11 CHAPTER ONE Introducing Networks

Objectives Identify and describe the functions of each of the seven layers of the OSI reference model Identify the reasons why the networking industry uses a layered model Define and explain the conversion steps of data encapsulation Define and describe the function of a MAC address Describe 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 history Today, networks include a wide variety of computers and peripheral components Systems that are part of a network do not have to be identical

Why Do We Use Networks? Convenience People expect interoperability from electronic devices Computer 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 servers You 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 network In a peer-to-peer network, all computers can share resources with other computers Peer-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 cable A wide area network (WAN) is a network that spans two or more geographically diverse locations A metropolitan area network (MAN) is an intermediate specification that defines networks confined to a fairly restricted geographic area A 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 network Networking 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 model Enables programmers to specialize in a particular level or layer of the networking model Provides design modularity Encourages interoperability Allows for standardized interfaces to be produced by networking vendors

Seven Layers of the OSI Model Physical layer Data Link layer Network layer Transport layer Sessions layer Presentation layer Application 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 communicat ion Figure 1-2: Peer communication

Layer Functions: Physical (Layer 1) Layer 1 in the OSI model Has 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 layer When 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 card The unique identifier is a 48-bit address represented as 12-digit hexadecimal number given to each network card during production Every 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 transportation Responsibilities 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 dialog Examples 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 network Components 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 packet The packet contains data from the sending application and additional information added by the protocol stack Prior 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 network The 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 process This process takes place as the data packet travels down the OSI stack