Random, malformed frames of data that are sent continuously by failed circuitry in a networking component. A network interface card (NIC) or other device that is jabbering generates a continuous stream of unwanted signals that can disrupt communication between other devices on the network. This is especially common on Ethernet networks, in which each device must compete for use of the line using the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) contention protocol. When a NIC jabbers on an Ethernet network, all network communication might cease until the offending NIC is replaced. Other causes of jabbering on a network include a faulty NIC, loose cabling, or a poorly grounded cable (for shielded cabling).
The term “jabber” is also used in the Institute of Electrical and Electronic Engineers (IEEE) 802.3 specification for any frame of data that exceeds the maximum frame length for that specification. For Ethernet networks, the maximum frame length is 1518 bytes (18 bytes of overhead and 1500 bytes of payload). A frame longer than 1518 bytes is often called a “jabber frame.” Another name for jabbering is “long packet error.”
Jabbering can also occur in Fast Ethernet networks, in which a combination of 100BaseT4 and 100BaseTX cabling schemes are employed in one network. The continuously generated idle stream signal of the TX network can appear to the T4 network as jabber and can bring down the entire network. This should not be a problem if all network devices implement autonegotiation in an IEEE-compliant fashion. In addition, Fast Ethernet repeaters generally implement jabber control by automatically disconnecting any port that transmits information in streams longer than 40 Kb.
NOTE
A frame shorter than 64 bytes on an Ethernet network is called a “runt frame.”
A receptacle into which you can insert a connector or plug to form a connection. An example is a category 5 (CAT5) jack, which can be attached to twisted-pair cabling and which accepts an RJ-45 connector. Jacks are usually built into networking devices such as hubs and switches, but in some modular expandable devices jacks can be inserted and removed to support different kinds of connectors. Patch panels and wall plates are other examples of items that can be modular and can be configured by inserting different kinds of jacks into them. This modularity allows flexibility for networks that must be reconfigured frequently, such as classroom computer networks.
Graphic J-1. Jack.
The outer covering of cabling that protects it from physical damage. The jacket of a cable protects the wiring from various kinds of damage, including
Damage caused by exposure to sun, heat, moisture, dust, and chemicals
Damage caused by contact with sharp edges of walls, furniture, or plenum ducts; by mechanical vibrations; or by mechanical stresses introduced during the cable installation process
The jacket is usually made of an insulating material, although its primary purpose is not insulation. (This function is provided by the insulation of individual wires within the jacket.) Common materials used for cable jackets include the following:
Polyvinyl chloride (PVC): This is the most common material used for cable jackets. PVC is a tough, fire-resistant material formed from resins and plastics that can withstand a wide range of temperatures. PVC cables are typically used in vertical rises, but they should not be used in plenums because PVC gives off toxic fumes when burned.
Kynar plastic (polyvinylidene fluoride): This material has good fire-retardant qualities, which makes it suitable for installation in plenums, but it is several times more costly than polyvinyl chloride.
Flamarrest: This chloride compound has high fire retardance and is suitable for plenum installations.
Neoprene: This dark-colored plastic is resistant to moisture and doesn’t degrade in sunlight. It’s a good choice for use in outdoor cable runs.
Polyethylene: This soft, moisture-resistant plastic has poor fire retardance and is suitable only for outdoor cable runs.
See also cabling
A signal sent by a device on an Ethernet network to indicate that a collision has occurred on the network. Collisions occur on Ethernet networks because access to media (usually a cable) is based on contention—that is, on a first-come, first-served basis. If two stations attempt to take control of the medium at the same time and begin transmitting, both stations will detect each other’s signal and realize that a collision has occurred. The two stations then issue a jam signal, which notifies all other stations on the network of the collision. They all must wait a short period of time before attempting to transmit again. The length of time is random for each station so that the retransmissions won’t cause more collisions. The jam signal sent by one transmitting station must start with a 62-bit pattern of alternating 0s and 1s, followed by a 32-bit sequence that provides a dummy checksum value for the other transmitting station. This 32-bit sequence cannot be equal to the cyclical redundancy check (CRC) value for the frame preceding the jam.
An object-oriented programming language developed by Sun Microsystems. Java is syntactically related to the C and C++ programming languages but is simpler, has libraries that are tuned for the Internet environment, and is designed to be portable across different operating systems and platforms. Java code can be reused and extended through inheritance, which simplifies the development process.
You can use Java to develop full-fledged distributed applications and application suites, but much of its appeal comes from its ability to create applications called “applets” that can be downloaded from Web servers by standard Web browsers and then executed on the client browser. These applets can add various interactive features to Web sites to enhance the user’s experience and provide features such as rotating ad banners or animated stock tickers.
How It Works
After you write an application (or applet) in Java, a Java compiler takes your source code (which is usually saved with the extension .java) and compiles it into something called bytecode. Bytecode is not true machine code—it is a set of generic instructions that are not specific to any particular hardware platform or operating system. Bytecode files generally have the extension .class and are referred to as class files. Here is a simple example of a Java application called HelloThere.java:
Class HelloThere { Public static void main (String args[]) { System.out.println("Hello there"); } } The HelloThere.java program defines a new class called HelloThere and contains only one method: main(). When this program is compiled into the class file HelloThere.class and then run, it displays the text “Hello there” on the screen (the standard output device).
To run the application (class file), you must have a bytecode interpreter called a Java Virtual Machine (JVM) on the system. The JVM reads the bytecode of your application and executes it by converting each bytecode instruction into a machine-native instruction or set of instructions. This translation process takes place regardless of whether the particular piece of bytecode has been previously executed. If the application is an applet, the Web browser that accesses it must have a JVM installed in it. The use of a bytecode interpreter instead of a “true” compiler, which generates platform-specific machine code, means that applications written in Java are often slower than those written in C or C++, but the ease of development and portability of code offset these disadvantages in most cases. To obtain additional functionality, you can link native code to Java applications.
The JVM has special security features built into it to ensure that any malicious Java applet downloaded from the Internet cannot perform any harmful actions on the user’s system. In this sense, you can consider the JVM running in the Web browser a “sandbox” that allows the applet to safely “play around” (execute) while preventing access to certain items, such as the client machine’s file system.
An alternative to the JVM is a just-in-time (JIT) Java compiler that dynamically compiles segments of Java bytecode into equivalent segments of platform-specific executable machine code at run time. JIT compilers offer better performance than the JVM for running Java applications, especially in applications in which segments of code run multiple times (for example, in loops).
NOTE
Other components of the Java platform include the following:
JavaBeans, the Java component architecture for developing Java applications. JavaBeans can be self-contained applications or portions of Java applications. You can use them as building blocks for creating more complex programs.
Java Foundation Classes (JFC), which are building blocks for creating interfaces for Java applications.
Java Database Connectivity (JDBC), which provides standard application programming interfaces (APIs) for connecting SQL (Structured Query Language) databases to Java applications.
JavaOS, an operating system designed to natively run Java programs.
On the Web
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Sun Microsystems Java home page : http://java.sun.com
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Microsoft technologies for Java : http://www.microsoft.com/java
A standard Structured Query Language (SQL) data access interface developed by Sun Microsystems that allows Java applications to access databases. Java Database Connectivity (JDBC) is based on open database connectivity (ODBC) and is used with the Java programming language. The JDBC application programming interface (API) is a standard component of Java 2.0 Platform.
How It Works
The JDBC API specifies a set of Java classes that represent database connections, SQL queries and their result sets, and other objects associated with accessing databases. Multiple drivers for JDBC exist that allow access to different database formats, and the JDBC drivers themselves can be implemented either within applets or as native methods on the operating system. JDBC also supports drivers that act as a bridge between ODBC and JDBC. This type of driver translates JDBC function calls into native ODBC calls, but this bridge cannot be run by untrusted applets within a Web browser environment.
See also Java
A cross-platform, object-based scripting language that is similar to C but simpler in syntax. JavaScript was developed by Netscape Communications and built into the Netscape Navigator Web browser version 2.0 and later. JavaScript enables Web developers to produce Web pages with dynamic functionality such as animated graphics, browser detection, cookies, scrolling text, form handlers, and authentication without having to write or invoke Common Gateway Interface (CGI) applications. JavaScript can be used to develop both client and server applications, but it is usually run on the client Web browser.
How It Works
JavaScript scripts are placed in standard Hypertext Markup Language (HTML) files using special tags. Here is a simple example:
<HTML> <HEAD><TITLE>Javascript Test</TITLE></HEAD> <BODY> <H1>Knock Knock</H1> <SCRIPT LANGUAGE="JavaScript"> alert("Who's there?"); </SCRIPT> </BODY> </HTML> The <SCRIPT> and </SCRIPT> tags identify and contain the actual JavaScript script. When this page is loaded into a Web browser such as Netscape Navigator version 3.0, the browser reads the script and interprets it, causing an alert dialog box to be displayed with the message “JavaScript Alert: Who’s there?” and an OK button that closes the dialog box when clicked.
NOTE
JavaScript is not the same as Java, which is an object-oriented programming language. A syntactically similar scripting language called JScript is supported on Microsoft Internet Explorer.
See also Java, JScript
Stands for “just a bunch of disks” and refers to any unstructured high-capacity disk-storage system. JBODs do not have the fault tolerance features of RAID disk systems. They are used for simple applications that require a large disk capacity. A JBOD has no specified format—anything will do, including a collection of daisy-chained external Small Computer System Interface (SCSI) disk drives, removable Jaz drives, optical drives, or a mixture of different types of disk drives. JBODs are difficult to manage and usually arise as a result of poor network planning as a company expands and upgrades its services. If you have a JBOD on your network, it is a good idea to migrate as much of your data as possible to a RAID-5 disk system, either an external hardware RAID-5 unit or a Microsoft Windows NT server with RAID-5 implemented in software.
See Java Database Connectivity (JDBC)
A Microsoft database engine technology developed in 1992 and included in a variety of Microsoft products, including Microsoft Access and Microsoft Visual Basic. Jet is not available as a separate product.
Jet, which stands for Joint Engine Technology, is an advanced 32-bit multithreaded database engine that uses transaction logs to track and maintain information and provide fault tolerance. The Jet database engine has its own processors for performing client-based processing of queries and for generating result sets. Jet databases can be accessed using Data Access Objects (DAO) and open database connectivity (ODBC). DAO provides the Component Object Model (COM) interface for accessing indexed sequential access method (ISAM) databases such as Microsoft FoxPro, dBASE, or Paradox through Jet. Native Jet database files usually have the extension .mdb. Jet is used by the directory and information store database services of Microsoft Exchange Server.
NOTE
Jet databases are best accessed through DAO (rather than ODBC) because they expose only a limited amount of functionality to ODBC drivers. Jet cannot use ODBC to build queries that use server-side cursors, and it has limited capability for stored procedures and multiple result sets.
A Microsoft Windows NT utility installed with the Dynamic Host Configuration Protocol (DHCP) and Windows Internet Naming Service (WINS) services that you use to manually compact the DHCP or WINS database on a server running Windows NT. Compacting a database usually improves its performance. These two databases should be compacted automatically, but jetpack gives administrators the option of manually compacting the databases when needed.
Example
To compact the DHCP database, you use the following procedure at the command prompt:
cd \%SystemRoot%\system32\dhcp net stop dhcpserver jetpack dhcp.mdb tmp.mdb net start dhcpserver
In this example, tmp.mdb is a temporary database that is used during the compaction process. Jetpack copies the database information to tmp.mdb as it compacts it, deletes the original database, and renames tmp.mdb to replace the deleted database.
Don’t use temp.mdb as the name of the temporary database instead of tmp.mdb as shown in the example; if you do, an error will result. Before you run jetpack, be sure that there is no file named temp.mdb in the DHCP or WINS database directory.
TIP
The DHCP or WINS database should be compacted if it approaches 30 MB in size.
A networking technology developed by Sun Microsystems that enables devices that run the Java Virtual Machine (JVM) to quickly form network connections between each other as needed for communication. You can plug together Jini devices to form an instant federation or community of devices without needing to install software or configure the devices. Each device in a Jini community can provide various kinds of services to other devices in the community. Jini technology is based on the Java programming language and defines protocols for dynamic discovery, joining, lookup, leasing, and transaction processes between devices. Sun hopes to implement Jini technology in everyday devices such as televisions, telephones, and pagers to increase the level of connectivity of these devices and to provide new functions for them.
On the Web
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Sun Microsystems Jini home page : http://www.sun.com/jini
Distortion in transmission that occurs when a signal drifts from its reference position. Jitter can be caused by variations in the timing or the phase of the signal in an analog or digital transmission line. Jitter typically results in a loss of data because of synchronization problems between the transmitting stations, especially in high-speed transmissions.
Jitter is inherent in all forms of communication because of the finite response time of electrical circuitry to the rise and fall of signal voltages. An ideal digital signal would have instantaneous rises and falls in voltages and would appear as a square wave on an oscilloscope. The actual output of a digital signaling device has finite rise and fall times and appears rounded when displayed on the oscilloscope, which can result in phase variation that causes loss of synchronization between communicating devices. The goal in designing a transmission device is to ensure that the jitter remains within a range that is too small to cause appreciable data loss.
In Microsoft Systems Management Server (SMS), a set of instructions about system actions to be performed, such as managing the distribution, installation, and removal of software from computers in the network. Using the Systems Management Server Administrator program, you can create, modify, cancel, and delete jobs.
How It Works
All jobs are stored in the SMS database using Microsoft SQL Server. Before you create a job, you must create a package, and you might also need to create machine groups, site groups, or queries depending on whether you want to use these tools to limit which computers receive the package. For example, if you want to create a job that will distribute Microsoft FrontPage to all computers in your development sites that have sufficient disk space, you first run a query to find all computers (from your development sites) with sufficient disk space. Then you create a package for FrontPage and a site group that represents your target systems. Finally, you create the job.
After you create a job, SMS monitors the database and starts the job at the scheduled time. SMS sends the package and job actions to the targeted systems, where they are carried out. As a job is carried out, its status is reported back to the server in the site where the job was created.
You can create three kinds of jobs using SMS:
Run Command On Workstation jobs
Share Package On Server jobs
Remove Package From Server jobs
A database operation that allows you to retrieve and modify data from more than one table at a time using a single Structured Query Language (SQL) SELECT statement. You can use the Microsoft Query tool for Microsoft SQL Server to graphically create joins for transact-SQL queries that join two or more tables or join a table to itself. Joins are a necessity because a single table might not provide all the information you want to retrieve about a specific database entity. By connecting tables using joins, you can retrieve information that resides in more than one table in a single operation.
You can create three kinds of joins:
Inner joins: Connect two tables to form a result set that contains only the matching rows that satisfy the join condition. Inner joins can be equijoins or natural joins.
Cross or unrestricted joins: Produce a result set that contains all combinations of all rows from the tables that are joined. A cross join between a table of 5 rows and another table of 4 rows creates a result set of 20 rows.
Outer joins: Maintain information that does not match the join condition. For example, a left outer join contains at least all rows from the first table.
NOTE
There are two kinds of syntax for creating joins, ANSI join syntax and SQL Server join syntax. You can use only one of these types of syntax in a given SELECT statement.
See Jet
An interpreted object-based scripting language supported by Microsoft Internet Explorer, Internet Information Services (IIS), and the Windows Scripting Host (WSH). Microsoft JScript version 5.0 is the first fully compliant implementation of the ECMA-262 specification for cross-platform, vendor-neutral scripting languages.
JScript enables Web developers to produce dynamic content for Web pages that can incorporate Microsoft ActiveX controls and Java applets. JScript cannot be used for developing stand-alone applications and has limited file input/output (I/O) features. It is a loosely typed language, which means that you don’t have to declare a variable’s data type before you use the variable.
How It Works
Scripts written in JScript are embedded directly into Hypertext Markup Language (HTML) using <SCRIPT> tags, and the HTML files are stored on the Web server. When a Web browser such as Internet Explorer 4 accesses the page, it downloads and executes the script locally on the client machine. Internet Explorer can do this because it has a built-in scripting engine that can understand and interpret scripts written in JScript and Microsoft Visual Basic, Scripting Edition (VBScript).
The following is a simple example of a script written in JScript. The script is embedded in an HTML page between the <SCRIPT> and </SCRIPT> tags and displays a confirm message box when the user opens the file using Internet Explorer. When the user makes a selection, the remaining portion of the page loads.
<HTML> <BODY> <H1>Welcome to our restaurant!</H1> <HR> <SCRIPT> var testing=window.confirm("Click OK for food, Cancel for drink."); if (testing) { window.alert("Hot dog"); } else window.alert("Beer"); </SCRIPT> <H1>Thanks for coming!</H1> </BODY> </HTML> 
Graphic J-2. A dialog box created using JScript.
NOTE
JScript 3 and JavaScript 1.2 are syntactically similar but have some differences. JScript 3 complies fully with the ECMA-262 specification, while JavaScript 1.2 does not. JScript also includes additional features not included in the ECMA-262 standard.
On the Web
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Microsoft Windows Script Technologies home page : http://msdn.microsoft.com/scripting/default.htm
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ECMA-262 specification : http://www.ecma.ch/stand/ecma-262.htm
