Transcript Wireless Communication: Satellites

Wireless Communication:
Satellites
Wireless Transmission
• Directional
– Focuses electromagnetic beam in direction of
receiver
• Terrestrial microwave
• Satellite microwave
• Omni directional
– Spreads the electromagnetic signal in all directions
• AM and FM radio
• 3G networks
• Smart watches
Terrestrial Microwave
• Parabolic dish antenna sends signal to
receiving dish
• Line-of-sight
• Typically on towers to avoid obstacles
• Frequencies in the gigahertz range
What is a telecommunications
satellite?
Telecommunications satellites
• Space-based cluster of radio repeaters
(called transponders)
• Link
– terrestrial radio transmitters to satellite
receiver (uplink)
– Satellite transmitters to terrestrial receivers
(downlink)
Orbits
• Mostly geostationary (GEO)
– Circular orbit
– 22,235 miles above earth
– Fixed point above surface
– Almost always a point on Equator
• Must be separated by at least 4 degrees
Satellite services
• Wide Area Broadcasting
– Single transmitter to multiple receivers
• Wide Area Report-Back
– Multiple transmitters to a single receiver
– Example VSATs (very small aperture terminals)
• Also have microwave transmitters and receivers
– Allows for spot-beam transmission (point- to-point data
communications)
• Can switch between beams upon request (Demand
Assigned Multiple Access –DAMA)
• Multi-beam satellites link widely dispersed mobile and
fixed point users
Earth-based equipment
• Original
microwave
transmitters and
receivers were
large installations
– Dishes
measuring 100
feet in diameter
• Modern antennas
about 3 feet in
diameter
A Modern GEO satellite (IntelSat
900 series)
• May have more than 72 separate microwave
transponders
• Each transponder handles multiple simultaneous
users (protocol called Time Division Multiple
Access)
• Transponder consists of
– Receiver tuned to frequency of uplink
– Frequency shifter (to lower frequency to that of
transmitter)
– Power amplifier
IntelSat 902 (launched August 30,
2001)
Frequency ranges
• Most transponders operate in 36MHz
bandwidth
• Use this bandwidth for
– voice telephony (400 2-way
channels/transponder)
– Data communication (120Mbs)
– TV and FM Radio
C-band, Ku-band, Ka-band
• Most GEO satellites operate in the C-Band
frequencies
– Uplink at 6 GHz
– Downlink at 4 GHz
• Ku-band also used
– Uplink at 14 GHz
– Downlink at 11 GHz
• Above bands best suited for minimal
atmospheric attenuation
• Few slots left… forcing companies to look at
Ka band (uplink:30 GHZ , downlink: 20 GHz)
MEO Satellites
• Exist between the first and second Van
Allen Radiation belts
• Peak height is ~ 9000 miles\
– Typical is about 4000 miles
• Need less power than GEO satellites to
reach.
• Example GPS satellites
Global Positioning Systems
• A constellation of 24 DoD satellites orbiting about
10,000 miles above earth’s surface
• First launched in 1978; complete set by 1994;
replaced every ten years or so..
• Solar-powered; Each circles earth about twice a day
• Also have 5 ground stations (control segments)
– monitor the GPS satellites, checking both their
operational health and their exact position in
space.
– Five monitor stations: Hawaii, Ascension Island,
Diego Garcia, Kwajalein, and Colorado Springs.
GPS Constellation
How they work
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To determine position
GPS satellites emit 3 bits of information in its signal (L1 for civilians; L2 for
military):
– Pseudorandom code (ID which identifies specific satellite)
– Ephemeris data (status of satellite and current data and time)
– Almanac data (tells exactly where that satellite and all others are supposed to be
at any given time during the day)
•
Finding your location
– Compare time a signal is transmitted to when it is received – tells how far away
satellite is… receiver knows it is on the surface of an imaginary sphere centered
around the GPS satellite
– With similar distance measurements from other satellites, receiver can determine
location (intersection of at least three spheres)
– GPS receiver must lock on to 3 satellites to give 2D location; 4 satellites to give
altitude as well.
– Accurate up to ~10-15 meters; DGPS and Augmented GPS can go down to a
few centimeters.
Sources of Error for GPS
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Ionosphere and troposphere delays — The satellite signal slows as it passes
through the atmosphere.
Signal multipath — This occurs when the GPS signal is reflected off objects such as
tall buildings or large rock surfaces before it reaches the receiver. This increases the
travel time of the signal, thereby causing errors.
Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic
clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.
Orbital errors — Also known as ephemeris errors, these are inaccuracies of the
satellite's reported location.
Number of satellites visible — The more satellites a GPS receiver can "see," the
better the accuracy. Buildings, terrain, electronic interference, or sometimes even
dense foliage can block signal reception, causing position errors or possibly no
position reading at all. GPS units typically will not work indoors, underwater or
underground.
Satellite geometry/shading — This refers to the relative position of the satellites at
any given time. Ideal satellite geometry exits when the satellites are located at wide
angles relative to each other. Poor geometry results when the satellites are located in
a line or in a tight grouping.
Intentional degradation of the satellite signal — Selective Availability (SA) is an
intentional degradation of the signal once imposed by the U.S. Department of
Defence. The government turned off SA in May 2000, which significantly improved
the accuracy of civilian GPS receivers.
Source: http://www.pocketgps.co.uk/howgpsworks.php
LEO Satellites
• Lowest of the satellites – below the first radiation
belt
– Typically orbit at ~600 miles
• Much less power needed than for GEO and
MEO
• Can be accessed using smaller devices such as
phones.
• Available anywhere in the world.
• Geostationary?
Companies on the forefront:
Teledesic
• Offer “Internet-in-the-Sky”
• Main shareholders Craig McCaw and
Bill Gates
• McCaw also has taken over ICO Global
Communications
• Wanted Iridium but has backed out
Teledesic
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Again, series of LEO satellites
24 pole orbiting satellite rings, 15 degrees apart
12 satellites in each ring (total = 288 LEO satellites)
Worldwide switching.. Satellites pass on data through
laser
• Will map IP packets on latitudes and longitudes ..
Average will be 5 satellite hops in 75 ms
• Supposed to start in 2002; offer 2Mbps Internet
access from terminals starting at $1000 each
– Postponed to 2005
Optical Transmission
• Cutting edge
• Uses modulated monochromatic light to
carry data from transmitter to receiver
• Optical wavelengths are suited for high
rate broadband communications
• Laser-based (up to 1000 times faster than
coaxial)
Other landline transmission
paths
T-Carrier Lines
• Dedicated telephone line
• T1 carries data at about 1.544 Mbps
• Each T1 is broken down into 24 channels
of 64Kbps each
• Each channel can carry either data or
voice
• T3 can go up to 44.736 Mbps (672
channels)
Cable Modems
• Designed to work over cable lines (HFC- hybrid fiber
coaxial)
• Speed is about 10Mbps
• Process
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Coaxial cable has enough free bandwidth
IP packets modulated and sent to user’s PC
Signal hits splitter that shunts data to modem
Cable modem demodulates into Ethernet packets
Slower on the upload
Users share bandwidth
• Comparison - download 857 pages of Moby Dick
– Cable Modem: all 857 pages in ~ 2 seconds
– 56K Modem: about 3 pages in 2 seconds
Digital Subscriber Lines (DSL)
• Pumps data at high rates to PCs using ordinary
copper lines.
• Based on the 4KHz frequency cut
• North American DSL market reaches 4.7 million
(11/27/2001) – Telechoice survey
Flavors of DSL
• Referred to as xDSL
• ADSL (asymmetric)
– Approximately 8Mbits/sec download
– Maximum of 640Kbits/sec upload
• SDSL (symmetric)
– Equal rates for upload and download (~ 1.5Mbits/sec)
• VDSL (Very high)
– Up to 55 Mbits/sec
– Only 1000’ from telco
Wireless Data Communication
Networks
• High frequency radio waves… mostly
for mobile users
• Send and receive data on a LAN or via
fax, email, Internet
• Services include
– Cellular Digital Packet Data
– Packet Radio Systems
– Personal Communication Systems
Data Transport Networks
• connect variety of computers and other
devices
• could be devices in same building
– local area networks
• could be devices in different countries
– packet switching networks vs. circuit switching
Packet Switching Network
Host
DC
Host
node
Host
Berlin
NY
node
node
Cairo
Host
node
PADs
X.25 Protocol (56K-64K bps)
• Popular protocol for PSNs in the 1970s
• Relatively slow… runs on 56K lines
• Packet Switched technology
– File broken down into discrete packets before being
transmitted
– Packets traverse different paths , at different times
before being reassembled at destination
– Efficient in apportioning bandwidth based on
availability
– Inefficient in that error control information is also
saved … unnecessary if network clean
Frame Relay (56K-45M bps)
• Dedicated, packet-switched service
• Sends data in variable length packets –
also called frames
• Variable length makes it efficient
• Works best when a few
branches/subsidiaries need to share files
with each other
International Frame Relay
• High speed packet-switching protocols in
WANs that span countries
• Variable length packets… best suited for
data and images… not for voice or video
• At highest speeds, can be used for realtime data
International Frame Relays
contd.
• Cuts costs of connections to foreign
countries
• Set up by one telecommunications carrier
• May not serve every country in an MNC’s
global network
• Many carriers overbook capacity of framerelay networks.. Can cause packet
discards
Asynchronous Transfer Mode
• A type of transport service on WANs
• Handles all types of data… including voice and
video… on single network
• Most Fortune 1000 companies have some form
of ATM
• Unlike TCP/IP, ATM is connection-oriented
– Sender, receiver set fixed path on network before
sending data
– Information arrives in order it was sent
ATM : How does it work?
• ATM network transfers data in small fixed-length
packets – 53 bytes each
• Packets are known as cells… all cells with same
source/destination follow same network path
• Real-time data takes precedence over other
types.. Voice always get priority over email cells
• Small, constant cell size allows more efficient
network usage – less delay at ATM switch
• “Cell tax” make Gigabit Ethernet more attractive
Local Area Networks
Topologies and Collision Detection
What do we know so far?
• Data communications involves
– Exchange of digital information
– Between two or more devices
– Across a transmission medium
• How are these devices connected?
Private Branch Exchanges
(PBX)
• Special computer that handles phone calls within a
company
• Carry both voice and data
• Can store, transfer, hold and redial calls
• Can also be used to transfer data between computers
• Does not require special wiring
– PCs can be plugged or unplugged anywhere in the building
• Supported by commercial vendors (no internal expertise
needed)
• Geographic scope limited to several hundred feet
• Cannot handle large amounts of data
Local Area Networks (LAN)
• Connect several buildings in close
proximity
• Typically within 2000 feet
• Requires own communication lines
• Have higher transmission speeds
• Typically used to connect PCs and shared
printers
Typical LAN Components
Network
Server (with network software)
LAN
Another
LAN
Gateway
Network Topologies
• In the case of LANs, the shape of the
network defines its topology
– Star
– Bus
– Ring
Star Network Topology
Host
Computer
- Used to connect a
smaller number of
computers
- depends on health of
host computer
Bus Network Topology
Central line (“bus”) that may be TP, Coaxial, or fiber
All messages broadcast to entire network
Software identifies which device receives message
-Bus network can only handle one message at a time
-Can slow down at peak hours
-Collisions may occur
Ring Network Topology
- Each computer part of a
closed loop
-messages passed from one
device to another
-Only passes in one direction
Ethernet
• Designed for multiple devices sharing a
single communication cable
• Devloped by Bob Metcalfe of Xerox in
1973
– Tried to link a Xerox Alto computer to a printer
Ethernet Terms
Medium, Segment, Node, frames
CSMA / CD
• An analogy
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Imagine a group of people sitting at a table
They are having a polite conversation
Everyone can hear others speak
They wait for conversations directed at them
Wait for a pause in conversation before speaking
Two people waiting for lull speak up at same time
Must repeat themselves
Contention issues
• All devices on a bus or ring can send messages
• Devices keep listening to the network to check
for messages meant for them
• What happens if messages are sent at the same
time?
• Messages can sometimes collide and be
garbled or lost
• LANs must have a predetermined way to deal
with these conflicts or contentions
CSMA/CD
(Collision Sensing Multiple
Access/Collision Detection)
• This is used in traditional bus network topologies
– Ethernet uses bus topology with CSMA/CD
• Any device on the bus can send a message
• If the line is idle two devices may send at same
time
• Device recognize collision and send message
again after random period of time
Limitations of Ethernet Networks
• Mostly relate to length of cable segments
• Electrical signals attenuate as they travel longer
distances
• Segment must be short enough for devices to
hear each other clearly
– Places limit on size on network
– Network diameter
• Since CSMA/CD only allows one device to
communicate at a time, limits number of devices
without degrading performance
Repeaters
• Repeaters connect multiple Ethernet
segments
• Any signals heard on one segment will be
heard and repeated on all other segments
connected to repeater
• Allows for expanding network diameter
Bridges
• What happens if there are a large number
of people at the table?
– Multiple simultaneous conversations
• In large networks, devices would
constantly be interfering and sending
colliding signals
• Bridges are like repeaters that echo
signals, but can also regulate traffic
-The bridge aims at reducing unnecessary traffic on Ethernet segments
-If signal from A is meant for B, there is no point echoing it on Segment 2
-If it is meant for C or D, the frame is forwarded to Segment 2.
-A can simultaneously transmit to B since it only uses Segment 1
Token Ring
• Used in ring topology networks
– Eg: IBM token rings
• A token (series of 0,1) floats along line
• A device wishing to send message on line
must first grab the token and keep it – only
then can it send
• Once the message has been sent, device
releases token back into the ring
• Collisions can never occur
• Token-ring networks typically transmit data at
either 4 or 16 Mbps.
IBM Token Ring
Problems with bridges
• In the Ethernet design, messages are broadcast
to every device (or node) on the network
segments.
• The bridge forwards these broadcasts to all
connected segments
• In very large networks, this can cause
congestion
– Many stations on different segments broadcast at the
same time
– Can be as bad as if all nodes were on one segment
Routers
• Routers are advanced network
components
• They divide the network into two virtual (or
logically independent) networks
• Broadcasts cross bridges in search of their
desired node
• They do not cross routers
– The router forms a logical boundary of the
network
Fujitsu GeoStream R900
Router
Research Question for Next Class
• What is Abilene?