DCTI Duty Cycle Timing Information

1
DCTIDuty Cycle Timing Information

• A synchronous method for the clock recovery in
  the transport of Constant bit rate services over
  ATM

2
DCTI the idea

• Lets consider the XOR (y) between the network
  clock divided by an integer D (cd) and the user
  clock divided by an integer Ds (cds).
• The y signal is a sort of PWM and the modulating
  signal dc (y duty cycle) contains the information
  about the relation between the 2 clocks.
• The problem is how to extract such info, how to
  code it and how to use it at the receiver in
  order to recovery the user clock.

3
DCTI the idea

• Lets assume that both at the transmitter and at
  the receiver the signal dc and dc are available
• Lets define ti the temporal distance between the
  i-th sample of dc and and the i-th sample of dc.
  
• The 2 user clocks will be locked if ti ti1 for
  each i gt io.
• Our goal is to get a close forme expression
  involving just the variable dc, dc and ti. In
  this way we could control a VCXO (Voltage
  Controlled Quarz Oscillator) keeping ti constant.

4
DCTI the idea

• Making a few computations we can easily
  obtain the following results
• So we can define a new function dcT at the
  transmitter and its homologous dcR at the
  receiver

5
DCTI the idea

• Lets consider at the receiver the difference
  function dcT-dcR, which has got the following
  mathematical expression
• This function has got the following graphical
  rapresentation

As you can see it still depends from the
parameter tT
6
DCTI the idea

• But if we finally consider the following
  function
• This function has got the following graphical
  rapresentation

It is constant respect to tT and this is the
reason why it can be used for our controlling
porpose as following fi fi-1 k (xi
xi-1) Where fi is the output frequency of the
VCXO related to the i-th DCTI sample received
7
DCTI Implementation

• Not considering the quantization process and a
  few other mathematical considerations
• here following an high level overview of the
  system

8
DCTI DCTI Generator

• This is the blocks shema of the DCTI Transmitter
• Which implements the DCTI signal and samples it
  each N fs cycles

9
DCTI Control Parameter Generator
This is the blocks shema of the Control Parameter
generator
10
DCTI Oscillator System

• This is the blocks schema of the Oscillator
  System
• The clipper has been introduced in order to avoid
  instability condition.
• Note The stability study of the system and
  others mathematical considerations are not
  matters of this persentation. For more detailed
  information please refer to my thesis (Un nuovo
  sistema di recupero del clock per segnali a bit
  rate costante in ambiente ATM, Università degli
  Studi di Palermo, DIE, Laureando Salvatore
  Valenza Relatore Prof. Matteo Campanella, 1998)
  or contact me at SalvoValenza_at_msn.com

11
DCTI vs SRTS
Long term oscillation
Locking time
12
DCTI vs SRTS
Frenquency variation during the locking time
Long time average error
13
DCTI Advantages

• Advantages with repect to the ITU raccomended
  method SRTS
• The locking time is between the 50 and the 82
  with respect to the SRTS one
• The Long term oscillation is between 25 and
  55 with respect to the SRTS one
• The frequency variation is between 99.4 and
  99.8 with respect to the SRTS one
• The DCTI is almost unaffected by any cell loss
  because of the limited variations of the
  recovered frequency
• The overhead of the timing information is 25
  less with respect to the SRTS one