WOW! - Technology Overview
A Global, Broadband Internet-in-the-SkyTM Network
The Teledesic Network is a high-capacity broadband network that combines the global coverage and low latency of a low-Earth-orbit (LEO) constellation of satellites, the flexibility and robustness of the Internet, and "fiber-like" Quality of Service (QOS). Essentially an Internet-in-the-SkyTM system, the Teledesic Network brings affordable access to interactive broadband communication to all areas of the Earth, including those areas that could not be served economically by any other means.
The Teledesic Network can serve as the access link between a user and a gateway into a terrestrial network, or as the means to link users or networks together. Covering nearly 100 percent of the Earth's population and 95 percent of the landmass, the Teledesic Network is designed to support millions of simultaneous users.
Seamless Compatibility
Geostationary satellite communications systems require changes to terrestrial network standards and protocols to accommodate their inherent high latency, a minimum half-second round-trip delay. Teledesic's objective is to meet current network standards rather than to change them. By using fiber optics as the guideline for service quality, the Teledesic Network is designed for compatibility with applications that are based on today's and tomorrow's protocols. This places stringent requirements on the system design, including low latency, low error rates, high service availability, and flexible, broadband capacity — all characteristics of fiber.
The Teledesic Network
The Teledesic Network consists of a ground segment (terminals, network gateways and network operations and control systems) and a space segment (the satellite-based switch network that provides the communication links among terminals). Terminals are the edge of the Teledesic Network and provide the interface both between the satellite network and the terrestrial end-users and networks. They perform the translation between the Teledesic Network's internal protocols and the standard protocols of the terrestrial world, thus isolating the satellite-based core network from complexity and change. (See Figure 1.)
Teledesic terminals communicate directly with the satellite network and support a wide range of data rates. The terminals also interface with a wide range of standard network protocols, including IP, ISDN, ATM and others. Although optimized for service to fixed-site terminals, the Teledesic Network is able to serve transportable and mobile terminals, such as those for maritime and aviation applications.
Most users will have two-way connections that provide up to 64 Mbps on the downlink and up to 2 Mbps on the uplink. Broadband terminals will offer 64 Mbps of two-way capacity. This represents access speeds up to 2,000 times faster than today's standard analog modems.
The ability to handle multiple channel rates, protocols and service priorities provides the flexibility to support a wide range of applications including the Internet, corporate intranets, multimedia communication, LAN interconnect, wireless backhaul, etc. In fact, flexibility is a critical network feature, since many of the applications and protocols Teledesic will serve in the future have not yet been conceived.
Terminals also provide the interconnection points for the Teledesic Network's Constellation Operations Control Centers (COCC) and Network Operations Control Centers (NOCC). COCCs coordinate initial deployment of the satellites, replenishment of spares, fault diagnosis, repair, and de-orbiting. The NOCCs include a variety of distributed network administration and control functions including network databases, feature processors, network management and billing systems.
Figure 1 - The Teledesic Network (Larger image)
Fast-Packet Switching
Teledesic's space-based network uses fast-packet switching. Communications are treated within the network as streams of short, fixed-length packets. Each packet contains a header that includes destination address and sequence information, an error-control section used to verify the integrity of the header, and a payload section that carries the digitally encoded user data (voice, video, data, etc.). Conversion to and from the packet format takes place in the terminals at the edge of the network.
The topology of a LEO-based network is dynamic. The network must continually adapt to these changing conditions to achieve the optimal (least-delay) connections between terminals. The Teledesic Network uses a combination of destination-based packet addressing and a distributed, adaptive packet routing algorithm to achieve low delay and low delay variability across the network. Each packet carries the network address of the destination terminal, and each node independently selects the least-delay route to that destination. Packets of the same session may follow different paths through the network. (See Figure 2.) The terminal at the destination buffers and if necessary reorders the received packets to eliminate the effect of timing variations.
Figure 2 - Teledesic's Distributed Adaptive Routing Algorithm
The Satellite Constellation
Each satellite is a node in the fast-packet-switch network and has intersatellite communication links with other satellites in the same and adjacent orbital planes. This interconnection arrangement forms a robust non-hierarchical mesh, or "geodesic," network that is tolerant to faults and local congestion. The network combines the advantages of a circuit-switched network (low delay "digital pipes"), and a packet-switched network (efficient handling of multirate and bursty data).
From a network viewpoint, a large constellation of interlinked switch nodes offers a number of advantages in terms of service quality, reliability and capacity. The richly interconnected mesh network is a robust, fault-tolerant design that automatically adapts to topology changes and to congested or faulty nodes and links. To achieve high system capacity and channel density, each satellite is able to concentrate a large amount of capacity in its relatively small coverage area. Overlapping coverage areas plus the use of on-orbit spares permit the rapid repair of the network whenever a satellite failure results in a coverage gap. In essence, the system reliability is built into the constellation as a whole rather than being vulnerable to the failure of a single satellite.
The lowest frequency band with sufficient spectrum to meet Teledesic's broadband service, quality and capacity objectives is the Ka band. The terminal-satellite communication links operate within the portion of the Ka frequency band that has been identified internationally for non-geostationary fixed satellite service and in the U.S., licensed for use by Teledesic. Downlinks operate between 18.8 GHz and 19.3 GHz, and uplinks operate between 28.6 GHz and 29.1 GHz. Communication links at these frequencies are degraded by rain and blocked by obstacles in the line-of-sight. To avoid obstacles and limit the portion of the path exposed to rain requires that the satellite serving a terminal be at a high elevation angle above the horizon. The Teledesic constellation assures a minimum elevation angle (mask angle) of 40¼ within its entire service area. Using this design, the Teledesic Network is able to achieve availability of 99.9 percent or greater.
Latency is a critical parameter of communication service quality, particularly for interactive communication and for many standard data protocols. To be compatible with the latency requirements of protocols developed for the terrestrial broadband infrastructure, Teledesic satellites operate at a low altitude, under 1,400 kilometers. The combination of a high mask angle and low-Earth orbit result in a relatively small satellite coverage zone, or footprint, that enables efficient spectrum reuse but requires a large number of satellites to serve the entire Earth. In the initial constellation, the Teledesic Network will consist of 288 operational satellites, divided into 12 planes, each with 24 satellites.
Multiple Access
Since the Teledesic Network uses wireless access, communication channels are not dedicated to terminals on a permanent basis. The channel resources associated with a cell are shared among terminals in that cell, with capacity assigned on demand to meet their current needs. This flexibility allows Teledesic to handle efficiently a wide variety of user needs: from occasional use to full-time use; from bursty to constant bit-rate applications; from low-rate to high-rate data; from low usage-density areas to areas of relatively high usage density.
A multiple access scheme implemented within the terminals and the satellite serving the cell manages the sharing of channel resources among terminals. Within a cell, channel sharing is accomplished with a combination of Multi-Frequency Time Division Multiple Access (MF-TDMA) on the uplink and Asynchronous Time Division Multiplexing Access (ATDMA) on the downlink.
Network Capacity
To make efficient use of the radio spectrum, frequencies are allocated dynamically and reused many times within each satellite footprint. The Teledesic Network supports bandwidth-on-demand, allowing a user to request and release capacity as needed. This enables users to pay only for the capacity they actually use, and for the Network to support a much higher number of users. Thus, the Teledesic Network is designed to support millions of simultaneous users. The Network scales gracefully to much higher capacity by adding additional satellites.
Figure 3 - The Teledesic Satellite
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