VOD: Video On Demand

The materials presented here are adapted from Dave Sincoskie's pioneer paper, "System Architecture for a Large Scale Video On Demand Service," Computer Network and ISDN Systems, 22, (1991) 155-162. The topic is related to real-time, multimedia storage/communications systems.

• Sincoskie's Globecom VOD paper.

• Copier memory description in Sincoskie's Computer Network paper. It is scanned in with ocr.

Video On Demand Example

Video On Demand Service

• It is provided from a centralized location over a digital network.
• A customer can signal the network to select a movie, the network will start transmission within a reasonable time.
• Coarse rewind and fast forward can also be provided.
• All customers could choose to watch the same movie but individual customers may be watching different part of the movie.

Technological Assumptions

• The North American telephone systems have a common 1.544 Mbps rate.
• It is possible to extend this rate into the subscriber loop, which is the last leg of the connection to the customer.
• For broadband, the lowest channel rate of SONET standard is 51.84 Mbps.
• Assume two coding rates:
• NTSC video encoded into a 1.5 Mbps channel.
• HDTV encoded into a 50 Mbps channel.
• NTSC service over upgraded copper technology.
• HDTV service over BISDN fiber loop.
• Use double star topology: with non-shared fiber running from the customer premise to a remote concentrator, and then shared fibers from the concentrator to the local central office.
• The central office switch is an ATM-based packet switch with multicast capability. The centralized video database is connected to the central office with a large amount of dedicated fiber bandwidth.
• The video on demand service may be possible sometime before year 2000. (This was predicted in 1991, -- it was pushed hard during 94-96 in several field trials but pace slows down due to network architecture cost/price problems.)
• Direct TV and Prime Star are distributing video using MPEG2.

Video Material

• consists of a large number of movies with a length about 100 minutes each.
• 1.125/37.5 GBytes for 100 minutes at 1.5/50 Mbps.

(100*60*1.5/8=1.125 GB; 100*60*50/8=37.5GB)
• video may be stored on video tapes or video disks.

D3, SVHS VTR tape are used in TCI.
• video may in analog or digital form but is converted into a digital stream at a constant rate of 1.5/50 Mbps. (mpeg stream has variable bit rate)
• the customer is responsible for conversion from digital to display on his TV.
• Three-level memory hierarchy will be used to store the video database.

Disk Technology Trend

Transfer rate:
The transfer rate is controlled by the linear bit density and rotational speed.
Ultra wide SCSI drives transfer data at 40MBps (8ms access time, 512k cache, 9.1GB, $800).
Enhanced IDE and Ultra DMA drives transfer data at 33MBps (9.5ms access time, 512k cache, 6.4GB, $259)
RAID data striping can multiply the transfer speed of single drive.
The real-time Audio/Video applications require fast response time and sustain delivering speed. Disk cache at hard drive may have to be turned off or speeded up to meet the requirements.
Assume no change in rotational speed, both linear density and radial density increase twice resulting four times increase in disk density,
by 1998 the disk may have 83 Mbps transfer rate.
 

Central Office Switch

• For 10,000 customers each watches 1.5/50 Mbps video stream, we need 15/500 Gbps.
• The switch design based on Batcher-banyan technology and 3D packaging is capable of delivering 2.4 Tb/s with 16K lines@150Mb/s

Video on Demand System Architecture

• Library is read-on memory.
• Each copier has one single input (from tape player) and one output per disk head.
• Each stop-start buffer has one input and one output.
• Inputs and outputs are multiplexed up to the switch rate of 150 Mbps or higher

View with More Detailed Multimedia Storage System

Usage Scenario

1. A single customer watching a movie that no other customer is viewing. A connection is setup between customer and tape player.
2. A popular movie, when the first customer requests it, it will be cached into the copier memory.
• The network allocates a copier disk to this movie,
• signals the library to load the tape,
• set up a multicast connection from the library tape to the copier disk and also to the customer.
• sometime later, customer 2 requests the same movie.
• customer 2 may needs to wait until disk heads move back to their beginning position. (For example, customer 1 uses 4th disk head while customer 2 uses the 1st head. Assume a disk may have 20 heads, each head covers 5 min of the movie).
• If customer 3 requests the same movie when customer 2 is waiting, the network can set up a multicast connection to connect customer 2 and 3 to the same head.
• This brings the customers into a fixed phase relationship, with granularity controlled by the number of heads per movie. Each phase contains 5 min movie (100min/20 heads=5min).

Copier Memory

• For HDTV case, each disk holds a movie (37.5 GB) and has 20 heads operating in parallel at 50 Mbps each. The relative position of the heads are fixed.
• Each head hold 5min movie.
• The head phase difference is 5 min.
• For NTSC case, assume
• We use two 637MB disks, 4 head each.

2*637MB=1.274GB > 1.125 GB for 100 min NTSC movie.
• Each head has a transfer rate of 20.4Mbps. With 4 heads, the disk can transfer 20.4*4=81.6Mbps.
• Each head can produce max. 13 phases at 1.5 Mbps per phase.

(floor(20.4/1.5)=13; 13*1.5=19.5Mpbs < 20.4Mbps)
• With 8 heads per movie, 13*8=104 different phases is achieved.
• Phase difference=100*60/104=57.7 sec/phase.
• To make the frame number dividable by 8/13, add 24 frame to make it 180024.
• Each head has 180024/8=22,503 frame and grouped into 22503/13=1731 blocks. each block has 13 frames and each frame holds 1 frame from each of the 13 phases.
• Assume uniformed frame size. Each frame contains 1.5Mbps/30fps=50kb.
• How long for a disk head to transfer the 12.5min length of the movie that it covers?

12.5*60s*1.5Mbps/20.4Mbps=55.14 s or more accurately
1731*13*50kb/20.4Mbps = 1125.15Mb/20.4Mbps = 55.15 s.

Is it correct that since the disk head covers 12.5 min movies, it will take 12.5min to read the same frame again?

Stop-Start Buffer

• It allows a customer to pause and resume.
• The size of the stop-start buffer is twice the phase difference (57.7*2=115.4)/(5min*2=10*60=600) s or (1.5Mbps*115.4/8=21.6MB)/3750 MB. The bandwidth is twice the coding rate at 3/100 Mbps.

In HDTV case, it contains 1/10 of the movie 100*60*1/10=600s.
In NTSC case, it contain 2/104 of the movie 100*60*2/104 = 115.4s
• When customer 2 requests a pause, the network allocates a stop-start buffer.
• redirect the transmission from the disk to this buffer. (change the connection)
• When customer 2 requests resume, the data comes from the head of the buffer while the transmission from the disk continues into the tail end.
• Pauses longer than twice the head phase difference can be handled by a combination of the stop-start buffer and a head change.
• It provides limited visual rewind or fast forward. (There may be problems with this design.)
• The stop-start buffer must be allocated on a per-user basis.
• Therefore it may be placed in the customer’s TV or digital decoder.

System Trade-offs

• The above discussion is only qualitative.
• Quantitative studies will require considerably more information on cost and performance of various components, and the statistics of customer’s viewing habits.
• Considering the placement of a video-on-demand system in a LATA (Local Access and Transport Area) instead of class 5 local office.
• Assume 625,000 customer homes per LATA, 60 class 5 offices per LATA, 2000 movies available.
• eliminate the library and stop-start buffers, keep movies in disks.
• $10,000 cost per disk
• The cost of the video database would come out to ($10,000*2000/625000=)$32 per customer, only small fraction of fiber loop and loop electronics.
• Assume every phase of every movie is multicast to every class 5 office, this would require a maximum of 324/2000 Gbps transmission.

For NTSC,  2000movies*104phases/movie*1.5Mbps/phase=312Gbps.
For HDTV,  2000movies*20phases/movie*50Mbps/phase=2000Gbps.
• The number of channels is much bigger than that of existing broadcast TV channel.
• NTSC video-on-demand system is well within today’s capability.