6 - Data Link Layer

Layer 2

What is another name for the OSI data link layer?

To determine whether the frame experienced transmission error

What is the function of the last field in a data link layer frame?

Provides the method to get the frame on and off the media

What is the responsibility of the MAC sublayer?

mesh Topic 6.2.0 - The mesh topology provides high availability because every node is connected to all other nodes. Mesh topologies can be found in WANs. A partial mesh topology can also be used where some, but not all, end points connect to one another.

A technician has been asked to develop a physical topology for a network that provides a high level of redundancy. Which physical topology requires that every node is attached to every other node on the network?

Bus

All end systems are chained to each other and terminated in some form on each end. Infrastructure devices such as switches are not required to interconnect the end devices. Legacy Ethernet networks were often bus topologies using coax cables because it was inexpensive and easy to set up.

the use of full-duplex capable Layer 2 switches Topic 6.2.0 - The use of Layer 2 switches operating in full-duplex mode eliminates collisions, thereby eliminating the need for CSMA/CD.

Although CSMA/CD is still a feature of Ethernet, why is it no longer necessary?

Full-duplex communication

Both devices can simultaneously transmit and receive on the shared media. The data link layer assumes that the media is available for transmission for both nodes at any time. Ethernet switches operate in full-duplex mode by default, but they can operate in half-duplex if connecting to a device such as an Ethernet hub. Click play in the figure to see the animation showing full-duplex communication.

Half-duplex communication

Both devices can transmit and receive on the media but cannot do so simultaneously. WLANs and legacy bus topologies with Ethernet hubs use the half-duplex mode. Half-duplex allows only one device to send or receive at a time on the shared medium. Click play in the figure to see the animation showing half-duplex communication.

Data

Contains the frame payload (i.e., packet header, segment header, and the data).

Ring

End systems are connected to their respective neighbor forming a ring. The ring does not need to be terminated, unlike in the bus topology. Legacy Fiber Distributed Data Interface (FDDI) and Token Ring networks used ring topologies.

Control

Identifies special flow control services such as quality of service (QoS). QoS gives forwarding priority to certain types of messages. For example, voice over IP (VoIP) frames normally receive priority because they are sensitive to delay.

Type

Identifies the Layer 3 protocol in the data field.

Physical topology

Identifies the physical connections and how end devices and intermediary devices (i.e, routers, switches, and wireless access points) are interconnected. The topology may also include specific device location such as room number and location on the equipment rack. Physical topologies are usually point-to-point or star.

Media Access Control (MAC)

Implements this sublayer (IEEE 802.3, 802.11, or 802.15) in hardware. It is responsible for data encapsulation and media access control. It provides data link layer addressing and it is integrated with various physical layer technologies.

Error Detection

Included after the data to form the trailer

Error detection

Includes a trailer used to detect transmission errors.

Addressing

Indicates the source and destination nodes on the media.

Addressing

Provides source and destination addressing for transporting the Layer 2 frame between devices on the same shared medium.

Logical topology

Refers to the way a network transfers frames from one node to the next. This topology identifies virtual connections using device interfaces and Layer 3 IP addressing schemes.

Logical Link Control Media Access Control

The IEEE 802 LAN/MAN data link layer consists of which two sublayers? (Choose two.)

Frame delimiting

The framing process provides important delimiters to identify fields within a frame. These delimiting bits provide synchronization between the transmitting and receiving nodes.

Media sharing Topology

The media access control method used depends on which two criteria?

Logical Link Control (LLC)

This IEEE 802.2 sublayer communicates between the networking software at the upper layers and the device hardware at the lower layers. It places information in the frame that identifies which network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IPv4 and IPv6, to use the same network interface and media.

Hub and Spoke

This is a WAN version of the star topology in which a central site interconnects branch sites through the use of point-to-point links. Branch sites cannot exchange data with other branch sites without going through the central site.

Point-to-Point

This is the simplest and most common WAN topology. It consists of a permanent link between two endpoints.

Mesh

This topology provides high availability but requires that every end system is interconnected to every other system. Therefore, the administrative and physical costs can be significant. Each link is essentially a point-to-point link to the other node.

Frame start and stop indicator flags

Used to identify the beginning and end limits of the frame.

Accepts a frame from a medium De-encapsulates the frame Re-encapsulates the packet into a new frame

What Layer 2 function does a router perform? (Choose three.)

MAC LLC Topic 6.1.0 - The data link layer of the OSI model is divided into two sublayers: the Media Access Control (MAC) sublayer and the Logical Link Control (LLC) sublayer.

What are the two sublayers of the OSI model data link layer? (Choose two.)

It accepts Layer 3 packets and encapsulates them into frames It provides media access control and performs error detection. Topic 6.1.0 - 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.

What are two services performed by the data link layer of the OSI model? (Choose two.)

MAC address Topic 6.3.0 - The data link layer describes media access and physical addressing. The encoding of a MAC address on a NIC places it at that layer. Ports and cables are placed at the physical layer of the OSI model. IP addresses are placed at the network layer. The TCP/IP protocol stack describes a different model.

What attribute of a NIC would place it at the data link layer of the OSI model?

header trailer

What does the data link layer add to a Layer 3 packet to create a frame? (Choose two.)

MAC address Topic 6.3.0 - Ethernet frames are identified at the data link layer by their MAC addresses, which are unique to each NIC. IP addresses are used at the network layer, and TCP and UDP port numbers are used at the transport layer. Sequence numbers are fields in TCP headers.

What identifier is used at the data link layer to uniquely identify an Ethernet device?

Logical topologies refer to how a network transfers data between devices. Topic 6.2.0 - Physical topologies show the physical interconnection of devices. Logical topologies show the way the network will transfer data between connected nodes.

What is true concerning physical and logical topologies?

WAN

What kind of network would use point-to-point, hub and spoke, or mesh topologies?

CSMA/CA Topic 6.2.0 - Carrier sense multiple access with collision avoidance (CSMA/CA) is used with wireless networking technology to mediate media contention. Carrier sense multiple access with collision detection (CSMA/CD) is used with wired Ethernet technology to mediate media contention. Priority ordering and token passing are not used (or not a method) for media access control.

What method is used to manage contention-based access on a wireless network?

star Topic 6.2.0 - Devices connected to the Ethernet star topology connect to either a hub or a switch.

What type of physical topology can be created by connecting all Ethernet cables to a central device?

extended star

Which LAN topology is a hybrid topology?

CSMA/CD Topic 6.2.0 - CSMA/CD is used by Ethernet networks using legacy Ethernet hubs. CSMA/CA is used by 802.11-based wireless networks.

Which data link layer media access control method does Ethernet use with legacy Ethernet hubs?

half-duplex

Which duplex communication method is used in WLANs?

to identify which network layer protocol is being used Topic 6.1.0 - Defining the media access processes that are performed by the hardware and providing data link layer addressing are functions of the MAC sublayer. The data link layer accepts Layer 3 packets and packages them into data units that are called frames.

Which is a function of the Logical Link Control (LLC) sublayer?

data link Topic 6.3.0 - Encapsulation is a function of the data link layer. Different media types require different data link layer encapsulation.

Which layer of the OSI model is responsible for specifying the encapsulation method used for specific types of media?

destination NIC address, source NIC address, source IP address, destination IP address

Which lists the Layer 2 and Layer 3 address fields in the correct order?

Carrier Sense Multiple Access / Collision Detection (CSMA/CD)

Which media access control method is used in legacy Ethernet LANs?

802.11 Ethernet PPP

Which of the following are data link layer protocols? (Choose three)

IEEE

Which organization defines standards for the network access layer (i.e., the OSI physical and data link layers)?

Data that is transmitted over the network flows in one direction at a time. Topic 6.2.0 - The data that is transmitted over the network can flow using one of three modes: Simplex - Data can only flow in one direction. Half-duplex - Data flows in one direction at a time. Full-duplex - Data flows in both directions at the same time.

Which statement describes the half-duplex mode of data transmission?

logical topology

Which topology displays networking device layer IP addresses?

International Telecommunication Union (ITU) Institute of Electrical and Electronics Engineers (IEEE) Topic 6.1.0 - The IANA is responsible for overseeing and managing IP address allocation, domain name management, and protocol identifiers. The EIA is an international standards and trade alliance for electronics organizations, and is best known for its standards related to electrical wiring, connectors, and the 19-inch racks used to mount networking equipment. IEEE defines open standards and protocols that apply to the network access layer.

Which two engineering organizations define open standards and protocols that apply to the data link layer? (Choose two.)

Hybrid

a variation or combination of any topologies. For example, a partial mesh is a hybrid topology in which some, but not all, end devices are interconnected.