By Bob Catanzarite
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Wiring and Setup
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  2001
Bob Catanzarite

 

 

 

 

 

Home Network Wiring and Setup - Chapter 3

 

A Brief and Simplified Description of Various Network Devices

The devices described below are actually quite complex. But fortunately for home networks you can look at these as relatively simple devices. I'll just describe these as simple devices and give you some links to sources that will describe them in more detail.

 

Hubs

If you want to network JUST TWO PC's together you can simply connect the ethernet ports on both PC directly together with a ethernet crossover cable. But, if you want to network three or more devices then you need some type of network distribution device. The simplest of these is a hub. Hubs have 4 or 8 or more ethernet ports. When a packet ( ethernet communicates by sending and receiving short messages called packets) comes into any port on a hub the packet is buffered and sent out to EVERY port the hub has. With a hub every PC will receive every packet that any of the other PC's send. 

Links to more detailed descriptions of hubs are:
HowStuffWorks.com - How LAN Switches Work -
http://www.howstuffworks.com/lan-switch.htm

 

Switches

Switches work in a similar fashion with one key difference. A switch will examine a packet as it comes in and will ONLY pass the packet on to the port that has the device the packet was intended for. A switch will 'learn' the addresses of the devices connected  to it so that it knows what port the packet need to go to.

With a hub every port carries every packet that any device sends while on a switch a port only carries traffic intended for the device connected to it. Hubs do not allow multiple devices to communicate at the same time but switches do. With a switch a device on port 1 can be communicating with a device on port 2 at the SAME TIME as another device on port 3 is communicating with a device on port 4. Switches make much more efficient use of a ports bandwidth.

Switches also allow Full Duplex communications. With Full Duplex a port can transmit and receive at the same time.

Links to more detailed descriptions of switches are:
HowStuffWorks.com - How LAN Switches Work -
http://www.howstuffworks.com/lan-switch.htm

 

Routers

Routers are a lot like switches in that they route packets to proper destination. The come in a lot of different forms but for a home network they usually have the additional task of acting as a bridge between your LAN (Local Area Network) and the WAN (the internet).  A router will examine packets and determine if they are destined for your LAN or the WAN and route them accordingly. With a router sitting in-between your LAN and the internet and deciding which packets are allowed to flow between the two, it is in an ideal position to protect the two from each other. This is why most home networking routers have built in firewalls. 

Routers have one other huge benefit for people with networked PC that want to share a single internet connection.  All your ISP (Internet Service Provider) will see is the router and not the multiple PC behind the router that are sharing the one connection. Without a router the ISP will see each PC and the IP address that each PC has. ISP's usually charge extra for more than one IP address. But, with the router, all the ISP will see is the IP address of the router and the router will make sure that packets to and from the internet get to the proper PC.

Links to more detailed descriptions of routers are shown below:
How Stuff Works - Routers -  http://www.howstuffworks.com/router.htm

 

Print Servers

A print server is a device that links a printer to a network so that any device on the network can use it. There are two basic way to share a printer among devices that are networked together. The cheapest is to connect the printer physically to one of the networked PC's and then on that PC set the printer up as a shared resource. More on how to do that is coming soon. After this shared resource is setup in the operating system, any of the networked PC's can print to the printer. One drawback to doing it this way is the PC that the printer is connected to needs to be turned on and running anytime another PC needs the printer. In other works, a PC is acting as an expensive print server. The other way of doing it is with a dedicated print server.  A print server can be as small as a deck of cards and has at least one printer port and one ethernet port. Print servers for a home network can cost as little as $40 and as much as $200. Mine is a Linksys EFSP42 and has a 4 port ethernet switch and a print server for two printers and cost $130. A print server is a relatively simple device with no hard drive or cooling fan to wear out so you can leave it powered up all the time ready to serve it's printer(s) on the network . The print server costs extra but it eliminates the need for a PC to be up and running around the clock acting as a print server. Nice to have one of these.

 

UPLINK Ports -vs- Crossover Cables

You'll notice the drawings on the previous page show ports called UPLINK ports. To understand the purpose of an UPLINK port you need to have a basic understanding of the two types of networked devices. You have NDD (Network Distribution Devices - I made that term up) and you have NU (Network Users - again, a term that I made up). An NDD will be a distribution device like a hub or switch. A NU will be a PC or a print server or a router or a broadband modem. So here is the rule of thumb:

When connecting a NDD to an NU - use a standard straight through cable.

When connecting like devices such as a hub to a hub or a PC to a PC - use a crossover cable.

A NDD (Network Distribution Device) such as a hub or a switch in intended to be connected to a NU (Network User)  such as a PC. If you connect LIKE devices you have to use a crossover cable. A crossover cable is the same as a straight through cable except the the TX (Transmit) and RX (Receive) pairs are reversed or swapped on ONE end of the cable.

To a network beginner it may not always be clear when you can use a standard cable and when you need a crossover cable.  Basically you need a crossover cable when connecting LIKE devices together.

Most manufacturers of hubs and switches know that users will often connect these NDD devices together to add additional ports to a network. With these being LIKE devices a crossover cable is needed. With this in mind, on MOST hubs and switches, one of the ports can usually be configured as an UPLINK port. An UPLINK port has the TX and RX pairs reversed internally to the device which eliminates the need for a crossover cable.  Some devices will have an electrical switch near the port that will change the port to a standard port or an UPLINK port. Other devices will have two jacks for one of the ports with one jack wired as a standard port and the other as an uplink port. When your shopping for a hub or switch look for one that has a configurable UPLINK port!

 

Crossover Cables

If you must use a crossover cable you can buy them or make them. I have seen 40-11 different descriptions of how a crossover cable is wired. All of them seemed to confuse me so I created my own 40-12th description as shown in the drawings below. To state it as simply as I can - A crossover cable differs from a straight through cable by exchanging or swapping the TX and RX pairs on ONE end of the cable.

The thing that adds to the confusion here, I think, is the fact that there are two wire color patterns - T568A and T568B.  As I have said earlier in this website - the meat of the matter is that these two different looking wiring standards are ELECTRICALLY BOTH THE SAME IF you use the SAME color pattern on BOTH ends of a given cable! Irregardless of which of the two color pattern you choose pin 1 on one end is connected to pin 1 on the other end. Pin 2 on one end is connected to pin 2 on the other end and so forth. The difference is just in the COLOR of the wires. 

Another way to describe a crossover cable is that one side is wired to the T568A color code and the other side s wired to the T568B color code.

So rather than explain all this in words I have made drawings of the 4 possible cable configurations. You have two wire color standards and you can have a straight through and a crossover version of each.  All 4 are shown below:

 

T568B Colors

 

 

T568A Colors

 

In summary-

- Use a crossover cable to connect LIKE devices. For un-like devices use a straight through cable.

- To make a crossover cable you reverse the TX and RX pairs on ONE end of the cable.

- When your connecting a hub to a hub or a switch to a switch - you don't have to use crossover cables if the hub or switch has an UPLINK port. Simplify things by choosing devices that have UPLINK ports!!

- When your connecting a hub to a hub or a switch to a switch using an uplink port, connect one end of a straight through cable to an UPLINK port and the other end of the cable to a standard port. Remember - you are essentially swapping the TX and RX pairs on ONE end of the cable only. This is why you connect one end of the cable to an uplink port and the other end to a standard port.

- Another thing. If you make or buy a crossover cable it will look a LOT like a straight through cable. Make sure you label a cable clearly as being a crossover cable. You can spend hours troubleshooting a problem that arises because you used the wrong kind of cable.

 

Here are some links to other sites that describe crossover cables:
The littlewhitedog.com - How to make a crossover cable -  http://www.littlewhitedog.com/reviews_other_00009.asp
DUX Computer Digest - How to make a crossover cable -  http://www.duxcw.com/digest/Howto/network/cable/cable5.htm
Linksys - What is a network cable -   http://www.linksys.com/tech_helper/cabling.html

The next page-

will discuss how to setup the operating system on your PC's to get them to work with a network.

 

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