Monday, August 23, 2010

Lesson 7 - Building a Home Network

"Dad! What are you doing?" - My son Mattie is looking over my shoulder while I'm connecting three old computers to the hub I'v salvaged from a certain death. "I'm building a small, home network." - I reply surprised at his sudden interest. "What do you need to build a small network?" - There's this little sparkle in his eyes only kids his age have. "And how do you do it?". There's nothing I can do but explain to him how it's done!

We will need few components to build this network. First of all, we'll need computers, running some operating system that can talk across the network (like Mac OS, Windows, Linux).

Pic. 1 - Computers.

Computers are already equipped with NICs (Network Interface Cards) by the manufacturer.
"What does the NIC look like dad?" - He interrupts. Nine years old kids can be very tenacious, almost obsessive. They are not easily dismissed. They won't stop unless they're fully satisfied with the answer. So, I'm taking one NIC out of the computer and explain that it is plugged in to the PCI slot in order to work. "PCI slot is connected to the motherboard of the computer and the operating system uses a special piece of software to talk to the NIC. It's called a driver. The driver translates between an operating system and the NICs hardware. The NIC sends bits down to the wire and knows what to do when bits come back from the network". Bits are small pieces of information (1s and 0s) which in software make data like music, pictures, text documents etc.

Pic. 2 - NIC (Network Interface Card).

"Okay." - Mattie says, "But what else will you need?

"I will need Unshielded Twisted Pair cables (UTP). One, for each computer. The cables (sometimes referred to as Ethernet cables), use RJ-45 connectors that the most commonly used nowadays." - I reply. "Here's the cable without the connector:

Pic. 3 - UTP cable without RJ-45 termination

"Mattie's looking at the cable and I know what's going to happen next. "Dad, why does it have so many wires and why are those twisted like that?"

One pair of the wires (two wires) are used to transmit data. Another pair, is used to receive data from the network. Other wires can be used to carry the power to some types of the devices (PoE devices) or to accomplish faster speeds (1Gbps etc.). They are twisted like that on purpose. The guy who invented that concept was Graham Bell. He invented it for the telephony purposes and patented that in 1881. He discovered that twisting wires (conductors) minimized or canceled Electromagnetic Interference (EMI) from external sources and, so called, cross talk from the neighboring wires.

The cable must be terminated at both ends with RJ-45 type of connector, like the one depicted below:

Pic. 4 - RJ-45 Connector.

Below is the cable with the connectors.

Pic. 5 - UTP cable with RJ-45 connectors.
The UTP cables can have different category numbers (CAT 1-6). The higher the category number is, the better quality of the cable, the faster, and better transmissions are going to be. Also, the UTP cables can be terminated in two different ways like explained below.

Straight-Through Cable
In straight through cable the transmitting pair of wires are 1 and 2, the receiving pair of are wires 3 and 6. There are two major standards (ways) of using the colored wires, but important thing is, that the colors on the both ends of the cable are terminated identically. Please, look at the picture below.

Pic. 6 - Straight Through Cable.

Cross-over Cable
In the cross-over cable, the position of the wires is changed such that the sending pair is terminated at the receiving pair on the other side of the cable. It is illustrated below.

Pic. 7 - Cross-over Cable

My son Mattie's holding both types of cable, looking at them and I know I cannot dismiss him with that explanation. So, I continue.

If you connected two computers together and the NICs are wired identically, you would connect the sending pair (pins 1 and 2) to the sending pair on the other end. This obviously would not work. You must connect sending pair on one end (pins 1 and 2) to the receiving pair (pins 3 and 6) on the other end. For instance, if you connect the following devices together, you'll need cross-over cable:
  • computer-to-computer
  • switch-to-switch
  • hub-to-hub
  • computer-to-router (directly)

Modern NICs can 'sense' the type of cable and adjust the operation regardless of the cable used. But this is not always the case.

Devices such as hubs and switches, are designed such way they can use straight-through cables. The cross between the transmitting and receiving pairs is done in their port controllers. So, the following device connections will use straight-through cable:
  • computer-to-hub
  • computer-to-switch
  • router-to-hub
  • router-to-switch
Now, all we have to do is to connect the cables to the the hub, configure IP addresses on the NICs and voila! They can talk to one another.

Ethernet Hub
It is a simple device that allows to connect a few computers together. Look at the typical, cheap hub you can buy for home purposes:

Pic. 8 - An Ethernet Hub.

"How does the hub work then?" - I can tell my son got some interest by now.
"Well, that is the topic of our next lesson." - I say smiling.


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