The OSI (Open System Interconnection) Model breaks the various aspects of a computer network into seven distinct layers. Each successive layer envelops the layer beneath it, hiding its details from the levels above.
The OSI Model isn't itself a networking standard in the same sense that Ethernet and TCP/IP are. Rather, the OSI Model is a framework into which the various networking standards can fit. The OSI Model specifies what aspects of a network's operation can be addressed by various network standards. So, in a sense, the OSI Model is sort of a standard's standard.
The first three layers are sometimes called the lower layers. They deal with the mechanics of how information is sent from one computer to another over a network. Layers 4–7 are sometimes called the upper layers. They deal with how applications relate to the network through application programming interfaces.
Layer 1: The Physical Layer
The bottom layer of the OSI Model is the Physical Layer. It addresses the physical characteristics of the network, such as the types of cables used to connect devices, the types of connectors used, how long the cables can be, and so on. For example, the Ethernet standard for 100BaseT cable specifies the electrical characteristics of the twisted-pair cables, the size and shape of the connectors, the maximum length of the cables, and so on.
Another aspect of the Physical Layer is that it specifies the electrical characteristics of the signals used to transmit data over cables from one network node to another. The Physical Layer doesn't define any particular meaning for those signals other than the basic binary values 0 and 1. The higher levels of the OSI model must assign meanings to the bits transmitted at the Physical Layer.
One type of Physical Layer device commonly used in networks is a repeater. A repeater is used to regenerate signals when you need to exceed the cable length allowed by the Physical Layer standard or when you need to redistribute a signal from one cable onto two or more cables.
An old-style 10BaseT hub is also a Physical Layer device. Technically, a hub is a multi-port repeater because its purpose is to regenerate every signal received on any port on all the hub's other ports. Repeaters and hubs don't examine the contents of the signals that they regenerate. If they did, they'd be working at the Data Link Layer, not at the Physical Layer.
Layer 2: The Data Link Layer
The Data Link Layer is the lowest layer at which meaning is assigned to the bits that are transmitted over the network. Data-link protocols address things, such as the size of each packet of data to be sent, a means of addressing each packet so that it's delivered to the intended recipient, and a way to ensure that two or more nodes don't try to transmit data on the network at the same time.
The Data Link Layer also provides basic error detection and correction to ensure that the data sent is the same as the data received. If an uncorrectable error occurs, the data-link standard must specify how the node is to be informed of the error so it can retransmit the data.
At the Data Link Layer, each device on the network has an address known as the Media Access Control address, or MAC address. This is the actual hardware address, assigned to the device at the factory.
You can see the MAC address for a computer's network adapter by opening a command window and running the ipconfig /all command.
Layer 3: The Network Layer
The Network Layer handles the task of routing network messages from one computer to another. The two most popular Layer-3 protocols are IP (which is usually paired with TCP) and IPX (normally paired with SPX for use with Novell and Windows networks).
One important function of the Network Layer is logical addressing. Every network device has a physical address called a MAC address, which is assigned to the device at the factory. When you buy a network interface card to install in a computer, the MAC address of that card can't be changed. But what if you want to use some other addressing scheme to refer to the computers and other devices on your network? This is where the concept of logical addressing comes in; a logical address gives a network device a place where it can be accessed on the network — using an address that you assign.
Logical addresses are created and used by Network Layer protocols, such as IP or IPX. The Network Layer protocol translates logical addresses to MAC addresses. For example, if you use IP as the Network Layer protocol, devices on the network are assigned IP addresses, such as 207.120.67.30. Because the IP protocol must use a Data Link Layer protocol to actually send packets to devices, IP must know how to translate the IP address of a device into the correct MAC address for the device. You can use the ipconfig command to see the IP address of your computer.
Another important function of the Network layer is routing — finding an appropriate path through the network. Routing comes into play when a computer on one network needs to send a packet to a computer on another network. In this case, a Network Layer device called a router forwards the packet to the destination network. An important feature of routers is that they can be used to connect networks that use different Layer-2 protocols. For example, a router can be used to connect a local-area network that uses Ethernet to a wide-area network that runs on a different set of low-level protocols, such as T1.
Layer 4: The Transport Layer
The Transport Layer is the basic layer at which one network computer communicates with another network computer. The Transport Layer is where you'll find one of the most popular networking protocols: TCP. The main purpose of the Transport Layer is to ensure that packets move over the network reliably and without errors. The Transport Layer does this by establishing connections between network devices, acknowledging the receipt of packets, and resending packets that aren't received or are corrupted when they arrive.
In many cases, the Transport Layer protocol divides large messages into smaller packets that can be sent over the network efficiently. The Transport Layer protocol reassembles the message on the receiving end, making sure that all packets contained in a single transmission are received and no data is lost.
Layer 5: The Session Layer
The Session Layer establishes sessions (instances of communication and data exchange) between network nodes. A session must be established before data can be transmitted over the network. The Session Layer makes sure that these sessions are properly established and maintained.
Layer 6: The Presentation Layer
The Presentation Layer is responsible for converting the data sent over the network from one type of representation to another. For example, the Presentation Layer can apply sophisticated compression techniques so fewer bytes of data are required to represent the information when it's sent over the network. At the other end of the transmission, the Transport Layer then uncompresses the data.
The Presentation Layer also can scramble the data before it's transmitted and then unscramble it at the other end, using a sophisticated encryption technique.
Layer 7: The Application Layer
The highest layer of the OSI model, the Application Layer, deals with the techniques that application programs use to communicate with the network. The name of this layer is a little confusing because application programs (such as Excel or Word) aren't actually part of the layer. Rather, the Application Layer represents the level at which application programs interact with the network, using programming interfaces to request network services. One of the most commonly used application layer protocols is HTTP, which stands for HyperText Transfer Protocol. HTTP is the basis of the World Wide Web.
