1. Network Fundamentals – Chapter 7
OSI Data Link Layer
Network Fundamentals – Chapter 7

2. 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.

3. 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

4. 7.1 Data Link Layer – Accessing the Media

5. 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

6. 7.1.1 Data Link Layer – Supporting & Connecting to Upper Layer Services
Data Link Layer Terms

7. 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.

8. 7.1.2 Data Link Layer – Controlling Transfer across Local Media
Layer 2 protocols specify the encapsulation of a packet into a frame .

9. 7.1.3 Data Link Layer – Creating a Frame
Data Link layer frame includes:

10. 7.1.3 Data Link Layer – Creating a Frame
The role the Data Link layer plays in linking the software and hardware layers

11. 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

12. 7.1.5 Data Link Layer – Standards
Standards for the Data Link layer

13. 7.2 Media Access Control Techniques

14. 7.2.1 Placing Data on the Media
Explain the necessity for controlling access to the media

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 7.2.5 Point-to-Point Topology
A point-to-point topology connects two nodes directly together.

23. 7.2.5 Point-to-Point Topology
A virtual circuit is a logical connection created within a network between two network devices.

24. 7.2.5 Point-to-Point Topology
A virtual circuit is a logical connection created within a network between two network devices.

25. 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.

26. 7.2.6 Multi-Access Topology
How nodes access the media in a multi-access topology

27. 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.

28. 7.2.7 Ring Topology
How nodes access the media in a logical ring topology

29. 7.3 Media Access Control Addressing and Framing Data

30. 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.

31. 7.3.2 Framing – Role of the Header
The role of the frame header in the Data Link layer

32. 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.

33. 7.3.3 Addressing – Where the Frame Goes
Addressing Requirements

34. 7.3.4 Framing – Role of the Trailer
The role of the frame trailer in the Data Link layer

35. 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.
Wireless
Frame
Wireless
Frame
HDLC
Frame Relay
PPP frame
Ethernet Frame

36. 7.3.5 Data Link Layer Protocol – The Frame
Ethernet Protocol for LANs

37. 7.3.5 Data Link Layer Protocol – The Frame
Point-to-point Protocol for WANs

38. 7.3.5 Data Link Layer Protocol – The Frame
Wireless Protocol for LANs
Power Management field - Set to 1 to indicate that a node will be in power-save mode
Retry field - Set to 1 if the frame is a retransmission of an earlier frame
More Data field - Set to 1 to indicate to a node in power-save mode that more frames are buffered for that node
More Fragments field - Set to 1 for frames that have another fragment
From DS field - Set to 1 in data frames exiting the distribution system
Frame Body field - Contains the information being transported; for data frames, typically an IP packet
FCS field - Contains a 32-bit cyclic redundancy check (CRC) of the frame
Receiver Address (RA) field - MAC address that identifies the wireless device that is the immediate recipient of the frame
Fragment Number field - Indicates the number for each fragment of a frame
Sequence Number field - Indicates the sequence number assigned to the frame; retransmitted frames are identified by duplicate sequence numbers
Transmitter Address (TA) field - MAC address that identifies the wireless device that transmitted the frame
Source Address (SA) field - MAC address of the node the initiated the frame
Duration/ID field - Depending on the type of frame, represents either the time, in microseconds, required to transmit the frame or an association identity (AID) for the station that transmitted the frame
To DS field - Set to 1 in data frames destined for the distribution system (devices in the wireless structure)
Destination Address (DA) field - MAC address of the final destination node in the network
Order field - Set to 1 in a data type frame that uses Strictly Ordered service class (does not need reordering)
Type and Subtype fields –
Identifies one of three functions and sub functions of the frame: control, data, and management
Wired Equivalent Privacy (WEP) field - Set to 1 if the frame contains WEP encrypted information for security
Protocol Version field - Version of frame in use

39. 7.4 Putting it All Together

40. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

41. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

42. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

43. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

44. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

45. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

46. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

47. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

48. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

49. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

50. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

51. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

52. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

53. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

54. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

55. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

56. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

57. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

58. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

59. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

60. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

61. 7.4.1 Follow Data Through an Internetwork
A simple data transfer between two hosts across an internetwork.

62. Summary