business data communications

Business Data Communications, by Allen Dooley, (c) 2005 Pearson Prentice Hall Chapter Three 1

Business Data Communications

Chapter ThreeData Link Layer Fundamentals


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


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 – 802.3 for Ethernet

Is responsible for: Line discipline Flow control Error control


Components of the Data Link Layer


Logical Link Control (LLC)

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:

Type 1 – connectionless without acknowledgement, the most used delivery service

Type 2 – connection-oriented with acknowledgement

Type 3 – connectionless with acknowledgement



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


Media Access Control (MAC)

Has various IEEE designations, with the most common being 802.3 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


Media Access Control – 802.3


Media Access Control – 802.5



Standard 802.3 networks using hubs and bridges can suffer from significant collision impairment under high traffic:

Modern 802.3 networks using switching technology have greatly eased this problem

Switches do not change the underlying 802.3 architecture



Format of a MAC Protocol Data Unit (PDU)


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


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


Flow Control

Stop-and-wait: 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


Stop-and-Wait Flow Control


Sliding Windows Flow Control


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


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


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


Parity Checking

An extra parity bit is added to the byte

Assuming even parity: 10000010 – data sent

10000110 - data received Error detected on receiver side (single bit)

10000010 – data sent10011010 – data received

No error detected on receiver side (multiple bit)

Simple parity detects only single bit errors


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


Longitudinal Redundancy Checking

01000010 01011001 01010100 01000101 – Before BCC


Longitudinal Redundancy Checking

01000010 01011001 01010100 01000101 00001010 – After BCC The BCC added to the end of the data block.


Checksum Checking – CC The message sender:

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


Checksum Checking – CC

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

Y 0 1 0 1 1 0 0 1

T 0 1 0 1 0 1 0 0

E 0 1 0 0 0 1 0 1

TOTAL

DecimalEquivalent

66898469308

308 / 255 = 1.21 CC = 21


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


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


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


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


Asynchronous Transmission


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


Simple Synchronous Transmissions


Data Link LayerDevices 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


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


A Linksys Wireless Bridge

Bridges filter network traffic


A Bridge Filtering Traffic


A standard Cisco Switch

Switches have mostly replaced hubs in modern Ethernet networks


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

business data communications

Documents

lostflow control

examplecontrolled access

common forms of flow

media access controldescribe

media access controlprovides

protocol data units

methods of delineating

form of controlled access