Frequently Asked Questions

Welcome to Professor Sync's FAQ forum. This interactive feature is designed to give you answers to all of your questions on synchronization and related topics. Read through the list of questions that have been asked by students or ask Professor Sync your own question.



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Recent questions:

 Is synchronization required for non-traditional services such as voice-over-IP?

 In the event of a GPS shut down by the US government, what would be the effects on a communications network that relies on GPS for its Primary Reference?

 What cause Frequency and MTIE Errors?

 Why are there discrepancies between standards bodies regarding tolerable jitter and wander parameters

 We're currently designing a T1 to DS3 multiplexer. It is our understanding that DS3 timing is independent of the embedded T1 timing (M13 mux allows the rate adaptation.) Given this, is there any reason why we need a PLL for retiming the DS3? Can we discard the received DS3 timing and always use a free-running oscillator for transmit DS3 timing?

 What development work has been done on Time Of Day? What is the current standard and are future standards planned?

 I had tested 2 different output (2MB & 2KHz) with a frequency counter. The result shown for o/p 2MB is stable and but for 2KHz the value is 2048001/2/3. Is this normal?

 I realize that a synchronization signal (DS1) derived from a GPS receiver, such as a PRR-10, and one derived from a Cesium PRS, such as a TimeCesium 4500, should be at least Stratum 1 accurate (+/-1X10-11). My question is: Which one would be the more accurate signal and what would be the expected accuracy of each?

 Simplistically explain how asynchronous DS1 signals setup in an "A" end master configuration to a "Z" end sync recovery mode in either network/line/loop/recovered configuration can operate over multinode SONET rings with some associated subtending rings.

 Is the 64k composite clock standard different in Japan than in the USA?

 I heard Japan was using 64kbps(CC) clock as one of their timing sources for their network transport system, my question is does any ITU or Bellcore requirement apply for the Japan application?

 What exactly is TDEV? How is it related to jitter and wander?

 Any ideas on the future possibility/potential of strontium and hydrogen maser technologies for TSG application?

 SONET clocks have a drift rate of about 1 part in 10^9. How long does it take for the drift to equal the timeframe of 1 bit?

 What are the implications of this calculation?

 What attributes determine the data rate of 148.608Mbits/s that are derived from SONET OC-3 parameters?

 What is the difference between cesium and rubidium timing products?

 Which text books do you recommend related to synchronization?

 Is a Rubidium atomic reference suitable for a SDH network to be a PRC?

 How can one GPS timeserver have accuracy to the microsecond and another only in milliseconds when they use the same GPS satellites?

 Why would there be antenna incompatibilities with different NTP servers?

 Can you explain pluses/minuses of using GPS antenna splitters?

 Which text books do you recommend related to synchronization?

 There are discrepancies between standards bodies regarding tolerable jitter and wander parameters - why?

 If a line card receives redundant external timing for its optical transmitter from a Stratum 3E reference, can the line card use phase buildout in its own external timing selection switch?

 What exactly is meant by "clock recovery"? How is it implemented? When is it needed? And, what is the notion of "clocked out"?

 A customer is using a private network of TDMs. They have clock slip problems on a number of leased lines which are provided as unframed E1s. The Customer TDMs provide clock. Diagnostics of the E1s show no error conditions. How should we go about diagnosing a 'clocking problem' with the TDMs?

 Is it a fair statement that, to properly operate, a digital PBX with digital interfaces to more than one carrier (local or long distance) with DS1 service into digital switches at the carriers' POPs, requires that timing be taken from one of them (should not matter which), and all timing has to be Stratum One traceable, and the PBX clock must be 'slaved' to one of the DS1s (or be fed from a Stratum One clock on site).That is, all digitally interconnected DS1 devices and lines must operate at a common rate; and since the carriers are Stratum One, everything else must be, too.

 What is IRIG-B? what is it used for?

 What's the relationship between the NIST-F1 Cesium Fountain Atomic Clock and GPS and LORAN-C/E-LORAN?

 What is the number of the Networks Elements that one Stratum 1 clock can handle? What ITU recommendations talk about this?

 Are E1 links used for data different from those used for voice?

 What is drift rate?

 Can I connect a Sync reference to a T1 tie cable to transport the signal from location A to B? The distance is less than 180ft and is an interior cable.

 What causes Frequency and MTIE Errors? How can I know if these type of errors are occuring, and how can I correct for them?

 What is meant by "pull-in" and "pull-out" range?

 What is "line-timing"?

 What is the difference between an E1 and 2048 kHz signal?

 Everything I read stresses that Composite Clock is required for DS0 interconnections. A DS3/DS1/DS0 cross-connect vendor I am dealing with only provides the capability to externally time their equipment with a DS1. Their equipment makes cross-connects at the DS0 level. They claim they have been doing this for a long time on numerous networks and have no problems. I don't understand how this can be true. What types of problems should this synchronization limitation (DS1) cause for the DS0 circuits?

 Why is an 8kHz signal always used as a frame synchronization signal in PDH and SDH Systems? Why not 4kHz or 2kHz?

 Can you suggest a better way of extracting a frame synchronization signal (8kHz)from E1(PDH)?

 What is the difference between a typical BITS/SSU feed and an IRIG-B feed?

 What is the relationship between time and frequency?

 I cannot seem to grasp the difference between jitter and wander. Can you explain?

 I have a question on GPS systems. Where is the satellite's reference clock? Is it on board the satellite or is the information relayed to the satellite via an uplink? Also what kind of clock is it?

 Are there any further synchronisation requirements that telecom operators need to be aware of to support SDSL technologies over & beyond what is currently provided to support ADSL technology? Has the ITU-T carried out any work in this area for example?

 The National Institute of Standards and Technology (NIST) has a sub-committee called T1X1. What does that stand for?

 On a SONET NE that is optioned for line timing from an OC-N signal, how is the synchronization extracted from the optical signal? Is it calculated from the input line signal in total, or does it use a specific bit (ie bit 1 of the A1 byte) or byte to establish an 8000 hz reference?

 How many bytes does the physical layer of ATM consist of?

 Can the multiplexing from DS1 to DS3 be done directly without passing intermediately through DS2?

 Can we generally claim that PDH links deals with synchronization transfer only if we abuse some bandwidth on a low hierarchy as DS1 or E1 for the sync derivation (that is not being used for traffic transfer simultaneously as well)?

 In regard to synchronization signals jitter and wander specifications: In G.823 and G.824, there is a full clause 6 (in both standards) dealing with synchronization interface requirements for E1 and DS1 respectively. Do the G.811 to G.813 standards contain also this information? If not, what do G.823 and G.824 synchronization requirements stand for?

 What is the relationship between ppm and sec? How accurate is a clock in sec, if it has 50ppm resolution? What is the accuracy of the clock in sec in one day?

 I work as a system engineer for the carrier-grade high density media gateway. Our media/signaling gateway interconnects PSTN, IP and ATM networks. We support E1/T1, SDH/SONET STM-1 PSTN trunks, Gigabit Ethernet IP STM-1 links and ATM STM-1 links. Is it logical to request Stratum 2 performance on this GW in order to support any carrier-grade applications/services? In the case that we provide E1/T1 PSTN interfaces and IP Ethernet interfaces only, do we need to provide performance better than Stratum 3? Can we declare stratum compatible performance with the limitation during the clock rearrangement?

 What is the impact of jitter and wander on frequency deviation?

 Who is responsible for synchronization in a network?

 How does synchronization differ from timing?

 Why can't my PBX just assume it gets good synchronization from the public network's DS1 or E1 signals?

 If I adopt sync status messages in my sync distribution plan, I will not have to worry about timing loops, right?

 If ATM is asynchronous by definition, why is synchronization even mentioned in the same sentence?

 If voice traffic is still intelligible to the listener in a relatively poor communication channel, why isn't it easy to pass it across a network optimized for data?

 Most network elements have internal stratum 3 clocks with 4.6ppm accuracy, so why does the network master clock need to be as accurate as 1 part in 1011?

 What are the acceptable limits for slip and/or pointer adjustment rates when designing a sync network?

 Why is it necessary to spend so much time and effort on synchronization in telecom networks when the basic requirement is so simple, especially when computer LANs have never bothered with it?

 How many stratum 2 and/or stratum 3E TSGs can be chained either in parallel or series from a PRS?

 Why is a timing loop so bad, and why is it so difficult to fix?