Overview

Power over IP (PoIP) – the technology allowing the remote device to transmit electrical energy along with the data over a single twisted pair cable. This technology is designed for various sensors, intelligent relays, ZigBee transmitters and other wireless technologies with the super-low energy consumption to which for any of several reasons it is undesirable or impossible to conduct a separate electrical cable. It is assumed that the Power over IP technology will be the most demanded for construction of the distributed sensor networks, and also for IoT support (Internet of Things).

The PoIP technology is currently under development, the publication of the first version of a standard draft is under construction, however a number of companies are already preparing for pilot use of the technology being developed in their equipment. Among the vendors actively involved in the development of prototype devices with PoIP support are: ARM, Cisco, Intel, ASUS, Broadcom, Realtek and STMicroelectronics.

Equipment and working principle

The PoIP technology doesn't affect the quality of the transmitted data. Transmission of energy is made by means of properties of the physical Ethernet layer. The main difference of PoIP from PoE (Power over Ethernet) is that PoE uses so-called out of band transmission of energy whereas a power of PoIP devices is made by means of the energy transferred by a signal itself.

For the operation the PoIP technology requires 10Base-T (IEEE 802.3) Ethernet support in the full-duplex mode from the switching equipment. It is worth noting that the recommended switch setup is automatic detection of speed and duplex though also manual configuring isn't forbidden. All equipment with support of PoIP shall support also automatic cabling (Auto MDI / MDIX). The 10Base-T standard defines a voltage value of ± 2.5 V for the transmitting pair, which together with a maximum current of 250 mA allows the device to operate at a power up to 0.5 Watts. The 10Base-T Ethernet standard supports cable segments up to 100 meters long, however when using PoIP it should be taken into account that in case of transmission of energy via the twisted pair cable there is a significant power loss depending on the distance and thickness of the conductor therefore in case when PoIP devices are connected it is recommended to use short and thick cables. Typically, the conductor's thickness for the twisted pair cable is specified in AWG (American Wire Gauge). In the table below compliance of thickness of a cable given in AWG to cross-sectional area of the conductor for solid and stranded cables is shown.

AWG Cross-section area, mm2
Solid Stranded
22 0,325 0,327-0,352
24 0,205 0,201-0,226
26 0,128 0,14-0,153

The resistivity of copper is approximately 17mOhm*mm2/m, that is the solid twisted pair cable 24 AWG 100 meters long will have resistance more than 16,5 Ohms. Thus, the maximum power which the PoIP device in this case can "expect" won't exceed 0,2 Watts.

In case your network is under congestion, UDP as a transport looks inappropriate for energy delivery, because of its unreliable nature. In this case, one should use unicast and TCP as a reliable transport protocol. To improve PoIP efficiency you need to use jumbo frames, because you can’t take advantage of any headers in PoIP process.

Until now we spoke only about a "client" part of PoIP technology. However the switch itself won't supply the devices connected to it with energy even of 0.5 Watts. In order the energy transmission was carried out, the flow of specially created traffic is necessary. Any device with IPv4 support can generate and transfer such traffic. For security reasons, and also to reduce the load on the network infrastructure, the equipment generating a traffic must be located in one local segment with PoIP clients. Reduction of network load happens also due to use of multicast. In case of each start and during an operating time in an automatic mode the choice of the most optimum traffic profile (pattern) allowing to maximize the energy received by clients is made. If for any reason the optimum traffic profile for one of devices differs from coordinated for remaining devices on this network, then for such client generation of unicast is allowed.

High availability

All PoIP clients shall register on all local generators and report to them about optimum traffic patterns. However at each moment sending a traffic is made by only one generator. All remaining generating devices are also joined for receiving a flow from the active source. The group traffic flow from the active source performs also the keepalive functions. In the absence of traffic flow during 50 msec the role of the active device takes over the equipment with the highest priority. The priority can assume values from 1 to 255, by default – 100.

Client PoIP device shall possess the built-in rechargeable battery and be capable to endure short-time miss of IP traffic flow.

Vendors

At the time of the publication three companies reported about development of the module generating IP traffic: ASUS, Cisco and Intel.

The ASUS company began to build in the traffic generator to top models of the routers. Control is made by means of the hidden PoIP gen tab of Network Utilities menu item.

 

All industrial routers, and also L3 switches of the Cisco Systems company working under control of the IOS (starting with the version 15.6(2)T), IOS XE (from version 16.04.01 Everest) and IOS XR (from version 6.1.3), possess a set of the hidden commands allowing to turn on the built in generator. PoIP support for NX-OS series switches is not planned.

cisco#sho poip ?
% Unrecognized command
cisco#sho poip devices
Capability codes:
    (G) Generator, (P) Probe, (L) LED, (W) WLAN Access Point
    (R) Relay, (S) Station, (B) Beeper, (O) Other
Device ID                          Hold-time  Capability      Port ID       IP Address
fox_temp_sensor_test1              50         P,R             Gi1/0/10      192.168.1.114
fox_humidity_probe_test1           50         P               Gi1/0/12      192.168.1.112
fox_alarm_test                     50         L,B             Gi1/0/24      192.168.1.140
fox_poip_gen                       50         G,S             Te1/0/4       192.168.1.1
Total entries displayed: 4
cisco#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
cisco(config)#poip ?
% Unrecognized command
cisco(config)#poip test
cisco(config-poip)#?
Configure commands:
  access-class                Filter client devices based on an IP access list
  authentication              Auth Manager PoIP Configuration Commands
  bandwidth                   Set max bandwidth to utilize
  default                     Set a command to its defaults
  description                 PoIP instance specific description
  interface                   Select an interface to support
  logging                     Handles logging operations
  mode                        PoIP operating mode
  network-policy              Network Policy
  password                    Secret key for MD5 authentication
  pattern                     Configure a packet pattern
  preempt                     Overthrow lower priority Active generators
  priority                    Priority level
  shutdown                    Shutdown this instance of PoIP
  static                      Define a static PoIP client
  timer                       Hold timer
  track                       Priority tracking
  version                     PoIP version

The utility of Intel company – Intel PoIP Gen, on announcement of the vendor, completely corresponds to RFC3251 (Electricity over IP) and at the moment passes a stage of internal testing, remaining unavailable to ordinary users.

Future of the standard

At the moment as a physical medium the twisted pair Ethernet cable is used. Also support of fiber optic and wireless connections is put in road-map if it is  possible to reduce energy consumption of devices even more.

The standard draft assumes a possibility of connection of PoIP device by means of two twisted pair cable for doubling of the energetic budget.

Also among innovations support of upcoming version of the IP protocol – IPv6 is declared.

Appearance of more energy efficient client devices on the basis of SoC with the low power consumption will allow expanding in the near future a scope of Power over IP technology considerably.

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