Foundation Summary

The "Foundation Summary" section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your INTRO exam, a well-prepared CCNA candidate should know, at a minimum, all the details in each "Foundation Summary" section before going to take the exam.

Table 2-6 summarizes the key points about how adjacent layers work together on a single computer and how one layer on one computer works with the same networking layer on another computer. These concepts are some of the most important concepts in this chapter.

Table 2-6 Summary: Same-Layer and Adjacent-Layer Interactions

Concept

Description

Same-layer interaction on different computers

Each layer of a networking model works with the same layer on another computer with which it wants to communicate. The protocol defined by each layer uses a header that is transmitted between the computers to communicate what each computer wants to do.

Adjacent-layer interaction on the same computer

A higher layer might need a particular service that is not included in that layer. To perform the missing function, the protocol at the higher layer requests that the next lower layer perform the needed function.

Data encapsulation is another key concept discussed throughout this chapter. You can think about the complete process generically or with the example five-step TCP/IP encapsulation process shown in the following list and in Figure 2-11:

Step 1 Create the application data and headers—This simply means that the application has data to send.

Step 2 Package the data for transport—In other words, the transport layer (TCP or UDP) creates the transport header and places the data behind it.

Step 3 Add the destination and source network layer addresses to the data—

The network layer creates the network header, which includes the network layer addresses, and places the data behind it.

Step 4 Add the destination and source data link layer addresses to the data—

The data link layer creates the data link header, places the data behind it, and places the data link trailer at the end.

Step 5 Transmit the bits—The physical layer encodes a signal onto the medium to transmit the frame.

Figure 2-11 Five Steps of Data Encapsulation—TCP/IP

Data

TCP Data

TCP Data

3.

IP

TCP

Data

4.

LH

IP

TCP

Data

Transmit bits

Application Transport

Internet

Network interface

* The letters LH and LT stand for link header and link trailer, respectively, and refer to the data link layer header and trailer.

You should know the names of all the OSI and TCP/IP layers, as shown in Figure 2-12.

Figure 2-12 Comparing OSI, TCP/IP, and NetWare

OSI TCP/IP

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Transport

Internetwork

Network Interface

NetWare

SAP, NCP

Mac Protocols

You should memorize the names of the layers of the OSI model. Table 2-7 lists a summary of OSI functions at each layer, along with some sample protocols at each layer.

Table 2-7 OSI Functional Summary

OSI Layer Name

Functional Description

Examples

Application (Layer 7)

Interface between network and application software.

Telnet, HTTP

Presentation (Layer 6)

How data is presented.

Special processing, such as encryption.

JPEG, ASCII, EBCDIC

Session (Layer 5)

Establishing and maintaining end-to-end bidirectional flows between endpoints. Includes managing transaction flows.

Operating systems and application access scheduling RPC

Transport (Layer 4)

Reliable or unreliable delivery. Multiplexing.

TCP, UDP, SPX

Network (Layer 3)

Logical addressing, which routers use for path determination.

IP, IPX

Data link (Layer 2)

Combination of bits into bytes, and bytes into frames.

Access to the media using MAC address. Error detection and error recovery.

802.3/802.2, HDLC

Physical (Layer 1)

Moving of bits between devices.

Specification of voltage, wire speed, and cable pinouts.

EIA/TIA-232, V.35

The following list summarizes the benefits of layered protocol specifications:

■ Easier to learn—Humans can more easily discuss and learn about the many details of a protocol specification.

■ Easier to develop—Reduced complexity allows easier program changes and faster product evolution.

■ Multivendor interoperability—Creating products to meet the same networking standards means that computers and networking gear from multiple vendors can work in the same network.

■ Modular engineering—One vendor can write software that implements higher layers— for example, a web browser—and another can write software that implements the lower layers—for example, Microsoft's built-in TCP/IP software in its operating systems.

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