The chained layers are the bottom-most three layers of the OSI Reference Model (OSI-RM). They include the Physical Layer, Data Link Layer, and Network Layer. They are referred to this because it is these three layers that form the chain that connects two end systems to one another for the purposes of communication. In other words, it is these layers that make up the fabric (apparently, it's chain-mail) of the network.
It is also in these layers that the common carriers have made most of their money since the dawn of data networking. They have historically made it their business to sell networks. So we see private line services, which are essentially Layer 1 services; frame relay, ATM, and Ethernet services, which are essentially Layer 2 services; and IP services, which are essentially Layer 3 services.
Chained Layer Devices
It is important to note that it is not necessary for every element within the network to implement all of the chained layers. Some of the devices within the network will play roles that focus on a specific layer, and these devices only need that layer and the layers below. For example, multiplexers, repeaters, DSUs, modems, and amplifiers are all concerned with the correct operation of the physical plant, and the transmission of bits along that cable plant. These devices are considered Layer 1 devices and have no need of Layers 2 through 7.
Ethernet switches, frame relay switches, and ATM switches are Layer 2 devices and have to understand both the Layer 2 protocol in use and the underlying Layer 1 protocol supporting transmission. They do not, however, have any need for Layers 3-7. Likewise, a router is a Layer 3 device and requires Layers 1 through 3, but has no explicit need for Layers 4-7.
And end system, however, needs all three of the chained layers to be able to connect to, and transmit across, the network. It will also need the remaining end-to-end layers to be able to do work over that network.
In modern networks, however, there is a strong emphasis on network management. In order for any device to be managed remotely, it must run appropriate management software (e.g., a management agent) and it must speak network management protocols (e.g., SNMP). This implies that most equipment in our network today actually implements all of the OSI RM layers. It only needs the layers defined by its core function, but it needs the rest of the layers to be manageable.
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