Satellitebased Internet

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Satellite-based Internet
Wireless Communication Systems Aitor Ferández
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Main features

• Appropiate mobility support
• Global coverage
• Inherent broadcast capacity
• Bandwith on demand capacity

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Appliable to

• Globally scattered users
• Aeronautical and maritime users
• Remote, infraestructure lacking areas
• Point to multipoint communications
• Multipoint to multipoint comm.

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Offered services

• Broadband access networks
• High speed backbone networks
• Connecting heterogeneous networks
• Comm. links between users with mobile or
  fixed terminals

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Satellite communication Fundamentals

• A satellite system consists of

Space segment
Ground segment
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Satellite communication Fundamentals

• Ground segment consists of

• Gateway stations (GS)
• Network control center (NCC)
• Operation control center (OCC)

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Satellite communication Fundamentals

• Space segment consists of

• Satellites

• Geostationary orbit (GSO)
• Nongeostationary orbit (NGSO)

• Medium earth orbit (MEO)
• Low earth orbit (LEO)

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Satellite communication Fundamentals

• GSO Orbit

- 35786 Km avobe the equator - Appears fixed to
an observer on the Earth - Sufficient for global
coverage - 1 GSO Satellite covers one third of
Earth surface - This area is called
footprint
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Satellite communication Fundamentals

• GSO Orbit

- High launching cost - Large antennas and
transmission power required - Biggest problem
Propagation delay - Around 250-280 ms
- Undesirable for real-time traffic
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Satellite communication Fundamentals

• GSO satellite coverage

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Satellite communication Fundamentals

• LEO and MEO Orbits

- Closer to the earth surface - Smaller antennas
and transmission power required - Footprint
is also smaller - Larger number of satellites is
necessary - Steerable antennas become useful
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Satellite communication Fundamentals

• MEO Orbit

- From 3000 Km up to the GSO orbit - Typical
round-trip propagation delay 110-300 ms

• LEO Orbit

- 200-3000 Km above earth surface - Typical
round-trip propagation delay 20-25 ms
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Satellite communication Fundamentals
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Satellite communication Fundamentals

• Satellite payloads

- Responsible of the satellite communications -
Once launched, a satellite is impossible to
upgrade

• Robust
• Simple

- No on-board processing (OBP) on traditional sats
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Satellite communication Fundamentals

• Satellite payloads

- Some satellites allow OBP (Conform ISL's) -
Demodulation/remodulation -
Decoding/recoding - Transponder/beam
switching - Routing - Connectivity in space
without terrestrial resource is possible
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Satellite communication Fundamentals

• Frequency bands

- Most commonly used are C, Ku, and Ka bands -
With higher frequency, and shorter wavelenght

• Smaller antenas can be used

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Satellite communication Fundamentals
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Satellite-basedInternet architectures

• Several implementation options due to diversity
• Suggestions to use hybrid GSO-NGSO network
• Sats can act as a backbone/Hi-speed network

• ARPANET became backbone of research network

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Satellite-basedInternet architectures

• The idea of using satellites in last mile is newer

• Satellites interact with GS's
• May be the only access method for remote areas
• Bent pipe architecture (Fig 1) suffers big latency

• Because of the lack of direct communication

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Satellite-basedInternet architectures

• Bent-pipe design

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Satellite-basedInternet architectures

• OBP and ISL's can be combined to create a network
  in the sky. Both combination access and backbone
  network. Teledisc or Iridium are examples of this
  kind of network.
• Flexibility at cost of more complex routing

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Satellite-basedInternet architectures

• OBP and ISL

Architecture Design
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Satellite-basedInternet architectures

• DBS Model (Direct Broadcast Satellites)

• Acces internet directly by satellites. Only
  download
• Created because of internet traffic asimetry in
  which the server transmits much more information
  than in reverse
• Upload from the client is made via terrestrial
  GS's

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Satellite-basedInternet architectures

• DBS architecture

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Satellite-basedInternet architectures

• Astrolink

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Satellite-basedInternet architectures

• Skybridge distribution

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Satellite-basedInternet architectures

• Spaceway architecture scheme

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Satellite-basedInternet architectures

• Teledisc satellite distribution

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Satellite-basedInternet architectures

• Teledisc satellite coverage

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Satellite-basedInternet architectures

• Iridium satellite distribution

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Satellite-basedInternet architectures

• Iridium satellite coverage

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Technical challenges

• Multiple Access Control (MAC)

- Is a set of rules - This rules decide how the
clients in the footprint of the satellite acces
its uplink channel, which is a limited resource -
Affects to the QoS and higher layers
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Technical challenges

• Fixed Assignement

• This is the most basic of MAC's
• It is based in frequency, time or code divixion
  basis (FDMA, TDMA and CDMA respectively)
• Very poor resource management. For small networks.

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Technical challenges

• Random Access

• Thanks to small and cheap terminals
• Less data usage, which turns in a bigger number
  of terminals which a satellite can cover.
• Each station transmit data regardless of the
  others.

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Technical challenges

• Demand Assignment

• Solves RA's lack of QoS providing
• Dynamically allocates system bandwidth depending
  on how many clients request access
• The transmission for permission becomes the
  problem, but it is affordable.

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Technical challenges

• After reservation, bandwidth is segmented in FDMA
  or TDMA
• Centralized or Distributed control
• Resource reservation can be explicitly or
  implicitly done.
• Priority Oriented Demand Protocol and First In
  First Out, combine explicit and implicit requests

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Routing Issues

• We will refer to LEO satellite systems, as
  Iridium can be.
• It is very attractive to be able to design a
  network in the sky, thanks to OBP and ISL's
• The routing becomes crucial.

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Routing Issues

• Dynamic Topology

• Satellites have very little visible period to us
• When a satellite goes out, and another comes in,
  intersatellite handover happens
• Each satellite is able to set up 4 to 8 ISL's
• These can be intraplane or interplane

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Routing Issues

• Discrete-time Dynamic Virtual Topology Routing

• DT-DVTR makes use of the periodic nature of sats
• Works completely offline
• System storages visibility data of each interval
• When topology changes, the best path is choosen

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Routing Issues

• Virtual Node (VN)

• Hidew the topologycal changes from the routing
  protocols, despite satellites are actually moving
• Keeps state information, such as routing tables
  and user data
• When the satellite covering the node disappears
  in the horizon, the node is covered by the one
  that comes

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Routing Issues

• IP Routing at satellites

• IP routing is adopted, based in the VN concept
• Integrates the space network with terrestrial
  Internet
• Supports IP multicast and IP QoS
• Despite is useful and desirable, implementation
  problems appear everywhere. Constantly improving.

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Routing Issues

• ATM Switching

• Many proposed systems use ATM as network protocol
  for the constellation
• A version of DT-DVTR based in ATM is
  investigated, grouping the ingress and engress
  satellites' virtual channel connections in a VPC
• Possibly, IP over ATM will be implemented

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Routing Issues

• External Routing Issues

• Internal design of communications will probably
  change constantly to fit manufacturers or
  clients.
• Satellite network is isolated of terrestrial
  network
• AS (astronomous system) concept. SatAS.
• Only Border Gateways will communicate with these,
  by BGP

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Routing Issues

• Unidirectional Routing

• DBS is unidirectional. Traditional Routing is
  invalid
• Static routing instead of dynamic routing is an
  option
• Options

• Routing Protocol Modification
• Tunnelling

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Routing Issues

• Routing Protocol Modification

• In unidirectional routing, we can only send
  (feeders)
• The clients at the other side can only receive
  (receivers)
• Make a receiver identify the potential feeders,
  ignoring unusable data while mantaining
  neighboring connection
• The receiver periodically delivers its own
  routing message to all feeders through terrestial
  link

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Routing Issues

• Tunneling

• Offers a link layer approach to hide the
  asymetry.
• A tunnel is a virtual link set between a DBS and
  a receiver by using encapsulation and
  decapsulation
• User encapsulates package -gt sends to routing
  protocol via terrestrial nw -gt arrives to
  satellite -gt the tunnel decapsulates the message
  - gt forwards it to the routing protocol

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Satellite Transport

• TCP/IP and UDP/IP protocols are the heart of the
  Internet. Their solidity and standarization makes
  them adaptable and unlikely to be discarded
• Satellite-based Internet will use UDP and TCP.
• TCP being connection oriented, will receive the
  great impact of high latencys and error rates.

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Satellite Transport

• TCP/IP performance over satellite

• High latencies will affect TCP's functionality.
• Timeout based protocols such as this may severely
  suffer large round-trip times delivered by
  satellite connections
• Also suffer from interferences, fading,
  shadowing, and rain atteunation. This causes Bit
  Error Rate (BER)

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Satellite Transport

• Performance enhacenments have been developed over
  TCP
• TCP selective acknowledgement (SACK)
• TCP for transaction (T/TCP)
• Persistent TCP Connection
• MTU mechanism
• FEC

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Satellite Transport

• TCP extensions solve some limitation of standard
  TCP
• TCP Spoofing
• TCP Splitting
• Web Caching

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THANK YOU!Aitor Fernández