SEMESTER 1 Chapter 7
Data Link Layer

 V 4.0




What are the 2 basic services the Data Link layer provides?

Allows the upper layers to access the media using techniques such as framing

Controls how data is placed onto the media and is received from the media using techniques such as media access control and error detection


List and define the 4 specific terms associated with the Data Link layer?

Frame - The Data Link layer PDU

Node - The Layer 2 notation for network devices connected to a common medium

Media/medium (physical)* - The physical means for the transfer of information between two nodes

Network (physical)** - Two or more nodes connected to a common medium

What happens the frame as moves from source to destination?

It changes based on the medium used


Define Media Access Control.

The technique used for getting the frame on and off media


What can the control information tell you?

Which nodes are in communication with each other

When communication between individual nodes begins and when it ends

Which errors occurred while the nodes communicated

Which nodes will communicate next


List and define the three elements of a Data Link Layer Frame.

Data - The packet from the Network layer

Header - Contains control information, such addressing, and is located at the beginning of the PDU

Trailer - Contains control information added to the end of the PDU

List and define the 5 typical field types.

Start and stop indicator fields - The beginning and end limits of the frame

Naming or addressing fields

Type field - The type of PDU contained in the frame

Quality - control fields

A data field -The frame payload (Network layer packet)


What is the most common physical device that is used for the Data Link Layer?


Describe the two sublayers of the Data Link Layer.

The upper sublayer defines the software processes that provide services to the Network layer protocols.

The lower sublayer defines the media access processes performed by the hardware.


What are the two functions of the Logical Link Control?

Frames the network layer packet

Identifies the network layer protocol


What are the two functions of the Media Access Control?

Addresses the frame

Marks the beginning and ending of each frame


What are the 4 organizations that define the Data Link standards?

International Organization for Standardization (ISO)

Institute of Electrical and Electronics Engineers (IEEE)

American National Standards Institute (ANSI)

International Telecommunication Union (ITU)


What analogy is made with the media access control?

traffic rules that regulate the entrance of motor vehicles onto a roadway


What does the method that media access control uses depend on?

Media sharing - If and how the nodes share the media

Topology - How the connection between the nodes appears to the Data Link layer


What are the two basic media access control methods for shared media?

Controlled - Each node has its own time to use the medium

Contention-based - All nodes compete for the use of the medium


What are two examples of controlled media access?

Token Ring



What are two examples of contention-based media access?




Why can controlled media access be inefficient?

Device has to wait its turn to access the media


What is CSMA?

Carrier Sense Multiple access


What is a data collision?

When two devices transmit at the same time


What is CSMA/CD?

Carrier Sense Multiple access with Collision Detection


What is CSMA/CA?

Carrier Sense Multiple Access with Collision Avoidance


Define Half-Duplex.

devices can both transmit and receive on the media but cannot do so simultaneously


Define Full-Duplex.

Both devices can transmit and receive on the media at the same time.


Define physical topology.

arrangement of the nodes and the physical connections between them


Define logical topology.

the way a network transfers frames from one node to the next


What logical and physical topologies are typically used in networks?





Define point-to-point topology.

connects two nodes directly together

Define virtual circuit.

a logical connection created within a network between two network devices


Which type of topology is used to determine the media access control?

logical point-to-point topology


Define logical multi-access topology.

enables a number of nodes to communicate by using the same shared media


What are the three types of media access control can be used in a multi-access topology?

CSMA/CD, CSMA/CA, or token passing


How does a data flow in a ring network?

From node to node until it reaches the destination.


What are the three basic parts of a data link layer frame?





What are the typical fields in the frame header?

Start Frame field - Indicates the beginning of the frame

Source and Destination address fields - Indicates the source and destination nodes on the media

Priority/Quality of Service field - Indicates a particular type of communication service for processing

Type field - Indicates the upper layer service contained in the frame

Logical connection control field - Used to establish a logical connection between nodes

Physical link control field - Used to establish the media link

Flow control field - Used to start and stop traffic over the media

Congestion control field - Indicates congestion in the media


Which part the network (WAN/LAN) is the data link address used in delivering?



What is the difference between Data Link Address and Logical Address?

Data Link address only defines the machine it is not logically assigned


Does a point-to-point link need a data link address?

No it only has one place to go


Define error detection.

accomplished by placing a logical or mathematical summary of the bits that comprise the frame in the trailer


Define Frame Check Sequence.

used to determine if errors occurred in the transmission and reception of the frame


How could the CRC be correct and the frame contain errors?

The error bits could cancel each other out


Do all protocols support using the FCS to determine errors?



What Layer 2 protocols will we study in this course?


Point-to-Point Protocol (PPP)

High-Level Data Link Control (HDLC)

Frame Relay

Asynchronous Transfer Mode (ATM)


Which usually performs at a higher bandwidth, LAN or WAN?


What do Ethernet standards define?

Layer 2 Protocols and Layer 1 technologies


What is the data link address?

MAC address


How is the data link address expressed?

48 bit


What type of architecture does PPP use?

Layered architecture


What are options available to use with PPP?

Authentication, compression, and multilink (the use of multiple physical connections).

What is the standard used for wireless LANs?



What services are supported by 802.11?

authentication, association (connectivity to a wireless device), and privacy (encryption)

Examine each of the processes and take notes. Develop questions on any step you do not understand.