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Networking TutorialsPart 3: Sharing DevicesDevice ServersA device server is defined as a specialized, network-based hardware device designed to perform a single or specialized set of server functions. It is characterized by a minimal operating architecture that requires no per seat network operating system license, and client access that is independent of any operating system or proprietary protocol. In addition the device server is a "closed box," delivering extreme ease of installation, minimal maintenance, and can be managed by the client remotely via a Web browser. Print servers, terminal servers, remote access servers and network time servers are examples of device servers which are specialized for particular functions. Each of these types of servers have unique configuration attributes in hardware or software that help them to perform best in their particular arena. Print ServersPrint servers allow printers to be shared by other users on the network. Supporting either parallel and/or serial interfaces, a print server accepts print jobs from any person on the network using supported protocols and manages those jobs on each appropriate printer. The earliest print servers were external devices, which supported printing via parallel or serial ports on the device. Typically, only one or two protocols were supported. The latest generation of print servers support multiple protocols, have multiple parallel and serial connection options and, in some cases, are small enough to fit directly on the parallel port of the printer itself. Some printers have embedded or internal print servers. This design has an integral communication benefit between printer and print server, but lacks flexibility if the printer has physical problems. Print servers generally do not contain a large amount of memory; printers simply store information in a queue. When the desired printer becomes available, they allow the host to transmit the data to the appropriate printer port on the server. The print server can then simply queue and print each job in the order in which print requests are received, regardless of protocol used or the size of the job. Multiport Device ServersThe ancestor of today's multiport device server was a "Terminal Server", used to enable terminals to transmit data to and from host computers across LANs, without requiring each terminal to have its own direct connection. And while the terminal server's existence is still justified by convenience and cost considerations, its inherent intelligence provides many more advantages. Among these is enhanced remote monitoring and control. Multiport Device Servers that support protocols like SNMP make networks easier to manage. Devices that are attached to a network through a multiport device server can be shared between terminals and hosts at both the local site and throughout the network. A single terminal may be connected to several hosts at the same time (in multiple concurrent sessions), and can switch between them. Multiport device servers are also used to network devices that have only serial outputs. A connection between serial ports on different servers is opened, allowing data to move between the two devices. Given its natural translation ability, a multi-protocol multiport device server can perform conversions between the protocols it knows, like LAT and TCP/IP. While server bandwidth is not adequate for large file transfers, it can easily handle host-to-host inquiry/response applications, electronic mailbox checking, etc. And it is far more economical than the alternatives of acquiring expensive host software and special-purpose converters. Multiport device and print servers give their users greater flexibility in configuring and managing their networks. Whether it is moving printers and other peripherals from one network to another, expanding the dimensions of interoperability or preparing for growth, multiport device servers can fulfill your needs, all without major rewiring. Access ServersWhile Ethernet is limited to a geographic area, remote users such as traveling sales people need access to network-based resources. Remote LAN access, or remote access, is a popular way to provide this connectivity. Access servers use telephone services to link a user or office with an office network. Dial-up remote access solutions such as ISDN or asynchronous dial introduce more flexibility. Dial-up remote access offers both the remote office and the remote user the economy and flexibility of "pay as you go" telephone services. ISDN is a special telephone service that offers three channels, two 64 Kbps "B" channels for user data and a "D" channel for setting up the connection. With ISDN, the B channels can be combined for double bandwidth or separated for different applications or users. With asynchronous remote access, regular telephone lines are combined with modems and remote access servers to allow users and networks to dial anywhere in the world and have data access. Remote access servers provide connection points for both dial-in and dial-out applications on the network to which they are attached. These hybrid devices route and filter protocols and offer other services such as modem pooling and terminal/printer services. For the remote PC user, one can connect from any available telephone jack (RJ45), including those in a hotel rooms or on most airplanes. Network Time ServersAs organizations become global in their scope, they can begin to encounter problems which never before affected them. Time is one of these problems. How does one begin to coordinate activities in several sites that require precise timing of events when network traffic between those sites is subject to delays or inconsistencies? The answer to this question is to obtain timing information in all locations from a single source. A network time server is a server specialized in the handling of timing information from sources such as satellites or radio broadcasts and is capable of providing this timing data to its attached network. Specialized protocols such as NTP or udp/time allow a time server to communicate to other network nodes ensuring that activities that must be coordinated according to their time of execution are synchronized correctly. GPS satellites are one source of information that can allow global installations to achieve constant timing. Device ServersA Device Server is an appliance that network-enables any device with a serial communications port. By virtue of its independent operating system, protocol independence, small size and flexibility, Device Servers are able to meet the demands of virtually any network-enabling application. The demand for Device Servers is rapidly increasing because organizations need to leverage their networking infrastructure investment across all of their resources. Many currently installed devices lack network ports or require dedicated serial connections for management -- Device Servers allow those devices to become connected to the network. Device Servers are currently used in a wide variety of environments in which machinery, instruments, sensors and other discrete devices generate data that was previously inaccessible through enterprise networks. Device Servers are also used for security systems, point-of-sale applications, network management and many other applications where network access to a device is required. As Device Servers become more widely adopted and implemented into specialized applications, we can expect to see variations in size, mounting capabilities and enclosures. Device Servers are also available as embedded devices, capable of providing instant networking support for builders of future products where connectivity will be required. ConclusionWe hope this introduction to networking has been helpful and informative. However, we cannot explain everything there is to know about planning, installing, administering and troubleshooting a network in this short tutorial. There are many Internet websites, books and magazines available that explain all aspects of computer networks, from LANs to WANs, network hardware to running cable. Check your local bookstore, software retailer or newsstand for more detailed information.
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