Читать книгу Understanding Infrastructure Edge Computing - Alex Marcham - Страница 43

Any detailed discussion of network technology would be incomplete without a shared understanding of the Open Systems Interconnection (OSI) model [1]. This is an often used conceptual model which allows us to create highly interoperable, open, and scalable communication systems by categorising the technological functions required by the network into a stack of layers that are numbered from 1 to 7, each of which has a set of interfaces to communicate with its directly adjacent layers (see Table 3.1).

This model is very powerful, as by isolating a set of technological functions into a specific layer that has interfaces to talk to the layers it is directly adjacent to, changes to any one specific layer do not need to impact the operation of any other layer in the system, allowing asynchronous evolution of the entire stack of network technologies where one or more layers experience more rapid change than their neighbouring layers. An example of this can be seen with each new generation of Wi‐Fi; significant advances in speed can be achieved by changing only layers 1 and 2, without any of the upper layers being aware of the change. Consider what the situation would be like if the entire network technology stack had to be remade to accommodate a change at any layer. The stack that would result would be highly inflexible as even an isolated change would require significant work. Over time, this would become a key barrier to keeping up with the edge of technological progress and prevent open contribution to the stack by other companies or individuals, limiting innovation.

Now that the reasoning behind the OSI model has been established, we will briefly describe the functionality of each layer as it is relevant to infrastructure edge computing, and the number of each layer will be used throughout this book to quickly refer to the concepts that they represent. In this example, we will be taking the perspective of a network endpoint receiving data that has been transmitted across the network, so our progression will be from layer 1 through layer 7. When considering the process of sending data, this progress through the layers is reversed in order as data flows from layer 7 down to layer 1 to be transmitted across the physical network.

Table 3.1 OSI model layer numbers, names, and examples.

OSI layer number	OSI layer name	Example entity or protocol
1	Physical	Copper cabling
2	Data link	Ethernet
3	Network	IPv4
4	Transport	TCP
5	Session	NFS
6	Presentation	PNG
7	Application	HTTP