Alarm indication signal. AIS is also known as the blue alarm. When no incoming signal is detected, a CSU/DSU transmits an unframed all-ones pattern to the network to maintain synchronization and announce its presence to the network.

Asynchronous control character map.

Alternate Mark Inversion. An encoding scheme, also known as bipolar return to zero, in which one bits (marks) are transmitted as pulses and zeros (spaces) are indicated by the absence of a pulse. Pulses alternate in polarity.

Automatic Number Identification. A similar service to CLID, which transmits the number of the calling party.

Data transmission that relies on the endpoints to send explicit stop and start instructions. While easier to do than synchronous transmission, the overhead of start and stop messages makes it impractical for high-speed links.

Bipolar with 8 Zero Substitution. An encoding scheme that replaces eight zeros with intentional bipolar violations. Eight zeros are replaced with a code that has two intentional bipolar violations. The substitution takes the form 000V10V1, where V is a pulse of the same polarity as the previous pulse. B8ZS enables clear channel transmission because it ensures adequate pulse density even when long strings of zeros must be transmitted.

Bit Error Rate. The fraction of bits in error on a particular circuit. If 100,000 bits are transmitted and 1,000 are received in error, the BER is .01, or 1%. BER is often expressed in scientific notation, where 1 bit error per 1,000 bits is equal to 1 x 10^-4 or 1E-4.

Bit Error Rate Test. A test that measures the BER.

Older glossterm for AIS. See AIS.

Bipolar violation. In AMI encoding schemes, successive ones are supposed to be sent with alternating polarities. When this pattern is broken, it is known as a bipolar violation. Bipolar violations are the result of line noise, unless expressly inserted by a clear-channel line code such as B8ZS.

For signaling purposes, T1 links may rob the least-significant bit out of every time slot in every sixth frame. The lost bit is called the robbed bit, which is an appropriate glossterm for data network engineers because the robbed bit takes away 12.5% of the capacity.

Another glossterm for telco.

Clear channel capability. Lines that do not need to rob bits from the data stream for signaling or timing purposes are said to be clear channel lines. This glossterm is used to describe 64k DDS lines and higher levels of the T-carrier hierarchy that employ 64k channels.

Calling Line Identification; also referred to as “Caller ID.” It is the transmission of the caller’s telephone number to the receiver.

Customer premises equipment. Hardware purchased by customers and used for the purpose of connecting to the network. In the context of T1, it means the CSU/DSU.

Controlled slip. Replication or deletion of the 192-bit data frame, usually due to buffer overrun or underrun caused by timing mismatch between the relatively inaccurate CPE clock and the much more accurate CO clock.

Channel Service Unit. The piece of your CSU/DSU that talks to the telco network, understands framing and line coding, and provides electrical isolation of your network from the telco network.

(After the Western Electric channel bank of the same name.) A mistaken identifier sometimes used to refer to the superframe (SF) format on T1 links. SF existed long before the D4 channel bank.

Digital Access and Cross-connect System. Also called a DCS.

Digital Cross-connect System. CO equipment that connects incoming digital lines to outgoing digital lines. DCSs are the foundation of data transmission over the telco network.

Short for demarcation point. To the telco, anything beyond the demarc is a customer problem. To the customer, anything beyond the demarc is the telco’s problem.

Data link. In this book, I have referred to the ESF facilities data link as the FDL, though some standards may refer to it as the DL.

Dialed Number Information Service. A channelized T1 service that transmits the dialed number to the recipient.

Digital Stream, level zero. A single 64-kbps channel that can be used to transmit a sequence of pulses across the telephone network.

Digital Stream, level 1. 24 DS0s are combined into a DS1, which supplies 1.536-Mbps connectivity, with 8 kbps framing and signaling overhead for a total speed of 1.544 Mbps.

Data Service Unit. The part of the CSU/DSU that interfaces with our familiar world of routers, switches, and packets. It has a serial port to interface with compatible data equipment.

Ear and Mouth signaling. A method switches use to tell neighboring switches which channels are being used for telephone calls. E&M signaling has both wink-start and immediate-start variants.

Errored second. A 1-second interval with path code violations, controlled slips, out of frame errors, or AIS defects.

Extended superframe. ESF is a newer frame format that allows for error detection across an entire span and includes an embedded data link that can be used for signaling and performance reporting. Now the default framing format on nearly all new T1s.

The telco will put the demarc where it is convenient. To make it useful, an extended demarc must be constructed so the jack is near the customer DTE.

Excessive zeros. An error recorded when the line code has had too many zeros and has lost synchronization. This is >15 for AMI and >7 for B8ZS. Most equipment does not track EXZ events.

Facilities data link. On ESF-framed T1s, the FDL provides a 4-kbps channel that can be used for control, signaling, and performance reporting.

Frame bit error. A single framing bit that does not have the expected value. Only framing bits are considered on ESF; SF framing bits are always considered, and SF glossterminal bits may be considered.

Foreign exchange office. The CO-side counterpart to a foreign exchange station (FXS).

Foreign exchange station. A telephone connected through a PBX, or the PBX itself.

The deadlock that results when both ends of a line attempt to use it at the same time. It is amusing to think of telephone switches staring at each other, but the word glare was probably chosen for a different reason.

A common ISO-standardized link layer protocol, and the grandfather of most subsequent link layer protocols, including all varieties of the ITU’s Link Access Procedure and the IETF’s PPP.

Integrated Services Digital Network.

International Telecommunications Union. The ITU is responsible for international telecommunications standardization through several of its component bodies.

The ITU organ responsible for telephone standardization. ITU-T was formerly known as the CCITT (Consultative Committee on International Telephone and Telegraph).

Link Access Procedure, Balanced. The ISO specification for the X.25 defines LAPB as its own network layer. LAPB may be used as part of an X.25 network or to provide a reliable lower layer for PPP (see RFC 1663).

Link Access Procedure, D Channel. An ITU specification used in ISDN for control signaling that was adopted for sending performance reports over the T1 FDL. Because telcos use performance reports to monitor service levels, this LAPD (unlike the more familiar LAPD—the Los Angeles Police Department) might actually help enforce your rights.

Line build out. Circuitry that attenuates the signals from the CSU/DSU to the appropriate amplitude for the nearest line repeater.

Loss of frame. When a T1 CSU/DSU is unable to synchronize framing patterns with the remote end for 2.5 seconds, LOF is declared. LOF is sometimes called the red alarm, after the color of the light on an old channel bank.

Loss of signal. When no incoming pulses are received by a T1 CSU/DSU for 100 to 250 bit times, LOS is declared. Even if only zeros were transmitted as data, some framing bits should result in pulses on the line during that time. LOS indicator lights are often red as well. If LOS persists, LOF will eventually be declared because there is no incoming signal with which to synchronize.

Network Facilities Associated Signaling. A means of sharing a single ISDN PRI D channel among several PRI lines. NFAS also allows for a backup D channel in case of the failure of the primary D channel.

Network interface. Used to refer to the RJ-48 jack on the CSU/DSU to connect to the telco network.

Non-Return to Zero Inverted. A line code in which zeros change the voltage from the previous bit and ones do not.

Out of frame. When frame synchronization is lost, an OOF event is recorded. If OOF persists, LOF is declared. Some T1 CSU/DSUs may have a light that blinks briefly on OOF events. OOF is cleared when frame synchronization is regained.

Private branch exchange. A customer-owned switch that creates a private telephone network at the customer location.

Plesiochronous digital hierarchy. Lower-speed digital lines are clocked with similar but not identical clocks. Successively higher speeds in the PDH are achieved by time-division multiplexing of signals from the lower levels. In North America, PDH is synonymous with the T-carrier hierarchy. Europe’s PDH is the E-carrier hierarchy.

Network in which timing is almost the same. It uses independent clocks that are within tight tolerances of each other.

Plain old telephone service. Used in North America to refer to traditional analog telephone service, as opposed to any fancier digital telephony services such as ISDN.

A glossterm telcos often use to mean “installing and configuring.” It may also refer to configuration that can be done at the CO switch to set up the telephone network to recognize your circuit.

Public switched telephone network. A fancy name for the telephone network, used frequently in job interviews.

Permanent virtual circuit. A dedicated logical path through a frame relay network.

Remote alarm indication. Also known as yellow alarm, after the color of the indicator on Western Electric equipment. When a CSU/DSU enters the red alarm state, an RAI is transmitted in the outgoing direction. The RAI signals to the remote end that the local end is unable to synchronize framing patterns.

See LOS and LOF.

Registered jack. A code used with the Universal Service Order Code to specify a standard jack pinout and connector. Common designations are RJ-11 (plain telephone jacks) and RJ-45 (Ethernet).

Severely errored framing. Refers to a certain density of frame bit errors, which almost always causes an out of frame defect.

Severely errored second. One-second interval that is likely to have garbage on data links because framing was lost, AIS was detected, or 320 or more frames failed the CRC check.

Superframe. The original multiframe format for T1, which was composed when 12 frames were bundled together. Errors were detected solely by any resulting bipolar violations. No error check was available over the entire T1 path. All the framing bits were used for maintaining frame synchronization, so no end-to-end communications were possible outside of the data channel. Full-bandwidth data could cause a false yellow alarm unless certain bits were constrained from being zero, so data transmission capability was limited. All these limitations spurred the development of the ESF format, and SF has largely been supplanted by ESF.

Procedures used for communicating control information.

When used to refer to data transmission, it means that data is streamed, with bits transmitted at regular intervals. Frequently, it also implies that a separate clocking signal is sent with the data.

DS1 delivered over a 4-wire copper interface.

A company that sells telecommunications services to the public; synonymous with carrier. Also known pejoratively to many as “the phone company.”

Unavailable second. Used in measuring the performance of T1 lines; carriers will often guarantee a certain uptime. Uptime is the number of seconds in the measuring period minus the number of unavailable seconds. Different monitoring specifications define this quantity in different ways.

Unit interval. Synonymous with bit time. The time slot for a single bit.

Universal Service Order Code. System of codes used by AT&T that make up the Registered Jack system (see RJ).

Van Jacobson (Compression), named for its inventor, is an ingenious method of compressing TCP/IP headers on slow serial links. Its formal specification is RFC 1144.

Also known as RAI; see RAI.

Zero Byte Time Slot Interchange. ZBTSI is a method of clear channel coding that is no longer widely used in T-carrier facilities.