HHMSM: A Hierarchical Hybrid Multicast Stream Merging Scheme For Large-Scale Video-On-Demand Systems Hai Jin and Dafu Deng Huazhong University of Science and Technology, Wuhan,China ICME 2003 Hsin-Hua, Lee

Outline Introduction HHMSM Scheme  Delivery technique  Optimal time interval Performance Evaluation Conclusion

Introduction The key performance bottleneck for large- scale VOD system==>server bandwidth. Two exiting stream scheduling schemes can save server bandwidth significantly: Batching and Patching.

Introduction Cont. Batching  Use a single multicast stream for clients that request the same video object in the same interval.  Long start-up latency and high reneging probability. Patching  Server schedules a unicast stream for every one to patch the lost data.  Guarantee the zero start-up latency and save the server bandwidth at low or moderate client request rate.  Mass retransmission for the same video data at high client request rates.

HHMSM Scheme Combines the advantages of the batching and patching scheme. Clients that requests the same video are repeatedly merged into larger groups, leading to a hierarchical hybrid merging scheme.

HHMSM delivery technique Key elements:  Time interval T must be selected firstly.  Server schedules stream only at the end of time slot.  Server maintains a stream list E (t, I, A) to record information of current transmitting streams.  Patching policy  t : the stream scheduled time,  I : the multicast group address of the stream,  A : an array to record the serial of video segments that shall be transmitted.

HHMSM delivery technique Cont. S0S0 S1S1 S2S2 S3S3 S4S4 S5S5 t0t0 t1t1 t2t2 t3t3 t4t4 t5t5 x y The patching multicast stream for client B The patching multicast stream for client C and D The patching multicast stream for client C,D and merged partition for client E The patching multicast stream for client E The complete multicast stream Skipped video A B C DE S0S0 S1S1 S2S2 S3S3 S4S4 S5S5 S0S0 S0S0 S1S1 S0S0 S2S2

Optimal time Interval b max : the maximum client bandwidth capacity. Ex. Client bandwidth: 100Mbps, MPEG-1(1.2~1.5Mbps) video b max =100/1.5 ≒ 65 streams B max : the maximum total number of streams that may be concurrently received by a client. T*: optimal time interval which not only guarantees that clients have enough bandwidth to concurrently receive all streams, but also minimize the client average start-up latency.

Optimal time Interval Cont. b max : maximum client bandwidth B max : maximum streams that may be concurrently received by a client. T*: the optimal time interval x y t0t0 tjtj tktk t k+1 SjSj SkSk SjSj PS j: The first patching stream 1. ∵ the start transmission time for S j must ≧ t k ∴ 2j ≧ k+1 2. The number of concurrently received streams access to to the client’s max bandwidth. B max ≦ (k+1)/2 ≦ ceiling (L/2T) 3. ∵ In order to guarantee that clients have enough bandwidth to receive all streams, b max must ≧ B max, ∴ T ≧ L/(2b max ) 4. ∵ The smaller the selected time interval is and the shorter start-up latency is. ∴ T* shall be the minimum value of T => T*=l/ceiling (2b max ) Complete multicast stream Client request

Performance Evaluation Experimental parameters  The popularity of each video was modeled using a Zipf-like distribution.  Client requests were generated using a Poisson arrival process with interval time of 1/λ.  Reneging function U min =0, and τ=15

Performance Evaluation Cont. The average start-up latency vs. request arrival rates T*=L/(2B max ) ≒ 120/(2*60)=1 min HHMSM scheme with T=1min HHMSM scheme with T=5mins HHMSM scheme with T=10mins HHMSM scheme with T=15mins Patching Scheme FCFS batching scheme with T=7mins

Performance Evaluation Cont. The client reneging probability vs. request arrival rates HHMSM scheme with T=1min Patching Scheme FCFS batching scheme with T=7mins HHMSM scheme with T=5mins HHMSM scheme with T=10mins HHMSM scheme with T=15mins

Performance Evaluation Cont. The average server bandwidth consumption vs. request arrival rates HHMSM scheme with T=1min HHMSM scheme with T=5mins HHMSM scheme with T=10mins HHMSM scheme with T=15mins Patching Scheme FCFS batching scheme with T=7mins

Conclusion HHMSM can significantly reduce the server bandwidth over the wide range of client request rates. HHMSM scheme utilizes multicast propagation method for all transmission streams so that the missed video segments can be both from the complete stream and existed patching streams. HHMSM scheme with optimal time interval can improve the start-up latency performance greatly compared with the batching scheme.