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• So let's say we have two computers here now

• We want to send some data from one computer to the other so if we've seen before

• We can connect the cable between them and then maybe vary the voltage

• across two of the wires within this cable like say between zero volts and 5 volts

• [in] zero volts is a symbol that represents a zero and [five] volts as a symbol that represents a one and so we can send

• Some binary Data this way zero one one zero one

• But something interesting is happening right here. Which is we have a bunch of zeros in a [row] and

• In order to know exactly how many zeros we have here?

• We need we need to make sure that both of these computers have a synchronized clock and this is something that's actually very important in

• Networking is is timing?

• To make sure that both [sides] of these of this link agree on the clock rate

• So if I add a clock rate here, this is a signal basically that just alternates between zero and one zero one zero one

• And what we can do is we can look at this clock

• Everywhere it transitions from Zero to one

• That is a point at which. [we] should read the value of the data. So we have a data signal here

• we have a clock signal here and

• so for this example here

• we have this whole string of zeros if

• We just look at how many transitions are exactly when these transitions occur from 0 to 1 on the [clock] we can see

• this transition

• The Data is a 1 this transition. It's a 0 this [transitions] to another 0 this transitions another 0

• another 0

• Another [0] and then a 1 [as] long as we keep track of these transitions. We know exactly how [many]

• Zeros we have no matter how long this stretch of Data is as long as we have this clock

• But [the] important thing is that both sides of this link agree on the same clock so that the computer that is

• Sending this data is using the same clock rate as the computer. That's receiving it

• So now you might [be] [wondering] what happens if these took two computers clock signals aren't running at exactly the same speed

• so like maybe maybe the receivers computer is a little bit slow so [I] can show you actually what [that] might look like if we

• Take that [out] and bring this in and then actually get rid of these little

• transitions, so this here is a clock that is running a little bit slower than

• Then the Data was originally clocked at and so what you see [here]. Is [that] you see

• Here's a transition, and we get a 0 another transition to 1 and these are these are actually lining up pretty well, so far

• So this 0 kind of is lining up

• but here you notice we missed a zero there should be a zero here and

• Then this stuff lines up pretty well

• and again we miss a zero and then

• [Things] are lining up [ok]

• But here in this stretch where we had 5 zeros before now with a slightly slower clock

• We're only reading four zeros, and then we have this last one here

• And so the places where we're missing bits because of this mismatch

• [I] guess between transmit and receive clocks are called clock slips

• So this is a clock slip because we're basically missing a bit of information

• And then you might have the sort of opposite of this if the receiving clock were running faster

• You might actually get extra bits in there

• But either way, we would refer to that as a clocks flip so this here is a clock slip

• and

• Of course if you know if we're missing bits like this then the data that we're receiving

• Isn't going to make any sense so so clearly we don't want the two computers clocks to be out of sync like this

• So even if they're a little bit out [of] sync

• [you] [know] of course this is kind of an extreme example here where you know we were sending 16 bits

• And we only received 13 bits

• but even if even if these clocks are just a little bit out of sync and

• Eventually over time we will have a clocks flip and that will corrupt our message

• And we would have to either we transmit our message or somehow

• figure out, what happened there, so

• It's important to make sure that these clocks are in sync. And there are a couple ways [that] we can do that

• One way that we can make sure the clocks are in sync is actually for both of these computers [to] [to] have synchronized clocks

• That are either synchronized, maybe through GPS

• [Antennae]

• So these computers would have little antennas that would connect or that would actually receive signals from GPS satellites which are the global positioning?

• System Satellites because the GPS satellites actually have atomic clocks onboard and have very very accurate clocks

• And so if these computers have those GPS receivers they can synchronize their own clocks to the GPS clocks?

• And then know that that the clocks between the two computers are in sync and and use those clocks for sending and receiving data

• There's obviously some disadvantages to that the GPS antenna is extra hardware, so it's a little bit a little bit expensive

• [and] you also need to be [able] to mount the antenna somewhere outside or on the roof of the [building] or something like [that]

• But there there's definitely some network equipment that is in use on the internet that uses GPS timing to synchronize

• To synchronize its clock and data

• so that is that is one solution another solution is actually to have an atomic clock in the computer itself and that's

• It you know occasionally that is done. It's fairly fairly uncommon

• Another approach that you could take is we get rid of our slow clock and bring back our

• Normal clock here another approach that you can take is to actually send a separate signal so we have a separate

• We have like another

• Another link or another pair [of] wires between these computers where we send this clock signal so we're sending both the data and the clock

• across two different links and so that way this computer doesn't need to use its own clock it can actually receive the clock from the

• Same computer that's sending the data. So it knows that these are in sync

• [and] there's actually a little bit [of] a problem with that as well or potential problem [with] that

• Which is that as as you increase [the] speed that you're sending this data?

• These clock pulses could could actually be as close as just a few nanoseconds apart

• And so it's very important that the clock and the data line up

• Because if you can imagine this shifted just a little bit to the left or to the right?

• [then] these these points where the clock transitions from a 0 to a 1 might not line [up] exactly with the bits

• and you could miss read a bit, so it's very important that these stay lined up correctly and

• And that's called clock Phase

• Phase and one of the problems with sending a clock and a data across two separate links is that it's possible that

• The you know the propagation of electrons literally across one of these links might be slightly slower

• Either because it takes a slightly [different] path or the conductivity is a little bit different. If this is a long path, so

• You tend to not [want] to do this on very high speed links over very long distances

• Or you could get into an issue where the clock gets slightly out of phase with the data and you start to miss read?

• some bits

• So another approach that we can take that's actually quite common is is kind of ingenious which is to combine?

• the clock and the Data by using different symbols to represent ones and zeros, so

• Just review what we've been doing here is

• When we're transmitting our data?

• We're transmitting it using two symbols

• And so when we want to send we want to send a one the symbol that we're using is five volts

• So you can see here every time we're sending a one our symbol is that we are

• Setting the voltage to five volts when we want to send a zero the symbol we use is zero volts

• So now let's see what happens when we change that so instead of making well instead

• Let's try to make the the symbol for sending a 1 instead of making it 5 volts. Let's make it

• Actually the symbol will be transitioning from [0] volts to 5 volts. This is 0 volts this is 5 volts and

• the symbol for a 1 is a transition from [0] to 5 and

• Then what we can do is the symbol for sending a 0 will be a transition from 5 volts to [0] volts?

• So this was 5 volts and we're transitioning to 0 volts so before the symbol that we were using

• was

• The symbol we were using for sending a one is a just a five volt signal

• Now let's try sending a transition from zero volts to five volts as the symbol

• So what we can do is here. Is that same signal right here?

• That we're sending up here me so we can see both of these

• So this is the same the same data the zero one zero zero one zero and so on

• Is now being sent?

• Using this scheme so for example

• Here we have a transition from five volts to zero volts, so this is five volts

• oops

• this is [a] transition from five volts to zero volts and

• So this is a zero and here we have a transition from Zero to five so that's a one here

• we have another transition from five volts to zero volts so that transition represents a zero and

• This is again a transition from five volts to zero volts so that represents a zero

• this represents a 1 0 a 1 a 1

• 0 a 1 and then here you can again see those transitions from 5 volts to 0 volts as a 0 0

• 0 0 [0] so very clearly there's five of those transitions, so we know there are [five] [zeroes]

• and then finally we have that transition from zero volts to 5 volts, and that's a 1

• So you might be wondering?

• You know what's going on like here for example

• That you were transitioning from 0 volts to 5 volts, so shouldn't we should we like count this as a 1 in here

• And so actually we shouldn't because there's still a clock and we still expect to see each bit at a regular time interval

• [so] for example you can see there's there's still like a a regular interval here where these bits are occurring

• and

• And the receiver can easily tell that that this

• This short interval here is is drastically different than the regular interval that we see everywhere else

• And so the receiver can can just ignore this

• Because it because it doesn't match the the symbol rate you know even if the receivers clock isn't completely perfect

• So this method of encoding that I've described here is called Manchester

• [Manchester] coding

• And I believe it's named after the I think it was

• s invented at the University of Manchester in

• the [uk] and so Manchester coding is is just an example of one of the simpler ways of

• Combining clock and data into one signal so that the transmitter and receiver don't need perfectly synchronized clocks

• And so Manchester coding also happens to be the type of line coding used by lower speed Ethernet which many many

• Computers use for connecting to wired networks so in the next video

• We'll look at exactly how ethernet uses Manchester coding in some more detail

So let's say we have two computers here now

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Clock synchronization and Manchester coding | Networking tutorial (3 of 13)

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林宜悉 posted on 2020/03/27
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