Wander occurs in networks for several reasons. The clock regeneration in SDH networks is never completely perfect; rather, each regenerated.clock will have variations in frequency and phase. The more nodes passed en route, the less stable the clock will be. Aging of the node clock oscillators and temperature changes can increase wander. Temperature changes cause the expansion and contraction of the transmission,cables, which in turn generates wander. For each degree Celsius that the temperature changes, 80 ps of wander is generated per kilometer of,optical fiber. For copper cable, the generated wander is 725 ps/km for each degree Celsius change. This may not sound like very much, but,consider how much the temperature can change in one day and add how many kilometers of optical fiber or copper cable there are, and it soon,adds up to a considerable amount of wander.
Wander vs. Jitter
Jitter, which is constant over time, might cause bit errors. However, most jitter can be filtered out in the SSUs and SECs. Wander can only partly be filtered out in the network nodes and it accumulates in the network, causing incorrect synchronization or even a total loss of synchronization.Incorrect synchronization in transport networks may cause severe transmission problems. Voice calls (fixed or cellular) will be lost, fax machineswill misprint, and data will be lost or frequently retransmitted. The network operator will have increased service costs and may lose customers—
Synchronization must exist at three levels: bit, time slot, and frame. Bit synchronization refers to the requirement that the transmit and receive ends of the connection operate at the same clock rate, so that bits are not misread. Bit synchronization
involves timing issues such as transmission line jitter and ones density.These issues are addressed by placing requirements on the clock andthe transport system.
The synchronisation into the frames coming in the uplink direction is done with synchronisation zeros and ones in the frames. The synchronisation is performed continuously and the search window is moved according to the changes in timing. The synchronisation is considered lost when at least three consecutive frames with at least one synchronisation error in each have been received. According to the 3GPP specifications, the loss of synchronisation\ should cause the muting of decoded speech in the speech state and after this the TRAU should wait for one second before any other procedure is undertaken. In order to avoid unpleasant sound effects, this has been implemented in the software in a faster way.
The synchronisation into the frames coming in the uplink direction is done with synchronisation zeros and ones in the frames. The synchronisation is performed continuously and the search window is moved according to the changes in timing. The synchronisation is considered lost when at least three consecutive frames with at least one synchronisation error in each have been received. According to the 3GPP specifications, the loss of synchronisation\ should cause the muting of decoded speech in the speech state and after this the TRAU should wait for one second before any other procedure is undertaken. In order to avoid unpleasant sound effects, this has been implemented in the software in a faster way.
The process of handover or handoff within any cellular system is of great importance. It is a critical process and if performed incorrectly handovercan result in the loss of the call. Dropped calls are particularly annoying to users and if the number of dropped calls rises, customer dissatisfaction increases and they are likely to change to another network. Accordingly GSM handover was an area to which particular attention was paid when developing the standard.
The process of handover or handoff within any cellular system is of great importance. It is a critical process and if performed incorrectly handovercan result in the loss of the call. Dropped calls are particularly annoying to users and if the number of dropped calls rises, customer dissatisfaction increases and they are likely to change to another network. Accordingly GSM handover was an area to which particular attention was paid when developing the standard.
Time slot synchronization aligns the transmitter and receiver so that time slots can be identified for retrieval of data. This is done by using a fixed frame format to separate the bytes. The main synchronization issues at the time slot level are reframe time and framing loss detection.
Frame synchronization refers to the need of the transmitter and receiver to be phase aligned so that the beginning of a frame can be
identified. The frame in a DS1 or E1 signal is a group of bits consisting of twenty four or thirty bytes, or time slots, respectively, and a
single framing pulse. The frame time is 125 microseconds. The time slots are associated with particular circuit users.In private networks, synchronization can cause additional impairments in the form of error bursts .
control slip rates, pointer adjustment events, and synchronization-caused error bursts.
Output jitter is a measurement of the jitter present on an outputfrom a system. This could have been generated within a single pieceof equipment (jitter generation or intrinsic jitter), or may have builtup as the signal traversed a large network (network jitter). It is specified in unit intervals and the result expressed as a Root MeanSquare (RMS), or peak-to-peak value. RMS values give information about the total amount of average jitter present, while peak-to-peak results tell more about the effect on performance, as it is the extremes that can cause errors. While jitter is defined as any phase variations above 10 Hz, most measurements use additional high-pass and low-pass filters, and some systems define more than one set.
Jitter tolerance is a measurement to check the resilience of equipment to input jitter. A signal is generated with added
sinusoidal jitter and applied to the DUT (Device Under Test). At each jitter frequency, the amplitude of the jitter is increased until
transmission errors are detected. Alternatively, a specified level of input jitter is generated and error-free operation checked. In the
real world, jitter is unlikely to be sinusoidal, but it is easy to generate and gives repeatable results. It allows results for different
systems to be compared and for system specifications to be written, usually in the form of a jitter tolerance mask.Jitter transfer is a measure of how much jitter is transferred between input and output of network equipment. As mentioned
earlier, this is a function of jitter frequency and the type of clock recovery used. As a signal traverses a network, the jitter generated by each piece of equipment becomes the input jitter to the following equipment. If this jitter is amplified as it passes through the networkthen it could exceed the jitter tolerance of subsequent equipment. To avoid this, a jitter transfer function is specified for equipment, typically allowing a maximum of 0.1 dB jitter gain.
A different set of measurements is used to characterize wander, the longer-term phase variations ranging from 10 Hz down to micro-Hertz and below. While jitter is normally measured with reference to a clock extracted from the data signal, wander is measured against an external reference clock . The fundamental measurement is of Time Interval Error (TIE). This represents the time deviation of the clocksignal under test relative to the reference source.
Several results requiring intensive computation can be calculated from TIE according to the ITU-T G-series recommendations:
• MTIE (Maximum Time Interval Error): The peak-to-peak variation of TIE within a defined observation interval τ.
• TDEV (Time Deviation): A measure of the spectral content of wander and again is a function of the observation interval τ.
• Frequency Offset: A measure of the degree to which the clock frequency deviates from its ideal value.
• Frequency Drift Rate: A measure of how the frequency offset changes with time (i.e. frequency stability).
Wander is a phase variation of synchronisation and traffic signals from their ideal position, where the variation is greater than 10Hz in frequency.
Diurnal Wander is a phase variation caused by the heating and cooling effects of a transmission medium throughout the course of a day. A transmission line, be it; optical fiber, copper pair,coaxial cable or microwave (air) is composed of a physical medium. The propagation speed and slight differences in length due to heating and cooling cause the phase of the emerging signal to move. The wander generated by optical fiber is approximately: 80pS/Km/°C
The wander generated by copper cable is approximately: 725pS/Km/°C High amplitude wander is also generated by SDH or SONET tributaries as a result of pointer activity.
Wander is impossible to filter out in a synchronisation network so it must be minimized nby network planning, for example; the avoidance of very long over-ground cables, subject to wide temperature variations, and SDH or SONET tributaries for synchronisation transport.
Jitter
Jitter is a phase variation of synchronisation andtraffic signals from their ideal position, where the variation is less than 10Hz in frequency.In order to derive a synchronisation signal suitable for clocking out going tributaries a network element must convert the gapped clock presented on its line interface to a regular clock. The gaps in the gapped clock will have been produced dynamically and therefore cannot be predetermined by the desynchroniser and a phase locked loop (PLL) is required to smooth out the clock gaps. Unfortunately this process is not perfect and a certain amount of phase variation is introduced to the clock known as justification jitter.
Wander and Jitter are two important issues to be resolved when distributing synchronisation. Wander cannot be filtered and should be minimized by design. Jitter accumulates as synchronisation is recovered and rebuilt at each network element but this can be filtered using a narrow band synchronisation filter elements commonly called; Stand Alone Synchronisation Elements (SASE), Source Synchronisation Units (SSU) or Building Integrated Timing Systems (BITS),
three names for the same device.
Period Jitter: Period jitter compares the length of each cycle to the average period of an ideal clock using
the long term averaging frequency.
Cycle to Cycle Jitter: Cycle to cycle jitter compares the difference in the cycle length of adjacent cycles.
Time Interval Error Jitter: TIE Jitter is the variation in the clock’s transition from its ideal position over
G.823 The Control of Jitter and Wander within Digital Networkswhich are
based on the 2048 b/shierarchy.
This standard setsnetwork performance expectations for PDH interfaces outside N. America. The parameters coveredin this specification include output jitter and input jitter/wander tolerance.
G.825 The Control of Jitterand Wander within Digital Networks which are based on the Synchronous Digital Hierarchy.
G.825 is the equivalent of G.823 and G.824 for any SDH interface.
O.171 Timing jitter and wander measuringequipment for digital systems which are based on the Plesiochronous DigitalHierarchy (PDH).
This publication details the minimum requirements for a test instrument in order to test and measure jitterand wander in PDH signals. This specification also contains appendices containing guidelines for the
measurement of jitter and wander.
O.172 Jitter and wander measuring equipment for digital systems which are based on the SynchronousDigital Hierarchy(SDH).
This document essentially provides information for SDHjitter and wander testing, equivalent toO.171 for PDH signals.
– Voice is still switched in synchronous switches poor synchronisation will
cause slips
• Air Interface Frequency stability
– Poor synchronisation will cause interference with other services and failure
of operation
• Cell Handover
– Poor synchronisation will cause handover failure causing dropped calls
• Time Synchronous CDMA systems require precise time and
phase synchronisationIf the synchronisation chain is not working correctly calls may be cut or call quality may not be the best possible.Ultimately it may even impossible to establish call.
Why do Mobile Networks need synchronisation
Voice is still switched in synchronous switches poor synchronisation will cause slips –
long term objective 1 x 10-11
• Air Interface Frequency stability
– GSM and CDMA systems require a Base Station accuracy of at least 5 x 10-8, US
TDMA only required 5 x 10-7
– GSM requires a Mobile Station (handset) frequency
In SONET/SDH networks, new requirements are being placed on network synchronization;
SONET/SDH standards have been defined to provide high-speed synchronous transport systems.
Network elements do not cause slips when synchronization is lost due to the fact that the payload in
the SONET/SDH service is transmitted asynchronously. These network elements use pointers to
identify the beginning of a frame, any mismatch in the sending and receiving rate causes an adjustment
in the pointer. This pointer adjustment, cause jitter and wander in the transported signal. Jitter is a fast
(>10 Hz) change in the phase of a signal, whilst wander is a slower (<10 Hz) phase change. Excessive
jitter from network elements can cause a loss of frame synchronization, whilst excessive wander
causes the terminating network element to slip its clock.
What is Network Synchronisation?
At its most basic, network synchronisation is simply the means by which all digital equipment in a communication network operates at the same average rate.
Quality of Service whilst network coverage is and has improved greatly (We are nearing total coverage), the biggest problem is call drop out
How can poor Synchronisation cause a call drop
out?
The basis of any robust and reliable GSM / UMTS network is good quality synchronisation references. This is particularly applicable to base stations, an area often overlooked in terms of synchronisation. The accuracy of the synchronisation at the base station is critical to the call hand over (when a subscriber moves from one cell/base station to another). This level of accuracy is required to
ensure hand over between base stations without drop out is +/- 50 parts per billion ppb. When a hand over occurs between two base
stations the potential for frequency difference can be as high as 100ppb. This in turn can be equated to a Doppler shift of 100kph in vehicle speed. If the mobile subscriber device i.e. the handset, datacard etc. cannot react quickly enough to this Doppler shift the
call will be dropped.
The Problems
without effective synchronisation a mobile network will not function correctly due to frequency disparities between base stations. The historical method of achieving synchronisation quality within the GSM environment (in particular at the base stations) was to co-locate a low quality oscillator at the base station site which would be retimed using a timing reference derived from either a T1 or E1 backhaul line. This was known as recovered clocking. However this approach creates another set of issues .Using leased lines (which are normally sourced from fixed line operators) the mobile network operator will have no direct control over the leased line reference, therefore the quality of the synchronisation reference cannot be guaranteed (remember we are dealing with an accuracy tolerance of -/+50 ppb While this is happening the base station will at best only have the co-located low quality oscillator in holdover mode. This will only provide frequency accuracy of around 10 x -5 meaning regular call drop outs at the user end.
Synchronisation must exist at three levels; bit,time slot, and frame.
Bit Synchronisation
We have just looked at a very simplistic example of bit synchronisation, where the transmit and receive ends of a transmission line must operate at the same clock rate, so bits are not misread or lost. Bit synchronisation is achieved by the receiving element attempting to align its sampling frequency with the frequency of the incoming
data. This can be compromised by short term events such as transmission line jitter and ones density. These issues are addressed by placing requirements on the alignment mechanism and the transport system.
Time Slot Synchronisation
Time slot synchronisation aligns the transmitter and receiver so that time slots within the structured transmission signal, can be identified for recovery. Time slot alignment is possible by using fixed frame formats to define their position. The main synchronisation issues at the time slot level are reframe time and framing loss detection.
Frame synchronisation
Frame synchronisation aligns the transmitter and receiver so that the beginning of a frame, within the structured transmission signal, can be identified for recovery.
Why is Synchonisation Important?Consider a very simple network comprising of just two elements, imaginatively named A & B Element A clocks digital levels into the transmission line at a clock frequency of f1. However, Element B is sampling the signals on the transmission line at clock frequency f2 - which is greater than f1.As the sample frequency is too high erroneous bits become added to the data stream at element B. Conversely if clock frequency f1 is greater than f2…
As the sample frequency is too low data is lost from the data stream recovered at element B. With today's SDH and SONET structured transmission technology operating at Giga-bits per second, synchronising the insertion and recovery rates is extremely important.
Wander is a phase variation of synchronisation and traffic signals from their ideal position, wherethe variation is greater than 10Hz in frequency.Diurnal Wander is a phase variation caused by the heating and cooling effects of a transmission medium throughout the course of a day. A transmission line, be it; optical fiber, copper pair,coaxial cable or microwave (air) is composed of a physical medium. The propagation speed and slight differences in length due to heating and cooling cause the phase of the emerging signal to move.The wander generated by optical fiber isapproximately: 80pS/Km/°C
The wander generated by copper cable is approximately: 725pS/Km/°C
High amplitude wander is also generated by SDH or SONET tributaries as a result of pointer activity.Wander is impossible to filter out in asynchronisation network so it must be minimized by network planning, for example; the avoidance of very long over-ground cables, subject to wide temperature variations, and SDH or SONETtributaries for synchronisation transport.
Jitter
Jitter is a phase variation of synchronisation and traffic signals from their ideal position, where the variation is less than 10Hz in frequency.In order to derive a synchronisation signal suitable for clocking out going tributaries a network element must convert the gapped clock presented on its line interface to a regular clock. The gaps in the gapped clock will have been produced dynamically and therefore cannot be predetermined by the desynchroniser and a phase locked loop (PLL) is required to smooth out the clock gaps. Unfortunately this process is not perfect and a certain amount of phase variation is introduced to the clock known as justification jitter.
Wander and Jitter are two important issues to be resolved when distributing synchronisation.Wander cannot be filtered and should be
minimized by design. Jitter accumulates as synchronisation is recovered and rebuilt at each network element but this can be filtered using a narrow band synchronisation filter elements commonly called; Stand Alone Synchronisation Elements (SASE), Source
Synchronisation Units (SSU) or Building Integrated Timing Systems (BITS), three names for the same device.
2G
PSN need to synchronize the packets to and from the 2G networks so that it meets the stringent frequency requirements of the 2G networks. BTS requires a frequency accuracy of 0.05ppm (50ppb) at the radio interface. Driver for this requirement is mobility and thec associated Doppler shift experienced by a moving Mobile Station.
For example: If a mobile experiences a handover between two BTS, the possible frequency difference of 0.1ppm could be equivalent to a velocity of over 100 km/h. To support accurate location of mobile the frequency accuracy required need to be two orders of magnitude more stringent.
RAN equipment needs to be fully synchronized to a common reference timing signal to ensure-
– sufficient frequency stability,
– radio framing accuracy and
– handoff control for RF channels.
Thus the mobile backhaul network needs to support distribution of frequency from the Radio Network Controller (RNC) to the
RAN equipment.
– Example: in the case where the air-interface is based on Time Division Duplexing (TDD), the base station clocks must be synchronized to ensure no overlap of their transmissions within the TDD frames.
Ensuring synchronization allows for tighter accuracies and reduced guard bands thereby ensuring higher capacity.
Synchronization is an important topic in many communi cations networks but particularly in the mobile RAN where numerous disparate elements must be kept in tight sync in order for communications to proceed effectively without dropped calls or distortion/noise etc. In the mobile RAN, the need for synchronization is focused in three areas as depicted in Figure 10.
5.2.1 RADIO FRAMING ACCURACY
Radio framing accuracy focuses on the correct insertion and extraction of protocol elements in the air interface. This allows for reliable communications, even under sub-optimal condi - tions, and maximum bandwidth usage. A typical clocking accuracy target for GSM/UMTS FDD (frequency division duplexing) is 50 parts per billion. Additionally, TDD (time division duplexing) mechanisms, as used in CDMA 2000 and WiMAX, requires a strict phase stability. The phase stability requirement is +/- 1.25 μS around UTC.
5.2.2 HANDOFF CONTROL
Soft handoff mechanisms are used as a mobile device moves into another cell or sector.By monitoring the radio power at the receiver (i.e. the mobile device) a correct decision can be made to switch to another signal. A make-before-break mechanism is used within strict timing constraints to ensure uninterrupted communications. Activities are orchestrated in separate elements: the base station, the handset and the controller. Accurate, synchronized timing is paramount for a successful , transparent handover completion.
5 . 2 . 3 B ACKHAUL TRANSPORT RELIABILITY
Wander and jitter in the backhaul and aggregation network can cause underflows and over flows in buffers, slips in the PDH framing can cause bit errors leading to packet rejections. TCP sessions will often exist between the handset and some application servers. A packet rejection will lead to end-to-end retransmissions at the TCP layer and perceptible slowdowns in application ‘goodput’.
5 . 2 . 4 P R I N C I PAL MECHANISMS FOR ACHIEVING SYNCHRONIZATION
There are a number of mechanisms for achieving synchronization in the mobile RAN: • Using the PDH/SDH hierarchy to furnish a clock with a known accuracy (a known deviation from a PRC (primary reference clock)
Voice/data/signaling information appears at various places in the frame
Were we to understand the proprietary signaling • we would know where to look for the various channels• but this signaling is vendor-dependent• and the formats are not always known
So we need to employ an intelligent detector/classifier/deframer• detect channel framing and return field positions• classify channel as voice/data/signaling/idle/unknown• maintain relative synchronization
Matching framer at egress needs to recreate the original frames
signalingHR voiceFR voice64K dataTDM sync FR voice 32K data
This detector/classifier needs to continually scan all1-bit positions for HR TRAU frameseven aligned 2-bit fields for FR TRAU frameseven aligned 2-bit fields for HDLCnibble-aligned nibbles for HDLCbyte-aligned octets for HDLCfields of idle bitsanything else
and then return the identifications and positions found
Unidentified non-idle information must be reliably transported
The processing involves searching for specific bit combinationsperforming bit correlations
and is extremely computationally intensive
Can be performed by a DSP with good bit-oriented operations
Synchronization is an important topic in many communications networks but particularly in the mobile RAN where numerous disparate elements must be kept in tight sync in order for communications to proceed effectively without dropped calls or distortion/noise etc. In the mobile RAN, the need for synchronization is focused in three areas as depicted in Figure 10 .5.2.1 RADIO FRAMING ACCURACY
Radio framing accuracy focuses on the correct insertion and extraction of protocol elements in the air interface. This allows for reliable communications, even under sub-optimal condi - tions, and maximum bandwidth usage. A typical clocking accuracy target for GSM/UMTS FDD (frequency division duplexing) is 50 parts per billion. Additionally, TDD (time division duplexing) mechanisms, as used in CDMA 2000 and WiMAX, requires a strict phase stability. The phase stability requirement is +/- 1.25 μS around UTC.
For example, in the case where the air-interface is based on Time Division Duplexing (TDD), the base station clocks must be synchronized to ensure no overlap of their transmissions within theTDD frames. Ensuring synchronisation allows for tighter accuraciesand reduced guard bands thereby ensuring higher capacity.
Soft handoff mechanisms are used as a mobile device moves into another cell or sector. By monitoring the radio power at the receiver (i.e. the mobile device) a correct decision can be made to switch to another signal. A make-before-break mechanism is used within strict timing constraints to ensure uninterrupted communications. Activities are orchestrated in separate elements: the base station, the handset and the controller. Accurate, synchronized timing is paramount for a successful , transparent handover completion.
5 . 2 . 3 B ACKHAUL TRANSPORT RELIABILITY
Wander and jitter in the backhaul and aggregation network can cause underflows and over - flows in buffers, slips in the PDH framing can cause bit errors leading to packet rejections. TCP sessions will often exist between the handset and some application servers. A packet rejection will lead to end-to-end retransmissions at the TCP layer and perceptible slowdowns in application ‘goodput’.
The lack of synchronization at the base station leads to RF interference. The resultant effect is degraded
call quality, increased dropped calls during handoffs, excessive call setup times, lower bandwidth and inefficient usage of spectrum. Since wireless carriers compete on all of these important customer quality issues and pay millions or billions of dollars to acquire spectrum licenses, one can see how important synchronization is for the operator
In order to guarantee that the air interface requirements listed in Table 1 are met, operators generally require that timing equipment provide a frequency output that is accurate to within ± 15 ppb. This stricter requirement is needed in order to provide margin for the base station, which takes the timing signal in as input to its internal phase locked loops and then modulates the signal for transmission over the air interfaceSynchronization Service Level Agreements
In addition to the synchronization requirements, the IP RAN backhaul network must meet fundamental performance levels and availability for the services it transports. Performance parameters for legacy services include Errored Second Ratio (ESR), severely Errored Second Ratio (SESR), Bit Error Ratio (BER), Frame Delay, Jitter (bit), Delay Difference, MTIE, TDEV, and frequency accuracy. Some representative values for these performance parameters are shown in Tables 2 through 4.
In order to meet the above requirements, the IP RAN backhaul provider has to meet additional performance criteria placed on the end-to-end packet transport network. Chief among these are the one-way delay, jitter (packet), packet loss rate, and throughput bandwidth. Representative values are shown in Table 5. Superior network synchronization is key to meet and monitor the objectives for delay and jitter in the milliseconds and throughput accuracy to 1ppm.
Ultimately, the wireless operator imposes the above requirements on the IP RAN backhaul provider, which then has to meet them. Contractually and financially, this is done in a Service Level Agreement (SLA), which specifies the performance parameters rolled up as Service Availability requirements. This is represented in terms of Error Free Seconds Ratio (EFSR), Annual Service Availability and Mean Time To Repair (MTTR) and perhaps other objectives as shown in Table 6. In order to bill for its services, the IP RAN backhaul operator must not only meet the service availability requirements but must also prove it is meeting the requirements by monitoring the network and providing reports.
In order to meet the above requirements, the IP RAN backhaul provider has to meet additional performance criteria placed on the end-to-end packet transport network. Chief among these are the one-way delay, jitter (packet), packet loss rate, and throughput bandwidth. Representative values are shown in Table 5. Superior network synchronization is key to meet and monitor the objectives for delay and jitter in the milliseconds and throughput accuracy to 1ppm.
Ultimately, the wireless operator imposes the above requirements on the IP RAN backhaul provider, which then has to meet them. Contractually and financially, this is done in a Service Level Agreement (SLA), which specifies the performance parameters rolled up as Service Availability requirements. This is represented in terms of Error Free Seconds Ratio (EFSR), Annual Service Availability and Mean Time To Repair (MTTR) and perhaps other objectives as shown in Table 6. In order to bill for its services, the IP RAN backhaul operator must not only meet the service availability requirements but must also prove it is meeting the requirements by monitoring the network and providing reports.
One main issue is the possibility to provide a synchronisation reference with a frequency accuracy better than 0.05 ppm at the Node B in order to properly generate signals on the radio interface.
One main issue is the possibility to provide a synchronisation reference with a frequency accuracy better than 0.05 ppm at the Node B in order to properly generate signals on the radio interface.
In FDD Radio Interface Synchronization is necessary to assure that the UE receives radio frames synchronously from different cells, in order to minimize UE buffers.
Slips can occur for two basic reasons. The first is the lack of frequency synchronization among the clocks in the
connection, resulting in differences in clock rates. The second is phase movement either on the communications link (such as jitter and wander) or between the source and receiver clock. The latter, phase movement between the source and receiver clock, will be shown to be the largest contributor to slips in communication networks.
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