1. Business Data Communications
Chapter Three
Data Link Layer Fundamentals

2. Primary Learning Objectives
Understand the function of the data link layer
Distinguish Logical Link Control from Media Access Control
Describe the two types of flow control
Explain line discipline
Define the components of error control
Recognize two methods of delineating data in a bit stream
Identify devices and components associated with the data link layer

3. The Data Link Layer – Its Function
Sits above the physical and below the Network Layers
Formats data bits into frames
Has two components:
Logical Link Control – 802.2
Media Access Control – for Ethernet
Is responsible for:
Line discipline
Flow control
Error control

4. Components of the Data Link Layer

5. Logical Link Control (LLC)
Designated by the IEEE as and sits above the Media Access Control
Provides three types of frame delivery service using protocol data units:
Type 1 – connectionless without acknowledgement, the most used delivery service
Type 2 – connection-oriented with acknowledgement
Type 3 – connectionless with acknowledgement

6. Logical Link Control (LLC)
LLC supports three types of frames:
I – Information
is connection-oriented
S – Supervisory
manages the Information frames
U – Unnumbered
used by connectionless services and terminates connection-oriented services
Only Type 2 delivery service uses all three types of frames

7. Logical Link Control (LLC)

8. Media Access Control (MAC)
Has various IEEE designations, with the most common being for Ethernet
Determines how devices share a common circuit
Falls into one of two categories:
Contention (802.3, Ethernet, for example)
Controlled access (802.5, Token Ring, for example)
FDDI, another form of controlled access, is an ANSI/ITU-T standard

9. Media Access Control – 802.3

10. Media Access Control – 802.5

11. Media Access Control (MAC)
Standard networks using hubs and bridges can suffer from significant collision impairment under high traffic:
Modern networks using switching technology have greatly eased this problem
Switches do not change the underlying architecture

12. Media Access Control (MAC)
Format of a MAC Protocol Data Unit (PDU)

13. Flow Control
Prevents a sender from overwhelming a receiver with traffic:
A sender and receiver each have a memory area in which they can store frames
This memory is sometimes referred to as a buffer
A sender can overwhelm, or overflow, a receiver’s memory buffer without proper flow control
If an overflow occurs, data would likely be lost

14. Flow Control Two common forms of flow control are: Stop-and-wait
Each single frame sent requires receipt of one acknowledgement
Sliding windows
The sending of multiple frames requires a single acknowledgement returned

15. Flow Control Stop-and-wait: Sliding windows:
Most efficient for messages containing a few large frames that traverse short links
Requires one acknowledgement for each frame sent
Sliding windows:
Most efficient for messages containing many small frames that traverse long links
Allows for one acknowledgement for multiple frames

16. Stop-and-Wait Flow Control

17. Sliding Windows Flow Control

18. Line Discipline
Can be viewed as a “polite” means of controlling a conversation between communicating devices
Associated with two types of network environments:
Point-to-point between communicating devices using half- or full-duplex circuits
Multipoint with communicating devices going through a central controlling device
The central control device is often a mainframe with connected terminals

19. Error Control No system is perfect; errors should be expected
Errors can result when data is lost, corrupted, or damaged, making error control critical
Error control has two components:
Error Detection
Error Correction
The two components are equally important

20. Error Detection Common error-detection methods include:
Parity checking
50% probability of detection
Longitudinal redundancy checking
98% probability of detection
Checksum checking
99.6% probability of detection
Cyclical redundancy checking
99.9%+ probability of detection

21. Parity Checking An extra parity bit is added to the byte
Assuming even parity:
– data sent
data received
Error detected on receiver side (single bit)
– data received
No error detected on receiver side (multiple bit)
Simple parity detects only single bit errors

22. Longitudinal Redundancy Checking -- LRC
Longitudinal literally means “lengthwise”
The sender, for each byte in the message, calculates a parity value, creating an additional block check character or BCC
As with parity checking, the parity value is odd or even
The BCC is added to the end of the message block
The receiver performs the same lengthwise LRC computation
If the receiver’s calculated BCC does not equal the sender’s calculated BCC, the receiver assumes a transmission error

23. Longitudinal Redundancy Checking
– Before BCC

24. Longitudinal Redundancy Checking
– After BCC
The BCC added to the end of the data block.

25. Checksum Checking – CC The message sender: The message receiver:
Evaluates each binary byte in the message to its decimal value
Totals the decimal values of all bytes
Divides the total by 255, creating a remainder
Using the remainder for the CC, adds the CC to the end of the message block
The message receiver:
Performs the same byte-by-byte calculation and creates his own CC
Compares his calculated CC to the sender’s
Assumes a transmission error if the two CC values differ

26. Checksum Checking – CC 308 / 255 = 1.21 CC = 21 66 89 84 69 308 Char
Col 1
Col 2
Col 3
Col 4
Col 5
Col 6
Col 7
Col 8 B 1 Y T E
TOTAL
Decimal
Equivalent 66 89 84 69
308
308 / 255 = CC = 21

27. Error Control
The most common error correction technique is to simply retransmit the data in error
Easy, but requires time for the retransmission
A second error correction technique is called forward error correction:
The core message is sent along with redundant data bits
The redundant data bits can, if necessary, be used by the receiving device to correct errors on site without retransmission
However, forward error correction results in inefficient use of a circuit if too many redundant data bits are sent and not used

28. Data Delineation
A transmitted bit stream contains not only the core message but control information as well
Control information could include:
Source address
Destination address
Length of message field
Error control data
Other “non-core” information
Data delineation differentiates between core and other data

29. Data Delineation Two key methods providing data delineation are:
Asynchronous data link protocols
Synchronous data link protocols
Asynchronous protocols:
Are used mostly by mainframes and their connected terminals
Provide byte-by-byte delineation
Synchronous protocols:
Are used in LANs, BNs, MANs, and WANs
Provide delineation for groups of bytes

30. Data Delineation Popular asynchronous protocols include:
XModem
YModem
ZModem
Kermit
Asynchronous protocols require that every data byte have a start and stop bit before and after it
Generally less efficient than synchronous protocols

31. Asynchronous Transmission

32. Data Delineation
Synchronous protocols are either bit- or byte-oriented
Bit-oriented protocols are more flexible:
They do not require a predetermined “byte” character format, such as EBCDIC or ASCII
They are more complicated
Bit stuffing may be required
HDLC is a formalized bit-based protocol
Byte-oriented protocols:
Are based upon known “byte” based data
Ethernet is a very common byte-based protocol

33. Simple Synchronous Transmissions

34. Data Link Layer Devices and Components
A Network Interface Card is:
A component rather than device
Essential to connect a device to a network
Bridges:
Link segments of the same logical network
Filter traffic, and so can improve network performance
Switches:
Offer more functionality than hubs and bridges
Provide point-to-point connections to devices plugged into them
Have transformed how standard Ethernet is configured

35. A 3Com Network Interface Card (NIC)
NICs have a physical address
NIC addresses must be unique
NIC addresses can be bypassed, or overridden, by software, but care must be taken when doing this to avoid address duplication

36. A Linksys Wireless Bridge
Bridges filter network traffic

37. A Bridge Filtering Traffic

38. A standard Cisco Switch
Switches have mostly replaced hubs in modern Ethernet networks

39. In Summary The data link layer:
Is stacked above the physical and below the network layers
Formats data bits into units called frames
Is composed of two stacks, the logical link and the media access controls
Performs error control
Has devices such as bridges and switches with which it is particularly associated