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Small-aperture earth stations can be an alternative to private-line networks.

Small-Aperture Earth Stations Can Be an Alternative to Private-Line Networks For years, private-network operators have been forced to use the private-line offerings of the common carriers. However, as demands for new applications have increased, these lines have become increasingly limited in terms of performance, flexibility and reliability of service. Additionally, as post-divestiture terrestial services have become less suited for advanced applications, their tariff costs have increased substantially.

One alternative to terrestial lines is satellite networks. In the past, these systems were cost-effective for a limited number of main-trunk applications between major corporate data centers, but were too expensive for applications requiring fast response-time transaction processing for a large number of distributed locations. Advances in satellite technology, coupled with explosive tariff increases, have fostered the widespread use of low-cost satellite small-aperture communications-station (CS) networks such as our Clearlink integrated data network.

In these networks, the CSs are located directly on the customer premises, enabling end-to-end communications between the facilities of a multi-branch company or a data-processing services organization and its customers. These full-solution networks thus enable network operators to completely bypass the problems, the costs and the lack of control of the terrestial multidrop, polled leased-line networks.

No Cost Increase for Distance

With satellite-based communications, a service area is created that provides data-communications capabilities to all points within the satellite footprint (the area on earth covered by its main beam). Cost doesn't increase with distance from the message source, as it does with terrestial lines. Additionally, every message is received by every point on the network. Thus, multi-address and broadcast messages are sent only once, instead of individually to every addressee.

Network flexibility is a second advantage. Since stations can be located anywhere within the satellite's footprint, they also can be relocated without any network reconfiguration.

Operational flexibility is also provided. Because a single broadcast channel is received by every station, network data-transmission bandwidth may be dynamically allocated to high-usage areas on either a tempoerary or a long-term basis.

A typical satellite bit-error rate (BER) is less than one error in 10.sup.7 bits, which greatly reduces the number of retransmissions required for error-free information exchange. This contrasts sharply with the typical BER for terrestial links--an average of one error in 10.sup.5 bits.

The key to harnessing satellite capabilities for private networks is to use an integrated-systems architecture. A typical systems architecture is shown in Figure 1. The integrated architecture includes small satellite-communications stations located on the customer's premises. These stations interface terminal cluster-controllers or single terminals to a central hub station via a dedicated 512-kb/s outbound channel and multiple inbound 32-kb/s satellite data channels. The hub station also contains a network control console (NCC) used to operate and manage all network operations. The hub station and NCC may be operated by the customer or by the system supplier.

Host Interface on Customer's Site

This integrated transmission network hub is interfaced to the customer's host computer/front-end processor (FEP) via high-speed terrestial lines or a dedicated satellite channel. To provide a transparent interface and minimize the lines needed for interconnection, a host interface is provided on the customer's site.

The communications station is an integrated satellite-transmission and data-communications protocol interfacing unit. It connects customer data-terminal equipment (DTE) to the network by providing a standard data-circuit terminating equipment interface (it "looks like a modem" to the terminal).

The CS consists of a small antenna containing a small RF electronics package that's located outdoors. A desktop-sized indoor unit contains an RF/IF baseband module and a remote terminal processor (RTP). The RTP performs protocol translation and formats data for transmission over assigned 32-kb/s channels. It also performs monitor, control and diagnostic services for the communications station.

The transmission architecture is organized into clusters, with each cluster containing a single 512-kb/s outbound channel received by all communications stations, and up to 50 inbound 32-kb/s channels that are shared by the communications stations for the transmission of inbound traffic.

Delays are minimized by this approach. The high outbound and inbound transmission rates readily accommodate peak system loads. Using an "Aloha" multi-access protocol allows efficient sharing of inbound (remote CS to hub) channels, with low delay for transaction-processing applications.

This approach eliminates the delays normally experienced in polled networks. Thus, the integrated satellite network offsets its normal satellite propagation delay to provide response time that equals, if not betters, the performance of many polled terrestial networks.

The hub station is a large-diameter antenna designed to provide reliable interconnection between remote terminals and the host computer system. The major components of the hub station are an antenna system, a complete RF subsystem, and a hub switching system. The switching system multiplexes and concentrates functions required to establish and maintain virtual circuits between ports on the host computer and terminal equipment attached to the remote communications stations.

A final portion of the hub station is the network-control computer. It's responsible for providing a central network-management and control function to the network operators. The host interface converts existing line protocols from the host computer into multiplexed virtual circuits for the trunk connection to the hub station.

One of the major considerations in selecting a satellite system is the selection of the frequency band for satellite transmission. Most commercial systems use either C-band frequencies or Ku-band frequencies. Each frequency has its pros and cons. In terms of impact on the user, probably the most-significant point is that Ku-band communications stations can be installed almost anywhere and easily licensed. Thus, Ku-band is better suited when operating a major data network where there is frequent movement and addition of equipment. For example, a company may decide to move a remote branch office. When it occupies the new site, it may learn that its C-band station can't be licensed there because of terrestrial interference. This can't happen in the Ku-band.

Satellite bypass-networks provide their best comparative cost savings when applied to high-performance multipoint polled lines and multi-application configurations. Many applications that replace a single voice-grade private line per remote office are cost-effective; however, satellite networks particularly shine in multipoint network environments when the peak capacity, reliability and response-time requirements of the network can no longer be met by a single line.

Batch Intermixing Is Accommodated

The major characteristic required is a heavy-transaction environment requiring short response times. Intermixing of batch data traffic is also accommodated. The bandwidth (capacity) of the entire network may be dynamically switched between these two differing types of applications, between cities in different time zones, or with other short-term, high-load requirements. This provides a degree of flexibility that's unattainable with conventional terrestrial-line-based approaches.

Estimating the costs of a satellite network for specific applications is quite straightforward, as the price of the remote communications station will be constant within a narrow range (for options) for all applications. The major variables are the data transmission-channel capacity (satellite capacity) and the number of remote communication stations. Some typical prices for a Ku-band system are: Remote communication station, including installation--$8,000 to $10,000; remote station monthly maintenance (each)--$60 to $100; remote station monthly space-segment cost--$10 to $200.

Another cost factor in the decision is whether to order a hub station for the exclusive use of the network (approximate costs are in the million-dollar range) or to use a shared-hub service. In a shared-hub service, the network operator incurs the cost of the hub hardware and prorates the cost of this service among several customers. This "virtual private network" approach provides substantial additional cost savings over the customer-owned hub-station approach. It also allows the advantages of customer control, because a remote CRT is located on the customer's premises to monitor and operate its portion of the network.

Terrestrial Lines Not Needed

The real value of a satellite bypass-network lies in the data communications services that it provides. The full solution must not only meet current needs at a lower cost than terrestrial networks, it also must provide increased performance, reliability and long-term growth capabilities. The satellite data network transparently connects the customer front-end processor to remote data terminals, thereby eliminating all intervening terrestrial lines, multiplexers, concentrators, modems, and so on.

A typical set of services would be a system data capacity of 512 kb/s provided on the outbound channel (hub to remote communications stations), and up to fifty 32-kb/s inbound channels (remotes to hub) configured into the system, depending on traffic volumes and response-time requirements. Additional capacity can be added for growth requirements.

Virtually any number of remote communications stations may be attached to the network. Each communications station provides one to eight RS-232-C or RS-449 ports for the connection of cluster controllers or data terminals. The host processor/FEP is interfaced to the network hub by a special high-speed port supporting multiple 56-kb/s data streams.

Systems services provide reliable end-to-end delivery of data between the customer host computer and remote terminals, or between terminals. Three classes of systems services may be obtained:

* Integrated services are optimized for transaction processing. Two-way communications are provided over a full-duplex facility. This service is optimized for terminal-to-host communications.

* Broadcast services are available within the network to distribute data from a single originator to either a group or to a single addressee on the network.

* Peer services are available within the network to provide a communications path between any connected entity on the network. This class of communications may be invoked to permit any terminal to be the source of the broadcast or interactive traffic described above. These different services may be mixed in any combination.

Dynamic reassignment of system resources between these services is permitted to accommodate time of day or other major traffic-load changes. In addition, these services are transparent to the applications being served. The applications may include the transmission of text, images (facsimile), voice, or one-way videoconferencing.

System response time is a function of the size and rates of transactions, the channel capacity implemented in the network, and the data rate of the host and terminal interfaces. High-performance satellite networks provide response times that are comparable to those of many terrestrial network implementations, as delays due to polling, concentration and multiple processing queues are eliminated. The satellite propagation delay of approximately 280 milliseconds is largely offset by its elimination of the above terrestrial delay factors.

Variety of Protocols Supported

A variety of the major terminal protocol standards can be supported by typical satellite bypass-networks. Within a network, the protocols are converted to a standard network packet format. This conversion is required to permit intermixing of multiple devices, communications between selected dissimilar devices and, importantly, to eliminate the delays associated with polling and other techniques used to operate terrestrial lines.

Therefore, the satellite network supplier provides protocol interfaces for the major standard protocols or writes special interface software for less-commonly used protocols. Major standard protocols typically supported are asynchronous. ASCII character transmission, IBM 3270 SNA/SDLC and IBM 3270 BSC. By providing these protocol interfaces, the network supplier ensures transparency for the network user.

A network management capability is provided to permit direct customer management of the network. Multiple CRT displays are used to enter commands and monitor network operation. These operator control functions are divided into those that perform actions in the communications station, the hub-station switching system, the host interface, and the network control-center resident data base of network components.

Typical command functions supplied for the remote communications station are: enable/disable, loopback test, read traffic statistics, and response-time measurement for the station. Switching-system commands include the capability to initiate and terminate systems operation, balance loads on 32-kb/s input channels, modify directories, and obtain traffic statistics for the system as a whole.

Diverse Requirements Can Be Met

Current terrestrial-network capabilities are being outstripped by the demands of new applications for higher capacity for the transmission of larger data blocks and the need to intermix multiple applications. Satellite data-networking provides the tools needed to meet these diverse requirements, including high capacity to meet the needs of advanced applications and future growth, high reliability to provide uninterrupted services during a time when corporations are becoming increasingly dependent upon their networks to sustain their operations, and high flexibility to accommodate any planned and unplanned changes as the telecommunications requirements of the corporation change or new opportunities become available.

This flexibility manifests itself in growth or reallocation of network transmission capacity to meet applications needs, such as accommodation of the physical moves and adds that are ongoing in any major network operation. The relocation of an office can be accommodated on short notice over a weekend, with new service installed at the new location while the old service is being phased out. This contrasts with the typical 45 to 60-day ordering times for leased lines.

Also, excellent response times are attainable from a satellite network, as polling and multiple-node queuing delays are eliminated. Importantly, the response times obtained from a satellite network are also more consistent.

There also is control over both network operations and long-term costs. By siting the network control CRT directly on customer premises, network operation may be directly measured and controlled. By dealing with a single responsive vendor, network operations such as moves, adds and the cutover of new applications may again be directly controlled.

Finally, costs may be controlled over the long term by obtaining equipment and services on a long-term (typically five years) lease or by purchasing equipment outright.

Immediate cost reduction over current terrestrial approaches is the extra dividend obtainable from this approach. Savings of up to 60 percent have been realizable for multiline applications studied to date. As applications requirements increase, cost reduction of smaller networks will also become significant.

Data networking represents a strategic resource to a company, whether it be used internally to improve operations or externally to provide better services to on-line customers. If your applications fit the general scale of requirements described in this article, its potentials should be explored.
COPYRIGHT 1986 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1986 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Huang, Larry
Publication:Communications News
Date:Mar 1, 1986
Previous Article:A user's guide for the evaluation of micro earth station networks.
Next Article:An introduction to how data communications works by networking of computers, terminals and modems.

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