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Terminal Adapters (TAs) are interface adapters for connecting one or more non-ISDN devices to an ISDN network. TAs, which are ISDN DCE, are equivalent to protocol or interface converters for use with equipment that does not have ISDN capability. The TAs must be exactly tuned to the specific terminal equipment. Network Termination (NT), in ISDN networks, is a function accomplished through the use of Network Termination logic embedded in the carrier network and the user equipment. NT2 is an interface to an intelligent device responsible for the users side of the connection to the network, performing such functions as multiplexing and switching; a NT2 would be the interface to an ISDN-compatible PABX or router. NT1 is responsible for interfacing to the carriers side of the connection, performing such functions as signal conversion and maintenance of the local loops electrical characteristics. These functions are similar to those provided by Data Service Units (DSUs) and Channel Service Units (CSUs).
Channel Service Units (CSUs) and Digital Service Units (DSUs) are devices which, in combination, interface the user to the digital network. In contemporary systems. CSUs and DSUs generally are combined into a single device, known variously as a CSU/DSU, CDSU, or ISU (Integrated Service Unit). It typically is found under the skin of another device, such as a multiplexer (MUX). They are used in a wide variety of digital voice and data networks, including DDS and T-carrier, which will be discussed in detail in Chapter 8.
CSUs
These provide the customer interface to the circuit. They also allow the isolation of the DTE/CPE from the network for purposes of network testing. CSU functions include electrical isolation from the circuit for purposes of protection from aberrant voltages, serving the same function as a protector in the voice world. Additionally, the CSU can respond to a command from the carrier to close a contact, temporarily isolating the DTE domain from the carrier domain. This allows the carrier to conduct a loopback test in order to test the performance characteristics of the local loop from the serving central office to the CSU and back to the central office.
The CSU also serves to interface the DTE domain to the carrier domain in an electrical environment. For instance, within the DTE, 1 bits are represented as positive (+) voltages and 0 bits as null (0) voltages. The network requires that 1 bits be alternating + and - voltages and that the 0 bits be 0 voltages. Further, the network requires assurance that ones density is achieved. Depending on the carrier network, from 15 to 80 zeros can be transmitted in a row, as long the density of ones is at least 12.5% over a specified interval of time. CSUs will insert, or stuff, 1 bits on a periodic basis in order to ensure that the various network elements maintain synchronization.
The CSU also provides signal amplification and will generate keep alive signals to maintain the circuit in the event of a DTE transmission failure. Finally, the CSU will store in temporary memory data describing its performance in order that it might be considered by an upstream network management system.
DSUs
DSUs convert the DTE unipolar signal into a bipolar signal that is demanded by the network. DSU functions include regeneration of digital signals, insertion of control signals, signal timing and reformatting.
FEPs combine the functions of a concentrator and a message switch. They have the ability to concentrate and switch traffic between multiple terminals and groups of terminals in order that a single circuit can be shared for access to mainframe resources. They also are an interface to Wide Area Network (WAN) circuits to serve mainframe resources to remote terminals. Most FEPs are midrange computers that, in turn, connect to the primary host mainframe; FEPs have their own databases. FEPs provide additional functions including error detection and correction, queuing, editing validation, and limited application processing. While the mainframe clearly could perform such tasks, it is more cost-effective to apply a lower order computer to the performance of such mundane responsibilities. In this fashion, the power of the mainframe is reserved for more difficult and demanding tasks in support of user-oriented applications.
Protocols are the procedures which are employed to ensure the orderly exchange of information between devices on a data link, data communications network, or system. Protocols comprise standards which, at a basic level, include the dimensions of line setup, transmission mode, code set, and non-data exchanges of information such as error control (detection and correction). Protocols have two major functions, handshaking and line discipline [7-4].
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