ITN modules 4-7

What is the purpose of the OSI physical layer?

• controlling access to media

• transmitting bits across the local media

• performing error detection on received frames

• exchanging frames between nodes over physical network media

Why are two strands of fiber used for a single fiber optic connection?

• The two strands allow the data to travel for longer distances without degrading.

• They prevent crosstalk from causing interference on the connection.

• They increase the speed at which the data can travel.

• They allow for full-duplex connectivity.

Which characteristic describes crosstalk?

• the distortion of the network signal from fluorescent lighting

• the distortion of the transmitted messages from signals carried in adjacent wires

• the weakening of the network signal over long cable lengths

• the loss of wireless signal over excessive distance from the access point

What are two characteristics of fiber-optic cable? (Choose two.)

• It is not affected by EMI or RFI.

• Each pair of cables is wrapped in metallic foil.

• It typically contains 4 pairs of fiber-optic wires.

• It is more expensive than UTP cabling is.

• It combines the technique of cancellation, shielding, and twisting to protect data.

Answer explanation

Fiber-optic cabling supports higher bandwidth than UTP for longer distances. Fiber is immune to EMI and RFI, but costs more, requires more skill to install, and requires more safety precautions.

What is a primary role of the Physical layer in transmitting data on the network?

• create the signals that represent the bits in each frame on to the media

• provide physical addressing to the devices

• determine the path packets take through the network

• control data access to the media

What is the function of the CRC value that is found in the FCS field of a frame?

• to verify the integrity of the received frame

• to verify the physical address in the frame

• to verify the logical address in the frame

• to compute the checksum header for the data field in the frame

Answer explanation

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.

Which two fields or features does Ethernet examine to determine if a received frame is passed to the data link layer or discarded by the NIC? (Choose two.)

• auto-MDIX

• Frame Check Sequence

• CEF

• source MAC address

• minimum frame size

Answer explanation

An Ethernet frame is not processed and is discarded if it is smaller than the minimum (64 bytes) or if the calculated frame check sequence (FCS) value does not match the received FCS value. Auto-MDIX (automatic medium-dependent interface crossover) is Layer 1 technology that detects cable straight-through or crossover types. The source MAC address is not used to determine how the frame is received. CEF (Cisco Express Forwarding) is a technology used to expedite Layer 3 switching.

What is a characteristic of the LLC sublayer?

• It provides the logical addressing required that identifies the device.

• It places information in the frame allowing multiple Layer 3 protocols to use the same network interface and media.

• It defines software processes that provide services to the physical layer.

• It provides delimitation of data according to the physical signalling requirements of the medium.

Answer explanation

The Logical Link Control (LLC) defines the software processes that provide services to the network layer protocols. The information is placed by LLC in the frame and identifies which network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IPv4 and IPv6, to utilize the same network interface and media.

During the encapsulation process, what occurs at the data link layer for a PC connected to an Ethernet network?

• An IP address is added.

• The logical address is added.

• The physical address is added.

• The process port number is added.

Answer explanation

The Ethernet frame includes the source and destination physical address. The trailer includes a CRC value in the Frame Check Sequence field to allow the receiving device to determine if the frame has been changed (has errors) during the transmission.

Which frame forwarding method receives the entire frame and performs a CRC check to detect errors before forwarding the frame?

• cut-through switching

• store-and-forward switching

• fragment-free switching

• fast-forward switching