Transcript How GPS Work Part 2 - ECE - Lehigh University

How GPS Works
Part 2
LUCID Summer Workshop
July 30, 2004
Outline for Today

We spend a little more time reviewing the GPS
system today.

Once we finish up GPS, we will switch gears and
gain a basic understanding of how the Internet
works.

This will be helpful for our WiFi discussion next
week.
Recall: What it is

GPS: Global Positioning System is a worldwide
radio-navigation system formed from a constellation
of 24 satellites and their ground stations.

Uses the principle of
triangulation and timeof-arrival of signals to
determine the location
of a GPS receiver.
Typical GPS Applications



Location - determining a basic position
Navigation - getting from one location to another
Tracking - monitoring the movement of people and
things.

Mapping - creating maps of the world

Timing - bringing precise timing to the world
GPS Triangulation Procedure
Triangulation Requirements

To triangulate, a GPS receiver measures distance
using the travel time of radio signals.

To measure travel time, GPS receiver needs very
accurate timing.

Along with distance, receiver need accurate data on
where satellites are in space.

System will also need to correct for any delays the
signal experiences as it travels through atmosphere.
Components of GPS System

Control Segment: five ground stations located on
earth.

Space Segment: satellite constellation (24 active
satellites in space).

User Segment: GPS receiver units that receive
satellite signals and determine receiver location from
them.
Ground Monitor Stations
Falcon AFB
Colorado Springs, CO
Master Control Monitor Station
Kwajalein
Monitor Station
Hawaii
Monitor Station
Ascension Island
Monitor Station
Diego Garcia
Monitor Station
Basic Functions of Monitor
Stations

These stations are the eyes and ears of GPS,
monitoring satellites as they pass overhead by
measuring distances to them every 1.5 seconds

This data is then smoothed using ionospheric and
meteorological information and sent to Master
Control Station at Colorado Springs.

The ionospheric and meteorological data is needed
to get more accurate delay measurements, which in
turn improve location estimation.
Functions of Monitor Stations
(Cont’d)

Master control station estimates parameters
describing satellites' orbit and clock performance,. It
also assesses health status of the satellites and
determines if any re-positioning may be required.

This information is then returned to three uplink
stations (collocated at the Ascension Island, Diego
Garcia and Kwajalein monitor stations) which
transmit the information to satellites.
Space Segment

Space segment is the satellite constellation.

24 satellites with a minimum of 21 operating 98%
of the time
6 Orbital planes
Circular orbits
20-200 km above the Earth's surface
11 hours 58 minute orbital period
Visible for approximately 5 hours above the
horizon





GPS Satellite Orbits

We can obtain updates of GPS satellites at
http://www.ngs.noaa.gov/GPS/GPS.html
GPS Satellite Orbits (Cont’d)

Orbits of GPS satellites need to be updated every
once in a while because orbit does not stay circular
without adjustments.

Adjustments needed because:
 Other objects exert gravitational force on each
satellite (e.g. sun, moon)
 Effect of gravity is non-uniform during orbit.
 Radiation pressure (due to solar radiation).
 Atmospheric drag
 Other effects
Interesting Aside on GPS
Orbits

When GPS satellites are decommissioned, they are
placed on a disposal orbit (outside the operating
GPS orbit).

Some studies show that satellites in disposal orbits
can eventually, perhaps over 20-40 years, encroach
into operating constellation.
Aside (Cont’d)

This is because disposal orbits, while circular
initially, become increasingly elliptical, mostly as
result of sun-moon gravitational perturbations.

Besides intersecting GPS constellation, these
satellites eventually could pose a threat to
operational satellites in low Earth and
geosynchronous orbits
Aside (Cont’d)
Aside (Cont’d)

Similar threats posed by other satellite systems.

The Russian Glonass constellation, a navigation
system similar to GPS, will also experience orbit
eccentricity growth and may pose a collision risk to
itself and GPS.
Glonass, which has about 100 failed satellites
within its constellation, is located about 1,000
kilometers (621 miles) lower than GPS and could
pose a collision problem in 40 years, the studies
show.

Aside (Cont’d)

Galileo satellites also may pose a threat to GPS.

Galileo is Europe’s own global navigation satellite
system.

First experimental satellite will be launched in
second half of 2005.

Galileo will be under civilian control.
Third Component of GPS: User
Segment

User segment comprises receivers that have been
designed to decode signals transmitted from
satellites for purposes of determining position,
velocity or time.

Receiver must perform the following tasks:
 select one or more satellites in view
 acquire GPS signals
 measure and track signal
 recover navigational data
Important Terminology

Satellite transmits Ephemeris and Almanac Data to
GPS receivers.

Ephemeris data contains important information
about status of satellite (healthy or unhealthy),
current date and time. This part of signal is essential
for determining a position.

Almanac data tells GPS receiver where each GPS
satellite should be at any time throughout day. Each
satellite transmits almanac data showing orbital
information for that satellite and for every other
satellite in the system.
Measuring Time Of Arrival
(TOA) in GPS
TOA Concept

GPS uses concept of time of arrival (TOA) of signals
to determine user position.

This involves measuring time it takes for a signal
transmitted by an emitter (satellite) at a known
location to reach a user receiver.

Time interval is basically signal propagation time.
TOA Concept (Cont’d)

Time interval (signal propagation time) is multiplied
by speed of signal (speed of light) to obtain satellite
to receiver distance.

By measuring propagation time of signals broadcast
from multiple satellites at known locations, receiver
can determine its position.

Assuming we have precise clocks, how do we
measure signal travel time?
Measuring Distance using a
PRC Signal

At a particular time (let's say midnight), the satellite
begins transmitting a long, digital pattern called a
pseudo-random code (PRC).

The receiver begins running the same digital pattern
also exactly at midnight.

When the satellite's signal reaches the receiver, its
transmission of the pattern will lag a bit behind the
receiver's playing of the pattern.
Measuring Distance



The length of the delay is equal to the signal's travel
time.
The receiver multiplies this time by the speed of light
to determine how far the signal traveled.
Assuming the signal traveled
in a straight line, this is the
distance from receiver to
satellite.
Synchronizing Clocks

In order to make this measurement, the receiver and
satellite both need clocks that can be synchronized
down to the nanosecond.

Accurate time measurements are required. If we
are off by a thousandth of a second, at the speed of
light, that translates into almost 200 miles of error.
Synchronizing Clocks (Cont’d)

To make a satellite positioning system using only
synchronized clocks, you would need to have atomic
clocks not only on all the satellites, but also in the
receiver itself.

But atomic clocks cost somewhere between $50,000
and $100,000, which makes them a just a bit too
expensive for everyday consumer use.

The Global Positioning System has a clever, solution
to this problem. Every satellite contains an
expensive atomic clock, but the receiver itself uses
an ordinary quartz clock, which it constantly resets.
Synchronizing Clocks (Cont’d)

The Global Positioning System has a clever,
effective solution to this problem.

Every satellite contains an expensive atomic clock,
but the receiver itself uses an ordinary quartz clock,
which it constantly resets.

In a nutshell, the receiver looks at incoming signals
from four or more satellites and gauges its own
inaccuracy.
Synchronizing Clocks (Cont’d)



When you measure the distance to four located
satellites, you can draw four spheres that all
intersect at one point.
Three spheres will intersect even if your numbers
are way off, but four spheres will not intersect at one
point if you've measured incorrectly.
Since the receiver makes all its distance
measurements using its own built-in clock, the
distances will all be proportionally incorrect.
Synchronizing Clocks (Cont’d)

The receiver can easily calculate the necessary
adjustment that will cause the four spheres to
intersect at one point.

Based on this, it resets its clock to be in sync with
the satellite's atomic clock.

The receiver does this constantly whenever it's on,
which means it is nearly as accurate as the
expensive atomic clocks in the satellites.
Synchronizing Clocks (Cont’d)

The receiver can easily calculate the necessary
adjustment that will cause the four spheres to
intersect at one point.

Based on this, it resets its clock to be in sync with
the satellite's atomic clock.

The receiver does this constantly whenever it's on,
which means it is nearly as accurate as the
expensive atomic clocks in the satellites.
Knowing Satellite Locations

In order to properly synchronize clocks and figure
out which PRC signal to listen to, the receiver has to
know where the satellites actually are.

This isn't particularly difficult because the satellites
travel in very high and predictable orbits.
Using Almanac Information

The GPS receiver simply stores an almanac that
tells it where every satellite should be at any given
time.

Things like the pull of the moon and the sun do
change the satellites' orbits very slightly.

However, the Department of Defense constantly
monitors their exact positions and transmits any
adjustments to all GPS receivers as part of the
satellites' signals.
2 Types of Errors

Errors can be categorized as intentional and
unintentional.

Intentional errors: government can and does
degrade accuracy of GPS measurements. This is
done to prevent hostile forces from using GPS to full
accuracy.

Policy of inserting inaccuracies in GPS signals is
called Selective Ability (SA). SA was single biggest
source of inaccuracy in GPS. SA was deactivated in
2000.
Sources of Unintentional
Timing Errors
Typical Errors
Source of Error
Satellite Clocks
Orbit Errors
Ionosphere
Troposphere
Receiver Noise
Multipath
SA
Typical Error in Meters
(per satellite)
1.5
2.5
5.0
0.5
0.3
0.6
30
Differential GPS

Technique called differential correction can yield
accuracies within 1-5 meters, or even better, with
advanced equipment.

Differential correction requires a second GPS
receiver, a base station, collecting data at a
stationary position on a precisely known point.

Because physical location of base station is known,
a correction factor can be computed by comparing
known location with GPS location determined by
using satellites.
Improved Offered by
Differential GPS
Source
Uncorrected
With Differential
Ionosphere
Troposphere
Signal Noise
Orbit Data
Clock Drift
Multipath
Receiver Noise
SA
0-30 meters
0-30 meters
0-10 meters
1-5 meters
0-1.5 meters
0-1 meters
~1 meter
0-70 meters
Mostly Removed
All Removed
All Removed
All Removed
All Removed
Not Removed
Not Removed
All Removed
Using GPS Data

A GPS receiver essentially determines the receiver's
position on Earth.

Once the receiver makes this calculation, it can tell
you the latitude, longitude and altitude of its current
position. To make the
navigation more userfriendly, most receivers
plug this raw data into
map files stored in
memory.
Using GPS Data (Cont’d)

You can
 use maps stored in the receiver's memory,
 connect the receiver to a computer that can hold
more detailed maps in its memory, or
 simply buy a detailed map of your area and find
your way using the receiver's latitude and
longitude readouts.

Some receivers let you download detailed maps into
memory or supply detailed maps with plug-in map
cartridges.
Using GPS Data (Cont’d)

A standard GPS receiver will not only place you on a
map at any particular location, but will also trace
your path across a map as you move.

If you leave your receiver on, it can stay in constant
communication with GPS satellites to see how your
location is changing.

This is what happens in cars equipped with GPS.
Using GPS Data
With this information and its built-in clock, the
receiver can give you several pieces of valuable
information:
 How far you've traveled (odometer)
 How long you've been traveling
 Your current speed (speedometer)
 Your average speed
 A "bread crumb" trail showing you exactly where
you have traveled on the map
 The estimated time of arrival at your destination if
you maintain your current speed
Internet
Background

One of the greatest things about the Internet is that
nobody really owns it.

It is a global collection of networks, both big and
small.

These networks connect together in many different
ways to form the single entity that we know as the
Internet. In fact, the very name comes from this idea
of interconnected networks.
The Internet Concept
The Internet Concept (Cont’d)
Background (Cont’d)

Since its beginning in 1969, the Internet has grown
from four host computer systems to tens of millions.

However, just because nobody owns the Internet, it
doesn't mean it is not monitored and maintained in
different ways.

The Internet Society, a non-profit group established
in 1992, oversees the formation of the policies and
protocols that define how we use and interact with
the Internet.
Outline for Remainder of
Slides

In the next few slides, we will review basic
underlying structure of the Internet.

We will learn about domain name servers, network
access points and backbones.

First, we review how your computer connects to
others.
Network of Networks

Every computer that is connected to the Internet is
part of a network, even the one in your home.

For example, you may use a modem and dial a local
number to connect to an Internet Service Provider
(ISP).

At school/work, you may be part of a local area
network (LAN), but you most likely still connect to
the Internet using an ISP that your school/company
has contracted with.
Network of Networks (Cont’d)

When you connect to your ISP, you become part of
their network.

The ISP may then connect to a larger network and
become part of their network.

The Internet is simply a network of networks.
Point of Presence

Most large communications companies have their
own dedicated backbones connecting various
regions.

In each region, the company has a Point of
Presence (POP).

The POP is a place for local users to access the
company's network, often through a local phone
number or dedicated line.
Hierarchy of Network


The amazing thing here
is that there is no
overall controlling
network.
Instead, there are
several high-level
networks connecting to
each other through
Network Access
Points or NAPs.
A Network Example

Imagine that Company A is a large ISP. In each
major city, Company A has a POP.

The POP in each city is a rack full of modems that
the ISP's customers dial into.

Company A leases fiber optic lines from the phone
company to connect the POPs together
Fiber Optic
Connections
POP
POP
…
POP
A Network Example (Cont’d)

Imagine that Company B is a corporate ISP.

Company B builds large buildings in major cities and
corporations locate their Internet server machines in
these buildings.

Company B is such a large company that it runs its
own fiber optic lines between its buildings so that
they are all interconnected.
Building 1
Building 2
…
Building N
Example (Cont’d)

In this arrangement, all of Company A's customers
can talk to each other, and all of Company B's
customers can talk to each other.

There is no way for Company A's customers and
Company B's customers to intercommunicate.

Therefore, Company A and Company B both agree
to connect to NAPs in various cities, and traffic
between the two companies flows between the
networks at the NAPs.
Connecting Networks

In the real Internet, dozens of large Internet
providers interconnect at NAPs in various cities, and
trillions of bytes of data flow between the individual
networks at these points.

The Internet is a collection of huge corporate
networks that agree to all intercommunicate with
each other at the NAPs.

In this way, every computer on the Internet connects
to every other.
Connecting Your Computer to
the Internet

Up until just a few years ago, there was really only
one way to connect to the Internet, dial-up.

Connection speed bottlenecks were simply
determined by the call letters (speed) of your PC’s
modem – 14.4Kbps, 28.8Kbps, 56Kbps, etc.

Well, now there are several, easily available options
for getting online, both at home and in the office.
Summary of Various
Connection Options
Connection
Dial-up
Cable Modem
ISDN
DSL
T1
T3
Download Upload
Cost/Month
56Kbps
56Kbps
$0-20
15-50Mbps 128Kbps
$30-70
128Kbps
128Kbps
$25-70
6-8.5Mbps 128Kbps
$0-80
1-10Mbps
1-10Mbps
$300+
40-100Mbps 40-100Mbps $1000+
Installation
$0
$0-100
$100-300
$0-250
$400+
NA
Availability
Universal
Limited
Universal
Limited
Fairly Universal
Fairly Universal
Connecting a Network of
Networks

All the networks that make up the Internet rely on
NAPs, backbones and routers to talk to each other.

What is incredible about this process is that a
message can leave one computer and travel
halfway across the world through several different
networks and arrive at another computer in a
fraction of a second!
Routers

Routers determine where to send information from
one computer to another.

Routers are specialized computers that send your
messages and those of every other Internet user
speeding to their destinations along thousands of
pathways.
Cable/DSL Router
Wireless Router
Industrial Router
Routers (Cont’d)

A router has two separate, but related, jobs:

It ensures that information doesn't go where it's
not needed. This is crucial for keeping large
volumes of data from clogging the connections of
"innocent bystanders."

It makes sure that information does make it to the
intended destination.
Routers (Cont’d)




A router is extremely useful in dealing with two
separate computer networks.
It joins the two networks, passing information from
one to the other. It also protects the networks from
one another, preventing the traffic on one from
unnecessarily spilling over to the other.
Regardless of how many networks are attached, the
basic operation and function of the router remains
the same.
Since the Internet is one huge network made up of
tens of thousands of smaller networks, its use of
routers is an absolute necessity.
Router (Cont’d)
Backbones

Backbones are high-speed lines that connect
networks together.

Backbones are typically fiber optic trunk lines. The
trunk line has multiple fiber optic cables combined
together to increase the capacity.

Fiber optic cables are designated OC for optical
carrier, such as OC-3, OC-12 or OC-48. An OC-3
line is capable of transmitting 155 Mbps while an
OC-48 can transmit 2,488 Mbps (2.488 Gbps).
Backbones (Cont’d)

Today there are many companies that operate their
own high-capacity backbones, and all of them
interconnect at various NAPs around the world.

In this way, everyone on the Internet, no matter
where they are and what company they use, is able
to talk to everyone else on the planet.
IP Addresses

Every machine on the Internet has a unique
identifying number, called an IP Address.

The IP stands for Internet Protocol, which is the
language that computers use to communicate over
the Internet.

A protocol is the pre-defined way that someone who
wants to use a service talks with that service. The
"someone" could be a person, but more often it is a
computer program like a Web browser.
IP Addresses (Cont’d)

A typical IP address looks like this:
216.27.61.137

To make it easier for us humans to remember, IP
addresses are normally expressed in decimal format
as a dotted decimal number like the one above. But
computers communicate in binary form. The same
IP address in binary:
11011000.00011011.00111101.10001001
IP Addresses (Cont’d)

It can be shown that there are a possible of
4,294,967,296 unique IP address values!

Of these about 4.3 billion possibilities, certain values
are restricted from use as typical IP addresses.

The values indicate the network the machine is on
and the identifier for the machine in that network.
Domain Name System

IP addresses are difficult to remember, especially
with so many of them.

In 1983, the University of Wisconsin created the
Domain Name System (DNS), which maps text
names to IP addresses automatically.

So, if you want to connect to the main machine at
Lehigh University, you connect to lehigh.edu. The
DNS maps this text to the binary IP address value.
Clients and Servers




Internet servers make the Internet possible. All of
the machines on the Internet are either servers or
clients.
The machines that provide services to other
machines are servers.
The machines that are used to connect to those
services are clients.
There are Web servers, e-mail servers, FTP servers
and so on serving the needs of Internet users all
over the world.
Clients and Servers (Cont’d)



When you connect to www.cnn.com to read a page,
you are a user sitting at a client's machine.
You are accessing the cnn Web server. The server
machine finds the page you requested and sends it
to you.
Clients that come to a server machine do so with a
specific intent, so clients direct their requests to a
specific software server running on the server
machine. For example, if you are running a Web
browser on your machine, it will want to talk to the
Web server on the server machine, not the e-mail
server.
Clients and Servers (Cont’d)




A server has a static IP address that does not
change very often.
A home machine that is dialing up through a
modem, on the other hand, typically has an IP
address assigned by the ISP every time you dial in.
That IP address is unique for your session -- it may
be different the next time you dial in.
This way, an ISP only needs one IP address for
each modem it supports, rather than one for each
customer.
Ports

Any server machine makes its services available
using numbered ports -- one for each service that is
available on the server.

For example, if a server machine is running a Web
server and a file transfer protocol (FTP) server, the
Web server would typically be available on port 80,
and the FTP server would be available on port 21.

Clients connect to a service at a specific IP address
and on a specific port number.
Packets

It turns out that everything you do on the Internet
involves packets.

For example, every Web page that you receive
comes as a series of packets, and every e-mail you
send leaves as a series of packets.

Networks that ship data around in small packets are
called packet switched networks.
Packets (Cont’d)

On the Internet, the network breaks an e-mail
message into parts of a certain size in bytes. These
are the packets.

Each packet carries the information that will help it
get to its destination -- the sender's IP address, the
intended receiver's IP address, something that tells
the network how many packets this e-mail message
has been broken into and the number of this
particular packet.
Packets (Cont’d)

The packets carry the data in the protocols that the
Internet uses: Transmission Control
Protocol/Internet Protocol (TCP/IP). More on this
later.

Each packet contains part of the body of your
message. A typical packet contains perhaps 1,000
or 1,500 bytes.
Shipping Packets from Source
to Destination

Each packet is then sent off to its destination by the
best available route -- a route that might be taken by
all the other packets in the message or by none of
the other packets in the message.

This (packet switching) makes the network more
efficient.
Benefits to Packet Switching

First, the network can balance the load across
various pieces of equipment on a millisecond-bymillisecond basis.

Second, if there is a problem with one piece of
equipment in the network while a message is being
transferred, packets can be routed around the
problem, ensuring the delivery of the entire
message.
Parts of Packets
Packets are split into three parts:



Header - contains instructions about the data
carried by the packet.
Payload – contains actual data that the packet is
delivering to the destination
Trailer - typically contains a couple of bits that tell
the receiving device that it has reached the end of
the packet. It may also have some type of error
checking. This error checking allows destination
to confirm the contents of the packets have
reached without errors.
Motivating Protocols

How the packet is exactly constructed (what goes
where in the header, payload, and trailer) depends
on the protocols adopted by the network.

A protocol is basically a common language enables
two people to understand what the other person
means.

Next, we review network protocols that enable one
user (or service) to communicate with a service (or
another user).
Network Protocols
Protocols and Protocol Layers

Two devices exchanging information need to follow
some simple rules or protocols so that information
can be interpreted correctly.

A network protocol gives a set of rules that are to be
followed by entities (machines) situated on different
parts of a network.

These protocols can be listed in order. The resulting
order can be used to defined protocol layers.
Protocol Layers

To communicate from information from one machine
to another, data has to be prepared in a special
format.

Think of protocol layers as an assembly line.

At each layer, certain things happen to the data that
prepare it for the next layer.

To understand this concept, let’s look at 3 layer
communication between two philosophers.
Layering Example


Assume there are two philosophers, A and B.
Philosopher A is in the U.S. and B is in France.

Philosopher A has thoughts (in English) and wishes
to communicate them to philosopher B, who only
understands French.

Clearly the data (the thought) has to be properly
prepared at Philosopher A’s office and sent to
Philosopher B’s office. There the information has to
be processed and conveyed to philosopher B in the
language he understands.
Layering Example

Assume no one in philosopher A’s office speaks
French and no one in philosopher B’s office speaks
English.

Assume that a translator and a secretary work at
each philosopher’s office.

Somehow an agreement had to have been
established between Philosopher A and B so that
they can talk to each other.
Layering Example (Cont’d)

The contents of this agreement are the protocols of
this communication link.

From these protocols, we will see that an assembly
line is constructed at both Philosopher A’s office and
Philosopher B’s office.

This assembly line will give us the protocol layer.
Philosopher-TranslatorSecretary Architecture
Location A
Dest: B
I like
rabbits
L: Dutch
Dest: B
Ik hou
Van
konijnen
Fax # -L: Dutch
Dest: B
Ik hou
Van
konijnen
Fax
Location B
Message
Information
For the remote
translator
Message
for the remote
secretary
J’aime
les
lapins
Philospher
L: Dutch
Dest: B
Ik hou
Van
konijnen
Translator
Fax # -L: Dutch
Dest: B
Ik hou
Van
konijnen
Secretary
Fax
Examining Layering Example

This communication architecture has three layers, at
each end of the communication link.

In the first layer, the philosopher generates a
thought. He/she decides this thought should be
conveyed to philosopher B (whose office may
employ several philosophers).

He/she writes this thought on paper and indicates
on it the “destination” of this message. He/she then
sends it to a translator.
Examining Layering Example
(Cont’d)

In the second layer, the translator looks at the
destination of the message and realizes that the
destination office does not speak English.

The translator then determines a common language
between the two offices, Dutch.

He/she converts the philospher’s message to Dutch
and adds a header to the message indicating that it
has been converted to Dutch.
Examining Layer Example
(Cont’d)

Note, the translator cares only about the conversion
of the message, not its meaning.

In the third layer, the secretary takes the message
(not caring about what language it is in or what it
means) and determines the fax number of the
destination office where philosopher B works.

He/she then faxes the message to the Philosopher
B’s office fax number.
Protocol Layers

This type of layered conversation also happens in
computer/telecommunication networks.

Most of these networks operate on either a 4, 5 or 7
layer protocol stack.

Layer n on one host carries a conversation with
layer n on another host.
Rules/conventions used in this conversation are
collectively known as the layer n protocol.

Example of Information Flow in
5 Layer Protocol Network
5
Layers
4
3
M
M
H4
H4
M
H3 H4 M1
H3 M2
2 H2 H3 H4 M1 T2 H2 H3 M2 T2
H3 H4 M1
M
H3 M2
H2 H3 H4 M1 T2 H2 H3 M2 T2
1
Source
Host
Destination
Host
TCP/IP Protocol Stack

Protocols used in internet communications
constitute a four layer protocol stack.
TCP/IP
Application
Transport (TCP, UDP)
Internet Protocol (IP)
Host
to
Network
Application Layer




This is the layer that actually interacts with the
operating system or application whenever the user
chooses to transfer files, read messages or perform
other network-related activities.
Typical applications: email, ftp, www, etc.
This layer also takes converts data into a standard
format that the other layers can understand.
And this layer establishes, maintains and ends
communication with the receiving device.
Transport Layer (TCP)

This layer maintains flow control of data and
provides for error checking and recovery of data
between the devices.

Flow control means that the Transport layer looks to
see if data is coming from more than one application
and integrates each application's data into a single
stream.
Transport Layer (Cont’d)

Sometimes information will be received out of order,
this layer also reorders the received data.

If application packets are too long, this layer will
segment them into smaller pieces that can pass
through the network.

The protocols used to do all this is collectively called
Transport Control Protocol (TCP).
Network (IP) Layer

This layer determines the way that the data will be
sent to the recipient device is determined in this
layer. It no longer cares about what application the
data corresponds to.

In this layer, packets may be further segmented,
addresses for packet source/destination are
determined, the routes used a particular packet to
get from source to destination are determined, etc.

In the internet, the protocols used for all this are
called the Internet Protocol (IP).
IP Layer

Sources and destinations are converted to IP
addresses.

Routes are determined using IP addresses.

Recall the internet is a packet-switched network, so
the layer determines the route for each packet
independently (packets for the same destination
may/may not follow the same route).
IP Layer (Cont’d)

Because of packet switching, packets may be
received at the destination out of order.

It is for this reason that the layer above (transport
layer) has to reassemble packets in order.

Essentially this packets performs functions on
packets so they can move from one network to
another.
Lowest Layer: Host-to-Network

This layer is has the responsibility of moving bit
streams thru a local network.

This layer deals with packets in the local network,
breaking them down to bit streams, and converting
them to voltage levels or radio signals that will be
transmitted over the physical media (optical fiber,
copper wire, radio spectrum, etc.).

This layer also handles multiple access within a
local network.

It allows performs error-checking.
How Wi-Fi Fits into the
Protocol Stack

Wi-fi is a protocol for this lowest layer.

For example, it allows a laptop to connect to the
internet. In the local network, the laptop
communicates to a wireless access point, which
may be connected to a wireless router. The wireless
router connects this local network to the Internet.

We will review wi-fi protocols next time.
Next Time

Next time, we start looking at the WiFi system.

Specifically, we look at how to setup a WiFi system
at home.

Dimitri Demergis, a graduate student from Lehigh,
will help us understand this setup via a
demonstration.