Communicating Between Layers
Each layer of the OSI model uses its own protocol
to communicate with its peer layer in the destination
device. The OSI model specifies how each
layer communicates with the layers above and
below it, allowing vendors to focus on specific layers
that will work with any other vendor’s adjacent
layers.
Information is exchanged between layers using
protocol data units (PDU). PDUs include control
information (in the form of headers and trailers)
and user data. PDUs include different types of
information as they go up or down the layers
(called “the stack”). To clarify where the PDU is
on the stack, it is given a distinct name at each of
the lower levels.
At-a-Glance: OSI Model
In other words, a PDU that is a segment (Layer 4)
includes all the application layer’s information. A
packet (Layer 3) includes network layer control
information in addition to the data and control
information contained at the transport layer.
Similarly, a frame (Layer 2) is a PDU that includes
data link layer control information in addition to
the upper layer control information and data.
Finally, PDUs at the physical layer (Layer 1) are
called bits. 24
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What Problems Need to Be Solved?
An OSI layer can communicate only with the layers
immediately above and below it on the stack,
and with its peer layer on another device. A
process must be used so that information (including
data and stack instructions) can be passed
down the stack, across the network, and back up
the stack on the peer device.
immediately above and below it on the stack,
and with its peer layer on another device. A
process must be used so that information (including
data and stack instructions) can be passed
down the stack, across the network, and back up
the stack on the peer device.
OSI Layers and Definitions
OSI Layers and Definitions
The OSI layers are defined as follows:
Layer 1: Physical
Layer 2: Data link
Layer 3: Network
Layer 4: Transport
Layer 5: Session
Layer 6: Presentation
Layer 7: Application
The four lower layers (called the data flow layers)
define connection protocols and methods for
exchanging data.
The three upper layers (called the application layers)
define how the applications within the end stations
communicate with each other and with users.
Several mnemonics have been developed to help you
memorize the layers and their order. Here’s one:
Please Do Not Throw Sausage Pizza Away
The OSI layers are defined as follows:
Layer 1: Physical
Layer 2: Data link
Layer 3: Network
Layer 4: Transport
Layer 5: Session
Layer 6: Presentation
Layer 7: Application
The four lower layers (called the data flow layers)
define connection protocols and methods for
exchanging data.
The three upper layers (called the application layers)
define how the applications within the end stations
communicate with each other and with users.
Several mnemonics have been developed to help you
memorize the layers and their order. Here’s one:
Please Do Not Throw Sausage Pizza Away
Why Should I Care About the OSI Model?
The Open Systems Interconnection (OSI) model is
a conceptual framework that defines network functions
and schemes. The framework simplifies complex
network interactions by breaking them into
simple modular elements. This open-standards
approach allows many independent developers to
work on separate network functions, which can
then be combined in a “plug-and-play” manner.
The OSI model serves as a guideline for creating
and implementing network standards, devices, and
internetworking schemes. Advantages of using the
OSI model include the following:
• It breaks interrelated aspects of network operation
into less-complex elements.
• It enables companies and individual engineers to
specialize design and development efforts on
modular functions.
• It provides standard interfaces for plug-and-play
compatibility and multivendor integration.
• It abstracts different layers of the network from
each other to provide easier adoption of new
technologies within a layer.
a conceptual framework that defines network functions
and schemes. The framework simplifies complex
network interactions by breaking them into
simple modular elements. This open-standards
approach allows many independent developers to
work on separate network functions, which can
then be combined in a “plug-and-play” manner.
The OSI model serves as a guideline for creating
and implementing network standards, devices, and
internetworking schemes. Advantages of using the
OSI model include the following:
• It breaks interrelated aspects of network operation
into less-complex elements.
• It enables companies and individual engineers to
specialize design and development efforts on
modular functions.
• It provides standard interfaces for plug-and-play
compatibility and multivendor integration.
• It abstracts different layers of the network from
each other to provide easier adoption of new
technologies within a layer.
Layer 7, application:
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.
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.
Layer 6, presentation:
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.
for the application layer. For example, file encryption happens at this layer,
as does format conversion.
Layer 5, session:
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.
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 4, transport:
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.
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 3, network:
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.
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 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
(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.
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).
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.
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.
BRANDWIDTH OVER BANDWIDTH
Branding will eventually replace the how. Just as consumers
talk about fueling up down at the local Texaco station but don’t
bother to explain if they filled up with diesel or gasoline, they
will talk about accessing the network via a particular brand.
Wireless Internet access brands will be not unlike the cellular
carriers of today; I’m an AT&T customer but often roam or use
another carrier’s network all while telling others I’m an AT&T
customer.
Even the most insightful futurists can’t guarantee exactly
what the interaction between culture and Wireless Internet
technology will result in. But even though the experts can’t
predict how the Wireless Internet will evolve, please keep one
thing in mind—the answer may someday be in your hand.
talk about fueling up down at the local Texaco station but don’t
bother to explain if they filled up with diesel or gasoline, they
will talk about accessing the network via a particular brand.
Wireless Internet access brands will be not unlike the cellular
carriers of today; I’m an AT&T customer but often roam or use
another carrier’s network all while telling others I’m an AT&T
customer.
Even the most insightful futurists can’t guarantee exactly
what the interaction between culture and Wireless Internet
technology will result in. But even though the experts can’t
predict how the Wireless Internet will evolve, please keep one
thing in mind—the answer may someday be in your hand.
WILL THE WIRELESS INTERNET SURVIVE?
In an age where rapid technology development produces concepts
and innovations that disappear often as quickly as they
come it’s only natural to ask the question—Will the Wireless
Internet survive? We believe the Wireless Internet will eventually
disappear.
It will be out of sight, but it will still exist. Not as the
wired or wireless Internet, but simply as “the Internet” or “the
network.” Access method and device will eventually become
irrelevant.
As the Wireless Internet evolves and embeds itself in the
society and culture of our modern world, the phrase “Wireless
Internet” will quietly go away. When is the last time you heard
someone refer to the “electric” light? Or the “gasoline powered”
automobile? Or even “indoor” plumbing? The descriptors
of how eventually fall away as society gets used to
assuming the obvious or irrelevant. What will matter in the
future is that a user is connecting to a network; whether that
user arrives via cable broadband, GPRS, or a public WLAN
won’t really matter.
and innovations that disappear often as quickly as they
come it’s only natural to ask the question—Will the Wireless
Internet survive? We believe the Wireless Internet will eventually
disappear.
It will be out of sight, but it will still exist. Not as the
wired or wireless Internet, but simply as “the Internet” or “the
network.” Access method and device will eventually become
irrelevant.
As the Wireless Internet evolves and embeds itself in the
society and culture of our modern world, the phrase “Wireless
Internet” will quietly go away. When is the last time you heard
someone refer to the “electric” light? Or the “gasoline powered”
automobile? Or even “indoor” plumbing? The descriptors
of how eventually fall away as society gets used to
assuming the obvious or irrelevant. What will matter in the
future is that a user is connecting to a network; whether that
user arrives via cable broadband, GPRS, or a public WLAN
won’t really matter.
THE FUTURE OF WIRELESS INTERNET
THE FUTURE OF WIRELESS INTERNET
IS CERTAIN—TO CHANGE!
This book has covered the technologies and applications of the
Wireless Internet in an attempt to give you a high-level glimpse
of the many challenges and issues surrounding its evolution.
On many levels it’s still anyone’s guess as to which protocols
and specific technologies will emerge as part of the standard of
the future.
The wireless industry has many players all working to provide
their contribution to this amazing future of Wireless
Internet access. Not everyone agrees on the best way to ensure
success but the momentum has generated a self-fulfilling
prophecy of sorts, led by industry: If they think it will happen,
it will (eventually anyway). But how much will it cost consumers
and industry? Who will profit?
IS CERTAIN—TO CHANGE!
This book has covered the technologies and applications of the
Wireless Internet in an attempt to give you a high-level glimpse
of the many challenges and issues surrounding its evolution.
On many levels it’s still anyone’s guess as to which protocols
and specific technologies will emerge as part of the standard of
the future.
The wireless industry has many players all working to provide
their contribution to this amazing future of Wireless
Internet access. Not everyone agrees on the best way to ensure
success but the momentum has generated a self-fulfilling
prophecy of sorts, led by industry: If they think it will happen,
it will (eventually anyway). But how much will it cost consumers
and industry? Who will profit?
REAL TIME ADDS VALUE
Remember the last time you went to a concert or show? Let’s
assume you have one friend who would have enjoyed the show
but wasn’t able to attend: The longer you wait to tell him about
it the less you will remember and the less emotion you will feel
about the event. As time passes, you’ll have a reduced ability to
recall the event details.
Now imagine that you could share images, sounds, and
your thoughts in text in almost real time. Ever watched a live
TV show? The value of sharing events while the event is occurring
is apparent on TV. Wireless multimedia messages won’t be
TV, but they will be more like a short commercial—images,
sounds, and text combined to communicate with detail,
efficiency, and emotion and to allow the person on the other end
to better understand you. 223
Remember the last time you went to a concert or show? Let’s
assume you have one friend who would have enjoyed the show
but wasn’t able to attend: The longer you wait to tell him about
it the less you will remember and the less emotion you will feel
about the event. As time passes, you’ll have a reduced ability to
recall the event details.
Now imagine that you could share images, sounds, and
your thoughts in text in almost real time. Ever watched a live
TV show? The value of sharing events while the event is occurring
is apparent on TV. Wireless multimedia messages won’t be
TV, but they will be more like a short commercial—images,
sounds, and text combined to communicate with detail,
efficiency, and emotion and to allow the person on the other end
to better understand you. 223
SPEED INFLUENCES THE VOLUME OF COMMUNICATION
The speed of our communication process influences the
amount of things we want to communicate. Real-time communications
allow us to share things while they still have relevance.
Human communication is often about human
experiences—things that somehow impact our five senses.
Even intense experiences eventually fade from our memory.
Communication of these experiences is best right after the
event, or ideally, during the event.
When was the last time you took pictures? Birthday party?
Vacation? Wedding? Pictures are usually taken at high-emotion
events so that we can capture the moment and remember it
later. How fun is it to share these kinds of photos with friends
soon after you take them? Many of us can’t wait to share our
pictures as soon as we get them. Trouble is, the longer you
wait, the less fun it usually is to share. As Wireless Internet
devices enable users to capture and transmit images, sound,
and other data the frequency of communication will increase.
amount of things we want to communicate. Real-time communications
allow us to share things while they still have relevance.
Human communication is often about human
experiences—things that somehow impact our five senses.
Even intense experiences eventually fade from our memory.
Communication of these experiences is best right after the
event, or ideally, during the event.
When was the last time you took pictures? Birthday party?
Vacation? Wedding? Pictures are usually taken at high-emotion
events so that we can capture the moment and remember it
later. How fun is it to share these kinds of photos with friends
soon after you take them? Many of us can’t wait to share our
pictures as soon as we get them. Trouble is, the longer you
wait, the less fun it usually is to share. As Wireless Internet
devices enable users to capture and transmit images, sound,
and other data the frequency of communication will increase.
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