Connecting to the Internet

 The internet is a vast and diverse place. Devices that connect to the internet fall into familiar silos:

Desktops and Laptops, Servers and Routers, Switches that direct network traffic, etc. Tablets, Cell Phones, ATM, Medical Devices, Cars.

The physical layer is not CAT5 and CAT6 and the data link layer is not entirely ethernet. A big part of your job is making sure people can get online. 

It became obvious there was a big need to connect computers to each other in order to share data. There were computer networks made up of technologies way more primitive. 2 graduate students at Duke University come up with a better way to connect computers. They realized the basic infrastructure existed, the telephone network (PSTN) which is referred to as the Plain Old Telephone Service. These Duke grad students weren't the first ones to think about using a phone line to transmit data, but they were the first to do it in a way that became a permanent precursor to the dial-up networks that follow. They built USENET, and colleges and universities used a permanent form of a dial-up connection, exchanging a series of messages to each other. The connection is established by actually dialing a phone number. The dial-up connection is done through devices called modems, which stands for modulator/demodulator. They take data and turn it into an audible wavelength known as POTS. 

The telephone system was developed to transmit sounds from one place to another, a similar way to how line programming changes charges to signals. A baud rate is a measurement of how many bits can be passed across a phone line in a second. Computers can only send each other data at 110 bits per second, increased to 300 bits per second, and 14.4 kilobits per second in the 1990s (14.4kps). Dial-up is still in some rural areas, but now broadband is now a better way to communicate. 

Broadband has a few definitions, which is any connectivity technology than dial-up internet and is almost always faster. They are links that are always present, the internet potential for businesses and home uses was harnessed. By the mid-1990s, it is common for businesses to use T-carrier technologies, invented by AT&T to transmit multiple phone calls over a single link. T-Carrier is also faster than dial-up connections. 

Home use became more prevalent. In the days of Dial-Up even a single image can take many seconds and high-resolution required a Long time, and the picture would take almost 20 minutes to load, but now it is a lot faster. Without broadband, the internet as we know it wouldn't exist, and couldn't be taking an online course. T-Carrier technologies require dedicated lines, and we usually only see them in use by businesses. There are 4 most common broadband technologies: T Carrier technologies, DSL, Cable Broadband, and Fiber Connections.

T-Carrier technologies provisions a system that allows lots of phone calls to travel across a single cable. Every phone call was made between copper wire before the transmission system. T-carrier technologies were first invented by AT&T in order to provide a system that allows lots of phone calls to travel through a single cable. The phone call was made over pairs of copper wire before transmission system 1, called T1 for short. With T1, AT&T invented a way to carry 24 simultaneous phone calls across a single piece of twisted copper. Later, it was repurposed for data transfers at 64 kps. T1 means any twisted-pair copper connection capable of speeds of 1.544 Mbps, even when doesn't follow the specification. 

T1 connects telecom sites to each other, but with the internet, in the 1990s, more and more businesses started to pay for T1 and more improvements allow multiple T1s to act as a single link. T3 is 28 T1s all multiplexed, achieving a total of 1.544 x 28 = 44.734 Mbps. T-Carrier technologies are surpassed by other broadband technologies. Different fiber technologies have all replaced older copper-based ones.

The public telephone network was a great option for connecting to the internet since it had infrastructure everywhere, and for a long time, dial-up connections were useful. As people wanted faster internet access, telephone companies think about using the same infrastructure but in a different way. Twisted-pair copper operating at a frequency range outside phone call, a technology known as digital subscriber line, or DSL sends much more data vs. dial-up and allows for voice phone calls and data transfer at the same line. They use DSLAM or digital subscriber access multiplexers, and these devices establish data connections across phone lines, and the connection is established when the DSLAM is on and isn't torn down until the DSLAM is off. There's a lot of DSLs available. The 2 most common types for a long time were ADSL (Asymmetric Digital Subscriber Line). ADSL are different speeds for outcoming and incoming data. Home users rarely need to upload as much data as they download. 

Webpage the upload or outbound data is pretty small but the download is much larger because it will contain the entire webpage. Asymmetric lines help provide this experience for a typical home user, but at a lower cost. SDSL is a Symmetric Digital Subscriber line, which is the same as ADSL, except the upload and the download speeds are the same. They were used by businesses that hosted servers that needed to send data to clients. SDSL is now more common for businesses and home users. Now it can run at 1.544 MPs. SDLS yielded HDSL (High Bit-Rate Digital Subscriber Lines), which provision speeds above 1.544 MPs. There are lots of other minor variations out in the wild, and these variations are super numerous, and contacting the ISP will provide you with enough details about the DSL line.

This history of both the telephone and computer networking say all communications are wireless, everything were going to be wireless. Initially, all televisions were wireless. In the late 1940s, the first cable television technologies were developed. In 1984, the Cable Communications Policy, deregulated cable television in the USA. Cable providers started trying to figure out if they could on in on the massive spike of the internet growth. The coaxial cables were capable of transmitting tons of data, and high speed internet access can be delivered on the same cable with different frequencies, called "cable broadband". 

Cable is a shared bandwidth technology. The connection from home or business goes to the central office or CO using a switch port to connect caller to callee. Then it became more automatic over time. Now technologies connected to CO can guarantee bandwidth. However at the time, we can only share a bandwidth, depending on cable wired for cable. Most cable operators try to update that end users don't see bandwidth decrease. Cable internet providers work with a cable modem. This sits at the edge of the consumer network and connects it with a cable modem termination system or CMTS. This connects cable connections to an ISP's core network. 

The core of the internet has used fiber for its connection, due to higher speed and further transmission. The maximum distance an electrical signal can travel across a copper cable before the signal degrades is thousands of feet. For a long time, it was a technology used by ISPs with core networks, but in recent years, it became much more popular to use Fiber. FTTX stands for Fiber To The X. The first team you might hear is FTTN which means Fiber To The Neighborhood, which delivers technology to a single physical environment. From this cabinet, coax might be used for the last distance.

FTTB is fiber to the building/business/basement, where cables to building enter. This is then Fiber technologies are used for data delivery for an individual building. A third version is FTTH, which is Fiber To The Home, where Fiber is run to each vistor or neighborhood in an apartment building. FTTH and FTTB may both also be referred to as Fiber to the premises. The demarcation point of fiber technologies is the Optical Network Terminator (ONT), which converts data from protocols that the fiber network to understand, to those that twisted pair copper networks can understand.

Let's say you're in charge of a network. At first, the business only has a few employees with a few computers in office. You use Non-routable address space and configure DHCP, Local DHL server and sign a contract with an ISP to deliver a link to the internet. Imagine the company grows. Maybe some people need to access the LAN, so you configure a  VPN server and make sure it is accessible via port forwarding. The CEO decides it's time to open a new office. This is where Wide Area Networks come to plan. WAN acts like a single network, but span across multiple physical locations, contacting a link with the ISP. A typical WAN has a Demarcation point with network of computers on some side of the country. Each network ends at a demarcation point (ISP network takes over). The area between each point is called a local loop, to the providers local regional office. 

WANs use a number of different protocols at a data link layer. The details of these protocols are out of the scope of this article. 

A popular alternative is point-to-point VPNs. WAN technologies can transfer large amounts of data to lots of sites. Over the last few years, companies have been moving more and more of their internal sources into the cloud. In the past, a company has to run its own email server. Now you can have an email in the cloud, or use email as a service provider, paying another cloud company. Companies can now use point-to-point VPNs to make sure the site can connect, and it establishes a VPN tunnel between 2 sites.

The VPN tunneling logic is handled by network devices on either side.  With so many portable computing devices, we also see the rise of wireless networking, which is a way to network without wires. The most common specifications are defined by the IEEE standards or the 802.11 families, making up WiFi. These networking devices communicate with each other through radio waves. A frequency band is a section of a band that is agreed upon for communications. FM radio operates between 88 and 108 Hz. There are lots of 802.11 specifications, for example 802.11b, 802.11a, 802.11g, 802.11n, and 802.11ac. 

You should think of 802.11 as defining how we operate at both the physical and data link layers. There's some fields. The first is the frame control field, 16 bits long describing how the frame itself should be processed. The next is the duration field, how long the total frame is to see how long the total frame is so the receiver knows how long to expect to listen to the transmission. The most common setup includes access points. The wireless access point bridges the wireless and wired version of a network. Devices with wireless networks associates with a certain access point. It can be determined by general signal strength and wireless interference. Associations isn't just important for wireless transmissions, it also allows for incoming transmissions to the wireless device as the same as the access point.

There are 4 address fields, so there needs to be room to see the access point to the frame. We also have the normal address, with the intended destination on the network, with a receiving and transmitter address. The receiver is the MAC address that should receive the access point of the frame, and the transmitter address is the MAC address of what has transmitted the frame. 

The destination/receiver address might often be the same. The source and transmitter address might also be the same. Sometimes, WANs might relay these frames to one another. Since all addresses are MC addresses, each of those fields is 6 bytes long. The sequence control field helps keep track of ordering the frame. Then, there's the data payload section, the data of the protocols on the stack, and the Frame Check Sequence field, for a cyclical redundancy check.

There are a few main ways that a wireless network can connect to each other.

1. Ad-hoc network - Nodes all speak directly to each other

2. WLAN - one or more access points acts as a bridge between wireless and wired networks.

3. Mesh networks - Hybrid between the two. 

In an Ad-hoc, there isn't really a network infrastructure, except that every device communicates with others within its proximity. Some smartphones can establish Ad-Hoc so people can exchange photos, videos, and contact information. 

They can be powerful tools during disaster situations, and professionals can use the network for search-and-rescue efforts. 

A wireless LAN (WLAN) contains one or more access points that act as bridges between wireless and wired networks. Wired networks operate as a normal LAN. The wired LAN contains the outbound internet link. Wireless devices communicate with access points and forward traffic to the gateway router.

Mesh networks are like Ad-Hoc networks, forming a mesh if you were to draw lines between all the nodes. This kind of network lets you deploy more access points to mesh without having to run a cable, helping the performance and range of a wireless network.

The concept of channels is important, and they are individual smaller sections of the overall frequency band used by the wireless network, and help address a collision domain, a segment where one computer can overlap another. Overlapping communications can't be properly understood by the receiving end, and all devices in question have to stop their transitions. The problem in collision domains is in switches, remembering which computers live on which interfaces, so traffic is only sent to the node it is intended for. Wireless networking doesn't have cable, so we can't have something that works as a wireless switch. Channels fixed the problem of wireless networks talking to each other to a certain extent. 

FM radio operates between 88MHz and 108MHz. Wireless networks operate on 2.4GHz, this roughly means that the network operate between 2.4GHz and 2.5GHz. Exactly how many channels are available for use depends on where in the world you are. How many channels are available.

For example, the signal of a channel operates between 24.01MHz and 24.03 MHz. Some channels overlap, but some are far enough so they won't interfere at all. We should find all the networks that don't overlap. The equipment now auto-sense which channels are overcrowded, and you can run into situations where you experience heavy channel congestion.

Understanding overlap is how you help troubleshoot bad wireless connectivity problems. You want to avoid collision domains wherever you can. Don't memorize the numbers, but understand how collision domains are a necessary problem with all wireless networks.

The only devices that only know what data is being transmitted are the 2 nodes on either end of the link. Anyone within range can hypothetically intercept any transmitted. WEP (Wired Equivalent Privacy) is an encryption technology for low-level privacy. It should really only be seen as unencrypted data. 

It doesn't take very long for a bad actor to get through this data. The number of bits in an encryption key corresponds to how secure it is. WEP only uses 40 bits, which isn't sufficient. WEP then was replaced with WPA or Wifi-Protected access which uses a 128-bit key. The most commonly used network is WPA2 using a 256-bit key. Another common way is through MAC filtering, allowing connections only from a specific set of MAC addresses, preventing unauthorized devices from connecting to the wireless network.

Cellular, or Mobile Networking, is another popular form of Wireless Networking. Cellular Networks are common all over the world, using a cellular network is the most common way.  One of the biggest differences is that these frequencies can travel over longer distances more easily. 

Cellular networks are built around cells, each cell having a different frequency band. The Cell Towers Broadcasting and Receiving transmissions can be thought of as access points. It's become more and more common to have automobiles to have built-in cellular access.