CCNA 1 Chapter 4 Exam Answers

The data link layer is responsible for the exchange of frames between nodes over a physical network media. Specifically the data link layer performs two basic services:
It accepts Layer 3 packets and encapsulates them into frames.
It provides media access control and performs error detection.
Path determination is a service provided at Layer 3. A Layer 2 switch builds a MAC address table as part of its operation, but path determination is not the service that is provided by the data link layer.

In an extended star topology, central intermediate devices interconnect other star topologies.

A hybrid topology is a variation or combination of a point-to-point, hub and spoke, or mesh topology. This may include a partial mesh or extended star topology.

When a cable to an RJ-45 connector is terminated, it is important to ensure that the untwisted wires are not too long and that the flexible plastic sheath surrounding the wires is crimped down and not the bare wires. None of the colored wires should be visible from the bottom of the jack.

Light can only travel in one direction down a single strand of fiber. In order to allow for full-duplex communication two strands of fiber must be connected between each device.

EMI and RFI signals can distort and corrupt data signals that are carried by copper media. These distortions usually come from radio waves and electromagnetic devices such as motors and florescent lights. Crosstalk is a disturbance that is caused by adjacent wires bundled too close together with the magnetic field of one wire affecting another. Signal attenuation is caused when an electrical signal begins to deteriorate over the length of a copper cable.

Throughput is the measure of the transfer of bits across the media over a given period of time. Throughput is affected by a number of factors such as, EMI and latency, so it rarely matches the specified bandwidth for a network medium. The throughput measurement includes user data bits and other data bits, such as overhead, acknowledging, and encapsulation. The measure of the usable data transferred across the media is called goodput.

Copper cabling is usually cheaper and easier to install than fiber optic cabling. However, fiber cables generally have a much greater signaling range than copper.

Encapsulation is a function of the data link layer. Different media types require different data link layer encapsulation.

To help prevent the effects of crosstalk, UTP cable wires are twisted together into pairs. Twisting the wires together causes the magnetic fields of each wire to cancel each other out.

The three areas of concern for wireless networks focus on the size of the coverage area, any nearby interference, and providing network security. Extensive cabling is not a concern for wireless networks, as a wireless network will require minimal cabling for providing wireless access to hosts. Mobility options are not a component of the areas of concern for wireless networks.

When data travels on the media, it is converted into a stream of 1s and 0s. The framing process inserts into the frame start and stop indicator flags so that the destination can detect the beginning and end of the frame.

The CRC value in the FCS field of the received frame is compared to the computed CRC value of that frame, in order to verify the integrity of the frame. If the two values do not match, then the frame is discarded.

A straight-through cable is commonly used to interconnect a host to a switch and a switch to a router. A crossover cable is used to interconnect similar devices together like switch to a switch, a host to a host, or a router to a router. If a switch has the MDIX capability, a crossover could be used to connect the switch to the router; however, that option is not available. A rollover cable is used to connect to a router or switch console port.

An encoding technique converts a stream of data bits in a predefined code that can be recognized by both the transmitter and the receiver. Using predefined patterns helps to differentiate data bits from control bits and provide better media error detection.

Bandwidth is the capacity of a medium to carry data in a given amount of time. It is typically measured in kilobits per second (kb/s) or megabits per second (Mb/s).​

The trailer in a data-link frame contains error detection information that is pertinent to the frame included in the FCS field. The header contains control information, such as the addressing, while the area that is indicated by the word “data” includes the data, transport layer PDU, and the IP header.

Single-mode fiber uses a laser as the light source. Its small core produces a single straight path for light and it is commonly used with campus backbones. Multimode fiber uses LEDs as the light source. Its larger core allows for multiple paths for the light. It is commonly used with LANs.

EMI and RFI can distort network signals because of interference from fluorescent lights or electric motors. Attenuation results in deterioration of the network signal as it travels along copper cabling. Wireless devices can experience loss of signals because of excessive distances from a access point, but this is not crosstalk. Crosstalk is the disturbance caused by the electric or magnetic fields of the signal carried on an adjacent wire within the same cable.

Network 1 represents an Ethernet LAN. Data on the wired LAN accesses the media using CSMA/CD. Network 2 represents a point-to-point WAN connection so no media access method is required. Network 3 represents a WLAN and data accesses the network using CSMA/CA.

Physical topologies show the physical interconnection of devices. Logical topologies show the way the network will transfer data between connected nodes.

Logical Link Control (LLC) is the data link sublayer that defines the software processes that provide services to the network layer protocols. LLC places information in the frame and that information identifies the network layer protocol that is encapsulated in the frame.