Streaming and Content Delivery SECTIONS 7.4 AND 7.5

Digital Audio  Audio waves are converted to a digital format by an ADC (Analog-to-Digital Converter) by taking electrical voltage as input and generates a binary number as output.  Reserve is to convert digital formats with DAC, so that sound can be projected by an audio device.  Problem with this is that doing so creates high transfer times and uses up large amounts of bandwidth.  In order to deal with this, we compress the audio files.  Compressing relies on encoding and decoding files for less intensive transfer.  Encoding is needed once whereas decoding is needed many times, so encoding is allowed to take longer.  Waveform encoding – transforms signals mathematically. Perceptual encoding – abuses certain flaws in the human auditory system.  Need to use different algorithms for live audio, otherwise we experience interrupts

Digital Video  Easiest way of having video is with frames that consist of rectangular grids of pixels – only allows for black and white (no shades of grey)  Using 8 bits allows us to use shades of gray for the pixels.  24 bits of color for the pixels allows us to produce 16 million different colors – more than we can even see.  For TV’s and computer monitors, pixels are instead made of red, green, and blue subpixels.  For television, which broadcasts at 25 frames/sec, motion isn’t steady, so instead they use two fields, one which is broadcast on even numbers, and the other broadcasting on odd numbers, a process known as interlacing.  Needs to be compressed like audio, done so using the MPEG standard.

Streaming  Rather than loading videos entirely before they are loaded, we use metafiles, things that point to where we can access the video from.  Video players have jobs to do – mainly dealing with errors, decompressing, and eliminating jitter.  For error handling, we attach parity packets to the end of our information, which duplicates the data.  Alternate approach is interleaving, where we mix up the media and descramble it at the end, so that if things go missing, its over a period of time rather than all at once, and hence less noticeable.  To help eliminate jitter, we simply use a playout buffer – simply, we load some of the video before we actually start playing it, rather than having to load and immediately display the same data.  Livestreaming is different – data is sent as its being created, so buffer has to be larger so things can be processed, in a way similar to the playout buffer.  Since livestreaming has many people accessing the same data, we use a method called multicasting, where each viewer is sent a packet to a group address, with information for audio/video being sent to this group address rather than many separate ones.

Traffic, Farms, and Proxies  Before 1994, most traffic was FTP, then became P2P(peer-to-peer) around  Traffic is highly skewed, small amounts of websites have large amounts of traffic, while large amounts of websites have small amounts of traffic.  Server farm – collection of computer servers that provide functionality far beyond that of a single machine.  Main problem with server farms is that the set of computers must look like a single logical website to clients.  With server farms, we map using load balance to evenly distribute the workload.  Proxy server – Computer that acts as mediator between a web browser and the Internet.  Web proxies are used to fetch web requests on behalf of users, and are more beneficial for popular requests.  Proxies are popular because they can also be used to filter content.

CDN’s  CDN – System of distributed servers that deliver web content based on the location of the user.  CDN’s use a structure that has three levels, an origin server, a node, and worldwide clients.  To support a CDN, we can use either a web proxy, mirroring, or DNS redirection.  CDN’s themselves can be used to support dynamic page creation, images, videos, and so on.

Peer-To-Peer Networks  Basic idea of Peer-to-Peer file sharing is that many computers come together and pool resources to form a content distribution system.  Computers in this system do not need to be intense, and are commonly home computers.  Peer-to-Peer networks are self-scaling, meaning their usable upload capacity grows in tandem with download demands from their users.  Commonly used because CDN’s have large amounts of personal information about many people, which can then be used to provide a better service.