Primer: Application Switching - ' REFERENCE'
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REFERENCE: Seven Layers for Seven Functions

The ISO's Open Systems Interconnection (OSI) model describes how data moves within a network, from actual electrons and radio waves (Layer 1) to information within an application (Layer 7). Each layer is dependent on the ones below it for certain services: Layer 3, for example, already translates network addresses, so Layers 4 through 7 can focus on performing other functions. This chart, created with the help of Arcturus Networks, provides a brief overview of the seven layers.

LAYER	PROTOCOLS	PURPOSE
7. Application	Telnet, ping, SSH, FTP, HTTP	Accept direct interaction—such as password entry, file requests—from the user.
6. Presentation	Character mapping, NFS	Ensure data is delivered in a format that the recipient can understand.
5. Session	DNS, RPC, SIP, H.323	Set up, manage and tear down virtual "circuits" between two computers on a network.
4. Transport	TCP, UDP	Move data across a network. Known as the "engine" of the OSI stack.
3. Network	IP, ICMP, NAT	Understand network addresses and choose a route to get there.
2. Data Links	ARP, PPP, LCP	Break data streams into packages appropriate for the physical connection; detect errors.
1. Physical	Fiber optics, ISDN, radio waves, 802.11	Connect to cables, wires and devices.
Source: Arcturus Networks

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