27
votes
Accepted
What's the point of auto-sensing 10/100 Mbit/s Ethernet?
Some devices could only run at 10 megabit/s, so the device at the other end would autosense the speed to match. If a device that has a maximum speed of 10 Mbit/s is connected to a 10 Mbit/s /...
- 2,102
15
votes
How exactly does an Ethernet collision happen in the cable, since nodes use different circuits for Tx and Rx?
A hub is really just a powered cable that repeats every signal it receives on one interface to all the other interfaces. If two devices transmit at the same time to the receive of the hub interfaces, ...
- 98.8k
15
votes
Accepted
How exactly does an Ethernet collision happen in the cable, since nodes use different circuits for Tx and Rx?
To understand this you need to understand the historical context.
Originally Ethernet used a shared coaxial cable. Only one device could successfully transmit on this at a time. If two devices ...
- 13.2k
14
votes
What's the point of auto-sensing 10/100 Mbit/s Ethernet?
Remember 10Mbps came first, then 100Mbps, then 1000Mbps.
The advantage of supporting multiple speeds and automatically switching between them is you can upgrade your network gradually without having ...
- 13.2k
11
votes
How exactly does an Ethernet collision happen in the cable, since nodes use different circuits for Tx and Rx?
Great question.
In full duplex, there is a dedicated channel for traffic from "left to right" and a dedicated channel from traffic from "right to left":
Therefore, in full duplex, collisions are ...
- 15k
7
votes
1000BASE-T with "no autonegotiation"
Autonegotiation should only be disabled where necessary. Properly compliant 802.3 hardware will send and respond to information in link pulses. Only in rare instances -- i.e. metro-ethernet -- are ...
- 31.8k
7
votes
Accepted
How does auto-negotiate determine link speeds?
A 10 Gb NIC connected directly to a 1 Gb either negotiates a 1 Gb connection, or it doesn't work at all. You cannot have the situation you describe unless you have something like a switch in between, ...
- 98.8k
7
votes
Enabling flow control on my ethernet NIC
You were closer than you think to getting it working. In fact the problem may have simply been that flow control wasn't enabled on the switch.
The ethtool source code (rev 3.18) and a register ...
- 81
6
votes
How exactly does an Ethernet collision happen in the cable, since nodes use different circuits for Tx and Rx?
With twisted pair and a repeater hub, the hub is not much more than a digital amplifier. For that it senses a carrier from an incoming signal on one port and switches all other ports to output mode. ...
- 82.7k
6
votes
The recommended settings for giga ethernet connected to Ethernet port
In reality, the legacy 10 Mbps ethernet interface probably can't negotiate, and it can probably only do half duplex (very few 10 Mbps interfaces can do full duplex). You should let the 1 Gbps ...
- 98.8k
5
votes
Accepted
Enabling flow control on my ethernet NIC
Most of this doesn't matter, ethernet flow control has never been widely supported and most switch devices will respect PAUSE frames, but not send them. That being said, your questions can be ...
- 508
5
votes
Accepted
The recommended settings for giga ethernet connected to Ethernet port
The very short answer: don't configure anything.
Auto negotiation (or the lack thereof as Ron's detailed) works only when it's left alone. Manual settings can very easily cause problems either right ...
- 82.7k
5
votes
Accepted
On RJ-45/UTP with 2 pairs, (Tx, Rx) which pair will be used in case of half-duplex?
For 10BASE-T and 100BASE-TX, the MDI side (NICs, routers) transmits on pairs 1-2 and receives on 3-6. On the MDI-X side (hubs, switches) the pairs are swapped. In general, concentrators use MDI-X ...
- 82.7k
5
votes
Accepted
Static speed and duplex/no-negotiate causing input error
If you disable Auto Negotiation (AN) you need to make sure that both sides are configured in exactly the same way. There's isn't any point in doing that manually, actually, so you should have AN ...
- 82.7k
4
votes
How exactly does an Ethernet collision happen in the cable, since nodes use different circuits for Tx and Rx?
Suppose machine A starts sending data to machine B. As the packet begins to be sent, machine C starts sending different data to machine B. There is only one signal path to machine B, so the ...
- 354
4
votes
Accepted
Verifying 802.3 auto negotiation
With gigabit endpoints (1000BASE-T) your injector needs to be gigabit capable as well.
Cheap injectors insert power on the pairs unused by 10BASE-T and 100BASE-TX while disrupting the data ...
- 82.7k
4
votes
Static speed and duplex/no-negotiate causing input error
By manually setting the speed and duplex on one side with automatic detection on the other side, the side with automatic speed will detect (not negotiate) the correct speed.
For the duplex, ...
- 98.8k
3
votes
How to connect an old network card (10 Mbps) to a new network (1000Mbps)
The router/switch you are plugging into should negotiate with the card. If you are plugging into a managed switch, you will likely be able to hard code the port's speed/duplex. In a Cisco device this ...
- 1,010
3
votes
On RJ-45/UTP with 2 pairs, (Tx, Rx) which pair will be used in case of half-duplex?
Half-duplex mode in Ethernet still uses 2 pairs. The difference to full-duplex is that it only uses one pair at a time.
Half-duplex mode with twisted pair cables only exists because of hubs:
Reason ...
- 356
3
votes
Accepted
Capture auto-negotiation ability flags
Auto negotiation is performed on the physical layer - the NIC driver may report what has been negotiated or it may not (only speed). If you want to look deeper into this I see two variants:
Use ...
- 82.7k
3
votes
Reliable transfer but unreliable auto-negotiation?
You simply cannot mix shielded and unshielded parts in a link. The shield only works if it continuous end-to-end and properly grounded on both ends. The actual wires in shielded cabling cannot meet ...
- 98.8k
3
votes
Accepted
Why is sometimes a good idea to disable auto-negotiation?
With rare exceptions, disabling Auto Negotiation is not a good idea.
Auto Negotiation (AN) is mandatory for 1000BASE-T and faster. It should stay enabled generally. Disabling it makes a default node (...
- 82.7k
3
votes
Why is sometimes a good idea to disable auto-negotiation?
It's important to understand that the cases where auto-negotiation should be disabled ARE VERY RARE. Many times study material for tests are a bit out of date. Virtually all equipment manufactured in ...
- 67.1k
3
votes
Accepted
How is the maximum speed of an ethernet cable detected?
There's Auto Negotiation and that's it. Both sides advertise the speeds and modes they support and the best mutual mode is chosen. The cable is not tested. If the cable quality doesn't support the ...
- 82.7k
2
votes
The recommended settings for giga ethernet connected to Ethernet port
A) offer only a subset of speeds
In cases where speed/duplex negotiation becomes difficult, you may want to consider the following to restrict a switch to offer only a (sub)set of speeds on a given ...
- 8,826
2
votes
Accepted
Forced full duplex and autonegotiation
When you specifically set the speed and duplex on one side of a link, that side will not negotiate, so there is no negotiation which takes place. The other side can detect, not negotiate, the link ...
- 98.8k
2
votes
How to connect an old network card (10 Mbps) to a new network (1000Mbps)
Have you tried simply plugging it in?
Many, even most, (but not all) gigabit ports will negotiate down there.
If the one you have won't, buy a small switch that will (look for specs like 10/100/...
- 2,746
2
votes
Do I need Auto-Negotiation in 1000BASE-X Ethernet if I have a point-to-point link always at 1 Gbps?
The link probably doesn't need auto-negotiation, but you probably should not change the default without a good reason. There is nothing about having it configured that will cause you any problem, and ...
- 98.8k
2
votes
Accepted
Auto negotiation and SFP+
For Ethernet, Auto Negotiation is only defined for twisted-pair and backplane variants (see IEEE 802.3 clauses 28, 37, and 73).
Other PHYs can use auto detection to fall back to a link partner's ...
- 82.7k
2
votes
cisco port status regarding the auto-negotiation
What we usually call auto-negotiation is the process by which 2 devices agree on the link speed and duplex. This is really fast (almost instant from a human point of view) and, while issue with this ...
- 19.5k
Only top scored, non community-wiki answers of a minimum length are eligible