Last Updated November 28, 1998

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 I try to give enough explanation of each term so that it adds to the readers 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 format.  If you find an error or have items that you feel should be added to the list please Email:


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 transport 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 10^23-1 test pattern, it is being asynchronously mapped for transport. And they call this SONET?

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 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.

Availability - 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 minuters/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, 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.

BIP-8 vs BER
BER Max. Detection Time BIP-8 Violations
10e-3 10 ms 344xN
10e-4 100 ms 492xN
10-5 1s 510xN
10e-6 10s 512xN
10e-7 100s 512xN
10e-8 1000s 512xN
10e-9 10,000s 512xN

Where N is the N in OC-N

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. The above values take into account the error detection saturation effect at high BER.

Design objectives for the for the average detection time depend on the level N of the Optical Carrier and are typically an order of magnitude lower than the Max. detection times shown in the above table for OC-3. As the optical rate increases the average detection time decreases proportionally.

BITS - Building Integrated Timing Supply. A single building master timing supply. BITS generally supplies DS1 and DS0 level timing throughout an office. The BITS concept minimizes the number of synchronization links entering an office, since only the BITS will receive timing from outside the office.

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 narrowband process that effects 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.

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 opticc 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-Nc - An STS 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.

CMI - Coded Mark Inversion. This is the STS-3 line code. This 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.

Dispersion - 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.

DS4NA - 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 a 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 errored 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 downstream line. 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:

  1. A GNE that interworks an X.25 DCN (Data Comm. Network) and the SONET DCC.
  2. A GNE that interworks an X.25 DCN and an intra-site LAN.
  3. 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 which 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.


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.


Jitter - 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 jitter 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 errored seconds cumulative, where these errored 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 errored 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.


Kerr-effect - 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 nonlinearity 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 - 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 - 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.


NRZ - The optical line coding used in SONET systems. A one or zero are designated by a 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.



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.


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.

Stratum Level - Clocks used to time signals in the digital hierarchy are divided into levels.  Stratum 1 is the highest level of accuracy and stability.  

Stratum Levels
Stratum Level Free Run Accuracy Holdover Stability


+/- 1x10^-11



+/- 1.6x10^-8

+/- 1x10^-10 per day


+/- 4.6x10^-6

+/- 1x10^-8 per day

for the first 24 hrs.


+/- 4.6x10^-6

+/- 3.7x10^-7 per day

for the first 24 hrs.

SONET minimum clock

+/- 20x10^-6

Under Study


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.







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