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CHAPTERONE

Introducing Networks

Objectives

Identify and describe the functions of each of the seven layers of the OSI reference modelIdentify the reasons why the networking industry uses a layered modelDefine and explain the conversion steps of data encapsulationDefine and describe the function of a MAC addressDescribe connection-oriented network service and connectionless network service, and identify the key differences between them

Introduction to Networking

Network Term that describes the connection of

two or more computers by some type of medium, including: Wire cable Fiber-optic cable Infrared Radio equipment

Origin of Networking

Difficult to place actual origin as many devices have been networked throughout historyToday, networks include a wide variety of computers and peripheral componentsSystems that are part of a network do not have to be identical

Why Do We Use Networks?

ConveniencePeople expect interoperability from electronic devicesComputer networks allow for the transfer of files, data, and even shared applications without copying anything to floppy disk

Connection Media

Media Cable, glass, or telephone lines that host

the signal from one computer to another on the network

Electromagnetic interference (EMI) Electric noise that disrupts signals on

cables

Wireless Communications that are not conducted

over physical wires or cables

Client/Server Networks

Networks that have computers that are servers and computers that act as clients to those serversYou may see a variety of servers on a network: Print server File server Database server Remote access server (RAS) Web server

Peer-to-Peer

When every computer on a network acts as both a client and a server, the network is a peer-to-peer networkIn a peer-to-peer network, all computers can share resources with other computersPeer-to-peer networks are also known as “workgroups”

LAN, WAN, MAN, SAN

A local area network (LAN) is a group of computers and other devices typically connected by a cableA wide area network (WAN) is a network that spans two or more geographically diverse locationsA metropolitan area network (MAN) is an intermediate specification that defines networks confined to a fairly restricted geographic areaA storage area network (SAN) is a subsystem of networked storage devices physically separated from servers

Network Operating System

Network operating system (NOS) Allows communication over a network

End system The location and/or set of controls

that the user can manipulate to interact with a computer or a network

NIC, Networking Hardware, and Networking Software

Network interface card (NIC) Hardware device that transmits and

receives electronic signals on a network

Networking hardware is a generic term that describes all the physical components of a networkNetworking software is a generic term that describes the software programs used to run a network

Understanding the OSI Model

International Organization for Standardization (ISO) Multi-industry association that attempts to

standardize and define items that increase communication and compatibility in many different countries

Open Systems Interconnection (OSI) Seven layer reference model created by the

ISO Defines and separates networking hardware

and software into distinct layers and functions

Reasons for Layering

Simplifies the network modelEnables programmers to specialize in a particular level or layer of the networking modelProvides design modularityEncourages interoperabilityAllows for standardized interfaces to be produced by networking vendors

Seven Layers of the OSI Model

Physical layerData Link layerNetwork layerTransport layerSessions layerPresentation layerApplication layer

Reasons for Layering

As a group, these layers form the OSI protocol stack

Figure 1-1:OSI Reference Model

Peer OSI Communication

The seven layers of the OSI reference model communicate with each other via peer communication

Figure 1-2:Peer communication

Layer Functions:Physical (Layer 1)

Layer 1 in the OSI modelHas the following responsibilities: Definition of the physical

characteristics of the network hardware

Representation of binary digits as voltages (encoding)

Transmission of signals on the wire

Layer Functions:Physical (Layer 1)

Figure 1-3:568B twisted pair wiring scheme

Layer Functions:Physical (Layer 1)

Connectors, cables, and devices like repeaters and hubs can be associated with the physical layerWhen choosing cable, consider the following: Expense Physical location Distance Security requirements Transmission speed required

Layer Functions:Data Link (Layer 2)

Has several responsibilities: NIC software functions, including

identification of source and destination nodes via physical addresses

Definition of how data is packaged for transport as frames

Error detection Flow control of information sent across

the link

Layer Functions:Data Link (Layer 2)

Data Link layer has two sublayers that further articulate its function: Logical Link Control (LLC) layer Media Access Control (MAC) layer

The Institute of Electrical and Electronics Engineers (IEEE) created these sublayers to identify and isolate the separate responsibilities required at this level of the protocol stack

Layer Functions:Data Link (Layer 2)

Figure 1-4:Data Link layer subdivision

Layer Functions:Data Link (Layer 2)

MAC layer defines the media access method and provides a unique identifier for the network cardThe unique identifier is a 48-bit address represented as 12-digit hexadecimal number given to each network card during productionEvery network interface card must have a unique physical address (also called the MAC address)

Layer Functions:Data Link (Layer 2)

Figure 1-5:MAC address

Layer Functions:Data Link (Layer 2)

Ethernet A standard networking architecture that

defines the physical layout, lengths, and types of media that can be used

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Network access method used by

Ethernet networks

Layer Functions:Network (Layer 3)

Has the following function: Error handling Software addressing for data packets Best path selection

The Network layer contains the logical address for the computer

Layer Functions:Transport (Layer 4)

Provides point-to-point data transportationResponsibilities include: End-to-end error-free transmission and

delivery Data segmentation into maximum

transmission unit (MTU) size Messaging service for the Session layer

Layer Functions:Transport (Layer 4)

Protocols that reside at the Transport layer can be: Connection-oriented Connectionless

A packet sent by a connectionless protocol is also called a datagram

Layer Functions:Session (Layer 5)

Enables two applications on the network to have an ongoing conversation or dialogExamples of Session layer protocols include: SQL RPC X-Windows

Layer Functions:Session (Layer 5)

Provides the following services: Control for data exchange Data synchronization definition Failure recovery Communication setup and teardown

Layer Functions:Presentation (Layer 6)

Prepares the data from Application layer for transmission over the networkComponents include extensions and coding schemes such as: BMP WAV EBCDIC ASCII

Layer Functions:Presentation (Layer 6)

Has these responsibilities: Data translation Data formatting Data syntax restructuring Data encryption Data compression

Layer Functions:Application (Layer 7)

Has the following responsibilities: Initiating the request for network

service Providing services to applications

Data Encapsulation

Data is sent from one computer to another in a data packetThe packet contains data from the sending application and additional information added by the protocol stackPrior to transmission across the network, the data is organized into a data frame at layer 2

Data Encapsulation

Protocol data unit (PDU) Information added to a data packet by

the layers of the protocol stack

Encapsulation Process that occurs during

transmission through the protocol stack in which data from the higher layers is wrapped in a protocol header and/or trailer

Data Encapsulation

Figure 1-6: Encapsulation

Data Encapsulation

Table 1-1:Five steps of data encapsulation

Chapter Summary

Two or more computers connected by media form a networkThe ISO developed the OSI model in the mid-1980s to standardize networking models Data transmission can be connection-oriented or connectionless

Chapter Summary

The OSI model has seven layers: Physical layer Data Link layer Network layer Transport layer Session layer Presentation layer Application layer

Chapter Summary

When the network user sends data to the network, it goes through a five-step data encapsulation processThis process takes place as the data packet travels down the OSI stack