SEMESTER 1 Chapter 7
Data Link Layer
V 4.0
7.1.1
What are the 2 basic
services the Data Link layer
provides?
List and define the 4
specific terms associated
with the Data Link layer?
7.1.1.2 What happens the frame as
moves from source to
destination?
7.1.2
Define Media Access
Control.
7.1.3
What can the control
information tell you?
List and define the three
elements of a Data Link
Layer Frame.
7.1.3.2 List and define the 5 typical
field types.
7.1.4
What is the most common
physical device that is used
for the Data Link Layer?
7.1.4.2 Describe the two sublayers
of the Data Link Layer.
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
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
It changes based on the medium used
The technique used for getting the frame on
and off media
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
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
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)
NIC
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.
Frames the network layer packet
Identifies the network layer protocol
Addresses the frame
Marks the beginning and ending of each frame
7.1.5
7.2.1
7.2.2
7.2.3
7.2.4
7.2.5
What are the two functions
of the Logical Link Control?
What are the two functions
of the Media Access
Control?
What are the 4
International Organization for Standardization
organizations that define the (ISO)
Data Link standards?
Institute of Electrical and Electronics Engineers
(IEEE)
American National Standards Institute (ANSI)
International Telecommunication Union (ITU)
What analogy is made with
traffic rules that regulate the entrance of motor
the media access control?
vehicles onto a roadway
What does the method that Media sharing - If and how the nodes share
media access control uses
the media
depend on?
Topology - How the connection between the
nodes appears to the Data Link layer
What are the two basic
Controlled - Each node has its own time to use
media access control
the medium
methods for shared media? Contention-based - All nodes compete for the
use of the medium
What are two examples of
Token Ring
controlled media access?
FDDI
What are two examples of
Ethernet
contention-based media
Wireless
access?
Why can controlled media
Device has to wait its turn to access the media
access be inefficient?
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
Point-to-Point
topologies are typically used Multi-Access
in networks?
Ring
Define point-to-point
connects two nodes directly together
topology.
7.2.5.2 Define virtual circuit.
7.2.6
7.2.7
7.3.1
7.3.2
7.3.3
7.3.4
Which type of topology is
used to determine the
media access control?
Define logical multi-access
topology.
What are the three types of
media access control can
be used in a multi-access
topology?
How does a data flow in a
ring network?
What are the three basic
parts of a data link layer
frame?
What are the typical fields in
the frame header?
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?
Does a point-to-point link
need a data link address?
Define error detection.
Define Frame Check
Sequence.
How could the CRC be
correct and the frame
a logical connection created within a network
between two network devices
logical point-to-point topology
enables a number of nodes to communicate by
using the same shared media
CSMA/CD, CSMA/CA, or token passing
From node to node until it reaches the
destination.
Header
Data
Trailer
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
LAN
Data Link address only defines the machine it
is not logically assigned
No it only has one place to go
accomplished by placing a logical or
mathematical summary of the bits that
comprise the frame in the trailer
used to determine if errors occurred in the
transmission and reception of the frame
The error bits could cancel each other out
7.3.5
7.3.5.2
7.3.5.3
7.3.5.4
7.4.1.2
contain errors?
Do all protocols support
using the FCS to determine
errors?
What Layer 2 protocols will
we study in this course?
Which usually performs at a
higher bandwidth, LAN or
WAN?
What do Ethernet standards
define?
What is the data link
address?
How is the data link address
expressed?
What type of architecture
does PPP use?
What are options available
to use with PPP?
What is the standard used
for wireless LANs?
What services are
supported by 802.11?
Examine each of the
processes and take notes.
Develop questions on any
step you do not understand.
No
Ethernet
Point-to-Point Protocol (PPP)
High-Level Data Link Control (HDLC)
Frame Relay
Asynchronous Transfer Mode (ATM)
LAN
Layer 2 Protocols and Layer 1 technologies
MAC address
48 bit
Hexadecimal
Layered architecture
Authentication, compression, and multilink (the
use of multiple physical connections).
802.11
authentication, association (connectivity to a
wireless device), and privacy (encryption)
* The Layer 2 protocol used for a particular network topology is determined by
- the technology used to implement that topology.
- the size of the network –
- the number of hosts
- the geographic scope
- the services to be provided over the network.
* The Data Link layer provides a means for exchanging data over a common local media.
 The Data Link layer performs two basic services:
– 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
* Layer 2 protocols specify the encapsulation of a packet into a frame and the techniques for
getting the encapsulated packet on and off each medium.
 The technique used for getting the frame on and off media is called the media access
control method.
* Make sure you know all the interaction of IP addresses and MAC address.
* There are 2 media access control methods for shared media.
 Controlled
o This method is also known as scheduled access or deterministic
o deterministic methods can be inefficient because a device has to wait for its turn
before it can use the medium
o For example: Token passing

Contention-based
o Also referred to as non-deterministic methods
o Contention-based media access control methods do not have the overhead of
controlled access methods.
o The two commonly used methods are: Carrier Sense Multiple Access/Collision
Detection (CSMA/CD) and CSMA/Collision Avoidance (CSMA/CA)
 CSMA/CD: Wired Ethernet network
 CSMA/CA: Wireless Ethernet network
* Token Ring:
 This allows a ring to use a controlled media access control technique called token passing
 If there is no data being transmitted, a signal (known as a token) may be placed on the
media and a node can only place a data frame on the media when it has the token.
 When using the controlled access method, network devices take turns, in sequence, to
access the medium.
* The topology of a network is the arrangement or relationship of the network devices and the
interconnections between them.
 The physical topology is an arrangement of the nodes and the physical connections
between them.
o The representation of how the media is used to interconnect the devices is the
physical topology.
 A logical topology is the way a network transfers frames from one node to the next. This
arrangement consists of virtual connections between the nodes of a network independent
of their physical layout.
* Regulating the placement of data frames onto the media is known as “media access control”.
 These media access control techniques define if and how the nodes share the media.
o For example: Traffic can enter the road by merging, by waiting for its turn at a
stop sign, or by obeying signal lights. A driver follows a different set of rules for
each type of entrance.
* The method of media access control used depends:
 Media sharing
– If and how the nodes share the media
• Shared or non-shared
 Topology
– How the connection between the nodes appears to the Data Link layer
• Point-to-point (Simple protocol and the media interconnects just two nodes)
• Multi-access (CSMA/CD; CSMA/CA)
• Ring (Token Ring)
* An Ethernet MAC address is 48 bits and is generally represented in hexadecimal format.
* Preamble---The alternating pattern of ones and zeros tells receiving stations that a frame is
coming (Ethernet or IEEE 802.3). It is used for timing synchronization.
* Trailer: Data Link layer protocols add trailer to the end of each frame.
 The trailer is used to determine if the frame arrived without error.
 This process is called error detection.
* Layer 2 addresses vs. layer 3 address.
 The data Link layer provides addressing that is used in transporting the frame across the
shared local media.
 Unlike Layer 3 logical addresses that are hierarchical, physical addresses do not indicate
on what network the device is located.
 Device addresses at this layer are referred to as physical addresses.
 If the device is moved to another network or subnet, it will still function with the same
Layer 2 physical address.
* The Data Link layer is often divided into two sublayers.
 Logical Link Control (The upper sublayer)
– defines the software processes that provide services to the Network layer
protocols.
 Media Access Control (The lower sublayer)
– defines the media access processes performed by the hardware.
* Make sure you understand what is CRC and the process of CRC calculation.
 Frame Check Sequence
– This is the cyclic redundancy check (CRC) value.
– This value is placed in the FCS field of the frame to represent the contents of the
frame.

When the frame arrives at the destination node, the receiving node calculates its own
logical summary, or CRC, of the frame.
– The receiving node compares the two CRC values. If the two values are the same,
the frame is considered to have arrived as transmitted.
– If the CRC value in the FCS differs from the CRC calculated at the receiving
node, the frame is discarded
* If the device is moved to another network or subnet,
– Layer 2 address will still function with the same Layer 2 physical address.
– Layer 3 address must be reassigned.
–