CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 11 pps

The OSI reference model defines the network functions that occur at each layer.. In addition, the OSI reference model describes how information, or data packets, travels from application

is responsible for a specific part of network communication These layers interact with

the layer above and below them only This interaction very narrowly defines a layer’s

purpose The two common network models that use layers are the Open System

Inter-connection (OSI) reference model and the TCP/IP reference model

The OSI Reference Model

The early development of LANs, MANs, and WANs was chaotic in many ways The

early 1980s saw tremendous increases in the number and size of networks As

compa-nies realized the money they could save and the productivity they could gain by using

networking technology, they added networks and expanded existing networks almost

as rapidly as new network technologies and products were introduced

By the mid-1980s, these companies began to experience difficulties from all the

imple-mented expansions It became more difficult for networks that used different

specifica-tions and implementaspecifica-tions to communicate with each other These companies realized

that they needed to move away from proprietary networking systems Proprietary

sys-tems are privately developed, owned, and controlled In the computer industry,

propri-etary is the opposite of open Propripropri-etary means that one company or a small group of

companies controls all usage of the technology Open means that free usage of the

technology is available to the public

To address the problem of network incompatibility and the inability to communicate

with one another, the International Organization for Standardization (ISO) researched

different network schemes, such as DECnet, Systems Network Architecture (SNA),

and TCP/IP, to find a set of rules As a result of this research, the ISO created a

net-work model that would help vendors create netnet-works that would be compatible and

operate with other networks

The process of breaking down complex communications into smaller discrete tasks

can be compared to the process of building an automobile When taken as a whole, the

design, manufacture, and assembly of an automobile is a highly complex process It

is unlikely that a single person would know how to perform all the required tasks to

build a car from scratch This is why mechanical engineers design the car,

manufactur-ing engineers design the molds to make the parts, and assembly technicians each

assemble a part of the car

TheOSI reference model, released in 1984, was the descriptive scheme that the ISO

created This reference model provided vendors with a set of standards that ensured

greater compatibility and interoperability among the various types of network

technol-ogies that were produced by many companies around the world

The OSI reference model is the primary model used as a guideline for network commu-nications Although other models exist, most network vendors today relate their prod-ucts to the OSI reference model, especially when they want to educate users on the use

of their products The OSI reference model is considered the best tool available for teaching people about sending and receiving data on a network

The OSI reference model defines the network functions that occur at each layer More importantly, it is a framework that facilitates an understanding of how information travels throughout a network In addition, the OSI reference model describes how information, or data packets, travels from application programs (such as spreadsheets and documents) through a network medium (such as wires) to another application program that is located in another computer on a network, even if the sender and receiver have different types of network media

The OSI reference model has seven numbered layers, each of which illustrates a partic-ular network function:

■ Layer 7—Application layer

■ Layer 6—Presentation layer

■ Layer 5—Session layer

■ Layer 4—Transport layer

■ Layer 3—Network layer

■ Layer 2—Data link layer

■ Layer 1—Physical layer

This separation of networking functions is called layering Dividing the network into

seven layers provides the following advantages:

■ It breaks network communication into smaller, simpler parts

■ It standardizes network components to allow multiple-vendor development and support

■ It allows different types of network hardware and software to communicate

■ It prevents changes in one layer from affecting the other layers so that they can

be developed more quickly

■ It breaks network communication into smaller components to make learning easier

By working through the layers of the OSI reference model, you will understand how data packets travel through a network and what devices operate at each layer As a result, you will understand how to troubleshoot network problems if they occur dur-ing data packet flow

OSI Layers and Functions

Each OSI layer has a set of functions that it must perform for data packets to travel

from a source to a destination on a network The following sections briefly describe

each layer in the OSI reference model

Layer 7: The Application Layer

Theapplication layer is the OSI layer that is closest to the user It provides network

services to the user’s applications It differs from the other layers in that it does not

provide services to any other OSI layer; instead, it provides services only to

applica-tions outside the OSI model Examples of such applicaapplica-tions are spreadsheet programs

and word-processing programs The application layer establishes the availability of

intended communication partners and also synchronizes and establishes agreement on

procedures for error recovery and control of data integrity Examples of the Layer 7

applications include Telnet and HTTP

Layer 6: The Presentation Layer

Thepresentation layer ensures that the information that the application layer of one

system sends out can be read by the application layer of another system If necessary,

the presentation layer translates among multiple data formats by using a common

for-mat One of the more important tasks of this layer is encryption and decryption The

common Layer 6 graphic standards are PICT, TIFF, and JPEG Examples of Layer 6

standards that guide the presentation of sound and movies are MIDI and MPEG

Layer 5: The Session Layer

As its name implies, the session layer establishes, manages, and terminates sessions

between two communicating hosts The session layer provides its services to the

pre-sentation layer It also synchronizes dialogue between the two hosts’ prepre-sentation

lay-ers and manages their data exchange In addition to handling session regulation, the

session layer offers provisions for efficient data transfer, class of service, and exception

reporting of session layer, presentation layer, and application layer problems

Exam-ples of Layer 5 protocols are the Network File System (NFS), X-Window System, and

AppleTalk Session Protocol (ASP)

Layer 4: The Transport Layer

Thetransport layer segments data from the sending host’s system and reassembles it

into a data stream on the receiving host’s system The boundary between the transport

layer and the session layer can be thought of as the boundary between application

pro-tocols and data-flow propro-tocols Whereas the application, presentation, and session

layers are concerned with application issues, the lowest four layers are concerned with data-transport issues

The transport layer attempts to provide a data-transport service that shields the upper layers from transport-implementation details Specifically, issues such as reliability of transport between two hosts are the concern of the transport layer In providing com-munication service, the transport layer establishes, maintains, and properly terminates virtual circuits Transport error detection and recovery and information flow control are used to provide reliable service Examples of Layer 4 protocols are Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Sequenced Packet Exchange (SPX)

Layer 3: The Network Layer

Thenetwork layer is a complex layer that provides connectivity and path selection between two host systems that might be located on geographically separated networks Additionally, the network layer is concerned with logical addressing Examples of Layer 3 protocols are Internet Protocol (IP), Internetwork Packet Exchange (IPX), and AppleTalk

Layer 2: The Data Link Layer

Thedata link layerprovides reliable transit of data across a physical link In so doing, the data link layer is concerned with physical (as opposed to logical) addressing, net-work topology, netnet-work access, error notification, ordered delivery of frames, and flow control

Layer 1: The Physical Layer

Thephysical layer defines the electrical, mechanical, procedural, and functional speci-fications for activating, maintaining, and deactivating the physical link between end systems Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications

Peer-to-Peer Communications

For data packets to travel from the source to the destination, each layer of the OSI model at the source must communicate with its peer layer at the destination This form

of communication is called peer-to-peer communication During this process, the

pro-tocols at each layer exchange information, called protocol data units (PDUs), between

peer layers Each layer of communication on the source computer communicates with

a layer-specific PDU and with its peer layer on the destination computer, as shown in Figure 2-17

Figure 2-17 Peer-to-Peer Communication

Data packets on a network originate at a source and then travel to a destination Each

layer depends on the service function of the OSI layer below it To provide this service,

the lower layer uses encapsulation to put the PDU from the upper layer into its data

field Each layer then adds whatever headers it needs to perform its function As the data

moves through the layers of the OSI model, additional headers are added The

group-ing of data at the Layer 4 PDU is called a segment

The network layer provides a service to the transport layer The network layer moves

the data through the internetwork by encapsulating the data and attaching a header to

create a packet (the Layer 3 PDU) The header contains information required to

com-plete the transfer, such as source and destination logical addresses

The data link layer provides a service to the network layer It encapsulates the network

layer information in a frame(the Layer 2 PDU) The frame header contains the physical

addresses required to complete the data link functions, and the frame trailer contains

the frame check sequence (FCS), which is used by the receiver to detect whether the

data is in error This then becomes the data that is passed down to the physical layer

The physical layer provides a service to the data link layer The physical layer encodes

the data link frame into a pattern of 1s and 0s (bits) for transmission on the medium

(usually a wire) at Layer 1

Network devices such as hubs, switches, and routers work at the lowest three layers

Hubs operate at Layer 1, switches operate at Layer 2, and routers at Layer 3 The first

layer that deals with the end-to-end transport between end users is the transport layer

(Layer 4)

Application Presentation Session Transport Network Data Link Physical

Segments Packets Frames Bits

Application Presentation Session Transport Network Data Link Physical Network

DoD (TCP/IP) Model

Although the OSI reference model is universally recognized, the historical and techni-cal open standard of the Internet is Transmission Control Protocol/Internet Protocol (TCP/IP) The TCP/IP reference model and the TCP/IP protocol suite make data com-munication possible between any two computers anywhere in the world at nearly the speed of light The TCP/IP model has historical importance, just like the standards that allowed the telephone, electrical power, railroad, television, and videotape industries

to flourish

The U.S DoD provided funding for the invention of the TCP/IP reference model because

it wanted a network that could survive any conditions, even a nuclear war To illus-trate further, imagine a world at war, criss-crossed by different kinds of connections, including wires, microwaves, optical fibers, and satellite links Then imagine that information/data (in the form of packets) must flow, regardless of the condition of any particular node or network on the internetwork (which, in this case, might have been destroyed by the war) The DoD wants its packets to get through every time, under any conditions, from any one point to any other point This very difficult design problem brought about the creation of the TCP/IP model, which has since become the standard

on which the Internet has grown

When reading about the TCP/IP model layers, remember the original intent of the Internet; it helps explain why certain things are as they are The TCP/IP model, as shown in Figure 2-18, has four layers:

■ The application layer

■ The transport layer

■ The Internet layer

■ The network access layer

Figure 2-18 The TCP/IP Model

Application (Layer 7) Presentation (Layer 6) Session (Layer 5) Transport (Layer 4) Network (Layer 3) Data Link (Layer 2) Physical (Layer 1)

OSI

Application

Transport Internet Network Access TCP/IP

It is important to note that some of the layers in the TCP/IP model have the same

names as layers in the OSI model However, do not confuse the layers of the two models

Even with the same name, most the layers have the same functions in each model, but

some do not

Detailed Encapsulation Process

All communications on a network originate at a source and are sent to a destination

The information that is sent on a network is called data or data packets If one

com-puter (Host A) wants to send data to another comcom-puter (Host B), the data must first be

packaged by a process called encapsulation.

Encapsulation

Encapsulation wraps data with the necessary protocol information before network

transit Therefore, as the data moves down through the layers of the OSI model, each

OSI layer adds a header (and also a trailer at Layer 2) to the data before passing it

down to a lower layer The headers and trailers contain control information for the

network devices and receiver, to ensure proper delivery of the data and to ensure that

the receiver can properly interpret the data For example, think of a header as an

address on an envelope An address is required on the envelope so that the letter inside

the envelope can be delivered to the desired recipient

To see how encapsulation occurs, examine the manner in which data travels through

the layers, as illustrated in Figure 2-19 After the data is sent from the source, it travels

through the application layer down through the other layers The packaging and flow

of the data that is exchanged go through changes as the layers perform their services

for end users

The data, in the form of electronic signals, must travel across a cable to the correct

destination computer and then be converted to its original form to be read by the

recipient As you can imagine, several steps are involved in this process For this

rea-son, developers of hardware, software, and protocols recognized that the most efficient

way to implement network communications would be as a layered process

Lab Activity OSI Model and TCP/IP Model

In this exercise, you describe and compare the layers of the OSI and TCP/IP models You also name the TCP/IP protocols and utilities that operate at each layer

NOTE

The word header

means that informa-tion was added to the front of the packet, just as trailers are added to the end In addition, an address is

an important piece of information that gets added.

Figure 2-19 Encapsulation

As illustrated in Figure 2-20, networks must perform the following five conversion steps to encapsulate data:

Step 1 Build the data—As a user sends an e-mail message, its alphanumeric

characters are converted to data that can travel across the internetwork

Step 2 Package the data for end-to-end transport—The data is packaged for

internetwork transport By using segments, the transport function ensures that the message hosts at both ends of the e-mail system can communicate reliably

Step 3 Append (add) the network address to the header—The data is put into a

packet or datagram that contains a network header with source and des-tination logical addresses These addresses help network devices send the packets across the network along a chosen path

Step 4 Append (add) the local address to the data link header—Each network

device must put the packet into a frame The frame allows connection

to the next directly-connected network device on the link Each device in the chosen network path requires framing to be connected to the next device

Step 5 Convert to bits for transmission—A clocking function lets the devices

distinguish these bits as they travel across the medium The medium on the physical internetwork can vary along the path used For example, the e-mail message can originate on a LAN, cross a campus backbone, and

go out a WAN link until it reaches its destination on another remote LAN Headers and trailers are added as data moves down through the layers of the OSI model

Figure 2-20 Data Encapsulation Process

De-Encapsulation

When the remote device receives a sequence of bits, the physical layer at the remote

device passes the bits to the data link layer for manipulation The data link layer does

the following:

Step 1 Verifies that the MAC destination address matches this station’s address

or is an Ethernet broadcast If neither of these situations is true, the frame is discarded

Step 2 If the data is in error, it can be discarded, and the data link layer might

ask for the data to be retransmitted If the data is not in error, the data link layer reads and interprets the control information in the data link header

Step 3 The data link layer strips the data link header and trailer and then passes

the remaining data up to the network layer based on the control informa-tion in the data link header

This process is called de-encapsulation Each subsequent layer performs a similar

de-encapsulation process Think of the de-de-encapsulation process as the process of reading

the address on a letter to see if it is for you and then removing the letter from the

enve-lope if the letter is addressed to you

More Information: Cyclical Redundancy Check

Each data packet has information added to the raw data itself, in the form of packet headers

The headers contain addressing information so that the packets reach the correct destination

They also contain sequencing information so the data can be reassembled accurately when all

packets reach the receiving computer.

continues

Networking Devices

Equipment that connects directly to a network segment is called a device These

devices are broken into two classifications:

■ End user devices—Include computers, printers, scanners, and other devices that

provide services directly to the user

■ Network devices—Include all devices that connect the end-user devices to allow

them to communicate

End-user devices that provide users with a connection to the network are also called

hosts Figure 2-21 shows an example of an end-user device—a workstation.

Figure 2-21 End-User Device: Workstation

Header information is placed at the head of the packet, in front of the original data Packets also can include trailer information, which is appended to the back of the packet, following the orig-inal data.

The error-checking component in the trailer is called a cyclical redundancy check (CRC) The CRC performs calculations on the packet before it leaves the source computer and again when

it reaches the destination If the results of these calculations are different, the data has changed This can occur because of a disruption of the electrical signals that represent the 0s and 1s making up the data If a discrepancy is found, that packet can be resent.

More Information: Cyclical Redundancy Check (Continued)