A slight variation of the TDMA access method is the Code-Division Multiple Access (CDMA) technique, also known as spread-spectrum systems. In this scheme, transmissions from the earth stations are spread over the time slots using a unique code identifier. This helps to combat signal jamming. For this reason, this scheme is used frequently by the military.
Another variation of the TDMA access method that is projected to be used in most future satellite systems is the Multifrequency Time Division Multiple Access (MF-TDMA). This method extends the single frequency scheme used by conventional TDMA into the use of multiple frequencies that can be shared by all earth stations. MF-TDMA therefore increases bandwidth and reduces antenna size.
The third existing satellite link access method is Demand-Assignment Multiple Access (DAMA). This technology allows dynamic allocation of bandwidth based on the needs of the network user. DAMA is suitable for use when communication between satellites is not required to be continuous. This permits the alternation of channels by which the ATM cells are transmitted as opposed to establishing a single channel and maintaining this connection continuously.
DAMA can be combined with other access methods such as MF-TDMA or SCPC. Using these separate technologies together will allow the system to take advantage of the benefits of both. For example, as mentioned above, DAMA is suited for non-continuous transmissions. Coupling this with SCPC which is suited for continuous connections can help to achieve greater efficiency in the ATM over satellite network configuration.
Error Control
A well-known problem facing satellite transmission systems is its susceptibility to burst errors. This characteristic is created by the variations in satellite link attenuation and the use of convolutional coding to compensate for channel noise. ATM systems are also suited for handling random errors in lieu of burst errors. Multiple burst errors in an ATM over satellite system may therefore cause many ATM cells to be discarded during transmission. In order to alleviate this problem, an efficient error checking and/or error correcting mechanism should be in place when implementing this type of system.
Implementing an automatic repeat request (ARQ) technique at the link layer of the protocol stack can help to reduce the high error ratio levels created by burst errors. There are three common versions of ARQ used in this situation; stop-and-wait, Go-back-N, and selective-repeat. Most existing ATM over satellite networks use of the Go-back-N scheme. Stop-and-wait is simple but not effective in the satellite environment due to the long propagation delay. Selective-repeat has the benefits of good throughput and error performance but suffers from the disadvantages of sender and receiver complexity and the potential for out-of-order packet reception.
Traffic Management
Traffic and congestion control are very important issues facing the designers of ATM over satellite networks who wish to maintain a high level of Quality of Service (QOS). The long propagation delays of satellite systems coupled with their limited bandwidths (as opposed to the bandwidths of optical fiber links typical of ATM land-based systems) make the efficient implementation of these control functions imperative. Poor overall system performance caused by the neglect of proper traffic and congestion control mechanisms can serve to make the ATM over satellite network unusable.
Three common traffic control techniques used with land-based ATM systems are traffic shaping, connection admission control (CAC), and deliberate (selective) cell dropping. Although these methods work well for the land-based systems, they need to be modified for acceptable use with an ATM over satellite network and to maintain the appropriate QOS.
Traffic shaping changes the characteristics of cell streams to improve performance. Some examples of traffic shaping are peak cell rate reduction, burst length limiting, and reduction of Cell Delay Variation (CDV) by positioning cells in time and queue service schemes. The limitation of this scheme in relation to the ATM satellite environment is the inability to dynamically change the traffic parameters during network congestion.
CAC is an effective traffic control mechanism when used with systems that experience occasional congestion. This method is a set of actions that the system can take to allow or disallow a network ATM connection to be established based on the amount of network congestion at the present moment. In the ATM satellite system, however, this scheme seems to be effective during the ATM connection phase only. The long propagation delay of the satellite portion of the system precludes this technique from being useful during transmission. If the system faces more than occasional congestion, the performance suffers from the inability to establish ATM connections.
The deliberate or selective cell dropping technique is based on the idea of potentially dropping a cell when the network becomes congested. The determining factor concerning which cells are to be dropped is the Cell Loss Priority (CLP) bit contained in the cell. (See Figure 2 for the location of the CLP bit in the ATM cell.) This scheme is not suited for the ATM satellite environment since it can cause many dropped cell retransmissions over long propagation delays thereby hindering overall performance.
Two additional schemes proposed for use with ATM over satellite networks are Explicit Forward Congestion Indication (EFCI), also known as Forward Explicit Congestion Notification (FECN), and Backward Explicit Congestion Notification (BECN). EFCI is a technique used to convey congestion notification information from the destination to the source via communication to its peer in the higher protocol layers. The source can therefore take appropriate action to reduce additional traffic through the present channel. The problem with this method is that at least a one-way propagation delay is required to notify the source of the congestion. BECN is a faster mechanism than EFCI since a congested network can use this technique to send congestion information in the reverse direction of the network flow to indicate the problem without requiring peer notification. However, like the EFCI technique, BECN is also subject to long propagation delays if the congestion is occurring at the destination.
Bandwidth Management
Since bandwidth for satellites is limited, proper bandwidth management in the ATM over satellite system is critical. A substantial degradation of the overall performance of the combined terrestrial and satellite system will severely inhibit its usefulness. Applications requiring high bandwidth allocations are particularly affected by this issue.
Bandwidth management is a difficult matter to handle within a satellite network. One possible way to help with this problem is to allocate bandwidth for the channel at the connection setup phase by using a Burst Time Plan (BTP). This traffic assignment scheme is a mapping tool that indicates the position and lengths of bursts in the transmission frame. The BTP restricts the number of ATM cells in bursts or subbursts that each earth station can transmit. The number of Virtual Paths (VP) and Virtual Channels (VC) of the ATM connection can also be restricted by the BTP to help with bandwidth management.
SUMMARY
The usefulness of a combination ATM and satellite network will be determined by its ability to maintain the QOS of terrestrial-based ATM systems. This will require a seamless integration of the two systems without producing serious performance degradations and/or error increases. The challenge facing designers desiring the combined benefits of the distance advantages of satellites and the speed and reliability of ATM is formidable. Many of the problems and concerns discussed in this document remain unresolved, precluding the worldwide implementation of an ATM over satellite system. As network technology advances, perhaps new schemes and techniques will be developed enabling some of the limiting factors of complexity, cost, and delay to be alleviated. These issues will have to be resolved if ATM technology over a satellite network is to play a significant role in the rapidly evolving information infrastructure.