Frequently Asked Question
UPDATE 21 August 2019
Because numerous third-party client devices have had problems with the Cisco 2960x POE circuit and their extremely short time threshold for overload detection, we wanted to publish this link on Cisco's site discussing potential work-arounds, such as using a longer (50-foot) cable to potentially slow down the inrush/response.
Cetis' First generation of SIP phones were introduced in 2008 and End-of-Lifed in 2018. During that time, SIP interoperability and certification focused on SIP protocol interoperability and NOT usually on the standard POE specification 802.3af. Almost all POE switches and client devices interoperate well....usually.
We found that as some hotels began to update their infrastructure that they sometimes settled on a Cisco 2960x series Power over Ethernet switch. Unfortunately, it's often the case that there is a failure to research and/or test ALL the existing equipment in-house before purchasing and installing new equipment.
In the case of the Cisco 2960x, sometimes the switch would detect an in-rush of current as the phone boots up and would turn off power to the port, thus turning off power to the phone endpoint as well. In other cases, this does NOT happen.
Over the 10-year life span of that 1st generation SIP phone family, the only POE LAN switches that Cetis 1GEN phones experienced incompatibility with are these Cisco switches (some of which have also been discontinued) that have “hair trigger” protection systems which rapidly and abruptly shut off the power feed to the far-end device (e.g. the telephone). All other brands/models of POE LAN switches give far-end devices “a little more time to start up and stabilize” before they shut off their power feed.
Cisco POE LAN switches are the ONLY popular POE LAN switches which have trouble with 1GEN and some 2GEN Cetis models. Cetis has tested plenty of HP, Dell, D-Link, and Adtran POE LAN switches. They always work perfectly with all generations and models of Cetis IP telephone sets.
The main problem with the Cisco POE LAN switches is that they contain a hyper-sensitive overload detector which instantly switches off the POE port when there is a very brief overload during POE power-up. Just a few msec of overload is sufficient for the Cisco switch to totally turn of its power feed.
In contrast every other POE LAN switch we have worked with limits the peak current during POE start-up to less than 500 ma, but all these other switches do this by temporarily reducing their output voltage. This power management technique appears to power up the connected device seamlessly every time. These other POE LAN switches DO NOT “shut off totally” when there is a brief overload during power-up.
Please note that the official POE maximum peak inrush limit is
Accurate measurement of POE inrush current is a challenge:
· The duration of the inrush current surge can be extremely short (less than 1 microsecond) or it can be larger (1 to 50 milliseconds).
· Many measurement techniques totally fail to detect surges with durations
· Also the 48v power source used for POE inrush testing must be capable of providing a much higher peak current than the expected values.