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Synchronization for QoS in networks
By A.K. Vanwasi

Synchronization distribution is essential for maintaining quality-of-service of optical and wireless networks. Here is a description of universal coordinated time (UTC) and its distribution in digital networks and the Internet

Network synchronization requires precise timing standards and distribution of timing information

Round-the-clock information flow is the invisible glue that binds the vast expanse of humanity today, and coordinates the functioning of the various constituents of society and essential services such as transport, energy, finance and so on.

The telecommunication infrastructure connects people to people, people to machine and machine-to-machine any time, any-where. It comprises a digital transmission network, switching nodes and synchronization sub-network. The transmission network comprises a wired network, optical fiber network, terrestrial wireless network, and satellite network etc.

Network synchronization is a key factor for ensuring quality-of-service of the telecommunication network. Network synchronization requires precise timing standards and distribution of the timing information. The world's first practical precise timing sources were crystal oscillators invented in the 1920s, and Atomic clocks came in only in the 1940s. Currently, there are clocks that provide accuracy of 10 ps/day.

Accurate timing information is distributed globally through 'Global Positioning System' (GPS) and communication links. GPS is a dual system that acts as a navigation system and time-transfer system. GPS provides stable timing information close to one ns/day.

The task of network synchronization would have been much easier had all the clocks and the propagation delay between two nodes been stable. Thus, with an initial adjustment, all clocks could run at the same rate and the network would operate without timing faults forever. However, available devices are not so perfect and path delays do vary. Thus, network synchronization maintains acceptable service quality at a reasonable cost.

Synchronization sub-network
It deals with the distribution of time and frequency over a network of timeservers spread over a geographical area. It comprises of a network of timing servers. The goal is to align the time and frequency scales of all timing servers using communication links.

The synchronization network must be reliable and survivable even under unstable network conditions and where connectivity may be lost for many days. Thus, there must be redundant timeservers and diverse transmission paths. It must also be possible to dynamically reconfigure the sub-network.

Full plesiochrony
It is a no-synchronization strategy and does not involve any synchronization distribution. Here, each network node is equipped with an independent clock. Thus, it is also known as synchronization anarchy.

The full-plesiochronous strategy demands independent accurate clocks at all nodes. This method is used in Frequency Division Multiplexing' (FDM) and 'Plesiochronous Digital Hierarchy' (PDH) networks.

Hierarchical master-slave synchronization
Here, timing reference is distributed from master clock (primary time server) to all other slave clocks (secondary time servers) of the network directly or indirectly. It is also known as despotism. It is generally considered unethical but it certainly ensures very tight control of slave clocks. Here, all clocks are synchronized with the master (primary) clock.

To ensure that the synchronization network does not fail, several protection mechanisms such as dual master clocks, multiple secondary timing clocks and routes are available. Thus, such a synchronization network is widely used to provide excellent timing performance and reliability at a reasonable cost.

Here, the accuracy of each timing server is defined by a number called the 'stratum' (level). The primary timing server is assigned stratum 1. Successive secondary servers are assigned a level higher than the preceding level.

Mutual synchronization
Also known as democratic synchronization, it is based on direct mutual control among the clocks so that the output frequency of each is the result of the 'suggestion' of the other peer clocks. Such a pure democracy is appealing due to the absence of master and slaves.

However, maintaining mutual synchronization is a very complex and expensive task. Normally, UTC is generated via mutual cooperative working of several national clocks.

Global Positioning System (GPS)
GPS is a reliable source of time and time-transfer system.

A versatile global tool, GPS is used to distribute time to an arbitrarily large number of users located anywhere on the earth. It also synchronizes clocks over long distances with a higher degree of precision and accuracy.

GPS system comprises of a constellation of 24 satellites launched by the US Department of Defence. Each satellite contains atomic clock that provides accurate and precise time within 10 ns of UTC.

The satellites transmit spread-spectrum signals on two frequency bands: L1 (1575.42MHZ) and L2 (1223.6MHz). The signals are modulated by two pseudo-random noise codes: P (Precision) code and C/A (Coarse/Acquisition) code. For civilian applications, C/A codes in the L1 band is used.

A GPS receiver generates a replica of the satellite's pseudo-random noise codes. For successful demodulation of GPS codes, exact matching of locally generated and received code is essential.

The received signals at GPS antenna are usually low (10 to power of minus 15W). For successful reception of signal, clear line-of-slight to the orbiting transmitter is essential.

Normally, for navigation purposes, a GPS receiver is required to acquire the signals from 4 GPS transmitters. However, for time keeping applications, the location of the timekeeping center is known beforehand and thus, tracking of one satellite signal is enough.

GPS synchronizes clocks over large distances using the GPS 'Common Views' (CV) technique. Here, two timekeeping stations simultaneously observe the same satellite. Each of these two stations must record the difference between his local clock time reference and GPS time at the same instant using the same satellite and using a GPS receiver known as GPS 'Time Transfer Unit' (TTU). A GPS TTU is a special GPS receiver programmed to compute and display items of interest to the timing community.

Here, user 'A' (Ta-GPS) and user 'B' observe (Tb-GPS). Thus, by computing the difference between the two sets of numbers we get the modulus ta-tb. This observed time difference could be compared against precisely known time difference between the two locations. This helps in applying necessary time correction. Presently, using coarse acquisition technique, it is possible to distribute time within 10-25n sec level and time synchronization in the range of 1-15 n/sec.

GLONASS is the Russian 'Global Navigation Satellites System'. It comprises of 24 satellites orbiting in three orbits at an altitude of 19100 Km. It provides comparable time synchronization service.

Synchronization in PDH systems
In the beginning, digital transmission was confined to isolated T1/E1 PCM links. These links were used to connect analog switching machines. The individual PCM terminals of a link operated with independent clocks having stability of ± 50 PPM.

To share the transport medium, the hierarchy of plesiochronous time division multiplexers was developed. This enabled transmission of thousands of 64 Kbps digital channels over the shared medium.

In PDH hierarchy, individual tributaries have plesiochronous clocks. Thus for 8448 Kbps (E2) digital multiplexer, each individual E1 tributary has clock variation of ± 50 PPM. To accommodate, these clock variations, each input tributary is translated to a common higher bitrate (2112Kb/s) and then multiplexed. This process is known as bit-justification. Here, additional bits are added to an individual tributary so that output bitrates of tributaries are same.

This bit-justification technique allows the transparent transfer of timing contents of a digital signal across a transmission chain. In this chain, clocks are asynchronous.

A PDH network deploys master-slave synchronization technique for transfer of timing in a PDH network.

Digital Switching Operations
Digital switching is based on moving octets (speech samples) from one time-slot to another time-slot and from one incoming channel to another output channel.

Time-slot interchange is achieved by writing the frames in a memory and then reading the time-slots in the desired order. Further, space-division switching is used for transferring an octet from one incoming channel to another channel.

However, for successful switching, it is required that all the incoming channels should be aligned/synchronized at the bit and frame level.

The frame and bit alignment is achieved by elastic store. Here, incoming data bits are written at clock rate 'fw' and subsequently read at clock rate 'fr'.

The elastic store absorbs zero-mean frequently fluctuations between the write and read clock within a given bound, due to buffer limits. In practice, a frequency offset (fw-fr) between the write and read clocks will make the buffer empty or overflow sooner or later.

This results in either missing or replication of an octet of data over some time period. This phenomenon is known as slip. Its limits are defined by ITU-T for plain old telephony (POT) applications. Few slips over a time period were tolerable.

Contrary to POT, the data transmission cannot tolerate slips, which results in either loss or repetition of transported data octet. This requirement necessitated the synchronization of digital switches. The synchronization of two digital switching exchanges is achieved by using master slave synchronization. Here, an exchange transmits the master clock. Such a clock is known as Standalone Synchronization Equipment (SASE). In ANSI terminology, it is known as 'Building Integrated Timing Supply' (BITS)

This master clock is recovered transparently at the remote-end and used to synchronize the slave clock. This ensures data transmission and reception at the same clock rate.

Synchronization in SDH transmission
ITU-T defined SDH standard in the year 1988. Of late, SDH is progressively replacing PDH in backbone networks worldwide.

The frames and signals of the SDH hierarchical levels are named 'Synchronous Transport Module-N' (STM-N) for N=1,4,16,64. Similarly, for the American standard SONET, corresponding hierarchical levels are named 'Synchronous Transport Signal-N' (STS-N) for N=1,3,12,48,192.

SDH hierarchy deploys byte multiplexing. Traditional PDH signals are mapped into 'Virtual Containers' (VCs). A VC is individually and independently accessible within a SDH frame through pointer information. The pointer information is directly associated with payloads. A pointer is located in a determined position within the multiplexing element.

VCs are allowed to shift independently in the output STM-N frame. However, their position is always tracked by the respective pointers. These pointers are incremented/ decremented according to specified rules. Such a mechanism is known as 'pointer-justification' and is analogous to bit-justification in PDH networks.

In SDH system, jitter affects the output tributary severely. Besides the jitter, pointer-justification and transmission line related jitter results in large shifts in VCs in STM-N frame. Though, it is possible to recover the PDH payload, but it is difficult to have timing transferred transparently.

Synchronization scheme of two digital switching exchanges is based on the SASE/BITS concept and hierarchical master-slave synchronization strategy. (see Figure-5)

The SASE in the first digital switching exchange synchronizes both the digital exchange and SDH multiplexer equipment using 2.048 MHz clock.

At the receiving end, the reference signal is extracted from the input STM-N signal and is used to synchronize the SASE clock. The SASE signal is now used to synchronize both SDH demux and digital switching exchange.

Synchronization in ATMs
The 'A' in ATM stands for Asynchronous. This makes one believe that ATM equipment can work in a system of independent and non-synchronized clocks. In practice, the asynchronous word does not refer to the equipment clock operation or to the physical level of information transfer. The word asynchronous only implies that the digitized information can be given to ATM equipment independently. Here, it is not necessary to input digital information in predefined time-slots as in primary PCM.

Thus, like PDH and SDH equipment, there is provision of a synchronization port in an ATM equipment The ATM equipment can accept external E1/T1 reference clocks. This ensures interworking of ATM equipment in a SDH/PDH network.

Cellular Wireless Network
The cellular wireless networks are structured in cells. A cell is a limited geographical area served by a dedicated base transceiver. A number of cells are so arranged that the seamless coverage of a specific area is possible. Thus, a wireless network must be capable of maintaining communication service to a mobile user.

In general, the smooth hand-over required in cellular network is achieved in the following ways:

1. In some networks, radio carrier frequencies of all the base stations in a region shall be within certain accuracy limits.

2. Other networks require that signal frames at the radio interface (antenna) of all base transceivers should be time aligned within certain accuracy.

Synchronization of computers over the Internet
Normally, every PC has an internal clock. However, this clock is inaccurate and cannot be used for accurate timekeeping. Thus, for demanding timing applications, it is necessary to synchronize widely distributed PC clocks to a network timeserver.

The 'Network Time Protocol' (NTP) is an Internet standard protocol. It is designed to distribute accurate and reliable time information to systems operating in diverse and widely distributed environment.

NTP is defined by RFC-1305. Current version of NTP is version-3. NTP is built on IP and typically operates over 'User Datagram Protocol' (UDP) but is also adaptable to other connectionless transport protocol.

In NTP, hierarchical master-slave strategy is deployed to distribute reference clock. Here, one or more primary timeservers synchronize directly to external reference source such as a GPS receiver. Further, secondary timeservers are synchronized to primary timeservers.

NTP servers can operate in broadcast and client-server mode.

Broadcast mode
In the broadcast mode, an NTP server broadcasts time to a group of hosts on a periodic basis. The clients then determine the time based on an assumed delay between them and the time server. This mode is suitable for use with a large number of hosts where modest accuracy reduced network traffic is desired.

Client server mode
In NTP client server mode, a client periodically polls the server for timing information in traditional client-server fashion. The synchronization involves the following steps:

  • The client sends an NTP packet to the server. The NTP packet contains the timestamp at the time of leaving.
  • The server sends the response NTP packet to the client. The NTP packet contains original timestamp of client and timestamps corresponding to arrival and dispatch at the server.
  • The Client takes fourth timestamp when it receives the server's NTP packet.
  • Now, these measurements enable a client to calculate the round-trip delay of packet exchange and the clock offset between the client and the NTP server.
  • The client uses the clock offset information to slew/adjust its clock into agreement with the NTP severs. The slewing operation typically advances or retards the clock by 0.1 per cent. In case, if the offset is too long, then the local clock is jammed with a new time.

It is impressive to note that the synchronization accuracy of the order of 1 milisec is typically reachable even with the inferior clocks found in most computers. Today, using GPS-based reference for primary NTP server, it is possible to achieve synchronization of the order of 1micro sec, among computers operating in several power stations, in western USA and Canada.

Network synchronization in India
National synchronization network is based on the hierarchical master-slave synchronization technique. The synchronization network supplies reference clock to all the digital switching exchanges.

The scheme envisages Main National Reference Clock Center (MNRC) at Mumbai and 'Back-up National Reference Clock Center' (BNRC) at Delhi. Each center has 3 Cesium beam clocks. However, the output of only one clock is being used at a time. If the reference clock at MNRC deteriorates, then it will be detected at MNRC by continuous comparison with other clocks. The MNRC itself can switch to a better clock.

The BNRC works as a standby primary reference clock. Both main and backup reference clock centers have connections via dedicated links to each other. This enables mutual monitoring of clocks by each other.

Level 2 reference clock centers have provision of dedicated links from both the MNRC and BNRC. This ensures continuous availability of the reference clock. Similarly, each lower layer clock center has at least three dedicated links for the clock reception from immediate higher-level clock centers.

Stability of various levels of clocks:

Level 1: Better than 1x (10 to power of minus 11)

(for the life of cesium beam tube)

Level 2: 1x (10 to power of minus 10)/day

Level 3: 2x (10 to power of minus 10)/day

Level 4: 1x (10 to power of minus 8)/day

Conclusion
Precise time standard and time-transfer is at the heart of modern society. Precise timing is essential for information flow, navigation and position-location systems. Conti-nuous efforts are being made to improve accuracy of clocks and devise better techniques to distribute the time and frequency.

Coordinated Universal Time (UTC): There is no universal agreement for a natural phenomenon i.e. Earth spin, Earth orbit, Moon/Sun rotation, that can be used as unique true 'time'. Thus, ITU by international agreement has defined 'Coordinated Universal Time' (UTC) as world's official time. Currently, the best accuracy for the determination of the second for UTC is equivalent to ± 1sec. in 10 million years.

Second: It is defined as the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the Cesium-133 atom.

Clock: It is a timing source. Normally, it is a two-part device. The first part provides equally spaced periodic events or intervals from some oscillating device. The second part counts the number of events and displays them.

Primary reference clock (source): It is usually a calibrated atomic clock.

Frequency stability: The stability of a clock is how well it can maintain a constant frequency. It measures the changes in frequency from one time interval to the next. A particular time interval such as day/month/year is chosen. It is known as averaging time. The frequency stability is then ascertained for different values of averaging time.

Time accuracy: It defines how well a clock duration compares with UTC.

Precision: It pertains to how precisely time can be resolved in a particular time keeping system.

Offset: The offset of two clocks is the time difference between them.

Skew: It is the frequency difference between two clocks.

Reliability: For a time keeping system, it is the fraction of time, it can be kept operating and connected in the network without considerations of stability and accuracy.

Timing server: A timing server must deliver continuous local time based on UTC. It should be able to accommodate leap seconds in the UTC time scale. The timing server must provide accurate and precise time, despite relatively large delay variation in network path and environmental conditions.

Primary timing server (clock): It is directly synchronized to a primary reference source. It is also known as master clock.

Secondary timing server: A secondary timing server derives synchronization from a primary server over network paths possibly shared with other services. A secondary timing server can also derive synchronization from other secondary servers.

Network: For any network topology, the basic elements are links and nodes. network comprises of terminals, links and nodes. (see Figure-1 A)

Terminal: It is a source of digitized information.

Links: The links are physical means to transport the digitized information from a terminal to a node and between nodes. No information processing is done in links. They simply restore the waveform by means of regenerative repeaters.

During transport delay, distortion and noise may cause impairments of the signals. Further, the propagation delay over a link also varies. The delay variation may be short-term as well as long-term. Thus, it is necessary to properly design the synchronization network.

The characteristics of a link differ according to the physical means used for signal transmission. The three main categories of transmission media are metallic wires or cables, optical fiber and radio transmission.

Wander: When a signal is transported over a link, due to slow temperature changes in medium, delay varies at a very slow rate. It is usually below 10 Hz. Thus, wander accounts for relatively slow variation below 10 Hz.

Jitter: The delay variations that are faster than 10 Hz are known as jitter. The contribution to jitter is mainly due to finite signal-to-noise ratio obtainable at the receiving end of the clock. Jitter accumulates over a chain of nodes.

Node: Here, incoming digitized information is processed so that it is forwarded correctly to another node and finally to the destination. Information processing is based on circuit switching and packet switching. Circuit switching establishes a continuous bi-directional connection between subscriber terminals for the duration of conversation or session.

Packet switching is analogous to store and forward mode of transferring units of information (packets). Each packet has a source and destination address. Using the destination address, at intermediate nodes, the packet is routed towards destination terminals.

Technical Glossary
Spread-Spectrum System A transmission system that transmits signals spread over a frequency band much broader than the minimum required.

Pseudo-random noise It is a binary code sequence. It comprises of a sequence of 0s and 1s. The sequence has statistical properties similar to noise.

UTC (USNO) A GPS system transmits UTC as determined by the 'US Naval Observatory' (USNO). The USNO is usually kept within about 10ns of UTC.

Isochronous digital signal It is a digital signal wherein the time interval between significant instants have, at least on an average same duration/s, which are integer multiples of the shortest one.

Synchronous Signals Two digital signals are synchronous if they are isochronous signals having the same average frequency and have controlled phase relationship. Conversely, two digital signals are asynchronous if they are not synchronous.

Plesiochronous signals Two plesiochronous digital signals are isochronous, asynchronous digital signals. Further, the respective time signals have the same frequency nominally but actually differ within a given tolerance range.

Justification—i) The term justification has origin in printing industry where it describes the process of adjusting spaces between words so that all the lines of print are of the same length.

ii) In digital multiplexing, bit-justification transforms plesiochronous tributaries into synchronous tributaries so that these can be multiplexed.

E1 PCM—It is the European standard for 'Pulse codes Modulation (PCM). It has nominal bitrate of 2.048 Mb/s. The clock tolerance is ±50ppm. It multiplexes 30 speech channels.

T1, PCM—It is the American standard for PCM. It has nominal bitrate of 1.54 Mb/s. The clock tolerance is ± 50 PPM. It multiplexes 24 speech channels.

References
For more information on synchronizing computers and workstations using network and direct connect time transfer techniques' visit http: // www.datum.com/bancomm/ettcpape.html.

Visit the University of Delaware's Time synchronization page at http://www.eecis. Udel.edu/~ntp/.
Visit the following site for ITU-T G-811 compliant clocks sources http: // www. Oscilloquartz.com

For more information on GPS visit www.gpsworld.com http://physics/nist.govt/Genint/Time/Time.html.
http://wwwhost.cc.utexas.edu/ftp/pub/grg/gcraft/notes/gps/gps/html.

A.K. Vanwasi is G.M. (R&D) ITI Ltd. Naini, Allahabad and can be reached at vanvasi_nni@tiltd.co.in

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