Data Transfer in the OSI Model

When communication occurs between two computers on different networks, as in the following graphic, data is transferred across the networks through routers.

routers

Devices that connect two or more networks. Routers work at the Network layer of the OSI model, receiving packets and forwarding them to their correct destination.

connection-oriented

A method of communication that is considered reliable because the sender is notified when data is not received or is unrecognizable.

Assume you have a file that you want to send from computer A to computer B. Many steps must occur:

1. The user on computer A creates an e-mail message using his or her favorite e-mail program. The e-mail program has support for specific network services that pertain to sending and receiving e-mail. The services that enable the e-mail program to communicate with e-mail servers are all part of the Application layer.

2. The next step is to translate the message into a language that is common to both computers. As humans, we see 'hello,' but each computer wants to see the message in a format that they can both interpret. The Presentation layer translates 'hello' into ASCII code, which is internationally recognized as 96 numbers and letters and 32 nonprinting characters. The computer can then convert the ASCII code into ones and zeros.

3. At the next layer of communication, a connection, or session, is established and maintained. This connection determines when requests are made so that appropriate responses can be returned. Just like human conversations, computer communications are usually a series of requests and responses that must be answered sequentially.

4. If you want to make sure that the connection is reliable (for example, like a letter that is sent with a return receipt to confirm that it was received as opposed to a letter sent via regular mail), you might use connection-oriented services that guarantee reliable delivery.

5. In this example, computer A and computer B are on different networks. The Network layer routes the packets to the correct network by identifying network addresses and calculating the best route the packet should take.

   node

   A connection point on a network. Nodes include computers, servers, and printers.

   Media Access Control (MAC) address

   A hexadecimal number that is allocated by an international organization and is burned into the network interface card by the NIC manufacturer. Media Access Control is a sublayer of the Data-Link layer.

6. After the data gets to the correct network, it must be delivered to the correct computer (sometimes called a node) on the network. On an Ethernet network, the packet is sent out on the network. Each computer receives the packet and reads the physical address-called the Media Access Control (MAC) address-to determine who the packet is intended for. The computer with the matching physical address accepts the packet and all other computers disregard the packet. Physical layer addressing occurs at the Data-Link layer of the OSI model.

7. The only physical connection that exists between the two computers is defined at the Physical layer of the OSI model. Network cables, connectors, and the hardware portion of the network interface card (rather than the software) are all part of the Physical layer specification.

Here are two mnemonic devices to help you remember the seven layers:

Layer	Memory Trick Top to Bottom	Memory Trick Bottom to Top
Application	All	Away
Presentation	People	Pizza
Session	Seem	Sausage
Transport	To	Throw
Network	Need	Not
Data-Link	Data	Do
Physical	Processing	Please