Survey of Satellite-Based Internet

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

Survey of Satellite-Based Internet 2

Content

Introduction

Satellite Communication Fundamentals

Satellite-Based Internet Architectures

Some Examples of Satellite Systems

Technical Challenges


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.


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


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


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)


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.


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.


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.


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


The satellite-based Internet with bent pipe architecture


The satellite-based Internet with OBP and ISL architecture


Next Generation Satellite Systems


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.


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.


Technical Challenges

Multiple Access Control

Routing Issues in Satellite Systems

Satellite Transport


Technical Challenges (MAC)

Multiple Access Control (MAC)

1. Performance

2. Schemes

3. Implementation



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



MAC Schemes

1. Fixed Assignment

2. Random Access

3. Demand Assignment



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.



Random Access

In RA schemes, each station transmits data regardless of the transmission status of others.

Retransmission after collision creates

- Packet delay - Frequent collisions



Demand Assignment

- DAMA protocols dynamically allocate systembandwidth in response to user accounts

- Resource Reservation can be made

- PODA and FIFO combine requests


Technical Challenges (Routing Issues)

Routing Issues in LEO Constellation

IP Routing

ATM Switching at the satellites

External 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



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



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



External Routing Issues

Internal routing done by Autonomous systems

Internal routing is handled by AS’s own internal routing protocol


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


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


Conclusion

Bent pipe and OBP were discussed

MAC, IP routing were investigated

Important research issues

- IP QoS support

- Traffic and congestion control

Survey of Satellite-Based Internet

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