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Ethernet Quartet: Old Speed, New Features


On February 5 of this year, a new standard for 10-megabit Ethernet was approved. Yes, you read it right: ten megabits per second.

Why is such a “small” speed needed in the 21st century? To replace the zoo, which is hiding under the capacious name of "field bus" - Profibus, Modbus, CC-Link, CAN, FlexRay, HART, etc. There are too many of them, they are incompatible with each other and relatively difficult to configure. And you just want to plug the cable into the switch, and that’s it. As with regular Ethernet.

And soon it will be possible! Meet: "802.3cg-2019 - IEEE Standard for Ethernet - Amendment 5: Physical Layer Specifications and Management Parameters for 10 Mb / s Operation and Associated Power Delivery over a Single Balanced Pair of Conductors."

What makes this new Ethernet interesting? First of all, it works on one twisted pair, not four. Therefore, it has fewer connectors and thinner cables. And you can use the already laid twisted pair, going to the sensors and actuators.

You might argue that Ethernet runs up to 100 meters and the sensors are much further away. Indeed, this used to be a problem. But 802.3cg works at a distance of 1 km! One pair! Not bad?

Actually, it’s even better: power supply can also be supplied for the same pair. Let’s start with him.

IEEE 802.3bu Power over Data Lines (PoDL)


I think many of you have heard of PoE (Power over Ethernet) and know that 2 pairs of wires are needed to transfer power. Power input / output is made at the midpoints of each pair of transformers. Using one pair is impossible to do. Therefore, I had to do it differently. How exactly - shown in the figure below. For example, classic PoE has been added.



Here:
PSE - power sourcing equipment (power source)
PD - powered device (far-end device that consumes electricity)

Initially, 802.3bu had 10 classes of power supply:



Three conditional gradations of the source voltage are highlighted in color: 12, 24 and 48V.

Designations:
V pse - power supply voltage,
V V pdmin is the minimum voltage per PD, V
I max is the maximum current in the line, A
P pd max is the maximum power consumption of PD, W

With the advent of the 802.3cg protocol, 6 more classes were added:



Of course, with such a variety, PSE and PD must agree on the power class before applying full voltage. This is done using SCCP (Serial Communications Classification Protocol). This is a low-speed protocol (333 bps) based on 1-Wire. It works only when the main power is not supplied to the line (including in sleep mode).

The flowchart shows how power is supplied:

  • a current of 10mA is applied and the presence of a 4V zener diode at the other end is checked
  • food class approval
  • main meal is served
  • if the consumption drops below 10 mA - sleep mode is activated (supply of standby power 3.3V)
  • if consumption exceeds 1mA - wakeup occurs



The coordination of the food class can be omitted if it is known in advance. This option is called Fast Startup Mode. It is used, for example, in automobiles, as there is no need to change the configuration of the connected equipment.

Both PSE and PD can initiate hibernation.

Now let's move on to the description of data transfer. It is also interesting there: the standard defines two operating modes - long-range and for small distances.

10BASE-T1L


This is an option for long reach. Key features are as follows:

  • range of action - up to 1km
  • 18AWG conductors (0.8mm 2 )
  • up to 10 intermediate connectors (and two terminal)
  • -
  • 7.5
  • PAM-3, 4B3T
  • 1 (1Vpp) 2.4
  • Energy Efficient Ethernet (“quiet/refresh” EEE)

Obviously, this option is intended for industrial use, access control systems, building automation, elevators. To control rooftop chillers, air conditioners, fans. Or located in technical rooms heating boilers and pumps. That is, a lot of all kinds of applications besides industry. Not to mention the Internet of Things (IoT).

It is worth mentioning that 10BASE-T1 is only one of the Single Pair Ethernet (SPE) standards. There are also 100BASE-T1 (802.3bw) and 1000BASE-T1 (802.3bp). True, they were developed for automotive applications, so there is a range of only 15 (UTP) or 40 meters (STP). However, the plans already have a long-range 100BASE-T1L. So in the future, auto-negotiation of speed will be added.

In the meantime, coordination is not used - the “quick start” of the interface is announced: less than 100ms from power supply to the start of data exchange.

Another option (optional) is to increase the transmission amplitude from 1 to 2.4V to improve the signal-to-noise ratio, reduce the number of errors, and counteract industrial interference.

Well, of course, EEE. This is a way to save electricity by turning off the transmitter if there is currently no data to transmit. The diagram shows how it looks:


No data - send the message “I went to bed” and turn off. We wake up occasionally and send the message “I'm still here.” When data appears, the warning “I wake up” is issued to the opposite side and transmission begins. That is, only receivers constantly work.

Now let's see what came up with the second version of the standard.

10BASE-T1S


Already the last letter makes it clear that this is a protocol for short distances (short reach). But why is it needed if at short distances the T1L also works? We read the characteristics:

  • range up to 15m in point-to-point mode
  • duplex or half duplex
  • 24-26AWG conductors (0.2-0.13mm 2 )
  • Symbol Rate 12.5Mbaud
  • DME 4B5B Encoding
  • signal with an amplitude of 1V (1Vpp)
  • up to 4 intermediate connectors (and two terminal)
  • no EEE support

It seems to be nothing special. So what is it for? But for this:

  • range up to 25m in multipoint mode (up to 8 knots)

And this:

  • PLCA RS (PHY-Level Collision Avoidance Reconciliation Sublayer) mode of operation

And this is already much more interesting, isn't it? Because helps to greatly reduce the number of wires in control cabinets, machines, robots, cars. And there are already proposals to use it as a replacement for I2C in servers, switches and other electronics.



But the multipoint mode has disadvantages. The main one is a shared data transmission medium. Of course, collisions are resolved using CSMA / CD. But it is not known what the delay will be. And for some applications this is critical. Therefore, in the new standard, the multipoint was supplemented with a special PLCA RS mode (see the next section).

The second drawback is that PoDL does not work in multipoint. That is, you have to supply power via a separate cable or take it somewhere on the spot.

However, in point-to-point mode, PoDL also works on T1S.

PLCA RS


This mode works as follows:

  • , ID=0
  • BEACON signal,
  • , 20 ,
  • ( ),

In general, it resembles TDMA. But with the peculiarity that the node does not use its time frame if it has nothing to transmit. And the frame size is not hard-set, because depends on the size of the data packet transmitted by the node. And all this works on top of standard Ethernet 802.3 frames (PLCA RS is optional, so there must be compatibility).

The result of using PLCA is at the bottom of the graphs. The first is the delay depending on the load, the second is the bandwidth depending on the number of transmitting nodes. It is clearly seen that the delay has become much more predictable. And it is in the worst case 2 orders of magnitude smaller than in the worst case CSMA / CD:



And the channel capacity in the case of PLCA is higher, because Not spent on resolving collisions:



Connectors


Initially, we chose from 6 options for connectors offered by different companies. As a result, we settled on these two options:



For ordinary operating conditions, the CommScope LC IEC IEC17171-1 was selected.



For harsh environments, the HARTING family of IEC 63171-6 connectors (formerly 61076-3-125). These connectors are rated for IP20 to IP67.



Of course, connectors and cables can be either UTP or STP.

Other


You can use a standard four-pair Ethernet cable, using each pair for a separate SPE channel. In order not to pull four separate cables somewhere into the distance. Or use a single-pair cable, and at the far end put a single-pair Ethernet switch.

And you can connect this switch directly to the local network of the enterprise, if in a distant distance the network is already stretched along the fiber. Stick sensors into it there, and read the readings from them here. Directly over the network. Without interface and gateway converters.

And it doesn’t have to be sensors. There may be video cameras, intercoms or smart bulbs. Actuators of any valves or turnstiles at the checkpoints.

So the prospects are interesting. It is unlikely, of course, that SPE will replace all field buses. But he will bite off a fair bit of them. Already in cars for sure.

PS I did not find the text of the standard in the public domain. The above information was collected in pieces from various presentations and materials available on the Internet. So there may be inaccuracies in it.

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