The Physical Layer - Systems (Part 1)

When you change the physical layer, it's likely because you're using a different type of channel. If we're communicating in a different type of channel, then your layer 2 has to change because there is the possibility of shared access over channel.

First system is satellite communication, they are effective for broadcast distribution and anywhere/anytime communications. A satellite orbits the earth and acts as a transponder, listens to some portion of the spectrum, amplifies signal and rebroadcasts it at another frequency to avoid interference. There are lots of uses for satellite communication for our daily lives. They do split the frequency spectrum to different ranges. Each satellite might be broadcasting multiple streams on different frequency bands. Listening to some portion, amplifying it and rebroadcasting it. The higher the satellite, the more power needed to communicate. The higher the satellite, the longer it takes to communicate. Howevver, the higher the satellite, the higher the coverage. 

There are 3 type of satellites. The Geostationary are highest with the highest latency with fewest as possible for global coverage. 

MEO is middle earth orbit and LEO is the low earth orbit. The reason GeoStationary Satellites is used for streaming is because you tune a disk and it always reaches out to the satellite. 

The reason GEO satellites is used is because you tune in your dish to a specfic direction and it's always reaching out to the same satellite. Your dish is called VSAT (very small Aperture Terminals) and typically used for receiving signals. They may be able to transmit and communicate with each other at least with some intermediate node. Satellite will recieve channel contents to earth through a hub. Sub sends signal to satellite, satellite agrees to a frequency band, and transfers the frequency band to transfer to VSATS. 

MEOs are not used for data/audio/video communication because they vary their coverage area over time and provides GPS support. Low-Earth Orbit Satellites are mainly used for real time communication. Iridium uses many low-latency satellites for coverage and route communications for them, routing 6 times around the earth. 

With satellites, it's easier to set up and broadcast, as well as code. It can provide services to relatively large regions. However there is limited bandwidth due to congestion. Wireless communication has more interference. For GEO satellites, we rely on frequency division multiplexing. I'm not going to get into the details, etc, it would be waste of time. 

Public telephone networks are old, and we should know how they work. It's the history of the internet. We started with phone networks, that were circuit switching. We progress overtime to packet switching, them moved away from them altogether. The progress covered multiple systems, going from networks to relying on cable. 

The first hub is a local loop connecting the phone to some end office. It connects to the box outside of the house managed by your service provider and multiple boxes will connect to a toll office. It's not really turning a channel to a digital channel. Local look is carrying digital and the end offices are connected with each other with switching offices in the middle, and switching offices are themselves. We pack several conversations together. We pack everything into a unit and we're going to see what these units are shifted towards toll offices. The core system is digitized to enable carrying multiple conversations with each other to appropriate destinations.

We can think of the carries like buses, the bus would carry a sample of your phone conversation, sample with other conversation. These sample would ride the bus and would go to some bus stops. These samples would then help by themselves to destinations to local loops. We have the version of local loops (twister pairs to houses) trunks (fiber optic link carrying calls) and switching offices, routing the code to destinations. 

 Local loops connect the end office to houses, often known as the last mile. Typically 3.3 kHz analog channel interface to the Plain Old Telephone Network (POTS), which is sufficient for a phone conversation. Compare that to the 222 MHz band used by Wi-Fi, we need high enough frequencies so that the antennas can be smaller in size. 

Trunks rely on TDM and FDM, and before multiplexing, you need to digitize the voice. Analog call signals (4000 Hz band) are sampled at a rate of 8,000 samples per second based on the Nyquist theorem). We need to convert our signals to digital form in order to manipulate them. 8000 samples per second is one sample every 125 μs. Each sample that is made on the voice is converted into a byte. This sample reflects a one-byte value. Each of these samples are converted into 8 digit digital values. 

I don't want to wait to sample for a few seconds, we need to shape them as soon as possible. We rely on the trunk transmission rate in order to do that. These samples can be packed together, but they need to be packed wisely. The phone would be digitized where they will ride bus/ride carriers every 125 μs to reach the desired destination. We collect all the symbols and make the bus leave carrying your symbols. With PSTN, we have a circuit being reserved end to end, weather. PSTN (phone) uses circuit switching, where in contrast the internet uses packet switching. Circuit switching decides on the exact pact voice will take then voice goes that direction. 

And here's a table requirement comparison:

Let's talk about phone carriers and buses. We have 2 typical carriers, T carriers and SONET carriers, and they rely on Time Division Multiplexing to place the samples within the carrier. The idea is I have samples from multiple conversations. They both rely on a frame where we can carry so many samples on it. For each carrier, what you place in there are going to be samples from many different conversations. You're sending many bytes every 125 μs. Let's say 1 carrier is carrying 24 samples. So we are not delaying voice in any way. Since  we are sending 24 bytes every 125 μs we are going at 1.544 Mbps. 

TDM allows multiple T1 carriers to by multiplexed into higher order carriers. These are higher order T carriers. T2 stream consists of 4 T1 streams (6.3 Mbps), indicating it can carry more samples every 125μs.

SONET is short for Synchronous Optical Network, where bits are going to be transmitted but they are transmitted synchronously. The base frame is 810 bytes (many which will be voice segments) and a frame. The left few columns are for control and overhead and all in all we are sending multiple samples at a higher rate so all the signals can be delivered on time to their destinations. Also like T1 T2 and T3 with SONET we have different terminology. 

These are carrier signals designed to leave synchronously every 125 μs. For a core perspective we have to use packet switching, and modulate our signals to digital form so core can understand the headers introduced in the packages. There maybe switches in some islands to route appropriately, then I need to do packet switching, route in different networks, so on and so forth. 

Now we want to use modems for data in addition to voice communication. Modem is Modeulator and Demodulator. Codec is coder decoder. We have routers and switches, and the trunks still exist, carrying data digitally. We want to somehow convert our data into a form that the code can understand. This usually relies on coders, decoders, and modems. 

Links  handles high speed transactions. We added some extra elements to the system, like intelligence at end offices, routers, coders, decoders, modems, to new modulation that the code can understand. Now we can send and receive data on a area based on sending and receiving voice, we made analog signals able to communicate digitally.  Modems convert digital data to analog, codec convert analog data to digital.  The only difference is the input which can be either in the analog or the digital form, and also the carrier which is being used is for modulation purpose is always meant to be in the analog form.