In this module, we will discuss the fundamental concepts of networking, Ethernet technology, and data transmission in Ethernet networks.
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
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.
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.
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).
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
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).
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.
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.
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.
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.
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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.
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.
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 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.
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
Repeaters and hubs
Ethernet bridges and switches
Multilayer switches and routers
Ethernet Virtual LANs (VLANs)
Ethernet beyond the LAN
Physical Layer Characteristics
Our discussion of physical layer characteristics covers both the physical media over which network communications flow and the rate at which communications occur. In fact, the nomenclature for the various types of Ethernet is based on both of these characteristics. The Ethernet type is referred to in the following format:
n-BASE-phy, such as 10BASE-T where:
n is the data rate in megabits per second (Mbps).
BASE indicates that the media is dedicated to only Ethernet services.
phy is a code assigned to a specific type of media.
A variety of media and transmission rates are available for Ethernet networks. The major media types used today are:
Unshielded Twisted Pair (UTP) copper cable
Shielded Twisted Pair (STP) copper cable
Fiber optic cables
The IEEE 802.3 standard identifies the following types of media for an Ethernet connection:
10BASE2: Defined in IEEE 802.3a, 10BASE2 Ethernet uses thin wire coaxial cable. It allows cable runs of up to 185 meters (607 feet). A maximum of 30 workstations can be supported on a single segment. This Ethernet type is no longer in use for new installations.
10BASE-T: Defined in IEEE 802.3i, 10BASE-T uses UTP copper cable and RJ-45 connectors to connect devices to an Ethernet LAN. The RJ-45 is a very common 8-pin connector.
Fast Ethernet: Defined in IEEE 802.3u, Fast Ethernet is used for transmission at a rate of 100 Mbps. It includes 100BASE-TX, which uses UTP copper cable. With this type of cable, each segment can run up to 100 meters (328 feet). Another media option specified in this standard is 100BASE-FX, which uses optical fiber supporting data rates of up to 100 Mbps.
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Gigabit Ethernet (GbE): Defined in IEEE 802.3z, GbE uses fiber for transmitting Ethernet frames at a rate of 1000 Mbps or 1 Gbps. GbE includes 1000BASE-SX for transmission over Multi-Mode Fiber (MMF), and 1000BASE-LX for transmission over Single-Mode Fiber (SMF). The differences between Multi-Mode and Single-Mode are the physical makeup of the fiber itself and the light source that is normally used – multi-mode normally uses an LED while single-mode uses a laser. Multi-mode has limited distance capability when compared to single-mode.
1000BASE-T: Defined in IEEE 802.3ab, 1000BASE-T provides GbE service over twisted pair copper cable.
10 GbE: Defined in IEEE 802.3ae, 10 GbE transmits Ethernet frames at data rates up to 10 Gbps.
Ethernet can operate in either of two communication modes, half-duplex or full-duplex. Ethernet MAC establishes procedures that all devices sharing a communication channel must follow. Half-duplex mode is used when devices on a network share a communication channel. Full-duplex mode is used when devices have no contention from other devices on a network connection. Let’s discuss each of these modes in more detail.
As shown in Figure 1-3, a device operating in half-duplex mode can send or receive data but cannot do both at the same time. Originally, as specified in the DIX80 standard, Ethernet only supported half-duplex operation.
Figure 1-3: Half-Duplex Transmission
Half-duplex Ethernet uses the CSMA/CD protocol to control media access in shared media LANs. With CSMA/CD, devices can share media in an orderly way. Devices that contend for shared media on a LAN are members of the same collision domain. In a collision domain, a data collision occurs when two devices on the LAN transmit data at the same time. The CSMA/CD protocol enables recovery from data collisions.
With CSMA/CD, a device that has data to transmit performs “carrier sense.” Carrier sense is the ability of a device to monitor the transmission media for the presence of any data transmission. If the device detects that another device is using the transmission media, the device waits for the transmission to end. When the device detects that the transmission media is not being used, the device starts transmitting data.
Figure 1-4 shows how CSMA/CD handles a data collision. When a collision occurs, the transmitting device stops the transmission and sends a jamming signal to all other devices to indicate the collision. After sending the jamming signal, each device waits for a random period of time, with each device generating its own time to wait, and then begins transmitting again.
Figure 1-4: CSMA/CD Operation
In the full-duplex communication mode, a device can send and receive data at the same time as shown in Figure 1-5. In this mode, the device must be connected directly to another device using a Point-to-Point (P2P) link that supports independent transmit and receive paths. (P2P is discussed later in this course.)
Figure 1-5: Full-Duplex Transmission
Full-duplex operation is restricted to links meeting the following criteria:
The transmission media must support the simultaneous sending and receiving of data. Twisted pair and fiber cables are capable of supporting full-duplex transmission mode. These include Fast Ethernet, GbE, and 10 GbE transmission media.
The connection can be a P2P link connecting only two devices, or multiple devices can be connected to each other through an Ethernet switch.
The link between both devices needs to be capable of, and configured for, full-duplex operation.
CSMA/CD is not used for full-duplex communications because there is no possibility of a data collision. And, since each device can both send and receive data at the same time, the aggregate throughput of the link is doubled. (Throughput is the amount of data that can be transmitted over a certain period of time.)
Let’s discuss another fundamental aspect of Ethernet transmission – the Ethernet frame. The Ethernet frame is used to exchange data between two Data Link layer points via a direct physical or logical link in an Ethernet LAN. The minimum size of an Ethernet frame is 64 bytes. Originally, the maximum size for a standard Ethernet frame was 1518 bytes; however, it is now possible that an Ethernet frame can be as large as 10,000 bytes (referred to as a jumbo frame).
As shown in Figure 1-6, an Ethernet frame consists of the following fields: (NOTE: The first two fields are added/stripped at Layer 1 and are not counted as part of the 1518 byte standard frame.)
Preamble: This 7-byte field establishes bit synchronization with the sequence of 10101010 in each byte.
Start Frame Delimiter: This 1-byte field indicates the start of the frame at the next byte using a bit sequence of 10101011.
Destination MAC Address: This field contains the MAC hardware address of the Ethernet frame’s destination.
Source MAC Address: This field contains the MAC hardware address of the device sending the frame.
Type / Length: The specific use of this field depends on how the frame was encapsulated. When type-encapsulation is used, the field identifies the nature of the client protocol running above the Ethernet. When using length-encapsulation, this field indicated the number of bytes in the Data field. The IEEE maintains a list of accepted values for this field, the list may be viewed at: http://standards.ieee.org/regauth/ethertype/
Data: This field contains the data or payload that has been sent down from Layer 3 for packaging to Layer 2.
Frame Check Sequence (FCS): This 32-bit field is used for checking the Ethernet frame for errors in bit transmission. FCS is also known as Cyclical Redundancy Check (CRC).
Figure 1-6: Ethernet Frame
Now that we have defined the basic structure of an Ethernet frame, let’s see how we can use the destination MAC address to create three different types of Ethernet frames.
An Ethernet frame intended for a single device on the network is a unicast frame. An example is shown in Figure 1-7. In this example, Station A is transmitting an FTP request to a specific FTP server on the network. The destination MAC address in the frames being sent for this request is the MAC address assigned to the FTP server by its manufacturer. Therefore, these frames are unicast frames, only intended specifically for one device on the network, the FTP server.
Figure 1-7: Unicast Frame Transmission
Multicast is a mechanism that provides the ability to send frames to a specific group of devices on a network – one sender to all who are set to receive. This is done by setting a frame’s destination MAC address to a multicast address assigned by a higher level protocol or application. However, devices must be enabled to receive frames with this multicast address.
An example of multicast frames is shown in Figure 1-8. In this example, the video server is transmitting the same video channel, via an Ethernet switch, to a group of video display devices on the network. The destination MAC address is the multicast address assigned by the video application. The receiving stations are configured to accept Ethernet frames with this multicast address.
Figure 1-8: Multicast Frame Transmission
Broadcasting is a mechanism for sending data in broadcast frames to all the devices in a broadcast domain. A broadcast domain is defined as a set of devices that can communicate with each other at the Data Link layer. Therefore, in a network that does not include higher layer devices, all of the network devices are in the same broadcast domain.
In broadcast frames, the hexadecimal destination MAC address is always ff:ff:ff:ff:ff:ff which, in binary notation, is a series of 48 bits, each set to a value of 1. All devices in the broadcast domain recognize and accept frames with this destination MAC address.
Be sure that students understand hexadecimal vs. binary notation, but do not take this topic beyond the scope of this course.
Since broadcasting reaches all devices within a broadcast domain, Ethernet can use this capability to perform various device setup and control functions. This is a very useful feature, allowing implementation and growth of a LAN with little intervention from a network administrator.
Figure 1-9 shows a broadcast transmission in which Station A is transmitting frames with this broadcast destination MAC address. All devices in the same broadcast domain as Station A receive and process the broadcast frames.
Figure 1-9: Broadcast Frame
Now that we have covered some basic concepts for LANs and Ethernet transmission, let’s continue by discussing how devices on Ethernet LANs are connected.
Check the existing knowledge of students on the differences among switches, hubs, routers, and gateways. Initiate a discussion around the differences among these devices and their suitability to different applications.
Repeaters and Hubs
A very simple LAN topology consists of network devices that are all connected directly to a shared medium as shown in Figure 1-10. If we need to connect more devices to the LAN, we are limited by the characteristics of the shared media. Devices such as repeaters and hubs can be used to overcome distance limitations of the media, allowing the reach of the network to be extended.
Figure 1-10: Simple LAN Topology
Repeaters are Physical layer devices that regenerate a signal, which effectively allows the network segment to extend a greater distance. As shown in Figure 1-11, we can use the additional segment length to add more devices to the LAN. Keep in mind that devices added through implementation of repeaters are still in the same collision domain as the original devices. This results in more contention for access to the shared transmission media. Such devices are in little use today.
Figure 1-11: LAN Extended with a Repeater
As shown in Figure 1-12, hubs can also be used to extend the distance of a LAN segment. Hubs are Layer 1 (physical) devices. The advantage of a hub versus a repeater is that hubs provide more ports. Increased contention for media access still exists since the additional devices connected to the hub(s) are still in the same collision domain.
Figure 1-12: LAN Extended with a Hub
Ethernet Bridges and Switches
Ethernet bridges and switches are Layer 2 (Data Link) devices that provide another option for extending the distance and broadcast domain of a network. Unlike repeaters and hubs, bridges and switches keep the collision domains of connected LAN segments isolated from each other as shown in Figure 1-13. Therefore, the devices in one segment do not contend with devices in another segment for media access.
Figure 1-13: LAN Extended with an Ethernet Switch
Frame Forwarding with Ethernet Switches
As Layer 2 devices, Ethernet switches make frame-forwarding decisions based on source and destination MAC addresses. One of the processes used in making these decisions is MAC learning.
To make efficient use of the data pathways that are dynamically cross connected within an Ethernet switch, the switch keeps track of the location of as many active devices as its design allows. When an Ethernet frame ingresses (enters) a switch, the switch inspects the frame’s source address to learn the location of the sender and inspects the destination address to learn the location of the recipient. This knowledge is kept in a MAC address table. Figure 1-14 shows an example of a MAC address table. As long as the sender remains connected to the same physical port that their MAC address was learned on, the switch will know which port to forward frames to that are destined for that particular sender’s address.
Figure 1-14: MAC Address Table
MAC address information stored in a MAC address table is not retained indefinitely. Each entry is time stamped; and if no activity is sensed for a period of time, referred to as an aging period, the inactive entry is removed. This is done so that only active devices occupy space in the table. This keeps the MAC address table from overloading and facilitates address lookup. The default aging period is typically five minutes.
Figure 1-15 shows how an Ethernet switch forwards frames based on entries in the MAC address table. The forwarding process consists of the following steps:
Inspect the incoming frame’s MAC destination address:
If the MAC destination address is a broadcast address, flood it out all ports within the broadcast domain.
If the MAC destination address is a unicast address, look for it in the MAC address table. If the address is found, forward the frame on the egress (exit) port where the NE knows the device can be reached. If not, flood it.
Flooding allows communication even when MAC destination addresses are unknown. Along with multicast, which is actually a large set of special-purpose MAC addresses, network traffic can be directed to any number of devices on a network.
Inspect the incoming frame’s MAC source address:
If the MAC source address is already in the MAC address table, update the aging timer. This is an active device on the port through which it is connected.
If the MAC source address is not currently in the MAC address table, add it in the list and set the aging timer. This is also an active device.
Periodically check for MAC address table entries that have expired. These are no longer active devices on the port on which they were learned, and these table entries are removed. If a device is moved from one port to another, the device becomes active on the new port’s MAC table. This is referred to as MAC motion.
An Ethernet switch will purposely filter (drop) certain frames. Whether a frame is dropped or forwarded can depend on the switch configuration, but normal switch behavior drops any frame containing a destination address that the switch knows can be reached through the same port where the frame was received. This is done to prevent a device from receiving duplicate frames.
Figure 1-15: Frame-Forwarding Process
A MAC Learning and Broadcast Domain Analogy – Mail Delivery
Consider this following analogy to understand the concept of MAC learning and broadcast domain:
Consider a situation where your friend wants to send you a birthday party invitation (the invitation represents an Ethernet frame). You and your friend live on the same street (the street represents a broadcast domain). However, there is a problem. Your friend does not know your house address so she writes her return (source) address on the birthday party invitation card and writes “the street name” as your (destination) address. Your friend drops the envelope in her mail box (your friend’s mail box represents a LAN) as shown in Figure 1-16.
Figure 1-16: Broadcast Analogy, Part 1
When the mail carrier picks up the mail, he notices that the destination address is “unknown.” The postman goes to a copier and makes enough copies so that he can deliver one copy to each possible destination address on the street. This would mean every house on the street, except for your friend’s house, will get a copy of the invitation. After the postman has delivered the envelopes to all the houses (this process is analogous to a broadcast transmission), you receive the birthday party invitation and recognize your name on the envelope. So, you open the envelope and read the invitation.
Figure 1-17: Broadcast Analogy, Part 2
All of your neighbors receive copies of the same envelope, but they see that the name is not theirs so they simply discard it. After reading the invitation, you send a thank you card back to your friend with your friend’s address; and you include a return (source) address. The postman sees that this envelope has a specific destination address so it can be delivered without broadcasting. It also has a source address, so the postman now knows your address. It is now possible to exchange mail directly with your friend without broadcasting letters to your neighbors. In other words, you can communicate using unicast transmission. If you and your friend were on different streets (broadcast domains), you would have never received your invitation card; and communication could have never occurred.
Multilayer Switches and Routers
In this course, our discussion of switching focuses on switching at the Data Link level since Ethernet is a Layer 2 technology. However, switching can also be
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