1 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

2 Outline Introduction: Broadband Satellite Access(BSA) Objective IP-Based BSA Systems TCP over BSA Systems Conclusion

3 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

4 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 GHz) and Ku (10.95 to 14.5 GHz) bands

5 Introduction (cont’) BSA system configuration

6 Introduction (cont’) Example (Digital Video Broadcast: DVB-RCS)

7 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

8 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.

9 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

10 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

11 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

12 Objective (method 2 vs. method 3)

13 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.

14 IP-Based BSA Systems (cont’) User Traffic Protocol Stack

15 IP-Based BSA Systems (cont’) T1, T2 handles DiffServ Packet

16 IP-Based BSA Systems (cont’) Simplified drawing of DiffServ Implementation

17 IP-Based BSA Systems (cont’) Representation of bandwidth assignment with coordinates of (time, frequency)

18 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),  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

19 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)

20 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

21 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

22 TCP over BSA Systems (cont’) Performance

23 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

24 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.

25 Question?