This is my documentation of Week 5, Course II in the Google Professional IT Certification Program from coursera.org.
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Introduction to Connecting to the Internet
This week we’re connecting to the internet. That should really help me do better in this online class. Okay, here we go.
This week we’re still watching what’s his face. The guy from the last couple weeks. He’s doing a pretty good job but I wish the teleprompter was aligned a little closer to the camera. The weird line-of-sight or whatever is still pretty distracting.
The video starts off with one of these fun statements like “The internet is a vast place with lots of devices.” We are to be aware that not only are laptops and desktops connected to the internet, there are millions of other things like phones, switches, industrial and medical devices, even automobiles.
All of these different devices have some pretty unique ways of connecting to each other. We’re going to learn about connectivity for different devices, along with WANs and cellular networks.
POTS and Dial-Up
Dial-Up, Modems, and Point-to Point Protocols
Before Ethernet and TCP computers were connected to each other to share data. These were machines that were relatively close together, and the way they connected reflected that.
But there was a growing need to connect machines at greater distances. How would this be accomplished? The phone lines!
The Public Switched Telephone Network (PSTN) is what we may also call the plain old telephone service, or POTS. By the late 1970s the phone system was a large, global and robust network, and some Duke University grad students thought it might be the best way to connect computers. They were not the first to use phone lines for data transmission, but they built the first precursor to a dial-up network known as USENET.
A dial-up connection is one that uses POTS for data transfer, and you actually had to dial a phone number to connect computers. Data was transferred using a device called a modem, or modulator/demodulator, which takes a data-stream from a computer and turns it into audible wavelengths that can be transmitted over POTS. That’s crazy!
Early modems were very slow. This transmission speed was measured in Baud Rate, which is how many bits can be passed over a phone line in one second. In the 1950’s this was about 110 bps; by the time USENET was developed this was around 300 bps; and once consumer-grade dial-up internet was available in the 90’s this had increased to 14.4 kbps.
While it is possible to have a career in IT these days without encountering dial-up, it is important to know that it is still out there, especially in rural areas.
What is Broadband?
When the term broadband is used in terms of internet connectivity, what is meant, according to old video guy here, is any connection type that isn’t dial-up internet. Broadband is always faster than dial-up (unless you have Charter amiright!!!?) and is a connection that is always “on.”
While the internet was always an important invention, its full potential for business and personal use was not fully established until broadband technologies were developed and deployed.
Before widespread broadband was available, it was common in the 1990’s for medium- to large-sized businesses to use what are known as T-carrier technologies, which were invented by AT&T and allowed multiple phone calls to be made over one link. These were then developed into transmission systems for data and the internet.
As the web grew in size and complexity, broadband service allowed for even further growth and complexity. We are presented with this comparison:
One photo from a smartphone is 2MB.
2MB = 16,777,216 bits
16,777,216 bits at 14.4 kbit/s = 1165 seconds
1165 seconds = 19.4 minutes
That is an insanely long time to spend on the garbage so many of use spend time looking at on the internet.
While T-carrier connections require dedicated lines and are therefore more expensive, other broadband technologies exist that are available to many consumers—cable, fiber, DSL.. We’ll look at those technologies in this section.
AT&T developed T-carrier technologies to have multiple phone lines function over a single wire. Before Transmission System 1 (T1) a single twisted pair of copper wires could carry 1 phone call. T1 allowed 24 simultaneous calls over a single twisted pair copper wire. Later, this same system was adopted for data, with 64 kilobits per second per 24 channels, meaning a single T1 line could transmit 1.544 megabits/second.
The designation “T1” has come to mean any twisted pair copper wire connection capable of carrying 1.544 megabits/second.
T1 was originally used to connect telecoms to their own facilities and to each other. But rise of the internet and the demands of the business world for better connections saw businesses pay for T1 connections.
T1 was improved further and a system was developed whereby multiple T1 lines could act as a single link. This 28xT1 multiplexed connection is known as a T3 line, which was capable of 44.736 megabits/second.
T-carrier connections are still in use today, but have mostly been surpassed by other broadband technologies.
Digital Subscriber Lines
Because the telephone infrastructure was already built, dial-up connections were a great way to get people online. But the dial-up technologies were limited. Telephone companies were eager to increase data transmission capacity and speed in order to satisfy the demands of businesses.
Digital Subscriber Line (DSL) allowed the same twisted pair copper wire that was used for voice calling to simultaneously carry data traffic, and at a faster rate than the audio signals carried over phone lines using dial-up modems. A DSL connection uses a device called a Digital Subscriber Line Access Multiplexer, or DSLAM. This device will typically make a connection when it is powered on and maintain that connection until it is powered off.
The two most common types of DSL are “Asymmetric” (ADSL) which have different download and upload speeds (faster download, slower upload). This is appropriate for home users, because they are generally just client machines. The home user just needs to send (upload) the packets requesting a web page, and then receive (download) the large amount of data that comprises the web page. ADSL is a practical system for the typical home user.
SDSL stands for Symmetric Digital Subscriber Line, which means that the upload and download speeds are the same. This is more appropriate for use by businesses that host servers that need to send data to clients. SDSL is now more common for home users as well as businesses. Most SDSL technologies have a data transmission cap of 1.544 megabits per second, the same as a T1 line.
There have been further developments, including HDSL, which is “High-Bitrate” DSL, and can supply speeds above 1.544 megabits per second.
There are also many minor varieties of DSL, but we are, thankfully, going to skip over those.
The video guy tells me to contact my ISP for more details. Great.
Telephone and computer networking all began as wired transmissions. That is all changing, as many devices now use wireless systems. Television began as a wireless service that became wired with the widespread adoption of cable television service.
Cable television began as early as the 1940’s to provide access to television in rural areas that were outside the range of broadcast towers. Cable slowly grew in popularity until 1984, when cable TV was deregulated with the passage of the Cable Communications Policy Act in 1984. This resulted in a cable TV boom that was soon replicated around the world. It was only natural that cable companies would want to get in on the new and promising market for internet service.
It was quickly discovered that the coaxial cables used for cable TV service can carry much more data than what was required for simple TV viewing. By using different frequencies a high-speed internet connection can be run over the same cable as TV service.
Cable, unlike other broadband technologies, is what is known as a shared bandwidth technology. DSL connections are made through a central office, or CO, which used to be an actual office of the phone company where calls were manually routed through switchboards by operators. These were eventually automated but the name remained.
A technology that connects through a CO can guarantee a certain bandwidth, because it is a point-to-point connection.
A cable broadband connection, with its shared bandwidth connection model, means that many users share a certain amount of bandwidth until the connection reaches the ISP’s core network. This could mean that one city block or a whole neighborhood could be sharing bandwidth, depending on how that area was originally wired for cable service.
Cable providers often try to mitigate the slowdowns that this can cause, but it is not uncommon for cable broadband to slow down at peak usage times.
A cable modem is used to provide cable broadband. This modem sits at the edge of a subscriber’s network and connects the user’s private network with the Cable Modem Termination System (CMTS), which is how many different cable connections are made to the core network of an ISP.
Fiber has to be the coolest transmission protocol. It is light travelling down through glass strands! That’s so cool!
Fiber has long been the transmission technology of the core of the internet due to its excellent speed. An electrical signal travelling over a copper wire will degrade (and require a repeater to maintain the signal) is only thousands of feet, while a signal over fiber optic lines can travel several miles.
Fiber is very expensive, so for a long time it was only used in core networks. Now it is coming closer and closer to consumers.
Fiber to the X (FTTX) is used to describe how far fiber is implemented, with X standing for many variables.
FTTN means fiber to the neighborhood, where fiber has been used to a single cabinet that serves an entire neighborhood. The remaining distance to the user is covered with copper or coaxial cable.
FTTB means to the building, business, or basement, and the final distance to the user is usually made with twisted pair copper.
FTTH means fiber to the home, and is exactly what it sounds like. Both FTTH and FTTB can be considered FTTP, or fiber to the premises.
Instead of using a modem a fiber network will have at the demarcation point a device called an optical network terminator (ONT). This device converts data from the fiber network protocols to those that are used by standard twisted pair copper networks.
Broadband Protocols (readings)
There are two readings here on the subject of broadband connection protocols. These will probably not be common in day-to-day IT work, but here they are anyway. The first is about point-to-point protocol¸ or PPP, and the second is about point-to-point over Ethernet, or PPPoE.
Wide Area Network Technologies
A wide are network (WAN) is similar to a LAN. It is a single network but it is composed of nodes in multiple physical locations. A WAN will usually involve a link from an ISP across the internet to other locations.
Most WANs are structured so that a one physical LAN ends at a demarcation point where the ISP manages the connection to the other location’s demarcation point. The space between the demarcation point and the ISP’s core network is called the local loop. This local loop is usually a T-carrier line or a fiber connection to the ISP’s core network connection and the internet.
WANs work using different data link layer protocols to move data from one site to another. These are often the same protocols running the core of the internet, as opposed to the Ethernet protocols we are familiar with. We can expect further reading on WAN protocols.
Supplemental reading: WAN Protocols
And here they are… Some WAN protocols, in case you want to become an ISP network engineer:
A point-to-point VPN is a popular alternative to a WAN. A WAN is a great way to move large amounts of data across sites, but they are also expensive to build. DSL or cable may be cheaper, but they often can’t provide the bandwidth needed by many organizations.
But as many companies move previously in-house services into the cloud, there is often no longer a need for a big expensive WAN setup. If email hosting, web hosting, and other essential services are all handled remotely by another company, using a point-to-point or site-to-site VPN tunnel to connect sites is a practical alternative.
A point-to-point VPN works much like a traditional VPN, except that the connection logic is handled by devices at either end of the tunnel so that a user does not have to establish their own connections.
Introduction to Wireless Networking Technologies
The modern world has lots of wireless devices now, which means there are lots of wireless networking protocols and technologies.
The most common wireless networking specs are defined by the IEEE 802.11 standards. This is known as the “802.11 family,” and it is what you are talking about when you talk about WiFi.
Wireless networking uses radio waves operating at different frequency bands to avoid interference with other devices and networks. A frequency band is a section of the radio spectrum that has been allocated for use by certain communications. For example, FM radio uses between 88 and 108 MHz. This is called the FM broadcast band.
WiFi commonly uses the 2.4 gigahertz and 5 gigahertz bands. There are many 802.11 specifications, but the most common are 802.11b, 802.11a, 802.11g, 802.11n, and 802.11ac. We will not go over these now, but they are listed here in the order that they were developed and adopted.
802.11 protocols define how things operate at the physical and data link layers.
An 802.11 frame has many fields:
- Frame Control Field: 16 bits; contains sub-fields that describe how the frame is to be processed.
- Duration/ID field: describes how long the total frame is so that the receiving devices knows how long to listen.
[There follow here four address fields because the Wireless Access Point in use will have to be defined. The WAP is a device that connects the wired and wireless sections of a network.]
- Source Address Field: 6 bytes; This is the MAC address of the sender
- Destination Address field: 6 bytes; the “intended destination on the network.”
- Receiving Address field: 6 bytes; The MAC address of the access point that should receive the frame.
- Sequence Control Field: 16 bits; “mainly contains” a sequence number to manage ordering the frames.
- Transmitter Address field: 6 bytes; the MAC address of whichever device has just transmitted the frame.
- Data Payload field: all of the data for the protocols further up the stack.
- Frame Check Sequence field: a checksum used in a cyclical redundancy check (just like how Ethernet does it.)
(This address stuff is not explained clearly in the video but I think it makes sense after re-watching five or so times.)
The destination and receiver addresses are often the same, and the source and transmitter are also often the same. This depends on the network architecture.
Supplemental reading: Alphabet Soup
My friends over at Wikipedia have put together this reading on all the different 802.11 specifications.
Wireless Network Configurations
Here are some common ways that wireless networks can be configured:
- Ad-hoc network: Nodes communicate directly with each other
- Wireless LAN (WLAN): One or more access points serve to connect wired and wireless networks
- Mesh Networks: Considered “kind of” a hybrid of the two.
In an ad-hoc networks there is no supporting infrastructure. Each device communicates with each other device. This may be the most simple network configuration, but it is not that common. Some smartphones and apps can create ad-hoc network connections to exchange data between phones. There are also industrial situations where pieces of equipment may need to communicate with each other but not with anything else. Ad-hoc networks are also used in disaster situations where the original network infrastructure was destroyed or damaged.
Wireless LAN configuration is the most common in the business world. WLAN networks are made up of access points which allow transmission between the wired and wireless networks. The wired section acts like a regular LAN, and contains the outbound internet link. In order to connect to devices outside the wireless network, wireless devices connect to access points, which connect to the gateway on the wired part of the network.
Mesh networks are somewhat similar to ad-hoc networks because many devices communicate wirelessly through many access points. Most mesh networks are composed of multiple access points, which are eventually connected to a wired network or gateway. Using a mesh network allows you to deploy many access points without having to run cable to each of them.
Channels are a very important concept in wireless networking. A channel is a smaller part of the frequency band used in a wireless network. Channels help eliminate the age-old networking problem of collision domains, where a network segment features devices that can interrupt each other. Overlapping transmissions—collisions—occur when two devices send data at the same time. They then both stop, wait, and try again. This slows the network down significantly. Switches alleviate this on wired networks by remembering which devices are connected to which physical interfaces, so traffic is only sent to the correct node.
But wireless devices don’t have interfaces they plug into. All those devices sending data at the same time could be a disaster for the network. Channels help solve this problem.
When we say that a wireless network is on the “2.4 gHz band,” what we mean is that the network is operating between 2.4 to 2.5 gHz. In this space there are several channels, operating within that 2.4 to 2.5 gHz range, each with their own width, in megahertz.
Wireless channels will overlap, so in an 802.11b network, for example, operating on 2.4 gHz, the only non-overlapping channels are 1, 6, and 11.
Many wireless access points and routers will sense channel use and select the least-used channels for itself. This is especially important in places with dense wireless communications.
Avoid collision domains! They are a necessary problem with wireless access points.
Sending data over a wired link implies a certain level of security and privacy. At the very least, access to one of the devices or the actual cable would be necessary to compromise the security of that data.
With wireless communications the signals are broadcast, making security much easier to compromise. Anyone within range could potentially be able to compromise those communications.
To mitigate this weakness WEP was developed. WEP (wired equivalent privacy) is a low-level encryption that protects your data just a little bit. It is weak, but it is better than nothing.
The number of bits in an encryption key directly corresponds to how secure that encryption is. The more bits, the longer it takes to crack. WEP is a 40-bit encryption, which, nowadays, can be cracked in minutes.
WEP has been largely replaced by with WiFi Protected Access, or WPA encryption. This is a 128-bit key encryption. This has been updated with WPA2, which uses a 256-bit key.
Another way of protecting a wireless network is with MAC filtering¸ where access points are configured to only allow connections with specific MAC addresses from devices you know and trust. This doesn’t do anything to secure the transmissions going out over the air, but it does help protect the network itself.
Cellular networks are widely used around the world, and conceptually have a lot in common with 802.11 networks.
There are many cellular specifications just like 802.11. They also operate using radio frequencies, just different ones that can travel over many miles.
The term “cellular” refers to the idea that adjacent networks use different frequencies which prevents interference and creates a “cell” of network communication segregated from adjoining cells. This makes cellular towers a lot like wireless access points, just with a much bigger range.
Many tablets and computers have cellular antennas now, as do cars and probably boats, I guess.
Keep a log of all the times and ways you connect to the internet for one full day.
I could not do this because I waited until the day it was due to start this part of the coursework. It is many times each and every day, especially work days.
This week’s graded assignment is another little illustration-game where you have to slide your orange WiFi network channels around the spectrum to minimize interference from neighboring networks. Very exciting stuff.
Okay, great week everyone. I can’t believe next week is already the end of Course II.
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