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36 MultiModem ZPX User Guide

CCITT (Consultative Committee for International Telephone and

Telegraph): An advisory committee created and controlled by the

United Nations and headquartered in Geneva whose purpose is

to develop and to publish recommendations for worldwide

standardization of telecommunications devices. CCITT has

developed modem standards that are adapted primarily by PTT

(post, telephone and telegraph) organizations that operate

phone networks of countries outside of the U.S. See also ITU.

Central Office (CO): The lowest, or most basic level of switching

in the PSTN (public switched telephone network). A business

PABX or any residential phone connects to the PSTN at a central

office.

Centrex: A multi-line service offered by operating telcos which

provides, from the telco CO, functions and features comparable

to those of a PBX for large busines++s users. See also “Private

Branch Exchange”, “Exchange”.

Channel: A data communicationspath between two computer

devices. Can refer to a physical medium (e.g., UTP or coax), or to

a specific carrier frequency.

Channel Bank: A device that acts as a converter, taking the digital

signal from the T1 line into a phone system and converting it to

the analog signals used by the phone system. A channel bank

acts as a multiplexer, placing many slow-speed voice or data

transactions on a single high-speed link.

Characteristics: Those qualities of a Universal Serial Bus device

that are unchangeable; for example, the device class is a device

characteristic.

Circuit-switched Network: A technology used by the PSTN that

allocates a pair of conductors for the exclusive use of one

communication path. Circuit switching allows multiple

conversations on one talk path only if the end-users multiplex

the signals prior to transmission.

Circuit Switching: The temporary connection of two or more

communications channels using a fixed, non-shareable path

through the network. Users have full use of the circuit until the

connection is terminated.

Clear Channel: A transmission path where the full bandwidth is

used (i.e., no bandwidth needed for signaling, carrier framing or

control bits). A 64 Kbps digital circuit usually has 8 Kbps used for

signaling. ISDN has two 64 Kbps circuits, and a 16 Kbps packet

service of which part is used for signaling on the 64K channels.

Client-Server: In TCP/IP, the model of interaction in distributed

data processing in which a program at one site sends a request to

a program at another site and awaits a response. The requesting

program is called a client; the answering program is called a

server.

Cluster Controller: A device that can control the input/output

operations of more than one device connected to it. A cluster

controller may be controlled by a program stored and executed

in the unit, or it may be entirely controlled by hardware.

Committed Burst Size: The maximum number of bits that the

frame relay network agrees to transfer during any measurement

interval.

Committed Information Rate (CIR): An agreement a customer

makes to use a certain minimum data transmission rate (in bps).

The CIR is part of the frame relay service monthly billing, along

with actual usage, that users pay to their frame relay service

provider.

Compression: 1. The process of eliminating gaps, empty fields,

redundancies, and unnecessary data to shorten the length of

records or blocks. 2. In SNA, the replacement of a string of up to 64-

repeated characters by an encoded control byte to reduce the

length of the data stream to the LU-LU session partner. The

encoded control byte is followed by the character that was

repeated (unless that character is the prime compression

character). 3. In Data Facility Hierarchical Storage Manager, the

process of moving data instead of allocated space during

migration and recall in order to release unused space. 4. Contrast

with decompression.

COMx Port: A serial communications port on a PC.

Congestion: A network condition where there is too much data

traffic. The ITU I.233 standard defines congestion managemennt

in terms of speed and burstiness.

Congestion notification: The function in frame relay that ensures

that user data transmitted at a rate higher than the CIR are

allowed to slow down to the rate of the available network

bandwidth.

Consecutive Severely Errored Seconds (CSES): An error condition

that occurs when from 3 to 9 SES (Severely Errored Seconds) are

logged consecutively.

Customer Premise Equipment (CPE): The generic term for data

comm and/or terminal equipment that resides at the user site

and is owned by the user with the following exclusions: Over

voltage protection equipment, inside wiring, coin operated or

pay telephones, “company-official” equipment, mobile

telephone equipment, “911” equipment, equipment necessary

for the provision of communications for national defense, or

multiplexing equipment used to deliver multiple channels to the

customer.

Data Communications Equipment (DCE): Any device which

serves as the portal of entry from the user equipment to a

telecommunications facility. A modem is a DCE for the telephone

network (PSTN) that is commonly on site at the user’s premises.

Packet Switched Networks have another level of DCE which is

most often located at a central office.

Data Link Connection Identifier (DLCI): One of the six

components of a frame relay frame. Its purpose is to distinguish

separate virtual circuits across each access connection. Data

coming into a frame relay node is thus allowed to be sent across

the interface to the specified “address”. The DLCI is confirmed

and relayed to its destination, or if the specification is in error,

the frame is discarded.

Dataphone Digital Service (DDS): A private line digital service

that offers 2400, 4800, 9600 and 56 Kbps data rates on an inter-

LATA basis by AT&T and on an intra-LATA basis by the BOCs.

Data Service Unit (DSU): A device that provides a digital data

service interface directly to the data terminal equipment. The

DSU provides loop equalization, remote and local testing

capabilities, and a standard EIA/CCITT interface.

Dedicated Line: A communication line that is not switched. The

term leased line is more common.

Default: This is a preset value or option in software packages, or

in hardware configuration, that is used unless you specify

otherwise.

Default Address: an address defined by the Universal Serial Bus

Specification and used by a Universal Serial Bus device when it is

first powered or reset. The default address is 00h.

Glossary

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