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Mobile Backhaul by Esa Metsala, Juha Salmelin

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5.1 Transport Systems

5.1.1 OSI-Model

In networking, and nowadays also in all transport, discussions often start with considering solutions in different layers, for example a Layer 2 solution for access or Layer 3 solutions for some part of backbone. These layers refer directly, or approximately, to the layer model developed originally for data transport.

Open System Interworking (OSI) model was formulated in International Standardization Organization (ISO) decades ago (Table 5.1). It was developed to define how data moves in open systems and it is used widely to abstract telecommunication/data communication systems. Systems are split into seven layers, each of which communicates with one above or below, and with similar layers elsewhere in the system. Functionalities of all equipment along the data path are mapped to a layer. The protocol standards that the ISO and other standards organizations like IEEE develop are associated with these layers.

One may argue that the original OSI model is today out of date and some protocols are hard to locate in a specific layer. It is, however, still a valid tool in conceptualization of networks. For example Ethernet switching is considered as L2 (Layer 2) functionality while IP routing takes place in L3. Some protocols like IP/MPLS fall in between two layers.

Also in MBH networks the roles of different layers are commonly discussed as well as pros and cons of locating certain functions in a specific layer.

Table 5.1 ISO OSI-layers [25].

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