How GPS Works Part 2 LUCID Summer Workshop July 30, 2004.

How GPS WorksPart 2

LUCID Summer WorkshopJuly 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 time- of-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 AFBColorado Springs, CO

Master Control Monitor Station

HawaiiMonitor Station

Ascension IslandMonitor Station

Diego GarciaMonitor Station

KwajaleinMonitor 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 Typical Error in Meters (per satellite)

Satellite Clocks 1.5Orbit Errors 2.5Ionosphere 5.0Troposphere 0.5Receiver Noise 0.3Multipath 0.6SA 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 0-30 meters Mostly Removed Troposphere 0-30 meters All Removed Signal Noise 0-10 meters All Removed Orbit Data 1-5 meters All Removed Clock Drift 0-1.5 meters All Removed Multipath 0-1 meters Not Removed Receiver Noise ~1 meter Not RemovedSA 0-70 meters 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 user-

friendly, 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

POP

POPPOP…

Fiber OpticConnections

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 Download Upload Cost/Month Installation AvailabilityDial-up 56Kbps 56Kbps $0-20 $0 UniversalCable Modem 15-50Mbps 128Kbps $30-70 $0-100 LimitedISDN 128Kbps 128Kbps $25-70 $100-300 UniversalDSL 6-8.5Mbps 128Kbps $0-80 $0-250 LimitedT1 1-10Mbps 1-10Mbps $300+ $400+ Fairly UniversalT3 40-100Mbps 40-100Mbps $1000+ NA 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 RouterWireless 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-by-millisecond 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-Translator-Secretary Architecture

Location A Location B

Fax # --L: DutchDest: BIk houVankonijnen

L: DutchDest: BIk houVankonijnen

Fax # --L: DutchDest: BIk houVankonijnen

Fax Fax

L: DutchDest: BIk houVankonijnen

Dest: BI likerabbits

J’aimeleslapins

Secretary

Translator

PhilospherMessage

Information For the remotetranslator

Messagefor the remotesecretary

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

M

MH4

M1 M2H3 H4H3

M1H3 H4H2 T2 M2H3H2 T2

M

MH4

M1 M2H3 H4H3

M1H3 H4H2 T2 M2H3H2 T2

5

4

3

2

1

SourceHost

DestinationHost

Laye

rs

TCP/IP Protocol Stack

Protocols used in internet communications constitute a four layer protocol stack.

Application

Transport (TCP, UDP)

Internet Protocol (IP)

Host toNetwork

TCP/IP

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.