Cisco ICND1 Foundation Learning Guide: LANs and Ethernet
Date: Jun 24, 2013
This chapter includes the following sections:
- Chapter Objectives
- Understanding LANs
- Connecting to an Ethernet LAN
- Chapter Summary
- Additional Resources
- Review Questions
- Production Network Simulation Question 3-1
Local-area networks (LAN) tend to spoil us as network users. These collections of high-speed network equipment allow us to achieve remarkable speeds in accessing network data and information. LANs are a relatively low-cost means of sharing expensive resources. LANs allow multiple users in a relatively small geographic area to exchange files and messages and to access shared resources such as file servers. LANs have rapidly evolved into support systems that are critical to communications within an organization. This chapter will ensure that you are comfortable describing these important network structures.
This chapter also describes different Ethernet media options (copper and fiber), which are presented together with a description of the most common connectors and cable types. Ethernet frame structure is introduced, and important fields are described. MAC addresses and their function are also elaborated on.
Chapter Objectives
Upon completing this chapter, you will be able to describe LAN networks. You will also be able to describe common Ethernet technologies typically found within these important areas of the overall network. These abilities include meeting these objectives:
- Define a LAN
- Identify the components of a LAN
- Describe the types of Ethernet LAN connection media
- Describe the fields of an Ethernet frame
- Define the structure and function of MAC addresses
Understanding LANs
A local-area network is a common type of network found in home offices, small businesses, and large enterprises. Understanding how a LAN functions, including network components, frames, Ethernet addresses, and operational characteristics, is important for an overall knowledge of networking technologies.
This section describes LANs and provides fundamental knowledge about LAN characteristics, components, and functions. It also describes the basic operations of an Ethernet LAN and how frames are transmitted over it.
The Definition of a LAN
A LAN is a network of computers and other components located relatively close together in a limited area. LANs can vary widely in their size. A LAN might consist of only two computers in a home office or small business, or it might include hundreds of computers in a large corporate office or multiple buildings. Figure 3-1 shows some examples of LANs.
Figure 3-1.Examples of LANs
A small home business or a small office environment could use a small LAN to connect two or more computers and to connect the computers to one or more shared peripheral devices such as printers. A large corporate office could use multiple LANs to accommodate hundreds of computers and shared peripheral devices, for departments such as finance or operations, spanning many floors in an office complex.
Components of a LAN
Every LAN has specific components, including hardware, interconnections, and software. Figure 3-2 highlights some typical hardware components of a LAN.
Figure 3-2.Typical Components of a LAN
Regardless of the size of the LAN, it requires these fundamental components for its operation:
- Computers: Computers serve as the endpoints in the network, sending and receiving data.
- Interconnections: Interconnections enable data to travel from one point to another in the network. Interconnections include these components:
- NICs: Network interface cards (NIC) translate the data produced by the computer into a format that can be transmitted over the LAN.
- Network media: Network media, such as cables or wireless media, transmit signals from one device on the LAN to another.
- Network devices: A LAN requires the following network devices:
- Hubs: Hubs provide aggregation devices operating at Layer 1 of the OSI reference model. However, hubs have been replaced in this function by switches, and it is very rare to see hubs in any LAN these days.
- Ethernet switches: Ethernet switches form the aggregation point for LANs. Ethernet switches operate at Layer 2 of the OSI reference model and provide intelligent distribution of frames within the LAN.
- Routers: Routers, sometimes called gateways, provide a means to connect LAN segments. Routers operate at Layer 3 of the OSI reference model.
- Protocols: Protocols govern the way data is transmitted over a LAN and include the following:
- Ethernet protocols
- Internet Protocol (IP)
- Internet Protocol version 6 (IPv6)
- Address Resolution Protocol (ARP) and Reverse Address Resolution Protocol (RARP)
- Dynamic Host Configuration Protocol (DHCP)
Functions of a LAN
LANs provide network users with communication and resource-sharing functions, including the following:
- Data and applications: When users are connected through a network, they can share files and even software application programs. This makes data more easily available and promotes more efficient collaboration on work projects.
- Resources: The resources that can be shared include both input devices, such as cameras, and output devices, such as printers.
- Communication path to other networks: If a resource is not available locally, the LAN, through a gateway, can provide connectivity to remote resources—for example, access to the web.
How Big Is a LAN?
A LAN can be configured in a variety of sizes, depending on the requirements of the environment in which it operates.
LANs can be of various sizes to fit different work requirements, including the following:
- Small office/home office (SOHO): The SOHO environment typically has only a few computers and some peripherals such as printers.
- Enterprise: The enterprise environment might include many separate LANs in a large office building or in different buildings on a corporate campus. In the enterprise environment, each LAN might contain hundreds of computers and peripherals..
Figure 3-3 demonstrates the dramatic differences that can exist with the size of LANs.
Figure 3-3.Different LAN Sizes
Ethernet
Ethernet is the most common type of LAN. It was originally developed in the 1970s by Digital Equipment Corporation (DEC), Intel, and Xerox (DIX) and was called DIX Ethernet. It later came to be called thick Ethernet (because of the thickness of the cable used in this type of network), and it transmitted data at 10 megabits per second (Mbps). The standard for Ethernet was updated in the 1980s to add more capability, and the new version of Ethernet was referred to as Ethernet Version 2 (also called Ethernet II).
The Institute of Electrical and Electronics Engineers (IEEE) is a professional organization that defines network standards. IEEE standards are the predominant LAN standards in the world today. In the mid-1980s, an IEEE workgroup defined new standards for Ethernet-like networks. The set of standards they created was called Ethernet 802.3 and was based on the carrier sense multiple access with collision detection (CSMA/CD) process. Ethernet 802.3 specified the physical layer (Layer 1) and the MAC portion of the data link layer (Layer 2). Today, this set of standards is most often referred to as simply “Ethernet.”
Ethernet LAN Standards
Ethernet LAN standards specify cabling and signaling at both the physical and data link layers of the OSI reference model. This topic describes Ethernet LAN standards at the data link layer.
Figure 3-4 shows how LAN protocols map to the OSI reference model.
Figure 3-4.Ethernet and the OSI Model
The IEEE divides the OSI data link layer into two separate sublayers:
- Logical link control (LLC): Transitions up to the network layer
- MAC: Transitions down to the physical layer
LLC Sublayer
The IEEE created the LLC sublayer to allow part of the data link layer to function independently from existing technologies. This layer provides versatility in services to the network layer protocols that are above it, while communicating effectively with the variety of MAC and Layer 1 technologies below it. The LLC, as a sublayer, participates in the encapsulation process.
An LLC header tells the data link layer what to do with a packet when it receives a frame. For example, a host receives a frame and then looks in the LLC header to understand that the packet is destined for the IP protocol at the network layer.
The original Ethernet header (prior to IEEE 802.2 and 802.3) did not use an LLC header. Instead, it used a type field in the Ethernet header to identify the Layer 3 protocol being carried in the Ethernet frame.
MAC Sublayer
The MAC sublayer deals with physical media access. The IEEE 802.3 MAC specification defines MAC addresses, which uniquely identify multiple devices at the data link layer. The MAC sublayer maintains a table of MAC addresses (physical addresses) of devices. To participate on the network, each device must have a unique MAC address.
The Role of CSMA/CD in Ethernet
Ethernet signals are transmitted to every station connected to the LAN, using a special set of rules to determine which station can “talk” at any particular time. This topic describes that set of rules.
Ethernet LANs manage the signals on a network by CSMA/CD, which is an important aspect of Ethernet. Figure 3-5 illustrates the CSMA/CD process.
Figure 3-5.CSMA/CD Process
In an Ethernet LAN, before transmitting, a computer first listens to the network media. If the media is idle, the computer sends its data. After a transmission has been sent, the computers on the network compete for the next available idle time to send another frame. This competition for idle time means that no one station has an advantage over another on the network.
Stations on a CSMA/CD LAN can access the network at any time. Before sending data, CSMA/CD stations listen to the network to determine whether it is already in use. If it is, the CSMA/CD stations wait. If the network is not in use, the stations transmit. A collision occurs when two stations listen for network traffic, hear none, and transmit simultaneously (see Figure 3-5). In this case, both transmissions are damaged, and the stations must retransmit at some later time. CSMA/CD stations must be able to detect collisions to know that they must retransmit.
When a station transmits, the signal is referred to as a carrier. The NIC senses the carrier and consequently refrains from broadcasting a signal. If no carrier exists, a waiting station knows that it is free to transmit. This is the “carrier sense” part of the protocol.
The extent of the network segment over which collisions occur is referred to as the collision domain. The size of the collision domain has an impact on efficiency and therefore on data throughput. In today’s LANs, switches have replaced hubs. The reason this occurs is that switches create tiny collision domains containing just one device. This eliminates the potential for collisions. This process is often called “microsegmentation” of the network.
In the CSMA/CD process, priorities are not assigned to particular stations, so all stations on the network have equal access. This is the “multiple access” part of the protocol. If two or more stations attempt a transmission simultaneously, a collision occurs. The stations are alerted of the collision, and they execute a backoff algorithm that randomly schedules retransmission of the frame. This scenario prevents the machines from repeatedly attempting to transmit at the same time. Collisions are normally resolved in microseconds. This is the “collision detection” part of the protocol.
While collisions are resolved quickly, it is still advantageous to eliminate them entirely from the network. This allows much more efficient communications. This is accomplished through the use of switches as described earlier.
Ethernet Frames
Bits that are transmitted over an Ethernet LAN are organized into frames. In Ethernet terminology, the “container” into which data is placed for transmission is called a frame. The frame contains header information, trailer information, and the actual data that is being transmitted.
Figure 3-6 illustrates all the fields that are in a MAC layer of the Ethernet frame, which include the following:
- Preamble: This field consists of 7 bytes of alternating 1s and 0s, which synchronize the signals of the communicating computers.
- Start-of-frame (SOF) delimiter: This field contains bits that signal the receiving computer that the transmission of the actual frame is about to start and that any data following is part of the packet.
- Destination address: This field contains the address of the NIC on the local network to which the packet is being sent.
- Source address: This field contains the address of the NIC of the sending computer.
- Type/length: In Ethernet II, this field contains a code that identifies the network layer protocol. In 802.3, this field specifies the length of the data field. The protocol information is contained in 802.2 fields, which are at the LLC layer. The newer 802.3 specifications have allowed the use of Ethertype protocol identifiers when not using the 802.2 field.
- Data and pad: This field contains the data that is received from the network layer on the transmitting computer. This data is then sent to the same protocol on the destination computer. If the data is too short, an adapter adds a string of extraneous bits to “pad” the field to its minimum length of 46 bytes.
- Frame check sequence (FCS): This field includes a checking mechanism to ensure that the packet of data has been transmitted without corruption.
Figure 3-6.Ethernet Frames
Ethernet Frame Addressing
Communications in a network occur in three ways: unicast, broadcast, and multicast. Ethernet frames are addressed accordingly. Figure 3-7 shows forms of Ethernet communications.
Figure 3-7.Ethernet Communications
The three major types of network communications are as follows:
- Unicast: Communication in which a frame is sent from one host and addressed to one specific destination. In a unicast transmission, you have just one sender and one receiver. Unicast transmission is the predominant form of transmission on LANs and within the Internet.
- Broadcast: Communication in which a frame is sent from one address to all other addresses. In this case, you have just one sender, but the information is sent to all connected receivers. Broadcast transmission is essential when sending the same message to all devices on the LAN.
- Multicast: Communication in which information is sent to a specific group of devices or clients. Unlike broadcast transmission, in multicast transmission, clients must be members of a multicast group to receive the information.
Ethernet Addresses
The address used in an Ethernet LAN, which is associated with the network adapter, is the means by which data is directed to the proper receiving location. Figure 3-8 shows the format of an Ethernet MAC address.
Figure 3-8.Ethernet MAC Address
The address that is on the NIC is the MAC address, often referred to as the burned-in address (BIA), and some vendors allow the modification of this address to meet local needs. A 48-bit Ethernet MAC address has two components:
24-bit Organizational Unique Identifier (OUI): The letter O identifies the manufacturer of the NIC. The IEEE regulates the assignment of OUI numbers. Within the OUI, the two following bits have meaning only when used in the destination address:
- Broadcast or multicast bit: This indicates to the receiving interface that the frame is destined for all or a group of end stations on the LAN segment.
- Locally administered address bit: Normally the combination of OUI and a 24-bit station address is universally unique; however, if the address is modified locally, this bit should be set.
- 24-bit vendor-assigned end station address: This uniquely identifies the Ethernet hardware.
MAC Addresses and Binary-Hexadecimal Numbers
The MAC address plays a specific role in the function of an Ethernet LAN. The MAC sublayer of the OSI data link layer handles physical addressing issues, and the physical address is a number in hexadecimal format that is actually burned into the NIC. This address is referred to as the MAC address, and it is expressed as groups of hexadecimal digits that are organized in pairs or quads, such as the following: 00:00:0c:43:2e:08 or 0000:0c43:2e08. Figure 3-9 shows the MAC address format compared to the MAC frame.
Figure 3-9.Hexadecimal MAC Address
Each device on a LAN must have a unique MAC address to participate in the network. The MAC address identifies the location of a specific computer on a LAN. Unlike other kinds of addresses used in networks, the MAC address should not be changed unless you have some specific need.
Connecting to an Ethernet LAN
In addition to understanding the components of an Ethernet LAN and the standards that govern its architecture, you need to understand the connection components of an Ethernet LAN. This section describes the connection components of an Ethernet LAN, including network interface cards (NIC) and cable.
Ethernet Network Interface Cards
A NIC is a printed circuit board that provides network communication capabilities to and from a personal computer on a network. Figure 3-10 shows an example of a NIC.
Figure 3-10.Network Interface Card
Also called a LAN adapter, the NIC plugs into a motherboard and provides a port for connecting to the network. The NIC constitutes the computer interface with the LAN.
The NIC communicates with the network through a serial connection, and with the computer through a parallel connection. When a NIC is installed in a computer, it requires an interrupt request line (IRQ), an input/output (I/O) address, a memory space within the operating system (such as DOS or Windows), and drivers (software) that allow it to perform its function. An IRQ is a signal that informs a central processing unit (CPU) that an event needing its attention has occurred. An IRQ is sent over a hardware line to the microprocessor. An example of an interrupt request being issued is when a key is pressed on a keyboard, and the CPU must move the character from the keyboard to RAM. An I/O address is a location in memory used by an auxiliary device to enter data into or retrieve data from a computer.
The MAC address is burned onto each NIC by the manufacturer, providing a unique, physical network address.
Ethernet Media and Connection Requirements
Distance and time dictate the type of Ethernet connections required. This section describes the cable and connector specifications used to support Ethernet implementations.
The cable and connector specifications used to support Ethernet implementations are derived from the EIA/TIA standards body. The categories of cabling defined for Ethernet are derived from the EIA/TIA-568 (SP-2840) Commercial Building Telecommunications Wiring Standards. EIA/TIA specifies an RJ-45 connector for unshielded twisted-pair (UTP) cable.
The important difference to note is the media used for 10-Mbps Ethernet versus 100-Mbps Fast Ethernet. In networks today, where you see a mix of 10- and 100-Mbps requirements, you must be aware of the need to change over to UTP Category 5 to support Fast Ethernet.
Connection Media
Several types of connection media can be used in an Ethernet LAN implementation. Figure 3-11 shows typical connection types.
Figure 3-11.Connection Types
The most common type of connection media is the RJ-45 connector and jack illustrated in Figure 3-11. The letters RJ stand for registered jack, and the number “45” refers to a specific physical connector that has eight conductors.
A Gigabit Interface Converter (GBIC), shown in Figure 3-12, is a hot-swappable I/O device that plugs into a Gigabit Ethernet port. A key benefit of using a GBIC is that it is interchangeable, allowing you the flexibility to deploy other 1000BASE-X technology without having to change the physical interface or model on the router or switch. GBICs support UTP (copper) and fiber-optic media for Gigabit Ethernet transmission.
Figure 3-12.1000BASE-T GBIC
Typically, GBICs are used in the LAN for uplinks and are normally used for the backbone. GBICs are also seen in remote networks.
The fiber-optic GBIC, shown in Figure 3-13, is a transceiver that converts serial electric currents to optical signals and converts optical signals to digital electric currents.
Figure 3-13.Fiber GBIC
Optical GBICs include these types:
- Short wavelength (1000BASE-SX)
- Long wavelength/long haul (1000BASE-LX/LH)
- Extended distance (1000BASE-ZX)
Unshielded Twisted-Pair Cable
Twisted-pair is a copper wire–based cable that can be either shielded or unshielded. UTP cable is frequently used in LANs. Figure 3-14 shows an example of a UTP cable.
Figure 3-14.UTP Cable
UTP cable is a four-pair wire. Each of the eight individual copper wires in UTP cable is covered by an insulating material. In addition, the wires in each pair are twisted around each other. The advantage of UTP cable is its ability to cancel interference, because the twisted wire pairs limit signal degradation from electromagnetic interference (EMI) and radio frequency interference (RFI). To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies. Both UTP and shielded twisted-pair (STP) cable must follow precise specifications regarding how many twists or braids are permitted per meter.
UTP cable is used in a variety of types of networks. When used as a network medium, UTP cable has four pairs of either 22- or 24-gauge copper wire. UTP used as a network medium has an impedance of 100 ohms, differentiating it from other types of twisted-pair wiring, such as that used for telephone wiring. Because UTP cable has an external diameter of approximately 0.43 cm, or 0.17 inches, its small size can be advantageous during installation. Also, because UTP can be used with most major network architectures, it continues to grow in popularity.
Here are the categories of UTP cable:
- Category 1: Used for telephone communications; not suitable for transmitting data
- Category 2: Capable of transmitting data at speeds of up to 4 Mbps
- Category 3: Used in 10BASE-T networks; can transmit data at speeds up to 10 Mbps
- Category 4: Used in Token Ring networks; can transmit data at speeds up to 16 Mbps
- Category 5: Capable of transmitting data at speeds up to 100 Mbps
- Category 5e: Used in networks running at speeds up to 1000 Mbps (1 Gbps)
- Category 6: Consists of four pairs of 24-gauge copper wires, which can transmit data at speeds of up to 1000 Mbps
- Category 6a: Used in networks running at speeds up to 10 Gbps
The most commonly used categories in LAN environments today are Categories 1 (used primarily for telephony), 5, 5e, and 6.
UTP Implementation
For a UTP implementation in a LAN, you must determine the EIA/TIA type of cable needed and also whether to use a straight-through or crossover cable. This topic describes the characteristics and uses of straight-through and crossover cables, as well as the types of connectors used when UTP is implemented in a LAN. Figure 3-15 shows an RJ-45 connector.
Figure 3-15.RJ-45 Connector
If you look at the RJ-45 transparent-end connector, you can see eight colored wires, twisted into four pairs. Four of the wires (two pairs) carry the positive or true voltage and are considered “tip” (T1 through T4); the other four wires carry the inverse of false voltage grounded and are called “ring” (R1 through R4). Tip and ring are terms that originated in the early days of the telephone. Today, these terms refer to the positive and negative wires in a pair. The wires in the first pair in a cable or a connector are designated as T1 and R1, the second pair as T2 and R2, and so on.
The RJ-45 plug is the male component, crimped at the end of the cable. As you look at the male connector from the front, the pin locations are numbered from 8 on the left to 1 on the right.
The jack is the female component in a network device, wall, cubicle partition outlet, or patch panel.
In addition to identifying the correct EIA/TIA category of cable to use for a connecting device (depending on which standard is being used by the jack on the network device), you need to determine which of the following to use:
- A straight-through cable (either T568A or T568B at each end)
- A crossover cable (T568A at one end; T568B at the other)
In Figure 3-16, the RJ-45 connectors on both ends of the cable show all the wires in the same order. If the two RJ-45 ends of a cable are held side by side in the same orientation, the colored wires (or strips or pins) can be seen at each connector end. If the order of the colored wires is the same at each end, the cable type is straight-through.
Figure 3-16.Straight-Through Cable
With crossover cables, the RJ-45 connectors on both ends show that some of the wires on one side of the cable are crossed to a different pin on the other side of the cable. Specifically, for Ethernet, pin 1 at one RJ-45 end should be connected to pin 3 at the other end. Pin 2 at one end should be connected to pin 6 at the other end, as shown in the Figure 3-17.
Figure 3-17.Crossover Cable
Figure 3-18 shows the guidelines for choosing which type of cable to use when interconnecting Cisco devices. In addition to verifying the category specification on the cable, you must determine when to use a straight-through or crossover cable.
Figure 3-18.When to Use a Straight-Through Cable Versus a Crossover Cable
Use straight-through cables for the following cabling:
- Switch to router
- Switch to PC or server
- Hub to PC or server
Use crossover cables for the following cabling:
- Switch to switch
- Switch to hub
- Hub to hub
- Router to router
- Router Ethernet port to PC NIC
- PC to PC
Auto-MDIX
After reading the previous section, you might be concerned about when you are cabling your network, you might make a critical mistake! Imagine, just one wrong type of cable, and your entire LAN might fail to access key resources. Fortunately, there is great news because of a new technology called Auto-MDIX. Auto-MDIX stands for automatic medium-dependent interface crossover, a feature that lets the interface automatically discover whether the wrong cable is installed. A switch that supports the Auto-MDIX feature detects the wrong cable and causes the switch to swap the pair it uses for transmitting and receiving. This solves the cabling problem, and the switch is able to communicate just fine regardless of the fact that you connected the “wrong” cable. Obviously, this new technology is very desirable and is making its way to more and more Cisco devices all the time.
Optical Fiber
An optical fiber is a flexible, transparent fiber that is made of very pure glass (silica) and is not much bigger in diameter than a human hair. It acts as a waveguide, or “light pipe,” to transmit light between the two ends of the fiber. Optical fibers are widely used in fiber-optic communications, which permit transmission over longer distances and at higher bandwidths (data rates) than other forms of communication. Fibers are used instead of metal wires because signals travel along them with less loss and with immunity to electromagnetic interference. Figure 3-19 shows an example of optical fiber.
Figure 3-19.Optical Fiber
The two fundamental components that allow a fiber to confine light are the core and the cladding. Most of the light travels from the beginning to the end inside the core. The cladding around the core provides confinement. The diameters of the core and cladding are shown in this illustration, but the core diameter can vary for different fiber types. In this case, the core diameter of 9 µm is very small—the diameter of a human hair is about 50 µm. The outer diameter of the cladding is a standard size of 125 µm. Standardizing the size means that component manufacturers can make connectors for all fiber-optic cables.
The third element in this picture is the buffer (coating), which has nothing to do with the confinement of the light in the fiber. Its purpose is to protect the glass from scratches and moisture. The fiber-optic cable can be easily scratched and broken, like a glass pane. If the fiber is scratched, the scratch could propagate and break the fiber. Another important aspect is the need to keep the fiber dry.
The most significant difference between single-mode fiber (SMF) and multimode fiber (MMF) is in the ability of the fiber to send light for a long distance at high bit rates. In general, MMF is used for shorter distances at a lower bit rate than SMF. For long-distance communications, SMF is preferred. There are many variations of fiber for both MMF and SMF. Figure 3-20 shows the two fiber types.
Figure 3-20.Fiber-Optic Types
The most significant physical difference is in the size of the core. The glass in the two fibers is the same, and the index of refraction change is similar. The core diameter can make a major difference. The diameter of the fiber cladding is universal for matching fiber ends.
The effect of having different-size cores in fiber is that the two fiber types will support different ways for the light to get through the fiber. The left image illustrates MMF. MMF supports multiple ways for the light from one source to travel through the fiber (the source of the designation “multimode”). Each path can be thought of as a mode.
For SMF, the possible ways for light to get through the fiber have been reduced to one, a “single mode.” It is not exactly one, but that is a useful approximation. Table 3-1 summarizes the characteristics of MMF and SMF.
Table 3-1. Summarizing MMF and SMF Characteristics
MMF Characteristics |
SMF Characteristics |
LED transmitter usually used |
Larger transmitter usually used |
Lower bandwidth and speed |
Higher bandwidth and speed |
Shorter distances |
Longer distances |
Less expensive |
More expensive |
An optical fiber connector terminates the end of an optical fiber. A variety of optical fiber connectors are available. The main differences among the types of connectors are dimensions and methods of mechanical coupling. Generally, organizations standardize on one kind of connector, depending on the equipment that they commonly use, or they standardize per type of fiber (one for MMF, one for SMF). Taking into account all the generations of connectors, about 70 connector types are in use today. Figure 3-21 shows some common connector types.
Figure 3-21.Common Fiber-Optic Connector Types
There are three types of connectors:
- Threaded
- Bayonet
- Push-pull
These materials are used for connectors:
- Metal
- Plastic sleeve
Most common connectors are classified as these types:
- ST: Typical for patch panels (for their durability)
- FC: Typical used by service providers for patch panels
- SC: Typical for enterprise equipment
- LC: Typical for enterprise equipment, commonly used on Small Form-Factor Pluggable (SFP) modules
In data communications and telecommunications applications today, small-form-factor connectors (for example, LCs) are replacing the traditional connectors (for example, SCs), mainly to pack more connectors on the faceplate and thus reduce system footprints.
Chapter Summary
LANs are a critical component in computer networks today. While these structures come in many different sizes, they are always used to carry data at speeds as fast as possible over short geographic distances.
Ethernet is the most common type of LAN used today. Standards unique to Ethernet specify Ethernet LAN cabling and signaling at both the physical and data link layers of the OSI reference model. Bits that are transmitted over an Ethernet LAN are organized into frames. Ethernet LANs manage the signals on a network using a process called CSMA/CD.
A NIC or LAN adapter plugs into a motherboard and provides an interface for connecting to the network. The MAC address is burned onto each NIC by the manufacturer, providing a unique, physical network address that permits the device to participate in the network.
The cable and connector specifications used to support Ethernet implementations are derived from the EIA/TIA standards body. The categories of cabling defined for Ethernet are derived from the EIA/TIA-568 (SP2840) Commercial Building Telecommunications Wiring Standards. Several connection media are used for Ethernet, with RJ-45 and GBIC being the most common.
A GBIC is a hot-swappable I/O device that plugs into a Gigabit Ethernet port on a network device to provide a physical interface.
UTP cable is a four-pair wire. Each of the eight individual copper wires in UTP cable is covered by an insulating material, and the wires in each pair are twisted around each other. A crossover cable connects between similar devices like router to router, PC to PC, or switch to switch. A straight-through cable connects between dissimilar devices like switch to router or PC to switch.
This chapter also examined fiber-optic media. Optical fiber is a flexible, transparent fiber that is made of very pure glass (silica) and is not much bigger than a human hair. It acts as a waveguide, or “light pipe,” to transmit light between the two ends of the fiber. Optical fibers are widely used in fiber-optic communications, which permit transmission over longer distances and at higher bandwidths (data rates) than other forms of communication.
Additional Resources
- Gigabit Ethernet, Wikipedia: http://en.wikipedia.org/wiki/Gigabit_Ethernet
- How Fiber Optics Work, How Stuff Works: http://computer.howstuffworks.com/fiber-optic.htm
Review Questions
Use the questions here to review what you learned in this chapter. The correct answers and solutions are found in Appendix A, “Answers to Chapter Review Questions.”
What organization is responsible for Ethernet standards?
- ISO
- IEEE
- EIA
- IEC
What are the characteristics of Ethernet 802.3? (Choose three.)
- Based on the CSMA/CD process
- Is a standard that has been replaced by Ethernet II
- Specifies the physical layer (Layer 1)
- Developed in the mid-1970s
- Specifies the MAC portion of the data link layer (Layer 2)
- Also referred to as thick Ethernet
Which statement about an Ethernet address is accurate?
- The address used in an Ethernet LAN directs data to the proper receiving location.
- The source address is the 4-byte hexadecimal address of the NIC on the computer that is generating the data packet.
- The destination address is the 8-byte hexadecimal address of the NIC on the LAN to which a data packet is being sent.
- Both the destination and source addresses consist of a 6-byte hexadecimal number.
Which statement about MAC addresses is accurate?
- A MAC address is a number in hexadecimal format that is physically located on the NIC.
- A MAC address is represented by binary digits that are organized in pairs.
- It is not necessary for a device to have a unique MAC address to participate in the network.
- The MAC address can never be changed.
Which statement about NICs is accurate?
- The NIC plugs into a USB port and provides a port for connecting to the network.
- The NIC communicates with the network through a serial connection and communicates with the computer through a parallel connection.
- The NIC communicates with the network through a parallel connection and communicates with the computer through a serial connection.
- A NIC is also referred to as a switch adapter.
Which minimum category of UTP is required for Ethernet 1000BASE-T?
- Category 3
- Category 4
- Category 5
- Category 5e
Match the UTP categories to the environments in which they are most commonly used.
- ___1. Category 1
- ___2. Category 2
- ___3. Category 3
- ___4. Category 4
- ___5. Category 5
- ___6. Category 5e
- ___7. Category 6
- ___8. Category 6e
- Capable of transmitting data at speeds up to 100 Mbps
- Used in networks running at speeds up to 1000 Mbps (1 Gbps)
- Consists of four pairs of 24-gauge copper wires, which can transmit data at speeds up to 1000 Mbps
- Used for telephone communications; not suitable for transmitting data
- Used in Token Ring networks; can transmit data at speeds up to 16 Mbps
- Capable of transmitting data at speeds up to 4 Mbps
- Used in 10BASE-T networks; can transmit data at speeds up to 10 Mbps
- Used in networks running at speeds up to 10 Gbps
Which type of UTP cable would you use to connect a router to a PC to have the devices pass user data?
- Straight-through
- Crossover
- Rollover
- None of these options are correct.
Which type of UTP cable would you use to connect a switch to another switch?
- Straight-through
- Crossover
- Rollover
- None of these options are correct.
What type of optical fiber provides higher speeds and bandwidths?
- MMF
- SMF
- MNF
- GMF
Production Network Simulation Question 3-1
Your colleague has come to you desperate for help. He needs to know what type of Ethernet cable he needs to use in each of these segments he is responsible for:
- The PC to the switch
- The switch to another switch
- The switch to a router
- The router to another router
None of these devices support the Auto-MDIX feature, so provide him with the correct cable type for each instance.