Time Synchronization in Telecom Networks

PBX

Private Branch Exchange: A telephone switch located on a customer's premises that primarily establishes voice-grade circuits over E1 links to PSTN (Public Switched Telephone Network).

PDH

The Plesiochronous Digital Hierarchy is a digital communication technology. PDH is based on the fundamental concepts of Time Division Multiplexing (TDM).

Plesiochronous: (Greek - plesio, meaning near and chronos, meaning time) The meaning of Plesiochronous therefore is "nearly synchronous".

Plesiochronous Digital Hierarchy is used to multiplex PCM30 transmissions to data streams with higher data rates and for transmission over digital transport equipment such as fiber optic and microwave radio systems. Although this transmission clock accuracy is not highly exact, the PCM30 data flow is transferred from synchronized switching centers, regarding data and clock transparency.

Digital switching centers transfers synchronous PCM30 - data flow with this transmission technique to other switching centers, and the recovered clock can be used for its own synchronization.

Much of the transport infrastructure based on plesiochronous digital hierarchy (PDH) is being replaced with SONET or SDH based infrastructure.

PRC

Primary Reference Clock: Usually a caesium standard that provides frequencies compliant with ITU G.811 or ETSI EN 300 462-6-1.

The accuracy of PRC must be > 1 x 10-11.

SLIP

Slip = PCM30 - Frame Slip - always arises in a PCM receiver. The data are stored error-free in a buffer, which can take up 2 PCM Frames. The buffer is read with the clock of the switching center.

If the receiving switching center has a lower frequency than the transmitting switching center, the buffer cannot read the data in the same rate as these is written by the transmitter. A buffer overflow occurs with the result of a frame slip.

If the receiving switching center has a higher frequency than the transmitting switching center, the buffer is faster read out, than it is filled. A buffer underflow occurs followed by a slip frame.

The slip rate is proportional to the frequency deviation between transmitter and receiver.

T3

Name of a clock input for clock generators (usually PLL) of a network element like e.g. switching center, BSC, BTS and SDH.

These inputs can be synchronized with 2,048 MHz or 2,048 MBit/s. The interfaces are either symmetrical or asymmetrical.

T4

Designation of the clock output of a clock generator in a network element e.g. switching center, BSC, BTS and SDH.

These output can deliver 2,048 MHz or 2,048 MBit/s. The interfaces are either symmetrical or asymmetrical.

E1

E1 transfers 32 time slots for each PCM frame - the first time slot of the first frame is the frame alignment signal (FAS). This signal makes the recognition of the frame in the PCM receiver, so that thereafter the time slots with speech data or data can be assigned to their goals correctly.

The first time slot of the second PCM frame is the non frame alignment signal (NFAS) to transfer alarms and checksums (CRC4).

Trau

Transcoder Rate Adaption Unit.
The TRAU belonged functionally to the mobile switching center. It has the GSM specific language coding and - decoding (from 64 kBit/s to 16 kBit/s per voice channel in both direcctions).

It is synchronized over the PCM30-Transmission from MSC and transmits the received clock accuracy over PCM30 transmissions to the base station control (BSC).

The TRAU is located at the BSC or directly at the MSC (mobile Switching center).

Sonet/SDH Definitions and Documentations

SDH (Synchronous digitally Hierarchy) is a standard technology for synchronous data communication on optical media (in the USA usually known as SONET).

This technology is characterised by higher data rates and makes the transmission of the existing data formats from the earlier PDH technology possible. Contrary to the PDH technology SDH network elements are synchronized.

The payload data is stored in "containers" with additional information - the "Overhead" and the "Payload" is transmitted as synchronous transport module (e.g. STM-1). This transmission is normally accomplished over optical media.

STM-1 designates a data rate of 155 MBit/s - e.g. 63 PCM30 transmissions can be transported. These 155 MBit/s data stream transports also the clock accuracy to the next SDH network element and synchronizes its clock generator. Thus a continuous synchronization of the SDH network is possible.

The clock outputs (T4) of the SDH network elements can be used for the synchronization of further SDH network components or other connected mobile equipment.

SDH uses the following Synchronous Transport Modules (STM) and rates:

STM-1 (155 MBit/s)
STM-4 (622 MBit/s)
STM-16 (2.5 Gbit/s)
STM-64 (10 Gbit/s)

SONET / SDH Technical Specs

SONET and SDH are related standards for synchronous data transmission via fiber optic networks. SONET is an acronym for Synchronous Optical NETwork, SDH is short for Synchronous Digital Hierarchy. SONET is the US version of the American National Standards Institutue (ANSI) standard. SDH is the international version of the International Telecommunications Union (ITU) standard.

SONET / SDH Digital Hierarchy

This table shows the SONET/SDH data rate 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-12	STS-12	622.080	601.344	20.736	STM-4
OC-48	STS-48	2488.320	2405.376	82.944	STM-16
OC-192	STS-192	9953.280	9621.504	331.776	STM-64
OC-768	STS-768	39813.120	38486.016	1327.104	STM-256

The "line rate" describes the raw bit rate transferred via the optical fiber. A certain amount of the bit rate are the overhead, that includes information that provides OAM&P: Operations, Administration, Maintenance, and Provisioning information like framing, trace, multiplexing, status, and performance monitoring. The payload rate which is the available bandwidth for user data as packets or ATM cells is calculated by the line rate minus the overhead rate.

Other rates like OC-9, OC-18, OC-24, OC-36, OC-96 were never widely implemented. Maybe higher rates like OC-3072 will defined in future times.

The SONET/SDH level designations including a "c" suffix (e.g. "OC-48c") indicate a "concatenated" or "clear" channel. This shows, that the entire payload rate is available as a single channel of communications. In these channels the entire payload rate can be used by a single flow of packets. Opposed to the concatenated or clear channel is the "channelized". In a channelized link the payload rate is divided into several fixed rate channels. E.g. the payload of an OC-48 link can be subdivided into four OC-12 channels. In this scenario the data rate of a single cell or packet flow has its limitations in the bandwidth of the individual channel.

ANSI SONET Standards

The ANSI coordinates and approves the SONET standards. These standards are developed by the T1 Committee that is sponsored by the Alliance for Telecommunications Industry Solutions (ATIS) and accredited by ANSI to develop network and interoperability standards for the US. T1X1, dealing with "digital hierarchy and synchronization" and T1M1, dealing with "internetworking operations, administration, maintenance, and provisioning" are the primary T1 Subcommittees developing SONET.

Below are some important standards from ANSI. For a complete set of SONET standards please visit the ANSI homepage.

ANSI T1.105: SONET - Basic Description including Multiplex Structure, Rates and Formats
ANSI T1.105.01: SONET - Automatic Protection Switching
ANSI T1.105.02: SONET - Payload Mappings
ANSI T1.105.03: SONET - Jitter at Network Interfaces
ANSI T1.105.03a: SONET - Jitter at Network Interfaces - DS1 Supplement
ANSI T1.105.03b: SONET - Jitter at Network Interfaces - DS3 Wander Supplement
ANSI T1.105.04: SONET - Data Communication Channel Protocol and Architectures
ANSI T1.105.05: SONET - Tandem Connection Maintenance
ANSI T1.105.06: SONET - Physical Layer Specifications
ANSI T1.105.07: SONET - Sub-STS-1 Interface Rates and Formats Specification
ANSI T1.105.09: SONET - Network Element Timing and Synchronization
ANSI T1.119: SONET - Operations, Administration, Maintenance, and Provisioning (OAM&P) - Communications
ANSI T1.119.01: SONET: OAM&P Communications Protection Switching Fragment

ITU-T SDH Standards

The International Telecommunications Union (ITU) (formerly known as the CCITT), coordinates the development of the actual SDH standards. It is financed by the United Nations and coordinates the development of telecom standards for the whole world. Below are the most important SDH standards available from ITU. Please visit the ITU homepage for a list of the complete SONET standards.

ITU-T G.707: Network Node Interface for the Synchronous Digital Hierarchy (SDH)
ITU-T G.781: Structure of Recommendations on Equipment for the Synchronous Digital Hierarchy (SDH)
ITU-T G.782: Types and Characteristics of Synchronous Digital Hierarchy (SDH) Equipment
ITU-T G.783: Characteristics of Synchronous Digital Hierarchy (SDH) Equipment Functional Blocks
ITU-T G.803: Architecture of Transport Networks Based on the Synchronous Digital Hierarchy (SDH)

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