network crossover cables

webspider

Hi
Can anyone please tell me the difference between a Ethernet Crossover and a Serial Crossover cable?

JDMurray

At the risk of sounding like a smart guy, one is for Ethernet network communications and the other is for serial communications.

Have a look at the Wikipedia pages:
Crossover cable - Wikipedia, the free encyclopedia
Ethernet crossover cable - Wikipedia, the free encyclopedia
Null modem - Wikipedia, the free encyclopedia

webspider

But Sir, those links dont explain the difference between serial and ethernet crossover cables

JDMurray

Are you looking for the differences in the physical cables and the connector wiring, or the differences in how and why they are used to connected equipment together? Please be more specific in what information you are looking for.

adam220891

Question...

A crossover cable is used to connect two like devices. Why is it the ethernet cable from my PC connects to my router via an ordinary straight through ethernet cable? Routers and PCs are like devices, correct?

mjb2424

Adam, this information might help you better understand the differences and why each type is used in different places:
Medium Dependent Interface - Wikipedia, the free encyclopedia

The important thing to consider is which pins the device's Ethernet adapter is using to send and receive data on. If you were to connect two devices together that both transmitted on pins 1 and 2, the communication would fail (think what would happen if everyone drove on the left side of the road for both directions of travel). The use of a crossover cable allows two "like" devices to avoid this barrier by correctly lining up the send and receive pins on each end. Further, a crossover cable is unnecessary between two "unlike" devices as each is using opposite pins for sending and receiving.

Additionally, when you mention you use a straight through cable to connect to your router I assume you're connecting to a home wireless router correct? This works because you are actually connecting to a switch. Home "routers" are composite devices that typically incorporate a router, switch, and wireless access point.

All of this frequently doesn't matter as much anymore for every day connections though as many devices are Auto-MDIX (See the link).

Jhon Arthur

if you want to connect 2 routers together use crossover cable, but if you want to connect a PC to a router or hub then use standard cable. the difference between the 2 is that crossover has the receive and send wires switched.

Darril

Good answers by mjb2424 and Jhon Arthur. Also, the term "like devices" refers to their usage. Routers and switches are networking devices and in this context are "like devices." A PC is a node on a network and in this context two PCs would be "like devices."

jvrlopez

I've been in networking/IT for 5 years and that's the best and most concicse explanation of XO vs Straight through that I've heard.

MrXpert

To understand this, you need to understand the pin configurations.
PCs, routers, WAPs send data on pins 1+2 and receive on 3+6
Switches and Hubs send on 3+6 and receive on 1+2

If you connect up a PC to another like device (i.e. a device which sends and receives using same pins) then you need to use a cross over cable because otherwise there is a conflict.
Cross over cable has pin 1 connected to pin 3, pin 2 connected to pin 6. Do you see the pattern? Take two computers called PC1 and PC2. PC1 sends on Pin1+2 and PC2 receives on Pin 3+6. When PC2 sends data to PC1, it does so using Pins 1+2. This is then receives on Pins 3+6 by PC2.

Whereas in a straight cable the pins from the local to the remote end all run in a straight line. i.e. pin 1 to pin 1, pin 2 to pin 2 etc.This is why this cable is used for unlike devices such as PC to Switch. When PC1 sends using Pins 1+2, they are received on Pins 1+2 by the switch.

Also worth remembering that Straight cables are 568A to 568A or 568B to 568B standard
Cross over is 568A to 568B.

adam220891

I appreciate the responses everyone!

The reason I ask, is in my Advanced Networking class, we use a CCENT book. I was reading chapters 1/2 and the table I came across said PC NICs, Routers, and WAPs transmit on Pins 1/2 where as hubs and switches transmit on Pins 3/6. I took out the cat5 cable from my PC connected to my home router, and the wires were the same on each end (and, from my understanding, if the cable was XO, the colored wires would not be in the same order on each end). However, mjb has stated that this is because it is a home router (composite device), which would explain things.

Is my interpretation correct?

MrXpert

adam220891 wrote: »

I appreciate the responses everyone!

The reason I ask, is in my Advanced Networking class, we use a CCENT book. I was reading chapters 1/2 and the table I came across said PC NICs, Routers, and WAPs transmit on Pins 1/2 where as hubs and switches transmit on Pins 3/6. I took out the cat5 cable from my PC connected to my home router, and the wires were the same on each end (and, from my understanding, if the cable was XO, the colored wires would not be in the same order on each end). However, mjb has stated that this is because it is a home router (composite device), which would explain things.

Is my interpretation correct?

as I said earlier, cross over cable is 568A to 568B and it connects pin 1 to 3, 2 to 6. This allows for devices who send and receive on the same pins to communicate. Examination of 568A on one end of the cable goes something like green/white, green, orange/white, blue, blue/white, orange, brown/white, brown. The other end will be orange/white, orange, green/white, blue, blue/white, green, brown/white,brown. So yes you can tell a cross over cable by looking at both ends and observing the colours. If they are the same on both ends then it is straight. if it is different then it is crossover.

Most new devices support Auto sensing(MJB mentioned this before) and this means you could get away plugging in a straight cable between two switches when forming trunk links as it internally switches the pins in a kind of logical manner. However probably not best to rely on auto sensing too much as it doesn't always work. A bit like when relying on auto settings to configure the correct speed and duplex of a port...sometimes it can auto set it to half duplex when the PC's NIC supports full. not good.