|Issue:||Latin America II 1996|
|Topic:||Advances in Satellite Based Telephony Networking|
|Author:||Dr Rein Luik|
During the fourth quarter of 1995, TIW Systems of Santa Clara, California began shipments of its Flexi-DAMA II satellite communications earth terminals. This new design incorporates new features which will reduce the system cost while at the same time enhancing its communications capabilities. The new system is primarily aimed at thin route voice communications but will also support interactive as well as high speed data communications requirements.
Intra-System Communications As is the case in many of the DAMA SCPC systems currently being deployed around the world, the intra-system communications channel, the so called Common Signalling Channel (CSC), between the remotes and the hub utilizes a contention access scheme in the inbound to the hub direction. In this type of channel the communications carrier is divided into time slots (Time Division Multiple Access)and any terminal requiring communications with the hub may contend for that time slot by transmitting its message in the next available time slot. If two terminals transmit into the same time slot a collision will occur and the information will be lost. In such a case the two or more terminals which collided will retransmit their messages after a randomly selected time delay in order to avoid colliding repeatedly. In a heavily loaded channel the system must prevent repeated collisions in order to prevent a total collapse of the channel throughput due to repeated retransmissions. Many of the products on the market today utilize a special modem for access to the contention access communications channel. In order to minimize the cost of the remote terminals, the TIW Flexi-DAMA system utilizes an idle channel unit to access this channel by retuning to the communications channel frequency when receiving and sending messages to the hub station. This is particularly useful for the thin route stations which are only equipped with one or two channel units. In the Flexi-DAMA system the channel unit is tuned to the intra-system communications channel when it is not involved in traffic service. When a call goes off the hook a call request is transmitted to the hub for assignment of traffic channel frequencies and for notifying the destination station of the incoming call. If the destination station is unavailable the call is busied out. However if the destination station is unavailable then both the originating and destination stations tune to the assigned traffic frequencies and establish communications. Since messages from the remotes to the hub are subject to collisions, there has to be a mechanism to detect such collisions in order to facilitate retransmissions. The common mechanism for ascertaining whether the message was transmitted successfully is to have the hub acknowledge the successful receipt of each message on the outbound communications channel. This channel operates on the Time Division Multiplex basis where outbound messages are continuously transmitted with each message containing the destination station address. All of the available remotes then monitor this channel and filter out the messages destined for them. The intelligence in the hub equipment keeps track of which remotes are busy and therefore can not receive messages. However since messages from the hub to the remotes can be lost for a variety of reasons, the messages are serial numbered and each call setup message is followed by a null packet. If the remote does not receive all the messages in serial order it notifies the hub that it has lost messages. For messages from the remote to the hub, the remote awaits the acknowledgment of messages transmitted to the hub and if no acknowledgment is received within a predefined time then the message is retransmitted after a randomly selected time delay in order to prevent repeated collisions. The intra-system communications channel is also utilized for three other purposes. This channel also handles system monitor and control messages to and from the hub and is utilized to download system operating software changes. The ability to download software changes is an important function of DAMA SCPC networks since it is often difficult and extremely costly to send personnel to the many sites in such networks. The TIW Flexi-DAMA system supports as many intra-system communications channels as are necessary to support the number of remote nodes and channels which are active in the network. Since the TIW Flexi-DAMA system utilizes a parallel processing approach for network control there is virtually no limit to the number of nodes or channels which can be supported in a single network. Since the network supports multiple intra-system communications channels, in large networks a dedicated channel may be assigned for downloading software to new stations entering the network or for updating software at existing nodes of the network. Another function of the intra-system communications channels is to transmit data packets to and from the hub to low throughput data terminals such as bank teller machines or point of sale terminals which may be attached to the remote stations. Since the intra-system communications channel functions identically to the commonly utilized Data-VSAT architectures it is eminently suitable for transmitting such traffic. From the system design standpoint the intra-system communications channels are designed so that half the capacity is allocated for telephone circuit setup messages and the other half is allocated for the transmission of VSAT type data packets. Thus the Flexi-DAMA system combines the functions of an SCPC DAMA voice network and a Data VSAT network. During 1996 the product line will be expanded to include a rate programmable high speed data modem which can be utilized for video conferencing and high speed data circuits with bandwidth assigned by the network system operator on a reservation basis. Thus the TIW Flexi-DAMA system will provide true multimedia support from a single low cost earth terminal. DAMA Control Card Each Flexi-DAMA chassis is equipped with a DAMA Control Card. It is the function of this card to handle the communications with the Network Control at the hub station and with the channel units (up to six Units) installed in that chassis. It is the DCC which formulates the message packets for outgoing messages and extracts the information from the incoming message packets. The DCC also assigns the channel units either to the traffic channels for call processing or to one of the intra-system communications channels. The DCC is also connected to the earth station tracking unit and RF equipment for the purpose of relaying system monitor and control messages. The DCC also downloads the operating software to the flash memory resident on the channel units installed in the terminal. The TIW Flexi-DAMA system includes a unique function to facilitate automatic tracking of the satellite position. Today many of the satellites operating around the world are allowed to drift in inclination angle in order to extend the service life of the satellites. Russian satellites, in particular are not closely station kept. This means that even small aperture antennas have to be equipped with an automatic tracking system in order to maintain system performance at acceptable levels. Since Russian satellites lack a beacon, it is necessary to track a carrier within ones own network. One possibility is to track the outbound from the hub intra-system communications carrier, which is a continous carrier that is always present This method, however, requires that each station has to include a receiver which is always tuned to this carrier. The TIW Flexi-DAMA system, however does not include a special channel unit for the intra-system communications and thus is not able to receive signals from that carrier when all channel units at a remote are assigned to carry telephone traffic. In order to provide for tracking in these circumstances, TIW utilizes the AGC voltage derived from the inbound traffic carrier demodulator for the tracking function. This however requires special processing since the voice traffic carriers are voice activated and therefore the AGC voltage is not stable. Thus, the Flexi-DAMA system provides for processing of the AGC voltage only when the inbound voice carrier is activated. The AGC voltage from the demodulator portion of the channel unit is then routed via the DCC to the Antenna Control Unit for use in the execution of the ‘Steptrack’ automatic tracking function. TIW’s strength as a designer of all the various subsystems involved allows such a integrated system solution to these types of problems which are difficult to resolve on a more piecemeal basis. Front End Processor At the hub station the intra-system communications are handled by a Front End Processor, which is a Pentium type PC computer as illustrated in Figure 5. Each Front End Processor is capable of interfacing with two intra-system communications channels each of which can support up to two hundred channel units. It is the function of the Front End Processor to handle all the messages between to and from the remotes assigned to the dialing group supported by this Front End Processor. The Front End Processors are connected to the Network Master Control Computer and to other Front End Processors via a Ethernet LAN. Call setup messages for calls which originate and terminate within different dialing groups and therefore are supported by different FEP’s are routed via the LAN between the FEP’s. Each FEP maintains in its memory tables of spacecraft channel assignments, call routing instructions, and call blocking instructions for handling the calls within its dialing group. For calls which transition two dialing groups the call originating station assigns a frequency from its channel pool and notifies the destination station of the channel assignments via the LAN. The Master Network Control Computer monitors each FEP channel pool and periodically revises the channel pools in accordance with the level of activity within each pool. Since the Front End Processors serve as the intra-system communications hub for up to 400 telephone channels their availability is extremely vital to the overall network availability. Therefore redundancy of the Front End Processors and their supporting modems is provided by simply equipping the hub with one or more unassigned Front End Processors which can take over the functions of any failed unit in the system. The Network Control Computer monitors the health of all the on line FEP’s and determines if any unit has failed. If a failure is detected then all the appropriate frequency allocation and call routing and blocking tables for the affected dialing group are downloaded to the spare FEP and it assumes the functions of the failed unit. In addition to serving as the hub for the intra-system communications traffic the Front End processors also serve to aggregate and to filter data traffic from the remote sites to the various data hubs being served by the network. This technique allows a single Flexi-DAMA network to service numerous data networks where the data hubs are not co-located with the network control hub. In order to support this function the Flexi-DAMA system incorporates optional rate selectable high speed data modems which are utilized for high rate data traffic or for video conferencing services. Therefore for data networks, with data hubs which are not co-located with the network control hub, pre-assigned high speed data channels route the traffic between the data hub and the network control hub. For such services the Front End Processor interrogates the data packet headers and aggregates the traffic destined for the various data hubs onto the dedicated high speed data carriers or conversely routes traffic from such data carriers to the various remote stations. TIW Systems believes that the Flexi-DAMA system is the only satellite networking equipment which offers this multiple data hub network support capability. In addition to the intra-system communications and data traffic routing functions the Front End Processors also maintain call records for downloading to the Network Control Computer and the Billing System Computer. Since the communications link between the Front End Processors and the Network Control Computer and the Billing Computer is via an Ethernet LAN a Redundant LAN is provided with manually controlled A/B switches for switching between LAN’s. The Network Control Computer and the Billing Computer serve respectively as the A and B LAN managers. Channel Unit The Channel Units are the voice traffic and intra-system communications modems. For voice communications the Channel Unit provides the telephone interface to either a 4 wire or 2 wire circuit with an interchangeable plug-in daughter card for each type of interface. The analog telephone channel is then digitized utilizing PCM coding. A DSP processor on board the Channel Unit provides echo cancellation and voice compression functions. The DSP processor also controls the voice activation feature of the channel unit by detecting the presence of speech in the channel. Once the voice has been compressed the signal enters the transmission modem portion of the circuitry where forward error correction is applied and removed and the signal is QPSK or BPSK modulated for transmission. The modem supports both BPSK and QPSK modulation at information rates of 9.6 Kbits/Sec, 16 KBits/Sec and 32 KBits/Sec with either rate 1/2 or rate 3/4 coding. Because of the use of discrete filters for optimal channel spacing only the rates which are compatible with two passband bandwidth are available in a given configuration. However any Channel Unit can be retrofitted for any transmission rate. Each channel unit incorporates a voice activation feature. This feature is supported by end and beginning of traffic messages which communicate the presence or absence of speech to the modem at the other end of the conversation. When the far end receives an end of speech message it removes its carrier and holds the receiving modem at its last frequency so that reacquisition can be accomplished quickly. When the end of speech message is received the outgoing telephone channel is muted with the exception of background noise which is artificially introduced into the channel. When voice activity returns a beginning of speech header is transmitted to the far end in order to allow lock up of the receive modem and to initialize the FEC decoder. Once those two functions have been accomplished the channel is again opened for voice traffic and the artificial noise generation is removed. Network Frequency Synchronization Because the various local oscillators on the satellite and within the earth station RF equipment drift, and because of Doppler shift effects resulting from the motion of the satellite, a means must be found for synchronizing the frequencies at the various stations in order to speed up the time of carrier acquisition and in order to minimize the guard band between the channels. Some providers of this type of equipment utilize the outbound CSC channel as a means of synchronizing but in the case of the Flexi-DAMA system this method of synchronization has been rejected due to a number of problems inherent with such an approach. Rather TIW Systems has elected to incorporate a GPS receiver into each remote terminal in order to derive atomic clock accuracy over the long term and in order to establish the exact location of each terminal for mobile terminals such as News Gathering Trucks. With the GPS the frequency accuracy of the entire network can be synchronized to a few Hertz including effects of Doppler. Also with the GPS implementation there is no inherent limit to the amount of Doppler which can be tolerated as is the case with many other systems. Highly accurate frequency synchronization will also result in shorter carrier acquisition and consequently faster call setup times. TIW is in the process of applying for a patent on the use of GPS receivers for SCPC DAMA network synchronization. High Speed Data Modems The TIW Flexi-DAMA system includes multiple rate high speed data modems which can be plugged into the remote terminal mechanics to provide high speed data and video conferencing capability. These modems operate at one of four rates 64, 128, 256 or 384 KBits/Sec with either 1/2 rate or 3/4 rate coding. Each high speed modem interfaces with the remote terminal bus architecture, so that the DAMA Control Card is able to assign frequencies, to set the rate of the modem and to turn the modem on and off when required. High speed data channels and video conferencing channels are assigned by the Network Control Center on a reservation basis. The modem interfaces to external equipment via either a V.41 or RS449 interface. Network Control Center The Network Control Center is the operator interface for managing the network. The following functions are performed by the Network Control Center: a. Entering, maintaining and editing the network configuration database. b. Entering and maintaining the call routing matrix. c. Entering, maintaining and editing the call blocking matrix. d. Entering, maintaining and editing the user database. e. Downloading new versions of system operating software. f. Entering, maintaining and editing the satellite channel pool. g. Entering and editing the satellite pool allocation to the dialing groups. h. Collecting displaying and storing network traffic statistics for network planning purposes. i. Archiving of network database information. j. Collecting and storing network call records. k. Planning and allocating high speed data channels on a reservations basis. l. Entering and editing data routing matrices for the various data networks supported by the network. The Network Control Center utilizes a Pentium based PC computer operating under the Windows NT operating system. The user interface is a user friendly Graphical User Interface. Conclusion Although this brief article covers only a few of the capabilities of the TIW Flexi-DAMA satellite communications networking product which was introduced into the market during the fourth quarter of 1995 it does provide an overview of the many powerful features built into this product line. The use of modern packaging and design techniques allows this product to be priced competitively with the older product in the market despite the many new features which are incorporated into the system. Information about this product line is available from TIW Systems Inc. in Santa Clara, California.