10mb vs 100 vs 1000

[d4mo]

So I feel I should know this but here goes anyways.

10 and 100 both use 2 pairs of wires. 1 pair tx and 1 rx. 1000 uses 4 pairs 2 tx and 2 rx. Correct?

What makes 100mb 10x faster than 10mb? Packet size stays the same correct? Can the packets just be sent at a higher frequency? Because if I recall correctly only 1 packet can be sent at a time.

 

[xSauronx]

http://www.hardwaresecrets.com/article/How-Gigabit-Ethernet-Works/231/2

 

[ScottMac]

GigE uses four pair to send and receive at the same time in both directions.

 

[spidey07]

GigE uses four pair to send and receive at the same time in both directions.

Correct. It uses modulation.

What really makes the speed difference is simply the clock. There are minor differences in encoding between 100 and 1000 but you don't need to worry about that unless you want to get really geeky and talk about interframe spacing (you can't put a frame on the wire directly after the previous one, the sender has to space the frames when transmitting)

 

[JWMiddleton]

What makes 100mb 10x faster than 10mb? Packet size stays the same correct? Can the packets just be sent at a higher frequency? Because if I recall correctly only 1 packet can be sent at a time.

You asked about 10 vs. 100 Mbps. 10Base-T (10 Mbps) Ethernet followed 10Base2(Thinnet) and 10Base5(Thicknet) all of which ran at 10Mbps. 10Base-T was a vast improvement with respect to cabling. This was in 1990. Then in 1995 with improvement in network electronics and a different coding scheme, 100Base-T was born. All of this takes place at the network or hardware layer. It does not affect the packets of data moving over the wires. So, same size restrictions apply.

 

[Mark R]

Correct. It uses modulation.

That's it.

100 Mbit transmits on a single wire pair at a time, and uses a crude type of modulation called MLT-3 (multi-level transmit - 3 levels) - this transmits 1 bit at a time. After overhead, 100 Mbit ethernet actually transmits 125 Mbit (25 Mbits is for control, etc.). The MLT-modulation, however, reduces the maximum frequency transmitted to 31.25 MHz - improving signal quality over relatively low-fidelity Cat5 cable.

1 Gbit uses a more sophisticated modulatation scheme called PAM-5. This encodes 2 bits into 1 'symbol' (where each symbol is one of 5 different voltages. 2 bits only gives 4, but the 5th is used for error correction). To transmit, a byte is split into 4 pairs, giving 4 voltages which are each transmitted over a pair - at 125 M symbols/sec. 125 M symbols x sec x 4 pairs x 2 bits per symbol gives 1 Gbit/s, with a maximum frequency of 125 Mhz. This is just about within the specification of Cat5 cable.

10 Gbit uses a more sophisticated modulation still. It uses 16 different voltages to represent 3.5 bits (transmitting 7 bits in 2 voltage pulses). 1 symbols is transmitted over each pair at a rate of 800 M symbols/sec. This works out at 11.2 Gbit/s (of which 1.2 Gbit/s is used for ECC and overheads). This is far beyond the capability of Cat5 or Cat5e cable - and requires Cat6.

 

[Eug]

Mellanox claims Cat5e can be used for 10GigE for short distances.

This is what I'm hoping for 15 years down the line. I didn't bother with Cat6a because of the hassle and the cost. If I ever do need 10GigE though, maybe I'll get lucky and it will work OK with the Cat6 (non-a) that I installed, over short distances.

 

[imagoon]

10gig in theory will work on those "Cat5E 550mhz" cables over a shorter distance.. The pure Cat5 spec is 125mhz which will likely fail hard. CAT6 is 250Mhz and CAT6A is 500mhz. 10gig is "specified up to 500mhz."

What it really comes down to is this: If you install better cables (IE cat5e 550mhz) you might be able to run out of spec. Of course running out of spec also means it might not work or work well. For home this may be fine. Data centers and enterprise should just install CAT6A and call it a day. I don't expect a lot of 10gig need in the near future for home / user end points. Longer term is hazy. 15 years is a long time.

Honestly though, the big issue will be the terminations. Improper terminations will wreck the drop faster than the cable will in lots of cases. We might sound like nut cases talking about properly installing wire all the time, but it really is the #1 issue out there for "weird and flaky" issues. Also don't assume your keystone is 550mhz even if your cable is.

Random thing for today though, got a hold of my first chunk of Cat7 cable. That will be a beast to work with.

 

[ScottMac]

(quote)
Honestly though, the big issue will be the terminations. Improper terminations will wreck the drop faster than the cable will in lots of cases. We might sound like nut cases talking about properly installing wire all the time, but it really is the #1 issue out there for "weird and flaky" issues. Also don't assume your keystone is 550mhz even if your cable is.
(end quote)

Amen.

Anything built on a poor foundation will be shaky and fail (and, according to Murphy, fail at the worst possible time). But, worse than failure is partial failure where you end up frustrated and hairless due to marginal signal propagation.