Transcript Data Link Layer Fundamentals Business Data Communications Chapter Three

Business Data Communications
Chapter Three
Data Link Layer Fundamentals
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
Chapter Three
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Primary Learning Objectives
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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
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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The Data Link Layer –
Its Function
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Sits above the physical and below the Network Layers
Formats data bits into frames
Has two components:
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Logical Link Control – 802.2
Media Access Control – 802.3 for Ethernet
Is responsible for:
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Line discipline
Flow control
Error control
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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Components of the Data Link Layer
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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Logical Link Control (LLC)
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Designated by the IEEE as 802.2 and sits above the
Media Access Control
Provides three types of frame delivery service using
protocol data units:
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Type 1 – connectionless without acknowledgement,
the most used delivery service
Type 2 – connection-oriented with acknowledgement
Type 3 – connectionless with acknowledgement
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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Logical Link Control (LLC)
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LLC supports three types of frames:
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I – Information
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S – Supervisory
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manages the Information frames
U – Unnumbered
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is connection-oriented
used by connectionless services and terminates
connection-oriented services
Only Type 2 delivery service uses all three types of frames
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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Logical Link Control (LLC)
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Media Access Control (MAC)
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Has various IEEE designations, with the most common
being 802.3 for Ethernet
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Determines how devices share a common circuit
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Falls into one of two categories:
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Contention (802.3, Ethernet, for example)
Controlled access (802.5, Token Ring, for example)
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FDDI, another form of controlled access, is an ANSI/ITU-T
standard
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Media Access Control – 802.3
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Media Access Control – 802.5
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Media Access Control (MAC)
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Standard 802.3 networks using hubs and bridges can
suffer from significant collision impairment under high
traffic:
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Modern 802.3 networks using switching technology
have greatly eased this problem
Switches do not change the underlying 802.3
architecture
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Media Access Control (MAC)
Format of a MAC Protocol Data Unit (PDU)
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Flow Control
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Prevents a sender from overwhelming a receiver with
traffic:
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A sender and receiver each have a memory area in
which they can store frames
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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
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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Flow Control
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Two common forms of flow control are:
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Stop-and-wait
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Each single frame sent requires receipt of one
acknowledgement
Sliding windows
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The sending of multiple frames requires a single
acknowledgement returned
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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Flow Control
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Stop-and-wait:
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Most efficient for messages containing a few large frames
that traverse short links
Requires one acknowledgement for each frame sent
Sliding windows:
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Most efficient for messages containing many small frames
that traverse long links
Allows for one acknowledgement for multiple frames
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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Stop-and-Wait Flow Control
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Sliding Windows Flow Control
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Line Discipline
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Can be viewed as a “polite” means of controlling a
conversation between communicating devices
Associated with two types of network environments:
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Point-to-point between communicating devices using
half- or full-duplex circuits
Multipoint with communicating devices going through a
central controlling device
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The central control device is often a mainframe with
connected terminals
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Error Control
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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:
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Error Detection
Error Correction
The two components are equally important
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Error Detection
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Common error-detection methods include:
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Parity checking
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Longitudinal redundancy checking
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98% probability of detection
Checksum checking
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50% probability of detection
99.6% probability of detection
Cyclical redundancy checking
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99.9%+ probability of detection
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Parity Checking
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An extra parity bit is added to the byte
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Assuming even parity:
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10000010 – data sent
10000110 - data received
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10000010 – data sent
10011010 – data received
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Error detected on receiver side (single bit)
No error detected on receiver side (multiple bit)
Simple parity detects only single bit errors
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Longitudinal Redundancy Checking -LRC
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Longitudinal literally means “lengthwise”
The sender, for each byte in the message, calculates a
parity value, creating an additional block check character
or BCC
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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
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Longitudinal Redundancy Checking
01000010 01011001 01010100 01000101 – Before BCC
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Longitudinal Redundancy Checking
01000010 01011001 01010100 01000101 00001010 – After BCC
The BCC added to the end of the data block.
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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Checksum Checking – CC
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The message sender:
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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:
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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
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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Checksum Checking – CC
Decimal
Equivalent
Char
Col
1
Col
2
Col
3
Col
4
Col
5
Col
6
Col
7
Col
8
B
0
1
0
0
0
0
1
0
66
Y
0
1
0
1
1
0
0
1
89
T
0
1
0
1
0
1
0
0
84
E
0
1
0
0
0
1
0
1
69
TOTAL
308
308 / 255 = 1.21 CC = 21
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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Error Control
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The most common error correction technique is to
simply retransmit the data in error
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Easy, but requires time for the retransmission
A second error correction technique is called
forward error correction:
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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
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Data Delineation
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A transmitted bit stream contains not only the core
message but control information as well
Control information could include:
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Source address
Destination address
Length of message field
Error control data
Other “non-core” information
Data delineation differentiates between core and other
data
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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Data Delineation
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Two key methods providing data delineation are:
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Asynchronous protocols:
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Asynchronous data link protocols
Synchronous data link protocols
Are used mostly by mainframes and their connected
terminals
Provide byte-by-byte delineation
Synchronous protocols:
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Are used in LANs, BNs, MANs, and WANs
Provide delineation for groups of bytes
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Data Delineation
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Popular asynchronous protocols include:
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XModem
YModem
ZModem
Kermit
Asynchronous protocols require that every data byte have
a start and stop bit before and after it
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Generally less efficient than synchronous protocols
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Asynchronous Transmission
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Data Delineation
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Synchronous protocols are either bit- or byte-oriented
Bit-oriented protocols are more flexible:
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They do not require a predetermined “byte” character
format, such as EBCDIC or ASCII
They are more complicated
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Bit stuffing may be required
HDLC is a formalized bit-based protocol
Byte-oriented protocols:
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Are based upon known “byte” based data
Ethernet is a very common byte-based protocol
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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Simple Synchronous Transmissions
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Data Link Layer
Devices and Components
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A Network Interface Card is:
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Bridges:
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A component rather than device
Essential to connect a device to a network
Link segments of the same logical network
Filter traffic, and so can improve network performance
Switches:
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Offer more functionality than hubs and bridges
Provide point-to-point connections to devices plugged into
them
Have transformed how standard Ethernet is configured
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A 3Com
Network Interface Card (NIC)
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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
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A Linksys Wireless Bridge
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Bridges filter network traffic
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A Bridge Filtering Traffic
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A standard Cisco Switch
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Switches have mostly
replaced hubs in modern
Ethernet networks
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In Summary
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The data link layer:
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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
Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall
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