CISC 370 - Class Today
• Project Notes
• Satellites Cell/Wireless
• Lab 2
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Project Notes
• Presentation schedule is available on
Blackboard
– Time: 15-20 minutes per group
• I need one exam question from each project
– No “true vs false”
– MUST be covered in your presentation!
– Good ideas:
• Define a key term – not something in the book
• List major elements – not in the book
• Describe a problem/solution presented in the project
– E-mail the exam question on Tuesday (paper deadline)
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Presentation Schedule
• Tuesday, May 6:
– Mahowald
Doyle, Roby
Lyons, Palchuk
Courtney
Vang
• Thursday, May 8:
– Albright, Rose
Beckman, Ward
Thornton, Kursis, Davis
Symoniak
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Cell Phones
• Based on low power transmitters and lots of
frequencies
– Each "cell" has a "base station" at the center
• Layout in a honeycomb (hexagons) =
equidistant base stations
– This is the default design
– Honeycomb is varied according to topography, call load, etc.
• Each cell is assigned a group of frequencies
• Adjacent cells use different frequencies
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Cell System Geography
• “Beehive” arrangement
– Squares are easier to lay out (‘sections’ in homesteading)
– Hexagons are simplest simulation of packed circles
• Equidistant centers between cells
• Not all cell towers are identical
– Impact of cell traffic load
• Geometry and frequency bands limit a cell’s load
• Most cells are far below their potential capacity
– Cell versus ‘microcell’
• Microcell has smaller transmitter power
• Covers smaller area, usually used in cities
• More cells = more capacity
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A somewhat busy tower
• Rural, not urban
• The “equipment enclosure”
– Hidden in the trees
– Formerly a small house
• 1 story farm/sharecropper house
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Antenna closeup
• Horizontal white thingies
–
–
–
–
Three of them above
Each is a cell antenna
Fixed capacity (#calls at a time)
“Wedge” coverage
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A modern cell
• Cell phones only
• Self contained
• (not in use, either)
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The antennae
• Triangle shape
– Cheaper than hexagon
– Provides good coverage for
the directional antennae
• Not in use
– “placeholder” antennae
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A busier tower
• 30 miles north of Boston
– pseudo-rural (expensive) suburb
• Count those antennae!
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A very, very rural tower
• Just south of Red Wing
– We are looking northwards to the tower
• Not too many antennae
– What conclusions can we draw about
where the cell customers are?
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More towers
Are these cell towers?
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Close ups
Clearer now?
Omni-directional
Unidirectional
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Summary
• How do we assess a cell phone network?
• What do the antennae tell us?
– Current traffic levels
– Direction of traffic levels
– Available room for growth
• What about cell phone antennae on buildings?
– Apartments on the Cretin/Grand corner
– Apartments along I-94 near Dale exit
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Cell Operation
• Organization
– Base stations connect to a mobile switching office
– Mobile switching office connects to POTS
• Handling a call
– Turn on the phone - it searches for a nearby cell
– Phone does handshake with strongest base station
• "Setup channel" is identified to handle calls
• Base station registers the phone's location for incoming
calls
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Operation, continued
• Outgoing call
–
–
–
–
Phone checks to see if setup channel is free
Once channel is free, sends the call request
Mobile switching office tries to make call.
If succeeds, the base station assigns a channel to the phone
• Handoff
– If the phone moves out of range, it contacts the nearest strong
base station
– New base station provides a new control channel
– If a call is in progress, they assign a new channel to the call
and new base station handles it
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Increasing cell system capacity
• Add new channels
– Look at a base station and count the vertical antenna elements
– Busy stations have lots of elements, less busy ones have fewer
• Frequency borrowing
– A more-busy station borrows a frequency from a less-busy neighbor
• Cell splitting
– Change the geographic coverage of adjacent cells to fit in another
base station
– If you get a lot of cells in a small area, you may get "microcells" that
use lower power base stations
• Cell sectoring
– Use directional antennas and assign specific channels to specific
directions.
– Yields wedge-shaped channel coverage areas
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Satellite System Architecture
• Earth stations
– Fixed locations with big satellite antennas
– or small, mobile devices like cel phones
• Satellites
– Orbiting devices that relay the messages,
– usually from a mobile device to a fixed station
– Or vice versa (pagers)
• Elements
– + Uplink - message going from earth station to satellite
– + Downlink - opposite direction
– + Transponder - typical satellite function - simply echoes the
uplink signal onto a downlink.
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Remember strong vs weak
• Portable devices usually have weak signals
– battery and portability limitations
• Satellites have signal limitations
– How much circuitry can we launch into orbit and keep there?
– How much power can we get from solar cells?
• Fixed Earth Station
– The Most powerful element is the fixed earth station:
• big antenna, lots of power
• Sends strong signal to satellite
• detects weak signals in a downlink
• Can reduce the satellite’s needs
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Classic Satellite Applications
• Earth surveillance (weather, spying, etc)
– Satellite collects sensor data, transmits to base station
– Powerful base station, huge antenna
• Network Television
– Powerful Earth station transmits the program signal
– Smaller (but still huge) antennas to receive the signal
• Pagers
– Earth station transmits the signal, bounces off the satellite
– Activates small, low powered receiver
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New Tech applications
• Home Television
– Powered Earth station transmitters, powered (small) receivers
– Satellite simply bounces the signal
• Telephones (Iridium, others)
– Weak base stations (phones)
– Satellites talk to phones, Earth stations, and each other
– Earth stations provide local phone company connection
• GPS
– Satellites transmit standard, coded signals
– Earth receivers are small, portable, vulnerable to signal
blockage
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Satellite constellations
• GEO
• LEO
• MEO
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o GEO - geostationary orbit
• + Fixed in one location
– 35,863km above earth (about 21K miles) on the Equator
• + "Geosynchronous“
– that height, but not necessarily a fixed location
• + Easiest type of satellite to handle
– location never changes
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GEO
• Benefits
– # No frequency shift problems from Doppler effect
– # Easy to track from earth, since it doesn't move
– # Fewer satellites (3) can cover entire habitable surface
• Problems
– # Signal gets really weak after travelling 35K km
– # Polar regions are hard to cover from the equator
– # Distance causes delays even at the speed of light
• Examples:
– TV - delay isn't a problem for 1-way broadcast video
– ICO started with GEO satellite
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LEO - low earth orbit
• Orbit at 500 to 1500 KM above the earth
• Benefits
– Very low transmission delay (20 ms)
– Strong signals
• Problem: lower altitude = higher speed
– Harder to track
– Doppler shifts affect frequencies
• Problem: lower altitude = smaller signal footprint
– Each satellite only covers circle with 8000 KM diameter
– Each satellite is visible for only 20 minutes at a time - must have
"hand off" between satellites or you lose the connection
• Need LOTS of satellites to cover the whole planet
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Examples
• ORBCOMM –
– called a 'little LEO' –
– small bandwidth satellites for paging and short text msgs
• Globalstar
– called a 'big LEO‘
– higher data rates
– 48 satellites, pure transponders
• Iridium
– 66 satellites @ 485 mile altitude
– they had to develop satellite mass production techniques to
make this work
– satellites will to each other as well as to ground stations
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o MEO - Medium Earth Orbit
•
•
•
•
+ Height of 5000 to 12,000 KM
+ Signal delay ~ 50ms
+ Footprint = 10K-15K km
+ Less handoff, less Doppler shift, fewer satellites than
LEO
• + Less round trip delay than GEO
• + Example: ICO ("New ICO") with planned set of 12
satellites at 10,400 km orbits
• Imagery http://www.ee.surrey.ac.uk/Personal/L.Wood/constellati
ons/globalstar.html
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Thursday
• Homework Due
• Will work Case Studies 6 and 7
Firewall Lab
• Configuring the Linksys router/firewall
– Blocking
– Network address translation
• A configuration problem
– We need to watch with WireShark
– Hub technology is lagging the network technology
• Due in a couple of weeks
Linksys Home Page
• Type in the router’s
IP address
• 10.10.10.10
– or 192.168.1.1
• Password
– Replace ‘1’ with ‘2’ in
the admin password
– or “admin”
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Five major headings of controls
• Setup
– Establishes the local address and configuration
• Security
– Filters traffic, enables/disables certain types of traffic
• Applications and Gaming
– Allows connections to servers on the LAN from the Internet
• Administration
– Change password, enable remote management features
• Status
– Check the status of the WAN connection
– Check status of LAN and its attached hosts
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Address Setup
• Set to “Obtain IP Automatically”
• Our local default internal addresses are Net 10
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Address Settings
• Set local address to 10.10.10.10
– That’s the address of this router
– Subnet mask 255.255.255.0
• Enable Local DHCP service
– Start assigning local addresses at 100, total of 50 addresses
– Renews address “leases” daily
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Looking at the Router Status
• Internal and external routing data
– The “Internet” addresses are for the “outside” of the router
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Looking at the LAN Status
• Gives addressing information about the router
as seen from the LAN side
– Click the button to see the DHCP client table
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DHCP Client Table
Lists all active
clients on the
LAN
Provides a map
to the LAN
Just like the lab
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The Management Screen
Starting point for
lower level
controls
Actually, password
changing is all
this is good for
PLEASE DON’T
CHANGE THE
PASSWORD.
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Traffic Filtering
• Blocks LAN machines
from the Internet
– Block by IP address
– Block by MAC address
• Block Port Numbers
• Other filters
– Multicast
– External Internet queries
• mostly Pings
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Port Forwarding
• Allows inbound connections – forwards particular
ports to specific PCs on the LAN
• Under the “Applications and Gaming” tab.
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Firewall Lab
• Overview
–
–
–
–
Rewire the lab to use the firewall
Map the rewired lab
Demonstrate host blocking through the firewall
Demonstrate NAT through the firewall
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