OSI Data Link Layer
Network Fundamentals – Chapter 7
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Objectives

Explain the role of Data Link layer protocols in data transmission.

Describe how the Data Link layer prepares data for transmission
on network media.

Describe the different types of media access control methods.

Identify several common logical network topologies and describe
how the logical topology determines the media access control
method for that network.

Explain the purpose of encapsulating packets into frames to
facilitate media access.

Describe the Layer 2 frame structure and identify generic fields.

Explain the role of key frame header and trailer fields including
addressing, QoS, type of protocol and Frame Check Sequence.
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Context Index
 7.1 Data Link Layer – Accessing the Media
 7.2 Media Access Control Techniques
 7.3 Media Access Control Addressing and Framing Data
 7.4 Putting it All Together
 7.5 Labs and Activities
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7.1 Data Link Layer –
Accessing the Media
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7.1.1 Data Link Layer – Supporting &
Connecting to Upper Layer Services
 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
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7.1.1 Data Link Layer – Supporting &
Connecting to Upper Layer Services
 Data Link Layer Terms
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7.1.1 Data Link Layer – Supporting &
Connecting to Upper Layer Services
 Numerous Data Link layer protocols are being used
over various types of LANs and WANs.
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7.1.2 Data Link Layer – Controlling Transfer
across Local Media
 Layer 2 protocols specify the encapsulation of a packet
into a frame .
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7.1.3 Data Link Layer – Creating a Frame
 Data Link layer frame includes:
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7.1.3 Data Link Layer – Creating a Frame
 The role the Data Link layer plays in linking the
software and hardware layers
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7.1.4 Data Link Layer – Connecting Upper
Layer Services to the Media
 Data Link layer is often divided into two sub-layers:
－Logical Link Control
－Media Access Control
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7.1.5 Data Link Layer – Standards
 Standards for the Data Link layer
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7.2 Media Access Control Techniques
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7.2.1 Placing Data on the Media
 Explain the necessity for controlling access to the
media
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7.2.2 Media Access Control for Shared Media
 There are 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
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7.2.2 Media Access Control for Shared Media
 There are 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
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7.2.2 Media Access Control for Shared Media
 There are 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
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7.2.3 Media Access Control for Non-Shared
Media
 Define Full Duplex and Half Duplex as it relates to
Media Access Control for non-shared media
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7.2.3 Media Access Control for Non-Shared
Media
 Define Full Duplex and Half Duplex as it relates to
Media Access Control for non-shared media
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7.2.3 Media Access Control for Non-Shared
Media
 Define Full Duplex and Half Duplex as it relates to
Media Access Control for non-shared media
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7.2.4 Logical Topology vs Physical Topology
 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.
 Topologies used in networks
are:
－Point-to-Point
－Multi-Access
－Ring
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7.2.5 Point-to-Point Topology
 A point-to-point topology connects two nodes directly
together.
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7.2.5 Point-to-Point Topology
 A virtual circuit is a logical connection created within a
network between two network devices.
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7.2.5 Point-to-Point Topology
 A virtual circuit is a logical connection created within a
network between two network devices.
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7.2.6 Multi-Access Topology
 Data from only one node can be placed on the medium
at any one time.
 Every node sees all the frames that are on the medium,
but only the node to which the frame is addressed
processes the contents of the frame.
 A Data Link media access control method is required to
regulate the transmission of data.
 The media access control methods are typically
CSMA/CD or CSMA/CA.
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7.2.6 Multi-Access Topology
 How nodes access the media in a multi-access
topology
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7.2.7 Ring Topology
 In a logical ring topology, each node in turn receives a
frame. If the frame is not addressed to the node, the
node passes the frame to the next node.
 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.
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7.2.7 Ring Topology
 How nodes access the media in a logical ring topology
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7.3 Media Access Control Addressing
and Framing Data
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7.3.1 Data Link Layer Protocols – The Frame
 The structure of the frame and the fields contained in
the header and trailer vary according to the protocol.
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7.3.2 Framing – Role of the Header
 The role of the frame header in the Data Link layer
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7.3.3 Addressing – Where the Frame Goes
 Data Link layer addressing is contained within the
frame header and specifies the frame destination node
on the local network.
 If the device is moved to another network or subnet,
physical addresses will still function with the same
Layer 2 physical address.
 The intermediate device - a router - will decapsulate the
original frame, create a new frame for the packet.
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7.3.3 Addressing – Where the Frame Goes
 Addressing Requirements
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7.3.4 Framing – Role of the Trailer
 The role of the frame trailer in the Data Link layer
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7.3.5 Data Link Layer Protocol – The Frame
 Given the wide range of physical media used in
networking, there are a correspondingly high number of
Layer 2 protocols in use.
802.11 Wireless
Frame
802.11 Wireless
Frame
HDLC
Frame Relay
PPP frame
Ethernet Frame
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7.3.5 Data Link Layer Protocol – The Frame
 Ethernet Protocol for LANs
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7.3.5 Data Link Layer Protocol – The Frame
 Point-to-point Protocol for WANs
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7.3.5 Data Link Layer Protocol – The Frame
 Wireless Protocol for LANs
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7.4 Putting it All Together
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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7.4.1 Follow Data Through an Internetwork
 A simple data transfer between two hosts across an
internetwork.
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Summary
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