Satellite Communications Modem Unit COM-U—
Enhanced Maintenance, Operations, and Spectrum Utilization Efficiency of Satellite Transponders for Remote Island Satellite Communications and Disaster Relief Satellite Communications

2.1 Enhanced utilization efficiency of satellite transponders

The NTT Group is currently providing services using satellite transponder bandwidth leased from satellite operators. Transponder bandwidth is costly, so it is critical to utilize this bandwidth as efficiently as possible. Guard band settings between adjacent carriers can be greatly reduced by adopting a HEGMM in the COM-U that is capable of allocating carriers at arbitrary speed over arbitrary frequencies. This makes it possible to allocate carriers much closer together than with the prevailing system, which means that transponder bandwidth is significantly reduced assuming the same number of carriers. To put it another way, this markedly increases the number of carriers able to use the same frequency bandwidth.

2.2 Improved maintenance and operations

The components of the current system are separated function by function, and several devices interwork to terminate satellite lines. This involves a lot of wiring between devices, which requires a lot of extra effort to assemble, install, and replace the wiring. Likewise, increased wiring increases the likelihood that more wiring-related mistakes will occur.

The COM-U is compactly integrated in a single housing by packaging functionally similar components together. When the equipment malfunctions, recovery time can be minimized since the technicians only have to replace the malfunctioned package. This single housing configuration means that the number of system components to be maintained is also greatly reduced, which further improves the maintenance and operability. In addition, the various packages for the disaster relief satellite communications system and the remote island satellite communications system are commonly integrated in the same housing.

3.1 Disaster relief satellite communications system

A schematic of the disaster relief satellite communications system is shown in Fig. 4. In this system, the COM-U connects to the Internet protocol (IP) network via the circuit interface, which interconnects via satellite channels to the PUE set up at the disaster site. The circuit interface of the COM-U for disaster relief communications is equipped with an Ethernet port for connecting to the IP network. In the outbound direction from the COM-U to the PUE, the COM-U unit supports up to thirteen 1536-kbit/s communication channels (with each channel shared by four earth stations), and in the inbound direction from the PUE to the COM-U, the COM-U supports a maximum of fifty-two 384-kbit/s communication channels. Since the current system in service uses the same frequency width and a maximum of 32 channels, the new system greatly expands the channel capacity. In addition to the channels already described, there are also control channels (CSC channels). The SAT-EMS sends control commands to the COM-U to start/stop communication and to set up channels, which are then sent on to the PUE over a CSC channel. Moreover, the COM-U can monitor the operations of up to 52 PUEs via CSC channels, and send an alarm to the SAT-EMS if a malfunction is detected.

Fig. 4. HESCS for disaster relief satellite communications system.

In terms of specific services, the PUE can simultaneously provide voice (voice over IP) and IP data communications. To prioritize voice communication from the COM-U to the PUEs, the COM-U has a priority control capability it can impose on IP packets. Priority control is implemented by referring to the PCP (Priority Code Point) value in the VLAN (virtual local area network) tag of the IEEE^* 802.1Q frame input from the network.

3.2 Remote island satellite communications system

In the remote island satellite communications system, the COM-U is connected to the SYS-U at the circuit interface. An RS-422 port is implemented at the circuit interface for remote island communication to connect to the SYS-U. Setting up COM-Us and SYS-Us on both the mainland and island means that the COM-Us are connected via satellite channels, while the transmission switching equipment is also connected by satellite links. The COM-U can simultaneously transmit and receive up to fifteen 1544-kbit/s channels.

Telephony, leased-line, and other existing services are delivered over networks using time-division multiplexing, and thus, the operation clocks of devices on the network must be precisely aligned and the entire network synchronized. To achieve network synchronization, the clocks of subordinate or slave devices must be synchronized to the clock supplied from the master clock. In terrestrial systems, clocks from the master clock are typically allocated to each building, where the CSM generates a clock based on the master clock, which the CSM distributes to all equipment in the building.

Similarly, network synchronization is required in the remote island satellite communications system in order to provide telephony and leased-line services. Network synchronization is achieved on the mainland side by the CSM, which is supplied over the terrestrial transport network, which then supplies clocks to the various COM-U and SYS-U devices.

The island side COM-Us and SYS-Us must also be synchronized to the mainland equipment, which means that the island side CSM must somehow receive a synchronized clock from the mainland CSM. In the remote island satellite communications system, the clock is transmitted to the island over a satellite channel. This is done by first generating a modulated signal in the mainland COM-U using the synchronized clock symbol rate of the mainland CSM. The modulated signal is then transmitted to the island over a satellite channel. The island COM-U then extracts the symbol rate clock from the received modulated signal. A 6.312-MHz signal is generated from the extracted clock, which is then supplied to the island CSM from the island COM-U clock transmitter. As a result of this procedure, the island CSM becomes network synchronized, and the clock is then supplied to all buildings with COM-U and SYS-U equipment on the island.

The clock is transmitted to the island over a satellite channel as we have described, but this raises the possibility that the quality of the clock sent from the COM-U to the island CSM could be degraded as a result of radio interference or another problem on the satellite channel. If a poor quality clock is sent to the island CSM, it could adversely affect all of the equipment that relies on the clock.

One solution would be to monitor the clock quality at the COM-U clock transmitter and stop the clock from being delivered if it is degraded. For example, if the satellite channel used to send the clock is out of sync, or if the reception level has fallen below a certain level, or the component in the COM-U that handles delivery of the clock has malfunctioned, clock output from the clock transmitter to the island CSM would be suspended. If any of these contingencies occur, services continue based on clock quality that is sufficient at least for the time being by having the island CSM switch over to a free-running clock.

A prototype of the new COM-U is shown in Fig. 5. To enhance maintenance and operations of the new system, functional capabilities are implemented as discrete packages, so problems can be quickly and easily resolved by simply swapping out the defective package. The packages are also hot pluggable, so they can be replaced with the power left on without affecting other packages or the main signal transmission. In addition, settings information for each package is backed up in the COM-U and is automatically written to any new package that is installed in the system. This permits fault handling and recovery without bringing in a technician to reset the new package when a failure occurs. The unit also includes remote lamp control, which sets off a blinking alarm on the defective package so the technician can spot the problem package immediately upon arriving at the site.

Fig. 5. Prototype of COM-U.