1. Chapter 7 Data Link Layer
CIS 81 Networking Fundamentals
Rick Graziani
Cabrillo College
Last Updated: 4/27/2008

2. This Presentation
For a copy of this presentation and access to my web site for other CCNA, CCNP, and Wireless resources please me for a username and password.
Web Site:

3. Data Link Layer Ethernet, PPP, ISDN, Frame Relay
The Data Link layer provides a means for exchanging data over a common local media.

4. Generic Data Link Header
Application Header + data

5. Node to node, not host to host
The Data Link layer provides a means for exchanging data over a common local media.

6. Focus on Data Link LayerIP IP

7. Reminder of encapsulation/decapsulation
Data Link Header
IP Header
TCP Header
HTTP Header
Data Link Trailer
Data
Data Link Header
Data Link Header
Data Link Trailer
Data Link Trailer
IP Packet
IP Packet
Data Link Header
Data Link Header
Data Link Trailer
Data Link Trailer
IP Packet
IP Packet
Data Link Header
Data Link Header
Data Link Trailer
Data Link Trailer
IP Packet
IP Packet
Data Link Header
IP Header
TCP Header
HTTP Header
Data Link Trailer
Data

8. Data Link Frame 1. 2.
The Data Link layer provides a means for exchanging data over a common local media.
The Data Link layer performs two basic services:
1. Allows the upper layers to access the media using techniques such as framing
2. Controls how data is placed onto the media and is received from the media.

9. Data Link Frame
The Data Link layer prepares a packet for transport across the local media by encapsulating it with a header and a trailer to create a frame.
The Data Link layer frame includes:
Data – Layer 3 (IP Packet or other Layer 3 information)
Header - Control information, such addressing
Trailer - Control information, such as error detection

11. Data Link Frame Typical field types may include:
Start and stop indicator fields
Addressing fields
Type field - The type of PDU contained in the frame
Quality - control fields
Data field -The frame payload (Network layer packet)
Not all protocols include all of these fields.

12. Data Link Layer Data Link layer
Connects the Network Layer with the Physical Layer
Network Layer and above is software (IP, TCP, HTTP, etc.)
Physical layer is implemented in hardware (converting bits to a transmission signal)
Data Link layer is implemented in both:
Software
Hardware
Data Link Layer prepares Network Layer packets for transmission across some form of media, be it copper, fiber, or the atmosphere.

13. Data Link Sublayers Data Link layer has two sublayers (sometimes):
Logical Link Control (LLC) – Software processes that provide services to the Network layer protocols.
Frame information that identifies the Network layer protocol.
Multiple Layer 3 protocols, (IP and IPX) can use the same network interface and media.
Media Access Control (MAC) - Media access processes performed by the hardware.
Provides Data Link layer addressing and framing of the data according to the protocol in use.

14. Data Link Standards
TCP/IP standards are defined by Internet Engineering Task Force (IETF).
Data Link layer protocols are defined by:
ISO - International Organization for Standardization
IEEE - Institute of Electrical and Electronics Engineers
ITU - International Telecommunication Union
ANSI - American National Standards Institute

15. Media Access Control
Media Access Control - Regulates the placement of data frames onto the media.
The method of media access control used depends on:
Media sharing
Do more than two nodes share the media?
If so, how? (Switches, hubs, etc.)

16. Serial vs multi-access
Point-to-Point
Point-to-Point networks
Only two nodes
/30 subnets are common
Protocols: PPP, HDLC, Frame Relay
Multi-access networks (LANs)
Multiple nodes
Subnets mask range depends upon the number of hosts (nodes)
Protocols: Ethernet, (wireless), Frame Relay Multipoint

17. Duplex Transmissions Simplex Transmission: One way and one way only.
One way street
Half-duplex Transmission: Either way, but only one way at a time.
Two way street, but only one way at a time (land slide).
Ethernet hubs use half-duplex
Full-duplex Transmission: Both ways at the same time.
Two way street
Ethernet switches use full-duplex
Most serial links are full-duplex

18. Physical Topology
Hub
The physical topology is an arrangement of the nodes and the physical connections between them.
Switch
Serial Connections

19. Logical Topology
A logical topology - The way a network transfers frames from one node to the next.
Defined by Data Link layer protocols.
Media Access Control used.
Type of network framing

20. Point-to-Point topology
A point-to-point topology connects two nodes directly together.
The media access control protocol can be very simple.
Frames from one devices are for the device at the other end.
Point-to-point topologies, with just two interconnected nodes, do not require special addressing.

21. Logical Point-to-Point Networks
Point-to-point networks may include intermediate devices.
No affect on logical topology.
The logical connection (in some cases) may be a virtual circuit.
A virtual circuit is a logical connection created within a network between two network devices.
The two nodes exchange the frames with each other.
Data Link Destination address is the device at the other end of the virtual circuit.

22. Multi-access Topology
A logical multi-access topology - Enables a number of nodes to communicate by using the same shared media.
“Data from only one node can be placed on the medium at any one time.”
(This is only true when using CSMA/CD (hubs), NOT true with switches or wireless)
Every node “may” see all the frames that are on the medium.
Data Link Destination Address denote which device the frame is for.

23. Multi-access Addressing
2222
4444
6666
3333
5555
6666
2222
Multi-access networks require an address to specifically identify the destination.

24. Media Access Control
The media access control methods used by logical multi-access topologies are typically:
CSMA/CD - Hubs
CSMA/CA - Wireless
Token passing – Token Ring
Later

25. Ring Topology Token Passing media access control
Each node in turn receives a frame.
If data link destination address is not for that device, passes frame to next node.

26. Data Link Frame
No one frame structure meets the needs of all data transportation across all types of media.
Depending on the:
Environment
Amount of control information needed
Topology

27. Data Link Frame Fields Data Link frame header fields may include:
Start Frame field - Indicates the beginning of the frame
Source and Destination address fields - Indicates the source and destination nodes on the media
Priority/Quality of Service field - Indicates a particular type of communication service for processing
Type field - Indicates the upper layer service contained in the frame
Logical connection control field - Used to establish a logical connection between nodes
Physical link control field - Used to establish the media link
Flow control field - Used to start and stop traffic over the media
Congestion control field - Indicates congestion in the media

28. Framing- The Trailer The signals on the media could be subject to:
Interference
Distortion
Loss
This would change the bit values that those signals represent.
The trailer is used to determine if the frame arrived without error.
Error detection.
The Frame Check Sequence (FCS) field is used to determine if errors occurred in the transmission and reception of the frame.

29. Cyclic Redundancy Check
Cyclic redundancy check (CRC) is commonly used.
Sending node includes a logical summary of the bits in the frame.
Receiving node calculates its own logical summary, or CRC.
Compares the two CRC values.
Equal – Accepts the frame
Different – Discards the frame

30. Bandwidth LAN typically uses a high bandwidth technology
Supporting large numbers of hosts
WAN
High bandwidth technology is usually not cost-effective across large geographic areas (cities or multiple cities, for example).
The cost typically results in lower bandwidth capacity.
Note: This is relative and the need for high bandwidth on WANs is increasing due to video, voice, and other applications.

31. Ethernet Protocol for LANs
Ethernet is a family of networking technologies that are defined in the IEEE and standards.
Uses 48 bit addressing (Ethernet MAC addresses) for Source and Destination
More next week!

32. Point-to-Point Protocol for WANs
Point-to-Point Protocol (PPP) is a protocol used to deliver frames between two nodes.
PPP can be used on various physical media, including:
Twisted pair
Fiber optic lines
Satellite transmission

33. Wireless Protocol for LANs
is an extension of the IEEE 802 standards.
It uses the same 48-bit addressing scheme as other 802 LANs.
Contention-based system using a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)

34. Chapter 7 Data Link Layer
CIS 81 Networking Fundamentals
Rick Graziani
Cabrillo College
Last Updated: 4/27/2008