Lesson 4 Terms

ES - End system. Generally, an end-user device on a network.

Data Link Layer - Layer 2 of the OSI reference model. This layer provides reliable transit of data across a physical link. The data link layer is concerned with physical addressing, network topology, line discipline, error notification, ordered delivery of frames, and flow control. The IEEE has divided this layer into two sublayers; the MAC sublayer and the LLC sublayer. Sometimes simply called the link layer.

MAC address - Standardized data link layer address that is required for every port or device that connects to a LAN. Other devices in the network use these addresses to locate specific ports in the network and to create and update routing tables and data structures. MAC addresses are 6 bytes long and are controlled by the IEEE. Also known as hardware address, a MAC-layer address, or a physical address.

NIC - Network interface card. Board that provides network communication capabilities to and from a computer system. Also called an adapter.

Node - Endpoint of a network connection or a junction common to two or more lines in a network. Nodes can be processors, controllers, or workstations. Nodes, which vary in routing and other functional capabilities, can be interconnected by links, and serve as control points in the network. Node is sometimes used generically to refer to any entity that can access a network, and is frequently used interchangeably with device.

Repeater - Device that regenerates and propagates electrical signals between two network segments.

Signal - Electrical or light impulses representing data that pass along the networking media from one networking device to another for the purpose of communication.

Introduction:
As you learned in Lesson 3, the first building block of the OSI reference model is the physical layer. The physical layer is the layer that defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Some of the network media used to connect computers are twisted pair, coaxial, and fiber-optic cable. Various criteria such as rate of data transfer and expense help determine which type of cable will be used.

The physical layer is where data is represented by either electrical pulses on copper wires or light pulses in optical fibers.

As you learned in Lesson 2, all data sent out on a network is from a source and is going to a destination. In this lesson you will learn access to the networking media occurs at the data link layer of the OSI model. In particular you will learn about how data is able to locate its intended destination on a network. You will also learn what some of the problems associated with large network are and how these problems can be resolved.

Where is the NIC card on the OSI model?
If you refer to the OSI model, below, you will see that the data link layer, layer 2, where the MAC address is located, is adjacent to the physical layer. Thus on a network the NIC card is where a device connects to the media and each NIC card has a unique MAC address.

How does the source establish a communication pathway to the destination?
On an Ethernet network, when one device wants to send data to another device, it can open a communication pathway to the other devices by using it's MAC address. Here's how; When data is sent out on a network by a source, it carries the MAC address of its intended destination. As this data travels along the network media, the NIC card in each device on the network checks to see if its MAC address matches the physical destination address carried by the data packet. If no match is made, the NIC card ignores the data packet, and the data packet continues along the network to the next station. However, when a match is made, the NIC card makes a copy of the data packet which it places in the computer where it resides at the data link layer. Even though this copy has been made by the NIC card and placed on the computer, the original data packet continues along the network where other NIC cards will be able to look at it to determine if a match can be made.

How does this method of establishing a communication pathway pose a problem for larger networks?
While sending out data to every device on a network might work for a relatively small network, it is easy to see that the larger an Ethernet network is, the more traffic there will be on that network. This can present a serious problem, since only one data packet can be on a cable at any one time. If there were only one cable interconnecting every device on a network, this could considerably slow down the flow of data over the network.

What are internetworking devices?
Internetworking devices are products used to connect networks. As computer networks grow in size and complexity, so do the internetworking devices used to connect them. However, regardless of the type of internetworking device used, they all share one or more common purposes. First, they allow a greater number of nodes to be connected to the network. Second, they extend the distance over which a network can extend. Third, they localize traffic on the network. Fourth, they can merge existing networks. And, Fifth, they isolate network problems so that they can be diagnosed more easily.

LAN devices include bridges, hubs, Ethernet switches, routers, and ATM switches. WAN devices include ATM switches, modems, and communications servers. You will be learning about many of these device in future lessons.

What internetworking devices operate at the physical layer (layer 1) of the OSI model?
In internetworking two or the more common problems that exist are that there are too many nodes or that there is not enough cable. A repeater can provide a simple solution if either of these two problems exits.

To understand how a repeater works, it is important to first understand that as data leaves a source and goes out over the network, it is transformed into either electrical or light impulses that pass along the networking media. These impulses are referred to as signals. When signals first leave a transmitting station, they are clean and easily recognizable. However, the longer the cable length, the weaker and more deteriorated the signals become as they pass along the networking media. For example, specifications for category 5 twisted pair Ethernet cable establish the maximum distance that signals can travel along a network as 100 meters. If a signal travels beyond that distance, there is no guarantee that a NIC card will be able to read it.

So that signals will not become unrecognizable to devices receiving them on the network, repeaters take in weakened signals, clean them up, amplify them, and send them on their way along the network. By using repeaters, the distance over which a network can operate is extended. Like networking media, repeaters exist at the physical layer, layer 1, of the OSI model.

A similar process exists when there are too many devices attached to a network. Each device attached to the network media causes the signal to degrade slightly. If a signal has to pass by too many stations or nodes, it can become so weak that it is unrecognizable to devices receiving it. To recap, repeaters take in weakened signals, clean them up, amplify them, and send them on their way along the network. By using repeaters in this manner, a greater number of nodes can be attached to a network.