Time-division multiplexing
TDM (time-division multiplexing) is a scheme in which numerous signals are combined for transmission on a single communications line or channel. Each signal is broken up into many segments, each having very short duration.
The circuit that combines signals at the source (transmitting) end of a communications link is known as a multiplexer. It accepts the input from each individual end user, breaks each signal into segments, and assigns the segments to the composite signal in a rotating, repeating sequence. The composite signal thus contains data from all the end users. At the other end of the long-distance cable, the individual signals are separated out by means of a circuit called a demultiplexer, and routed to the proper end users. A two-way communications circuit requires a multiplexer/ demultiplexer at each end of the long-distance, high-bandwidth cable.
If many signals must be sent along a single long-distance line, careful engineering is required to ensure that the system will perform properly. An asset of TDM is its flexibility. The scheme allows for variation in the number of signals being sent along the line, and constantly adjusts the time intervals to make optimum use of the available bandwidth. The Internet is a classic example of a communications network in which the volume of traffic can change drastically from hour to hour. In some systems, a different scheme, known as frequency-division multiplexing (FDM), is preferred.
time-to-live
TTL (time-to-live) is a value in an Internet Protocol (IP) packet that tells a network router whether or not the packet has been in the network too long and should be discarded. For a number of reasons, packets may not get delivered to their destination in a reasonable length of time. For example, a combination of incorrect routing tables could cause a packet to loop endlessly. A solution is to discard the packet after a certain time and send a message to the originator, who can decide whether to resend the packet. The initial TTL value is set, usually by a system default, in an 8-binary digit field of the packet header. The original idea of TTL was that it would specify a certain time span in seconds that, when exhausted, would cause the packet to be discarded. Since each router is required to subtract at least one count from the TTL field, the count is usually used to mean the number of router hops the packet is allowed before it must be discarded. Each router that receives a packet subtracts one from the count in the TTL field. When the count reaches zero, the router detecting it discards the packet and sends an Internet Control Message Protocol (ICMP) message back to the originating host.
The default Windows 95/98 TTL value is 32 hops. Some users recommend changing this to 128 if you have difficulty reaching certain sites.
The ping and the traceroute utilities both make use of the TTL value to attempt to reach a given host computer or to trace a route to that host. Traceroute intentionally sends a packet with a low TTL value so that it will be discarded by each successive router in the destination path. The time between sending the packet and receiving back the ICMP message that it was discarded is used to calculate each successive hop travel time.
Using the multicast IP protocol, the TTL value indicates the scope or range that a packet may be forwarded. By convention:
• 0 is restricted to the same host
• 1 is restricted to the same subnet
• 32 is restricted to the same site
• 64 is restricted to the same region
• 128 is restricted to the same continent
• 255 is unrestricted
Tivoli
Tivoli Systems is an IBM-owned company that develops software that allows a business to manage its computing environment. In a large organization, keeping track of anc upgrading computer equipment and various levels o operating systems and applications is a formidable task Tivoli offers products and solutions in these areas:
• E-business infrastructure management
• E-marketplace solutions
• Industry-specific solutions
• Service provider capabilities
• Availability and application management
• Network management
• Operations management
• OS/390 management
• Pervasive management (management of mobile devices and point-of-sale terminals)
• Storage and storage area network (SAN) management
• Security management
• Service management
• Web management
TMN
TMN (Telecommunications Management Network) originated formally in 1988 under the auspices of the International Telecommunication Union (ITU-TS) as a strategic goal to create or identify standard interfaces that would allow a network to be managed consistently across all network element suppliers. The concept has fostered and tracked a series of interrelated efforts at developing standard ways to define and address network elements. TMN uses the OSI Management Standards as its framework. TMN applies to wireless communications and cable TV as well as to private and public wired networks.
token ring
A token ring network is a local area network (LAN) in which all computers are connected in a ring or star topology and a bit- or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time. The token ring protocol is the second most widely-used protocol on local area networks after Ethernet. The IBM Token Ring protocol led to a standard version, specified as IEEE 802.5. Both protocols are used and are very similar. The IEEE 802.5 token ring technology provides for data transfer rates of either 4 or 16 megabits per second. Very briefly, here is how it works:
1. Empty information frames are continuously circulated on the ring.
2. When a computer has a message to send, it inserts a token in an empty frame (this may consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame.
3. The frame is then examined by each successive workstation. If the workstation sees that it is the destination for the message, it copies the message from the frame and changes the token back to 0.
4. When the frame gets back to the originator, it sees that the token has been changed to 0 and that the message has been copied and received. It removes the message from the frame.
5. The frame continues to circulate as an "empty" frame, ready to be taken by a workstation when it has a message to send.
The token scheme can also be used with bus topology LANs.
The standard for the token ring protocol is Institute of Electrical and Electronics Engineers (IEEE) 802.5. The Fiber Distributed-Data Interface (FDDI) also uses a token ring protocol.
Tool Kit
Tool Kit (Tk) is a companion program to Tool Command Language (Tcl) for creating graphical user interfaces. Together with Tcl, Tk is a rapid program development tool from Scriptics. Both can be ordered from the Scriptics Web site.
Tk provides a collection of standard buttons, menus, listboxes, and scrollbars and two special features, the text widget and the canvas widget, that the Tcl programmer can use to speed up program development.
toolbar
In the graphical user interface (GUI) for a computer, a toolbar is a horizontal row or vertical column of selectable image "buttons" that give the user a constantly visible reminder of and an easy way to select certain desktop or other application functions, such as saving or printing a document or moving pages forwards or backwards within a Web browser. Word processing, spreadsheet, and many other types of application programs come with one or more toolbars as part of their user interface.
In addition to the toolbars that come with a desktop or an application, some software developers or third parties provide supplementary toolbars that can be downloaded and installed. Supplementary toolbar functions may include quick access to news, sports, and weather headlines; instant form completion; instant access to favorite sites; and faster searches.
Topic Map
A Topic Map (ISO/IEC Standard 13250:2000) is an SGML or XML document used for navigation—by people or by machine—within an information set. A single information set can include any number of types of source data: for example, HTML files, PDF files, and databases could all be part of one information set. A Topic Map (TM) can be either embedded within a document, or may be, itself, a separate document. A Topic Map provides a particular view of topics within the information set. There could be, potentially, an infinite number of different TMs for any information set: for example, an information set dealing with the plays of Shakespeare could have a TM for navigation within the perspective of Elizabethan culture, or the perspective of themes throughout the plays. Each TM explicitly declares topics and provides links to relevant information.
The TAO of Topic Maps
Literally, Tao means the way; the term is expanded to mean the underlying unifying principle of the universe. In the case of TM, the unifying principle underlies the informational universe of the Web. The TAO of TMs stands for topics, associations, and occurrences. A topic is a subject: in terms of Shakespeare, for example, Avon, Romeo and Juliet, and
Anne Hathaway would all be topics. Topics are broken down into types: "Avon" would be defined as a town, and "Romeo and Juliet" as a play, for example. The word topic refers to both the topic link and the topic itself. Generally, topics have three types of characteristics: Names, which explicitly identify topics; occurrences, which are topic-relevant information resources; and roles in associations, which are details of how topics are related.
TMs are being heralded as an important information management tool for the development of the Semantic Web, because they can enable very precise data access—they are sometimes called "the Global Positioning System (GPS) of the Web”—within a vast, complex, and intricately interconnected global information repository. One way of looking at the way a TM works is to think of it as similar to a book’s index, and to compare seeking specific information within an indexed and an unindexed book. Although the information available on the Web now is linked, it is not linked in a systematic, standardized, and autonomously intelligent way (which explains why Web searches can often turn up so many irrelevant documents). Eventually, Topic Maps may be part of a comprehensive, user-friendly index to the body of human knowledge.
Topic Map Query Language
Topic Map Query Language (TMQL) is an XML-based extension of Structured Query Language (SQL), a query language developed for use in meeting the specialized data access requirements of Topic Maps (TMs). Two types of data access for Topic Maps are: information retrieval (IR), which is focused on separate search instances of a single user looking for specific information; and information filtering, which is a query process that builds up a sort of user profile, filtering information to construct a selection of data relevant to a particular user.
TMQL is intended to be easier to learn by developers, most of whom are likely to already be familiar with SQL. However, SQL was created to be used on the data in a relational database, which has a well-defined pre-existing structure; TMQL must be able to retrieve information from a vast and constantly changing body of information. A relational database may be expressed in terms of a simplified Topic Map, but TMQL will need to work with a much more complex data repository. A typical SQL select query (used to retrieve data from a table in a database) could be rewritten to retrieve data from a Topic Map.
top-level domain
On the Internet, a top-level domain (TLD) identifies the most general part of the domain name in an Internet address. A TLD is either a generic top-level domain (gTLD), such as "com" for "commercial," "edu" for "educational," and so forth, or a country code top-level domain (ccTLD), such as "fr" for France or "is" for Iceland.
topology
A topology (from Greek topos: place) is a description of any kind of locality in terms of its physical layout. In the context of communication networks, a topology describes pictorially the configuration or arrangement of a (usually conceptual) network, including its nodes and connecting lines.
toroid
A toroid is a coil of insulated or enameled wire wound on a donut-shaped form made of powdered iron. A toroids is used as an inductor in electronic circuits, especially at low frequencies where comparatively large inductances are necessary.
A toroid has more inductance, for a given number of turns, than a solenoid with a core of the same material and similar size. This makes it possible to construct high-inductance coils of reasonable physical size and mass. Toroidal coils of a given inductance can carry more current than solenoidal coils of similar size, because larger-diameter wires can be used, and the total amount of wire is less, reducing the resistance.
In a toroid, all the magnetic flux is contained in the core material. This is because the core has no ends from which flux might leak off. The confinement of the flux prevents external magnetic fields from affecting the behavior of the toroid, and also prevents the magnetic field in the toroid from affecting other components in a circuit.
touch pad
A touch pad is a device for pointing (controlling input positioning) on a computer display screen. It is an alternative to the mouse. Originally incorporated in laptop computers, touch pads are also being made for use with desktop computers. A touch pad works by sensing the user’s finger movement and downward pressure. The first touch pad was invented by George E. Gerpheide in 1988. Apple Computer was the first to license and use the touch pad in its Powerbook laptops in 1994. The touch pad has since become the leading cursor-controlling device in laptops. Many laptops use a trackball. IBM ThinkPad laptops use a ”pointing stick” (called a TrackPoint) that is set into the keyboard.
How the Touch Pad Works
The touch pad contains several layers of material. The top layer is the pad that you touch. Beneath it are layers (separated by very thin insulation) containing horizontal and vertical rows of electrodes that form a grid. Beneath these layers is a circuit board to which the electrode layers are connected. The layers with electrodes are charged with a constant alternating current (AC). As the finger approaches the electrode grid, the current is interrupted and the interruption is detected by the circuit board. The initial location where the finger touches the pad is registered so that subsequent finger movement will be related to that initial point. Some touch pads contain two special places where applied pressure corresponds to clicking a left or right mouse button. Other touch pads sense single or double taps of the finger at any point on the touch pad.
touch screen
A touch screen is a computer display screen that is sensitive to human touch, allowing a user to interact with the computer by touching pictures or words on the screen. Touch screens are used with information kiosks, computer-based training devices, and systems designed to help individuals who have difficulty manipulating a mouse or keyboard. Touch screen technology can be used as an alternative user interface with applications that normally require a mouse, such as a Web browser. Some applications are designed specifically for touch screen technology, often having larger icons and links than the typical PC application. Monitors are available with built-in touch screen technology or individuals can purchase a touch screen kit.
A touch screen kit includes a touch screen panel, a controller, and a software driver. The touch screen panel is a clear panel attached externally to the monitor that plugs into a serial or Universal Serial Bus (USB) port or a bus card installed inside the computer. The touch screen panel registers touch events and passes these signals to the controller. The controller then processes the signals and sends the data to the processor. The software driver translates touch events into mouse events. Drivers can be provided for both Windows and Macintosh operating systems. Internal touch screen kits are available but require professional installation because they must be installed inside the monitor.
There are three types of touch screen technology:
• Resistive: A resistive touch screen panel is coated with a thin metallic electrically conductive and resistive layer that causes a change in the electrical current which is registered as a touch event and sent to the controller for processing. Resistive touch screen panels are generally more affordable but offer only 75% clarity and the layer can be damaged by sharp objects. Resistive touch screen panels are not affected by outside elements such as dust or water.
• Surface wave: Surface wave technology uses ultrasonic waves that pass over the touch screen panel. When the panel is touched, a portion of the wave is absorbed. This change in the ultrasonic waves registers the position of the touch event and sends this information to the controller for processing. Surface wave touch screen panels are the most advanced of the three types, but they can be damaged by outside elements.
• Capacitive: A capacitive touch screen panel is coated with a material that stores electrical charges. When the panel is touched, a small amount of charge is drawn to the point of contact. Circuits located at each corner of the panel measure the charge and send the information to the controller for processing. Capacitive touch screen panels must be touched with a finger unlike resistive and surface wave panels that can use fingers and stylus. Capacitive touch screens are not affected by outside elements and have high clarity.
traceroute
Traceroute is a utility that records the route (the specific gateway computers at each hop) through the Internet between your computer and a specified destination computer. It also calculates and displays the amount of time each hop took. Traceroute is a handy tool both for understanding where problems are in the Internet network and for getting a detailed sense of the Internet itself. Another utility, ping, is often used prior to using traceroute to see whether a host is present on the network.
The traceroute utility comes included with a number of operating systems, including Windows and UNIX-based operating systems (such as IBM’s AIX/6000) or as part of a TCP/IP package. If your system doesn’t include the utility, you can install it. There are freeware versions that you can download.
How It Works
When you enter the traceroute command, the utility initiates the sending of a packet (using the Internet Control Message Protocol or ICMP), including in the packet a time limit value (known as the ”time to live” (TTL) that is designed to be exceeded by the first router that receives it, which will return a Time Exceeded message. This enables traceroute to determine the time required for the hop to the first router. Increasing the time limit value, it resends the packet so that it will reach the second router in the path to the destination, which returns another Time Exceeded message, and so forth. Traceroute determines when the packet has reached the destination by including a port number that is outside the normal range. When it’s received, a Port Unreachable message is returned, enabling traceroute to measure the time length of the final hop. As the tracerouting progresses, the records are displayed for you hop by hop. Actually, each hop is measured three times. (If you see an asterisk (*), this indicates a hop that exceeded some limit.)
If you have a Windows operating system, try traceroute out by clicking on Start->Programs->MS-DOS Prompt, and then at the C:WINDOWS prompt, enter: tracert www.whatis.com or whatever domain name for a destination host computer you want to enter. You can also enter the equivalent numeric form of the IP address.
Trackball
A trackball is a computer cursor control device used in many notebook and laptop computers. The trackball is usually located in front of the keyboard toward the user. Essentially, the trackball is an upside-down mouse that rotates in place within a socket. The user rolls the ball to direct the cursor to the desired place on the screen and can click one of two buttons (identical to mouse buttons) near the trackball to select desktop objects or position the cursor for text entry. IBM’s ThinkPad series of notebook computers uses a ”pointing stick,” called a TrackPoint, that is integrated into the middle of the keyboard keys.
