Motorola’s Powerline Multiple-Unit (MU) Gateway is designed to offer a quick and easy way of providing buildings with LAN connectivity over the mains electricity supply. Motorola is a member of the HomePlug Powerline Alliance, and the system works by converting Ethernet signals into the HomePlug protocol before injecting it directly into a building’s electrical system.

The gateway operates in the 4-21MHz spectrum and can be used in single-phase or three-phase electrical systems, with security being taken care of by the 56bit Data Encryption Standard (DES).

Raw Powerline data rates are currently 85Mbit/s and Motorola quotes typical user data transfer rates of about 8-12Mbit/s per gateway. The maximum transmission distance between Gateway and modem is 100 metres, and one Powerline MU Gateway can serve up to 50 users simultaneously.

Firms thinking of installing a system for LAN connectivity over mains power must have a site visit from Motorola to establish the layout of the electricity supply in the building. Installation also requires the co-operation of the firm’s maintenance contractor.

The system can be used in large single properties, multiple units and across enterprise campuses. Powerline MU products are designed to be used on low-voltage wiring, which in the UK means 230V and 50Hz AC.

The system can use two methods of injecting the signal into the mains: inductively coupled injection or capacitive injection. The inductive coupler consists of non-conductive ferrite rings that can be wrapped around the electric cables in the electricity distribution cabinet, with one coupler being used per electrical phase. The signal penetrates the cable’s insulation and enters the electric lines without touching the bare wire. Motorola says this is its preferred signal injection method for multi-metered buildings.

For our test we used capacitive injection to send signals between two modem s, a desktop PC and a laptop. We connected the gateway to a switch to get internet access, although users can configure the gateway’s onboard WAN link to be front-ended by a firewall before connection to a router.

The modems would normally be connected to a switch or wireless access point to give connectivity to desktop systems. This is the method we pursued while the gateway was connected to a switch and then through our four-port BT ADSL router. We also attached our 3Com AP7250 wireless access point to one of the modems, allowing us to access the internet wirelessly.

The Gateway is fairly simple and can be managed through a web-based interface or by using a dedicated management package called Prizm, which we installed on our Windows Server 2003 system.

Data transfer rates

We checked data transfer rates by measuring the time it took to send a 160MB file between the laptop and a shared drive on the desktop system. By placing the modem attached to our laptop at increasing distances from the gateway we could build up a picture of how transfer rates fared as the modem got further away from the gateway’s signal injection point.

The desktop was connected to one of the four 10/100Mbit/s LAN ports on the gateway. An initial measurement with the modem next to the gateway’s injection point gave a transfer rate of 10.9Mbit/s, while moving the laptop to a desk a few feet away resulted in a data transfer rate dropping to 3.6Mbit/s. We then plugged the laptop into a socket some 18 feet from the unit and recorded a rate of 1.2Mbit/s. We could not ascertain whether these varied results were down to the electrically hostile Labs environment or due to a non-optimal signal injection position.

To tackle situations where the transfer rate falls off rapidly, Motorola offers modem distribution units that act as repeaters to increase signal strengths. These units can service up to four modems.

Each Powerline modem has four LEDs that indicate whether the unit is receiving AC power, whether it can detect Powerline signals on the wires, whether it has established an Ethernet link to connected devices, and finally whether it has registered with the nearest Powerline gateway.

Managing the system by browsing to the gateway’s web page was straightforward. We could set network encryption keys, whether the gateway was to use DHCP and which authentication servers to use. We could also set up SNMP traps for problem condition alerts. It was easy to see which modems were currently up and running, and to get access to systems logs.