Chapter 2Networking Standards

and the OSI Model

Collected and CompiledBy JD WillardMCSE, MCSA, Network+, Microsoft IT Academy AdministratorComputer Information Systems InstructorAlbany Technical College

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Objectives

• Identify organizations that set standards for networking• Describe the purpose of the OSI Model and each of its

layers• Explain specific functions belonging to each OSI Model

layer• Understand how two network nodes communicate through

the OSI model• Discuss the structure and purpose of data packets and

frames• Describe the two types of addressing covered by the OSI

Model

Networking Standards Organizations

• Standards: documented agreements containing technical specifications or other precise criteria stipulating how particular products or services should be designed or performed. Standards assist in network design.– Define minimum acceptable performance

• Many different organizations have evolved to oversee computer industry’s standards

ANSI• The American National Standards Institute or ANSI accredits

standards that are developed by representatives of standards developing organizations, government agencies, consumer groups, companies, and others. These standards ensure that the characteristics and performance of products are consistent, that people use the same definitions and terms, and that products are tested the same way.

• In addition, ANSI promotes the use of U.S. standards internationally, advocates U.S. policy and technical positions in international and regional standards organizations and encourages the adoption of international standards as national standards where appropriate.

• The Institute is the official U.S. representative the International Organization for Standardization. (ISO).

EIA and TIA

• Electronic Industries Alliance (EIA): Trade organization composed of representatives from electronics manufacturing firms across US– Sets standards for its members– Helps write ANSI standards – Lobbies for legislation favorable to growth of computer

and electronics industries

• Telecommunications Industry Association (TIA): Focuses on standards for information technology (IT), wireless, satellite, fiber optics, and telephone equipment

IEEE

• Institute of Electrical and Electronics Engineers• International society composed of engineering

professionals • Goals are to promote development and education

in electrical engineering and computer science• IEEE technical papers and standards are highly

respected in the networking profession– Can purchase IEEE documents online from IEEE’s

Web site (www.ieee.org)

ISO

• International Organization for Standardization• Collection of organization standards representing

146 countries• Goal is to establish international technological

standards to facilitate global exchange of information and barrier-free trade

• Fewer than 300 of ISO’s more than 14,250 standards apply to computer-related products and functions

ITU• International Telecommunication Union• Regulates international telecommunications:

– Radio and TV frequencies– Satellite and telephony specifications– Networking infrastructure– Tariffs applied to global communications

• Typically, documents pertain more to global telecommunications issues than to industry technical specifications

• The ITU is a specialized United Nations agency that provides developing countries with technical expertise and equipment to advance those nations’ technological bases.

ISOC• Internet Society• Professional membership society that helps to

establish technical standards for the Internet• Oversees groups with specific missions:

– Internet Architecture Board (IAB): Technical advisory group of researchers and professionals

• Interested in overseeing Internet’s design and management

– Internet Engineering Task Force (IETF): Sets standards for how systems communicate over the Internet

• How protocols operate and interact

IANA and ICANN• Internet Protocol (IP) addresses: Addresses used to identify

computers on the Internet and other TCP/IP-based networks• Internet Assigned Numbers Authority (IANA): Used to keep

records of available and reserved IP addresses and determines how addresses were doled out

• Internet Corporation for Assigned Names and Numbers (ICANN): Ultimately responsible for IP addressing and domain name management– IANA still performs system administration

• Individuals and businesses lease addresses from Internet Service Provider (ISP)– Business providing access to Internet and other services

The OSI Model• Open Systems Interconnection

(OSI) Model: divides network communications into seven layers:

– Physical, Data Link, Network, Transport, Session, Presentation, and Application

• Protocols perform services unique to layer

• Protocols interact with protocols in layers directly above and below

• Protocol: set of instructions to perform a function or group of functions

– Written by a programmer

Development and Reason for Model Demo

The OSI Model Demo

What is the OSI Model? Demo

OSI Model Layer Mnemonics

Application All Away

Presentation People Pizza

Session Seem Sausage

Transport To Throw

Network Need Not

Data-Link Data Do

Physical Processing Please

OSI Layer Down Up

The OSI Model• Theoretical representation of

what happens between two nodes communicating on a network– Does not prescribe type of

hardware or software that should support each layer

– Does not describe how software programs interact with other software programs or how software programs interact with humans

• Each layer communicates with same layer from one computer to another (peer protocols)

• Model is imperfect

Overview of Layered Architecture Demo

The OSI Model

•The Application, Presentation and Session layers are known as the Upper Layers and are implemented in software•The Transport and Network layer are mainly concerned with protocols for delivery and routing of packets and are implemented in software as well•The Data Link is implemented in hard- and software •The Physical layer is implemented in hardware only, hence its name. These lower two layers define LAN and WAN specifications.

The layers of the model Demo

Upper Layers Demo

Lower Layers Demo

The OSI Model

Flow of data through the OSI Model

A more detailed description of each layer follows later, but here's what basically happens when data passes from Host A to Host B:

• The Application, Presentation and Session layers take user input and converts it into data

• The Transport layer adds a segment header converting the data into segments

• The Network layer adds a network header and converts the segments into packets / datagrams

• The Data Link layer adds a frame header converting the packets/datagrams into frames, the MAC sublayer converts the frames into bits, which the Physical layer can put on the wire.

• These steps are known as data encapsulation.

• When the bits stream arrives at the destination, the process is reversed and each layer will remove their corresponding header while the data flows up the OSI model until it is converted back to data and presented to the user. This is also known as decapsulation.

Packet Assembly and Disassembly Process

Each successive layer adds formatting and peer control information to the data in the form of a header. At the receiving end the headers are stripped off by the corresponding layers to determine how to handle the data. At the Data Link layer an error checking mechanism known as the Frame check sequence is added as a trailer.

Packet Assembly and Disassembly Process

Source Computer Destination Computer

Application Layer (7)• Topmost layer

• Represents services that directly support user applications

• Window to network services

• Services facilitate communication between software and lower-layer network services– Helps software applications negotiate formatting, procedural,

security, synchronization, and other requirements with network• Hypertext Transfer Protocol (HTTP): formats and sends requests from

client’s browser to server– Also formats and sends Web server’s response back to client’s

browser• Application program interface (API): set of routines that make up part

of a software application• Handles network access, flow control, and error recovery

• SNMP, FTP, Telnet, DHCP, DNS, WINS, POP, IMAP and SMTP protocols function at this level

• The Application layer separates data into PDUs (protocol data units), or discrete amounts of data.

The Application Layer Demo

Presentation Layer (6)• Network translator

• Protocols accept Application layer data and format it – So that one type of application and host can understand data from

another type of application and host• e.g., translation and conversion between graphics file types

• Examples of specifications defined at this layer are GIF, JPEG, MPEG, MIME, and ASCII.

• On sending end, determines formatting used to exchange data among computers and adds formatting so data can be understood by network

• On receiving end, translates data from application format to a common intermediate format

• Manages data compression, translation, encryption

• I/O redirectors work to redirect resources to a server

• The Server and Workstation Services work at this layer

The Presentation Layer Demo

Session Layer (5)• Allows two applications on different computers to open, use, and close connections • Protocols coordinate and maintain communications between two network nodes

– Establish and maintain communications link for duration of session – Synchronize dialogue between two nodes– Terminate communications

• Performs name recognition and provides security• Monitors identification of session participants

– Ensures that only authorized nodes have access• Provides synchronization by placing checkpoints in the data stream

– Determine if communications have been cut off• Determine where to restart transmission

• Implements dialog control between communication processes• Examples of protocols/API's that operate on this layer are RPC and NETBIOS.• Sets terms of communication

– Decides which node will communicate first– Decides how long a node can communicate

The Session Layer Demo

Transport Layer (4)• Protocols accept data from Session layer and manage end-to-end delivery of data

– Ensure data transferred reliably, in correct sequence, and without errors

• Protocols also handle flow control

– Gauging appropriate rate of transmission based on how fast recipient can accept data

• Transmission Control Protocol (TCP): Takes care of reliably transmitting HTTP requests from client to server and vice versa

• Connection-oriented protocols: ensure that data arrives exactly as it was sent

– Establish connection before transmitting data

– TCP is connection-oriented

• Client’s TCP protocol first sends synchronization (SYN) packet request to server

• Server responds with synchronization-acknowledgment (SYN-ACK) packet

• Client responds with own acknowledgment (ACK)

Transport Layer (4)• Acknowledgments also used to ensure that data was

properly delivered– For every data unit sent, connection-oriented protocol

expects acknowledgment from recipient• If no acknowledgment, data retransmitted

• Connection-oriented protocols (TCP & SPX) use a checksum– Unique character string allowing receiving node to

determine if arriving data unit exactly matches data unit sent by source

• Connectionless protocols ( UDP) do not establish connection before transmitting – No effort to ensure data delivered error-free

The Transport Layer Demo

Transport Layer (4)

• Transport layer protocols break large data units received from Session layer into smaller segments (segmentation)

• Maximum transmission unit (MTU): largest data unit a given network will carry

• Reassembly: process of reconstructing segmented data units

• Sequencing: method of identifying segments that belong to same group of subdivided data– Indicates where unit of data begins– Indicates order in which groups of data were issued– Transport layer protocols of two nodes must synchronize timing

and agree on starting point for the transmission

Transport Layer

Segmentation and Reassembly

Network Layer (3)• Network layer protocols accept Transport layer segments and add logical

addressing information in network header to form packets• Primary functions of protocols:

– Translate network addresses into physical counterparts (ARP)– Decide how to route data from sender to receiver (RIP OSPF IGMP BGP)

• Addresses the package using network address scheme (IP IPX)

• Each node has two types of addresses:– Network address: follows hierarchical addressing scheme

• Can be assigned through OS software• Network layer addresses, logical addresses, or virtual addresses

– Physical address• Network layer handles routing• Determines the best route on the network based on network conditions, priority

of service

• Performs packet switching, routing, traffic management, and controls congestion of data

• IP: Network layer protocol that underlies most Internet traffic• Fragmentation: Network layer protocol subdivides segments it receives from

Transport layer into smaller packets• Troubleshooting network connectivity (ICMP) The Network Layer Demo

Data-Link Layer (2)• Takes packets received from the Network layer and encapsulates them into frames

• Protocols divide received data into distinct frames – Can then be transmitted by Physical layer

• Appends Physical address and Frame check sequence to the frame

• Parts of data frame: Destination ID, Sender ID, Control Data

• Acknowledges data frames, error checking, and verification

• Frame: structured package for moving data – Raw data

• “payload” – Sender’s and receiver’s network addresses– Error checking and control information

• Error checking accomplished by 4-byte Frame Check Sequence (FCS) field– Ensures data at destination exactly matches data issued from source– When source node transmits data, performs Cyclic Redundancy Check (CRC) to get

FCS– Destination node’s Data Link layer services unscramble FCS via same CRC

algorithm• Data Link layer divided into two sub-layers:

– Logical Link Control – Media Access Control

• NDIS works at this level

The Data Link Layer Demo

Ethernet Data Frame

IEEE 802 Data Link sub layers

LLC sublayer - Short for Logical Link Control •Provides a common interface to the Network Layer, reliability and flow control •Defines SAPs (Service Access Points) MAC sublayer - Short for Media Access Control•Appends the physical address (MAC address) to the frame and performs error checking•Communicates directly with the media through the network interface card•It converts the frames into bits and puts them on the wire •Defines IEEE LAN standards such as 802.3, 802.4, 802.5, and 802.11

Data Link Layer

A NIC’s MAC address contains two parts: a block ID and a device ID. The block ID is a six-character sequence unique to each vendor. The remaining six characters known as the device ID are added at the factory, based on the NIC’s model and manufacture date.

Physical Layer (1)• Bottommost Layer Referred to as the “hardware layer”

• Hardware-oriented, establishes and maintains physical link between communication computers

• Defines how the cable is attached to the NIC

• Packet sent as an unstructured raw bit stream over physical medium

• Protocols accept frames from Data Link layer and generate voltage to transmit signals

• When receiving data, protocols detect voltage and accept signals

• Protocols also set data transmission rate and monitor data error rates – Cannot perform error correction

• Network administrators mostly concerned with bottom four layers of OSI Model

The Physical Layer Demo

Applying the OSI Model

Communication Between Two Systems

Data transformation through the OSI Model

TCP/IP Compared to the OSI Model

The TCP/IP suite of protocols can be divided into four layers that roughly correspondto the seven layers of the OSI Model.

■ Application layer —Roughly equivalent to the Application, Presentation, andSession layers of the OSI Model. Applications gain access to the networkthrough this layer, via protocols such as the File Transfer Protocol (FTP),Trivial File Transfer Protocol (TFTP), Hypertext Transfer Protocol (HTTP),Simple Mail Transfer Protocol (SMTP), and Dynamic Host ConfigurationProtocol (DHCP).■ Transport layer —Roughly corresponds to the Transport layer of the OSIModel. This layer holds the Transmission Control Protocol (TCP) and UserDatagram Protocol (UDP), which provide flow control, error checking, andsequencing. All service requests use one of these protocols.■ Internet layer —Equivalent to the Network layer of the OSI Model. This layerholds the Internet Protocol (IP), Internet Control Message Protocol (ICMP),Internet Group Message Protocol (IGMP), and Address Resolution Protocol(ARP). These protocols handle message routing and host address resolution.■ Network Interface layer —Roughly equivalent to the Data Link and Physicallayers of the OSI Model. This layer handles the formatting of data and transmissionto the network wire.

TCP/IP and OSI Models DemoModels Demo

Layers at Which Network Components Operate

Device Layer

Hubs Physical

Switches Data Link

Bridges Data Link

Routers Network

Network Interface Card

Data Link and Physical

Wireless Access Point

Data Link

Frame Specifications

• The two major categories of frame types:– Ethernet

• Four types of Ethernet frames

• Most popular form characterized by unique way in which devices share a common transmission channel (described in IEEE 802.3 standard)

– Token Ring: relies on direct links between nodes and a ring topology

• Nodes pass around tokens (control frames that indicate to network when a node is about to transmit data)

IEEE Networking Specifications

• Apply to connectivity, networking media, error checking algorithms, encryption, emerging technologies, and more

• Specifications fall under IEEE’s “Project 802”– Effort to standardize physical and logical

elements of a network

IEEE Networking Specifications

Summary• Standards are documented agreements containing

precise criteria that are used as guidelines to ensure that materials, products, processes, and services suit their purpose

• ISO’s OSI Model divides networking architecture into seven layers

• Each OSI layer has its own set of functions and interacts with the layers directly above and below it

• Application layer protocols enable software to negotiate their formatting, procedural, security, and synchronization with the network

Summary (continued)

• Presentation layer protocols serve as translators between the application and the network

• Session layer protocols coordinate and maintain links between two devices for the duration of their communication

• Transport layer protocols oversee end-to-end data delivery

• Network layer protocols manage logical addressing and determine routes based on addressing, patterns of usage, and availability

Summary (continued)

• Data Link layer protocols organize data they receive from the Network layer into frames that contain error checking routines and can then be transmitted by the Physical layer

• Physical layer protocols generate and detect voltage to transmit and receive signals carrying data over a network medium

• Data frames are small blocks of data with control, addressing, and handling information attached to them

The End