Chapter 2: Fundamentals of Ethernet LANs

Unicast address

Generally, any address in networking that represents a single device or interface, instead of a group of addresses (as would be represented by a multicast or broadcast address).

10/100BASE-T Devices that transmit on pins 3 and 6.

Hubs Switches

Straight-through cable

In Ethernet, a cable that connects the wire on pin 1 on one end of the cable to pin 1 on the other end of the cable, pin 2 on one end to pin 2 on the other end, and so on.

Core

In fiber-optic cabling, the center cylinder of the cable, made of fiberglass, through which light passes.

Fast Ethernet

The common name for all the IEEE standards that send data at 100 megabits per second.

Which of the following is true about the Ethernet FCS field? a. Ethernet uses FCS for error recovery. b. It is 2 bytes long. c. It resides in the Ethernet trailer, not the Ethernet header. d. It is used for encryption.

c., The 4-byte Ethernet FCS field, found in the Ethernet trailer, allows the receiving node to see what the sending node computed with a math formula that is a key part of the error-detection process. Note that Ethernet defines the process of detecting errors (error detection), but not error recovery. Error Detection with FCS Page number:107 Prev

Frame Check Sequence (FCS)

A field in many data-link trailers used as part of the error-detection process.

Ethernet

A series of LAN standards defined by the IEEE, originally invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation.

MAC address

A standardized data-link layer address that is required for every device that connects to a LAN. Ethernet MAC addresses are 6 bytes long and are controlled by the IEEE. Also known as a hardware address, a MAC layer address, and a physical address.

Transceiver

A term formed from the words transmitter and receiver. The hardware used to both send (transmit) energy over some communications medium (e.g., wires in a cable), as well as to process received energy signals to interpret as a series of 1s and 0s.

Ethernet frame

A term referring to an Ethernet data-link header and trailer, plus the data encapsulated between the header and trailer.

Gigabit Ethernet Interface Converter (GBIC)

The original form factor for a removable transceiver for Gigabit interfaces; larger than SFPs

The network in the figure uses CAT5 cabling with RJ-45 connectors. Which of the following statements are true about the wires inside the cable and the RJ-45 pins to which they are connected? a. The cable from SW1 to SW2 connects pin 6 on one end to pin 2 on the other. b. The cable from PC1 to SW1 connects pin 6 on one end to pin 3 on the other. c. The cable from PC1 to SW1 connects pin 2 on one end to pin 3 on the other. d. The cable from SW2 to R1 connects pin 3 on one end to pin 6 on the other. e. The cable from SW2 to R1 connects pin 6 on one end to pin 6 on the other. f. The cable from SW1 to SW2 connects pin 2 on one end to pin 3 on the other.

a., e. Straight-through cables are used on the links between a switch and a PC, and between a switch and a router. The link between switches uses a crossover cable, which connects pins 1 and 3, 2 and 6, 3 and 1, and 6 and 2.

Which fields are in the IPv4 header? a. Destination port b. Protocol c. Time to Live d. DS Select 3 answers

b., c., d. The IPv4 header includes: - Version - Length - DS Field - Packet length - Identification - Flags - Fragment offset - Time to Live - Protocol - Header checksum - Source and destination IP TCP and UDP heads include both a source and destination port field, but the IP header does not include port numbers.

The network in the figure uses CAT5 cabling with RJ-45 connectors. Which of the following statements are true about the wires inside the cable and the RJ-45 pins to which they are connected? a. The cable from PC1 to SW1 connects pin 1 on one end to pin 3 on the other. b. The cable from PC1 to SW1 connects pin 1 on one end to pin 1 on the other. c. The cable from SW2 to R1 connects pin 2 on one end to pin 2 on the other. d. The cable from SW1 to SW2 connects pin 1 on one end to pin 3 on the other. e. The cable from SW1 to SW2 connects pin 1 on one end to pin 1 on the other. f. The cable from SW2 to R1 connects pin 1 on one end to pin 3 on the other. Select 3 answers

b., c., d., Straight-through cables are used on the links between a switch and a PC and between a switch and a router. The link between switches uses a crossover cable, which connects pins 1 and 2 on SW2 to pins 3 and 6 at SW1, and pins 3 and 6 on SW2 to pins 1 and 2 on SW1.

In the figure, each link is labeled with a number. Switches have been given names that begin with SW, routers names that begin with R, and hubs names that begin with H. Which of the following statements are true about this network? a. Link 6 uses a rollover cable. b. Link 15 uses a straight-through cable. c. Link 1 uses a crossover cable. d. Link 11 uses a crossover cable.

b., d. Crossover cables are used between pairs of PCs, pairs of routers, between a switch and a hub, pairs of hubs, and pairs of switches. For Ethernet, all other links use straight-through cables. Rollover cables connect a serial port on a PC to a router or switch console; they are not used for Ethernet connections between any types of devices, because the cabling pinouts are wrong.

Which of the following are true about the format of Ethernet addresses? (Choose three answers.) a. Each manufacturer puts a unique OUI code into the first 2 bytes of the address. b. Each manufacturer puts a unique OUI code into the first 3 bytes of the address. c. Each manufacturer puts a unique OUI code into the first half of the address. d. The part of the address that holds this manufacturer's code is called the MAC. e. The part of the address that holds this manufacturer's code is called the OUI. f. The part of the address that holds this manufacturer's code has no specific name.

B, C, and E. The pre-assigned universal MAC address, given to each Ethernet port when manufactured, breaks the address into two 3-byte halves. The first half is called the organizationally unique identifier (OUI), which the IEEE assigns to the company that builds the product as a unique hex number to be used only by that company.

Broadcast address

Generally, any address that represents all devices, and can be used to send one message to all devices. In Ethernet, the MAC address of all binary 1s, or FFFF.FFFF.FFFF in hex. For IPv4, see subnet broadcast address.

Half duplex

Generically, any communication in which only one device at a time can send data. In Ethernet LANs, the normal result of the CSMA/CD algorithm that enforces the rule that only one device should send at any point in time.

Full duplex

Generically, any communication in which two communicating devices can concurrently send and receive data. In Ethernet LANs, the allowance for both devices to send and receive at the same time, allowed when both devices disable their CSMA/CD logic.

10BASE-T

The 10-Mbps baseband Ethernet specification using two pairs of twisted-pair cabling (Categories 3, 4, or 5): one pair transmits data and the other receives data. 10BASE-T, which is part of the IEEE 802.3 specification, has a distance limit of approximately 100 m (328 feet) per segment.

Small Form Pluggable (SFP)

The replacement for GBICs, used on Gigabit interfaces, with a smaller size, taking less space on the side of the networking card or switch.

10/100BASE-T LAN switch receivers use the wire pair at pins ___ and ___, and their transmitters use the wire pair at pins ___ and ___.

1 and 2, 3 and 6

10/100BASE-T As a rule, Ethernet NIC transmitters use the pair connected to pins ___ and ___. NIC receivers use a pair of wires at pin positions ___ and ___.

1 and 2, 3 and 6.

1000BASE-T Ethernet crossover cable pin pairs

1,2 3,6 4,5 7,8

Ethernet address

A 48-bit (6-byte) binary number, usually written as a 12-digit hexadecimal number, used to identify Ethernet nodes in an Ethernet network. Ethernet frame headers list a destination and source address field, used by the Ethernet devices to deliver Ethernet frames to the correct destination.

RJ-45

A popular type of cabling connector used for Ethernet cabling. It is similar to the RJ-11 connector used for telephone wiring in homes in the United States. RJ-45 allows the connection of eight wires.

Multicast IP address

A class D IPv4 address. When used as a destination address in a packet, the routers collectively work to deliver copies of the one original packet to all hosts who have previously registered to receive packets sent to that particular multicast address.

Network Interface Card (NIC)

A computer card, sometimes an expansion card and sometimes integrated into the motherboard of the computer, that provides the electronics and other functions to connect to a computer network. Today, most NICs are specifically Ethernet NICs, and most have an RJ-45 port, the most common type of Ethernet port.

Ethernet link

A generic term for any physical link between two Ethernet nodes, no matter what type of cabling is used.

Ethernet port

A generic term for the opening on the side of any Ethernet node, typically in an Ethernet NIC or LAN switch, into which an Ethernet cable can be connected.

Wired LAN

A local-area network (LAN) that physically transmits bits using cables, often the wires inside cables. A term for local-area networks that use cables, emphasizing the fact that the LAN transmits data using wires (in cables) instead of wireless radio waves. See also wireless LAN.

Wireless LAN

A local-area network (LAN) that physically transmits bits using radio waves. The name "wireless" compares these LANs to more traditional "wired" LANs, which are LANs that use cables (which often have copper wires inside).

100BASE-T

A name for the IEEE Fast Ethernet standard that uses two-pair copper cabling, a speed of 100 Mbps, and a maximum cable length of 100 meters.

1000BASE-T

A name for the IEEE Gigabit Ethernet standard that uses four-pair copper cabling, a speed of 1000 Mbps (1 Gbps), and a maximum cable length of 100 meters.

Fiber-optic cable

A type of cabling that uses glass fiber as a medium through which to transmit light.

Multimode fiber (MM)

A type of fiber cable that works well with transmitters like LEDs that emit multiple angles of light into the core of the cable; to accommodate the multiple angles of incident, the cable has a larger core in comparison to single-mode fiber cables.

Single-mode fiber (SM)

A type of fiber cable that works well with transmitters like lasers that emit a single angle of light into the core of the cable, allowing for a smaller core in comparison to multimode fiber cables.

1. In the LAN for a small office, some user devices connect to the LAN using a cable, while others connect using wireless technology (and no cable). Which of the following is true regarding the use of Ethernet in this LAN? a. Only the devices that use cables are using Ethernet. b. Only the devices that use wireless are using Ethernet. c. Both the devices using cables and those using wireless are using Ethernet. d. None of the devices are using Ethernet.

A. The IEEE defines Ethernet LAN standards, with standard names that begin with 802.3, all of which happen to use cabling. The IEEE also defines wireless LAN standards, with standard names that begin with 802.11, which are separate standards from Ethernet.

Crossover cable

An Ethernet cable that swaps the pair used for transmission on one device to a pair used for receiving on the device on the opposite end of the cable. In 10BASE-T and 100BASE-TX networks, this cable swaps the pair at pins 1,2 to pins 3,6 on the other end of the cable, and the pair at pins 3,6 to pins 1,2 as well.

4. Each answer lists two types of devices used in a 100BASE-T network. If these devices were connected with UTP Ethernet cables, which pairs of devices would require a straight-through cable? (Choose three answers.) a. PC and router b. PC and switch c. Hub and switch d. Router and hub e. Wireless access point (Ethernet port) and switch

B, D, and E. Routers, wireless access point Ethernet ports, and PC NICs all send using pins 1 and 2, whereas hubs and LAN switches transmit on pins 3 and 6. Straight-through cables connect devices that use opposite pin pairs for sending, because the cable does not need to cross the pairs.

3. Which of the following is true about Ethernet crossover cables for Fast Ethernet? a. Pins 1 and 2 are reversed on the other end of the cable. b. Pins 1 and 2 on one end of the cable connect to pins 3 and 6 on the other end of the cable. c. Pins 1 and 2 on one end of the cable connect to pins 3 and 4 on the other end of the cable. d. The cable can be up to 1000 meters long to cross over between buildings. e. None of the other answers is correct.

B. Crossover cables cross the wire at one node's transmit pin pair to the different pins used as the receive pins on the other device. For 10- and 100-Mbps Ethernet, the specific crossover cable wiring connects the pair at pins 1 and 2 on each end of the cable to pins 3 and 6 on the other end of the cable, respectively.

5. Which of the following are advantages of using multimode fiber for an Ethernet link instead of UTP or single-mode fiber? a. To achieve the longest distance possible for that single link. b. To extend the link beyond 100 meters while keeping initial costs as low as possible. c. To make use of an existing stock of laser-based SFP/SFP+ modules. d. To make use of an existing stock of LED-based SFP/SFP+ modules.

B. Multimode fiber works with LED-based transmitters rather than laser-based transmitters. Two answers mention the type of transmitters, making one of those answers correct and one incorrect. Two answers mention distance. The answer that mentions the longest distance possible is incorrect because single-mode cables, not multimode cables, provide the longest distances. The other (correct) answer mentions the tradeoff of multimode being used for distances just longer than UTP's 100 meter limit, while happening to use less expensive hardware than single mode.

6. Which of the following is true about the CSMA/CD algorithm? a. The algorithm never allows collisions to occur. b. Collisions can happen, but the algorithm defines how the computers should notice a collision and how to recover. c. The algorithm works with only two devices on the same Ethernet. d. None of the other answers is correct.

B. NICs (and switch ports) use the carrier sense multiple access with collision detection (CSMA/CD) algorithm to implement half-duplex logic. CSMA/CD attempts to avoid collisions, but it also notices when collisions do occur, with rules about how the Ethernet nodes should stop sending, wait, and try again later.

The IEEE defines two general categories of group addresses for Ethernet:

Broadcast address: Frames sent to this address should be delivered to all devices on the Ethernet LAN. It has a value of FFFF.FFFF.FFFF. Multicast addresses: Frames sent to a multicast Ethernet address will be copied and forwarded to a subset of the devices on the LAN that volunteers to receive frames sent to a specific multicast address.

9. Which of the following terms describe Ethernet addresses that can be used to send one frame that is delivered to multiple devices on the LAN? (Choose two answers.) a. Burned-in address b. Unicast address c. Broadcast address d. Multicast address

C and D. Ethernet supports unicast addresses, which identify a single Ethernet node, and group addresses, which can be used to send one frame to multiple Ethernet nodes. The two types of group addresses are the broadcast address and multicast address.

7. Which of the following is true about the Ethernet FCS field? a. Ethernet uses FCS for error recovery. b. It is 2 bytes long. c. It resides in the Ethernet trailer, not the Ethernet header. d. It is used for encryption.

C. The 4-byte Ethernet FCS field, found in the Ethernet trailer, allows the receiving node to see what the sending node computed with a math formula that is a key part of the error-detection process. Note that Ethernet defines the process of detecting errors (error detection), but not error recovery.

2. Which of the following Ethernet standards defines Gigabit Ethernet over UTP cabling? a. 10GBASE-T b. 100BASE-T c. 1000BASE-T d. None of the other answers is correct.

C. The number before the word BASE defines the speed, in megabits per second (Mbps). 1000 Mbps equals 1 gigabit per second (1 Gbps). The T in the suffix implies twisted-pair or UTP cabling, so 1000BASE-T is the UTP-based Gigabit Ethernet standard name.

Cladding

In fiber-optic cabling, the second layer of the cable, surrounding the core of the cable, with the property of reflecting light back into the core.

IEEE

Institute of Electrical and Electronics Engineers. A professional organization that develops communications and network standards, among other activities.

Small Form Pluggable Plus (SFP+)

Same size as the SFP, but used on 10-Gbps interfaces. (The Plus refers to the increase in speed compared to SFPs.)

10/100BASE-T Devices that transmit on pins 1 and 2

PC NICs Routers Wireless access point (Ethernet interface)

802.11 Ethernet header and trailer fields

Preamble Start Frame Delimiter (SFD) Destination MAC address Source MAC address Type Data and Pad Frame Check Sequence (FCS)

CMSA/CD (carrier-sense multiple access with collision detection) logic

Step 1. A device with a frame to send listens until the Ethernet is not busy. Step 2. When the Ethernet is not busy, the sender begins sending the frame. Step 3. The sender listens while sending to discover whether a collision occurs; collisions might be caused by many reasons, including unfortunate timing. If a collision occurs, all currently sending nodes do the following: A. They send a jamming signal that tells all nodes that a collision happened. B. They independently choose a random time to wait before trying again to avoid unfortunate timing. C. The next attempt starts again at Step 1.

Gigabit Ethernet

The common name for all the IEEE standards that send data at 1 gigabit per second.

Electromagnetic Interface (EMI)

The name of the effect in which electricity passes through one cable as normal, inducing a magnetic field outside the conductor. That magnetic field, if it passes through another conductor, like a nearby cable, induces new electrical current in the second cable, interfering with the use of electricity to transmit data on the second cable.

Ethernet Type/EtherType

Type field in the Ethernet header. The Type field identifies the type of packet encapsulated inside an Ethernet frame. Its purpose is to directly help the network processing on routers and hosts. Basically, the Type field identifies the type of network layer (Layer 3) packet that sits inside the Ethernet frame.

Which of the following is true about the CSMA/CD algorithm? a. Collisions can happen, but the algorithm defines how the computers should notice a collision and how to recover. b. The algorithm never allows collisions to occur. c. None of the other answers is correct. d. The algorithm works with only two devices on the same Ethernet.

a. NICs (and switch ports) use the carrier sense multiple access with collision detection (CSMA/CD) algorithm to implement half-duplex logic. CSMA/CD attempts to avoid collisions, but it also notices when collisions do occur, with rules about how the Ethernet nodes should stop sending, wait, and try again later.

In the LAN for a small office, some user devices connect to the LAN using a cable, while others connect using wireless technology (and no cable). Which of the following is true regarding the use of Ethernet in this LAN? a. Only the devices that use cables are using Ethernet. b. None of the devices are using Ethernet. c. Both the devices using cables and those using wireless are using Ethernet. d. Only the devices that use wireless are using Ethernet.

a. The IEEE defines Ethernet LAN standards, with standard names that begin with 802.3, all of which happen to use cabling. The IEEE also defines wireless LAN standards, with standard names that begin with 802.11, which are separate standards from Ethernet.

A device uses MAC address 000c.42b9.46ee. Which part of the MAC address indicates the originating networking equipment vendor? a. 000c.42 b. 000c c. b9.46ee d. 42b9

a. The first six hex characters (24 bits) of a MAC address are assigned to the equipment manufacturer, and the second six hex characters are assigned by that vendor. That first half is called the Organizationally Unique Identifier.

Which of the following terms describe Ethernet addresses that can be used to send one frame that is delivered to multiple devices on the LAN? (Choose two answers.) a. Broadcast address b. Multicast address c. Burned-in address d. Unicast address Select 2 answers

a., b. Ethernet supports unicast addresses, which identify a single Ethernet node, and group addresses, which can be used to send one frame to multiple Ethernet nodes. The two types of group addresses are the broadcast address and multicast address.

Each answer lists two types of devices used in a 100BASE-T network. If these devices were connected with UTP Ethernet cables, which pairs of devices would require a straight-through cable? (Choose three answers.) a. PC and switch b. Wireless access point (Ethernet port) and switch c. PC and router d. Router and hub e. Hub and switch

a., b., d. Routers, wireless access point Ethernet ports, and PC NICs all send using pins 1 and 2, whereas hubs and LAN switches transmit on pins 3 and 6. Straight-through cables connect devices that use opposite pin pairs for sending, because the cable does not need to cross the pairs.

Which of the following describe CSMA/CD? a. Waits for a quiet link before transmitting b. Avoids loops c. Handles collisions by retransmitting d. Same-sized frames e. Many devices can share a link f. Transmits with a token Select 3 answers

a., c., e. CSMA/CD causes an Ethernet NIC to first listen, making sure that the link is quiet before sending. This allows many devices to share the link and minimizes the number of collisions. When collisions do occur, the NIC handles the problem by waiting a random amount of time for a quiet link and then retransmitting the Ethernet frame.

Which of the following are true about the format of Ethernet addresses? (Choose three answers.) a. The part of the address that holds this manufacturer's code is called the OUI. b. Each manufacturer puts a unique OUI code into the first 2 bytes of the address. c. The part of the address that holds this manufacturer's code has no specific name. d. Each manufacturer puts a unique OUI code into the first half of the address. e. Each manufacturer puts a unique OUI code into the first 3 bytes of the address. f. The part of the address that holds this manufacturer's code is called the MAC.

a., d., e., The pre-assigned universal MAC address, given to each Ethernet port when manufactured, breaks the address into two 3-byte halves. The first half is called the organizationally unique identifier (OUI), which the IEEE assigns to the company that builds the product as a unique hex number to be used only by that company.

The diagram shows a simple internetwork in which all the LAN connections use FastEthernet. Select the answers that list the correct cable types. a. A straight-through between SW1 and SW2 b. An Ethernet crossover cable between SW3 and Hub1 c. A rollover cable between R1 and host F d. An Ethernet crossover cable between SW1 and R1 e. A rollover cable between SW3 and AP1 Select 2 answers

b., c., A crossover cable is needed between two devices that transmit on the same wire pair, and a straight-through cable is needed for devices that transmit on opposite wire pairs. For the listed answers, SW1 uses the opposite pairs from router R1, so they use a straight-through cable. SW3 and Hub 1 transmit on the same wire pair, requiring a crossover cable. SW3 and AP1 use opposite pairs, so they need a straight-through cable. Finally, SW1 and SW2 use the same pair, so a crossover cable is needed. The console of a router or switch requires the use of a rollover cable.

You have been given a Category 5 cable, terminated with RJ-45 connectors. The cable was terminated on each end based on T568A/T568B pinouts. What is the appropriate use for this cable? a. Switch to router b. Switch to switch c. Switch to PC d. Router to PC e. Router to hub f. PC to hub

b., d. Some Ethernet standards define that one pair should be used to send data in one direction from a given hosts perspective, with the other pair used to receive data. Ethernet NICs should send data using the pair connected to pins 1 and 2. Ethernet NICs inside routers and hosts should expect to receive data using the pair at pins 3 and 6. Knowing what the Ethernet NICs do, hubs and switches do the opposite; they receive on the pair at pins 1,2 and send on the pair at pins 3,6. If we have two devices that are sending on pins 1 and 2, we need a crossover cable so that the two similar devices can communicate with each other. All the other combinations listed require a straight through cable from hubs/switches connected to routers/PCs/printers/hosts.

What is the bandwidth associated with the IEEE standard 1000BASE-T? a. 10Gbps b. 100Mbps c. 1000Mbps d. 10Mbps e. All of these are correct.

c. Here are some of the IEEE standards and the associated bandwidth/speed: 10BASE-T = 10Mbps 100BASE-T =100Mbps 1000BASE-T =1000Mbps 10GBASE-T =10Gbps

Which of the following are advantages of using multimode fiber for an Ethernet link instead of UTP or single-mode fiber? a. To make use of an existing stock of LED-based SFP/SFP+ modules. b. To achieve the longest distance possible for that single link. c. To extend the link beyond 100 meters while keeping initial costs as low as possible. d. To make use of an existing stock of laser-based SFP/SFP+ modules.

c., Multimode fiber works with LED-based transmitters rather than laser-based transmitters. Two answers mention the type of transmitters, making one of those answers correct and one incorrect. Two answers mention distance. The answer that mentions the longest distance possible is incorrect because single-mode cables, not multimode cables, provide the longest distances. The other (correct) answer mentions the tradeoff of multimode being used for distances just longer than UTP's 100 meter limit, while happening to use less expensive hardware than single mode.

In the figure, CAT5 cabling with RJ-45 connectors is used. Which of the following statements are true about the wires inside the cable and the RJ-45 pins to which they are connected? a. The cable from R1's console to PC3's serial port connects pin 3 on one end to pin 1 on the other. b. The cable from PC1 to SW1 connects pin 2 on one end to pin 7 on the other. c. The cable from R1's console to PC3's serial port connects pin 8 on one end to pin 1 on the other. d. The cable from SW1 to SW2 connects pin 6 on one end to pin 3 on the other. e. The cable from SW1 to SW2 connects pin 1 on one end to pin 8 on the other. f. The cable from PC1 to SW1 connects pin 6 on one end to pin 6 on the other.

c., f. Straight-through cables are used on the links between a switch and a PC, and between a switch and a router. The link between switches uses a crossover cable, which connects pins 1 and 3, 2 and 6, 3 and 1, and 6 and 2. For the console, a rollover cable is used, which connects 1 to 8, 2 to 7, 3 to 6, 4 to 5, and so on.

The new connection that is going between Campus 1 and Campus 2 routers is required to provide at least 500 Mbps of bandwidth; the routers are located 3000 meters away from each other. Which standard of Ethernet should be used in this situation? a. 802.3u b. 802.3ab c. 802.3an d. 802.3z

d. The IEEE 802.3z standard supports a Gigabit Ethernet connection using fiber-optic cabling; this allows for a connection that supports more than 500 Mbps of bandwidth and will allow for the long 3000-meter connection requirement.

Which of the following describe a MAC address? a. Layer 1 address b. 32 bits c. Layer 3 address d. Layer 2 address e. 48 bits f. 128 bits

d., e. MAC addresses are 48 bits (6 bytes) long, usually written as 12 hexadecimal digits. In Cisco products, MAC addresses are often displayed 4 hex digits (2 bytes) at a time, separated by periods, for example, 0200.1234.BEEF.