Why does Ethernet transmitter side use termination resistors?

I'm working on a design based on the Wiznet W5300, and the schematic is like many other Ethernet designs, with 49.9 ohm resistors from each transmitter differential pin to ground (through a cap).

The schematic of their module is attached and can be found here: http://www.wiznet.co.kr/product-item/wiz830mj/?ckattempt=1

My understanding of transmission lines and termination has been that you only need a termination at the receiving end, so that signals launched from the transmitter "see" the same impedance at the end of the line, just like they "saw" throughout the cabling, 100 ohms differential for Ethernet.

Do these resistors at the transmitter add an unnecessary load? It seems like the transmitter is driving these resistors plus the cable, which is 100 || 100, or an effective load of 50 ohms. Are they intended to help with some type of noise rejection? Assuming the drivers are very low impedance, I would think that a series resistor on each of the differential driver pins would work without adding any extra load.

Thanks for any thoughts or explanations.


Update: I just realized that because this chip has auto MDIX capability, the TX and RX sides are interchangeable. I suppose the increased power on the transmitter is just an acceptable side effect of adding this feature.

Wiznet module schematic

Hi,

From what I know, the 50 Ω is what we call the characteristic impedance of the transmission line.

You check this link for more information on transmission lines:
"Transmission Lines("")":http://www.allaboutcircuits.com/textbook/alternating-current/chpt-14/finite-length-transmission-lines/

Best,
Kevin

Depends on the rise time of the signal and the cable length. To be sure, it pays to put a driving and terminating resistance matched to the characteristic impedance of a transmission line, at either end, since you have no control over the cable length. I believe the drivers are current output, in that case the driving resistance would go to ground. That is the equivalent of a voltage source driving a series terminating resistor. There are application notes on laying out Ethernet chipsets, it pays to have a good read, particularly if you are operating above 10Mb,
Cheers,
Richard

Hi Abe Shukert,
As per the communication theory transmission line must be terminated by an
impedance that is equal to the line characteristic impedance .using the proper
impedance matched resistor for termination on both master and slave end ,it attenuates signal reflections that degrades the quality of the transmitted data.once the termination impedance is properly matched it helps in increasing the data to be transmitted to the max distance allowed cable length or data helping to avoid garbage/ unwanted data during transmission.

Regards Sambath kumar.