Survey of Satellite-Based InternetSurvey of Satellite-Based Internet
20. November 200320. November 2003M.Sc. Lei Ma M.Sc. Lei Ma
M.Sc Rajesh ShankarM.Sc Rajesh Shankar
Department of Informatics VIIDepartment of Informatics VIIBayerische Julius-Maximilians Universität WürzburgBayerische Julius-Maximilians Universität Würzburg
Seminar Telematiksysteme in der RaumfahrtSeminar Telematiksysteme in der Raumfahrt
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Content
Introduction
Satellite Communication Fundamentals
Satellite-Based Internet Architectures
Some Examples of Satellite Systems
Technical Challenges
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Introduction
Source Material:Y.Hu and V.Li. Satellite-based Internet: a Tutorial,
IEEE Comm., March 2001.J.Farserotu and R.Prasad. A Survey of Future
Broadband Multimedia Satellite Systems, Issues and Trends, IEEE Comm., June 2000.
E.Lutz, M.Werner and A.Jahn. Satellite Systems for Personal and Broadband Communications, Springer, Berlin, 2000.
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Introduction
Technical challenges to Internet developmentProliferation of applicationsExpansion in the number of hostsUser imposeHigh-speed high-quality services needed to
accommodate multimedia applications with diverse quality of service
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Introduction
Satellite Network Global coverage Inherent broadband capabilityBandwidth-on-demand flexibilityMobility supportPoint-to-multipoint, multipoint-to-multipoint comm.
Satellite communication system is a excellent Satellite communication system is a excellent candidate to provide broadband integrated Internet candidate to provide broadband integrated Internet services to globally scattered usersservices to globally scattered users
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Satellite Communication Fundamentals
Construction of a satellite systemSpace segment: satellites
• Geostationary orbit (GSO)• Nongeostationary orbit (NGSO)
– Medium earth orbit (MEO)– Low earth orbit (LEO)
Ground segment • Gateway stations (GSs)• Network control center (NCC)• Operation control centers (OCC)
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Orbit Selection
GSO option: Larger Coverage (1/3 of Earth’s Surface)
Distance challenge:• Large delay (round-trip delay 250-280 ms)• Large propagation loss (requires higher transmitting powers
and antenna gains)
NGSO option: Smaller Delay (LEO round-trip delay ~20ms)
Variable looking angle challenge:• Requires sophisticated tracking techniques or, most of the
times, omni-directional antennas.• Requires support to handoff from one satellite to another.
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Frequency Bands
C Band (4-8 GHz): very congested already.
Ku Band (10-18 GHz): Majority of DBS systems, as well as current Internet DTH systems (DirectPC and Starband).
Ka band (18-31 GHz): Offers higher bandwidth with smaller antennas, but suffers more environmental impairments and is less massively produced as of today (more expensive) when compared to C and Ka.
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Satellite Payload
Bent pipe Satellites act as repeaters. Signal is amplified and retransmitted
but there is no improvement in the C/N ratio, since there is no demodulation, decoding or other type of processing. No possibility of ISL, longer delay due to multiple hops.
Onboard processing (OBP) Satellite performs tasks like demodulation and decoding which
allow signal recovery before retransmission (new coding and modulation). Since the signal is available at some point in baseband, other activities are also possible, such as routing, switching, etc. Allows ISL implementation.
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Satellite-Based Internet Architectures
The satellite-based Internet with bent pipe architectureLack of direct communication pathLow spectrum efficiency and long latency
The satellite-based Internet with OBP and ISL architectureRich connectivityComplex routing issues
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The satellite-based Internet with bent pipe architecture
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The satellite-based Internet with OBP and ISL architecture
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Next Generation Satellite Systems
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Case Study: Teledesic
Constellation consists of 288 satellites in 12 planes of 24 satellites.Ka-band system. Uplink operates at 28.6–29.1 GHz, downlink at 18.8–19.3 GHz. It usesSignals at 60 GHz for ISLs between adjacent satellites in each orbital plane.Full OBP and OBS (on-board switching)."Internet in the sky."Offers high-quality voice, data, and multimedia information services. QoS performance designed for a BER < 10–10.Multiple access is a combination of multifrequency TDMA (MF-TDMA) on the uplink and asynchronous TDMA (ATDMA) on the downlink.
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Case Study: Teledesic
Network capacity planned to be 10 Gb/s. User connections of 2 Mb/s on the uplink and 64 Mb/s on the downlink possible.Minimum elevation angle of 40.25 enables achievement of an availability of 99.9 percent.Enormous complexity to the table in terms of untried technology, onboard switching and inter-satellite capabilities.
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Technical Challenges
Multiple Access Control
Routing Issues in Satellite Systems
Satellite Transport
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Technical Challenges (MAC)
Multiple Access Control (MAC)
1. Performance
2. Schemes
3. Implementation
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Technical Challenges (MAC)
Performance of MAC
- Depends on shared communication media and traffic.
- Long latency in Sat-channels excludes some MAC schemes that are used in terrestrial LAN
- Limited power supply on board constrains computational capacity
- Implementation of priorities required
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Technical Challenges (MAC)
MAC Schemes
1. Fixed Assignment
2. Random Access
3. Demand Assignment
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Technical Challenges (MAC)
Fixed Assignment
- Techniques include FDMA,TDMA and CDMA- FDMA and TDMA uses dedicated channels- In CDMA, each user is assigned a unique code
sequence- Data signal is spread over a wider brand width
than the required to transmit the data.
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Technical Challenges (MAC)
Random Access
In RA schemes, each station transmits data regardless of the transmission status of others.
Retransmission after collision creates
- Packet delay - Frequent collisions
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Technical Challenges (MAC)
Demand Assignment
- DAMA protocols dynamically allocate systembandwidth in response to user accounts
- Resource Reservation can be made
- PODA and FIFO combine requests
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Technical Challenges (Routing Issues)
Routing Issues in LEO Constellation
IP Routing
ATM Switching at the satellites
External Routing Issues
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Technical Challenges (Routing Issues)
Routing Issues in LEO Constellation
Dynamic Topology- Handles Topological variations- ISL Maintenance
DT-DVTR- Works offline- Sets time intervals and remains constant until next
time interval
- No of consecutive routing tables are stored and then retrieved when topology changes
VN -Hiding of topology changes from routing
protocols
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Technical Challenges (Routing Issues)
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Technical Challenges (Routing Issues)
IP Routing at Satellites
Seems to be straightforward
Dealing with variable-length packets
Scalability problems
Computational and processing capacity
Research yet to be made on this scheme
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Technical Challenges (Routing Issues)
ATM Switching at the satellites
Many proposed systems use ATM as the network protocol
An ATM version of DT-DVTR is investigated
Modified S-ATM packet
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Technical Challenges (Routing Issues)
External Routing Issues
Internal routing done by Autonomous systems
Internal routing is handled by AS’s own internal routing protocol
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Technical Challenges (Satellite Transport)
TCP/IP UDP/IP
These 2 protocols will continue for now as they have tremendous legacy
• Performance will be any way affected by long latency and error prone characteristics of satellite links
• Researchers are still working in NASA on TCP/IP
•TCP performance will definitely improve
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Technical Challenges (Satellite Transport)
TCP performance over satellite
- Positive feedback mechanism
- Achieve rate control and reliable delivery
Performance enhancement
- TCP selective acknowledgement
- TCP for transaction
- Persistent TCP connection
- Path Maximum Transfer Unit
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Conclusion
Bent pipe and OBP were discussed
MAC, IP routing were investigated
Important research issues
- IP QoS support
- Traffic and congestion control
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