DCE  vs  DTE

DCE (Data Communications Equipment) and DTE (Data Terminal Equipment) are descriptions of the role an Ethernet interface (also other Interfaces but we will concentrate on Ethernet here) plays in 2-way communication.  DCE is typically the upstream device (network end), and DTE the downstream device (user end).  DTE is an end station, while DCE is a server, router, or hub.  But this can vary !!!

These terms are very important, because they dictate the send/receive pinouts on both devices, which will then let you know whether to use a straight-thru cable, or a crossover cable.

Straight-Thru vs Crossover Cables

Standard Ethernet cables are "straight-thru", and use two pairs of wires (one for transmit and one for receive)  .  .  .  Pair 1-2 and Pair 3-6.  The other 4 wires are available but are typically not used.  The reason you need 2 wires for each data transmission, is because a complete electrical circuit is required to pass electricity.  This is true for all electrical devices, such as lights, Television, Toaster, etc - a minimum of 2 wires are needed.

For the straight-thru, Pin 1 on one end connects to Pin 1 on the far end, Pin2 to Pin2  .  .  .  Pin8 to Pin8.  There are only 4 pins that are used (2 pairs), since you only need two wires for transmit, and two for receive. The two pairs used are 1-2, and 3-6.  With a crossover cable, the wires connected to Pins 1-2 on one end are "crossed over" and connected to Pins 3-6 on the far end.  The same is done with the wires connected to Pins 3-6 - they are crossed over and connected to Pins 1-2 on the far end.

Cardinal Rule

DCE/DTE refer to interfaces - not the actual device.  A device can have both DCE and DTE ports.  However, in general:

         NOTE:  Internet routers have serial WAN ports, so the descriptors do not apply to them

The Hub's Uplink Port

The uplink port takes the leads from the LAN port next to it and reverses the two pairs of wires (transmit and receive), making it a DTE port. The last LAN port and the Uplink port are shared.  The LAN ports are DCE and are used to connect to DTE wired Ethernet interfaces, such as workstations.  The uplink port is used to talk to other DCE devices such as hubs, routers, or switches - without having to use a crossover cable. 

When connecting two hubs, only use the uplink port on one of them.  If you do connect two uplink ports together, as with any DTE-to-DTE connection, you will need a crossover cable.  Since the Uplink is nothing more than a reversal of the adjacent LAN port - you can hook any hubs, switches, or routers together whether or not uplink ports are present. Simply use one of the LAN ports instead, and connect them to another hub, or a router or switch, using a crossover cable.  

Many hubs have 4 or 8 LAN ports and 1 Uplink port.  If you do not use the Uplink port, then you can use all LAN ports.  But if you use the Uplink port . . . you cannot use the shared port that sits next to it.  It's taken.  The adjacent port is usually Port 4 with a 4-port hub, or Port 8 with an 8-port hub).