1. Data Link Layer
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2. 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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3. Data Link Terms
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4. 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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5. 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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6. 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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7. 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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8. 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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9. 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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10. 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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11. Transfer of Frames
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12. 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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13. 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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14. 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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15. 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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16. Framing : Creating a frame
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17. Framing
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18. Connecting upper layer to the Media
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19. 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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20. Data Link Standards
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21. MAC - again
Regulating the placement of data frames onto the media is known as media access control.
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22. 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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23. 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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24. MAC
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25. 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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26. 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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27. 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 wireless networking technologies.
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28. 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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29. 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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30. Logical Topology
What about physical topology?
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31. Logical Point to Point
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32. Adding intermediate physical connections to Logical Point to Point
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33. 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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34. Logical multiple-access topology
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35. Ring topology
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36. Framing - again Data Link layer frames has three basic parts Header
Trailer
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37. In a fragile environment …
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38. In a protected environment …
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39. Role of header & trailer
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40. Addressing in multiple access topology
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41. Addressing in point-to-point topology
the frame has only one place it can go
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42. Examples of Data Link Frames
HDLC frame
PPP frame
Frame Relay frame
Ethernet frame
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43. Ethernet Frame
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44. PPP Frame
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45. Frame
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46. 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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47. Follow data through an internetwork - 1
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48. Follow data through an internetwork - 2
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49. Follow data through an internetwork - 4
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50. Follow data through an internetwork - 5
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51. Follow data through an internetwork - 7
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52. Follow data through an internetwork - 8
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53. Follow data through an internetwork - 9
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54. Follow data through an internetwork - 10
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55. Follow data through an internetwork - 12
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56. Follow data through an internetwork - 13
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57. Follow data through an internetwork - 14
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58. Follow data through an internetwork - 15
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59. Follow data through an internetwork - 16
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60. Follow data through an internetwork - 17
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61. Follow data through an internetwork - 19
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62. Follow data through an internetwork - 20
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63. Follow data through an internetwork - 21
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64. Follow data through an internetwork - 22
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