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Chapter Two
Networking Standards
and the OSI Model
Standards
Documented agreements containing
technical specifications or other precise
criteria that stipulate how a particular
product or service should be designed or
performed
Many different industries use standards to
ensure that products, processes, and
services suit their purpose
Networking Standards
Organizations
American National Standards Institute
(ANSI)


Comprised of industry and government
representatives
Determine standards for electronics industry in
addition to other fields
Electronic Industries Alliance (EIA)


Trade organization composed of
representatives from electronics manufacturing
firms across United States
Sets standards for members, helps write ANSI
standards, and lobbies for legislation favorable
to the industry
Networking Standards
Organizations
Institute of Electrical and Electronic Engineers
(IEEE)


International society of engineering professionals
Promotes development and education in electrical
engineering and computer science fields
International Organization for Standardization
(ISO – Greek for equal)



Collection of standards organizations (130
countries)
Goal is to establish international technological
standards to facilitate global exchange of
information and barrier-free trade
ISO-9000/QS-9000
Networking Standards
Organizations
International Telecommunication Union
(ITU)




Formerly called Consultative Committee on
International Telegraph and Telephony
(CCITT)
United Nations agency that regulates
international telecommunications
Founded in Paris in 1865
Assists developing countries
The OSI Model
Open Systems Interconnection (OSI) Model



Model for understanding and developing
computer-to-computer communication
Developed in the 1980s by ISO
Divides network architecture into seven layers
The OSI Model
Figure 2-1: The OSI Model
Phil Donahue Never Televises Sick People Anymore!
Physical Layer
Data Link Layer
Physical layer


First layer of OSI Model
Contains physical networking media
NIC, cabling, connectors, hubs, and repeaters
Sets data transmission rate and monitors data error rates
Data Link layer


Second layer of OSI Model
Primary function is to divide data it receives from
Network layer into distinct frames that can be
transmitted by Physical layer
Frames
Structured package for moving data
Includes raw data (or payload) along with sender’s
and receiver’s:


Network addresses
Error-checking and control information
Figure 2-2:
A simplified
data frame
Network Layer
Third layer of OSI Model
Translates network addresses into their
physical counterparts
Decides how to route data from sender to
receiver
Network Layer
Because Network layer handles routing,
routers belong in Network layer

To route means to direct data based on
addressing, usage patterns, and availability
Transport Layer
Fourth layer of OSI Model
Ensures that data are transferred between
points reliably and without errors
Handles flow control

Method of gauging appropriate rate of data
transmission
Transport layer protocols also accomplish:


Segmentation – decreasing the size of the data
units (and sequencing)
Reassembly – reconstructing the segmented data
units
Protocols TCP & IPX operate here
Transport Layer
Acknowledgement (ACK)


Response generated in Transport layer
Confirms to sender that its frame was received
correctly
Session Layer
Fifth layer of OSI Model
Establishes and maintains communication
between two nodes on the network
Session


Refers to a connection for data exchange
between two parties
Term session is most often used in context of
terminal and mainframe communications
Presentation Layer
Application Layer
Presentation layer



Sixth layer of OSI Model
Translates between application and network
Manages data encryption/decryption
Application layer



Seventh, or top, layer of OSI Model
Provides interface to software enabling programs to
use network devices, or API
API (Application Programming Interface) - Routine
that allows a program to interact with the
operating system
Applying the OSI Model
Table 2-1: Functions of the OSI layer
Communication Between Two
Systems
Figure 2-3: Data transfer between two systems
Formulates a request from remote node
Determines format or encryption
Assigns data token (approval)
Break down into manageable chunks
Adds addressing information
Data blocks packaged into individual frames
Deliver the data to the cabling
Communication Between Two
Systems
Figure 2-4: Data transformed through the OSI Model
Communication Between Two
Systems - Terms
Token

Special control frame indicating to rest of
network that a particular node has right to
transmit data
Frame Check Sequence (FCS)

Field in a frame responsible for ensuring that
data carried by frame arrives intact
Frame Specifications
Ethernet


Networking technology originally developed in
1970s by Xerox
Four types of Ethernet technology are used
on LANs today
Each type is governed by a set of IEEE standards
Frame Specifications
Token Ring


Networking technology developed by IBM in
the 1980s
Relies upon direct links between nodes and a
ring topology, using tokens to allow nodes to
transmit data
A Typical Ethernet Frame
802.3 standard (p. 49)

IEEE standard for Ethernet networking devices and
data handling
Figure 2-5: Ethernet frame as specified by the IEEE 802.3 standard
Components of the Ethernet
802.3 Frame
Preamble

Marks beginning of entire frame
Start of Frame Delimiter (SFD)

Indicates beginning of addressing frame
Destination Address

Contains destination node address
Components of the Ethernet
802.3 Frame
Source Address

Contains address of originating node
Length (LEN)

Indicates length of packet
Data

Contains data, or segmented part of data,
transmitted from originating node
Components of the Ethernet
802.3 Frame
Pad

Used to increase size of the frame to its
minimum size requirement of 46 bytes
Frame Check Sequence (FCS)

Provides an algorithm to determine whether
data were correctly received
Most commonly used algorithm is Cyclic
Redundancy Check (CRC)
Typical Token Ring Frame
802.5 Standard

IEEE standard for Token Ring networking devices and
data handling
Figure 2-6: Typical Token Ring frame
Components of Token Ring
Frame
Start Delimiter (SD)

Signifies beginning of packet
Access Control (AC)

Contains information about priority of the
frame
Frame Control (FC)

Defines type of frame
Components of Token Ring
Frame
Destination Address

Contains destination node address
Source Address

Contains address of originating node
Data

Contains data transmitted from originating
node
Components of Token Ring
Frame
Frame Check Sequence (FCS)

Used to check integrity of the frame
End Delimiter (ED)

Indicates end of the frame
Frame Status (FS)

Indicates whether destination node
recognized and correctly copied the frame
Addressing Through the Layers
Data Link layer address

Also called MAC address, after Media
Access Control (MAC) sublayer



Number uniquely defining a network node
Composed of Block ID and Device ID
Manufacturer-hard codes the address on the
NIC
3COM Ethernet – 00608C
Intel Ethernet – 00AA00
Addressing Through the Layers
Network layer address





Resides at Network level of OSI Model
Follows hierarchical addressing scheme
Can be assigned through operating system
software
More useful to internetworking devices, such
as routers, since data is sorted more logically
IP address (similar to long distance phone)
127.0.0.1

Ex. WINIPCFG or IPCONFIG /all
IEEE Networking Specifications
Table 2-2:
IEEE 802
standards
IEEE Networking Specifications
To accommodate shared access for
multiple network nodes, IEEE expanded
OSI Model by separating Data Link layer
into two sublayers:


Logical Link Control (LLC) sublayer
Media Access Control (MAC) sublayer
Data Link Layer Sublayers
LLC



Upper sublayer
Provides common interface
Supplies reliability and flow control services
MAC


Lower sublayer
Appends physical address of destination
computer onto the frame
Subdivided Data Link Layer
Figure 2-7:
LLC and
MAC
sublayers