Expand paging system coverage with satellite communications Demand-allocated, multiple-access (DAMA) systems with mesh technology offer advantages, including the ability of each remote earth station to exchange data with any other remote earth station.
North America’s communications business sector has seen explosive growth since deregulation.
Does this describe you? Your business has a modern messaging system for providing alphanumeric paging or digitized voice, and you want to expand your coverage area. You also want a platform for delivering messages to and from all areas. One answer might be to connect with a public switched telephone network (PSTN) or a private interexchange carrier (IXC) and to use frame relay technology. This transport technology moves large amounts of digitized messages to distant transmitter sites. What will you do if the expanded coverage and service area lacks the solid, reliable PSTN system you have come to expect in urban North America?
Consider using a satellite-based system to move digital information to distant or remote sites not served by a high-quality PSTN. Satellite-based systems may offer advantages such as stable bandwidth costs for data, flexible site location options and few worries about loss of service caused by political troubles and strikes. Complex and expensive system components are not a concern with satellite data transport. Modern technology has brought changes.
The building blocks of a satellite data transmission system are:
> Digital modem — It produces an IF signal (typically 50MHz to 80MHz) modulated in one of several formats. Its output is delivered to an indoor unit.
> Indoor unit (IOU) or upconverter — It takes the IF signal and converts it to L-band frequencies between 900MHz and 1,200MHz. Its output is delivered to the outdoor unit via an interfacility link (IFL) cable, a type with extremely low loss.
> Outdoor unit (ODU) — It has a solid-state power amplifier and low-noise block (LNB) downcoverter. Transmit power levels vary. Some units provide as much as 5W, and larger units furnish as much as 20W of RF energy to the feedhorn. The ODU is what you see with the feedhorn assembly mounted in front of the dish antenna, and it performs both transmit and receive functions. On receive-only units, this component is small, especially on Ku-band systems.
> Antenna or dish — Antenna size depends on the frequency band used (C or Ku) at power levels required to reach the system’s space segment reliably and the space segment’s transmitter power. These power levels are, in part, driven by location. Generally, the farther north or south the earth site is from the equator, the larger the antenna needed to overcome increased path losses. This entire system, except for antenna, can be as small as an ordinary desktop PC, and it uses regular power mains.
Two types of satellite systems and network topologies are in general telecommunications use. The first is single carrier per channel (SCPC) with a star topology. The second is demand-allocated, multiple-access (DAMA) with mesh topology. Both types use geosynchronous earth-orbiting (GEO) space segments. (See Figure 1 on page 12.) Common satellite frequencies in North America are C-band (4GHz to 6GHz) and Ku-band (8GHz to 12GHz).
The following information explains how to use these components in a system. The space segment is treated as a simple “bent pipe” that ensures satisfactory data delivery at the distant earth station. This simplification does not do justice to what actually are sophisticated space systems, but it serves to focus on data traffic movement from ground site to ground site.
The first system, SCPC, commonly is used as a very small aperture terminal (VSAT). It typically has a low-power (20W or less) digital carrier and a small, truncated parabolic antenna linking the remote site to the space segment. The space segment relays the signal to a centralized hub. At the hub, data are processed and used or rebroadcast to other remote systems. Unless polled, each remote terminal requires a small amount of bandwidth permanently allocated on the space segment.
This implementation has some advantages and disadvantages. As seen in Figure 2 above (a star network), all remote-station data must go to the hub first. To send data to any other remote station, the hub must provide a link to the space segment. Station B has data for station C, so remote station B must send data to the hub, which then retransmits it to remote station C. For a polled or broadcast application, where data transfer is continuous, where the same data are used by all ground stations or where data are returned to the remote site after centralized processing, SCPC systems using star topology may make sense. This technology is mature and robust, with a good selection of vendors able to provide the necessary components for a complete, “off-the-shelf” system package. Costs vary according to ground segment location and frequency band.
As a disadvantage, the delay incurred in this double-hop (remote up to satellite, down to hub, then up to satellite and back down to remote), made necessary by the star topology, may adversely affect time- or delay-sensitive data transfer protocols. The required permanent allocation of bandwidth on the space segment can be costly for applications with an intermittent data stream, such as with paging or messaging. Over time, the bandwidth cost on a GEO satellite system may be significantly higher than the entire cost of the ground segment. A newer type of equipment provides an alternative to SCPC implementation.
DAMA with mesh topology has virtually the same basic RF system building blocks as SCPC. (See Figure 3 at the right.) The difference is the amount of electronic intelligence contained within the digital satellite modem. With DAMA, each remote earth station can exchange data with the hub and, more importantly, with any other remote earth station. This communication among sites is the essence of a mesh topology system. Data can be exchanged between any two (or more) ground stations.
Implementing this topology is not as simple as Figure 3 may imply. Because data must flow over an RF link, a high-speed data protocol may require two frequencies (for a duplex circuit), but if data are sent using a bi-synchronous protocol, then only a single frequency (for a simplex circuit) is required. The implementation of DAMA mesh networking in most systems uses the hub controller as a kind of traffic cop. This controller function may require a separate, permanent frequency for passing directions to remote site terminals.
When, for example, remote system B has data for remote system C, system B signals the hub controller with a request to contact remote system C. The hub controller sends data to systems B and C, directing them to “meet” on an assigned frequency, relayed through the satellite, where the data are exchanged. When the transaction is complete, both system modems return to a common control frequency. Thus, data can be transferred between any two sites under the control of a central hub station. It takes longer to explain this function than it takes for modern satellite modems to change frequencies and to transfer data.
Monitoring remote system status and collecting billing information can be done at the control hub for either a SCPC or DAMA topology-based communications system. Each satellite system manufacturer has a different means to accomplish network control and remote messaging, so specific questions on hardware and available controller software should be directed to the manufacturers.
To extend your broadcast coverage over an extremely wide area or internationally, a receive-only satellite relay system may make sense. To extend your service area, certain types of this equipment can be upgraded simply and easily with changes to the hub controller and site modems–assuming care and planning for future growth were considered at the initial purchase. With the right kind of initial equipment purchase, the RF link portion of the system can remain the same when the system is upgraded. Modern, modular satellite communications equipment is reliable and simple for technical staff to maintain.
Acknowledgments: Thanks to George Molczan of General Communications (GCI), Anchorage, AK, for permission and access to photograph the company antenna farm.
