SONET GLOSSARY
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-1 |
STS-1 |
51.840 |
50.112 |
1.728 |
- |
|
OC-3 |
STS-3 |
155.520 |
150.336 |
5.184 |
STM-1 |
|
OC-9 |
STS-9 |
466.560 |
451.008 |
15.552 |
STM-3 |
|
OC-12 |
STS-12 |
622.080 |
601.344 |
20.736 |
STM-4 |
|
OC-18 |
STS-18 |
933.120 |
902.016 |
31.104 |
STM-6 |
|
OC-24 |
STS-24 |
1244.160 |
1202.688 |
41.472 |
STM-8 |
|
OC-36 |
STS-36 |
1866.240 |
1804.032 |
62.208 |
STM-13 |
|
OC-48 |
STS-48 |
2488.320 |
2405.376 |
82.944 |
STM-16 |
|
OC-96 |
STS-96 |
4976.640 |
4810.752 |
165.888 |
STM-32 |
|
OC-192 |
STS-192 |
9953.280 |
9621.504 |
331.776 |
STM-64 |
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 |
Stability |
Time to first
frame slip |
|
1 |
1 x 10-11 |
N/A |
N/A |
72 days |
|
2 |
1.6 x 10-8 |
±
1.6 x 10-6 |
1 x 10–10
/ day |
7 days |
|
3E |
1.0 x 10-6 |
±
4.6 x 10-6 |
1 x 10-8
/ day |
3.5 hours |
|
3 |
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 |
N/A |
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.
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 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.
Broadband
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.
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 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.
Interleave
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.
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 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.
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 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.
Narrowband
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.
NRZ
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.
Orderwire
A channel used by installers to expedite the provisioning of
lines.
Overhead
Extra bits in a digital stream used to carry information
besides traffic signals. Orderwire, for
example, would be considered overhead information.
Payload
The portion of the SONET signal available to carry service
signals such as DS-1, DS-2, and DS-3.
Plesiochronous
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.
Slip
An overflow (deletion) or underflow (repetition) of one
frame of a signal in a receiving buffer.
Synchronous
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.
Wideband
Service requiring 1.5 – 50 Mbps transport capacity.