Читать книгу CCNP Enterprise Certification Study Guide: Implementing and Operating Cisco Enterprise Network Core Technologies - Ben Piper, David Higby Clinton - Страница 28

The purpose of a network is to allow applications running on different hosts to communicate with one another. Robert Metcalfe, one of the inventors of the original Ethernet, said it succinctly in 1972: “Networking is interprocess communication.” Thus, at a minimum, a network needs to perform three basic functions:

Layer 1: Physical Connectivity between Nodes A node can be a workstation, server, router, switch, firewall, or any network-connected device that has a processor and memory.

Layer 2: Node-to-Node Data Transfer Data transfer between two nodes physically connected to a shared medium.

Layer 3: Forwarding/Routing Data transfer between any two nodes, regardless of whether they're physically connected to the same medium.

The OSI model sorts these three functions along with many others into the first four layers of the OSI model, as shown in Table 1.2. Not all protocols that operate in a given layer implement all the functions listed for that layer.

Table 1.2 Networking functions provided by each layer

Function	1 Physical	2 Data Link	3 Network	4 Transport
Transmission of bitstreams over physical media	X
Enabling/disabling physical network interface	X
Node-to-node data transfer over a shared medium		X
Forwarding/routing		X	X
Error control		X	X	X
Flow control		X	X	X
Multiplexing/splitting		X	X	X
Ordering		X	X	X
Fragmentation/reassembly			X	X

The OSI replicates some functions in most layers, blurring the distinction among them. It becomes apparent that what distinguishes the layers isn't what they do but what they don't do. Higher layers lack functionality provided by lower layers, something you'd expect given the hierarchical structure of layers. One layer whose functions differ starkly from the others is the Physical layer.