Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
2001-12-21
2002-11-26
Hsu, Alpus H. (Department: 2662)
Multiplex communications
Communication over free space
Repeater
C370S347000, C370S437000, C370S442000, C455S450000, C455S453000, C455S525000
Reexamination Certificate
active
06487183
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates generally to an improved communications system and more specifically to an improved satellite communications system. Still more particularly, the present invention relates to a method and system of assigning communication channel resources of an interactive multimedia satellite communications system to increase throughput efficiency while maintaining low access delays.
2. Description of the Related Art
Factors driving the need for broadband communications arise from changing user needs and demands. The International Telecommunications Union (ITU, formerly known as CCITT) defines broadband service as a service requiring transmission channels capable of supporting rates greater than 1.5 Mbps or a primary rate in ISDN or T
1
or DS
1
in digital terminology. A broadband integrated services digital network (BISDN) technology framework involves asynchronous transfer mode (ATM) as a protocol for coordinating information between source and destination nodes. Previously, public network needs were driven by telephoning, voice data. Data traffic has grown slowly until recently. With the lower cost in telecommunications and the higher increase in processing power of computers, the numbers of users accessing communications networks has increased. The needs of these users include, for example, video telephone, low cost video conferencing, imaging, high definition television (HDTV), and other applications requiring multimedia data transfers. Multimedia combines different forms of media in the communication of information between a user and a data processing system, such as a personal computer. A multimedia application is an application that uses different forms of communications within a single application. Multimedia applications may, for example, communicate data to a user on a computer via audio, text, and video simultaneously. Such multimedia applications are usually bit intensive, real time, and very demanding on communications networks.
Satellite communication systems are generally known to facilitate communication with user terminals across most areas of the Earth. In a satellite communication system, at least one satellite operates from an orbit above the earth. International telecommunication satellites typically operate from a geostationary orbit (GEO) that is approximately 36,000 kilometers above the surface of the earth.
Satellite communications, in particular GEO orbit satellites communications, are subject to long access delay. Contention access methods which can serve a large number of low duty cycle terminals well in low delay terrestrial radio environment do not perform efficiently in satellite systems.
Traditional very Small Aperture Terminal (VSAT) systems are designed to handle smaller numbers (a hundred or less) of terminals of relatively higher duty cycle. These VSAT systems often utilize a reservation based system, wherein a small amount of bandwidth is dedicated to each terminal for communicating a terminal's request to the gateway or hub. However, such reservation based system tends to be inefficient when a large number of low duty cycle terminals are sharing a pool of transmission channels.
Two specific examples of current contention access protocols are ALOHA and Reservation based. An ALOHA contention protocol will give very poor delay and throughput performance when utilized for a system with a large, active terminal population. In a system utilizing an ALOHA contention protocol, every terminal at every instant has the right to transit over the channel. There will be collisions, and the colliding packets will be destroyed. When this happens, each terminal waits a random period and then resends its packet. This provides low delay with minimal control overhead when the system is very lightly loaded or has relatively few active terminals. As the loading increases, so does the number of colliding packets, and thus the average delay associated with each packet. Further, the efficiency drops as more of the channel is wasted by colliding packets. The maximum throughput in an ALOHA system is limited to about 36% of the system capacity. As the load offered by the terminals approaches and passes this limit, the system throughput drops and the delay performance deteriorates rapidly.
There are many performance limitations of the circuit switched and burst-by-burst media access control (MAC) protocols. Circuit switched protocols assign fixed bandwidth to each active terminals. This works very well for relatively constant rate terminals where the channel size can be matched to the terminal transmission needs. However, for large populations of bursty multimedia users, circuits sized to the average terminal transmission need will give very poor delay performance, and circuits sized to give good delay performance to handle the terminal bursts will result in very poor system utilization.
Considering circuit switched performance for a simple example wherein 100,000 subscribers share 100 time division multiple access (TDMA) channels, and wherein the multimedia traffic offered by the user to the terminal is a block of data equivalent to 1 second of the TDMA channel every 100 seconds. This gives the average offered load of 1%. With 50 simultaneous sessions, the average assignment to each user is at most 2% of the TDMA channel. The delay performance is poor in that a terminal with this assignment would require 50 seconds to transmit each block of data. The channel utilization is 50% in this case. The delay performance could be improved by raising the connection rate. However, a higher connection rate reduces the number of subscribers that each channel would be able to support. The higher connection rate will also result in poorer utilization of the TDMA channel as each connection sits idle between bursts.
Burst-by-burst MAC protocols have the potential to provide better delay and channel utilization performance, but at the cost of significantly higher signaling overhead. In the burst-by-burst protocol, the subscriber makes a request for a specific number of TDMA slots and after receiving an allocation, transmits only in those slots. There is an intrinsic 0.5 second double hop delay in this approach. In addition, it requires significantly greater signaling overhead associated with the resource requests and allocations. Still greater delays may occur if the reservation request channel operates with a contention protocol with-the result that simultaneous requests collide and must be resent. With this MAC approach, it is possible for the signaling overhead to swamp the traffic data. The burst-by-burst MAC protocol would perform very well for traffic with characteristics described above. However for the case where the same quantity of traffic arrives as a constant stream of bursts, the signaling overhead inefficiency becomes apparent.
Assume that the traffic arrives in blocks of data equivalent to 0.01 second of the TDMA channel every second. The terminal needs to make a request and receive a burst assignment for each of these blocks. The request/assignment exchange introduces a delay of at least 0.5 second. Further, there is a delay waiting for the allocated slot time. Also, the high signaling requirement can result in a low overall efficiency.
Interactive multimedia traffic is characterized by intermittent bursts at very high rate surrounded by periods of inactivity and a resulting low average rate. A switched circuit connection approach performs poorly for multimedia traffic. With circuit switching, a terminal must establish and maintain a connection for the duration of its session. If the circuit rate is near the average data rate, the link can have extremely poor delay performance. If it is set near the burst rate, the link will be very inefficient. In the first case, the subscriber is likely to be dissatisfied with the performance. In the latter case, the subscriber is likely to be dissatisfied with his usage charges.
Terrestrial wireless broadband networks have been studied extensively in recent years
Lo Wing F.
Watson Jeff L.
Bracewell & Patterson L.L.P.
Crane John D.
Hsu Alpus H.
Nortel Networks Limited
Qureshi Afsar M.
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