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PoE (Power over Ethernet) Midspan units are installed between standard Ethernet switches and IP phones, WiFi access points, and IP security cameras without modifying the existing network infrastructure. Midspans add 48V DC power to each Ethernet cable, to remotely power IP devices that support the Power over Ethernet IEEE 802.3af standard centrally from the switch closet. To enable use in legacy installations, a midspan adds PoE capabilities to existing hubs and switches by injecting power into the twisted-pair cabling. Additionally, you can provide battery backup to all of your dispersed IP phones, access points, and IP cameras using a single centrally located UPS system.
One of the leaders in the PoE midspan market is PowerDsine, who has shipped over 2 million Power over Ethernet Ports. They sent us a couple of their midspans to test, including the industry�s first 24-port Gigabit midspan (PD6024G) (Figure 1) as well as their 48-port 10/100Base-T midspan (PD6548), which is the industry�s largest and first 48-port midspan.
Each of the PowerDsine models lets you assign an IP address for SNMP management as well as for Web-based access to the unit. Also, the latest version defaults to IP address 192.168.0.50 so you can change the IP address without needing a serial cable along with HyperTerminal to first assign an IP address, which is always a pain. The Web interface is a nice addition since many SMBs don�t have HP OpenView or other SNMP software to manage SNMP devices. Having a standard Web interface gives them another option for managing their network equipment. Additionally, the units sport a RS-232 serial interface which supports a very fast 38,400 baud rate perfect for uploading new software/firmware updates.
The real trick is to prevent the midspan from damaging equipment that�s not PoE- compatible. To do this, the PowerDsine midspan performs a discovery process that runs at power-on, as well as every time the user plugs a device into a PoE port. It also detects when the user disconnects a device and disconnects the port within a few hundred milliseconds. According to PowerDsine, the discovery process was one of PoE�s major challenges and in fact, PowerDsine helped write and ratify the IEEE 802.3af standard. PowerDsine actually leveraged their expertise in designing ring voltage generators and applied that knowledge from the telecom world to the datacom world. In any event, the method used to detect devices connected and disconnected uses �resistance discovery� looking for signature impedance by applying a current limited �test signal� to the cable and looking for return voltage.
Both the PD6024G (Gigabit 24-port midspan) and the PD6548 (48-port midspan) are constantly calculating power consumption in real time to ensure an overload condition does not occur. If it is near an overload condition it won�t let a new device to be added. It assigns priority for the power by the port number, so if you reach an overload it will disconnect the lowest priority device. For instance, this lets you prioritize placing the CEO�s IP phone at port #1. In addition, both units keep a �power gap,� a little bit of �wiggle room� for safety to make sure you never reach an actual overload condition. Each of these midspans supports up to the maximum standard 15.4W per port with 200W of total power from the power supply. Of course, if you take the either model and attempted to draw the maximum 200W you would only be able to connect 12 ports at the maximum before you�d exceed the 200W capacity with the 13th device. However, very few devices require a full 15.4W of power, so you wouldn�t need a stronger
power supply any way. In fact, most IP phones only use 5.6W, so you�d be able to power 24 IP phones with power to spare.
The midspans have temperature sensors both on a per port basis as well as system-wide to make sure you don�t overheat. One really nice capability of independent sensors is that if a port overheats only that port is turned off. Obviously, if the system-wide sensor reaches a certain threshold then the entire system is shutdown. LEDs for each port indicate the status of the units including yellow to indicate an overload, as well as orange to indicate a system fault. Both models are compatible with MIB-based network management platforms (SNMP � Figure 2). The PowerView software monitors IP Phones for possible failure as well as IP phones status alerts: power fall, disappearance and malfunction.
TMC Labs performed extensive tests on each of the models and used a variety of PoE-compatible IP phones to see if the midspans would detect and properly power the IP phones. We connected Cisco phones, Pingtel xPressa phones, and snom phones. All of the phones were successfully powered by the PowerDsine models. PowerDsine also shipped us a special test unit that, when plugged into the midspans and using a special dial, we were able to select the amount of power drawn. As we increased the dial setting we were able to draw enough wattage to simulate an overload condition. The port would then go offline and the LED would indicate an overload condition. Once we turned the dial back down to draw less power the port would then go back online.
We were quite impressed with both the PowerDsine 24-port Gigabit midspan (PD6024G) and the 48-port 10/100Base-T midspan (PD6548). With a built-in Web interface and SNMP capabilities, making sure your mission-critical IP phones or even PoE-aware WiFi access points and PoE-aware cameras are online and trouble-free is very easy to do. TMC Labs would not hesitate to recommend either of these PowerDsine models to suit your power over Ethernet needs. IT
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