Those who have become accustomed to router-speak are encountering the new terminology applied to optical transmission devices, such as Dense Wavelength Division Multiplexing (DWDM) gear.
Although the technology is new to us, DWDM has been a staple of the carrier networks for several years. Anyone who has worked with the telephone carriers knows that they employ unique terminology. Because DWDM gear descended into the enterprise space via the carriers, we must learn "carrier speak" to properly understand it.
For example, DWDM equipment operates at layer one, so it can't be considered a layer-two/three switch or a layer-three router. It changes connections, but also has intelligence, so it can't be considered strictly a patch panel. So what do we call it?
Some networkers might prefer to describe DWDM with the generic term "equipment" or "gear." However, the carriers refer to this kind of equipment as a "shelf." This is not our definition of a shelf (someplace to place your test equipment). But since DWDM gear comes tagged with the carrier "shelf" nomenclature, we might as well get used to it.
The differences extend beyond simple terminology. Even the functionality of DWDM gear is not what we expect from most network devices. Unless you understand the difference, things can become confusing very quickly. For example, DWDM shelves are not the least bit shy when it comes to triggering alarms.
Critical and major alarms are serious business to networkers. When we see such an alarm on our networks, our training tells us to jump into the problem feet first. Critical alarms usually mean a net-down is in progress or even in full swing -- there isn't a minute to lose.
With DWDM gear, that kind of response can be just a waste of adrenaline. A critical or major alarm LED on a shelf doesn't necessarily mean the shelf is in trouble. It might be operating just fine, thank you. You have to think differently.
Critical or major alarms can be triggered by what most networkers would consider to be innocent events; for instance, manual configuration changes made to the shelf. Critical alarms can also be triggered if a card that has not been configured is inserted in the shelf. (How many times do you slip a new card in a router, expecting to configure it later?) This is a normal event for network equipment, but not for carrier equipment. Why? The installer may have driven for 4 hours in the snow to reach the equipment. The vendor doesn't want the cards in the shelf left unconfigured because of a simple oversight by the installer -- hence the critical alarm.
For the same reason, if one of the redundant power supplies has failed, or has even been turned off, a critical alarm LED will light.
Speaking of LEDs, when some DWDM shelves are powered down, their power status indicators don't go off; they turn red and stay on. But have no fear -- we may become more familiar with such events. Our newer home gear seems to be following the carriers' lead. For example, my new Sony DVD player alarms when there is no DVD in the player. When I turn it off, the green power LED doesn't shut off. It turns red, just like the power switch on a DWDM shelf. As with the carrier optical shelves, there is nothing wrong with the player -- I just have to think differently.