You must be thinking, “What a silly topic for a blog targeted at Wireless LAN Professionals”?
On the surface it does seem that way.
But … in my decade of teaching and consulting with thousands of like-minded individuals I’ve been shocked to hear the answers to this very simple question. We who’ve been in the networking industry for a while have worked through an entire series of network hardware devices. And now we are working with these funny Access Points, which don’t quite fit into any one category.
So on our way to getting to the answer to this simple question, let’s back up a bit and do a little review of where we’ve come from, and how different devices on our networks actually work.
First – and the oldest – is a Repeater this is a very simple Layer 1 device. It works with Bits. A bit comes in one port, and is regenerated and comes out the other port nice and fresh. Since it’s digital and not analog, it comes out clean and fresh, with no amplified noise.
Next in our line up will be the Hub. This is also a Layer 1 device, and also works on Bits. When a bit comes in one port it is regenerated but this time it is sent out all the ports on the hub at the same time. This leads us to a new term – Collision Domain. Because all ports get flooded with the same bits, at the same time, they are considered to be in the same Collision Domain. (when one talks, all the others must listen) This is a problem with a shared medium. Another way to describe a Hub is a Multi-Port Repeater.
Moving on. We come to a Bridge. This is now moving up the protocol stack to be a Layer 2 device. It works on Frames. In the front of a frame there is the Destination Address – the MAC Address of the targeted NIC. A bridge has two sides, let’s call them Side A and Side B. If a frame with the Destination Address of Side A is found on Side A – nothing happens. The frame is already on that network. But if a frame comes along with a Destination Address on Side B, and is found on Side A, then the Bridge now does its job and regenerates that frame out the Side B port. Bridges have techniques to learn what MAC addresses are on which port.
Note each of these devices regenerates the data from one side to the other …
Next up – the Switch. A simple way of explaining a switch is that it is a Multi-Port Bridge. It’s also at Layer 2, and also uses Frames. The difference is it has multiple ports. So the decision to regenerate the frame (also called forwarding) is not just a simple Side A vs. Side B. But the decisions range across all ports in the switch. The decision is still made on Destination MAC address. (We could get into VLANs here… but we won’t)
And finally let’s talk about a Router. A router is a Layer 3 device. It works with packets. A router’s network port is the Destination MAC address of a packet. The router receives the packet, and de-encapsulates the frame to find the Destination IP Address, then puts a new MAC Header with the next hop as the Destination MAC address. Then it sends the packet along on its way. (There are also Layer 3 Switches… but that’s for a different blog)
So let’s say you are at home, with your home router as the Default Gateway to your local area network. On your laptop you type http://blogs.aerohive.com - what is the Destination MAC Address for that frame? If you said the Default Gateway, you are correct! Your local IP stack compares the IP Address of the http://blogs.aerohive.com returned from the DNS query, and compares it with the Subnet Mask – and notes that IP address is NOT on your local network. To get off your LAN, you must send the packet to the Default Gateway. Your laptop received the Default Gateway’s IP Address during the DHCP request, and so sends off an ARP request. The Default Gateway responds with its MAC address.
Now we are ready for the frame to leave your laptop and head over to the Aerohive website. The Destination IP Address is for http://blogs.aerohive.com, but the Destination MAC Address is for your local Router.
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Now we’ve quickly reviewed the bulk of network devices you might encounter on a daily basis.
So given all of that review… back to my initial question:
“What is a Wireless Access Point?”
(Now I’m not talking about little SoHo devices that are mixtures of Access Points, Switches, and Routers all in one little box … I’m talking about the features of just the Access Point itself)
Is it a Repeater? A Hub? A Bridge? A Switch? Or perhaps even a Router?
I’ve asked this question to literally thousands of network professionals … and it usually stumps them. I’ve heard all sorts of answers to this very simple question.
It’s actually quite sad that folks who have installed hundreds or even thousands of these network devices don’t actually know what they are doing on their networks.
So, to put you out of your misery. Here’s the answer.
On the Wireless Side, every Access Point is a Hub. The RF frequency it uses is a shared media, and thus an Access Point on the wireless side uses a shared media, collision domain, just like a Hub. And shares this media with all other devices using the same channel. It shares the frequency’s capacity with all other Access Points, and all other Client Devices that are within ‘hearing range’ on the same channel.
On the Wired Side, every Access Point is a Bridge. Not a Switch, since there are only the two sides, wireless and wired. An Access Point bridges traffic between the two media types. (And a bit of conversion as well, from 802.3 Ethernet to 802.11 Wi-Fi). So from the wired side, each AP looks like a Bridge.
There you have it. Every Access Point is a Hub on the wireless side, and a Bridge on the wired side. This knowledge alone will help a lot in your design and troubleshooting of your WLAN systems.
In a future blog post, we’ll next talk about the differences between Autonomous Access Points, Light-Weight Access Points, and Cooperative Control Access Points. They are quite different in how they perform these very same Hub and Bridge functions.
