Transcript Document

Data Link Layer
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Two basic services of Data Link
• 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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Data Link Terms
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Note …
• Medium or media refer to the material that
actually carries the signals representing the
transmitted data.
– Not audio, video, and the likes …
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Physical vs logical network
• Logical networks are defined at the
Network layer by the arrangement of the
hierarchical addressing scheme.
• Physical networks represent the
interconnection of devices on a common
media.
– Sometimes, also referred to as a network
segment.
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Recall …
• A network model allows each layer to
function with minimal concern for the roles
of the other layers.
– The Data Link layer relieves the upper layers
from the responsibility of putting data on the
network and receiving data from the
network.
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Data Link: supporting its upper layer
• In any given exchange of Network layer packets,
there may be numerous Data Link layer and media
transitions.
• At each hop along the path, an intermediary
device - usually a router - accepts frames from a
medium, de-capsulates the frame, and then
forwards the packet in a new frame appropriate to
the medium of that segment of the physical
network.
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A layer 3 packet may have to go
through various data link networks
Satellite
Satellite
Wifi
Wifi
Ethernet
Ethernet
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Data Link: supporting its upper layer
• The Data Link layer effectively insulates the
communication processes at the higher
layers from the media transitions that may
occur end-to-end.
• A packet is received from and directed to an
upper layer protocol that does not need to
be aware of which media the
communication will use.
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Data Link: supporting its upper layer
• Due to the higher number of
communication services provided by the
Data Link layer …
– it is difficult to generalize their role and provide
examples of a generic set of services.
– any given upper layer protocol may or may
not support all these Data Link layer
services.
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Transfer of Frames
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Media Access Control
• Each network environment that packets encounter
as they travel from a local host to a remote host
can have different characteristics.
– one network environment may consist of many hosts
contending to access the network medium on an ad
hoc basis.
– Another environment may consist of a direct
connection between only two devices over which data
flows sequentially as bits in an orderly way.
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Media Access Control (MAC)
• Define the processes by which network
devices can access the network media and
transmit frames in diverse network
environments.
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MAC
• A node that is an end device uses an adapter
to make the connection to the network.
– For example, to connect to a LAN, the device
would use the appropriate Network Interface
Card (NIC) to connect to the LAN media.
– The adapter manages the framing and media
access control.
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Network Interface Card (NIC)
• Different physical interfaces on the router
are used to encapsulate the packet into the
appropriate frame, and a suitable media
access control method is used to access each
link.
– E.g., a router may have an Ethernet interface to
connect to the LAN and a serial interface to
connect to the WAN.
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Framing : Creating a frame
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Framing
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Connecting upper layer to the Media
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Data Link Sub-layers
defines the software processes that
provide services to the Network layer
protocols
defines the media access processes
performed by the hardware
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Data Link Standards
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MAC - again
• Regulating the placement of data frames
onto the media is known as media access
control.
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MAC
• Regulating the placement of data frames onto the
media is known as media access control
• The absence of any media access control would be
the equivalent of vehicles ignoring all other traffic
and entering the road without regard to the other
vehicles.
– However, not all roads and entrances are the same.
– 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.
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MAC
• The method of media access control used
depends on:
– Media sharing
• If and how the nodes share the media
– Topology
• How the connection between the nodes appears to
the Data Link layer
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MAC
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MAC for shared media
• Two basic media access control methods for
shared media:
– Controlled - Each node has its own time to use
the medium
• Token Ring (deterministic)
– Contention-based - All nodes compete for the
use of the medium
• CSMA/CD (non-deterministic)
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CSMA/CD
• Multiple Access (MA)
– Describing the fact that media is shared
• The device monitors the media for the presence of
a data signal  Carrier Sense (CS)
– If a data signal is absent, indicating that the media is
free, the device transmits the data.
• If signals are then detected that show another
device was transmitting at the same time, all
devices stop sending and try again later 
Collision Detection (CD)
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CSMA/CA
• The device examines the media for the
presence of a data signal.
– If the media is free, the device sends a
notification across the media of its intent to use
it.
– The device then sends the data.
– Used by 802.11 wireless networking
technologies.
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MAC for non-shared media
• Require little or no control before placing
frames onto the media.
• These protocols have simpler rules and
procedures for media access control.
– Such is the case for point-to-point topologies.
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In Point-to-point connection …
• The Data Link layer has to consider whether
the communication is
– half-duplex
• devices can both transmit and receive on the media
but cannot do so simultaneously
– full-duplex
• both devices can transmit and receive on the media
at the same time
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Logical Topology
What about physical topology?
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Logical Point to Point
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Adding intermediate physical
connections to Logical Point to Point
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Virtual Circuit
• In some cases, the logical connection
between nodes forms what is called a virtual
circuit.
– A virtual circuit is a logical connection created
within a network between two network devices.
– Virtual circuits are important logical
communication constructs used by some Layer
2 technologies.  like frame relay
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Logical multiple-access topology
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Ring topology
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Framing - again
• Data Link layer frames has three basic parts
– Header
– Data
– Trailer
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In a fragile environment …
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In a protected environment …
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Role of header & trailer
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Addressing in multiple access
topology
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Addressing in point-to-point
topology
the frame has only one place it can go
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Examples of Data Link Frames
HDLC frame
PPP frame
Frame Relay frame
Ethernet frame
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Ethernet Frame
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PPP Frame
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802.11 Frame
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A simple data transfer between 2
hosts
1. Assume all routing tables are converged
2. ARP tables are complete
3. A TCP session is already established between the client and server.
4. DNS lookup for the WWW server is already cached at the client.
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Follow data through an internetwork - 1
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Follow data through an internetwork - 2
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Follow data through an internetwork - 4
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Follow data through an internetwork - 15
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Follow data through an internetwork - 19
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