- Fact:
- No 2 digital clocks
will ever run
on the same frequency
- E.g., temperature can affect the frequency (= speed) of digital clocks !!!
- No 2 digital clocks
will ever run
on the same frequency
- Fact of (normal) life:
- Two "identical" clock signals (used in computer communication) will run at slightly different speeds
- The difference
in clock speed
is called:
- "drift".
- Consequence of clock drift:
- Clock drift
can cause
reception (decoding) errors
if:
- A large number of bits is being decoded
Example:
- Clock drift
can cause
reception (decoding) errors
if:
- Important conclussion:
- The receiver must (from time to time) re-synchronize it clock with the sender's clock
Example:
- The receiver can
not
"see" the
sender's clock:
- The receiver can
only
"see" the
transmission from the
sender:
- This is
what the receiver will
"see":
- When
can the receiver
perform
a
clock (re-)synchronization ???
I.e.:
- Which moments can the receiver know for sure what the state of the sender's clock is ???
- $64,000 question:
- Which moment(s) in time can you know for sure that the sender's clock is at the beginning of a clock period ???
Pictorially:
Answer:
- Receiver can
re-synchronize its
clock
when the received signal
makes the
following transition:
- from 0 → 1 or
- from 1 → 0:
Graphically:
Example:
- General practice:
- Each time when the
received signal
makes a transmission:
- The receiver clock is re-synchronized !!!
(Re-synchronize means: force the clock signal to restart at the start of a period)
- Each time when the
received signal
makes a transmission:
Problem:
|
|
- There is still
a problem:
- If the sender transmits
a long
serie of 0's or 1's:
the receiver can not perform re-synchronize
- If the sender transmits
a long
serie of 0's or 1's:
- Solution ???
- More complicated transmission encoding !!!