Video-on-Demand Nick Caggiano Walter Phillips
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Video-on-Demand
Nick Caggiano
Walter Phillips
Video-on-Demand
• What is Video-on-Demand?– Storage, transmission, and display of archived
video files in a networked environment– Most popularly used to watch movies offered by
cable provider– Many companies banking on prospect of
bringing Video-on-Demand to educational institutions
Video-on-Demand
• Components of Video-on-Demand system– Client
• e.g. Set-top box• Buffers signals sent from server
– More buffering leads to less expensive decoding hardware– Decoding can be done while displaying, as opposed to real-
time decoding
• Decodes (usually from MPEG-2) signals• Ensures synchronization of audio and video• Also acts as interface between user and server
– Set-top box sends “STOP”, “PAUSE”, and “REWIND” signals upstream to the server
Video-on-Demand
• Components of Video-on-Demand system– Network
• Continuous and long-lived connections– unlike traditional bursty, short-lived computer connections
• Require bandwidths in the range of 1.5Mbps to 5Mbps.
• Delay and jitter must be minimized to preserve presentation.
• Packets which miss deadline must be dropped
Video-on-Demand
• Components of Video-on-Demand– Server
• Random access
• Short seek time
• Reliability
• Availability
• Scalability
Video-on-Demand
• Server architectures– Centralized system
• Server and archives stored in central location• Easy to manage• Doesn't scale well• Low throughput• May add local servers with video buffers
– no archives at local servers, but can forward requests to central server
» “Matrix” stored at local server, “Police Academy 12” kept in archive
» Similar to Blockbuster “New Releases” section
Video-on-Demand
• Server architectures– Distributed system
• Local processing servers with archives
• Reduced delay/congestion
• Scales well
• Higher availability and throughput
• More difficult to manage
Video-on-Demand
• Berkeley Distributed Video-on-Demand System– Composed of
• Database– Stores metadata for each video
– Keyword (for searches), genre, cast, runtime, etc
– Where the video is currently stored/cached
• Video Manager (VMGR)– Locates video and prepares for playback
– Initiates billing to user account
• Video File Server (VFS)– Stores video on magnetic disks
– May be replicated for availability/reliability
Video-on-Demand
• Berkeley Distributed Video-on-Demand System– Composed of
• Archive Server (AS)– Stores video on inexpensive storage (magnetic disk, tape, etc)
– May be replicated
– User selects video from supplied UI
– VMGR locates video on AS or VFS• May select best server due to locality, network load, etc
– VMGR initiates and dynamically manages playback
Video-on-Demand
• Video storage architectures– One movie per disk
• Disk is random access = good for rewind, fast-forward, etc
• Disk failure only affects one movie (and therefore it's streams)– Can easily move to another replicated disk
• Easy scheduling
• Under-utilizes resources (disk bandwidth)– some movies more popular than others (“Matrix” v. “Police
Academy 12”)
• Creates bottlenecks
• Can achieve an order of mag. in response time with replication
Video-on-Demand
• Video storage architectures– Stripe video across array of disks
• Each disk can service a small number of requests for different movies
• Less popular videos don't waste disk bandwidth• Load balancing• Scheduling is much more difficult
– New video must wait for disk scheduling window– Fast-forward or rewind must wait for scheduling window
in next disk
• Disk failure affects many movies, not just one• Best cost/stream of two architectures
Video-on-Demand
• Viola – Chinese University of Hong Kong– Video striping across servers
• RAIS – Redun. Array of Inexpensive Servers
• Provides additional hardware to merge video blocks into a single data stream
• Good scalability– Simply add another server
• Good reliability– Same parity protection as RAID
Video-on-Demand
• Quality of Service and Admission Control– Server must maintain some quality of service (QoS)
• Prompt set-up time
– User doesn't want to wait when he selects a movie
• Synchronization/continuity of streams
– Minimized delay/jitter
• Fast repsonse to “VCR” functions
– In order to do so, must maintain some admission control
• Disk bandwidth, memory buffers, network bandwidth, etc
• Must be determined ahead of time, to ensure QoS throughout session
Industry Perspective
Side Note: Why even bother with VOD servers?
• Personal Alternatives– Tivo -- Replay TV -- VCR
• Centrally managed benefit• “Interactive”
• Shopping and advertisement delivery. Usage profiling
• Play, pause, fast forward and rewind
• Billing• Monthly billing vs. usage billing (also Hybrid billing)
• Convenient access to the latest/dynamic content• Higher value to the user
• Marketing ploy• Competition with the satellite providers
Who wants Video On Demand?
• Some e-Poll findings– Two-thirds of those surveyed have heard of VOD
(mostly male and younger demographics)
– People prefer the subscription payment method vs. pay per view method (both methods are utilized)
– Scheduled premium movies (every half-hour) might be acceptable for most viewers (sporting events)
– Results were from December 2002 • Time Warner San Diego released VOD in September
Big Names• SeaChange
– ITV 12024– Maynard, Maryland– http://www.seachangeinternational.com/Products/On_Demand_television/
• Concurrent Computer Corporation– Media Hawk– Duluth, Georgia– http://www.ccur.com/vod/
• nCube– n4x – Beaverton, Oregon– http://www.ncube.com/vod/
Common features
• All use RAID5 or some proprietary variant– Why RAID5?
– RAID5 gives slow write performance but good read performance which is what we are concerned about
– Obviously fault tolerance and efficient space usage (compared to mirroring)
• Off the shelf processors (i.e. Pentium class)• High speed I/O
– SCSI ~160MBps
– FIBRE channel ~260MBps
Why such diversity?
• Time Warner is a big company so why would San Diego use Concurrent, Palm Desert use nCube, and Los Angeles use SeaChange?
• Answer: Competition amongst VOD vendors
Local Industry• Cox Communications
– Distributed Infrastructure
– Servers Used?
– Employees were not very helpful
• Time Warner– Centralized Infrastructure
– Media Servers used: Concurrent Computer Corporation “Media Hawk 2000” (7 of them)
– Covers a large geographic area North County to Coronado
– Not all Time Warner locations use the same equipment configuration
Centralized vs. Distributed
• Centralization is easy to manage
• Simpler• Requires high bandwidth
throughout the system
• Distributed replication can be a problem
• Might be more fault tolerant
• Better if limited bandwidth between the core and the hubs
What about scalability?
Time Warner• Capacity
– 16 On Demand Channels (3 more planned)
– 800 hours (expanding to 3200 hours)
– Each coax cable can carry 10 streams
– Designed for 6% of digital subscriber use
– Each node has 4 coax outputs
– The 41st subscriber would get a denial of service
– This means 40 movies can be delivered to a neighborhood
– Remember, this is VOD only. Regular PPV and digital channels still work
From the server to your house
• Media comes out of the server over 160 Mbps ASI (Asynchronous Serial Interface) cables
• Converted to optical signal and transmitted via a hub to a node in the neighborhood
• The node converts the signal back to an RF signal that can be transmitted over regular coax
– Scientific Atlanta D9477 MQAM Modulator– QAM Quadrature Amplitude Modulation
Side Notes• 160 Mbit/second ASI• A movie requires 3.75 Mbits/seconds• ~ 40 streams per ASI cable• Analog coax can carry 10 movies
• Nodes are logically grouped in 4’s• Can be reassigned dynamically as needed• Groupings are dictated by the number of set top
boxes served
Managing the system
• Sunfire 280R (http://www.sun.com/servers/entry/280r/)
• Business Management System (BMS)
• Responsible for things such as:– Billing / Ordering– Scheduling– Content management
Conclusion
• Its here now
• Is it all that exciting?
• Could it do more?
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