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In this module, we will discuss the fundamental concepts of networking, Ethernet technology, and data transmission in Ethernet networks.

Module Objectives

At the end of this module, you will be able to:

Explain the seven network layers as defined by the Open Systems Interconnection (OSI) Reference model

Describe, at a high level, the history of Ethernet

List physical layer characteristics of Ethernet

Explain the difference between half-duplex and full-duplex transmission in an Ethernet network

Describe the structure of an Ethernet frame

Explain how networks can be extended and segmented using various Ethernet devices, including hubs and switches

Describe how frames are forwarded in an Ethernet network

Explain, at a high level, how Virtual Local Area Networks (VLANs) function

Network Fundamentals

This section provides a brief overview of Local Area Network (LAN) technology. We will discuss LAN architecture from a functional perspective. A network is commonly divided into seven functional layers referred to as the OSI Reference model. In addition, we will briefly discuss the use of addressing in LANs.

Instructor Note

Point out that this section only touches briefly on LAN concepts, and students may want to explore LAN technology in more depth on their own.

Network Layers

A complete LAN implementation involves a number of functions that, in combination, enable devices to communicate over a network. To understand how Ethernet fits into this overall set of functions, we can use the OSI Reference model. The OSI Reference model was developed in 1984 by the International Organization for Standardization (ISO).

Instructor Note

You can introduce the discussion of the OSI Reference model by comparing analysis of the model to “peeling an onion.”

Shown in Figure 1-1, the OSI Reference model defines seven functional layers that process data when data is transmitted over a network. When devices communicate over a network, data travels through some or all of the seven functional layers.

The figure shows data being transmitted from Station A, the source, to Station B, the destination. The transmission begins at the Application layer. As data (referred to as the payload) is transmitted by Station A down through the layers, each layer adds its overhead information to the data from the layer above. (The process of packaging layer-specific overhead with the payload is referred to as encapsulation – discussed later in this course.) Upon reaching the Physical layer, the data is placed on the physical media for transmission.

The receiving device reverses the process, unpackaging the contents layer by layer, thus allowing each layer to effectively communicate with its “peer” layer. Ethernet operates at Layer 2, the Data Link layer.

Using Figure 1-1 as a reference, we will briefly discuss what occurs at each layer.

Figure 1-1: The OSI Reference Model

Application Layer

The Application layer, Layer 7 (L7), is responsible for interacting with the software applications that send data to another device. These interactions are governed by Application layer protocols, such as Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).

Presentation Layer

The Presentation layer, Layer 6 (L6), performs data translation, compression, and encryption. Data translation is required when two different types of devices are connected to each other, and both use different ways to represent the data. Compression is required to increase the transmission flow of data. Encryption is required to secure data as it moves to the lower layers of the OSI Reference model.

Session Layer

The Session layer, Layer 5 (L5), is responsible for creating, maintaining, and terminating communication among devices. A session is a logical link created between two software application processes to enable them to transmit data to each other for a period of time. Logical links are discussed later in this course.

Transport Layer

The Transport layer, Layer 4 (L4), is responsible for reliable arrival of messages and provides error checking mechanisms and data flow controls. The Transport layer also performs multiplexing to ensure that the data from various applications is transported using the same transmission channel. Multiplexing enables data from several applications to be transmitted onto a single physical link, such as a fiber optic cable. The data flow through the Transport layer is governed by transmission protocols, such as Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), which are beyond the scope of this course.

Network Layer

The Network layer, Layer 3 (L3), is responsible for moving data across interconnected networks by comparing the L3 source address with the L3 destination address. The Network layer encapsulates the data received by higher layers to create packets. The word packet is commonly used when referring to data in the Network layer. The Network layer is also responsible for fragmentation and reassembly of packets.

Data Link Layer

The Data Link layer, Layer 2 (L2), responds to requests sent by the Network layer and sends service requests to the Physical layer. The Data Link layer is responsible for defining the physical addressing, establishing logical links among local devices, sequencing of frames, and error detection. The Ethernet frame is a digital data transmission unit on Layer 2. The word frame is commonly used when referring to data in the Data Link layer.

The Data Link layer has been subdivided into two sub-layers: Logical Link Control (LLC) and Media Access Control (MAC). LLC, defined in the Institute of Electrical and Electronics Engineers (IEEE) 802.2 specification, manages communications among devices over a link. LLC supports both connection-oriented (physical, ex an Ethernet switch) and connectionless (wireless, ex a wireless router) services. The MAC sub-level manages Ethernet frame assembly and dissembly, failure recovery, as well as access to, and routing for, the physical media. This will be discussed in more detail in this module.

Physical Layer

The Physical layer, Layer 1 (L1), performs hardware-specific, electrical, and mechanical operations for activating, maintaining, and deactivating the link among communicating network systems. The Physical layer is responsible for transmitting the data as raw bits over the transmission media.

Now that we have reviewed the OSI Reference model, let’s discuss addressing of network devices.


Network devices that operate at the Data Link layer or higher are referred to as stations. Stations are classified as either end stations or intermediate stations. End stations run end-user applications and are the source or final destination of data transmitted over a network. Intermediate stations relay information across the network between end stations.

A characteristic of stations is that they are addressable. In the next section, we discuss the specifics of addressing.


Each device in an Ethernet network is assigned an address that is used to connect with other devices in the network. This address is referred to as the MAC address and is typically a permanent address assigned by the device manufacturer. Addressing is used in the network to identify the source station and the destination station or stations of transmitted data.

As shown in Figure 1-2, the MAC address consists of 48 bits (6 bytes), typically expressed as colon-separated, hexadecimal pairs.

Figure 1-2: MAC Address Structure

The MAC address consists of the following:

Individual / Group (I/G) Bit: For destination address, if the I/G bit = 0, the destination of the frame is a single station. This is referred to as a unicast address. If the I/G bit = 1, the destination is a group of stations. This is referred to as a multicast address. In source addresses, the I/G bit = 1.

Universal / Local (U/L) Bit: The U/L bit identifies whether the MAC address is universally unique (U/L bit = 0) or only unique in the LAN in which it is located. Vendor-assigned MAC addresses are always universally unique. A locally unique MAC address is assigned by the network administrator.

Organizationally Unique Identifier (OUI): This typically identifies the network equipment manufacturer. OUIs are assigned to organizations by the IEEE. To locate information on the OUI associated with a manufacturer go to the following website: http://standards.ieee.org/regauth/oui/index.shtml

Vendor-Assigned Bits: These bits are assigned by the vendor to uniquely identify a specific device.

Following is an example of a MAC address:


Later in this module, we discuss how MAC addresses are used in Ethernet networks.

Introduction to Ethernet

Ethernet is an internationally-accepted, standardized LAN technology. It is one of the simplest and most cost-effective LAN networking technologies in use today. Ethernet has grown through the development of a set of standards that define how data is transferred among computer networking devices.

Although several other networking methods are used to implement LANs, Ethernet remains the most common method in use today. While Ethernet has emerged as the most common LAN technology for a variety of reasons, the primary reasons include the following:

Ethernet is less expensive than other networking options.

Easy is easy to install and provision the various components.

Ethernet is faster and more robust than the other LAN technologies.

Ethernet allows for an efficient and flexible network implementation.

History of Ethernet

Ethernet was invented in 1973 by Bob Metcalfe and David Boggs at the Xerox Palo Alto Research Center (PARC). Ethernet was originally designed as a high-speed LAN technology for connecting Xerox Palo Alto graphical computing systems and high-speed laser printers.

In 1979, Xerox® began work with Digital Equipment Corporation (DEC) and Intel® to develop a standardized, commercial version of Ethernet. This partnership of DEC, Intel, and Xerox (DIX) developed Ethernet Version 1.0, also known as DIX80. Further refinements resulted in Ethernet Version 2, or DIX82, which is still in use today.

Project 802

In 1980, the Institute of Electrical and Electronics Engineers (IEEE) formed Project 802 to create an international standard for LANs. Due to the complexity of the technology and the emergence of competing LAN technologies and physical media, five working groups were initially formed. Each working group developed standards for a particular area of LAN technology. The initial working groups consisted of the following:

IEEE 802.1: Overview, Architecture, Internetworking, and Management

IEEE 802.2: Logical Link Control

IEEE 802.3: Carrier Sense Multiple Access / Collision Detection (CSMA/CD) Media Access Control (MAC)

IEEE 802.4: Token Bus MAC and Physical (PHY)

IEEE 802.5: Token Ring MAC and PHY

Additional working groups have since been added to address other areas of LAN technology.

The standards developed by these working groups are discussed as we move through this course. However, let’s look at IEEE 802.3, which addresses standards specific to Ethernet.

IEEE 802.3

IEEE 802.3 was published in 1985 and is now supported with a series of supplements covering new features and capabilities. Like all IEEE standards, the contents of supplements are added to the standard when it is revised. Now adopted by almost all computer vendors, IEEE 802.3 consists of standards for three basic elements:

The physical media (fiber or copper) used to transport Ethernet signals over a network

MAC rules that enable devices connected to the same transmission media to share the transmission channel

Format of the Ethernet frame, which consists of a standardized set of frame fields

We will discuss the transmission media used in Ethernet networks, the MAC rules, and the Ethernet frame later in this module.

Instructor Note

Tell the class that we will discuss the transmission media used in Ethernet networks, the MAC rules, and the Ethernet frame later in this module.

You can briefly explain the differences among LANs, WANs, and MANs to the students.

Ethernet Transmission Fundamentals

This section covers basic fundamentals of data transmission on Ethernet networks. Specifically, we will cover the following topics:

Physical layer characteristics

Communication modes

Ethernet frames

Repeaters and hubs

Ethernet bridges and switches

Multilayer switches and routers