1. University of Houston Networking 102 Dr Fred Zellner fzellner@uh.edu
Clocking
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2. Clocking in the Digital Network
Stratum Levels
Clocking Distribution
— Single Point versus Multiple Points
Timing Differences
Correcting Timing Errors
— Buffers
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3. Clocking
The most important aspect in digital networking is clocking. The clock that quantifies the analog signal must be the same clock that reconstructs the signal at the other end. The only way to assure that the clock is accurate within the entire digital network is to have only one clock.
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4. Clocking
This is the way the first digital network was designed and setup in America. AT&T set up a atomic clock which sent its time signals to each of the switching offices by means of the internal data network.
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5. Clocking
The different levels of clocking accuracy are called Stratum levels. This source of clock was called Stratum-1. Telephone companies distribute the analog clock frequency to the first working level which was called Stratum-2. This clock source is still very accurate but is starting to show signs of a measurable difference.
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6. Clocking 1) Clock Recovery 2) Isochronous Clocking 3) Ones Density
4) The Phase Locked Loop (PLL)
5) Digital Data Services or 56 Kbps Data
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7. Clocking
Getting from a Stratum Level-2 to a Level-3 telephone company office is achieved primarily over the digital network. The accuracy of a Stratum Level-3 clock relies on averaging time from a number of incoming digital lines, which results in a new nodal clock. Telephone company offices with Digital Cross-Connect Systems (DCCS) and DDS Hub offices use this Stratum level-3 nodal clock. It is the highest level of timing accuracy accessible to end-user facilities. .
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8. Clocking
This now brings us down to the clock internal to the end-user’s terminal. Since the terminal relies on phase locking (PLL) this frequency from an internal crystal , the results are going to be less accurate than the clock coming to your location. In order to connect to the public network, you must have “receive timing on transmit”. This means you must use the clock coming in and the clock generator for the signal going out.
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9. Clocking Distribution
Master Clock
Regional Message Switch
Network Timing Source
Digital Cross-Connect
Toll Office Switch
Digital Office Timing
End Office Switch
Digital Channel Bank
End User Terminal Equipment - Customer Owned Switch
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10. Clocking Distribution
Telephone Company Distribution
How many level-1 clocks exist in a particular telephone company network depends on their basic needs. Trans-Canada Telephone selected two master clocks--One at Ottawa and the other in Calgary. AT&T had one master Clock at Hillsboro.
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11. Clocking Distribution
Telephone Company Distribution
Other United States telephone companies have opted for independence from AT&T’s Reference frequency and have installed their own master clocks. Both Sprint and MCI have their own clocking schemes and employ multiple high level clock sources.
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12. Telephone Company Distribution
Many private companies have their own clock to time their networks because multiple carriers used or the number of different types of equipment at the customer site. As more Stratum Level-1 sources spring up across the country, look for a slight increase in bit slips between networks.
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13. Clocking Distribution
Satellite
Telephone companies and some other operators of Large area networks use timing sources based on Stratum-1 clock sources located in satellites. Time can be broadcast to all of the network office facilities at the same time, there-by assuring the same clock time at all locations.
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14. Typical Clocking Distribution
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15. Clocking Differences
Integrated digital networks are timing clock oriented. Clocks open and close logic gates to expected data pulses. When two clock sources appear in the same network, confusion abounds and errors ensue. Staying within one network only lessons the problems encountered. Your main concern is to reduce timing difference to a minimum, and thereby limit the number of bit slips in the network.
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16. Clocking Differences
There are several ways to reduce the clock differences. One method is adequate buffering between equipment and facilities. Another is slave-timing analog portions of your network to the main clock source.
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17. Clocking Differences BIT SLIPS
If you have two clocks in the same network, the difference between them will eventually result in a bit slip. A bit-slip results from one one clock source being faster or slower than the other. How often a bit-slip occurs depends on the timing difference. Digitized voice circuits go unscathed from even very high rates of bit-slips.
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18. Clocking Differences
Analog data circuits without built-in error correction may experience an error. Never the less, digital data circuits feel even the lowest number of bit-slips. Bit-slip can be either an extra bit inserted in the data stream or the omission of an expected bit.
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19. Clocking Differences BIT STUFFING
Reducing the clock difference between telephone company offices is a controlled timing difference. Telephone multiplexes purposely adds bits into the composite stream to adjust for the differences. Bit stuffing is a better name for this process. Stuff bits always appear in the aggregate data stream. If the transmitting end senses a need for a stuff bit, it notifies the other end to use one of the bits.
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20. Clocking Distribution - Error Correction
If data comes in too slow the buffer runs runs low and the pointer will reset at the middle. Data comes in either too fast or too slow If data comes in too fast the buffer overflows & the pointer will reset at the middle
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21. Clocking Differences BUFFERING
Buffering provides a way to reduce the number of slips between two clock sources. It doesn’t eliminate slips, but instead delays when they will happen. The buffer, after a bit slip goes back to the staring position. When the read clock is faster than the write clock, there comes a time when the buffer doesn’t have any space in it or any bits to give. This results in a bit gap in the data output. Bit gaps also become bit slips.
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22. Clocking Differences BUFFERING
If the Write Clock is faster than the Read clock the buffer will fill up, resulting in the buffer emptying, causing a bit slip. Buffers have a major drawback adding delay to the network. Increasing the buffer’s size to extend the interval between bit slips, increasing the delay within the system.
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23. COMPOSITE FREQUENCY SUPPLIES
Clocking Differences
COMPOSITE FREQUENCY SUPPLIES
Network offices have primary frequency generators that phase lock to an in coming master frequency source. The primary operating frequency in any telephone network is 4kHz. Multiples of 4kHz run anything from the individual channel to the very highest facility carrier system. A composite clock supply furnishes both 8kHz and 64kHz. Timing the Digital Signal Equipment (1 digital voice call) needs both the 8kHz and the 64Kbps clock
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24. Clocking Differences CLOCK RECOVERY
At each point in the network the clock is recovered by a Phase Locked Loop circuit. Every device that has a connection to the network must conform to loop timing on Transmit. What this means is that every data stream coming into a customer interface must trigger a PLL circuit. This clock generated by this PPL will loop the input clock back to the transmit side of the interface. This is done to make sure the timing accuracy level of the clock coming into the customer equipment is the same clock that will go back into the network facility.
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25. Network Clocking Repeater Repeater Repeater Repeater Regenerator
T1 Multiplexer
T1 Multiplexer
CPE Equipment
CPE Equipment
CPE Equipment
Regenerator
Phase-Locked Loop
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26. Clocking
ISOCHRONOUS
Isochronous clocking is the type of clocking used in the digital telephony network. The PLL produces a constant clock signal signal using the incoming bits as triggers to maintain the accuracy of the circuit. You can view this circuit as kind of a flywheel. Once the flywheel gets up to speed it will maintain the same RPM with only a small amount of energy to keep it at the same speed. The energy used in the PLL is the incoming pulse.
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27. Clocking
ISOCHRONOUS
If the incoming signal comes in before the circuit expects it the PLL speeds up a little as to adjust the clock output. The opposite occurs when the signal comes in after it is expected. It is necessary to make sure the network has a certain ones density to keep all of the PLL circuits in the network connection at the optimum timing accuracy.
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28. Thank You !
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