Layer 1, physical

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. 21

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.

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).

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.