Abbreviations

Term

Definition

Description

3G

Third Generation Mobile

Also known as UMTS

3GPP

Third Generation Partnership Project

The 3rd Generation Partnership Project (3GPP) is a collaboration between a number of international standards bodies (ARIB, CWTS, ETSI, T1, TTA, and TTC).

The 3GPP produce globally applicable Technical Specifications and Technical Reports for a 3rd Generation Mobile System and the maintenance and development of the Global System for Mobile communication (GSM) Technical Specifications and Technical Reports including evolved radio access technologies (e.g. General Packet Radio Service (GPRS)).

3R

Reamplification, Reshaping and Retiming

Technique employed in regenerators, where the signal is re-transmitted at a much higher signal strength, the signals shape is restored and it is retimed from a recovered clock from the received signal.

AAL

ATM Adaptation Layer

A layer in ATM used to adapt client services into the ATM payload.  Different AAL types are used for different client services.

ACR

Adaptive Clock Recovery

Method of transporting a client clock through an ATM network without the need to provide a local PRC traceable frequency reference.

Where ATM cells traverse a number of different ATM networks, it may not be possible to provide a common network clock to the transmitting and receiving end-stations. Also, some services may not require the purity of clock available from network synchronous or SRTS schemes. In these instances it may only be possible or more cost effective to use an adaptive clock recovery technique.

The adaptive clock method is actually a collective designation for a large number of end-to-end clock recovery methods using particular adaptive algorithms. Since all adaptive methods are based on the fact that the mean rate of cell arrivals is a measure of the source clock frequency, they are sensitive to the cell delay variation (CDV) induced in the network.

Playout buffer wander will render the MTIE of the output clock unusable for wander sensitive applications, e.g. Transporting PRC quality clock or synchronising GSM base stations.

ADM

Add Drop Multiplexer

Plesiochronousand lower bit rate synchronous signals can be extracted from or inserted into high speed SDH bit streams by means of ADMs. This feature makes it possible to set up ring structures, which have the advantage that automatic back-up path switching is possible using elements in the ring in the event of a fault.

ADEV

Allan Deviation

Frequency offset and drift
Any constant frequency offset of a timing signal, relative to the reference clock, has no influence on ADEV(t).
For observation intervals t where a linear frequency drift dominates, the ADEV(t) behaves as t.

Pros and cons
The behaviour of ADEV(t) is substantially independent of sampling period t0.
ADEV gives more information on the clock noise than MTIE, but it is not suited for buffer characterisation.
ADEV is sensitive to systematic effects, which might mask noise components; Adequate filtering must be done on the measured signal before processing ADEV calculation. Diurnal wander is an example of systematic effect.
ADEV results coming out of network measurement could be heavily influenced by systematic effects.

ADSL

Asymmetric Digital Subscriber Line

A solution for providing high speed data to subscribers over their existing copper (or aluminum!) network feed.  The signal shares the copper with existing voice services using a higher frequency spectrum.  It is asymmetrical as the data rate to the network is lower than the rate sent from the network.

AIS

Alarm Indication Signal

A signal used in transmission networks it is used by a function detecting a fault condition to indicate the fault to downstream equipment.  It can be used to enable alarm reporting only at the first location a fault is detected and to suppress consequential alarms.

ANSI

American National Standards Institute

American standards body which defines the characteristics and guidelines for telecommunications networks in the USA and countries adopting the T1 (1.544 Mbit/s) primary rate.

ATM

Asynchronous Transfer Mode

Packet orientated transmission technology, theoretically ATM does not need precise synchronisation to move packets through the network. However ATM is required to interface to STM-based applications. When transporting a CBR stream over an ATM network and when interworking with PSTN networks, the ATM network must provide correct timing at the service interfaces. This means that there are requirements for synchronisation transport functionality in ATM networks, especially on the boundary of the ATM networks, and these are dependent on the services carried over the ATM network.

Modes of synchronisation in ATM networks include-
Network Synchronous
- Synchronous Residual Time Stamp
- Adaptive Clock Recovery

An ATM network is not suitable for transporting PRC quality synchronisation. Generally all ATM switches will need a PRS such as GPS receiver co-located for correct working or a non-ATM transport layer for PRC traceable sync. ACR is the only method not requiring a local PRC quality synchronisation element. However ETSI TR 101 685 states that this method is not suitable for transporting synchronisation for wander sensitive application because of the playout buffer wander.

AU-n

Administrative Unit, level n

A logical entity in SDH which consists of a high order virtual container (e.g. VC-4) and its associated pointer.

AN

Access Network

Traditionally the part of the network between the customer and the service providers local exchange.  For some services the access network may extend further into the service providers network and exten beyond the local exchange.

ASP

Application Service Provider

Service provider who provides more than just connectivity.

AVAR

Allan Variance

The square of Allan deviation

B-ISDN

Broadband ISDN

ITU concept where ATM is used to provide a universal broadband network.  Has been overtaken by the internet revolution. 

BITS

Building Integrated Timing Source

American equivalent of SSU/SASE. Generally the outputs will be T1/1.5 Mbit/s. Connectors will be wire wrap rear facing.

BSC

Base Station Controller

Element of a GSM network which controls all the Base Stations. If co-located with the MSC, it should be provided with PRC quality synchronisation from the SSU/SASE.

BTS

Base Transceiver Station

GSM base station. Usually derives its RF stability of 5 x 10-8 from the incoming E1. If this has been passed over an SDH network, there will be problems if Pointer activity is experienced. Some base station designs do not react well when subjected to pointers. The solution is to use a Retimer function.

C/A Code

Coarse Acquisition Code

One of the two GPS transmission codes. Sometimes referred to as Common Availability Code. All non-military users of GPS work with C/A code.

CBR

Constant Bit Rate

CBR services include voice and video over SDH transmission systems. This can also refer to SDH clients (i.e. The line rate of the STM-N) over Optical Transport Networks as defined below.

CBR2G5 - A constant bit rate signal of 2,488,320 kbit/s ± 20 ppm. E.g. STM-16 signal.

CBR10G - A constant bit rate signal of 9,953,280 kbit/s ± 20 ppm. E.g. STM-64 signal.

CBR40G - A constant bit rate signal of 39,813,120 kbit/s ± 20 ppm. E.g. STM-256 signal.

CCITT

International Consultative Committee on Telephone and Telegraphy

Old name for ITU-T.  Originally in french, hence the acronym and its expansion doesn't align.

CDMA

Code Division Multiple Access

Technique to multiplex onto common bandwidth used in spread spectrum communication.  The data is transmitted using a unique code and can be detected by a receiver which uses the same code.

CDV

Cell Delay Variation

As cells are transported through an ATM network, they may be delayed by congestion, or other phenomena. This variable delay is known as Cell Delay Variation.

CLEC

Competitive Local Exchange Carrier

Term originating in the USA for new carriers who set up in a local regional area with a switch and SDH access rings.

CPE

Customer Premises Equipment

Equipment installed by carriers in their customer's office to terminate the service. E.g. This could be an STM-1 or STM-4 multiplexer.

CPN

Customer Premises Network

From a public network perspective it is the network on the customers premises for example a LAN normally owned and run by the customer.

CRC

Cyclic Redundancy Check

A code that is added to data to enable error detection and correction.

Cs

Caesium

Primary reference source with a guaranteed accuracy of parts in 10-12 , using the resonance of Caesium atom at 9.192631770 GHz

CWDM

Coarse Wave Division Multiplexing

Lower cost version of DWDM, less susceptible to the problems associated with dispersion because less wavelengths are multiplexed down the same fibre.

DWDM

Dense Wave Division Multiplexing

Process of multiplexing optical channels of different wavelengths onto a fibre.

DNU

Do Not Use

See QL-DNU

DSL

Digital Subscriber Line

A local loop access technology that provides high-speed connections over copper wire to deliver data, voice and video information over a dedicated digital network. Variations of DSL services are also known as ADSL, HDSL, SDSL, VDSL and ISDN.

DXC

Digital Cross-Connects

This network element allows mapping of PDH tributary signals into virtual containers as well as switching of various containers up to and including VC-4.

E1

Traffic Interface/Span Line

E1 is the name given to a 2048 kbit/s traffic interface or span line. Timing should not be taken from an E1 which has been passed through an SDH network unless it has been retimed using a Retimer Function.

ETSI

European Telecommunications Standards Institute

European standards body which defines the characteristics and guidelines for telecommunications networks.

FDD

Frequency Division Duplex

This is a scheme to ensure data traveling from A to B does not interfere with data traveling from B to A down the same media.  Different frequency ranges are used for the two directions ensuring there is no interference.

FEC

Forward Error Correction

Forward Error Correction in an OTN for the OTUk uses 16 byte interleaved codecs using a Reed-Soloman RS(255,239) code. FEC coding is also found in satellite communications modems using a Viterbi FEC algorithm with constraint length 7.

FFM

Flicker Frequency Modulation

Phase Noise on a clock signal having a characteristic spectral distribution proportional to f ^-1

FPM

Flicker Phase Modulation

Phase Noise on a clock signal having a characteristic spectral distribution proportional to f

FTTB

Fibre To The Building

Access technology where the fibre is connected all the way from the operators network to the customer building.  In a large multi tenant building fibre still might not reach the end user (see FTTO)

FTTCab

Fibre To The Cabinet

Access technology where the fibre is connected from the operators network to the street cabinet near the customer.

FTTC

Fibre To The Curb

Access technology where the fibre is connected from the operators network to the Curb (kerb) near the customer.

FTTH

Fibre To The Home

Access technology where the fibre is connected all the way from the operators network to the home.  This is nirvana for optical transmission networks but economically hard to justify.

FTTO

Fibre To The Office

Access technology where the fibre is connected all the way from the operators network to the customers office.

GPRS

General Packet Radio Service

So called 2.5G digital cellular

GPS

Global Positioning System

US government owned system using a number of Satellites orbiting the earth, these satellites are primarily intended to give positioning information for navigation but can also be used to derive a highly accurate timing source of PRC Quality. To use GPS, an antenna and a post processing unit are required which can output signals meeting the G.811 and ETS 300 462-6-1 standards.

GPSDO

GPS Disciplined Oscillator

A piece of equipment, usually 19" rack mount with an OCXO or Rubidium reference which is corrected for ageing by the GPS signal.

GSM

Global System for Mobile communications

2^nd generation of mobile telephony developed in Europe and standardized by ETSI.   The first generation had been analogue this is a digital system.

HTTP

Hyper Text Transfer Protocol

Message format to enable web pages to be sent over the internet and displayed on a variety of end systems.

ILEC

Incumbent Local Exchange Carrier

Traditional carrier with copper links to home telephony users. (mostly a US term)

I/O

Input/Output Interface

An interface to a device for transfer of data in and out of the device.

IP

Internet Protocol

Network layer protocol used in IP networks such as the Internet. IP provides features for addressing, type of service specification, fragmentation and reassembly, and security.

IPTV is the delivery of programming by video stream encoded as a series of IP packets. IPTV can be free or fee-based and can deliver either live TV or stored video. It can be bundled with other Internet Protocol services, including VoIP and high-speed Internet access. IPTV primarily uses multicasting with Internet Group Management Protocol (IGMP) version 2 for live television broadcasts and Real Time Streaming Protocol for on-demand programs

ISDN

Integrated Services Digital Network

A digital subscriber line network with circuit and packet switching capabilities for voice and data communications.

ISP

Internet Service Provider

A company who provide connection to the internet.  They Usually provide email services and web page space etc..

ITU-T

International Telecommunications Union
(Telecommunication Standardisation Sector)

International organisation which defines the characteristics and guidelines for telecommunications networks.

Iu

Interconnection point between an RNC and a Core Network. It is also considered as a reference point.

Iub

Interface between an RNC and a Node B.

Iur

A logical interface between two RNC. Whilst logically representing a point to point link between RNC, the physical realisation may not be a point to point link.

IWF

Interworking Function

The input stage of an ATM element.

JPEG

Joint Photographic Experts Group

International standards organization group that specify encoding for still images.  Also used to describe the data encoded using the standards developed by this group.

k

Index mark in Optical Networks

The index "k" is used to represent a supported bit rate and the different versions of OPUk, ODUk and OTUk. These are: -
- k=1 for an approximate bit rate of 2.5 Gbit/s
- k=2 for an approximate bit rate of 10 Gbit/s
- k=3 for an approximate bit rate of 40 Gbit/s

L1

Layer 1

Physical layer - UMTS Network

L2

Layer 2

Data Link layer - UMTS Network

L3

Layer 3

Network Layer - UMTS Network

LE

Line East

The STM-N line in a SDH network diagram going to the east or right.

3G UTRA-UTRAN Long Term Evolution (LTE) and 3GPP System Architecture Evolution (SAE).
Radio Side (LTE – Long Term Evolution)
Improvements in spectral efficiency, user throughput, latency
Simplification of the radio network
Efficient support of packet based services: MBMS, IMS, etc.

LW

Line West

The STM-N line in a SDH network diagram going to the west or left.

Mcps

Mega-chips per second

A measure of the number of bits (chips) per second in the spreading sequence of a direct sequence spreading code

MDEV

Modified Allan Deviation

Frequency offset and drift
Any constant frequency offset of a timing signal, relative to the reference clock, has no influence on MDEV(t).
For observation intervals t where a linear frequency drift dominates, the MDEV(t) behaves ast.

Pros and cons
For observation intervals where the WPM noise dominates, the behaviour of MDEV(t) significantly depends on sampling period t0.
MDEV gives more information on the clock noise than MTIE, but it is not suited for buffer characterisation. MDEV is sensitive to systematic effects which might mask noise components; Adequate filtering must be done on the measured signal before processing MDEV calculation. Diurnal wander is an example of systematic effect.
MDEV result coming out of network measurement could be heavily influenced by systematic effects.

MMF

Multi-Mode Fibre

A fibre type used for transport of signals in the LAN, it supports multiple modes and therefore exhibits modal dispersion making it unsuitable for long distance applications.  It has a wider core diameter than single mode fibre making it cheaper and easier to interconnect.

MML

Man-Machine Language

Language used to communicate between a person and a computer.

MPEG

Moving Pictures Experts Group

A working group of ISO/IEC in charge of the development of standards for coded representation of digital audio and video.  Also used to describe the data encoded using the standards developed by this group.

MRTIE

Maximum Relative Time Interval Error

MRTIE is defined as "The maximum relative peak-to-peak delay variation of a given input timing signalin an observation time (t=nt₀) for all observation times of that length within the measurement period (T)." e.g. Time error measured between output and input of a slave clock.

MS-AIS

Multiplex Section Alarm Indication Signal

An all ’1’s signal inserted in the Multiplex section overhead and payload by an SDH Regenerator when its input has failed. MS-AIS is used to indicate a failure upstream and suppress consequential alarms downstream.

MSC

Mobile Switching Centre

Main switch in a GSM network. Best practice recommendation is that it should always have an SSU/SASE co-located to supply it with PRC quality synchronisation traceable to a network PRC of G.811 or EN 300 462-6-1 quality.

MSOH

Multiplex Section Overhead

The multiplex section overhead in SDH networks is used for the needs of the multiplex section. The multiplex section covers the part of the SDH link between multiplexers. The carriers (VC, virtual containers) are available as payload at the two ends of this section.

MTIE

Maximum Time Interval Error

One of the key quality metrics for telecom Synchronisation. MTIE is defined as "The maximum peak-to-peak delay variation of a given timing signal with respect to an ideal timing signal within an observation time (t=nt0) for all observation times of that length within the measurement period (T)".

Frequency offset and drift
For observation intervals t where a constant frequency offset dominates, the MTIE(t) behaves as t.
For observation intervals t where a linear frequency drift dominates, the MTIE(t) is not theoretically bounded. From the measurement viewpoint this circumstance is expected to cause increasing value of estimated MTIE(t) as the total observation time, (i.e. The length N of the xi data) is increased.

Pros and cons
The behaviour of MTIE(t) is substantially independent of sampling period t0. MTIE (and MRTIE) is well suited for characterisation of buffer size. MTIE is however adversely effected by phase transients.

MUX

Multiplexer

An equipmentcapable of mapping multiple signals onto a single output signal.  The Multiplexer can be operating any on of a number of multiplexing schemes e.g: TDM, WDM, CDMA or FDM .  An example of a TDM Multiplexer is a PDH or SDH terminal capable of mapping multiple low speed signals onto a higher bit rate signal.

NE

Network Element

An item of transmission or switching equipment such as a switch or cross connect, multiplexer etc.

NNI

Network - Network Interface

The boundary between two independent carriers.

Node-B

UMTS base station, equivalent to a BTS in a GSM network.

A logical node responsible for radio transmission/reception in one or more cells to/from the User Equipment. Terminates the Iub interface towards the RNC.

OAN

Optical Access Network

An optical network between the customer and operators exchange or POP

OCh

Optical Channel

The OCh is the information structure used to support the OCh trail. OCh client signals are the OTUk signals

OC-N

Optical Container - level N

SONET Optical Container bit rates
- OC-3 155.52 Mbit/s. Equivalent to STM-1
- OC-12 622.08 Mbit/s. Equivalent to STM-4
- OC-48 2.488,320 Gbit/s. Equivalent to STM-16
- OC-192 9.953,280 "10" Gbit/s. Equivalent to STM-64
- OC-768 39.813,120 "40" Gbit/s. Equivalent to STM-256

ODC

ODUk Clock

Four Optical Data Unit Clocks are currently defined in the latest draft version of G.otnjit.

These are: -
- ODCa - for asynchronous mapping of clients into the ODUk
- ODCb - for bit-synchronous mapping of clients into the ODUk
- ODCr - for 3R regeneration
- ODCp - for de-mapping of constant bit rate clients

ODUk

Optical Channel Data Unit-k

The ODUk is an information structure consisting of the information payload (OPUk) and ODUk related overhead. (See above for a definition of "k")

OLT

Optical Line Termination

A network element that is part of the optical access Network for example a Passive Optical Network (PON).  The OLT is the head end connecting the access network back into the operators Network

ONU

Optical Network Unit

A network element that is part of the optical access Network for example a Passive Optical Network (PON).  The OLT is the customer end connecting customers to the access network

OPUk

Optical Channel Payload Unit-k

The OPUk is the information structure used to adapt client (e.g. STM-N, ATM, IP, Ethernet) information for transport over an optical channel. It comprises client information together with any overhead needed to perform rate adaptation between the client signal rate and other OPUk overhead supporting the client structure. This overhead is adaptation specific. (See above for a definition of "k")

OCXO

Oven Controlled Crystal Oscillator

Quartz oscillator where the resonator is encapsulated in a thermal chamber to keep the temperature constant and thus make the frequency more stable. OCXO's are more stable than TCXO's or VCXO's and are suitable for Local and Transit SSU designs.

OTN

Optical Transport Network

A WDM network meeting the ITU specifications defined in G.709.

OTUk

Optical Channel Transport Unit-k

The OTUk is the information structure used for the transport of an ODUk over one or more optical channel connections. It consists of the optical channel data unit and OTUk related overhead (FEC and overhead for management of an optical channel connection - sometimes referred to as the "Digital Wrapper"). It is characterised by its frame structure, bit rate and bandwidth. (See above for a definition of "k")

PANS

Pretty Amazing New Stuff

SDH, ATM, Optical networks, IP, etc.

Consider this if you want some real "PANS". If STM-1 were one lane on a highway, the M4 outside HeathrowAirportwould be an STM-4 i.e. 622 Mbit/s.

So now lets convert to 40 Gbit/s with DWDM. That means 256 lanes in each direction. Now that's big! But before you go Wow!, just put the red cars on this highway and stack 80 more above for all the different coloured cars. Don't stop there, you will have to upgrade the M25, and then you will have to rebuild all the intersections...........

Hmmm. I wonder if Alexander Graham Bell would have ever thought that his hearing aid experiment would lead to this?

Nortel has proposed PBT as a metro-ethernet technology with the support of some of the world's largest service providers and standards bodies. PBT removes some of the auto-learning and discovery processes from standard Ethernet to make it more like current carrier networks, more connection oriented.

PCM

Pulse Code Modulation

A scheme for transporting a sampled analogue signal as a series of digital pulses.

PCMCIA

People Can't Memorise Communications Industry Acronyms

You don't need to now, just bookmark these pages into your favourites and don't forget to email us with requests if you think we should add some more.

P-Code

Precise Code

The most accurate form of encoding used by the GPS satellites. Only available to military users.

PJE

Pointer Justification Event

A VC-12 pointer creates a 3.57µs phase hit and a VC-4 pointer creates a 160 ns phase hit at 2.048 Mbit/s This is the reason that sync must not be transported across the payload of a STM-N transmission layer.

PDH

Plesiochronous Digital Hierarchy

A digital multiplexing scheme superceded by SDH.

pk-pk

Peak-to-peak

A measurement term used to measure the maximum between two extremes.  Used in jitter and MTIE measurements.

PLL

Phase Locked Loop

A mechanism for locking a voltage controlled oscillator to a frequency reference of higher stability.

PLMN

Public Land Mobile Network

A PLMN operator in the entity which offers a GPRS.

POTS

Plain Old Telephone Systems

Good old legacy stuff, PDH, black Bakelite telephones with rotary diallers, analogue, Strowger, wiped lead joints - for a bonus prize, with what did they wipe the joint?

ppm

Parts per million

Parts in 10^-6.

PPP

Point-to-Point Protocol

Protocol used to transport IP over a dedicated connection.

PRBS

Pseudo-Random Binary Sequence

A test signal used to simulate random data but to enable errors to be detected.

PRC

Primary Reference Clock

A reference frequency standard comprising an ensemble of PRS's that provides a reference frequency signal compliant with ITU G.811 or ETSI EN 300 462-6-1. The free run accuracy of PRC must be 1 x 10-11.

PRS

Primary Reference Source

The free run accuracy of PRS must be 1 x 10-11 according to G.811 or EN 300 462-6-1.

Caesium and Telecom quality GPS receivers are examples of PRS's suitable for use in telecom networks.

PSTN

Public Switched Telephone Network

The basic telephone network providing a dial up connected voice service.

QL

Quality Level

QL is a configurable parameter used in the squelching of clock output signals from SDH Multiplexers. E.g. If the Quality Level (QL) of the signal used to derive the output falls below QL_Minimum then the output will be squelched (cutoff or set to Alarm Indication Signal (AIS)).

QL-PRC

Quality Level - PRC

One of the SSM codes. In a fully synchronized network all sources should be ultimately traceable to a PRC and this should be indicated using a '0010' code.

QL-SSU T

Quality Level - SSU Transit

One of the SSM codes. This indicates that the clock source is traceable to a transit node SSU and should be indicated using a '0100' code.

QL-SSU L

Quality Level - SSU Local

One of the SSM codes. This indicates that the clock source is traceable to a local node SSU and should be indicated using a '1000' code. QL-SEC Quality Level - SEC One of the SSM codes. This indicates that the clock source is traceable to a SEC and should be indicated using a '1011' code.

QL-DNU

Quality Level - Do Not Use

One of the SSM codes. "Do Not Use for Synchronisation" code is used to prevent timing loops and is transmitted in the opposite direction on interfaces used to synchronise an equipment's clock. Denoted by '1111'.

QoS

Quality of Service

The performance of a telecoms service specified in observable performance parameters for example Availability performance, error performance and Jitter and Wander Performance.

QPSK

Quadrature (Quaternary) Phase Shift Keying

A modulation scheme for the transport of digital signals where the phase of the carrier is modulated (shifted) to one of four different phases to carry the digital information.

Rb

Rubidium

Like Caesium, an atomic frequency standard, but not as accurate as Caesium

rms

Root Mean Square

A mathematical averaging used to enable the equivalent power of a signal to be considered.  For example the root mean square Voltage through a resistor is calculated so that its power dissipation = (Vrms)²/R.  rms is also used in the calculation of TDEV to give an indication of the spectral power density of a noise source at a particular observation internal.

RNC

Radio Network Controller

UMTS equivalent of the BSC in a GSM network. One RNC controls a number of Node B's.

This equipment in the RNS is in charge of controlling the use and the integrity of the radio resources in a UMTS network.

RNS

Radio Network Subsystem

Either a full network or only the access part of a UTRAN offering the allocation and the release of specific radio resources to establish means of connection in between an UE and the UTRAN. A Radio Network Subsystem is responsible for the resources and transmission/reception in a set of cells.

RS422

Specification for electrical parameters of an interface only. RS422 is compatible with V11 and X21.

RSOH

Regenerator Section Overhead

SDH networks are subdivided into various layers that are directly related to the network topology. The lowest layer is the physical layer, which represents to the transmission medium. This is usually a glass fiber or possibly a radio-link or satellite link. The regenerator section is the path between regenerators. Part of the overhead (RSOH, regenerator section overhead) is available for the signalling required within this layer.

RTG

Retiming Generator

RTG's are very simple timing sources. As opposed to network elements equipped with a SETS supplying timing to all functions, the RTG consists of a separate timing source for each signal direction of the bi-directional signal, so that each signal keeps its timing. Under normal operation an RTG receives a reference signal extracted from the incoming STM-N signal and distributes a timing signal to the signal processing functions and the output interface. If one of the incoming STM-N signals is lost, the RTG provides timing for the generation of an AIS (Alarm Indication Signal) downstream.

Regenerators operate in through timing mode, in which the timing from the outgoing line west signal is derived from the incoming line east signal and vice versa.

The RTG should not be confused with the Retimer function described elsewhere.

RWFM

Random Walk Frequency Modulation

Phase Noise on a clock signal having a characteristic spectral distribution proportional to f ^-2

SA

Selective Availability

A deliberate degrading of the GPS signal. This was switched off by the US government in May 2000

3G UTRA-UTRAN Long Term Evolution (LTE) and 3GPP System Architecture Evolution (SAE).
Network Side (SAE – System Architecture Evolution)
Improvement in latency, capacity, throughput
Simplification of the core network
Optimization for IP traffic and services
Simplified support and handover to non-3GPP access technologies

SASE

Stand Alone Synchronisation Equipment

One of the most important elements within a Synchronisation network, second only to PRC. The stand-alone implementation of the logical SSU function, which incorporates its own management function.

This is a piece of Synchronisation equipment that contains an SSU. This term is used to differentiate from the SSU clock function itself which can be located within another piece of equipment for instance an SDH digital cross connect or switch.

SASE implementations of the SSU may have the following additional features:
- distribution of 2 MHz and / or 2 Mbit/s Synchronisation reference output signals via a scalable number of G.703-9 or G.703-13 interfaces;
- continuous monitoring of the input signals for frequency offset and excessive jitter and wander
- provisioning of dedicated monitoring interfaces for continuous sync network performance measurement and
- surveillance;
- provisioning of processed phase measurement data: MRTIE and TDEV;
- 2 Mbit/s retiming function.

See SSU-T for more information.

SDH

Synchronous Digital Hierarchy

An ETSI and ITU time division multiplexing standard for the flexible transport of services in fixed bandwidth virtual containers.  In Europe it replaced the previous TDM multiplexing standard PDH and was first deployed in the early 1990’s.  Its advantages over PDH include its inbuilt management capabilities and its ability to easily add and drop traffic without fully demultiplexing.

SEC

SDH Equipment Clock

The logical function representing the equipment clock of a SDH network element having the timing characteristics given in ITU G.813 or ETSI EN 300 462-5-1

These specifications provide for:-
- moderate frequency accuracy in holdover mode in order to allow the use of relatively low cost oscillator
- tight intrinsic phase noise generation in order to allow cascading of many SEC's in the Synchronisation distribution network without suffering from excessive phase noise accumulation
- filtering bandwidth optimised for tight tracking of the input reference signals but with sufficient jitter filtering capability
- Synchronisation performance to be supported by the SSM protocol which allows for automatic reconfiguration of SEC sub-networks in order to limit the occasions for holdover mode operation after Synchronisation faults

The SEC has been defined as the performance feature set for the equipment timing generator of synchronous (especially SDH) telecom equipment (SETG). Its transfer performance is defined between the (SDH) traffic port input and the (SDH) traffic port output.

SETS

Synchronous Timing Source

The logical function representing all Synchronisation related functions to be considered in an SDH network element.

SETG

Synchronous Equipment Timing Generator

Contains the SEC within the SETS

SMC

SONET Minimum Clock

SONET equivalent of SEC

SMF

Single Mode Fibre

A fibre type used for transport of signals over short, medium and long distances. It supports a single mode and therefore exhibits no modal dispersion making it suitable for long distance applications.  It has a small core diameter making more expensive connectorize and splice than Multi Mode Fibre.

SMTP

Simple Mail Transfer Protocol

The main protocol used to exchange e-mail on the Internet.

SONET

Synchronous Optical Network

The American equivalent of SDH base transports the 1.5Mbit/s hierarchy.

SRTS

Synchronous Residual Time Stamp

A method of transporting a client's clock through an ATM network. This method requires accurate network synchronisation to be delivered to both ends of the ATM network by a non-ATM transport layer.

With SRTS the number of derived ATM network clock cycles (fn/X) in N cycles of the client Service clock is counted in a p-bit counter and this p-bit SRTS value transmitted across the network to the receiving network terminating point. At the receiving terminal the local service clock can be adjusted against the reference network clock to ensure that the same number of derived network clock cycles are counted over the same N cycles of local service clock, thus locking the local service clock to the transmitter service clock by use of the received residual time stamps and the reference network clock.

For 1,544 Mbit/s and 2,048 Mbit/s based CBR hierarchies the value of N = 3 008 and a p = 4-bit SRTS value has been agreed and specified in ITU-T Recommendation I.363.1. The use of 4 bits allows up to ±200 ppm offset from the nominal clock frequency.

SS7

Signalling System No. 7

The seventh standardized signaling system used in the Public Switched Voice Network (PSTN).

SSM

Synchronisation Status Message

A coding of the reference level of the timing source as specified in ITU G.707 and ETSI ETS 300 147 for STM-N, ITU G.704 for 2 048 kbit/s, ITU-T G.832 and ETSI EN 300 337 for synchronous 34 Mbit/s and 140 Mbit/s. A good overview can be found in ETSI EN 300 417-6-1

SSM is a signal that is passed over a Synchronisation interface to indicate the Quality-Level of the clock sourcing the Synchronisation signal. This signal was originally defined for use over STM-N interfaces in the S1 byte. It has since been proposed for use over 2 Mbit/s interfaces as well. The SSM code transmitted reflects the quality of the clock that the interface is ultimately traceable to; i.e. The grade-of-clock to which it is synchronised directly or indirectly via a chain of network element clock's (the Synchronisation trail), or how long this chain of clocks is. For example, the clock-source quality-level may be a Primary Reference Clock (PRC) complying with G.811 or EN 300 462-6-1, or it may be a Slave Clock in holdover-mode, complying with G.812 or EN 300 462-4-1, or a G.813 or EN 300 462-5-1 Clock in holdover or free-run.

The clock-source quality-level is essentially, therefore, an indication only of the long-term accuracy of the network element clock. See QL definitions above for further details.

NB Old SDH equipment may not be able to support SSM via their STM-N interfaces.

SSU

Synchronisation Supply Unit or Synchronisation Source Utility

One of the most important elements within a Synchronisation network, second only to PRC. A logical function for frequency reference selection, processing and distribution, having the frequency characteristics given in ITU G.812 and ETSI EN 300 462-4-1 and EN 300 462-7-1

These specifications aim at:-
- low bandwidth jitter filtering for removing efficiently jitter and short term wander from the Synchronisation reference signals
- high degree of frequency accuracy in the holdover mode for providing a local Synchronisation backup and to allow for delayed maintenance response on Synchronisation faults
- limited phase transient response on switching between input reference signals.

The SSU gives two key benefits: it filters out short term phase noise (jitter) and short term wander and provides a highly accurate clock if there is a failure of Synchronisation supply from the PRC. There are a number of different SSU implementations, these are usually differentiated on their frequency accuracy in holdover mode or PRC autonomy period. This is typically 2 x 10-10 per day for a quartz based SSU-T and 1 x 10-9 per day for the SSU-L. They vary from more expensive Rubidium based oscillators to lower cost quartz oscillators. There are also a number of higher quality quartz oscillators which use improved techniques to reduce the temperature and ageing effects of quartz.

Up to 10 SSU-T's may be deployed in SDH networks before a PRC needs to be fitted.

The physical implementation of the SSU function may either be integrated as the timing generator of a network element or it may be stand alone equipment, the so called SASE (See above).

An SSU is normally not connected directly to traffic signals: The network elements (NE's) terminating the transport signals connected to the node extract a Synchronisation reference signal out of suitable transport signals and deliver it via the T4 output to the node clock.

SSU-L

SSU Local version

The Local SSU is a high quality slave clock deployed as the last SSU in a sync trail in the Synchronisation network and is defined in G.812 or EN 300 462-7-1.

Since the SSU-L terminates a chain of SSU's its frequency accuracy in holdover mode is not as good as the SSU-T. The SSU-L will use quartz to achieve 1 x 10-9 per day. See SSU and SASE definitions.

SSU-T

SSU Transit version

The Transit SSU represents a set of performance specifications for a clock generator function which is defined in G.812 or EN 300 462-4-1. Typically the holdover specification is 2 x 10-10 per day. See SSU and SASE definitions.

STM-1

Synchronous Transport Module -1

A frame with a bit rate of 155.52 Mbit/s is defined in ITU-T recommendation G.707. This frame is called the synchronous transport module (STM). Since the frame is the first level of the synchronous digital hierarchy, it is known as STM-1. The format of this frame is made up from a byte matrix of 9 rows and 270 columns. Transmission is row by row, starting with the byte in the upper left corner and ending with the byte in the lower right corner. The frame repetition rate is 125 µs. Each byte in the payload represents a 64 kbit/s channel.

The STM-1 frame is capable of transporting any PDH tributary signal (< 140 Mbit/s).

STM-N

Synchronous Transport Module, level N

SDH Synchronous Transport Module bit rates:-
- STM-1 155.52 Mbit/s. Equivalent to OC-3
- STM-4 622.08 Mbit/s. Equivalent to OC-12
- STM-16 2.488,320 Gbit/s. Equivalent to OC-48
- STM-64 9.953,280 "10" Gbit/s. Equivalent to OC-192
- STM-256 39.813,120 "40" Gbit/s. Equivalent to OC-768

T1

Traffic Interface/Span Line

T1 is an abbreviated form of the US 1544 kbit/s primary rate signal.

T0

Terminology defined in EG 201 793 used to denote the port for internal NE Synchronisation distribution in a SETS within a SDH multiplexer.

T1

NB Beware alternative definition above!

Terminology defined in EG 201 793 used to denote the timing signal derived from an STM-N signal in a SETS within a SDH multiplexer.

T2

Terminology defined in EG 201 793 used to denote the timing signal derived from a 2 Mbit/s data signal in a SETS within a SDH multiplexer.

T3

Terminology defined in EG 201 793 used to denote the timing signal derived from a 2 MHz (2 Mbit/s) station clock input signal in a SETS within a SDH multiplexer.

T4

Terminology defined in EG 201 793 used to denote the 2 MHz (2 Mbit/s) station clock output signal in a SETS within a SDH multiplexer. The T4 signal as 2 Mbit/s G.703-9, EN 300 166 with SSM according to G.704 will allow a SDH element e.g. Multiplexer to interface to a SSU.

TCP/IP

Transmission Control Protocol/Internet Protocol

The basic protocols controlling applications on the Internet.

TCXO

Temperature Compensated Crystal Oscillator

A quartz crystal oscillator which relies on internal monitoring of the temperature to enable the quartz resonant frequency to be corrected. TCXO's are generally small, low cost and low power consumption devices.

TDD

Time Division Duplex

This is a scheme to ensure data traveling from A to B does not interfere with data traveling from B to A down the same media.  Different timeslots are used for the two directions ensuring that they do not use the media simultaneously.

TDEV

Time Deviation

One of the main quality metrics for telecom Synchronisation networks. TDEV is a measure of the expected time variation of a signal as a function of integration time. It can also provide information about the spectral content of the phase (or time) noise of a signal. TDEV is in units of time usually expressed in ns. It is calculated from a sequence of time error samples. It is recommended to have at least 12 times the number of samples as the longest TDEV observation period required.

Frequency offset and drift
Any constant frequency offset of a timing signal, relative to the reference clock, has no influence on TDEV(t).
For observation intervals t where a linear frequency drift dominates, the TDEV(t) behaves as t.

Pros and cons
For observation intervals where the WPM noise dominates, the behaviour of TDEV(t) significantly depends on sampling period t0.
TDEV gives more information on the clock noise than MTIE, but it is not suited for buffer characterisation.
TDEV is sensitive to systematic effects, which might mask noise components; Adequate filtering must be done on the measured signal before processing TDEV calculation. Diurnal wander is an example of systematic effect.
TDEV result coming out of network measurement could be heavily influenced by systematic effects.

A mobile TV and multimedia solution based on the 3GPP Release 6 Multimedia Broadcast and Multicast Services (MBMS) standard. TDtv operates in the universal unpaired 3G spectrum bands that are available across Europe and Asia at 1900MHz and 2010MHz, using all the available bandwidth for the downlink.

TETRA

Trans European Terrestrial Radio

Cellular radio system using bandwidth in the 400 MHz area of spectrum. Requirements for synchronisation are similar to GSM, i.e. a SSU/SASE at every major switch site.

TIE

Time Interval Error

The difference between the measure of a time interval as provided by a clock and the measure of that same time interval as provided by a reference clock.

TMN

Telecommunications Management Network

Network management as specified by the ITU and ETSI.

TS0

Time Slot 0 (Zero)

The first tie slot in a framed 2Mbit/s signal it is used to transport the frame word used for frame alignment.

TU-m

Tributary Unit, level m

A lower order container used to transport a virtual container.  It includes the payload overhead and its associated ponter.

TVAR

Time Variance

The square of the Time Deviation. A measure of the expected time variation of signal as a function of integration time.

UI

Unit Interval

Reciprocal of the bit rate e.g.
- 1 UI for a 2.048 Mbit/s signal is 488 ns
- 1/8 UI at 2.048 Mbit/s is 61 ns.

UIPP

Unit Interval peak-to-peak

A measure of the peak to peak variation of a signal.  It is measured in terms of the number of the bit periods Unit Interval (UI ) : 1 UI = 1 /bit rate

UMTS

Universal Mobile Telecommunications System

The telecommunications system, incorporating mobile cellular and other functionality, that is the subject of standards produced by 3GPP.

UTC

Universal Coordinated Time

The time scale, maintained by the Bureau International des Poids et Measures (BIPM) and the International Earth Rotation Service (IERS), which forms the basis of a coordinated dissemination of standard frequencies and time signal.
NB - The reference frequency for network Synchronisation is the frequency which generates the UTC time scale. It is recommended to use the words "UTC frequency" instead of "UTC".

UTRAN

Universal Terrestrial Radio Access Network

A conceptual term identifying that part of the network which consists of RNCs and Node Bs between Iu and Uu interfaces.

V11

See RS422

VBR

Variable Bit Rate

A signal that does not have a constant bit rate but its demand for bandwidth varies with time.  Used in ATM to categorize traffic types.

VC-n

Virtual Container, level-n

Two VC layers represent a part of the mapping process on SDH networks. Mapping is the procedure whereby the tributary signals, such as PDH and ATM signals are packed into the SDH transport modules. VC-4 mapping is used for 140 Mbit/s or ATM signals and VC-12 mapping is used for 2 Mbit/s signals.

VCO

Voltage Controlled Oscillator

Normally refers to the oscillator used in a PLL.

VCXO

Voltage Controlled Crystal Oscillator

A quartz crystal oscillator whose frequency can be controlled by applying a voltage. Usually a low grade oscillator. OCXO can be VCXO's

VC-TCXO

Voltage Controlled, Temperature Controlled Crystal Oscillator

A combination of a voltage controlled oscillator where the crystal oscillator is temperature controlled to provide improved holdover stability.

VoIP

Voice Over IP

A scheme where a voice service is packetized into IP packets and transported over an IP network.

VOD

Video On Demand

System for the reception of televised programs in digital form via telephone lines.

VPN

Virtual Private Network

A network implemented by a service provider but tailored to behave like private network. 

WAP

Wireless Application Protocol

Internet access over GSM

WCDMA

Wideband Code Division Multiple Access

A Multiplexing scheme used in 3G Mobile networks, unique codes are used to enable multiple users to share the spectrum.

WDM

Wave Division Multiplexing

A multiplexing scheme used in optical networks.  Different circuits use different wavelengths in order for them to share the same fibre.

WFM

White Frequency Modulation

Phase Noise on a clock signal having a flat spectral distribution

WPM

White Phase Modulation

Phase Noise on a clock signal having a characteristic spectral distribution proportional to f ²

X21

Interface electrically compatible with V11/RS422.

XDSL

The combined technologies used for the transmission of data via a pair of copper wires.