Sunday, September 18, 2011

Interesting Enagadget article on Whitespace

I know that I have been hoping to drip-feed information here, but occasionally I will need to jump the gun.

For those of you that don't know about the "White Space," it is referring to the spaces in the RF Spectrum that will be left between Television Channels, leaving clear bandwidth for devices to communicate on.

This spectrum remains unlicensed, and thus manufacturers can use this space for data transfer. Wireless Microphones operate in the same space; they are designed to work in the TV channel space, but in the "gaps" between channels. Of course, the introduction of more "White Space" devices sends shivers up most Radio Mic Technician's spine, because it potentially means that a device will suddenly appear in the middle of your spectrum... when you least expect it!

The reason White Space Devices didn't work so well in the past is due to the difference between Digital and Analogue TV transmission. Analogue is much less predictable, however Digital Channels stick out like a sore thumb to even the most basic devices. So, when you turn off the Analogue stations, you make it a lot easier to make White Space Devices.

It shouldn't be confused with the Digital Dividend, which is the space at the high end (~800MHz) of the spectrum that will be "empty" (i.e. no TV channels) once the Analogue TV stations are turned off. This spectrum will be sold off, and in the US it made a hell of a lot of money.

I will go into all of these topics in much greater detail, but for those of you that are hanging on any bit of Spectrum Information, please enjoy the link.

Thursday, September 15, 2011

The layers of the Internet - Part 2

The Internet Layer
In the last post I explained the basics of Layer 1 of the Internet Protocol; the Link Layer.

The Link Layer is where all devices are physically connected, either by wireless (e.g. Wireless LAN or 3G) or by a wired (e.g. Ethernet "Blue String" or ADSL) connection.

Whilst this is a great concept, if every device were to receive all of the data transmitted on the Internet, then we would be slowing down the process beyond belief.

Enter Layer 2 - The Internet Layer

This is Layer 2

On a network drawing, a network was always drawn using standardised symbols. Since everything that was on the "Internet" was connected at Layer 1, it no longer mattered how things were connected. You only had to show that there was some kind of connection.
And so the network symbol for the "Internet" became a cloud. It was some ethereal entity, floating out of the reach of Network Administrators across the globe.

Unfortunately marketing departments caught hold of this analogy, and thus Cloud Computing was born. You can see the "Cloud", and it brings you good things (like shade, and rain for your crops) but you have little to no power over it. It's there whether you like it or not. 

So how does it work?

Packet-Switching Networks
In this previous blog post I explained the anatomy of a standard Internet Protocol (IP) Packet. Packets are the currency for IP networks, and indeed the entire Internet. Without repeating myself too much, they contain two main parts; a "Payload" (the data that you want to move around the network) and a "Header" (which contains the addresses relevant to the Data).

In order to understand how the Internet works, we are going to have to introduce our first specific piece of network hardware: the Switch.

The image above is that of a "Switch," and it is a common thing to be found in data centres across the world. However you are reading this blog, somewhere along the line you are connected to a Switch. It might be a little 4-port switch that came with your ADSL plan, or you might be connected to a commercial-grade switch (like the one above) at work.

Switches are the building blocks of the Internet, and they elevate matters from Layer 1 (Link Layer) to Layer 2 (Internet Layer).

Every blue or pink cable in the above image connects to a device; a telephone, a computer, a printer etc. This is the Layer 1 connection. You can tell they are working by the blinking green lights. The Orange cables connect those switches to other switches, which the connect to other switches... until they reach whatever destination they need to get to. These connections are called "Uplink Ports", as they are headed up towards the "Cloud".

When a packet is sent to a switch, it "opens" it up and reads the "Header" (not the "Payload"). In the "Header" is all of the addressing information that the Switch needs to send the packet to where it needs to go. If the destination address is connected directly to the switch, then the packet will be sent directly to that device. If not, then the switch will send the packet to the "Uplink" port, at which point the next Switch will repeat the same process until the packet arrives at its destination.

By doing this, Switches make sure that you only receive the packets that you need to read your emails, browse your websites, control your motors, or route audio. Switches don't care what your packet has in it, so long as the address in the Header is valid.

The address used by the Internet Layer is the Internet Protocol (IP) Address. I will go into (much) more details about IP Addresses in a later post as the topic is as broad as the Internet itself. Suffice to say, a common IP address is an 8-byte address, usually rendered in four groups of numbers from 0 to 255, e.g.

Once two devices are connected at Layer 2 they are considered "Networked" and can now communicate as if they were in the same room. Layers 3 and 4 deal with how they communicate, and we will cover these in the next blog post.

Monday, September 12, 2011

The Layers of the Internet - Part 1

The Internet is a complex place. There are many articles about the history of the internet, and it's a little beyond the scope of what this blog is about, so I won't go into it too much.
Suffice to say, the only way the internet works is through rigorous adherence to global standards. These standards were developed over the 15-year genesis of the Internet from research labs to commercial use.

One of the best ways to describe how these standards, and indeed the entire internet, works is known as the "layer" method.

Layers are a great way to explain many things, and the layers that we will be looking at today not only apply to networking, but to almost every form of computing or digital signal processing.

There are four layers in the "standard" Internet topology:

  • Layer 1: Link Layer. This is the physical link between devices
  • Layer 2: Internet Layer. This is the "virtual" layer where the data moves around networks
  • Layer 3: Transport Layer. This layer defines how data moves around devices
  • Layer 4: Application Layer. This is the layer that shows how data is shared between programs.

Those short little descriptions probably mean very little to most of you, so let me break it down a little bit more.

The Link Layer
When I think of the best way to describe the Link Layer, this image comes to mind:

What you're looking at there is a fairly typical "Distribution" switch, and a lot of optical fibre.
The Link Layer is the only physical connection layer in the Internet Protocol. It defines all the different ways that you can connect devices together if you want them to be on the internet.
If you put your mind to it, you could easily rattle off a lot of the different standards that are in the Link Layer, for example:

  • Wireless Networking 
  • Ethernet networking (a.k.a. "Blue String" - those blue cables that we are all familiar with)
  • Fibre Networking 
  • ADSL (Asymmetric Digital Subscriber Line - The way most of us get our Home internet)
  • 3G/HSDPA - The wireless Broadband that most of us use on our phones.
  • DOCSIS, a.k.a "Cable Internet" - networking over Coaxial cable, similar to Cable TV (Thanks djzort)
The list goes on. The greatest thing about the "Layer" system of the Internet is that it doesn't matter how you connect devices together at the Link Layer, so long as they follow the Link Layer Standards. As soon as you have that "blinking light" that shows you are connected then you can start passing information around the Internet Layer.

The Link Layer extends across the entire Internet. Just ponder on that for a second; every device that is connected to the Internet is in some way, shape or form, connected. The Link Layer is the only layer at which every device is connected; once you start moving into the "Virtual" layers (layers 2-4) you start segregating devices into separate virtual networks (or "subnets"). But for now, let's just muse on the topic of every device acting together in synchronism.

The last thing that I will mention about the Link Layer is the address that applies to it. Obviously there is no point in connecting every device on the planet unless you knew which one you wanted to talk to. Therefore every device that connects to the internet has a unique address. This address, known as the Media Access Control (MAC) address, is a unique number assigned by the manufacturer.

The MAC Address is a 48-bit number (that is, 48 "1's" or "0's"), which makes for about 300 Trillion different addresses. Every single device that is capable of connecting to a network has a MAC address. This laptop, for example, has two addresses; one for the Wireless connection and one for the hard-wired connection. Even so, the IEEE doesn't expect that we'll run out of MAC addresses this century.

We'll look at the Internet Layer next time (probably in a couple of days). I thought it best to break things up for now.