Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-11-19
2003-12-09
Rao, Seema S. (Department: 2666)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S343000, C370S436000, C370S468000
Reexamination Certificate
active
06661781
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the efficient and reliable transmission of packet or cell-based information, such as frame relay, SS#7, ISDN, Internet or asynchronous transfer mode (ATM) based information, via wireless (terrestrial or satellite) telecommunication network. The techniques and the resulting wireless network are applicable to both terrestrial and satellite wireless environment and is particularly applicable to an Asynchronous Transfer Mode (ATM) network or frame relay network, although the application to other networks having cell or packet transmissions would be understood by one of ordinary skill in the art without undue experimentation. Although the examples of the applicability of this invention are drawn mostly to an ATM satellite communications network, it would be clear that the underlying invention is applicable to other cell and packet systems.
The implementation of the invention results in a mesh heterogeneous satellite network with the following major characteristics:
Single satellite hop connectivity among terminals of widely varying capabilities (uplink rates varying from 8 Kbps to 8 Mbps); and
Simultaneous Multimedia/Multiservice Communications Sessions from a terminal with multiple destinations via a single modem and a single uplink and downlink chain.
BACKGROUND OF THE INVENTION
While the present discussion uses Asynchronous Transfer Mode (ATM) transmission by way of example to define the problems with conventional bandwidth on demand systems, this invention is not limited to ATM systems. Distinct advantages are available in other transmission systems, including those based on frame relay, Internet and ISDN/SS#7 protocols, when this MCDD architecture is implemented. Problems with conventional ATM and advantages of the present invention are presented in detail, but one of skill in the art would understand that these advantages would also benefit other transmission schemes such as ISDN, Internet and SS#7.
ATM is a transmission and switching technology which has been standardized to carry statistically multiplexed traffic of different kinds—voice, date, and video. The bandwidth requirements for ATM traffic varies with the fluctuations in the user generated traffic, which can vary over a wide range. Also, the traffic generated from a particular source could be intended for a number of geographically dispersed destinations.
These salient features of ATM traffic demand simultaneous connectivity from a source to multiple destinations. The bandwidth requirements for each of these connections can vary widely.
The traditional Frequency Division Multiple Access (FDMA) (either single channel or multiple channels per carrier) method to provide single hop wireless and satellite ATM mesh connectivity among several terminals is very impractical for the following reasons. Simultaneous connectivity among multiple terminals needs a bank of modems and a number of uplink and downlink chains making the system very expensive. Also, the ATM cell transfer delay becomes large for the lower rate carriers, even though the terminal may be transmitting at a much higher aggregate rate.
The traditional Time Division Multiple Access (TDMA) or the enhanced multi-frequency (MF) TDMA systems work well at high rates (e.g. 2 MBPS or higher). However, an MF-TDMA system is not efficient if the carriers are of widely different rates of lower speeds (e.g. 8 Kbps) and higher speeds (e.g. 2 Mbps or 8 Mbps) which need to be considered together as a whole system. Considering that ATM is a scalable technology and that a terminal may be capable of transmitting at only low rates, such as 8 Kbps, to an MF-TDMA system cannot handle ATM traffic from such terminals in an efficient-manner.
Bandwidth on demand is typically achieved in the MF-TDMA system by a separate channel (a time slot on a frequency) from each terminal to a Network Control Center (NCC). According to the present invention, a request for bandwidth can be multiplexed with the traffic on any existing transmitting carrier, thereby reducing the requirement for a request channel considerably. Furthermore, since the bandwidth request can be received by all terminals, a distributed bandwidth allocation algorithm can be easily implemented.
In an MF-TDMA system, traffic from a terminal to multiple destinations is carried in different time slots with different rates. Thus, the traffic at lower rates suffers greater ATM cell delay. According to the present invention, the traffic from a terminal to different destinations can be multiplexed and aggregated on a larger carrier thereby reducing the ATM cell delay.
Finally, the satellite environment leads to a number of major challenges in the ability to provide satellite-based ATM services. These challenges stem from the fundamental differences in the satellite/wireless and fiber environments and from the fact that the ATM network architecture and protocols are designed for fiber optic cable infrastructure.
Specific problems arise because of three inherent characteristics of satellite links—(a) errors, (b) delay, and (c) bandwidth limitations, as discussed in D.M. Chitre, D. Gokhale, T. Henderson, J. Lunsford and N. Mathews, 1994, “Asynchronous Transfer Mode Operation Via Satellite: Issues, Challenges and Resolutions”.
International Journal of Satellite Communications
, Vol. 12, pp. 211-222.7.
Satellite RF links have considerably degraded error performance with respect to fiber optic links. For example, ATM protocols assume that a transmission medium has a very low Bit Error Ratio (BER)(10
−12
) and that bit errors occur randomly. In contrast, the bit error ratios associated with terrestrial wireless and satellite communications are much higher (on the order of 10
−3
to 10
−8
) and tend to fluctuate with atmospheric conditions.
In addition, in satellite systems, typical forward error corrected satellite links will have bursty errors with variable error rate as opposed to random errors on the fiber links. ATM operation is intolerant of burst errors, and the ATM quality of service requirements for multimedia applications are much more stringent than what a typical satellite link will provide. The one-way propagation delay of around 250 msec., which is intrinsic to geosynchronous satellite communications has an adverse impact on the data communication protocols, such as TCP at high speed (fractional T
1
or more), and on the ATM traffic and congestion control procedures.
Furthermore, satellite bandwidth, and terrestrial wireless bandwidth to a lesser extent, is a precious asset. Thus, the cost of bandwidth in a wireless network is much higher than in a fiber optic network. Thus, wireless systems tend to be unable to accommodate the bandwidth inefficient operation of ATM protocols. For example, ATM constant bit rate (CBR) speech will require a 70 kbps satellite bearer channel to carry one voice channel. As a result, the onus is on the operator of a bandwidth limited network to either tolerate the bandwidth inefficiencies of ATM technologies or to design special signal processing equipment that reduces the overhead across a bandwidth constrained network path. Therefore, if ATM technologies are to be implemented in wireless networks, more efficient bandwidth utilization schemes must be achieved, as in the present invention.
Current methods for bandwidth on demand/ATM products rely on the TDMA multiple access technique. Single carrier TDMA is not an attractive solution for bandwidth on demand/ATM because the amplifiers at the user transmitting stations must be sized for the peak power, which is much larger than the average power for single carrier TDMA. This is in addition to the additional complexity usually associated with the synchronization requirements of TDMA.
Traditionally, the selection of the multiple access scheme for ATM transmission over satellites is largely based on tradeoffs such as space segment efficiency, cost and complexity of earth stations, and single versus multiple carrier operation, where multiple carrier operation results in n
Chitre Dattakumar M.
Sayegh Soheil I.
Comsat Corporation
Duong Frank
Rao Seema S.
Sughrue & Mion, PLLC
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