The 7 Layers of the OSI Model

The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

 The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

Application (Layer 7)

This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network softwareservices. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

Presentation (Layer 6)

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Session (Layer 5)

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Transport (Layer 4)

This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

Network (Layer 3)

This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling,congestion control and packet sequencing.

Data Link (Layer 2)

At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls framesynchronization, flow control and error checking.

Physical (Layer 1)

This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

Local Area Network

A Local Area Network (LAN) is a network that is confined to a relatively small area. It is generally limited to a geographic area such as a writing lab, school, or building. Rarely are LAN computers more than a mile apart.

In a typical LAN configuration, one computer is designated as the file server. It stores all of the software that controls the network, as well as the software that can be shared by the computers attached to the network. Computers connected to the file server are called workstations. The workstations can be less powerful than the file server, and they may have additional software on their hard drives. On many LANs, cables are used to connect the network interface cards in each computer; other LANs may be wireless. See the Topology, Cabling, and Hardware sections of this tutorial for more information on the configuration of a LAN.

What is a Network?

A network consists of two or more computers that are linked in order to share resources (such as printers and CDs), exchange files, or allow electronic communications. The computers on a network may be linked through cables, telephone lines, radio waves, satellites, or infrared light beams.

The two basic types of networks include:

• Local Area Network (LAN)
• Wide Area Network (WAN)

You may also see references to a Metropolitan Area Networks (MAN), a Wireless LAN (WLAN), or a Wireless WAN (WWAN).

What Skills Do You Need to Become a CCNA?

To meet the CCNA certification skill level, you must be able to understand or do the
following:

A CCNA certified professional can install, configure, and operate LAN, WAN, and wireless
access services securely, as well as troubleshoot and configure small to medium networks
(500 nodes or fewer) for performance.

This knowledge includes, but is not limited to, use of these protocols: IP, IPv6, EIGRP,
RIP, RIPv2, OSPF, serial connections, Frame Relay, cable, DSL, PPPoE, LAN switching,
VLANs, Ethernet, security, and access lists.

Why Become a CCNA?

Cisco, not unlike Microsoft and Novell (Linux), has created the certification process to give
administrators a set of skills and to equip prospective employers with a way to measure skills
or match certain criteria. Becoming a CCNA can be the initial step of a successful journey
toward a new, highly rewarding, and sustainable career.
The CCNA program was created to provide a solid introduction not only to the Cisco Internetwork
Operating System (IOS) and Cisco hardware, but also to internetworking in general,
making it helpful to you in areas that are not exclusively Cisco’s. At this point in the certification
process, it’s not unrealistic that network managers—even those without Cisco equipment—
require Cisco certifications for their job applicants.
If you make it through the CCNA and are still interested in Cisco and internetworking,
you’re headed down a path to certain success.

Cisco Certified Network Associate (CCNA)
The CCNA certification was the first in the new line of Cisco certifications and was the precursor
to all current Cisco certifications. Now you can become a Cisco Certified Network
Associate for the meager cost of this book and either one test at $150 or two tests at $125
each—although the CCNA exams are extremely hard and cover a lot of material, so you have
to really know your stuff! Taking a Cisco class or spending months with hands-on experience
is not out of the norm.
And once you have your CCNA, you don’t have to stop there—you can choose to continue
with your studies and achieve a higher certification, called the Cisco Certified Network Professional
(CCNP). Someone with a CCNP has all the skills and knowledge he or she needs to
attempt the Routing and Switching CCIE lab. But just becoming a CCNA can land you that
job you’ve dreamed about.

Introduction
Welcome to the exciting world of Cisco certification! You have picked up this book because
you want something better—namely, a better job with more satisfaction. Rest assured that you
have made a good decision. Cisco certification can help you get your first networking job or
more money and a promotion if you are already in the field.
Cisco certification can also improve your understanding of the internetworking of more
than just Cisco products: You will develop a complete understanding of networking and how
different network topologies work together to form a network. This is beneficial to every networking
job and is the reason Cisco certification is in such high demand, even at companies
with few Cisco devices.
Cisco is the king of routing, switching, and security, the Microsoft of the internetworking
world. The Cisco certifications reach beyond the popular certifications, such as the MCSE, to
provide you with an indispensable factor in understanding today’s network—insight into the
Cisco world of internetworking. By deciding that you want to become Cisco certified, you are
saying that you want to be the best—the best at routing and the best at switching. This book
will lead you in that direction.