Computer Network. Additionally there are two levels of ISDN access that may be provided. These are known as BRI and PRI.

Transcription

1 ISDN & Terminal Handling:- ISDN was developed by ITU-T (International Telecommunications Union-Telecommunication standards sector) in 1976.It is a set of protocols that combines digital telephony and Data transport services. The whole idea is to digitize the telephone network to permit the transmission of audio, video and text over existing telephone line. ISDN is the most powerful tool for various services such as voice, images, video and multimedia data communication over telephone network. ISDN may be viewed as a logical extension to the digitalization of telephone network. ISDN Configurations There are two types of channel that are found within ISDN. These are the 'B' and 'D' channels. The B or 'bearer' channels are used to carry the payload data which may be voice and / or data, and the d or 'Delta' channel is intended for signaling and control, although it may also be used for data under some circumstances. Additionally there are two levels of ISDN access that may be provided. These are known as BRI and PRI. BRI (Basic Rate Interface) - This consists of two B channels, each of which provides a bandwidth of 64 kbps under most circumstances. One D channel with a bandwidth of 16 kbps is also provided. Together this configuration is often referred to as 2B+D. The basic rate lines connect to the network using a standard twisted pair of copper wires. The data can then be transmitted simultaneously in both directions to provide full duplex operation. The data stream is carried as two B channels as mentioned above, each of which carry 64 kbps (8 k bytes per second). This data is interleaved with the D channel data and this is used for call management: setting up, clearing down of calls, and some additional data to maintain synchronization and monitoring of the line. The network end of the line is referred to as the 'Line Termination' (LT) while the user end acts as a termination for the network and is referred to as the 'Network Termination' (NT). Within Europe and Australia, the NT physically exists as a small connection box usually attached to a wall etc, and it converts the two wire line (U interface) coming in from the network to four wires A.K.Maurya Page 1

2 (S/T interface or S bus). The S/T interface allows up to eight items or 'terminal equipments' to be connected, although only two may be used at any time. The terminal equipments may be telephones, computers, etc, and they are connected in what is termed a point to point configuration. In Europe the ISDN line provides up to about 1 watt of power that enables the NT to be run, and also enables a basic ISDN phone to be used for emergency calls. In North America a slightly different approach may be adopted in that the terminal equipment may be directly connected to the network in a point to point configuration as this saves the cost of a network termination unit, but it restricts the flexibility. Additionally power is not normally provided. PRI (Primary Rate Interface) - This configuration carries a greater number of channels than the Basic Rate Interface and has a D channel with a bandwidth of 64 kbps. The number of B channels varies according to the location. Within Europe and Australia a configuration of 30B+D has been adopted providing an aggregate data rate of Mbps (E1). For North America and Japan, a configuration of 23B+1D has been adopted. This provides an aggregate data rate of Mbps (T1). The primary rate connections utilise four wires - a pair for each direction. They are normally 120 ohm balanced lines using twisted pair cable. Primary rate connections always use a point to point configuration. Although ISDN is has been overtaken by technologies such as ADSL it is nevertheless still widely used in many areas, particularly where existing services need to be maintained, or where compatibility needs to be guaranteed. As such it is still an important technology that will be encountered for many years to come. Terminal Handling:- A computer terminal is an electromechanical or electronic hardware device which is used for entering data into, and displaying data from a computing system or computer. The early terminals were inexpensive but slower than punched cards or paper tape for input. Video displays were introduced with the advancement in technology. Time sharing systems also developed. Multiple users could work on the same machine at their own terminals. A smart terminal has a significant local programmable data processing capability. A terminal which depends on the host computer for its processing power is known as a thin client. A A.K.Maurya Page 2

3 personal computer can run software which emulates the function of a terminal and can sometimes allow concurrent use of local programs and access to a distant terminal host system. Capabilities:- Text terminals:- A text terminal, or often just terminal (sometimes text console) is a serial computer interface for text entry and display. Information is presented as an array of pre-selected formed characters. When such devices use a video display such as a cathode-ray tube, they are called a "video display unit" or "visual display unit" (VDU) or "video display terminal" (VDT). The System console is a text terminal used to operate a computer. Modern computers have a built-in keyboard and display for the console. Some Unix-like operating systems such as Linux and FreeBSD have virtual consoles to provide several text terminals on a single computer. The fundamental type of application running on a text terminal is a command line interpreter or shell, which prompts for commands from the user and executes each command after a press of Enter. This includes Unix shells and some interactive programming environments. In a shell, most of the commands are small applications themselves. Dumb terminals:- Dumb terminals are those that can interpret a limited number of control codes (CR, LF, etc.) but do not have the ability to process special escape sequences that perform functions such as clearing a line, clearing the screen, or controlling cursor position. In this context dumb terminals are sometimes dubbed glass Teletypes, for they essentially have the same limited functionality as does a mechanical Teletype. This type of dumb terminal is still supported on modern Unix like systems by setting the environment variable TERM to dumb. Smart or intelligent terminals A.K.Maurya Page 3

4 are those that also have the ability to process escape sequences, in particular the VT52, VT100 or ANSI escape sequences. Graphical terminals:- A graphical terminal can display images as well as text. Graphical terminals are divided into vector-mode terminals, and raster mode. A vector-mode display directly draws lines on the face of a cathode-ray tube under control of the host computer system. The lines are continuously formed, but since the speed of electronics is limited, the number of concurrent lines that can be displayed at one time is limited. Vector-mode displays were historically important but are no longer used. Practically all modern graphic displays are raster-mode, descended from the picture scanning techniques used for television, in which the visual elements are a rectangular array of pixels. Since the raster image is only perceptible to the human eye as a whole for a very short time, the raster must be refreshed many times per second to give the appearance of a persistent display. The electronic demands of refreshing display memory meant that graphic terminals were developed much later than text terminals, and initially cost much more. Most terminals today are graphical, that is, they can show images on the screen. The modern term for graphical terminal is "thin client". A thin client typically uses a protocol like X11 for Unix-terminals, or RDP for Microsoft Windows. The bandwidth needed depends on the protocol used, the resolution, and the color depth. Modern graphic terminals allow display of images in color, and of text in varying sizes, colors, and fonts (type faces). Emulation:- A terminal emulator is a piece of software that emulates a text terminal. In the past, before the widespread use of local area networking and broadband internet access, many computers would use a serial access program to communicate with other computers via telephone line or serial device. When the first Macintosh was released, a program called MacTerminal was used to communicate with many computers, including the IBM PC. A.K.Maurya Page 4

5 Dec Terminal was one of the first terminal programs for the popular Altair. The Win32 console on Windows does not emulate a physical terminal that supports escape sequences so SSH and Telnet programs (for logging in textually to remote computers) for Windows, including the Telnet program bundled with some versions of Windows, often incorporate their own code to process escape sequences. The terminal emulators on most Unix-like systems, such as, for example, gnome-terminal, qterminal, xterm, terminal.app, do emulate physical terminals including support for escape sequences; e.g. xterm can emulate the VT220 and Tektronix 4010 hardware terminals. A.K.Maurya Page 5