So, how does this work in the real world? Rich Hernandez lays out some great examples in a paper titled “Gigabit Ethernet Auto-Negotiation,” published by Dell. Let’s take a look at his three cases. In the first one, you connect a 10Base-T half-duplex device to a GigE switch. In the second case, you connect a 100Base-T half-duplex device to the same switch. In the last example, you connect a 100Base-T auto-negotiation capable device to the GigE switch. Remember, the first two devices do not have auto-negotiation capabilities. Let’s look at some diagrams that will help illustrate what is going on as each pair negotiates a link.
In the first case, both devices begin in a link fail state. (See Figure 1.) The GigE switch begins sending Fast Link Pulses. The old 10Base-T device begins sending NLPs. When the GigE switch receives the NLPs, it immediately hands off negotiation to a legacy 10Base-T Physical Medium Attachment module.
From this point on, the GigE switch behaves as if it were a legacy 10Base-T device. It begins sending NLPs. Both devices detect NLPs and transition from link fail to link UP state. As Hernandez notes, the default condition when a GigE device detects a 10Base-T device is half-duplex. If the 10Base-T device is set to full duplex, it will not work.
In the second situation, we are connecting a 100Base-T device without auto-negotiation to a GigE switch. (See Figure 2.) Again, both devices begin in a link fail state. The switch begins transmitting Fast Link Pulses. At the same time, the device, which knows nothing about FLPs, begins transmitting idle symbols. The GigE switch detects the idle symbols and immediately knows that it is trying to negotiate with a 100-Base-T device. As with the 10Base-T device, only half-duplex is supported when a GigE device is establishing a link with a non auto-negotiation device.
The GigE switch begins sending out idle symbols, both devices then recognize the other, and they change their status from link fail to link UP.
In the final case, both devices are auto-negotiation capable, but there is a speed mismatch. One is a 100Base-T device, and the switch is GigE. Figure 3 shows that the devices begin in link fail. Both of them transmit FLPs. The highest capability of the 100Base-T device is 100Base-T, full duplex. Both devices recognize that this is the fastest, shared capability mode, so the GigE switch configures itself for 100Base-T full-duplex. At that point, the devices change state from link fail to link UP.
When auto-negotiation works, it works well. But, problems can arise when a GigE switch expects a slower device to be in half-duplex mode and it is instead in full duplex. When this happens, links may either become extremely slow, or they may not work at all. It is critical that engineers who maintain professional media networks understand that incorrect duplex configurations in older equipment can cause serious performance problems.
The root of the issue is in the original Ethernet collision and backoff method of sharing a single piece of media. There is no guarantee in Ethernet that two cards will not try to talk at the same time. When they do, both cards detect a collision, stop transmitting and then retry after a random period of time. In a multiplex mis-configuration situation, the card in half-duplex detects a collision and retries. The full-duplex card does not retry, but it does detect that the incoming packet is bad and discards it. So, be sure you configure cards correctly.
Looking ahead through the coming months, we will explore additional basic networking concepts.
—Brad Gilmer is executive director of the Video Service Forum, executive director of the Advanced Media Workflow Association and president of Gilmer & Associates.