This layer is responsible for content delivery to the consumer. It consists of huge number of delivery nodes, which can be organized also in clusters. We can call these nodes or clusters as distribution points.
Every distribution point contains big storage capacity on local or external disk arrays. This capacity is used for the content storage.
File based content is sequence of bytes which have started and ended in the past.
File based content can be downloaded without QoS, but almost always requires zero tolerance to change or modification at any level of content.
This type of content is majority of all content delivered over the Internet and also over the CDNs. For this purpose the ideal transfer protocol is HTTP, which is widely used in current Internet.
A stream is sequence of bytes which started and ended in the past.
The stream based content contains useful information per parts, so it can be transferred continuously and during the transfer it can be also consumed. Streaming content is mostly sensitive to QoS of transfer channel, especially to delay and jitter.
Typical content of this type is a media stream. In contrast to data transfers in Internet the video and audio content is becoming more and more popular. CDN like part of Internet is helping to fulfill these requirements by spreading the content and the load over more location and distribution points.
Media consumers are sensitive to audio information, because every country is using different language. To save resources in the network it is effective to transfer only that audio stream which contains required language. To do this a distribution point can contain content with video stream and multiple audio streams. Only one audio stream (elementary stream) is transferred to consumer in a session according consumer set up. Every elementary stream can be transferred over different paths from source to destination.
The same concept can be also used for multiple angle content, where single content is recorded from more video cameras from different angles (views). Again only video is transferred from distribution point to consumer.
This stream is a sequence of bytes which have started in the past and is not finished until now and continues.
Because this stream is live, it cannot be cached for further streaming and all the efficiency in content distribution is in single transfer over the same parts of network.
More clear explanation would be on simple example: imagine that two consumers in Europe want to see live video show from China Olympic Games. In a normal unicast network, which Internet is, the stream will be duplicated at China at a streaming platform and transferred to consumers in two separate streams (Figures above).
By redirection of consumers to CDN, CDN can join the video stream from China Streaming server, transfer it to its location in Europe (Figures above). The consumers then connect to CDN and stream is duplicated in CDN distribution point and transferred to the consumers in two separate streams, but only inside the European network. This concept can save Internet connectivity from Europe to China. In this case bandwidth is 1/2 of previous concept. If there are three consumers of same content saving is 1/3 of previous concept, etc.