section 3.1 describe data packets explain the role of network interface cards

Section 3.1• Describe data packets• Explain the role of network interface cards• Compare analog and digital signals

Section 3.2• Explain the OSI reference model• Identify the layers of the OSI reference model

Section 3.3• Explain the role of protocols• Identify industry standard protocol stacks• Describe how protocols work within a reference model

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Main Ideas

Two important components needed to transfer data are data packets and network interface cards. Data can be transmitted using a digital or analog signal.

Key Terms

data packetcyclical redundancy check (CRC)busparallel transmissionserial transmissionanalogbroadband transmissiondigitalbaseband transmissionmultiplexing

Guide to Reading

How Network Traffic Gets fromHere to There

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Networks are very fast, but there is a limit to the amount of traffic networks can transport at one time. The solution to this problem is to break this traffic into small pieces known as data packetsdata packets.

A data packet, also called a packet, is a very small part of the entire piece of data that needs to be sent.

data packet Data that consist of three parts—a header, the data itself, and a trailer. (p. 70)

Data Packets


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A cyclical redundancy cyclical redundancy check (CRC)check (CRC) is performed at the sending and receiving computers to make sure the data packet arrived with no errors.

cyclical redundancy check (CRC) Error checking used in networks. A sending node performs a mathematical calculation on the packet and the result is attached to the packet’s trailer. The receiving node performs the same calculation on the packet it received. If the calculations differ, the CRC signals the source computer to retransmit the packet. (p. 71)

Data Packets


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Data Packets

The job to neatly chunk, address, and deliver the information in a network is handled by the network interface card (NIC) in each computer.

The NIC does the following:

• Provides the source, or hardware, address of the computer.• Prepares the data from the computer for the network cable.• Sends data to another computer.• Controls the flow of data between the computer and the cabling.• Receives incoming data from another computer.• Translates electrical impulses from the cable into binary code that the computer understands.


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Within the computer itself, information travels along pathways known as buses. A busbus consists of multiple “lanes” (wires) set side by side.

bus A data pathway that connects components in a computer using parallel cabling or wire. (p. 72)

Data Packets


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Data packets travel in parallel transmissionparallel transmission within the computer, but in serial transmissionserial transmission on a cable.

Data on a network move from the “multilane” computer cable to a “single-lane” network cable.

parallel transmission The orderly procession of transmitted data in computers in which groups of bits are transferred simultaneously side by side over two or more wires. (p. 72)

serial transmission The procession of transmitted data in which bits are sent over network cables and travel bit by bit one at a time. (p. 73)

Data Packets


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Transmitting Data Packets

Baseband and broadband are two techniques used to transmit data packets over cable.

Bandwidth is the maximum speed at which a particular communications medium, such as cables, can transfer information. Bandwidth measurement depends on the type of signal (analog or digital) and the media used to carry the information.

Type of Signal (analog or digital) + Type of Media = Type of Signal (analog or digital) + Type of Media = BandwidthBandwidth


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In an analoganalog signal, information travels as a continuously changing wave. This signal cycles up and down in a wavelike pattern.

analog A signal that travels as a continuously variable wavelike pattern. (p. 74)



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Broadband transmissionsBroadband transmissions can be divided into multiple channels separated by small bands of unused frequencies to avoid one channel interfering with the signal being transmitted on its neighbors.

Broadband is unidirectional—it moves in one direction only. To send and receive, the communications bandwidth is divided into two channels, one for each direction.

broadband transmission Transmissions that rely on an analog signal, a range of frequencies, and a communications medium. Broadband signals can be divided into multiple channels separated by small bands of unused frequencies to avoid interference. (p. 74)



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An example of broadband transmissions are digital signals. Digital signals encode information numerically, using 0s and 1s. These simple signals convey information in separate on/off pulses over the communications medium, such as cables.


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BasebandBaseband transmissionstransmissions send digital signals over a single channel. Multiple transmissions can be sent through the channel simultaneously through a technique known as multiplexingmultiplexing.

baseband transmission Transmissions of digital signals over a single channel, typical of most current LANs. One signal at a time travels over the network cable. (p. 75)

multiplexing Multiple transmissions sent through a channel simultaneously and are interwoven into a single signal. (p. 75)



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This is an example of multiplexing, in which several signals from different sources can be fed into one cable for transmission.


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Main Ideas

Network models describe how different network devices communicate with each other. The OSI model consists of seven layers that describe the tasks a network component must handle.

Key Terms

protocolOpen Systems Interconnection (OSI) reference modelinterfacesessiondata frameTransmission Control Protocol/Internet Protocol (TCP/IP) reference modelAdvanced Program to Program Communications (APPC)Internetworking

Guide to Reading

Network Models

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To ensure that products from various manufacturers can communicate with each other, network models have been created.

A protocolprotocol, which is part of software, is what makes a network work. The model describes what needs to be done, and the protocol performs the work.

protocol Rules that define how network devices communicate with each other and perform specific tasks. (p. 78)

Models Versus Protocols

Network Models

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The Open Systems Open Systems Interconnection (OSI) Interconnection (OSI) reference modelreference model consists of seven layers that define how data are sent from a computer, through the network, and into a receiving computer.

Understanding each layer and how each one interacts with other layers helps you understand how networks operate.

Open Systems Interconnection (OSI) reference model An international standard that is a guide for networking. Defines how data are sent from a computer, through the network, and into a receiving computer. Also called the OSI model. (p. 79)

The OSI Reference Model

Network Models

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The OSI model provides a description of how network hardware and software work together in a layered fashion to make communications between computers possible. Each layer includes different network activities, equipment, or protocols.

Network Models

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Layers are separated from each other by boundaries called interfacesinterfaces.

interface The connection that provides communication between layers. It also shields neighboring layers from the details of how services are implemented. (p. 80)


Network Models

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OSI Layers

Beginning at the top of the OSI model (Layer 7), we work down to the bottom (Layer 1). The lower layers in the OSI model support the tasks that are performed at the upper layers.

Network Models

Layer Number

Layer Name Description

7 Application Relates to services that directly support user applications

6 Presentation Defines the format used to exchange data among networked computers

5 Session Allows two applications on different computers to open, use, and close a connection between computers

4 Transport Ensures that data packets are delivered error-free, in sequence, and without data losses or duplications

3 Network Responsible for addressing messages and finding the best path to move data across the network

2 Data Link Sends data frames from the Network Layer to the Physical Layer

1 Physical Carries the signals to transmit the data over the cable to the network

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OSI Layers

Network Models

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Layer 5, the Session Layer, allows two applications on different computers to open, use, and close a connection called a session.

The Session Layer is responsible for managing this dialogue.

session A highly structured dialog between two workstations. (p. 81)

OSI Layers

Network Models

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Layer 2, the Data Link Layer, sends data frames from the Network Layer to the Physical Layer. A data framedata frame is like a vehicle, in which the data are the people, or cargo, who ride inside the vehicle across the network.

data frame An organized, logical structure in which data can be placed. (p. 82)

OSI Layers

Network Models

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OSI Layers

A data frame is used to transport data across the network.

Network Models

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Two other models, also layered, tend to pop up in descriptions of network architectures.

• Systems Network Architecture (SNA)

• Transmission Control Transmission Control Protocol/Internet Protocol Protocol/Internet Protocol (TCP/IP) reference model(TCP/IP) reference model

Transmission Control Protocol/Internet Protocol (TCP/IP) reference model An Internet-related network architectural model. (p. 84)

Other Networking Models

Network Models

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SNA was originally designed around the mainframe/terminal relationship, but was later modified, in a specification known as Advanced Program to Advanced Program to Program Program Communications Communications (APPC)(APPC), to include minicomputers and personal computers.

Advanced Program to Program Communications (APPC) The SNA mainframe communication model was modified in a specification known as APPC to include minicomputers and personal computers. (p. 84)


Network Models

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The birth and evolution of the Internet gave rise to yet another model. This is the TCP/IP reference model, also referred to as the Internet reference model.

The TCP/IP reference model reflects the world of internetworkinginternetworking.

internetworking The transfer and routing of information between and among varied workstations and networks. (p. 84)


Network Models

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This image below shows the difference between the TCP/IP and OSI models, as well as rough—though not exact—equivalents between the layers in the two models.

Network Models

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You Try It

• Activity 3A – Learning the OSI Reference Model (p. 83)

Network Models

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Main Ideas

Protocols are rules and procedures used to make network communications possible. Multiple protocols can be used by a NIC. Many protocols that operate in conjunction perform the various tasks needed to send and receive data.

Key Terms

protocol stack

binding process

binding order

Media Access Control (MAC)

Guide to Reading

Protocols

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The Function of Protocols

When data are transmitted over a network, they are sent in steps. A single step includes certain actions that cannot take place at any other step. Each step has its own rules and procedures, or protocols.

There are four points to keep in mind about protocols:

• There are many protocols.

• Some protocols work only at particular OSI layers.

• Some protocols work at multiple layers.

• Protocols are consistent.

Protocols

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A protocol stackprotocol stack is a combination of protocols. Layers within the stack, which generally correspond to the OSI model, specify various protocols for handling a function or subsystem of the communication process. Each layer in the stack has its own set of rules.

protocol stack Multiple layers of protocols that work together. (p. 88)

Protocols in a Layered Architecture

Protocols

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The binding processbinding process is used to connect protocols, or protocol stacks, to the NIC. Protocols and NICs can be mixed and matched on an as-needed basis.

The binding order determines the sequence in which the operating system runs the protocol.

binding process The process of tying the protocols together to provide data with a route from the application level to the NIC. (p. 88)

binding order The sequence in which the operating system runs the protocol. (p. 88)


Protocols

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The computer industry has designated several kinds of protocol stacks as standard models. Hardware and software manufacturers can develop their products to meet any one or a combination of these protocol stacks. The most important protocol stacks include:

• OSI protocol suite

• Digital DECnet

• Novell NetWare

• AppleTalk

• TCP/IP

• IBM SNA

Protocols

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Protocols exist at each layer of these stacks, performing the tasks specified by that layer. However, the communication tasks that networks need to perform are grouped into one of the three following types:

• application protocols

• transport protocols

• network protocols

Protocols

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The Data Link Layer is divided into two sublayers:

• Logical Link Control (LLC)

• Media Access Control Media Access Control (MAC)(MAC)

Media Access Control (MAC) The lower of two sublayers that make up the Data Link Layer. The MAC manages access to the physical network, delimits frames, and handles error control. (p. 92)


Protocols

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You Try It

• Activity 3B – Discovering What Protocols You Use (p. 89)

Protocols

Resources

For more resources on this chapter, go to the Introduction to Networks and Networking Web site at http://networking.glencoe.com.

Chapter 3

section 3.1 describe data packets explain the role of network interface cards

Documents

data packet data

data pathway

flow of data

incoming data

data packetsthe job

entire piece of data

readinghow network traffic

network cables