CS485 Sylabus

### The problem of clock drift

• Click drift

• Fact:

• No 2 digital clocks will ever run on the same frequency

 E.g., temperature can affect the frequency (= speed) of digital clocks !!!

• 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

• Consequence of clock drift:

• Clock drift can cause reception (decoding) errors if:

 A large number of bits is being decoded

Example:

• Important conclussion:

 The receiver must (from time to time) re-synchronize it clock with the sender's clock

Example:

• When can you re-synchronize the receiver's clock ???

Problem:

• 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:

• 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)

• Wait !!! There is still a 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

• Solution ???

 More complicated transmission encoding !!!