Data Link Layer

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Data Link Layer

• Chap 7, Course 1
• Cisco CCNA Exploration 1

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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
Satellite
Wifi
Wifi
Ethernet
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 layerprotocols
defines the media access processesperformed 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 converged2. 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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