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ON MANAGING INTELLIGENT SATELLITE NETWORKS AN EVOLUTIONARY APPROACH IN POLICY BASED DISTRIBUTED MANAGEMENT
Greg Totsline and Rajeev Gopal Hughes Network Systems, LLC
11717 Exploration Lane Germantown, MD 20876
Early satellite based communication systems for voice and data consisted of bent pipe communications and could be managed through a traditional TT&C system. Newer systems are emerging where satellite payloads are now doing more than just repeating a signal, they are interpreting the information received and making intelligent decisions on how to process the data. Satellite payloads exist and new ones are being designed to operate like network nodes in the internet. Capacity is also allocated by these latest generation payloads dynamically, in response to changing user needs and environments. Managing these systems with a COTS network management system is not a viable solution. The advent of intelligent payloads requires network management systems that provide more than just hardware status. Instead, higher layer functions such as capacity management, dynamic bandwidth allocation, quality of service in a packet switched environment, and packet routing must be planned and managed. Policy based network management and supporting standards based management protocols serve as the building blocks for distributed, intelligent management. These satellite network management topics are surveyed in this paper.
The primary focus of this paper is on managing communication network services in an evolving satellite technology environment. The outline of this paper is as follows:
Discuss the functions and issues concerning the management of satellite communication systems
Review the current state of network management standards and technologies
Describe the emergence and growing trend towards the deployment of intelligent satellite networks
Discuss the unique capabilities of intelligent satellite networks and propose an evolutionary approach to managing those networks
MANAGING TRADITIONAL SATCOM SYSTEMS The majority of todays voice and data traffic that is carried via satellites is done so with transparent transponder (or bent pipe) based payloads. A typical transparent transponder based system receives a terminals signal, performs frequency conversion and filtering of the signal, then separates them into individual transponders finally re-amplifying the signal for transmission to a fixed or pre-configured set of destinations. Many of these systems operate in Ku, C, and L band and are now being launched in higher bands such as Ka. As with any engineered system, there are always trade-offs and compromises. The simple design of a transparent system must be traded with the following consequences of that design. Any noise not filtered by the transponder is amplified and passed on to the destination terminal. This impacts terminal design since it limits minimum antenna size and power requirements. Since the baseband signal is not processed by the payload, any packet based switching or routing must be done via a ground system. This can lead to inefficient use of available spectrum since multiple satellite hops are required which in turn results in longer propagation delay.
Figure 1 Typical System Architecture
Satellite Control Center
Network Control Center
Network Management Segment
TT&C User Data
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Figure 1 illustrates the typical communication satellite system architecture; its management is described next.
Space Segment Management
There are two overarching missions of the typical space segment and its operators or stake holders: (1) the mission of the space segment is ultimately to provide [secure] radio links at a specified quality of service to support fixed and mobile terminal communications, (2) the mission of the space segment provider is to optimize utilization and availability of space segment resources to maximize returns (i.e. monetary or mission support) on a substantial initial investment. The primary elements of the typical space segment are: the satellite bus and its payload, the frequency bands and channels used to support radio communications, a satellite control center, and depending on the system architecture, one or more geographically diverse satellite gateways. With respect to network management functions TT&C primarily plays a role in fault and performance management by supplying data, typically in the form of status, events, alarms, and processed measurements that can be used to provide the network manager (with the aid of network management tools) a picture of how the bus is performing. In first generation bent pipe systems, the payload implements the filtering, conversion, amplification, and forwarding of received radio signals for the purposes of supporting terminal and gateway traffic. Second generation systems support multiple spot beams to maximize spectrum efficiency. Another characteristic of second generation systems like ACeS, Inmarsat B, and DIRECWAY is the concept of demand assigned bandwidth and power. These systems employ a ground based network control to manage available pools of bandwidth and allocate power to down link spot beams. In order to support second generation payloads (still considered bent pipe, albeit with enhanced capabilities) space segment network management systems need to support the fault, configuration, accounting, and security (FCAPS) functions associated with managed objects such as spot beams and their associated channels and polarization configuration, frequency sub-band switches, and power allocation tables. Note that with each additional capability of the payload there is an additional responsibility in managing it. Depending on the level of automation within the network management system, this
can in turn lead to additional complexity for the network manager. Support for multiple spot beams exists in many fixed and mobile satellite communication systems. Like terrestrial cellular networks, the objectives of satellite spot beams (i.e., cells) are to provide coverage where markets exist and within defined regulatory boundaries, and to maximize utilization of available spectrum through frequency reuse. Demand assigned capacity is another technique used to maximize its utilization. In second generation satellite systems this typically involves a request from the terminal through the satellite to a network control center that manages bandwidth allocations across the entire system. Finally, variable modulation and coding schemes implemented by the satellite payload and terminal are utilized to support throughputs within prescribed error rates. Network management plays a supporting role in satellite radio channel configuration management. This may include storage, report, and status functions regarding the configuration of spot beam channels, displays of their coverage areas (often with geographic overlays). The network management system can process information from multiple sources to provide the network manager with the operational status of the network. Operational status and alarms received from the satellite bus and payload are reported through telemetry to the network control center to provide the health status of physical assets. Performance statistics regarding bandwidth assignments and capacity utilization are collected by the network management system, correlated to geographic location and provided to a capacity planning system which can analyze traffic trends and help the network planner make decisions regarding distribution of capacity across coverage areas. Performance statistics are also used by the network management system to help the network manager spot trends that might indicate a problem. For example, a sudden drop in traffic over a normally busy coverage area might indicate a problem with the spot beams serving the coverage area.
Managing Satellite Gateways Satellite gateways are a specialized type of satellite terminal that provide high capacity / volume services (relative to individual satellite terminals). Like terminals, gateways convert a customers terrestrial or land mobile network interface into a satellite network interface and vice versa. Since gateways are considered high value assets and like satellite operation centers have their own computing and network infrastructures, FCAPS network management functions apply to satellite gateways.
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Gateways have the additional complexity of supporting terrestrial PSTN and mobile network interfaces as well as terrestrial data network interfaces.
Managing Satellite Terminals The mission of the terminal segment is to support end user services by providing the bridge between the users access network (i.e. its physical interface and protocol stacks) and the satellite network. In many ways the terminal operates like a much smaller scale gateway; in the most extreme case a satellite terminal can be for personal use and be the size of a mobile handset. Satellite terminal management can pose unique challenges for satellite network management systems. Satellite networks can contain tens or hundreds of thousands of satellite terminals so a scalable network management solution is required to accommodate a growing population of subscribers. Furthermore, a satellite terminal typically implements a host of protocols (both standard and proprietary) and functions that must be configured and monitored. Satellite terminals also support various authentication, access control, and encryption functions that req