Last Updated on September 18, 2001

This glossary is a work in progress and will be updated from time to time.  The glossary is meant to be more than just a list of acronyms, but to give enough explanation of each term so that it adds to the understanding of SONET in general.  The source for this material comes mostly from Bellcore, ANSI standards, and publications.  It has been aggregated and condensed here in a glossary form from A to Z except SONET/SDH and Stratum Levels.


SONET (Synchronous Optical Network) / SDH (Synchronous Digital Hierarchy)


Optical Level

Electrical Level

Line Rate (Mbps)

Payload Rate (Mbps)

Overhead Rate (Mbps)

SDH Equivalent





























































OC-9, OC-18, OC-24, OC-36, OC-96 are considered orphaned rates.

Stratum Levels

Level of clock source used to categorize accuracy.  Stratum 1 is the highest level of accuracy and stability.

Stratum level

Free Run Accuracy

Pull-in-range / Capable of synchronizing to clock with accuracy of


Time to first frame slip


1 x 10-11



72 days


1.6 x 10-8

± 1.6 x 10-6

1 x 10–10 / day

7 days


1.0 x 10-6

± 4.6 x 10-6

1 x 10-8 / day

3.5 hours


4.6 x 10-6

± 4.6 x 10-6

3.7 x 10-7 / day

255 in 24 hours

SONET Minimum Clock

20 x 10-6

± 20 x 10-6

Same as accuracy



ADM - Add-Drop-Multiplexer

This is one of SONET's claim to fame.  The tributaries of a SONET transport stream, are synchronously multiplexed to the line rate, i.e. there are no stuff bits or stuff opportunity bits as is the case in the plesiochronous hierarchy.  As such an ADM can insert or extract lower rate tributary data without demultiplexing the aggregate line rate.

AIS - Alarm Indicator Signal

This is a coded signal that is sent to downstream network elements to indicate that an upstream failure has been detected and alarmed.

Asynchronous Mapping

These mappings are defined for clear channel transports of digital signals that meet the standard DSX cross connect requirements, typically DSX-1 and DSX-3 in most practical applications.  At the asynchronous mapping interface, frame acquisition and generation is not required.  For example if your system can transport a BERT (bit error test set) signal with a 1023-1 test pattern, it is being asynchronously mapped for transport.

APS - Automatic Protection Switching

A 1+1 protection switch architecture is one in which the head end signal is permanently bridged (at the electrical level) to service and protection equipment to enable the same payload to be transmitted identically to the tail end service and protection equipment.  At the tail end, each service and protection optical signal is monitored independently and identically for failures.  The receiving equipment selects either the service or protection channel based upon the switching criteria.  A 1+N protection switch architecture is defined as an architecture in which any one of N service channels can be bridged to a single optical protection channel.  Head end to tail end communications are accomplished by using the SONET APS channel, bytes K1 and K2.

ATM – Asynchronous Transfer Mode

A type of multiplexing which broadband ISDN will use, where payload is multiplexed into cells.


The foundation for many Bellcore reliability criteria is an end-to-end two-way availability of objective of 99.98% for interoffice applications (0.02% unavailability or 105 minutes/year down time).  The objective for loop transport between the central office and the customer premises is 99.99%.  For interoffice transport the objective refers to a two-way broadband channel, e.g. SONET OC-N, over a 250-mile path.  For loop applications the objective refers to a two-way narrowband channel, e.g. DS0 or equivalent.

BIP-8 – Bit Interleaved Parity-8

BIP-8 is a method used for error monitoring where each bit of the BIP-8 code word or byte, corresponds to even parity as calculated across matching bit positions for the distinct bytes in a SONET frame.  That is, the first BIP-8 bit would correspond to even parity across bit number 1 of a certain number of bytes in the SONET frame.  The certain number of bytes depends upon whether you are calculating section, line, or path BIP-8.  The BIP violations are counted over a sliding time window equal to the maximum detection time.  The BIP-8 violation counts of individual STS-1s of an STS-N are added together.  Design objectives for the average detection time depend on the level N of the Optical Carrier and are typically an order of magnitude lower than the maximum detection times.  As the optical rate increases the average detection time decreases proportionally.

BITS - Building Integrated Timing Supply

Synchronization networks provide timing signals to all synchronization network elements at each node in a digital network.  These timing signals are traceable to a highly accurate Primary Reference Source (PRS) clock of Stratum 1 quality.  The aim is to ensure that all outgoing transmissions from a digital network node have the same average frequency.  Buffer elements are used at important transmission interfaces to absorb differences between the average local frequency and the actual short-term frequency of incoming signals, which may be affected by phase wander and jitter accumulated along the transmission paths.  A synchronized network has two major parts: inter-office and intra-office.  The inter-office network consists of a primary and a secondary T1 (DS1) link, carrying timing between offices in a hierarchical relationship.  Intra-office timing distribution is based on the concept of a BITS master clock, providing timing to all other digital equipment in the office.  The BITS concept minimizes the number of synchronization links entering an office, since only the BITS will receive timing from outside the office.


Service requiring 50 – 600 Mbps transport capacity.

B3ZS - Bipolar with 3 zero substitution

This is the STS-1 line code. In the B3ZS technique each block of 3 consecutive zeros is removed and replaced with {B0V} or {00V}.  The choice is made such that the number of B pulses between consecutive V pulses is odd.  Where B represents the normal bipolar pulse and V represents a bipolar violation.

Category I

Terminal options that perform an asynchronous multiplex function.  Examples of Categrory I transport NE interfaces are 1) low speed interfaces to ADMs, 2) digital radio terminals and fiber optics terminals (excluding terminals that function solely as digital repeaters or regenerators), and 3) low-speed interfaces to DCSs (e.g., the DS-1 interface to a DCS 3/1). Asynchronous DS-1, DS-2, DS-1C, DS-3 interfaces to SONET NE are considered Category I.

Category II

Equipment interfaces whose behavior with respect to timing jitter is governed exclusively by input timing recovery circuitry.  Examples of Category II transport NE interfaces are 1) digital terminals at a DLC system, 2) repeaters for metallic cables, 3) regenerators for fiber optic cables, and 4) high speed interfaces to DCSs (e.g. with respect of jitter behavior the DS-3 interface to a DCS 3/1. STS-N and OC-N interfaces to a SONET NE are considered Category II.

Concatenated STS-1 – STS-NC

An STS (Synchronous Transport Signal) line signal in which the STS envelope capacities from N STS-1s are transported as a single entity as opposed to being treated as separate signals.  The STS-NC shall be multiplexed, switched, and transported over the network as a single entity.  It is used to transport signals that do not fit into an STS-1 (51 Mbps) payload.

CMI - Coded Mark Inversion

This is the STS-3 line code.  T his is a two level non-return to zero code. A binary 1 is coded by either of the amplitude levels, +A or -A, for one full unit time interval (T) in such a way that the level alternates for successive binary ones.  For a binary zero there is always a positive transition (-A to +A) at the mid point of the binary unit interval (T/2).

Clock Free Run Mode

An operating condition of a clock in which its local oscillator is not locked to an external synchronization reference, and is using no storage techniques to sustain it's accuracy.

Clock Holdover Mode

An operating condition of a clock in which its local oscillator is not locked to an external synchronization reference but which is using storage techniques to maintain its accuracy with respect to the last known frequency comparison with a synchronization reference.

Collapsed Ring

A topology where the ring resides entirely within a single fiber bundle.  In the event of a cable cut the ring is severed in two places.

DCC - Data Communications Channel

In the section layer, 3 bytes (D1, D2, D3) are allocated in STS-1 number 1 of an STS-N signal for section data communications.  These 3 bytes are treated as one 192kbs data channel for the transmission of alarms, maintenance, control, administration as well as other network element communication needs.  In the line layer, 9 bytes (D4-D12) are used as a 576kbs data channel for similar purposes.


More correctly known as chromatic dispersion.  Dispersion results when light of different wavelengths propagates at different velocities down a fiber span.  The result is pulse spreading that is a function of length.  Total dispersion is measured in units of ps/nm and the dispersion coefficient of a fiber is in units of  ps/nm-km, or pico-seconds per nanometer per km of fiber length.

Drop and Broadcast

A cross connect typically used to enable a broadcast transmission.  A signal in the high-speed time slot is used to provide simultaneous drops at more than one node.  A distance learning application would use drop and continue to feed multiple classrooms.


The DS4NA (where NA stands for North America) is specified for a 139.264 Mb/s interface (not 274 Mb/s as referenced in some literature with regard to DS-4 systems.  This specification is compatible with CCITT Recommendations G.755 for multiplexing 45Mb/s signals into 139 Mb/s signals, but does not specify the multiplexing of other signals into the 139 Mb/s signal.

Dual Ring Internetworking

A topology where two rings are connected at two different nodes providing traffic an alternate path from one ring to another.

FEBE - Far End Block Error

A signal returned to the transmitting network element indicating that an error block has been received at the receiving network element.

FERF - Far End Receive Failure

A line FERF alerts the upstream network element that a failure has been detected along the down streamline.  A line FERF is different from a yellow signal in that a yellow signal can be used for trunk conditioning.

Four Wave Mixing

FWM is basically an intermodulation and cross talk phenomenon that occurs in WDM systems due to the non-linear nature of the fiber optic cable.  The effect occurs in areas of zero dispersion, as the signals need to be traveling at the same velocity in the fiber for the effect to occur.  FWM does not occur in the 1550nm window unless the fiber is dispersion shifted.

GNE - Gateway Network Element

A GNE internetworks two different kinds of networks. In SONET there are three different types of GNEs:


A GNE that interworks an X.25 DCN (Data Comm. Network) and the SONET DCC.

A GNE that interworks an X.25 DCN and an intra-site LAN.

A GNE that interworks an intra-site LAN and the SONET DCC.


Another GNE network function is message concentration for the X.25 DCN.  Instead of having one X.25 virtual circuit to each SONET network element, the gateway can provide and X.25 virtual circuit it and the OS that can be used for messages to and from the OS and subtending network elements on the SONET network.

HDB3 - High Density Bipolar 3 coding

The European equivalent of B3ZS.


The ability of SONET to mix together and transport different types of input signals in an efficient manner, thus allowing higher transmission rates.

Intermediate NE

Intermediate Network Element (INE), has one or more subtending NEs and performs routing for tandem traffic. An INE must support IS-IS (Intermediate System) level 1 routing and the IS role of the ES-IS (End System) protocol.  The role a given SONET NE supports (GNE, INE, ENE) depends upon the operations communications network architecture.

Intermediate Reach

IR optical interfaces refer to optical sections with system loss budgets from 0 db to 12 db.  Typically low power, e.g. 50uW or -13 dbm, SLM or MLM lasers are used.

ISDN – Integrated Services Digital Network

An international telecommunication network standard that provides end-to-end digital connections for both voice and non-voice service.


Timing jitter is the short-term variation of a digital signal's significant instant from their ideal positions in time, where short term implies phase oscillations of frequency greater than or equal to 10Hz.  Significant instants include for instance, optimum sampling instants.  Long-term variations, where the variations are of frequency less than 10Hz, are called wander.

Jitter Generation

The process whereby jitter appears at the output port of an individual piece of digital equipment in the absence of applied jitters at the input.  When looped back at the high speed rate, whether or not a standard interface exists at the higher rate, Category I equipment must produce less than 0.3 Unit Intervals (UI) of RMS jitter and less than 1.0 UI of peak-to-peak timing jitter at the output of the terminal receiver.  This is as specified in TR-499.  In TR-253 for SONET a DS-3 interface shall generate jitter less than 0.4 UI peak-to-peak.

Jitter Tolerance

For STS-N electrical interfaces input jitter tolerance is the maximum amplitude of sinusoidal jitter at a given jitter frequency, which when modulating the signal at an equipment input port, results in no more than two error seconds cumulative, where these error seconds are integrated over successive 30 second measurement intervals.  Requirements on input jitter tolerance as just stated are specified in terms of compliance with a jitter mask, which represents a combination of points.  Each point corresponds to a minimum amplitude of sinusoidal jitter at a given jitter frequency which when modulating the signal at the equipment input port results in two or fewer error seconds in a 30 second measurement interval.  For the OC-N optical interface it is defined as the amplitude of the peak-to-peak sinusoidal jitter applied at the input of an OC-N interface that causes a 1 db power penalty.

Jitter Transfer

This is the relationship between jitter applied at the input port and the jitter appearing at the output port.


Optical Kerr-effect is where the index of refraction of a fiber optic varies with the intensity of the transmitted light.  This is a non-linear process that occurs when the product of the laser power and the effective system length becomes a significant fraction of the nonlinear coefficient y.  At 1550 nm, 1/y  ranges from 700 mW-km for unshifted single mode fiber to 500 mW-km for NZDSF (Non Zero Dispersion Shifted Fiber).  In systems with milliwatt transmitters and hundreds of km span lengths nonlinear diffraction will occur.  The main effect of this is self phase modulation of the signal

Line AIS

It is generated by the section terminating equipment upon loss of an input signal, loss of frame, or equipment failure.  The line AIS maintains operation of downstream regenerators preventing generation of unnecessary alarms.

Long Reach

LR optical interfaces refer to optical sections with system loss budgets from 10 db up to 28 db at OC-3, to 24 db at OC-12, and to 20 db at OC-48.  Typical of long haul telecommunications systems, LR interfaces are based on high power, e.g. 500uW or -3dbm, Multi-Longitudinal Mode (MLM) or Single-Longitudinal Mode (SLM) lasers.

Mediation Device

It is a device that facilitates communications between a SONET network and an OS.

Multiplex Process

STS-N signals are formed by byte interleaving STS-1 signals.  Three STS-1 signals shall be interleaved, one byte at a time, to form an STS-3 signal.  The first byte of the STS-3 signal shall be the A1 byte of STS-1 number 1, followed sequentially by the A1 byte from STS-1 number 2, then the A1 byte from STS-1 number 3.  The first bit to be transmitted in the STS-3 is the most significant bit of the A1 framing byte from STS-1 number 1.


Services requiring up to 1.5 Mbps transport capacity.

NE – Network Elements

In SONET, the five basic network elements are add/drop multiplexer, broadband digital cross-connect, wideband digital cross-connect, digital loop carrier, and switch interface.


The optical line coding used in SONET systems.  A one or zero is designated by constant levels of opposite polarity.

NZDSF - Non Zero Dispersion Shifted Fiber

This type of fiber was designed to introduce a small amount of dispersion without the zero point crossing being in the WDM passband.  With this type of fiber you can eliminate, or at least greatly reduce the degradation due to four wave mixing, a distortion mechanism that requires the spectral components to be phase matched along the fiber.


A channel used by installers to expedite the provisioning of lines.


Extra bits in a digital stream used to carry information besides traffic signals.  Orderwire, for example, would be considered overhead information.


The portion of the SONET signal available to carry service signals such as DS-1, DS-2, and DS-3.


A network with nodes timed by separate clock sources with almost the same timing.

PMD - Polarization Mode Dispersion

Light transmitted down a single mode fiber can be decomposed into two perpendicular polarization components.  Distortion results due to each polarization propagating at a different velocity.  PMD causes pulse spreading as the polarizations arrive at different times.  The longer the span, the worse the PMD.  Total PMD = PMDc x (L)1/2, where PMDc is the PMD coefficient and L is the length of the fiber.   PMDc has the units of   ps/(km)1/2, that is pico-seconds per root km.  PMD is generally not a factor at OC-48 but will be a factor at OC-192.  Corning has stated that they have conducted field measurements on various installed SMF-28 fibers and have typical installed link centered at less than 0.1ps/(km)1/2.  Beginning in 1994, Corning also implemented a fiber PMD specification of <0.5ps/(km)1/2 for SMF-28 and Titan single mode fibers.  For OC-192 this level of PMD probably will meet most common span engineering requirements.

Pulse Density

At all digital interfaces, digital bit streams must contain sufficient energy for self-extraction of a timing signal.  The level of energy is controlled by ensuring that the signal has a sufficient number of pulses as specified by a pulse density.  In general, as the bit rate increases the desired level of pulse density also increases, resulting in different requirements being applied to different levels in the digital hierarchy.

Raman Scattering

Stimulated Raman Scattering is the result of interaction between the optical signal and silica molecules in the fiber.  This process is broadband and applies to the overall optical spectrum being transmitted.  SRS manifests itself as a transfer of power from the shorter wavelengths to the longer wavelengths, i.e. from higher photon energy wavelengths to lower photon energy longer wavelengths.  This will result in the optical spectrum having a tilt.  The effect increases with power and the width of the DWDM spectrum.  One way to mitigate the effect is to use moderate channel powers and a densely packed spectrum.

SBS – Stimulated Brillouin Scattering

Stimulated Brillouin scattering is an interaction between the optical signal and the acoustic waves in the fiber that causes the optical power to be scattered backwards towards the transmitter.  It is a narrow band process that affects each channel in a DWDM system individually.  It is noticeable in systems that have channel powers in excess of 5 - 6 dBm.  In most cases SBS can be suppressed by modulating the laser transmitter to broaden the line width.

Short Reach

SR optical interfaces refer to optical sections having system loss budgets from 0 db to 7 db.  Depending on the SONET hierarchical level, SR transmitters may be either LEDs or low power MLM (multi-longitudinal Mode) lasers.


An overflow (deletion) or underflow (repetition) of one frame of a signal in a receiving buffer.


A network where transmission system payloads are synchronized to a master (network) clock and traced to a reference clock.

STM - Synchronous Transfer Module

A measure 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-64 are also defined.

STS-1 – Synchronous Transport Signal Level 1

The basic SONET building block signal transmitted at 51.84 Mbps data rate.

STS-N – Synchronous Transport Signal Level N

The signal obtained by multiplexing integer multiples (N) of STS-1 signals together.

TARP - TID Address Resolution Protocol

This is used on a NE-NE interface when there is a need to translate the TID of TL-1 messages to the CLNP address (NSAP, network service access point) of an NE.  The protocol would typically be used by a GNE in a TL-1/X.25 network that needs to map TIDS to NSAPs in a subtending network.

Transmission Delay

To control echo and to minimize the effect on digital throughput, the maximum (one way absolute delay for steady state operation of a 100 mile transport system with no intermediate terminals is 1ms. This applies for all interface options provided. The required maximum delay for shorter systems is to be decreased in direct proportion to the route mileage.

VT – Virtual Tributary

A signal designed for transport and switching of sub-STS-1 payloads.


Service requiring 1.5 – 50 Mbps transport capacity.