So let's start with layer one so layer one is the physical layer and the job of layer one is to transfer a stream of bits a stream of ones and zeros from one end of a link to the other end of a link so for example it could be a long uh cop cable it could be a long fiber optic or it could even be radio waves across free space but its job is to take ones and zeros and get them to the other end of some kind of physical medium and so at layer one everything to do with how that series of ones and zeros is transferred is defined so what kinds of cables what the connectors are what pins are active what voltages are used and the mechanism that's used to transfer ones and zeros into the physical representation so that's modulation and converting them back again to ones and zeros at the other end which is demodulation so if you're talking about fiber optics as well you'll be talking about what kinds of fiber what the wavelength the lambdas are and so on another important characteristic at layer 1 is the transmission rate in terms of bits per second if you're transmitting at 10 megabits per second or 100 megabits per second that's very much characteristic of layer one but layer one doesn't have any concept about bytes or frames it doesn't have any concept of how to route data it's just trying to send ones and zeros from one end of a link to the other end of a link so what kinds of equipment can you have at layer one well the textbook example of a layer one device is a hub now you won't see hubs anymore you haven't seen them since probably the 1990s unless you're in a museum or something like that but i have one here this is a four port hub it's got uh four rj45 ports on the back and it's got some leds on the front and if you saw this um right now you'd probably think it was a switch um the way i can tell this isn't a switch but it's a hub is it has the word hub printed on it um and it also has an led marked collision so what does a hub actually do well a hub waits for data to appear on one of the ports and when it does it re-transmits the same stream of ones and zeros down all of the other ports simultaneously so it doesn't understand anything about ethernet framing it can tell that the signal is present or not so when a signal a port becomes active all the other ports just get a copy of the same data that was on that first port and it's just as if all the devices were plugged onto the same cable so that means that the total amount of bandwidth that's available in this case 10 megabits this is a 10 meg hub is shared between all the devices only one device can be transmitting at a time and if two devices do happen to try and transmit at the same time and the hub sees two ports active at the same time it can't deal with that that's called a collision and that's what the led on the front is for and in that case it simply blocks the data and the two transmitting devices they have to back off and they have to try again a little bit later so this is a device you're thankfully not going to be using but there are plenty of other real world layer 1 devices that exist so repeaters you will find those on for example long distance submarine networks you'll find them on long distance radio networks where the signal is regenerated ready to go on the next leg of its journey more commonly this is a an sfp a small form factor pluggable and this plugs into an sfp port on a switch and it converts on this side it has a copper interface and on this side this particular sfp has a fiber interface so it's converting a stream of ones and zeros from copper to fiber media converters are another device that you may come across again they take a signal on copper and convert it to fiber and those are again layer one devices they are dumb devices they just retransmit the streams of ones and zeros so that's the fundamental thing about layer one devices they work at the level of individual bits so ones and zeros come in and ones and zeros go out and if the device has more than one port then the data will be sent out all of the ports and so in the case of the ethernet hub that's what you saw so the data may end up in places where it's not needed well that's because a layer 1 device doesn't understand anything about frames or routing so it can't make any decision as to where to send the data to now layer 1 is all about the physical the real world and therefore the real world is not perfect and therefore errors can occur so a transmission error is when a data was sent as a one or a zero but at the other end it was received wrongly the wrong bit was received and so transmission engineers talk about things like the bit error rate which is what proportion of data isn't received correctly and the signal-to-noise ratio which is how much stronger the signal is compared to the background noise these things are related and so obviously you will try and reduce the bit error rate as far as you can to reduce the number of errors on the link but because it's the real world errors always can occur

© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.

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