CNA 101 - Module 6: Data Link Layer

Layer 3

Addresses in the packet header are carried from the source host to the destination host, regardless of the number of network hops along the route.

Data Link Layer

Data Link Layer

Ethernet 802.11 Wireless Point-to-Point Protocol (PPP) High-Level Data Link Control (HDLC) Frame Relay

Data link layer protocols include:

Control

Identifies special flow control services such as quality of service (QoS). QoS gives forwarding priority to certain types of messages.

IEEE 802.11 WLANs

What is another form of CSMA used is carrier sense multiple access/collision avoidance (CSMA/CA)?

Layer 2

What is another name for the OSI data link layer?

To determine whether the frame experienced transmission errors.

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?

Logical Topologies refer to how a network transfers data between devices.

What is true concerning physical and logical topologies?

Ring Topology

A LAN configuration in which all nodes are connected in a closed loop.

Bus Topology

A network layout in which there is one main trunk, or backbone, that all the various computers and network devices are connected to.

IP address

A router can determine the network location of the destination device and the best path to reach it by using the __________________.

Extended Star Topology

A star network with multiple hubs connected to the central hub.

Mesh

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?

Cyclic Redundancy Check (CRC)

A transmitting node creates a logical summary of the contents of the frame __________________ value.

Frame Check Sequence (FCS)

A value that is placed in the _________________ field to represent the contents of the frame. In the Ethernet trailer, this field provides a method for the receiving node to determine whether the frame experienced transmission errors.

Hybrid

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

Third step (CSMA/CD) Hub sending a frame

All devices attached to the hub will receive the frame. However, because the frame has a destination data link address for PC3, only that device will accept and copy in the entire frame. All other device NICs will ignore the frame, as shown in the figure.

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.

Star Topology

All nodes are connected to a central computer.

Contention-Based Access

All nodes operating in half-duplex compete for the use of the medium, but only one device can send at a time.

Ethernet Hub

Also known as a multiport repeater.

The use of full-duplex capable Layer 2 switches.

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

1. Header 2. Data 3. Trailer

Although there are many different data link layer protocols that describe data link layer frames, each frame type has three basic parts:

1. Accepts a frame from a medium 2. De-encapsulates the frame 3. Re-encapsulates the packet into a new frame 4. Forwards the new frame appropriate to the medium of that segment of the physical network

At each hop along the path, a router performs the following Layer 2 functions:

Wireless LANs

Carrier sense multiple access with collision avoidance (CSMA/CA) are used on:

legacy bus-topology Ethernet LANs

Carrier sense multiple access with collision detection (CSMA/CD) are used on:

Full Duplex

Communicate in both directions at the same time

Half Duplex

Communicate in only one direction at a time.

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.

1. Wireless LAN (uses CSMA/CA) 2. Legacy bus-topology Ethernet LAN (uses CSMA/CD) 3. Legacy Ethernet LAN using a hub (uses CSMA/CD)

Examples of contention-based access networks include the following:

Legacy Token Ring Legacy ARCNET

Examples of multiaccess networks that use controlled access include the following:

1. Header 2. Packet (Data) 3. Trailer 4. Frame start and stop indicator flags 5. Addressing 6. Type 7. Control 8. Data 9. Error Detection 10. Frame Stop

Frame fields include the following:

Type

Identifies the Layer 3 protocol in the data field.

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.

Controlled-based multiaccess network

In this network each node has its own time to use the medium.

PPP

In this protocol a node does not have to make any determination about whether an incoming frame is destined for it or another node.

LAN

In this type of topology, end devices are connected to a central intermediary device.

Error Detection

Included after the data to form the trailer used to determine if the frame arrived without error.

Error detection

Includes a trailer used to detect transmission errors.

Addressing

Indicates the source and destination nodes on the media.

Data

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

CMSA/CA

It does not detect collisions but attempts to avoid them by waiting before transmitting. Each device that transmits includes the time duration that it needs for the transmission. All other wireless devices receive this information and know-how long the medium will be unavailable.

same duplex mode

It is important that two interconnected interfaces, such as a host NIC and an interface on an Ethernet switch, operate using the __________________. Otherwise, there will be a duplex mismatch creating inefficiency and latency on the link.

Addressing

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

Point-to-Point Protocol (PPP) High-Level Data Link Control (HDLC) Frame Relay Asynchronous Transfer Mode (ATM) X.25

Some of the common WAN protocols over the years have included:

Logical Link Control (LLC)

Sublayer that 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.

Second step (CSMA/CD) Hub receiving a frame

The Ethernet hub receives and sends the frame. Any bits received on an incoming port are regenerated and sent out all other ports, as shown in the figure.

Logical Link Control Media Access Control

The IEEE 802 LAN/MAN data link layer consists of which two sublayers?

1. Frame delimiting 2. Addressing 3. Error detection

The MAC sublayer provides data encapsulation:

First step (CSMA/CD) PC sending a frame

The PC NIC needs to determine if any device is transmitting on the medium. If it does not detect a carrier signal (in other words, it is not receiving transmissions from another device), it will assume the network is available to send.

data link layer

The __________________ address is only used for local delivery.

Layer 2 protocol

The __________________ used depends on the logical topology and the physical media.

end

The data link layer appends information in the form of a trailer at the __________________ of the frame.

encapsulating

The data link layer prepares the encapsulated data (usually an IPv4 or IPv6 packet) for transport across the local media by __________________ it with a header and a trailer to create a frame.

NIC-to-NIC

The data link protocol is responsible for __________________ communications within the same network.

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?

1. Contention-based access 2. Controlled access

There are two basic access control methods for shared media:

Layer 2

These addresses are only used to connect devices within the same shared media, on the same IP network.

Full-duplex

These communications do not require access control.

point-to-point

This connection over Ethernet requires the device to determine if the incoming frame is destined for this node.

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.

Data Link

This layer prepares network data for the physical network.

multiaccess

This network is a network that can have two or more end devices attempting to access the network simultaneously.

MAC

This sublayer controls the NIC and other hardware that is responsible for sending and receiving data on the wired or wireless LAN/MAN medium.

MAC sublayer

This sublayer provides media access control, allowing multiple devices to communicate over a shared (half-duplex) medium.

LLC

This sublayer takes the network protocol data, which is typically an IPv4 or IPv6 packet, and adds Layer 2 control information to help deliver the packet to the destination node.

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.

PPP Wan

Topology limited to two nodes.

WLANs Legacy bus topologies

Two topologies with Ethernet hubs that use the half-duplex mode.

physical addresses

Unique to the specific device and does not indicate on what network the device is located.

Frame start and stop indicator flags

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

1. Accepts a frame from a medium 2. De-encapsulates the frame 3. Re-encapsulates the packet into a new frame 4. Forwards the new frame appropriate to the medium of that segment of the physical network

What Layer 2 function does a router perform?

LLC and MAC

What are the two sublayers of the OSI model data link layer?

802.11 Ethernet PPP

What are three data link layer protocols?

1. Accepts Layer 3 packets and encapsulates them into frames. 2. Provides media access control and performs error detection.

What are two services performed by the data link layer of the OSI model?

MAC Addresses

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

determines the best path

What does a router do after de-encapsulating a received frame?

Header and trailer

What does the data link layer add to a Layer 3 packet to create a frame?

MAC Address

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

WAN

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

CSMA/CA

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

Star

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

higher overhead slower transmission rates

When greater effort is needed to ensure delivery it means there is __________________ and __________________.

Extended Star Topology

Which LAN topology is a hybrid topology?

CSMA/CD

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

Half-duplex

Which duplex communication method is used in WLANs?

To identify which network layer protocol is being used.

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

data link

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

Destination NIC Source NIC Source IP Destination IP

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

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

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

IEEE

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

The data that is transmitted over the network flows in one direction at a time.

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

Star / Extended Star Topology

Which topologies are easy to install, very scalable (easy to add and remove end devices), and easy to troubleshoot?

Logical

Which topology displays networking device layer IP addresses?

ISO and ITU

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

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.