I code, therefore I am... nerdy

Grokking Sockets: Physical Layer

After a short time of thinking, I’ve decided to focus on the 7 layers of the OSI model. We will be building off the knowledge of each layer to see how they all work together harmoniously and how we can trick them into going just a little bit faster. I guess that makes this series an eight part series, so it’s going to be one long journey. Of course, this series isn’t called “Kinda knowing sockets,” or even “Mastering sockets.” It’s called Grokking sockets. That requires a deep, xen like understanding of networking. And that includes layer one of the OSI model, with all of its hardware goodness.

Yes, if you look again at the URL for this site, it will still say Of course, you can’t master the art of software without knowing how what your write turns into physical changes. In this case, how does the information from one computer electronically get sent over to another. This is not black magic (as much as the electrical engineers want us to believe).

Let’s first talk about the physical layer you may be most familiar with, ethernet. You may see it as 10BASE-T, 100BASE-TX, or 1000BASE-T. These are the standards for communicating over copper wire, which is what hides inside the ethernet cable. The wires are paired and twisted together inside the shielding. They are twisted and shielded to avoid electromagnetic interference. The trick here is the twisted pairs carry opposite signals, known as differential signaling. This means that if one wire gets interference, the other will too because the twisted wires coupled together.

The other standard you probably know and use is 802.11, or really IEEE 802.11. This is the standard for consumer grade wireless networks and is pretty standardized in the corporate world. It transmits wireless signals from one wireless adapter to another using strange modulation acronyms such as DSSS, FHSS, and OFDM. Basically, all of these describe how to use the wireless frequencies assigned to each standard. The basic idea is that wireless has to deal with more interference than a physical wire. Think of your radio in your last car (because maybe you have one of those fancy HD radios now). When you were far from a radio station, sometimes you heard two stations switching back and forth within the static. This is what radio frequency, or RF, interference “sounds” like.

These radio stations are given dedicated bands of space by the FCC (in the U.S.). That means they are guaranteed that band by the government. WiFi does not have that luxury. Everyone needs to play nice and share the same bands. Worse yet, the transmission performance is so poor on these bands, that the FCC didn’t want it and has opened it up to anything. That’s why we see 2.4GHz cell phones, 2.4GHz wireless keyboards/mice, and 2.4GHz bluetooth. Everything is on this band and we want to get over 100Mbps to our PCs.

That’s where the complicated spread spectrum type modulations come into play. At its most basic level, a signal is transmitted on a coded set of frequencies that is agreed upon by the sender and receiver. This is called code division multiple access. Since the frequencies are coded, another sender can send on a different code simultaneously. The receiver then takes the full spectrum, and using advanced math that is above my level, extracts or de-spreads the original transmission.

There are further ways of handling interference such as 802.11n’s use of orthogonal frequency-division multiplexing. It uses multiple signals, subcarries, and higher level math than the previous versions. If you’re really interested, or just a glutton for punishment, you can read more about it on Wikipedia.

So what does this all mean for us coders? Well for one, you can now impress your engineer friends by using acronyms such as CDMA and complex vocabulary such as “orthogonal subcarries”. But in reality, we need to know one thing out of all of this. Interference is our main source of slowdowns in layer 1 of the OSI model. So, we have a few options to think about. If you need the dedicated bandwidth, go with a wired connection. This will give you some level of guaranteed bandwidth, although there are some things to worry about higher up the OSI model.

If you have to use wireless, know that you may lose large chunks of your data. If you’re using TCP, this equates to large lags and drops. You can avoid some interference by switching channels on your routers and making sure two routers near each other are on orthogonal frequencies. This will avoid interference between the routers and minimize the crosstalk between computers on different routers.

You can also switch bands all together to the 5GHz band. There will be generally less interference on this band right now since most consumer grade electronics don’t use it. Plus the added benefit of 802.11n’s four simultaneous open channels using multiple input, multiple output and its multiple antenna array for allowing directional transmission will give you much higher performance with much less broadcast noise.

Also, since 802.11n is an open ended standard in some regards, you will find that the expensive hardware will actually out perform the cheap stuff. So, if you need the bandwidth, get ready to pony out a lot more money for a router that has higher coding rates and more simultaneous streams. These babies can get up to 600 Mbps (theoretical), if you have the right hardware.

So now that you know the basics of the first layer of the OSI Model, we can continue building upon that knowledge to understand the next article in this series about layer 2, the data link layer.