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Chapter 7Data Link Layer
Modified by Profs. Chen and Cappellino
Learning Objectives
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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.
Connecting to Upper Layer Services
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The ____________ layer provides a means for
_______________________________________
_____________________________________:
The Data Link layer performs ___ basic services:
Allows
the _____________________ the media using
techniques such as _______________
Controls ________________________________ and is
received from the media using techniques such as
_____________________________________
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Common Layer 2 terms:
_________________ - The Data Link layer PDU
________________ - The Layer 2 notation for network
devices connected to a common medium
_____________________________- The physical
means (cables, atmosphere) for the transfer of
information between two nodes
______________________________ - Two or more
nodes connected to a common medium
Connecting to Upper Layer Services
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The Data Link layer ___________________________
_______________________________________- over
which data is to be transmitted.
At
each hop along the path, a router (or similar device) ________
______________________________________________________
________________ appropriate to the medium of the next segment.
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Example, as an IP packet travels from PC to a remote
laptop via a satellite link:
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it will be encapsulated into Ethernet frame at the source
decapsulated,and then encapsulated into a new data link frame to cross the
satellite link.
Several hops with different data link frames could occur in the middle…
For the final link, the packet will use a wireless data link frame from the final
router to the laptop.
As packet moves up the OSI layer at the destination, the
higher layer protocols do not need to be aware of which
media the communication used
Controlling Transfer across Local Media
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Layer 2 protocols specify :
The
technique used for ______________________________ which
is called the _______________________________________
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_______________________ media access control methods may be
required during the course of a ________________________________
_____________________________________________________
For example, the device (such as PC or laptop) would use
the appropriate NIC to connect to the LAN media.
The
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NIC manages the framing and media access control.
At intermediary devices such as a _________________
______________________________________________________
________________________________________________
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To
____________________ interfaces connect to ________________
______________________ interfaces connect to _______________.
process a frame, the router uses Data Link layer to receive the
frame from medium, decapsulate it up to the Layer 3 PDU, reencapsulate the PDU into a new frame, and place the frame on
the medium of the next link of the network.
Creating a Frame
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When a node is receiving long streams of 1 and 0 bits, how does it determine
where a frame starts and stops or which bits represent the address?
Data Link layer protocols require the following _______
__________________ to enable protocols to function:
__________ are in communication with each other
______________________ between individual
nodes begins and when it ends
________________ while the nodes communicated
_____________________________________
A packet is encapsulated within a _______________
______________________________________
____________ - Contains __________________,
such as addressing, and is located at the beginning
of the PDU
More to
______________________________________
come…
_______________ - Contains ________________
added to the end of the PDU
Creating a Frame
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Typical field types include:
____________________________
- The beginning and end limits of
the frame
______________________________
____________________ - The type of PDU contained in the frame
________________________________________
_____________________________________________________
_______________ Fields at end of frame used for error detection
and mark the end of the frame.
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The standards for a specific Data Link protocol define the
actual frame format and actual fields used for that protocol
Connecting Upper Layer Services to the Media
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The Data Link layer ______________
_______________________________
____________________________ it.
The ________________ performs
some of the jobs of the Data Link layer
NIC
includes both _________________
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Software associated with the NIC enables the NIC to
perform its intermediary functions of ___________
_____________________________ the data as
signals to be sent on the associated media.
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Prepares the Network layer packets for
_________________________________
Separating Data Link into Sublayers...
 Allows one type of frame defined by the upper layer to access
different types of media defined by the lower layer.
 _________________ (Upper sublayer)
__________________________
that _______
____________________________ protocols.
Logical Link Control (LLC) places information in
the frame that identifies which _____________
_________________ is being used for the frame.
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Allows multiple Layer 3 protocols to utilize the
same network interface and media.
___________________ (Lower sublayer)
defines
the ______________________
performed by the __________________.
 Media
Access Control (MAC) provides _________
_____________________________
 Provides delimiting of data based on the physical
signaling requirements of the medium and the type of
Data Link layer protocol in use.
Standards
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See the chart at the right for standards making
bodies involved in the development of standards
at the Data Link layer
Unlike TCP/IP suite, Data Link layer protocols
are generally not defined by Request for
Comments (RFCs).
Engineering organizations that define open
standards and protocols that apply to the Data
Link layer include:
 International
Organization for Standardization
(ISO)
 Institute of Electrical and Electronics Engineers
(IEEE)
 American National Standards Institute (ANSI)
 International Telecommunication Union (ITU)
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Data Link layer processes occur __________
_____________________________
 The
protocols at this layer are implemented within
the electronics of the NIC with which the device
connects to the physical network.
Placing Data on the Media
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Recall: Regulating the placement of
data frames onto the media is
known as “media access control”.
Data
link layer protocols, using media
access control techniques, define the
rules for access to different media
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Different methods of media access
control used depend on the
following:
_______________________
 If and how the nodes share the media
_______________________
 How the connection between the nodes
appears to the Data Link layer
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Media Access Control for Shared Media
 Shared media: meaning _____________________
_____________________________________
There are _____ media access control
methods for shared media:
1. ________ - Each node __________
___________, accessing the medium.
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This
method is also known as __________.
Only ____________________________
If a device does not need to transmit, the
opportunity to use the medium passes to
the next device.
Deterministic methods ______________
because a device has to wait for its turn
before it can use the medium.
Example: _______________________
M A C for Shared Media cont…
2. _________________________________________________
___________________________________________
Also referred to as ________________________
 Allows _______________________________ whenever it has data to send.
 Contention-based media access control methods have ______________
than controlled access methods
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Carrier
Sense Multiple Access/Collision
Detection (___________________).
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The device monitors the media for the presence of a
data signal. __________________, indicating that
the media is free, the device _________ the data.
If a signal from another node is detected, another
__________________________
If two devices transmit at the same time, data from
both devices is destroyed in a __________, all
devices stop sending and try again later.
Traditional forms of Ethernet use this method.
CSMA/Collision Avoidance (___________),
 the device examines the media for the presence of a
data signal. If the media is free, ________________
_________________________________
 This method is used by _____________________.
•Note: CSMA/CD will be covered
in more detail in Chapter 9.
Media Access Control for Non-Shared Media
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Non-Shared media: meaning _______________
_______________________________________
Media access control protocols for non-shared
media require little before placing frames onto the
media.
___________________________________
for
controlling non-shared media access.
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Communication is either Full or Half Duplex
____________________
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communication
Devices can both _________________ on the media but ________
_________________________________
Arbitration rules used for resolving conflicts
_______________________
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communication
Both devices can __________________________________________
Logical Topology vs Physical Topology
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The ____________________ of a network is the
__________________________________________
___________________ between them.
The
__________________ is an _________________________
_______________________ between them.
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Configuration of cables, computers and other peripherals
A
_______________________ is the _______________________
from one node to the next.
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Consists of virtual connections between the nodes of a network independent of
their physical layout and defined by Data Link layer protocols.
It is the ___________________________________________________
_______________________________________
Logical topologies typically used in networks are:
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More to follow…
Logical Point-to-Point Topology
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Review: A point-to-point topology
connects two nodes directly
together, as shown
Data that only flows in one direction
at a time is referred to as what?
Flow of data across the link from
each node simultaneously is called?
The end nodes communicating in a
point-to-point network can be
_____________________________
___________________________
Still considered a logical topology
Logical Point-to-Point cont…
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As shown, the source and
destination nodes may be
___________________ to each
other over some geographical
distance and with a number of
devices between them
A ______________________
__________________________
__________________________
__________________________
Source
and destination nodes
exchange frames through
intermediary devices forming a
virtual circuit.
Virtual circuits used by some Layer
2 technologies.
Logical Multi-Access Topology
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A logical multi-access topology enables a number
of nodes to communicate by using the same
shared media.
This is also known as what?
How does Contention-based media access
control work?
How does Controlled-based media access control
work?
The media access control methods used by
logical multi-access topologies are typically
__________________________________
Token
passing rarely used
Logical Ring Topology
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Each node in turn receives a frame.
Nodes
in a logical ring topology __________________
___________________________, and ____________ if
it is _______________________________
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all nodes around the ring- between the source and destination
node- examine the frame.
A signal (known as a ___________) may be
placed on the media and a _______________
_______________________________________
Recall: this allows a ring to use a controlled media
access control technique called token passing.
The Data Link layer Frame
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Three basic parts common to all different
types of Data Link layer frames:
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All Data Link layer protocols ___________
__________________________________
_________ of the frame.
___________________ and the ______
contained in the header and trailer
____________ according to the protocol.
There
is no one frame structure that meets the
needs of all data transportation across all types
of media
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The Data Link layer protocol describes the
features required for the transport of
packets across different media.
Framing – Role of the Header
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Frame information is unique to each type of protocol.
Typical frame header fields include:
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See definitions
below…
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Other fields which we will not focus on include:
 Priority/Quality of Service field
 Logical connection control field
 Physical link control field
 Flow control field
 Congestion
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control field
Different Data Link layer protocols may use different fields
from those mentioned
depending on
media etc.
When is the Data Link address needed?
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Data Link addresses are also referred to as __________
_____________________________________________
Address
is _______________________________________
_________________________________________________
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The data Link layer provides addressing that is used in
transporting the frame across the shared _______ media.
Physical addresses _______________________ the
device is located (unlike Layer 3 logical addresses which
are hierarchical)
If
the device is moved to another network or subnet, it will still
function with the same Layer 2 physical address.
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Data Link layer address is only used to transport data
____________________________________
Addresses
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at ______________________________________.
If the packet in the frame must ____________________
segment, the router will ________________________,
__________________________ for the packet, and send
it onto the new segment.
Is the physical address always needed?
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The need for the physical address
at the Data Link layer depends on
the logical topology.
_____________________
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With just two interconnected nodes, do
__________________________
____________________
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topologies
topologies
Connect many nodes on a common
medium so _______________________
____________________________
When a frame reaches each node in the
topology, the node examines the
destination MAC address in the header to
determine if it is the destination of the
frame.
Packet propagation and switching
within a router EXAMPLE…
The next several slides give a step-bystep example of packet flow from a LAN
through a P-to-P WAN onto a remote
LAN.
See if you can determine the original
source and ultimate destination…
Let’s take a look…
Packet propagation and switching within a
router EXAMPLE…
Step 1
Packet propagation and switching within a
router EXAMPLE…
Step 2
Step 3
Packet propagation and switching within a
router EXAMPLE…
Step 3
Packet propagation and switching within a
router EXAMPLE…
Step 4
Packet propagation and switching within a
router EXAMPLE…
Step 5
Step 6
Packet propagation and switching within a
router EXAMPLE…
Step 6
Packet propagation and switching within a
router EXAMPLE…
Step 7
Packet propagation and switching within a
router EXAMPLE…
Step 8
Framing – Role of the Trailer
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Data Link layer protocols add a trailer to the end of
each frame.
Trailer contains the ________________________
_______________________ as follows…
Frame Check Sequence field
Used
to determine if the frame arrived without error.
More details to come on next slide…
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____________________ field
Optional
field used when the length not specified in
Type/Length field.
Indicates ____________________________
____________________ field…
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Used to determine if the ________________________
This process is called _______________________
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Achieved by placing a mathematical summary of the bits in
the trailer called a _____________________________
value.
 Summary of bits represent the contents of the frame.
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When the frame arrives at the ___________________,
that node __________________________________,
or CRC, of the frame.
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The receiving node compares the two CRC values. If the
two ________________________, the frame is considered
to have ________________________
 If the ________________________ from the CRC
calculated at the receiving node, the
_________________________
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Note: Error detection is added at the Data link layer
because this is where data is transferred across media
which can sometimes be an unsafe environment for
data
The Frame
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The actual Layer 2 protocol used depends on the logical
topology of the network and the of the Physical layer.
Protocols that will be covered in CCNA courses include:

Point-to-Point
Protocol (
)
High-Level Data Link Control (
)
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Each protocol performs media access control for specified
Layer 2 topologies given the technology in use.
This
means that a number of different network devices can act as nodes
that operate at the Data Link layer when implementing these protocols.
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Network devices include NICs on computers and interfaces on routers
SUMMARY
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.
Ethernet Protocol for LANs
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______________ is a family of networking technologies
that are _________ in the __________________________
Ethernet
standards define both the Layer 2 protocols and the Layer
1 technologies.
Ethernet is the most widely used LAN technology and supports data
bandwidths of 10, 100, 1000, or 10,000 Mbps.
The ______________________________________________
_________________ despite using many different types of media
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Ethernet provides unacknowledged connectionless service
over a shared media using CSMA/CD.
Shared
media requires that the Ethernet frame header to _______
_______________________________________ of the nodes.
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______________________ is the Ethernet frame format
used in TCP/IP networks.
More
to come on Ethernet in a later chapter…
Summary Ethernet Frame
Summary of the
function of the fields
-- notice the Data field
contains a packet – so
that field contains raw
data plus header info
from higher layers
Ethernet vs IEEE 802.3 -- FYI
Frame type
used in TCP/IP
networks so
our focus…
• There is a slight
difference in the
fields of these two
frame formats
•Good to be
aware…
Point-to-Point Protocol for WANs
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Point-to-Point Protocol (_____________) is a protocol ___
________________________________________
PPP was developed as a _____________ and remains the
protocol of choice to implement many serial WANs.
PPP
can be used on _____________, including twisted pair, fiber
optic lines, and satellite transmission, as well as for _________
________________________
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PPP establishes _____________________, called
___________, between two nodes.
The PPP session hides the underlying physical media from
the upper PPP protocol and ________________________
________________________________________
PPP offers the option to use ______________________,
and multilink (the use of multiple physical connections).
Summary Point-to-Point Frame
WAN protocols are
covered in more
detail in CCNA 4
Summary of the
function of the fields
-- notice the difference
in the size and content
of the fields vs the
Ethernet frame.
-- notice the size of the
address field?
Wireless Protocol for LANs
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Defined by IEEEE _______________ and many variations
Many _________________ at the MAC sublayer and
Physical layer vs frames used over a wired infrastructure
Standard IEEE 802.11, commonly referred to as ________,
is a ________________ system using a Carrier Sense
Multiple Access/Collision Avoidance (_______________).
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What is CSMA/CA?
802.11 networks also use _______________________ to
confirm that a data frame is received successfully.
If
the sending station does ______________________________
frame, the _________________________________

Other services supported by 802.11 are ______________,
_________________ (connectivity to a wireless device),
and ______________________
Summary of the Wireless Frame for LANs
Summary of the many
fields of a wireless LAN
frame.
-- All fields described in
curriculum…
Summary- Putting it all together
Section 7.4 in the curriculum contains an
excellent- 22 step- example of a simple data
transfer.
You are expected to understand each step –
they are a review of what we have seen so
far this semester.
QUESTIONS? Please ask….