The length of the cycle is dynamically varied according to the overall system load • A lower bound on the assignment is set as a minimum guaranteed bandwidth, whereas an upper bound is given by the capacity of a carrier 13th Ka-band conference

5. Diversity Slotted Aloha (1/2) i-th frame time time slot DSA for k=2 A packet is sent twice station #3 station #1 station #2 13th Ka-band conference

6. Diversity Slotted Aloha (2/2) • The NCC maintains a target throughput by allocating the number of DSA slots per frame that are necessary to achieve it; it corresponds to the average number of successful slots in a frame divided by the target throughput itself • For design purposes, we can consider the asymptotic values of DSA collision probability and throughput as functions of G, the number of users per slot, and k, the number of copies delivered 13th Ka-band conference

7. Case study: mobile user acts as an FTP client • Assumptions • Users are synchronized with the system and receive the burst time plan, sent at each frame time by the NCC in TDM mode (DVB-S2) • Any user is allowed to transmit in a frame only upon receiving a valid burst time plan for that frame • DAMA first access & connection termination • A user issues the initial request for bandwidth on reception of a SYN message from the TCP server by sending a request packet in the DSA area • Successive requests for bandwidth are piggy-backed with data • A user releases the bandwidth when the data transfer is completed • System parameters • DAMA: the minimum amount of bandwidth equal to one slot every m frames allocated by the NCC upon the reception of the access request • DSA: the target throughput and, consequently, the collision probability that derives by the best choice of the k value • Target throughput 10%  k=4, collision probability = 0.012 • Target throughput 20%  k=3, collision probability = 0.121 13th Ka-band conference

8. Simulative Scenario (ns-2) LMS channel parameters UPLINK: TDMA/DSA DOWNLINK: TDM ON/OFF mobile channel HUB station SERVER Terrestrial Segment (50 ms) CLIENT Internet Gateway RTT=600 ms, Segment size=1416B (equivalent to 8 DVB packets), Frame size=20 ms, TCP/Sack 13th Ka-band conference

9. Short-lived connections (highway) Average Uplink Bandwidth Cost Average Completion Time 13th Ka-band conference

10. Short-lived connections (rural) Average Uplink Bandwidth Cost Average Completion Time 13th Ka-band conference

11. Long-lived connections (highway) DSA TDMA TDMA DSA • DSA may not be suitable for long-lived connections in highway • DSA 10% is outperformed by DAMA 1f/s in both duration and cost, while DSA 20% has a duration very close to DAMA 4f/s, but the latter exhibits a significantly lower cost 13th Ka-band conference

12. Long-lived connections (rural) DSA TDMA TDMA DSA • In a rural environment • DSA 10% and DAMA 2f/s exhibit very similar performance in both duration and cost, while DSA 20% has a duration similar to DAMA 8f/s, but the latter exhibits a significantly lower cost 13th Ka-band conference

13. Summary of simulations results • The smaller the size of TCP messages, the higher the gain of DSA with respect to DAMA in bandwidth expenditure • Since DSA cost depends on data successfully delivered only, it is less sensible to changes in the channel blocking probability • DSA is convenient for very small transfers in terms of completion time in both environments as well • the connection duration in DAMA 1f/s is comparable with DSA 10% in that very low losses of ACKs due to collision cause negligible performance degradations to TCP • Limited to our investigations, DSA is not convenient for long-lived connections in both environments • The choice of system parameters is a matter of more complex investigations (e.g. the mobile acts as server). Nevertheless, once the parameters have been selected, cost diagrams will be used to evaluate convenience thresholds. For instance • In case DAMA 4f/s and DSA10% are selected as the most convenient options, the threshold file size is about 17 kB and 65 kB for highway and rural environments, respectively 13th Ka-band conference

14. Conclusions & Future works • We analyzed the behaviour of two different access methods, based on DSA and DAMA respectively, in providing the basis for a FTP • The two methods have been compared in terms of bandwidth cost and connection completion time under the case of users acting as clients • Simulation results have shown an advantage of DSA over DAMA up to certain convenience thresholds in the file size as functions of DSA throughput and DAMA m parameter • The non compliance with the convenience thresholds would imply an increased cost for users and, likely, higher revenue for the service provider • DSA does not result convenient for long-lived connections in both environments • This study will be completed with the analysis of the case in which the mobile user acts as a server • Server and client cases are necessary to operate the choice of the target throughput in DSA and the m factor in DAMA • For the server case, in order to obviate to the collisions’ effect in DSA, a packet level FEC might be necessary 13th Ka-band conference