All computer networks have some things in common

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All computer networks have some things in common. Some computers are independent units, with little need to exchange information with other computers near or far. At most, these computers may use local interfaces such as USB or RS-232 to communicate with printers or other devices close at hand. But with a network connection, a computer can reach beyond its local interfaces to send and receive information of any kind, over distances large and small, via wires or through the air. Computers of different types can communicate using network protocols supported by all. In a network of embedded systems, each system can communicate with the other systems in the network, sharing information and sending and responding to requests as needed. Desktop computers in the network can monitor and control the operation of the embedded systems. Many local networks follow the networking standard popularly known as Ethernet.

Ethernet networks are capable and flexible. Many products designed for use in networks have support for Ethernet built in. A router, or gateway, enables an Ethernet network to communicate with computers in other networks, including computers on the Internet. Two or more computers that share a network connection form a local area network, or LAN. The smallest network links just two computers. For example, a data logger might connect to a remote computer that receives and displays the logger’s data. Or a personal computer (PC) may use a network connection to monitor and control a piece of equipment. At the other extreme, the Internet is the largest network. With an Internet connection, the computers in a local network can access resources on the Internet and make local resources available to any computer on the Internet. To design and program embedded systems for networking, you need to understand the elements that make up a network, so this networking tutorial begins with the basics of how networks are structured. Following this is an introduction to Ethernet, including its capabilities and how Ethernet networks manage network traffic.

The Elements of a Network

All computer networks have some things in common. Every network must have the physical components that enable the computers in the network to exchange data. And in every network, the computers must agree about how to share the data path that connects the computers, to help ensure that transmitted data gets to its destination.

Components

All networks include the following physical components:

• Two or more computers that need to communicate with each other. In the networks described in this networking tutorial, at least one of the computers is an embedded system, which is a device that contains a computer dedicated to a specific task or a series of related tasks.

• A defined physical interface, to ensure that the output of a transmitting computer is compatible with the inputs of the receiving computers. For Ethernet networks, the Ethernet standard specifies this interface.

• Cables or wireless transceivers to connect the computers. Ethernet networks have several options for cables. An Ethernet interface may also connect to a device called a wireless access point, which enables the embedded system to access a wireless network. The computers in the network must also agree on the following aspects of sharing the network:

• Rules for deciding when a computer may transmit on the network. When multiple computers share a data path, whether in a cable or wireless medium, the computers need to know when the path is available for transmitting. The Ethernet standard contains rules that specify when a computer may transmit.

• A way of identifying a transmission’s intended destination. In Ethernet networks, multiple computers may receive a message intended for one computer in the network. When a message arrives at a computer’s network interface, the computer needs to know whether the message is intended for itself or another computer. Every communication in an Ethernet network includes a hardware address that identifies the Ethernet interface of the intended receiver. Some communications also use Internet protocols that contain additional addressing information, such as an addresses that identify the sending and receiving computers on the Internet and a port, or process, that receives the communication at the destination computer.

• A defined format for the information sent on the network, so a computer can understand and use the information it receives from the network. In Ethernet networks, all data travels in structures called frames. Each frame includes fields for data, addressing, and other information that helps the data reach its destination without errors. The information in a frame’s data field may also use protocols that help the receiver of the frame decide what to do with the received data.