1. Dynamic Access for a Space Communications Network with IP Functionality Hui Zeng and Michael Hadjitheodosiou Center for Satellite & Hybrid Communication Networks, University of Maryland, College Park, MD michalis@umd.edumichalis@umd.edu, zengh@umd.eduzengh@umd.edu June 8, 2004 4 th Space Internet Workshop

2. 4 th Space Internet Workshop (SIW4) 2 Outline  Overview  Space Communications Network Architecture  Traffic Modeling  Hybrid-mode TDMA Protocol  Dynamic Bandwidth Allocation  Simulation Configuration & Results  Future Work

3. 4 th Space Internet Workshop (SIW4) 3 Evolving to the Future Space Communications Network  An increasing number of single or constellation scientific spacecraft with IP-addressable instruments and the new NASA initiative for Moon, Mars and other planets  Internet protocols connecting everybody together and leading a new development phase with wireless extensions of this network in a variety of environments  The ability to use recent advances in communications technologies by investigators on Earth to enjoy a virtual presence in space  Dynamic communication support for bursty mission traffic vs. current pre-planned assignments  Commercial technology and standard protocols reducing development and deployment costs

4. 4 th Space Internet Workshop (SIW4) 4 Significance and Objective  To support both NASA's need to reduce operational costs by using commercial technology and commercial assets and to enable new types of collaborative science, where eventually investigators can access their data from space "anytime, anywhere" via direct communication with the instruments on the spacecraft  To aid the transition to NASA's Vision of enabling an on- demand mission operation that efficiently supports multiple spacecraft with priorities, security and QoS guarantees

5. 4 th Space Internet Workshop (SIW4) 5 Future IP-Based Space Network vs. Terrestrial Mobile Network In the dynamic configuration we are studying, the operation of the future IP-Based mission support network becomes similar to terrestrial cellular network, where  Gateways act as Base Stations  Spacecraft act as Mobile Platforms, registering with respective Gateways as they move in their orbit And in most cases:  Motion and approximate position can be predicted in advance  Active number of users (network size) is known and growth pre-determined (missions launched or decommissioned).

6. 4 th Space Internet Workshop (SIW4) 6 Challenges for Future IP-Based Space Network  Intermittent communication links  Highly asymmetric or unidirectional communication links  Higher bit error rates (BER) than most terrestrial wired links  Multiple mobile nodes forming a dynamic network topology  High mobility (velocity), but often very predictable, since most spacecraft move along pre-defined orbits and their locations may be easily predicted.  Security issues  Tradeoff among the precedence, delay, throughput and reliability

7. 4 th Space Internet Workshop (SIW4) 7 Network Scenario Multi-access for the downlink channel of the TDRS relay system

8. 4 th Space Internet Workshop (SIW4) 8 Network Architecture  Multi-access sharing among a large number of spacecraft for the downlink channel  On-board scheduling and queuing among several streams for each spacecraft  Dynamic bandwidth allocation determined by the scheduler in the ground station  GEO relay satellite system (or non-GEO)

9. 4 th Space Internet Workshop (SIW4) 9 Traffic Modeling for Space Mission (1)  As a typical EOS mission with heavy load, TERRA has five different instruments on board and a total high data rate of 108Mbps to relay through TDRSS.  Multi-state multi-mode for different data rates, on/off switch for active/inactive periods Mission ModeData Rate (Mbps) Day Full Mode89.2 VNIR Mode62.038 TIR Mode4.109 Stereo Mode31.019 NightTIR Mode4.109 SWIR + TIR Mode27.162 ASTER observation modes

10. 4 th Space Internet Workshop (SIW4) 10 Traffic Modeling for Space Mission (2)  By using the ESE TERRA mission, determined the details of the data generation of the instruments and packetization and queuing on-board  Multi-state multi-mode traffic source model with on/off periods to represent realistic traffic generation  On-board Priority queuing for different streams

11. 4 th Space Internet Workshop (SIW4) 11 Hybrid-mode TDMA Protocol  Besides the control slots, use the “piggy-back” method to send the information in the data slots  “Out-of-date” collected information in NCC because of the long propagation delay (Need estimation)

12. 4 th Space Internet Workshop (SIW4) 12 Delay and Time Synchronization  Delay for reservation-mode TDMA : T r-mode = T transmission + T queueing + T propagation + T reservation  270-350ms RTD between GEO and ground station  More than 480ms RTD between LEO and ground station through GEO relay system

13. 4 th Space Internet Workshop (SIW4) 13 Dynamic Bandwidth Allocation (1)  Controls applied to deal with variations in traffic dynamics and with the presence of multiple services, in order to guarantee different performance requirements and avoid congestion.  Direct algorithm for distinct penalties { ν kl } and definite known demands {D kl }  Computation or estimation of the demands from the recent or long-term collected information during multiple frames (stability)  To users with same penalties, fairness and maximum throughput considered together

14. 4 th Space Internet Workshop (SIW4) 14 Dynamic Bandwidth Allocation (2)  Two levels of scheduling:  Burst-level scheduling: performed only once during each frame and allocates timeslots to a stream within a frame in a contiguous fashion.  Packet-level scheduling: performed during each timeslot and one timeslot is assigned at a time.  Burst-level scheduling is more practical and stable for our long delay satellite communications network.  The scheduler generates a bandwidth allocation table (BAT) and sends it back to all the spacecraft. A BAT contains several information fields such as User_ID, First_slot, and Last_slot.

15. 4 th Space Internet Workshop (SIW4) 15 Simulation Configuration  RTD > 0.66 sec, Frame duration = 0.1372 sec  Number of data slots per frame = 64, number of control slots per frame = 4.  Channel capacity = 2Mbps, error-free

16. 4 th Space Internet Workshop (SIW4) 16 Performance of Hybrid-mode TDMA

17. 4 th Space Internet Workshop (SIW4) 17 Hybrid-mode VS Static TDMA

18. 4 th Space Internet Workshop (SIW4) 18 Queuing Delay On-board

19. 4 th Space Internet Workshop (SIW4) 19 Summary  Discussed the architecture for the future IP- based space network and identify some challenges  Developed traffic models of the spacecraft- generated science traffic  Proposed a hybrid-mode reservation-based TDMA protocol with optimal bandwidth allocation, and compared its performance with that of a fixed-assignment scheme by using discrete-event simulation

20. 4 th Space Internet Workshop (SIW4) 20 Future work  Continue optimization of the MAC protocol for different services by modeling different traffic sources, and then re-formulating the resource-allocation problem with consideration of the QoS and other constraints (on-board queue size, fairness, etc.).  Focus on network security we are looking into solutions to make this flexible access to space more secure, with emphasis on efficiency in this constrained environment. Attempt to develop implementable solutions that can be supported by the available bandwidth utilization, storage and computational complexity.  Use our results to study design trade-offs for a future broadband relay satellite constellation with on-board switching and inter-satellite links between relay spacecraft.  Focus this work to address requirements for supporting the new office of space exploration initiative.