VOD insert for Web tutorial 
	Digital Video Servers and Video Transport
		HPDC95 Tutorial
		August 1, 1995

		Marek Podgorny, Miguel Del Rosario
		NPAC
		Syracuse University
		111 College Place
		Syracuse NY 13244-4100
	Abstract of Video Server Tutorial 
		This presentation describes issues involved in Video server and transport
		As well as server technology, we cover data transport over ATM, and MPEG compression  
		VBR (Variable Bit Rate) and CBR (Constant Bit Rate) are two data delivery options and we present comparisons
		We discuss the realAudio digital audio and extensions to video
	Video Server Simulation 
		Prerequisite to software implementation
		Full nCUBE VOD server simulation
		Integration of the following simulation modules:
			Simulation kernel -- Proteus (MIT)
			Disk simulation code (Dartmouth)
			VOD file system simulation code (NPAC)
			VOD network interface simulation code (NPAC)
		VOD network protocol simulation
			Opnet simulation kernel
			Protocol stack simulation code (NPAC)
	VBR(Variable Bit Rate) to CBR(Constant Bit Rate) 
		Technological Hurdles
			Video File Server Bandwidth
				Needs real-time O/S support for I/O (network and disk)
				Need better understanding of parallel I/O for multimedia
				Need faster I/O hardware, especially tertiary storage:
					high-end tape -- 1MBps
					low-end tape -- 100 KBps
					optical disk -- 0.5 MBps
					(long to very long seek times)
	VBR(Variable Bit Rate) to
	 CBR(Constant Bit Rate) II
		Technological Hurdles (conŐt ...)
			ATM Switch Bandwidth
				1.2 Gbps => ~400 streams
				For 1000 streams, need more switches
			Network Protocols
				Need real-time video protocol
				Cable networks not desinged for interactive digital data
	VBR(Variable Bit Rate) to
	 CBR(Constant Bit Rate) III
		Key issue in VOD : Network Traffic 
		Central debate in CBR vs. VBR : 
			Is there really a performance gain in VBR?
		CBR Encoding:
			Constant bit rate, variable picture quality
			Advantage: fixed bandwidth requirement
			Disadvantage: picture quality degradation
		VBR Encoding:
			Variable bit rate, constant picture quality
			Advantage: constant picture quality
			Disadvantage: difficult to allocate network bandwidth
	VBR-to-CBR Transmission- I
		Terms
			        : Total cycle time.
			        : Constant bit rate requirement (e.g., Ave. bit rate)
			        : Bits of data per cycle to satisfy consumption.			
			        : Actual bit rate required for transmission.					
			        : Portion of cycle time allocated to each stream.
		Assumptions
			Round-robin scheduling -- most simple. (An area of future work) 
			Fixed time slice for each stream
	VBR-to-CBR Transmission - II
		Buffer Allocation Requirements
			             : Number of frames consumed by client in slot  
			             : Number of frames in buffer at beginning of time slot  
			             : Number of frames arriving in buffer during time slot  
		Continuous Media Requirement					
			True for all slots, therefore true for any slot                                                                                                                          							
			For any point during transmission, buffer must never be empty.
	VBR-to-CBR Transmission -III
		Buffer Underflow Condition
			Arriving frames less than number consumed:   
			At start, the buffer is empty : 
			For continuous media requirement, we require for any slot N						
			To prevent underflow, we buffer frames							
			where F is given by:
	VBR-to-CBR Transmission - IV
		Buffer Overflow Condition
			Arriving frames greater than number consumed:
			At start, buffer according to underflow equation.
			Buffer  computation is in units of bits.
			From the continuous media requirement, the required overflow buffer size is given by   (modulo some timing details).							
		This also defines the maximum VBR-to-CBR buffer size requirement.
	Preliminary CBR versus VBR Results
		Sample Description
			10,000 frame (approx. 15 min.) full-motion videos.
			Compressed with MPEG-1 VBR encoder.
		Bandwidth requirement and multiplexing gain 
			Using ave. bit rate as CBR transmission rate:
	Preliminary Results (conŐt ...)
		Same 10,000 frame sample
		Buffer requirement
			Optimal bit rate = Ave. rate  + (Ave. rate  x  Std. Dev. factor)
			Reduce buffer size by selecting best CBR rate 
				(All buffer quantities in units of MBytes)