Transcript EBCOT - Washington University in St. Louis

Interactive Multimedia Satellite
Access Communications
Tho Le-Ngoc, McGill University
Victor Leung, University of British Columbia
Peter Takats and Peter Garland, EMS Technologies
Appear in IEEE Communications Magazine, July 2003
Reviewed by Huakai Zhang, 12/4/2003
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Outline

Introduction: Broadband Satellite Access(BSA)

Objective

IP-Based BSA Systems

TCP over BSA Systems

Conclusion
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Introduction

Collaborative research in telecommunications
A Canadian experience:

Dynamic satellite bandwidth allocation

An architecture for DiffServ provisioning over BSA
systems

A dynamic TCP Vegas protocol as a proxy service for
split-TCP connections over BSA systems
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Introduction

Satellite Communication

An alternative to traditional(terrestrial) communication

Most satellites operate in the microwave region

Microwave satellites operate on assigned frequency
bands designated by a letter. Common communications
satellite bands are the C (3.4 to 6.425 GHz) and Ku
(10.95 to 14.5 GHz) bands
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Introduction (cont’)

BSA system configuration
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Introduction (cont’)

Example (Digital Video Broadcast: DVB-RCS)
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Introduction (cont’)

Broadband Satellite Access(BSA)

Pros
1. Provide connectivity in remote areas, continental coverage
2. Avoid unpredictable congestion and delay (only one hop)
3. Ideal for real-time multicast and broadcast services

Cons
1. Lack of economical satellite-based return/interaction
2. High cost compared to terrestrial technologies
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Objective

Dynamic Capacity Allocation
All User Terminals(UTs) share
the same return link using a
Multi-Frequency Time-Division
Multiple Access (MF-TDMA)
scheme, which needs to be dynamic
to ensure efficient return link
utilization.
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Objective (media access method 1)

Random Access: ALOHA

No channel setup/tear-down, but limited channel
utilization

Ideal for low data rates/fast response app., such as
bank transactions
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Objective (media access method 2)

Demand Assigned Multiple Access (DAMA)

High QoS, but need setup phase to reserve capacity

Ideal for VoIP and Video Conf. use pre-defined
constant bit rate, i.e PAR ~ 1
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Objective (media access method 3)

Combined Free/DAMA (CFDAMA)

Freely allocate the remaining capacity to the UTs to make
the response time small, while keeping good QoS and high
channel utilization

Ideal for bursty multimedia services with diverse QoS
requirements
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Objective (method 2 vs. method 3)
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IP-Based BSA Systems

Goal

End users are assumed to be IP-based.

It is desirable for BSA systems to interoperate
seamlessly with the terrestrial IP networks and to
be compatible with IP-based technologies and
protocols.
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IP-Based BSA Systems (cont’)
User Traffic Protocol Stack
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IP-Based BSA Systems (cont’)
T1, T2 handles DiffServ Packet
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IP-Based BSA Systems (cont’)
Simplified drawing of DiffServ Implementation
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IP-Based BSA Systems (cont’)

Representation of bandwidth assignment with
coordinates of (time, frequency)
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IP-Based BSA Systems (cont’)

DVB-RCS bandwidth allocation mechanisms






Continuous Rate Assignments (CRA),
VoIP
Rate Based Dynamic Capacity (RBDC),
Web traffic
Volume Based Dynamic Capacity (VBDC), Email
Absolute Volume Based Dynamic Capacity (AVBDC)
Free Capacity Assignment (FCA)
Mapping for the DiffServ PHB and DVB-RCS



Expedited Forwarding(EF) <> CRA / RBDC
Assured Forwarding(AF) <> RDBC
Default(DE) <> FCA / VBDC
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TCP over BSA Systems
Why

TCP throughput is degraded in satellite links?
High delay-bandwidth networks with short
connection and TCP flow control (e.g. WWW traffic)
(high packet loss rate, long RTTs)
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TCP over BSA Systems (cont’)

Solutions

Link Layer: Retransmission and error correction

End-to-end: Extension/options of TCP, e.g. slow start
modification, but very limited

Performance Enhancement Proxy (PEP):
The most effective one, containing virtual TCP
senders/receivers between terrestrial IP networks and
satellite links
Solutions have to cope with end-user transparency
and/or protocol stack redefinition
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TCP over BSA Systems (cont’)

Proposed TCP Proxy Service

Dynamic congestion control mechanism (DVgas),
uncoupling of flow control and error recovery
mechanisms.

Active Queue management by RED

Immediate Feedback Mechanism results in few
dropped packets
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TCP over BSA Systems (cont’)
Performance
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TCP over BSA Systems (cont’)
Effects
of Traffic Load and BER on throughput
100% with 10^-7 BER, 80% with 10^-7 and 20% with 10^-6 BER, 20% with 10^-6 BER
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Conclusion
This
paper gives an overview of key innovations on
BSA system architecture.
CFDAMA offer
short delay, high channel utilization.
It is suitable for DiffServ provisioning over BSA
systems
A proxy
service to improve TCP performance is
presented.
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Question?
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