Networks Fundemantals 1
Lecture 5 Wireless Technologies
Wi-Fi
Wi-Fi is radio signal transmitted over the air that your phone can intercept and encode
because it contains a network interface card that specializes in doing this translation. The
network interface card also helps identify the device to the router because it establishes and
contains the MAC address. In some systems , MAC addresses are used as sources of truth
to identify a device, but MAC addresses are just a configuration value and easy to spoof, so
newer systems do not use that for identity.
The radio transmission signal comes from your router.
The reason why some parts of your home have better wifi signal than others is that wifi
signal doesn’t travel well through walls. There are two radio bands that are allocated for
wifi: 2.4 GHz and 5 GHz. If you have both at home, you may have noticed that the 5 GHz
band provides slightly faster connection. There are 4 reasons for this:
• First, 5 GHz is literally a faster (higher) frequency. Higher frequencies are less likely
to penetrate walls, this helps to contain the signal and keeps the 5 GHz band less
crowded.
• Second, because 5 GHz is newer, fewer routers support it and there is less congestion
in a crowded place like an apartment building.
• Third, before the 2.4 GHz band was allocated to wifi, many appliances such as
microwaves used that band, so if you have an older microwave at home, you may
have noticed you cannot use the wifi and the microwave at the same time.
Networks Fundemantals 2
Lecture 5 Wireless Technologies
• Finally, the 2.4 GHz band has 14 channels and the 5 GHz band has more, which helps
with congestion.
802.11
There are all sorts of acronyms for the different flavors of WiFi networks, but they all
basically mean a few things:
• How far can the wireless signal reach
• How much data can the signal send
• Is it backwards compatible with other standards
Think of it like cars:
• A race car can’t go far on a tank of gas or carry a lot, but it can go very fast
• An 18 wheel truck can’t go very fast, but it can carry a lot of things in it and go a long
way
• A station wagon isn’t very fast and can’t carry as much as an 18 wheeler, but it can
still go pretty far
There are various flavors of WiFi. They’re all called the IEEE 802.11 standard with the
letter after the 802.11 numbers. The usual rule is the higher the letter, the faster the speed of
the network. Almost all of these function over a range of about 30 meters (150 feet).
Each network can be broken down by a few different settings:
• Speed: How much data the network can transmit. This is calculated in Mbps (1 million
bits per second)
• Frequency: What radio frequency the network is carried on. These are either 5 Ghz
or 2.4 Ghz.
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Lecture 5 Wireless Technologies
Name Speed Frequency Notes
802.11a 54 Mbps 5 GHz Not compatible with b or g networks. This is one of the oldest
standards, but still in use by many devices today.
802.11b 11 Mbps 2.4 GHz Compatible with g networks. Really, g was made to be backwards
compatible with b to support more devices.
802.11g 54 Mbps 2.4 GHz The most popular network type in the early 2000’s. Its combination of
speed and backwards compatibility makes it a good match for those
days networks.
802.11n 100 Mbps 2.4 and 5 GHz The most common type of network. 100 Mbps is common, though
speeds of up to 600 Mbps is possible under perfect conditions. It does
this by using multiple frequencies at once and joining that speed
together.
Then 802.11ac has came after the n, 5 GHz with 1.73 Gbps of data rate. But it’s not over
yet! WiFi 6 is here! Officially, it’s labeled as 802.11 ax. X? Yes, X. If we go by the 1990’s
standards it would stand for “eXtreme!” It is made to work in the 2.4 GHz and 5 GHz range,
offering backwards compatibility for the 802.11a networks out there and up to 4.8 Gbps data
rate.
Local networks
The local network described here is a home network, but this could apply to any local
network, such as the network internal to a data center. The router is responsible for
dispatching messages to devices within the home network. To do this it needs to assign a
local IP address to each device within the network. There are address ranges reserved for
local networks so that the router can use these addresses within the network and there won’t
be a colliding address somewhere else on the internet.
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Lecture 5 Wireless Technologies
One critical job the router plays is keeping track of all the devices in the network and making
sure packets get sent to the right device. The router keeps track of open connections by
managing the port numbers each device connects to. Each device’s traffic within the home
network runs over a different port number, so that even if two people within the house are
going to spotify.com at once, their individual responses from Spotify’s servers get sent back
to the right device.
Bridging from local to global
The router receives internet connection from the access point, which bridges the global
internet to your home. Many home routers act as access points. The access point connects
through the wall to a physical cable that is wired up to your building.
Undersea cables
The physical internet is a series of cables that connect the different continents so that a
packet can route around the Earth. This allows, for example, a person in Dubai to reach a
server in Frankfurt. At the time of writing (April 2018), there are 393 cables that connect
the planet. You can see them all at submarinecablemap.com
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Lecture 5 Wireless Technologies
Each undersea cable has 4-6 fibers inside it, and to help communications make their way
across the cable, the cables have a repeater every 400 kilometers.
802.15.1
Bluetooth is a short-range wireless technology standard used for exchanging data between
fixed and mobile devices over short distances using UHF radio waves in the ISM bands,
from 2.402 GHz to 2.48 GHz, and building personal area networks (PANs). It was originally
conceived as a wireless alternative to RS-232 data cables. It is mainly used as an alternative
to wire connections, to exchange files between nearby portable devices and connect cell
phones and music players with wireless headphones. In the most widely used mode,
transmission power is limited to 2.5 milliwatts, giving it a very short range of up to 10 meters
(30 feet).
The IEEE standardized Bluetooth as IEEE 802.15.1. Bluetooth has encountered many
changes in range and data rate, the latest covers 240 m and 2 Mbps.
