Early attempts at wireless data transmission evolved around
proprietary technologies. Usually, the cost to deploy such networks
limited their use to that of large companies. The services
offered usually revolved around some form of dispatch
service. Today, however, cellular networks are ubiquitous and
quite capable of serving the data requirements of not only large
companies, but individuals. Cellular is changing the way we
communicate on an everyday basis.
Another factor in moving to a Wireless Internet is the size
of computing devices. Miniaturization and improved batteries
are providing smaller, better mobile tools. Laptops and PDAs
are small enough to be very mobile but powerful enough to
tackle anything that we might do on a desktop computer.
Portability and connectivity can be readily achieved. Now a
mobile businessman can be more productive because he can
access his data in his office or retrieve data stored from a global
Internet connection. As the performance of the Wireless
Internet approaches that of a fixed connection, there is no
longer a need to remain tethered to a desk.
There are two types of Wireless Internet connections: those
through cellular and those through a mobile data network.
There is a very big distinction between these two types of connection.
Cellular has traditionally been a circuit-switched connection,
whereas mobile data networks are packet based. The
next generation cellular standards will eliminate this difference.
Data on the Wireless Internet will be packet-based using
TCP/IP, the same protocol used on the Internet. The spectrum
resource or channel will become a shared resource, and new
methods of usage billing rather than airtime billing will emerge.
Data speed will also increase from 64 Kbps to more than 2
Mbps depending on the technology.
The Wireless Internet is a natural and inevitable progression
from the wired Internet when you consider today’s wireless
communications devices, a very mobile society, and a free
market economy where anything can be sold if it has the right
sales approach. Cellular penetration is very high, with over 1
billion cellular users projected by the end of 2002. Some estimates
put data revenue streams in 2006 higher than today’s
voice revenue streams. This may be a bit optimistic but it is
clear the demand for wireless data transmission is growing. A
cellular device is a personal device and the value of wireless
data is in the knowledge of the user, his buying habits, his location
and other personal information. The proper use this
knowledge will create new revenue streams. Applications must
be created that the user cannot live without.
The terminal market varies to users’ demand. As we stated earlier,
a Wireless Internet device does not have to resemble a cellular
phone or even possess the functionality of a cellular phone.
The new Wireless Internet will be accessed by many new devices
and methods. Voice functionality does not necessarily need to
reside on the device. Voice recognition and text-to-speech may be
the solution for access. User interfaces must change to reflect the
wider spectrum of data throughput. Displays and keyboards may
no longer be of primary importance: If you have voice recognition
and text-to-speech capabilities, do you need a bigger display and
keyboard? Maybe the Wireless Internet device will require no
human interface. Technologies used in terminal devices are large ly determined by the application required. (In Chapters 4 and 5
we discuss this subject further.) 36
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
THE INTERNET, A NEW IDEA
The 1990s saw the emergence of the Internet as a dominant
communications media but actually its beginnings can be traced back to the late 1960s. The Internet started out as an
idea born within the Rand Corporation, America’s premier
think-tank for Cold War strategy. The Defense Department
wanted to create a method for communications of defense
command and control information in a post-nuclear world. It
required a decentralized network that could function even if
several nodes were destroyed. Ultimately, a request for proposal
was issued from the Defense Advanced Research Projects
Agency for a packet-switching network that DARPA was planning
to build. Several graduate students and faculty at the
University of California–Los Angeles presented a proposal to
DARPA for establishing a communications network model
known as the ARPANET project. There were no formal standards
written for ARPANET, so a method of documentation
was devised: the RFC or Request for Comment. (This method
continues today and throughout this book, references to RFC
numbers will be made.) By December of 1969, a four-node
network was working and by 1972, thirty-seven nodes were
working in the ARPANET. The network enabled researchers
across the country to share computational resources.
Somewhere along the way, an interesting observation took
place that the ARPANET was really a government subsidized
person-to-person communications service more than a sharing
of resources. The advent of personal user accounts enabled an
electronic mail service. Shortly after this the mailing list was
invented, which enabled the broadcasting of messages to large
numbers of users simultaneously. Researchers could now communicate
personally or share ideas with a group. ARPANET in
its infancy was shared by academic institutions and their financial
backers, the Department of Defense. The network grew
throughout the 1970s because of its ability to add nodes using
many different computers as long as they “spoke” the same language
of ARPANET. The original language was Network
Communications Protocol (NCP) but it was superseded by
TCP/IP in the early 1980s. Transmission Control Protocol
(TCP) breaks each message into packets at the source and then
reassembles them at the destination. Each packet contains a
source and destination address so that Internet Protocol (IP) can route the packets through multiple nodes and multiple networks
successfully, even if the nodes or networks operate with
different standards.
The principles behind TCP/IP and the Internet are that
data can go many different paths. Figure 1-3 shows how two
different data sources, A and B, can travel along separate paths
1 and 2, through multiple nodes (n), and them come together
at the destination.
Concurrent to the development of this vast network of
supercomputers was the invention of the personal computer in
the early 1980s. Suddenly thousands of individuals had access
to a computer on their desktop or in their homes. This played
heavily in expanding the ARPANET although it remained
closely controlled until 1983, when the Defense Department
split off the MILNET. By this time many groups of people had
access to computers and through the simplicity of TCP/IP public-
domain protocols, they could link to the network and essentially
add another node. Thus the “network of networks” was
created which ultimately became known as the Internet.
By 1984 the National Science Foundation (NSF) jumped
into the fray, promoting technological advances for ARPANET.
Faster speeds were achieved through upgraded links and newer
supercomputers. Other government agencies joined in expanding
the network and the cumulative knowledge base of information.
A method of identifying users was devised to create
domains with unique identifiers such as com, gov, org, mil, edu, and net. At the same time two-letter country designations such
as uk, dl, and fr were created because the network now crossed
international boundaries. By 1989, ARPANET passed into history,
a victim of its own success. The birth of the Internet had
occurred as a result of many people’s long, arduous hours of
research and development. It was not invented by a politician
as some would like us to believe!
The 1990s saw explosive growth in the Internet, with hundreds
of companies supplying enabling technology, thousands
of companies providing service, and even more users sharing
the combined wealth of knowledge on the Internet. By 1993
there were over 1.3 million computers connected to the
Internet. Today there over 20 million hosts and 500 million
users, sharing ideas and knowledge, swapping emails, and buying
and selling through e-commerce.
The explosive use of the Internet during the 1990s could be
compared to other momentous events such as the invention of
the wheel, the Industrial Revolution, and the invention of the
transistor or integrated circuit. No single “invention” or “revolution”
has affected our lives more than the Internet. A whole
new generation has grown up with access to the ’Net, accepting
its promise to communicate with anyone, anywhere. That
same generation considers the cellular phone a necessity and
would not go anywhere without it.
Although the original intended use of the Internet was file
sharing and electronic mail, it soon became apparent that it
really was a tool to connect people to people. It created the
world’s largest, easily accessible marketplace and gave birth to
e-commerce. The under-25 age group represents a huge market
segment, and an entire industry has grown-up in the “dot
com” market segment catering to the needs of these Internet
users. However, until recently, this was a “tethered” connection.
If the Internet could be extended beyond wired connections,
it could be accessed anytime and anywhere.
This brings us back full circle to wireless technologies and
raises the question, “How can I access the Internet from
where I am at this moment? How do we put all of this together
to benefit us?” Let’s consult our crystal ball to look into the future. We know that the Internet will expand our horizons
and opportunities for new services. We also know that wireless
communications is becoming very popular and that data rates
are increasing. Added to all of this, we can know wireless user
location thanks to GPS. An application environment to connect
the wireless user to the Internet is ready. All that the
Wireless Internet needs are applications to drive user adoption.
For service providers, developing and launching these
applications is a gamble. If user satisfaction is low, the
Wireless Internet will be a huge disaster. The financial implications
will be staggering to the world economy, considering
that service providers have committed billions of dollars on
licenses, infrastructure, and development all predicated on
the success of the Wireless Internet.
communications media but actually its beginnings can be traced back to the late 1960s. The Internet started out as an
idea born within the Rand Corporation, America’s premier
think-tank for Cold War strategy. The Defense Department
wanted to create a method for communications of defense
command and control information in a post-nuclear world. It
required a decentralized network that could function even if
several nodes were destroyed. Ultimately, a request for proposal
was issued from the Defense Advanced Research Projects
Agency for a packet-switching network that DARPA was planning
to build. Several graduate students and faculty at the
University of California–Los Angeles presented a proposal to
DARPA for establishing a communications network model
known as the ARPANET project. There were no formal standards
written for ARPANET, so a method of documentation
was devised: the RFC or Request for Comment. (This method
continues today and throughout this book, references to RFC
numbers will be made.) By December of 1969, a four-node
network was working and by 1972, thirty-seven nodes were
working in the ARPANET. The network enabled researchers
across the country to share computational resources.
Somewhere along the way, an interesting observation took
place that the ARPANET was really a government subsidized
person-to-person communications service more than a sharing
of resources. The advent of personal user accounts enabled an
electronic mail service. Shortly after this the mailing list was
invented, which enabled the broadcasting of messages to large
numbers of users simultaneously. Researchers could now communicate
personally or share ideas with a group. ARPANET in
its infancy was shared by academic institutions and their financial
backers, the Department of Defense. The network grew
throughout the 1970s because of its ability to add nodes using
many different computers as long as they “spoke” the same language
of ARPANET. The original language was Network
Communications Protocol (NCP) but it was superseded by
TCP/IP in the early 1980s. Transmission Control Protocol
(TCP) breaks each message into packets at the source and then
reassembles them at the destination. Each packet contains a
source and destination address so that Internet Protocol (IP) can route the packets through multiple nodes and multiple networks
successfully, even if the nodes or networks operate with
different standards.
The principles behind TCP/IP and the Internet are that
data can go many different paths. Figure 1-3 shows how two
different data sources, A and B, can travel along separate paths
1 and 2, through multiple nodes (n), and them come together
at the destination.
Concurrent to the development of this vast network of
supercomputers was the invention of the personal computer in
the early 1980s. Suddenly thousands of individuals had access
to a computer on their desktop or in their homes. This played
heavily in expanding the ARPANET although it remained
closely controlled until 1983, when the Defense Department
split off the MILNET. By this time many groups of people had
access to computers and through the simplicity of TCP/IP public-
domain protocols, they could link to the network and essentially
add another node. Thus the “network of networks” was
created which ultimately became known as the Internet.
By 1984 the National Science Foundation (NSF) jumped
into the fray, promoting technological advances for ARPANET.
Faster speeds were achieved through upgraded links and newer
supercomputers. Other government agencies joined in expanding
the network and the cumulative knowledge base of information.
A method of identifying users was devised to create
domains with unique identifiers such as com, gov, org, mil, edu, and net. At the same time two-letter country designations such
as uk, dl, and fr were created because the network now crossed
international boundaries. By 1989, ARPANET passed into history,
a victim of its own success. The birth of the Internet had
occurred as a result of many people’s long, arduous hours of
research and development. It was not invented by a politician
as some would like us to believe!
The 1990s saw explosive growth in the Internet, with hundreds
of companies supplying enabling technology, thousands
of companies providing service, and even more users sharing
the combined wealth of knowledge on the Internet. By 1993
there were over 1.3 million computers connected to the
Internet. Today there over 20 million hosts and 500 million
users, sharing ideas and knowledge, swapping emails, and buying
and selling through e-commerce.
The explosive use of the Internet during the 1990s could be
compared to other momentous events such as the invention of
the wheel, the Industrial Revolution, and the invention of the
transistor or integrated circuit. No single “invention” or “revolution”
has affected our lives more than the Internet. A whole
new generation has grown up with access to the ’Net, accepting
its promise to communicate with anyone, anywhere. That
same generation considers the cellular phone a necessity and
would not go anywhere without it.
Although the original intended use of the Internet was file
sharing and electronic mail, it soon became apparent that it
really was a tool to connect people to people. It created the
world’s largest, easily accessible marketplace and gave birth to
e-commerce. The under-25 age group represents a huge market
segment, and an entire industry has grown-up in the “dot
com” market segment catering to the needs of these Internet
users. However, until recently, this was a “tethered” connection.
If the Internet could be extended beyond wired connections,
it could be accessed anytime and anywhere.
This brings us back full circle to wireless technologies and
raises the question, “How can I access the Internet from
where I am at this moment? How do we put all of this together
to benefit us?” Let’s consult our crystal ball to look into the future. We know that the Internet will expand our horizons
and opportunities for new services. We also know that wireless
communications is becoming very popular and that data rates
are increasing. Added to all of this, we can know wireless user
location thanks to GPS. An application environment to connect
the wireless user to the Internet is ready. All that the
Wireless Internet needs are applications to drive user adoption.
For service providers, developing and launching these
applications is a gamble. If user satisfaction is low, the
Wireless Internet will be a huge disaster. The financial implications
will be staggering to the world economy, considering
that service providers have committed billions of dollars on
licenses, infrastructure, and development all predicated on
the success of the Wireless Internet.
COMPUTING POWER IN MOBILE COMMUNICATIONS
Computing power can mean many things depending on where
the term is applied. The computing power of today’s cellular
handset is much greater than just ten years ago. The mobile
phones of the 1980s used 8-bit microcontrollers with very little
memory. A typical phone operated with 6 Kb of RAM
(scratchpad memory) and 32 Kb of ROM (program memory).
That was fine, because AMPS was an analog communications
system based on FM and the data rate was low because all data
requires a modem to convert digital data to analog modulation.
Data rates for wire-line modems in the early 1980s were initially
300 baud progressing to, at best, 19,200 baud by 1990.
Therefore, rates of 1200 baud to 2400 baud were acceptable
for wireless device communications. (One important point
should be noted—gross baud rates and throughput are two different
things entirely!)
The wireless cellular communications channel is a dirty,
nasty place for data communications signals. Impairments to an
analog cellular channel are noise, weak signals, interference,
and signal dropouts caused by handoffs from one channel to
another. Today’s digital cellular channel problems are compounded
by signal degradation, multipath fading, and delay.
But today’s cellular phones have more computing power
than the average workstation of the early 1990s. They will contain
16-bit, 32-bit, or 64-bit RISC microcontrollers, digital signal
processors capable of 400 MIPS or more by 2002, and they
will contain megabytes of memory. This translates to computational
power sufficient to make a digital voice call or send highspeed
data such as full motion video while simultaneously
reading your email on the display. Data rates will soon
approach ISDN or DSL rates and may go higher, anywhere
from 114 Kbps to 2 Mbps or higher.
This vastly increased computing power (and the interest of
the Defense Department) has brought one other very important
new feature to wireless communications devices—Global
Positioning Services (GPS). Using GPS, a wireless device can
communicate its location to anywhere in the world. Orwell
look out: Location-based marketing to a mobile customer base
is coming.
the term is applied. The computing power of today’s cellular
handset is much greater than just ten years ago. The mobile
phones of the 1980s used 8-bit microcontrollers with very little
memory. A typical phone operated with 6 Kb of RAM
(scratchpad memory) and 32 Kb of ROM (program memory).
That was fine, because AMPS was an analog communications
system based on FM and the data rate was low because all data
requires a modem to convert digital data to analog modulation.
Data rates for wire-line modems in the early 1980s were initially
300 baud progressing to, at best, 19,200 baud by 1990.
Therefore, rates of 1200 baud to 2400 baud were acceptable
for wireless device communications. (One important point
should be noted—gross baud rates and throughput are two different
things entirely!)
The wireless cellular communications channel is a dirty,
nasty place for data communications signals. Impairments to an
analog cellular channel are noise, weak signals, interference,
and signal dropouts caused by handoffs from one channel to
another. Today’s digital cellular channel problems are compounded
by signal degradation, multipath fading, and delay.
But today’s cellular phones have more computing power
than the average workstation of the early 1990s. They will contain
16-bit, 32-bit, or 64-bit RISC microcontrollers, digital signal
processors capable of 400 MIPS or more by 2002, and they
will contain megabytes of memory. This translates to computational
power sufficient to make a digital voice call or send highspeed
data such as full motion video while simultaneously
reading your email on the display. Data rates will soon
approach ISDN or DSL rates and may go higher, anywhere
from 114 Kbps to 2 Mbps or higher.
This vastly increased computing power (and the interest of
the Defense Department) has brought one other very important
new feature to wireless communications devices—Global
Positioning Services (GPS). Using GPS, a wireless device can
communicate its location to anywhere in the world. Orwell
look out: Location-based marketing to a mobile customer base
is coming.
CELLULAR, TRUE MOBILITY FOR THE MASSES
Meanwhile in a separate segment of communications, another
revolution was taking place—cellular telephones. In the late
1970s AT&T Bell Laboratories began working with several leading
United States and Japanese companies to create a cellular
telephone system based on dividing coverage areas into small
cells and reusing frequencies. Previous mobile telephone technologies
operated on limited numbers of channels, thus limiting
the number of users in any given coverage area to a very small
number. The result was low user use and costly service and
equipment. A core group was created to develop a standard
called the Advanced Mobile Phone Service (AMPS). In
December 1983, AMPS was launched in Chicago, Illinois with
great fanfare. It proved immensely popular. Now before someone
says, “Hey, wait a minute, AMPS wasn’t the first cellular
system!,” let’s give that credit to the Nordic Mobile Telephone
(NMT) system. NMT was launched in 1981 in Scandinavia, but
in terms of market size, AMPS potential market in the United
States was vastly larger. AMPS quickly spread to other countries
in North and South America, Korea, and Australia. A similar
standard, Total Access Communications System (TACS), was
developed in the United Kingdom as well.
Today, there are many competing standards in mobile telephones
worldwide. In fact the word “mobile” means something
entirely different today than it did in 1983. The majority of cellular
telephones sold today are hand-held, not permanently
installed in vehicles. Each competing standard is incompatible
with others on the basic technology used, but to the end user,
all cellular telephones should perform the basic functions
expected. (Even though many new carriers would like to distinguish
themselves from “cellular” companies by calling themselves
“PCS” companies, we consider both as cellular
applications in this book. This is not to say that companies with
PCS spectrum in the 1900 MHz band may or may not have
some advantages over carriers with traditional spectrum allocations
in the 800 MHz band. But because many carriers own
spectrum in both bands this is a moot point.)
Cellular radio got its name from the physical layout of a system
in a pattern resembling a honeycomb figuratively. In
Figure 1-2, a vehicle traveling from point A to C, will initially
be communicating through cellsite 1. As it moves to position B,
communications is handed off to cell site 2 and similarly for
position C. Each cell site will operate on a different frequency
so that neighboring cells do not interfere with one another.
However, frequencies can be re-used if they are separate by
sufficient distance. This is referred to as the re-use pattern.
revolution was taking place—cellular telephones. In the late
1970s AT&T Bell Laboratories began working with several leading
United States and Japanese companies to create a cellular
telephone system based on dividing coverage areas into small
cells and reusing frequencies. Previous mobile telephone technologies
operated on limited numbers of channels, thus limiting
the number of users in any given coverage area to a very small
number. The result was low user use and costly service and
equipment. A core group was created to develop a standard
called the Advanced Mobile Phone Service (AMPS). In
December 1983, AMPS was launched in Chicago, Illinois with
great fanfare. It proved immensely popular. Now before someone
says, “Hey, wait a minute, AMPS wasn’t the first cellular
system!,” let’s give that credit to the Nordic Mobile Telephone
(NMT) system. NMT was launched in 1981 in Scandinavia, but
in terms of market size, AMPS potential market in the United
States was vastly larger. AMPS quickly spread to other countries
in North and South America, Korea, and Australia. A similar
standard, Total Access Communications System (TACS), was
developed in the United Kingdom as well.
Today, there are many competing standards in mobile telephones
worldwide. In fact the word “mobile” means something
entirely different today than it did in 1983. The majority of cellular
telephones sold today are hand-held, not permanently
installed in vehicles. Each competing standard is incompatible
with others on the basic technology used, but to the end user,
all cellular telephones should perform the basic functions
expected. (Even though many new carriers would like to distinguish
themselves from “cellular” companies by calling themselves
“PCS” companies, we consider both as cellular
applications in this book. This is not to say that companies with
PCS spectrum in the 1900 MHz band may or may not have
some advantages over carriers with traditional spectrum allocations
in the 800 MHz band. But because many carriers own
spectrum in both bands this is a moot point.)
Cellular radio got its name from the physical layout of a system
in a pattern resembling a honeycomb figuratively. In
Figure 1-2, a vehicle traveling from point A to C, will initially
be communicating through cellsite 1. As it moves to position B,
communications is handed off to cell site 2 and similarly for
position C. Each cell site will operate on a different frequency
so that neighboring cells do not interfere with one another.
However, frequencies can be re-used if they are separate by
sufficient distance. This is referred to as the re-use pattern.
FIRST VOICE COMMUNICATIONS
Voice communication became possible when Alexander
Graham Bell invented the telephone on March 10, 1876. His
experiments with his assistant Thomas Watson finally proved
successful when the first vocal sentence was transmitted:
“Watson, come here; I want you.” The telephone was demonstrated
to the world at the 1876 Centennial Exposition in
Philadelphia, Pennsylvania, and led to the creation of the Bell
Telephone Company in 1877.
By 1906 American inventor, Lee De Forest, invented a
three-element vacuum tube that revolutionized the entire field
of electronics by allowing amplification of signals, both telegraphy
and voice. The first radio broadcast in the United States
was made in 1906, and within four years the first broadcast
from the Metropolitan Opera House was transmitted.
Wireless voice communications using amplitude modulation
(AM) was a reality. The ensuing years of the 1920s saw
tremendous growth in radio station broadcasting that brought
the possibility of real-time information to the public. Society
changed forever…again. The radio became a necessity for people
to communicate information and ideas over vast distances
without wires.
Of course, wires still had their place because radio was not
always the most reliable medium. The environment, weather,
time of day, and man-made interference could interrupt communications.
Telephone technology advanced steadily, and telegraphy still found a place in data communications in the
form of the telegram.
Radio technology advanced throughout the 1930s with the
notable invention of frequency modulation (FM), which provided
better sound quality and was more resistant to interference
than the older AM broadcasting system. One of the first
applications for FM was police radio; it was ideal for mobile
communications. Commercial FM broadcasting did not develop
until much later in the twentieth century. It should be noted
here that FM technology became the cornerstone of the analog
cellular system launched in 1983.
World War II accelerated the advancement of radio communications
and electronics. Transatlantic cables between Europe
and North America improved but we were still limited to realtime
communications by copper cables or high-frequency (HF)
radio spectrum under 30 megahertz. Data was still limited to
telegraphy or some analog signals representing data. This was
acceptable because demand for data was also low.
However, the post-war period saw an explosion of innovation
with the development of the transistor (December 1947)
and the birth of the computer. In the Moore School of
Engineering, ENIAC, the world’s first electronic, large-scale,
general-purpose computer, was activated at the University of
Pennsylvania in 1946. Unfortunately, the computer preceded
the transistor so ENIAC contained about 18,000 tubes. This
was much to the chagrin of the graduate students who had to
replace the burned out ones—often! Some refer to this as the
Birth of the Information Age, but we like to think of it as the
Re-Birth of the Information Society. Computers provided a tool
for people to process data, lots of data; now we needed a better
way to move that data faster.
The 1950s had many “Ages” to ponder, the Atomic Age, the
Information Age, and if that was not enough, another almost 100
years after the first transoceanic cable, another society-altering
event occurred, one that changed the way we communicate and,
perhaps even more so, the way we think globally. The Space Age
began with the launch of the Soviet satellite Sputnik on October
4, 1957. Satellite communications provided reliable long distance communications by augmenting or replacing cables. This
created the demand for reliable, anytime, anywhere communications.
The beginning of an idea for a truly mobile, global society
was planted; the capability to link people around the world
with nearly instantaneous voice and data communications was a
reality, but it was still fixed point-to-point communications.
The Space Age brought changes to the way we think and
the technology we create. It brought us integrated circuits,
fiber optics, photonics, ceramics, freeze-dried food, and ultimately
digital electronics. Digital technology enabled the creation
of computers, as we know them today, and the
transmission of data at higher speeds. It also provided wireless,
high-bandwidth communications. Communications satellites
and transoceanic cables—including technologically advanced
fiber optic cables with high bandwidth—continue to be
installed around the world.
It took almost 26 years after Sputnik before cellular communications
brought mobile voice communications to the
masses (at least those who could afford $4,500 for a mobile
phone in December of 1983). Mobile data took a couple more
years to become common, but speed and reliability remained
issues to its success. Outside the military, access to large databases
of information was still limited to commercial and educational
institutions with their internal mainframe computers.
(Because sharing this data over wireless connections has been
impractical, data networks have remained mostly wired.)
During the 1980s, no compelling need for wireless data transmission
existed. That was about to change…. Figure 1-1 illustrates
the communication timeline.
Graham Bell invented the telephone on March 10, 1876. His
experiments with his assistant Thomas Watson finally proved
successful when the first vocal sentence was transmitted:
“Watson, come here; I want you.” The telephone was demonstrated
to the world at the 1876 Centennial Exposition in
Philadelphia, Pennsylvania, and led to the creation of the Bell
Telephone Company in 1877.
By 1906 American inventor, Lee De Forest, invented a
three-element vacuum tube that revolutionized the entire field
of electronics by allowing amplification of signals, both telegraphy
and voice. The first radio broadcast in the United States
was made in 1906, and within four years the first broadcast
from the Metropolitan Opera House was transmitted.
Wireless voice communications using amplitude modulation
(AM) was a reality. The ensuing years of the 1920s saw
tremendous growth in radio station broadcasting that brought
the possibility of real-time information to the public. Society
changed forever…again. The radio became a necessity for people
to communicate information and ideas over vast distances
without wires.
Of course, wires still had their place because radio was not
always the most reliable medium. The environment, weather,
time of day, and man-made interference could interrupt communications.
Telephone technology advanced steadily, and telegraphy still found a place in data communications in the
form of the telegram.
Radio technology advanced throughout the 1930s with the
notable invention of frequency modulation (FM), which provided
better sound quality and was more resistant to interference
than the older AM broadcasting system. One of the first
applications for FM was police radio; it was ideal for mobile
communications. Commercial FM broadcasting did not develop
until much later in the twentieth century. It should be noted
here that FM technology became the cornerstone of the analog
cellular system launched in 1983.
World War II accelerated the advancement of radio communications
and electronics. Transatlantic cables between Europe
and North America improved but we were still limited to realtime
communications by copper cables or high-frequency (HF)
radio spectrum under 30 megahertz. Data was still limited to
telegraphy or some analog signals representing data. This was
acceptable because demand for data was also low.
However, the post-war period saw an explosion of innovation
with the development of the transistor (December 1947)
and the birth of the computer. In the Moore School of
Engineering, ENIAC, the world’s first electronic, large-scale,
general-purpose computer, was activated at the University of
Pennsylvania in 1946. Unfortunately, the computer preceded
the transistor so ENIAC contained about 18,000 tubes. This
was much to the chagrin of the graduate students who had to
replace the burned out ones—often! Some refer to this as the
Birth of the Information Age, but we like to think of it as the
Re-Birth of the Information Society. Computers provided a tool
for people to process data, lots of data; now we needed a better
way to move that data faster.
The 1950s had many “Ages” to ponder, the Atomic Age, the
Information Age, and if that was not enough, another almost 100
years after the first transoceanic cable, another society-altering
event occurred, one that changed the way we communicate and,
perhaps even more so, the way we think globally. The Space Age
began with the launch of the Soviet satellite Sputnik on October
4, 1957. Satellite communications provided reliable long distance communications by augmenting or replacing cables. This
created the demand for reliable, anytime, anywhere communications.
The beginning of an idea for a truly mobile, global society
was planted; the capability to link people around the world
with nearly instantaneous voice and data communications was a
reality, but it was still fixed point-to-point communications.
The Space Age brought changes to the way we think and
the technology we create. It brought us integrated circuits,
fiber optics, photonics, ceramics, freeze-dried food, and ultimately
digital electronics. Digital technology enabled the creation
of computers, as we know them today, and the
transmission of data at higher speeds. It also provided wireless,
high-bandwidth communications. Communications satellites
and transoceanic cables—including technologically advanced
fiber optic cables with high bandwidth—continue to be
installed around the world.
It took almost 26 years after Sputnik before cellular communications
brought mobile voice communications to the
masses (at least those who could afford $4,500 for a mobile
phone in December of 1983). Mobile data took a couple more
years to become common, but speed and reliability remained
issues to its success. Outside the military, access to large databases
of information was still limited to commercial and educational
institutions with their internal mainframe computers.
(Because sharing this data over wireless connections has been
impractical, data networks have remained mostly wired.)
During the 1980s, no compelling need for wireless data transmission
existed. That was about to change…. Figure 1-1 illustrates
the communication timeline.
HISTORY OF MODERN COMMUNICATIONS
It could be argued that the Information Age began in 1837
with the invention of the telegraph in the United States. The
first public telegraph was completed in 1844 and ran 64 km or
about 40 miles between Washington, D.C., and Baltimore,
Maryland. Obviously, Samuel B. Morse was aware of his place
in history when he transmitted the first message, “What hath
God wrought?”
Morse realized in that instant that communications
between individuals and nations had been dramatically altered.
Today we take the first steps toward another milestone—the
Wireless Internet. To understand the significance of a Wireless
Internet, we should look at some of the milestones along the
way. It has been said that “Rome wasn’t built in a day,” and the
Wireless Internet will not happen instantly either. It has taken
164 years to get this far—from the invention of the telegraph
to today’s Wireless Internet.
In this chapter we review a little of the history of wired and
wireless communications and the reason for its progress.
Technology can drive applications but sometimes, applications
create a need for new technology; thus it is with the Wireless
Internet—one must understand that having the capability
does not mean that the capability will be used. This chapter
introduces some of the technological achievements that will
culminate in the Wireless Internet:
• Voice and data communications
• Birth of the cellular telephone
• Wireless communication devices
• 2G and 3G cellular
• Technologies driving the Wireless Internet
with the invention of the telegraph in the United States. The
first public telegraph was completed in 1844 and ran 64 km or
about 40 miles between Washington, D.C., and Baltimore,
Maryland. Obviously, Samuel B. Morse was aware of his place
in history when he transmitted the first message, “What hath
God wrought?”
Morse realized in that instant that communications
between individuals and nations had been dramatically altered.
Today we take the first steps toward another milestone—the
Wireless Internet. To understand the significance of a Wireless
Internet, we should look at some of the milestones along the
way. It has been said that “Rome wasn’t built in a day,” and the
Wireless Internet will not happen instantly either. It has taken
164 years to get this far—from the invention of the telegraph
to today’s Wireless Internet.
In this chapter we review a little of the history of wired and
wireless communications and the reason for its progress.
Technology can drive applications but sometimes, applications
create a need for new technology; thus it is with the Wireless
Internet—one must understand that having the capability
does not mean that the capability will be used. This chapter
introduces some of the technological achievements that will
culminate in the Wireless Internet:
• Voice and data communications
• Birth of the cellular telephone
• Wireless communication devices
• 2G and 3G cellular
• Technologies driving the Wireless Internet
Classful Routing: Summarization at Boundary Routers
A question arises from the preceding discussion: How does a RIP process interpret the subnet of a major
network if it has no interfaces attached to that network? Without an interface on the class A, B, or C
network of the destination, the router has no way of knowing the correct subnet mask to use and therefore
no way of correctly identifying the subnet.
The solution is simple: If a router has no direct attachments to the network, then it needs only a single
route entry pointing toward a router that is directly attached.
NOTE
Boundary routers perform route summarization, also known as subnet hiding.
network if it has no interfaces attached to that network? Without an interface on the class A, B, or C
network of the destination, the router has no way of knowing the correct subnet mask to use and therefore
no way of correctly identifying the subnet.
The solution is simple: If a router has no direct attachments to the network, then it needs only a single
route entry pointing toward a router that is directly attached.
NOTE
Boundary routers perform route summarization, also known as subnet hiding.
Classful Routing: Directly Connected Subnets
Classful route lookups can be illustrated with three examples (referring to Figure 5.5):
1. If a packet with a destination address of 192.168.35.3 enters this router, no match for network
192.168.35.0 is found in the routing table and the packet is dropped.
2. If a packet with a destination address of 172.25.33.89 enters the router, a match is made to class B
network 172.25.0.0/24. The subnets listed for this network are then examined; no match can be
made for subnet 172.25.33.0, so that packet, too, is dropped.
3. Finally, a packet destined for 172.25.153.220 enters the router. This time 172.25.0.0/24 is
matched, then subnet 172.25.153.0 is matched, and the packet is forwarded to next-hop address
172.25.15.2.
Another look at Figure 5.3 reveals that there is no provision for RIP to advertise a subnet mask along with
each route entry. And accordingly, no masks are associated with the individual subnets in the routing
table. Therefore, if the router whose forwarding database is depicted in Figure 5.5 receives a packet with
a destination address of 172.25.131.23, there is no positive way to determine where the subnet bits end
and the host bits begin, or even if the address is subnetted at all.
The router's only recourse is to assume that the mask configured on one of its interfaces attached to
172.25.0.0 is used consistently throughout the internetwork. It will use its own mask for 172.25.0.0 to
derive the subnet of the destination address. As the routing tables throughout this chapter illustrate, a
router that is directly connected to a network will list the network in a heading along with the subnet mask
of the connecting interface and will then list all the known subnets of the network. If the network is not
directly connected, there is a listing only for the major-class network and no associated mask.
Because the destination addresses of packets being routed by a classful routing protocol are interpreted
according to the subnet masks locally configured on the router's interfaces, all subnet masks within a
major, class-level network must be consistent.
1. If a packet with a destination address of 192.168.35.3 enters this router, no match for network
192.168.35.0 is found in the routing table and the packet is dropped.
2. If a packet with a destination address of 172.25.33.89 enters the router, a match is made to class B
network 172.25.0.0/24. The subnets listed for this network are then examined; no match can be
made for subnet 172.25.33.0, so that packet, too, is dropped.
3. Finally, a packet destined for 172.25.153.220 enters the router. This time 172.25.0.0/24 is
matched, then subnet 172.25.153.0 is matched, and the packet is forwarded to next-hop address
172.25.15.2.
Another look at Figure 5.3 reveals that there is no provision for RIP to advertise a subnet mask along with
each route entry. And accordingly, no masks are associated with the individual subnets in the routing
table. Therefore, if the router whose forwarding database is depicted in Figure 5.5 receives a packet with
a destination address of 172.25.131.23, there is no positive way to determine where the subnet bits end
and the host bits begin, or even if the address is subnetted at all.
The router's only recourse is to assume that the mask configured on one of its interfaces attached to
172.25.0.0 is used consistently throughout the internetwork. It will use its own mask for 172.25.0.0 to
derive the subnet of the destination address. As the routing tables throughout this chapter illustrate, a
router that is directly connected to a network will list the network in a heading along with the subnet mask
of the connecting interface and will then list all the known subnets of the network. If the network is not
directly connected, there is a listing only for the major-class network and no associated mask.
Because the destination addresses of packets being routed by a classful routing protocol are interpreted
according to the subnet masks locally configured on the router's interfaces, all subnet masks within a
major, class-level network must be consistent.
Classful Routing
NOTE
RIPv1 can perform equal-cost load balancing.
The routing table in Figure 5.5 contains RIP-derived routes, which are recognized from the key to the left
of each entry. Of significance in these entries are the bracketed tuples; as discussed in Chapter 3, "Static
Routing," the first number is the administrative distance, and the second number is the metric. It is readily
seen that RIP has an administrative distance of 120, and as already stated, the metric for RIP is hop count.
Therefore, network 10.8.0.0 is 2 hops away, via either E0 or S1. If more than one route exists to the same
destination with equal hop counts, equal-cost load balancing will be performed. The routing table of
Figure 5.5 contains several multiple, equal-cost routes.
Figure 5.5. This routing table contains subnets of networks 10.0.0.0 and 172.25.0.0. All networks not
directly connected were derived by RIP.
When a packet enters a RIP-speaking router and a route table lookup is performed, the various choices in
the table are pruned until a single path remains. First, the network portion of the destination address is
read and the routing table is consulted for a match. It is this first step of reading the major class A, B, or C network number that defines a classful routing table lookup. If there is no match for the major network,
the packet is dropped and an ICMP Destination Unreachable message is sent to the packet's source. If
there is a match for the network portion, the subnets listed for that network are examined. If a match can
be found, the packet is routed. If a match cannot be made, the packet is dropped and a Destination
Unreachable message is sent.
NOTE
Definition of a classful route lookup
RIPv1 can perform equal-cost load balancing.
The routing table in Figure 5.5 contains RIP-derived routes, which are recognized from the key to the left
of each entry. Of significance in these entries are the bracketed tuples; as discussed in Chapter 3, "Static
Routing," the first number is the administrative distance, and the second number is the metric. It is readily
seen that RIP has an administrative distance of 120, and as already stated, the metric for RIP is hop count.
Therefore, network 10.8.0.0 is 2 hops away, via either E0 or S1. If more than one route exists to the same
destination with equal hop counts, equal-cost load balancing will be performed. The routing table of
Figure 5.5 contains several multiple, equal-cost routes.
Figure 5.5. This routing table contains subnets of networks 10.0.0.0 and 172.25.0.0. All networks not
directly connected were derived by RIP.
When a packet enters a RIP-speaking router and a route table lookup is performed, the various choices in
the table are pruned until a single path remains. First, the network portion of the destination address is
read and the routing table is consulted for a match. It is this first step of reading the major class A, B, or C network number that defines a classful routing table lookup. If there is no match for the major network,
the packet is dropped and an ICMP Destination Unreachable message is sent to the packet's source. If
there is a match for the network portion, the subnets listed for that network are examined. If a match can
be found, the packet is routed. If a match cannot be made, the packet is dropped and a Destination
Unreachable message is sent.
NOTE
Definition of a classful route lookup
Request Message Types
A RIP Request message may request either a full routing table or information on specific routes only. In
the former case, the Request message will have a single route entry in which the address family identifier
is set to zero, the address is all zeros (0.0.0.0), and the metric is 16. A device receiving such a request
responds by unicasting its full routing table to the requesting address, honoring such rules as split horizon
and boundary summarization (discussed in "Classful Routing: A Summarization at Boundary Routers,"
later in this chapter).
Some diagnostic processes may need to know information about a specific route or routes. In this case, a
Request message may be sent with entries specifying the addresses in question. A device receiving this request will process the entries one-by-one, building a Response message from the Request message. If
the device has an entry in its routing table corresponding to an address in the request, it will enter the
metric of its own route entry into the metric field. If not, the metric field will be set to 16. The response
will tell exactly what the router knows, with no consideration given to split horizon or boundary
summarization.
As noted previously, hosts may run RIP in silent mode. This approach allows them to keep their routing
tables up-to-date by listening to RIP updates from routers without having to send RIP Response messages
uselessly on the network. However, diagnostic processes may need to examine the routing table of these
silent hosts. Therefore, RFC 1058 specifies that if a silent host receives a request from a UDP port other
than the standard RIP port of 520, the host must send a response.
the former case, the Request message will have a single route entry in which the address family identifier
is set to zero, the address is all zeros (0.0.0.0), and the metric is 16. A device receiving such a request
responds by unicasting its full routing table to the requesting address, honoring such rules as split horizon
and boundary summarization (discussed in "Classful Routing: A Summarization at Boundary Routers,"
later in this chapter).
Some diagnostic processes may need to know information about a specific route or routes. In this case, a
Request message may be sent with entries specifying the addresses in question. A device receiving this request will process the entries one-by-one, building a Response message from the Request message. If
the device has an entry in its routing table corresponding to an address in the request, it will enter the
metric of its own route entry into the metric field. If not, the metric field will be set to 16. The response
will tell exactly what the router knows, with no consideration given to split horizon or boundary
summarization.
As noted previously, hosts may run RIP in silent mode. This approach allows them to keep their routing
tables up-to-date by listening to RIP updates from routers without having to send RIP Response messages
uselessly on the network. However, diagnostic processes may need to examine the routing table of these
silent hosts. Therefore, RFC 1058 specifies that if a silent host receives a request from a UDP port other
than the standard RIP port of 520, the host must send a response.
RIP Message Format
The RIP message format is shown in Figure 5.3. Each message contains a command and a version
number and can contain entries for up to 25 routes. Each route entry includes an address family identifier,
the IP address reachable by the route, and the hop count for the route. If a router must send an update with
more than 25 entries, multiple RIP messages must be produced. Note that the initial portion of the
message is four octets, and each route entry is 20 octets. Therefore the maximum message size is 4 + (25
X 20) = 504 octets. Including an eight-byte UDP header will make the maximum RIP datagram size (not
including the IP header) 512 octets.
Command will always be set to either one, signifying a Request message, or two, signifying a Response
message. There are other commands, but they are all either obsolete or reserved for private use.
Version will be set to one for RIPv1.
Address Family Identifier is set to two for IP. The only exception to this is a request for a router's (or
host's) full routing table, as discussed in the following section.
IP Address is the address of the destination of the route. This entry may be a major network address, a
subnet, or a host route. The section titled "Classful Route Lookups" examines how RIP distinguishes
among these three types of entries.
Metric is, as previously mentioned , a hop count between 1 and 16.
Several historical influences contributed to the inelegant format of the RIP message in which far more bit
spaces are unused than are used. These influences range from RIP's original development as an XNS
protocol and the developer's intentions for it to adapt to a large set of address families to the influence of
BSD, and its use of socket addresses to the need for fields to fall on 32-bit word boundaries.
number and can contain entries for up to 25 routes. Each route entry includes an address family identifier,
the IP address reachable by the route, and the hop count for the route. If a router must send an update with
more than 25 entries, multiple RIP messages must be produced. Note that the initial portion of the
message is four octets, and each route entry is 20 octets. Therefore the maximum message size is 4 + (25
X 20) = 504 octets. Including an eight-byte UDP header will make the maximum RIP datagram size (not
including the IP header) 512 octets.
Command will always be set to either one, signifying a Request message, or two, signifying a Response
message. There are other commands, but they are all either obsolete or reserved for private use.
Version will be set to one for RIPv1.
Address Family Identifier is set to two for IP. The only exception to this is a request for a router's (or
host's) full routing table, as discussed in the following section.
IP Address is the address of the destination of the route. This entry may be a major network address, a
subnet, or a host route. The section titled "Classful Route Lookups" examines how RIP distinguishes
among these three types of entries.
Metric is, as previously mentioned , a hop count between 1 and 16.
Several historical influences contributed to the inelegant format of the RIP message in which far more bit
spaces are unused than are used. These influences range from RIP's original development as an XNS
protocol and the developer's intentions for it to adapt to a large set of address families to the influence of
BSD, and its use of socket addresses to the need for fields to fall on 32-bit word boundaries.
RIP Timers and Stability Features
After startup, the router gratuitously sends a Response message out every RIP-enabled interface every 30
seconds, on average. The Response message, or update, contains the router's full routing table with the
exception of entries suppressed by the split horizon rule. The update timer initiating this periodic update
includes a random variable to prevent table synchronization.[6] As a result, the time between individual
updates from a typical RIP process may be from 25 to 35 seconds. The specific random variable used by
Cisco IOS, RIP_JITTER, subtracts up to 15% (4.5 seconds) from the update time. Therefore, updates
from Cisco routers vary between 25.5 and 30 seconds (Figure 5.1). The destination address of the update
is the all-hosts broadcast 255.255.255.255.
Several other timers are employed by RIP. Recall from Chapter 4, "Dynamic Routing Protocols," the
invalidation timer, which distance vector protocols use to limit the amount of time a route can stay in a
routing table without being updated. RIP calls this timer the expiration timer, or timeout. Cisco's IOS
calls it the invalid timer. The expiration timer is initialized to 180 seconds whenever a new route is
established and is reset to the initial value whenever an update is heard for that route. If an update for a
route is not heard within that 180 seconds (six update periods), the hop count for the route is changed to
16, marking the route as unreachable.
Another timer, the garbage collectionor flushtimer, is set to 240 seconds–60 seconds longer than the
expiration time.[8] The route will be advertised with the unreachable metric until the garbage collection
timer expires, at which time the route is removed from the routing table. Figure 5.2 shows a routing table
in which a route has been marked as unreachable, but has not yet been flushed.
This command applies to the entire RIP process. If the timing of one router is changed, the timing of all
the routers in the RIP domain must be changed. Therefore, these timers should not be changed from their
default values without a specific, carefully considered reason.
RIP employs split horizon with poison reverse and triggered updates. A triggered update occurs whenever
the metric for a route is changed and, unlike regularly scheduled updates, may include only the entry or
entries that changed. Also unlike regular updates, a triggered update does not cause the receiving router to
reset its update timer; if it did, a topology change could cause many routers to reset at the same time and
thus cause the periodic updates to become synchronized. To avoid a "storm" of triggered updates after a
topology change, another timer is employed. When a triggered update is transmitted, this timer is
randomly set between 1 and 5 seconds; subsequent triggered updates cannot be sent until the timer
expires.
NOTE
Silent hosts
Some hosts may employ RIP in a "silent" mode. These so-called silent hosts do not generate RIP updates,
but listen for them and update their internal routing tables accordingly. As an example, using routed with
the -q option enables RIP in silent mode on a UNIX host.
seconds, on average. The Response message, or update, contains the router's full routing table with the
exception of entries suppressed by the split horizon rule. The update timer initiating this periodic update
includes a random variable to prevent table synchronization.[6] As a result, the time between individual
updates from a typical RIP process may be from 25 to 35 seconds. The specific random variable used by
Cisco IOS, RIP_JITTER, subtracts up to 15% (4.5 seconds) from the update time. Therefore, updates
from Cisco routers vary between 25.5 and 30 seconds (Figure 5.1). The destination address of the update
is the all-hosts broadcast 255.255.255.255.
Several other timers are employed by RIP. Recall from Chapter 4, "Dynamic Routing Protocols," the
invalidation timer, which distance vector protocols use to limit the amount of time a route can stay in a
routing table without being updated. RIP calls this timer the expiration timer, or timeout. Cisco's IOS
calls it the invalid timer. The expiration timer is initialized to 180 seconds whenever a new route is
established and is reset to the initial value whenever an update is heard for that route. If an update for a
route is not heard within that 180 seconds (six update periods), the hop count for the route is changed to
16, marking the route as unreachable.
Another timer, the garbage collectionor flushtimer, is set to 240 seconds–60 seconds longer than the
expiration time.[8] The route will be advertised with the unreachable metric until the garbage collection
timer expires, at which time the route is removed from the routing table. Figure 5.2 shows a routing table
in which a route has been marked as unreachable, but has not yet been flushed.
This command applies to the entire RIP process. If the timing of one router is changed, the timing of all
the routers in the RIP domain must be changed. Therefore, these timers should not be changed from their
default values without a specific, carefully considered reason.
RIP employs split horizon with poison reverse and triggered updates. A triggered update occurs whenever
the metric for a route is changed and, unlike regularly scheduled updates, may include only the entry or
entries that changed. Also unlike regular updates, a triggered update does not cause the receiving router to
reset its update timer; if it did, a topology change could cause many routers to reset at the same time and
thus cause the periodic updates to become synchronized. To avoid a "storm" of triggered updates after a
topology change, another timer is employed. When a triggered update is transmitted, this timer is
randomly set between 1 and 5 seconds; subsequent triggered updates cannot be sent until the timer
expires.
NOTE
Silent hosts
Some hosts may employ RIP in a "silent" mode. These so-called silent hosts do not generate RIP updates,
but listen for them and update their internal routing tables accordingly. As an example, using routed with
the -q option enables RIP in silent mode on a UNIX host.
Operation of RIP
NOTE
The metric for RIP is hop count.
The RIP process operates from UDP port 520; all RIP messages are encapsulated in a UDP segment with
both the Source and Destination Port fields set to that value. RIP defines two message types: Request
messages and Response messages. A Request message is used to ask neighboring routers to send an
update. A Response message carries the update. The metric used by RIP is hop count, with 1 signifying a
directly connected network of the advertising router and 16 signifying an unreachable network.
On startup, RIP broadcasts a packet carrying a Request message out each RIP-enabled interface. The RIP
process then enters a loop, listening for RIP Request or Response messages from other routers. Neighbors
receiving the Request send a Response containing their routing table.
When the requesting router receives the Response messages, it processes the enclosed information. If a
particular route entry included in the update is new, it is entered into the routing table along with the
address of the advertising router, which is read from the source address field of the update packet. If the
route is for a network that is already in the table, the existing entry will be replaced only if the new route
has a lower hop count. If the advertised hop count is higher than the recorded hop count and the update
was originated by the recorded next-hop router, the route will be marked as unreachable for a specified
holddown period. If at the end of that time the same neighbor is still advertising the higher hop count, the
new metric will be accepted.
The metric for RIP is hop count.
The RIP process operates from UDP port 520; all RIP messages are encapsulated in a UDP segment with
both the Source and Destination Port fields set to that value. RIP defines two message types: Request
messages and Response messages. A Request message is used to ask neighboring routers to send an
update. A Response message carries the update. The metric used by RIP is hop count, with 1 signifying a
directly connected network of the advertising router and 16 signifying an unreachable network.
On startup, RIP broadcasts a packet carrying a Request message out each RIP-enabled interface. The RIP
process then enters a loop, listening for RIP Request or Response messages from other routers. Neighbors
receiving the Request send a Response containing their routing table.
When the requesting router receives the Response messages, it processes the enclosed information. If a
particular route entry included in the update is new, it is entered into the routing table along with the
address of the advertising router, which is read from the source address field of the update packet. If the
route is for a network that is already in the table, the existing entry will be replaced only if the new route
has a lower hop count. If the advertised hop count is higher than the recorded hop count and the update
was originated by the recorded next-hop router, the route will be marked as unreachable for a specified
holddown period. If at the end of that time the same neighbor is still advertising the higher hop count, the
new metric will be accepted.
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