Excerpt from Cisco Networking Simplified, 2nd Edition.

 

Buy this book!

Newsletters: Sign-Up & Save! Receive Special Offers, Free Chapters, Articles Reference Guide Updates, and plug into the pulse of what's happening in your corner of the industry by subscribing to InformIT newsletters! FREE coupon after sign-up!

Try Safari Books Online NOW! Access the largest fully searchable e-reference library for programmers and IT professionals!

Read the authors' blog for Cisco Subnet

More chapters from new and classic Cisco Press books  

Rate your favorite Cisco Press books

How Computers Communicate

TCP/IP and IP Addressing

Internet Applications

How Computers Communicate

Networking Fundamentals

Before we begin talking about specific networking technologies and applications, it's worth taking a few pages to go over some networking fundamentals. Networks exist for the sole purpose of sharing information between people or machines. However, to share information, rules must be followed to ensure that the myriad combinations of devices, transports, hardware, and software can communicate smoothly.

In "How Computers Communicate," we cover the most basic aspects of computer networking, starting with the OSI model. This communication model is the basis for all other topics discussed in this book, so it's a great place to start.

In "TCP/IP and IP Addressing," we explore how two of the most popular protocols in use today work. TCP/IP is the communication protocol that drives the Internet as well as most corporate traffic. We then go a bit deeper into the Internet Protocol with a discussion of IP addressing, the concept that allows shared information to reach its intended destination. We end the chapter with an overview of IPv6. The addressing scheme discussed here (known as IPv4) has been in service for years. However, there has been some concern in recent years that Internet has grown beyond the current IP addressing scheme's ability to serve an ever-growing demand. Changing addressing schemes this far into networking's history provides some interesting challenges, which we will also explore.

"Internet Applications" provides a look at two of the most common applications—e-mail and web browsing. This chapter provides some background on how these applications came about and provides a summary of how they work. This should be helpful, because you probably use these applications every day.

The OSI Model

At some point, everyone involved with networking comes across a reference to the Open Systems Interconnection (OSI) seven-layer model. Because this model provides the architectural framework for all of network and computing communication, it's a good place to start. Even if you don't ever plan on setting up your own network, being familiar with this model is essential to understanding how it all works.

The OSI seven-layer model describes the functions for computers to communicate with each other. The International Organization for Standardization (ISO) published this model in 1984 to describe a layered approach for providing network services using a reference set of protocols called OSI. The basis of the definition is that each of the seven layers has a particular function it must perform, and each layer needs to know how to communicate with only the layers immediately above and below it.

The advantages of the OSI approach may not be readily apparent. But this simple concept of having layers understand only those adjacent to themselves allows communications systems to be easily adapted and modified as technologies evolve. For example, as new technologies are introduced in a lower layer, such as Layer 1, upper layers do not necessarily need to be changed. Instead, the adaptations at Layer 2 allow the layers above to use the new technologies transparently. Imagine if all web browsers and e-mail programs had to be replaced every time a new wireless network standard were introduced.

When the OSI networking model was defined, there was little standardization among network equipment manufacturers. Customers generally had to standardize on a particular vendor's often proprietary hardware and software to have devices communicate with each other. As a result of the ISO's and other standardization efforts, networking customers can mix and match hardware when running open-standards protocols, such as Internet Protocol (IP).

Open Versus Proprietary Systems

Although the open-source model is well-known today, when the OSI model was being developed, there was an ongoing struggle to balance technical openness with competitive advantage. At that time, each individual network equipment vendor saw it as an advantage to develop technologies that other companies could not copy or interact with. Proprietary systems let a vendor claim competitive advantage as well as collect fees from other vendors it might choose to share the technology with.

However, proprietary systems can complicate the network administrator's job by locking him or her into one vendor, reducing competitiveness and allowing the vendor to charge higher prices. If the vendor goes out of business or abandons the technology, no one is left to support or enhance the technology.

The alternative is an open-systems approach in which standards bodies, such as the Institute of Electrical and Electronic Engineers (IEEE) or ISO, define technologies. Ethernet, Transmission Control Protocol/Internet Protocol (TCP/IP), and Spanning Tree Protocol (STP) are examples of technologies that became standards. Today it is almost impossible to gain market traction with a product that does not at least allow an open interface for other vendors to work with. Any network-equipment vendor can implement an open standard.

Seven Layers

The following list outlines the seven layers of the OSI model from the bottom up:

• Layer 1, physical: The physical layer is responsible for converting a frame (the output from Layer 2) into electrical signals to be transmitted over the network. The actual physical network can be copper wiring, optical fiber, wireless radio signals, or any other medium that can carry signals. (We often joke about running networks over barbed wire. It's just a joke, but it actually can be done.) This layer also provides a method for the receiving device to validate that the data was not corrupted during transmission.

• Layer 2, data link: The data link layer is responsible for establishing the most elemental form of communication session between two different devices so that they may exchange Layer 3 protocols. For computer networks, the data link layer adds a header, which identifies the particular Layer 3 protocol used and the source and destination hardware addresses (also known as Media Access Control [MAC] addresses). At this point, the packet (the Layer 3 output) is successfully processed into a Layer 2 Frame and is ready to go onto the network. Ethernet switching and bridging operate at this level.

• Layer 3, network: The network layer is where the majority of communications protocols do their work, relying on Layers 2 and 1 to send and receive messages to other computers or network devices. The network layer adds another header to the front of the packet, which identifies the unique source and destination IP addresses of the sender and receiver. The process of routing IP packets occurs at this level.

• Layer 4, transport: The transport layer is responsible for taking the chunk of data from the application and preparing it for shipment onto the network. Prepping data for transport involves chopping the chunk into smaller pieces and adding a header that identifies the sending and receiving application (otherwise known as port numbers). For example, Hypertext Transfer Protocol (HTTP) web traffic uses port 80, and FTP traffic uses port 21. Each piece of data and its associated headers is called a packet.

• Layer 5, session: The session layer manages connections between hosts. If the application on one host needs to talk to the application on another, the session layer sets up the connection and ensures that resources are available to facilitate the connection. Networking folks tend to refer to Layers 5 to 7 collectively as the application layers.

• Layer 6, presentation: The presentation layer provides formatting services for the application layer. For example, file encryption happens at this layer, as does format conversion.

• Layer 7, application: The application layer provides networking services to a user or application. For example, when an e-mail is sent, the application layer begins the process of taking the data from the e-mail program and preparing it to be put onto a network, progressing through Layers 6 through 1.

The combination of the seven layers is often called a stack. A transmitting workstation traverses the stack from Layer 7 through Layer 1, converting the application data into network signals. The receiving workstation traverses the stack in the opposite direction: from Layer 1 to Layer 7. It converts the received transmission back into a chunk of data for the running application.

When the OSI model was created, there was an industry initiative that tried to implement a universal set of OSI network protocols, but it was not adopted. Most popular protocols today generally use design principles that are similar to and compatible with the OSI model, but they deviate from it in some areas for various technical reasons. That said, the OSI model is still considered the basis of all network communication.

Whitepapers

• Best Practices for HP Servers and HP Enterprise Virtual Array in a Microsoft Exchange
• Network-Wide Change Management Visibility with Route Analytics
• REMOTE CONTROL: How to Reduce the Cost, Complexity and Risk of Managing Your Distributed IT Infrastructure
• Steal This Script: Hands-On Tips for Service Impact Network Management
• The Case for an Untethered Enterprise
• Best Practices for WAN Optimization with Data Replication

View more LANS & WANS whitepapers

Webcasts

• WAN Optimization Editorial Webcast
• Best Practices for Deploying WAN Optimization for Disaster Recovery
• Tips and Techniques for Remote Data Backup and Delivery
• WAN Optimization Landscape
• High Availability for SQL Server 2005 Using Array-Based Replication and Host-Based Mirroring Technologies
• Technical Overview of Exchange Server 2007 with HP Servers and Storage

View more LANS & WANS webcasts

Special Reports

• Network World Executive GUide: The Virtualization Equation
• Get More From Your WAN
• Learn how to Keep your Mobile Workforce Connected
• WAN Optimization: How to rev up sluggish applications
• The Wireless Explosion
• Network World Executive Guide: Perfecting Application Performance Management

View more LANS & WANS special reports