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Friday, August 1, 2008

The Two Broad Types Of Networking Equipment

There are two main types of networking equipment; Data Communications Equipment (DCE) which is intended to act as the primary communications path, and Data Terminal Equipment (DTE) which acts as the source or destination of the transmitted data.

Data Terminal Equipment

DTE devices were originally computer terminals located at remote offices or departments that were directly connected modems. The terminals would have no computing power and only functioned as a screen/keyboard combination for data processing.

Nowadays most PCs have their COM and Ethernet ports configured as if they were going to be connected to a modem or other type of purely networking-oriented equipment.

Data Communications Equipment

A DCE is also known as Data Circuit-Terminating Equipment and refers to such equipment as modems and other devices designed primarily to provide network access.

Using Straight-Through/Crossover Cables to Connect DTEs And DCEs

When a DCE is connected to a DTE, you will need a straight-through cable. DCEs connected to DCEs or DTEs connected to DTEs require crossover cables. This terminology is generally used with Ethernet cables.

The terminology can be different for cables used to connect serial ports together. When connecting a PC's COM port (DTE) to a modem (DCE) the straight-through cable is frequently called a modem cable. When connecting two PCs (DTE) together via their COM ports, the crossover cable is often referred to as a null modem cable.

Some manufacturers configure the Ethernet ports of their networking equipment to be either of the DTE or the DCE type, and other manufacturers have designed their equipment to flip automatically between the two types until it gets a good link. As you can see, confusion can arise when selecting a cable. If you fail to get a link light when connecting your Ethernet devices together, try using the other type of cable.

A straight-through Ethernet cable is easy to identify. Hold the connectors side by side, pointing in the same direction with the clips facing away from you. The color of the wire in position #1 on connector #1 should be the same as that of position #1 on connector #2. The same would go for positions #2 through #8, that is, the same color for corresponding wires on each end. A crossover cable has them mixed up. Table 2-3 provides some good rules of thumb.

Table 2-3: Cabling Rules of Thumb

Scenario Likely Cable Type
PC to PC Crossover
Hub to hub Crossover
Switch to switch Crossover
PC to modem Straight-Through
PC to hub Straight-Through
PC to switch Straight-Through

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