Hotspot | Wireless | Networking

As the package delivery example demonstrates, the information itself is only part of the process. When information moves across a network, it’s essential that all of the parties involved the originator, the ultimate recipient, and everything in between agree that they will use the same formatting, timing, and routing rules and specifications. These rules (also called protocols) define the network’s internal “plumbing” and the form of the information that moves through it.

As network communication has become more complex, the community of network designers has accepted the International Organization for Standardization’s (ISO) Open Systems Interconnection (OSI) model to identify the individual elements of a network link. The OSI model applies to just about any kind of data communication system, including the broadband wireless network.

Because everybody in the communication industry uses the OSI model, it encourages hardware and software designers to create systems and services that can exchange information with similar products from other manufacturers. Without the OSI model or something like it, it would not be possible to expect equipment from more than one source to work together.

The OSI model also allows a designer to change just one element of the network without the need to design everything else from scratch. For example, a wireless network uses radio signals instead of cables at the physical layer and adds routing information at the data linklayer, but it keeps the existing protocols and specifications for everything else. A complex network (such as the Internet) can use wired connections for one part of the signal path and wireless connections for another.

As the name suggests, the physical layer defines the physical media or hardware that carries signals between the end points of a network connection. The physical layer might be a coaxial cable, a pair of telephone wires, flashing lights, or radio waves. The specifications of a network’s physical layer might include the shape of the shell and the pin numbers in a cable connector, the voltages that define the 0 and 1 (on and off) values, the durations of individual data bits, and the radio frequencies and modulation methods used by a radio transmitter and receiver.

The data link layer handles transmission of data across the link defined by the physical layer. It specifies the format of each data packet that moves across the network, including the destination of each packet, the physical structure of the network, the sequence of packets (to make sure that the packets arrive in the correct order), and the type of flow control (to make sure that the transmitter doesn’t send data faster than the receiver can handle it). Each packet also includes a checksum that the receiver uses to confirm that the data was not corrupted during transmission, as well as the string of bits and bytes that contains the actual data inside the packet. Therefore, it contains the software that creates and interprets the signals that move through the physical layer.

In both wired and wireless Ethernet, every physical device that is connected to the network has a unique 48-bit media access control (MAC) address that identifies it to the network. The header (the first part of the data string inside of a packet) includes the MAC addresses of both the origin and destination of that packet.

The network layer specifies the route that a signal uses to move from the source to the destination independently of the physical media. At the network level, it doesn’t matter whether the data moves through a cable, radio waves, or if it uses some combination of both because that’s all handled at a lower level.

Within the Internet, the exchange of data between LANs, wide area networks (WANs), and the core Internet trunk circuits occurs at the network layer.

Starting at the transport layer, the OSI model is concerned with communication between programs on two different computers rather than the process of moving data from point A to point B. For example, when you view a web page on the Internet, the connection between the browser on your computer (such as Internet Explorer or Firefox) and the webserver that contains that page occurs at the transport layer (but the commands you send to the server occur at the application layer).

The session layer defines the format that the programs connected through the transport layer use to exchange data. If the programs use passwords or other authentication to assure that the program at the distant end of the connection is allowed to use a local program, that authentication happens in the session layer.

The presentation layer controls the way each computer handles text, audio, video, and other data formats. For example, if a distant computer sends a picture in JPEG format, the software that converts the data string to a picture on a monitor or a printer operates at the presentation layer.

The application layer handles the commands and data that move through the network. For example, when you send an email message, the content of your message (but not the address or the formatting information) is in the application layer. Most of the words, pictures, sounds, and other forms of information that you send through a network enter the system through the application layer.