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«A Survey of Congestion Control Techniques and Data Link Protocols in Satellite Networks1 Sonia Fahmy, Raj Jain, Fang Lu2 and Shivkumar Kalyanaraman ...»

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A Survey of Congestion Control Techniques and Data Link

Protocols in Satellite Networks1

Sonia Fahmy, Raj Jain, Fang Lu2 and Shivkumar Kalyanaraman

Department of Computer and Information Science

The Ohio State University

Columbus, OH 43210-1277, USA

Phone: 614 292-3989, Fax: 614 292-2911

Email: ffahmy, jain, u, shivkumag@cis.ohio-state.edu


Satellite communication systems are the means of realizing a global broadband integrated

services digital network. Due to the statistical nature of the integrated services tra c, the resulting rate uctuations and burstiness render congestion control a complicated, yet indispensable function. The long propagation delay of the earth-satellite link further imposes severe demands and constraints on the congestion control schemes, as well as the media access control techniques and retransmission protocols that can be employed in a satellite network. The problems in designing satellite network protocols, as well as some of the solutions proposed to tackle these problems, will be the primary focus of this survey.

1 Introduction Satellite communication systems play a crucial role in the integration of networks of various types and services, and will potentially be used for a wide range of applications. Satellites will continue to play an ever-increasing role in the future of long-range communications 21.

Satellite applications range from navigation, weather monitoring and terrain observations to deep-space exploration.

Satellites of various sizes and capabilities have been launched to serve almost all countries of the world 21. The main advantages inherent to satellite communications are the broadcasting capability, the full connectivity of stations, the exibility of station organization, the capacity to support mobile users, and the high transmission quality. In addition, satellite networks can sometimes have the ability to obtain bandwidth on demand.

Several constraints are, however, also inherent in satellite systems. The resources of the satellite communication network, especially the satellite and the earth station, are essentially of a high cost and limited capabilities and exibility. The most crucial problem, however, is the long propagation delay of the user-satellite links, which renders many traditionally employed schemes such as congestion control schemes, media access control techniques and Submitted to the International Journal of Satellite Communications, 1995.

Fang Lu is currently with Lucent Technologies, New Jersey, USA.

retransmission protocols ine cient. The integration of various types of tra c on the links further complicates these issues.

This survey carefully examines some of the solutions proposed to develop e cient protocols for satellite networks, and exposes several problems that are yet to be tackled. The issues pertaining to devising a congestion control mechanism suitable for integrated services tra c on satellite networks are particularly emphasized.

The remainder of this survey is organized as follows. First, the various media access control techniques for satellite networks are examined, exposing several limitations in satellite networks. Then, the use of satellites in interconnecting local area networks is highlighted. The adaptation of retransmission protocols to compensate for the long propagation delay of satellite links is outlined next, followed by a brief discussion of routing schemes. The integration of various types of tra c in Broadband Integrated Services Digital Networks connected via satellites is then explored, and a discussion of the problems in devising a congestion control mechanism for satellite networks concludes the survey.

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as on-board processing OBP. Several schemes developed for OBP systems will be discussed 42, 33, 52, 36, 25, 2, 14, 27.

2 Media Access Control The development of a multiple access protocol suitable for satellite networks has been the focus of extensive research. A multiple access protocol can be de ned as a set of rules for controlling the access to a shared communication channel capacity among various contending users 44.

Earth stations in a satellite network must share the network transmission capacity, and the satellite should be able to handle many simultaneous uplinks and downlinks. Hence some means of controlling access to the transmission medium is needed to provide for an e cient use of its capacity.

Several reservation and random access protocols have been developed for media access control in satellite networks. This section investigates several mechanisms for arbitrating media access, and compares their features and performance. Several problems associated with satellite networks are also brie y examined in this context.


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2.1 Time Division Multiple Access TDMA Most of the research that investigates multiple access schemes focuses on the Time Division Multiple Access TDMA and the slotted-ALOHA schemes. A number of researchers have argued that TDMA is the most suitable mechanism for future broadband integrated services networks, since the e ciency in the use of the satellite channel is high 30, 38. This section is devoted to examining some of the major issues related to the TDMA access scheme, while the next section investigates slotted-ALOHA.

TDMA allows a station to transmit only during its allotted time interval. Some of the issues pertaining to burst synchronization, time-slot-assignment, fading and data link layer design of TDMA satellite networks are discussed in this section. In addition, several variations on the basic TDMA scheme have been developed and analyzed, and a few of them are presented here.

2.1.1 Satellite-switched Time Division Multiple Access Satellite-switched Time Division Multiple Access SS-TDMA is one of the connection schemes of a multibeam satellite communication system, where beam switching is performed in the satellite. This scheme is expected to be used for integrated services networks. Burst synchronization and time slot assignment for SS-TDMA are discussed next.

The traditional single-feedback loop for burst synchronization does not achieve good results for many stations in an SS-TDMA multibeam satellite system. In 39, it is proposed that the timing would still be synchronized to the reference station, but the burst synchronization be controlled by the stations for each beam. This still maintains the advantages of the feedbackloop scheme over the closed-loop scheme, while eliminating the performance degradation resulting from simply applying a single feedback loop.

Several sequential and parallel algorithms have been presented for time-slot assignment in SS-TDMA systems with variable-bandwidth beams 11. The algorithms are based on formulating the time-slot assignment problem as a network- ow problem to nd a circulation in a graph model representing the switching system. An SS-TDMA system with variablebandwidth uplink beams and downlink beams is presented as an N tra c matrix M N M

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given tra c matrix that satis es some constraints to get maximum tra c handling capacity and minimum overall time slot assignments. The divide-and-conquer approach is used for constructing the algorithms, where a given tra c matrix is decomposed into two smaller tra c matrices. The new algorithms are proved to achieve an improved time complexity 11.

2.1.2 TDMA Enhancements and Experiments Several other modi cations to the basic TDMA scheme have been proposed to improve its performance, and make it dynamically adapt to integrated services tra c. A load adaptive TDMA communications link interconnecting broadband multimedia packet streams has been analyzed in 43. The channels are dynamically assigned to the network stations on the satellite backbone link, and the stations support packetized voice and data message streams.

Statistical multiplexing is employed, as well as a variable bit rate packet voice encoding scheme. Algorithms are proposed for allocating the shared backbone channels to the stations, and their performance is analyzed using voice and data packet delays and packet blocking probabilities as metrics. Fairness issues are examined, and e ects of propagation delay are discussed. Performance degradations are observed as the allocation delay increases, as well as when the mean call-on" and call-o " state durations of data sources decrease.

An on-board switching satellite network employing a modi ed TDMA technique is described in 42. The main objective of the scheme is to reduce the end-to-end delay. For up-link access, DTDMA is used where slots are assigned to an earth station on demand, with a minislot for each earth station to transmit its request, so that the access delay is shorter than the xed TDMA when the tra c is light or unbalanced. For the on-board switching, the multiple input queuing approach is used with separate queues for each down-link destination.

This solves the problem where a packet is blocked in the queue even if the out port is available until the packet is at the head of the queue. Thus the switching delay decreases.

Asynchronous TDMA is used in transmission on to the down-links to further reduce the transmission delay. An independent queuing system is used as the system model for M=G=1 analysis.

Another adaptive satellite TDMA scheme is the FIFO Ordered Demand Assignment FODA access scheme. FODA supports both real-time and non-real-time tra c 10. The quality of service achieved by a VBR video application characterized as a Markov process with Bernoulli scene changes is analyzed and the queuing model of the system is solved in 15.

A conservative approach in bandwidth allocation is adopted, because a low packet loss ratio is desired. Bandwidth allocation is centrally controlled by a station sending a reference burst every 20 milli-seconds. Each FODA frame is divided into a stream sub-frame allocated at setup time and a datagram sub-frame. The bu er occupancy and bounds on the maximum access delay are computed using a new algorithm.

The fading of the satellite links under atmospheric attenuation is an important problem, and a method to counteract this fading in TDMA systems is discussed in 26. When the bandwidth of a satellite link narrows by rain or clouds, the rst step to make up this loss is to increase the up-link power up to 8 of attenuation. If the fading is still worse than a certain dB threshold, di erent transmission bit rates and Forward Error Correction FEC coding rates are used. If the total link attenuation exceeds the range of the above countermeasures, the transmission parameters are reset to nominal values. Because of the correlation of the events that cause the link attenuation, many of the links in the system can become attenuated at one time, thus the e ciency of full adaptive resource sharing is constrained. An extra time slot is used in each TDMA frame to compensate for this problem so as to maintain the required system availability 26.

As previously mentioned, the propagation delay is signi cantly higher in a satellite packet communication network than in a local or wide area network. The isolation of the Media Access Control MAC and Logical Link Control LLC layers is contrasted to combining them in the satellite packet communication network in 47. The throughput and the average response time are evaluated for both types, using two MAC protocols: the TDMA-Reservation and the slotted-ALOHA. For TDMA-Reservation which exhibits a high overhead, combining the two layers shows signi cantly better performance, while for slotted-ALOHA, no real di erence can be observed. A connection-oriented LLC sublayer is employed 47.

In the isolated-type system, LLC frames are handled simply as data frames in the MAC sublayer. Thus in the TDMA-reservation case, not only the transmission of the reservation frame, but also the transmission of various control frames in the LLC sublayer must be executed using the minislots. In contrast, in the combined-type system, the reservation frame must be sent only when the transmission request for the data frame is produced in the LLC sublayer. Various control frames can be sent without reservation. Because of this, the overhead is smaller in the combined type than in the isolated type.

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