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the process where a part of the information carried in a transmission system is demodulated (dropped) at an intermediate point and different information is entered (added) for subsequent transmission;
add/drop multiplexer (ADM)
the process where a part of the information carried in a transmission system is demodulated (dropped) at an intermediate point and different information is entered (added) for subsequent transmission;
alarm indicating signal (AIS)
a code sent downstream indicating an upstream failure has occurred; SONET defines the following four categories of AIS: line AIS, STS path AIS, VT path AIS, DS–n AIS
alternate mark inversion (AMI)
the line-coding format in transmission systems where successive ones (marks) are alternatively inverted (sent with polarity opposite that of the preceding mark)
American National Standards Institute (ANSI)
a membership organization that develops U.S. industry standards and coordinates U.S. participation in the International Standards Organization (ISO)
asynchronous
a network where transmission system payloads are not synchronized, and each network terminal runs on its own clock
asynchronous transfer mode (ATM)
a multiplexing or switching technique in which information is organized into fixed-length cells with each cell consisting of an identification header field and an information field; the transfer mode
attenuation
reduction of signal magnitude or signal loss, usually expressed in decibels
bandwidth
information-carrying capacity of a communication channel; analog bandwidth is the range of signal frequencies that can be transmitted by a communication channel or network
bidirectional
operating in both directions; bidirectional APS allows protection switching to be initiated by either end of the line
bit 7
one binary digit; a pulse of data
bit error vs. block error
error rate statistics play a key role in measuring the performance of a network; as errors increase, user payload (especially data) must be retransmitted; the end effect is creation of more (nonrevenu
bit interleaved parity (BIP)
a parity check that groups all the bits in a block into units (such as byte), then performs a parity check for each bit position in a group
bit interleaved parity–8 (BIP–8)
a method of error checking in SONET that allows a full set of performance statistics to be generated; for example, a BIP–8 creates eight-bit (one-byte) groups, then does a parity check for each o
bit stuffing
in asynchronous systems, a technique used to synchronize asynchronous signals to a common rate before multiplexing
bit synchronous
a way of mapping payload into VTs that synchronizes all inputs into the VTs, but does not capture any framing information or allow access to subrate channels carried in each input; for example, bit sy
bits per second (BPS)
the number of bits passing a point every second; the transmission rate for digital information
broadband
services requiring 50–600 Mbps transport capacity
broadband integrated services digital network (BISDN)
a single ISDN that can handle voice, data, and eventually video services
byte interleaved
bytes from each STS–1 are placed in sequence in a multiplexed or concatenated STS–N signal; for example, for an STS–3, the sequence of bytes from contributing STS–1s is 1, 2, 3, 1,
byte synchronous
a way of mapping payload into VTs that synchronizes all inputs into the VTs, captures framing information, and allows access to subrate channels carried in each input; for example, byte synchronous ma
CCITT
the technical organs of the United Nations specialized agency for telecommunications, now the International Telecommunications Union–Telecommunications; they function through international commit
channel
the smallest subdivision of a circuit that provides a type of communication service; usually a path with only one direction
circuit
a communications path or network; usually a pair of channels providing bidirectional communication
circuit switching
basic switching process whereby a circuit between two users is opened on demand and maintained for their exclusive use for the duration of the transmission
coding violation (CV)
a transmission error detected by the difference between the transmitted and the locally calculated bit-interleaved parity
concatenate
the linking together of various data structures—for example, two bandwidths joined to form a single bandwidth
concatenated STS–Nc
A signal in which the STS envelope capacities from the N STS–1s have been combined to carry an STS–Nc SPE; it is used to transport signals that do not fit into an STS–1 (52 Mbps) payloa
concatenated VT
a VT x Nc that is composed of N x VTs combined; its payload is transported as a single entity rather than separate signals
cyclic redundancy check (CRC)
a technique for using overhead bits to detect transmission errors
data communications channels
OAM&P channels in SONET that enable communications between intelligent controllers and individual network nodes as well as internode communications
defect
a limited interruption in the ability of an item to perform a required function
demultiplexing
a process applied to a multiplex signal for recovering signals combined within it and for restoring the distinct individual channels of the signals
digital cross-connect system (DCS)
an electronic cross-connect that has access to lower-rate channels in higher-rate multiplexed signals and can electronically rearrange (cross-connect) those channels
digital signal
an electrical or optical signal that varies in discrete steps; electrical signals are coded as voltages; optical signals are coded as pulses of light
DSX–1
may refer to either a cross-connect for DS–1 rate signals or the signals cross-connected at an DSX–1
DSX–3
may refer to either a cross-connect for DS–3 rate signals or the signals cross-connected at an DSX–1
envelope capacity
the number of bytes the payload envelope of a single frame can carry; the SONET STS payload envelope is the 783 bytes of the STS–1 frame available to carry a signal; each VT has an envelope capac
European Conference of Postal and Telecommunications Administrations (CEPT)
the CEPT format defines the 2.048–Mbps European E1 signal made up of 32 voice-frequency channels
Exchange Carrier Standards Association (ECSA)
an organization that specifies telecommunications standards for ANSI
failure
a termination of the ability of an item to perform a required function; a failure is caused by the persistence of a defect
far end block error (FEBE)
a message sent back upstream that receiving network element is detecting errors, usually a coding violation
far end receive failure (FERF)
a signal to indicate to the transmit site that a failure has occurred at the receive site
fixed stuff
a bit or byte whose function is reserved; fixed-stuff locations, sometimes called reserved locations, do not carry overhead or payload
floating mode
a VT mode that allows the VT synchronous payload envelope to begin anywhere in the VT; pointers identify the starting location of the VT SPE; VT SPEs in different superframes may begin at different lo
framing
method of distinguishing digital channels that have been multiplexed together
frequency
the number of cycles of periodic activity that occur in a discrete amount of time
grooming
consolidating or segregating traffic for efficiency
interleave
the ability of SONET to mix together and transport different types of input signals in an efficient manner, thus allowing higher transmission rates
isochronous
all devices in the network derive their timing signal directly or indirectly from the same primary reference clock
jitter
short waveform variations caused by vibration, voltage fluctuations, control system instability, etc.
line
one or more SONET sections, including network elements at each end, capable of accessing, generating, and processing line overhead
line alarm indication signal (AIS–L)
AIS–L is generated by section terminating equipment (STE) upon the detection of a loss of signal or loss of frame defect, on an equipment failure; AIS–L maintains operation of the downstream
line overhead (LOH)
18 bytes of overhead accessed, generated, and processed by line terminating equipment; this overhead supports functions such as locating the SPE in the frame, multiplexing or concatenating signals, pe
line remote defect indication (RDI–L)
a signal returned to the transmitting line terminating equipment (LTE) upon detecting a loss of signal, loss of frame, or AIS–L defect; RDI–L was previously known as line FERF
line terminating equipment (LTE)
network elements such as add/drop multiplexers or digital cross-connect systems that can access, generate, and process line overhead
locked mode
a VT mode that fixes the starting location of the VT SPE; locked mode has less pointer processing than floating mode
map/demap
a term for multiplexing, implying more visibility inside the resultant multiplexed bit stream than available with conventional asynchronous techniques
mapping
the process of associating each bit transmitted by a service into the SONET payload structure that carries the service; for example, mapping a DS–1 service into a SONET VT1.5 associates each bit
mesochronous
a network whereby all nodes are timed to a single clock source; thus, all timing is exactly the same (truly synchronous)
multiplex/demultiplex (MUX/DEMUX)
multiplexing allows the transmission of two or more signals over a single channel; demultiplexing is the process of separating previously combined signals and restoring the distinct individual channel
multiplexer
a device for combining several channels to be carried by one line or fiber
narrowband
services requiring up to 1.5–Mbps transport capacity
operations system (OS)
sophisticated applications software that overlooks the entire network
operations, administration, maintenance, and provisioning (OA&M or OAM&P)
provides the facilities and personnel required to manage a network
orderwire
a channel used by installers to expedite the provisioning of lines
OSI seven-layer model
a standard architecture for data communications; layers define hardware and software required for multivendor information-processing equipment to be mutually compatible; the seven layers from lowest t
overhead
extra bits in a digital stream used to carry information besides traffic signals; orderwire, for example, would be considered overhead information
packet switching
an efficient method for breaking down and handling high-volume traffic in a network; a transmission technique that segments and routes information into discrete units; packet switching allows for effi
parity check
an error-checking scheme that examines the number of transmitted bits in a block that hold the value one; for even parity, an overhead parity bit is set to either one or zero to make the total number
path
a logical connection between a point where an STS or VT is multiplexed to the point where it is demultiplexed
path overhead (POH)
overhead accessed, generated, and processed by path-terminating equipment; POH includes 9 bytes of STS POH and, when the frame is VT–structured, 5 bytes of VT POH
path terminating equipment (PTE)
network elements, such as fiber-optic terminating systems, which can access, generate, and process POH
payload
the portion of the SONET signal available to carry service signals such as DS–1 and DS–3; the contents of an STS SPE or VT SPE
payload pointer
indicates the beginning of the synchronous payload envelope (SPE)
photon
the basic unit of light transmission used to define the lowest (physical) layer in the OSI seven-layer model
plesiochronous
a network with nodes timed by separate clock sources with almost the same timing
point of presence (POP)
a point in the network where interexchange carrier facilities like DS–3 or OC–N meet with access facilities managed by telephone companies or other service providers
pointer
a part of the SONET overhead that locates a floating payload structure; STS pointers locate the SPE; VT pointers locate floating mode VTs; all SONET frames use STS pointers; only floating mode VTs use
poll
an individual control message from a central controller to an individual station on a multipoint network inviting that station to send
regenerator
device that restores a degraded digital signal for continued transmission; also called a repeater
remote alarm indication (RAI)
a code sent upstream in a DS–n network as a notification that a failure condition has been declared downstream; RAI signals were previously referred to as yellow signals
remote defect indication (RDI)
a signal returned to the transmitting terminating equipment upon detecting a loss of signal, loss of frame, or AIS defect; RDI was previously known as FERF
remote error indication (REI)
an indication returned to a transmitting node (source) that an errored block has been detected at the receiving node (sink); this indication was formerly known as far end block error (FEBE)
remote failure indication (RFI)
a failure is a defect that persists beyond the maximum time allocated to the transmission system protection mechanisms; when this situation occurs, an RFI is sent to the far end and will initiate a pr
section
the span between two SONET network elements capable of accessing, generating, and processing only SONET section overhead; this is the lowest layer of the SONET protocol stack with overhead
section overhead
nine bytes of overhead accessed, generated, and processed by section terminating equipment; this overhead supports functions such as framing the signal and performance monitoring
section terminating equipment (STE)
equipment that terminates the SONET section layer; STE interprets and modifies or creates the section overhead
slip
an overflow (deletion) or underflow (repetition) of one frame of a signal in a receiving buffer
stratum
level of clock source used to categorize accuracy
STS path remote defect indication (RDI–P)
a signal returned to the transmitting STS path terminating equipment (PTE) upon detection of certain defects on the incoming path
STS path terminating equipment (PTE)
equipment that terminates the SONET STS path layer; STS PTE interprets and modifies or creates the STS POH; an NE that contains STS PTE will also contain LTE and STE
STS POH
nine evenly distributed POH bytes per 125 microseconds starting at the first byte of the STS SPE; STS POH provides for communication between the point of creation of an STS SPE and its point of disass
superframe
any structure made of multiple frames; SONET recognizes superframes at the DS–1 level (D4 and extended superframe) and at the VT (500 µs STS superframes)
synchronous
a network where transmission system payloads are synchronized to a master (network) clock and traced to a reference clock
synchronous digital hierarchy (SDH)
the ITU–T–defined world standard of transmission whose base transmission level is 52 Mbps (STM–0) and is equivalent to SONET's STS–1 or OC–1 transmission rate; SDH standards w
synchronous optical network (SONET)
a standard for optical transport that defines optical carrier levels and their electrically equivalent synchronous transport signals; SONET allows for a multivendor environment and positions the netwo
synchronous payload envelope (SPE)
the major portion of the SONET frame format used to transport payload and STS POH; a SONET structure that carries the payload (service) in a SONET frame or VT; the STS SPE may begin anywhere in the fr
synchronous transfer module (STM)
an element of the SDH transmission hierarchy; STM–1 is SDH's base-level transmission rate equal to 155 Mbps; higher rates of STM–4, STM–16, and STM–48 are also defined
synchronous transport signal level 1 (STS–1)
the basic SONET building block signal transmitted at 51.84–Mbps data rate
T1X1 subcommittee
a committee within ANSI that specifies SONET optical interface rates and formats
virtual tributary (VT)
a signal designed for transport and switching of sub–STS–1 payloads
VT group
a 9-row by 12-column structure (108 bytes) that carries one or more VTs of the same size; seven VT groups can be fitted into one STS–1 payload
VT path remote defect indication (RDI–V)
a signal returned to the transmitting VT PTE upon detection of certain defects on the incoming path
VT path remote failure indication (RFI–V)
a signal, applicable only to a VT1.5 with the byte-synchronous DS–1 mapping, that is returned to the transmitting VT PTE upon declaring certain failures; the RFI–V signal was previously know
VT path terminating equipment (VT PTE)
equipment that terminates the SONET VT path layer; VT PTE interprets and modifies or creates the VT POH; an NE that contains VT PTE will also contain STS PTE, LTE, and STE POH
VT POH
four evenly distributed POH bytes per VT SPE starting at the first byte of the VT SPE; VT POH provides for communication between the point of creation of an VT SPE and its point of disassembly
wander
long-term variations in a waveform
wideband
services requiring 1.5– to 50–Mbps transport capacity
