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• 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• Ionosphere and troposphere delaysIonosphere and troposphere delays — The satellite signal slows as it passes

through the atmosphere. • Signal multipathSignal 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 errorsReceiver 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 errorsOrbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.

• Number of satellites visibleNumber 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/shadingSatellite 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 signalIntentional degradation of the satellite signal — Selective AvailabilitySelective 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

• 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

– 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 networkslocal area networks

• could be devices in different countries– packet switching networks vs. circuit switchingpacket switching networks vs. circuit switching

Packet Switching Network

Host

Host

Host

Hostnodenode

nodenode

nodenode

nodenode

DC

NY

Cairo

Berlin

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 real-time 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 frame-relay 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-orientedconnection-oriented

– Sender, receiver set fixed path on network beforebefore 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 ComponentsTypical LAN Components

LAN

Network Server (with network software)

AnotherLAN

Gateway

Network Topologies

• In the case of LANs, the shape of the network defines its topology– Star– Bus– Ring

Star Network Topology

HostComputer

- 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 fiberAll messages broadcast to entire networkSoftware 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– 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?