Midspan PoE

3 posts by 3 authors in: Forums > CWNA - Enterprise Wi-Fi Admin
Last Post: August 22, 2006:
  • There's a question in Pool D about replacing a hub with a Midspan PoE switch. The correct answer alleges that such a switch does not repeat an ethernet signal like a hub (or normal switch) would.

    Since I've never heard of a "Midspan PoE Switch," I Googled. And found almost nothing. A single page of hits. Only one of which seemed to have any real info, but it was short on details about whether this was a switch in the Ethernet sense or a switch in some other marketing fantasy.

    I did find:

    which had some useful info. They call their product a Poe Controller, not a switch. But of course, I disagree with some of their financial analysis.

    We're considering such hardware at the hospital where I work because, as Devin's podcast notes, there's no way we can afford to replace all our switches with PoE model switches, financially or electrically due to increased heat and need for additional power wiring.

    So I think we need a good definition for "Midspan PoE Switch" or perhaps the question should use some other term, like Controller?


  • Hi Seymour:

    My opinion: The word "switch" boosts sales as reliably as lowering prices one penny to $29.99. Early vendors of Wireless LAN (WLAN) thin access points (AP) solutions created a new breed of equipment that implemented the AP functionality left out of the thin APs. They wanted to support power over Ethernet (PoE) for the thin APs but knew many customers didn't have the PoE infrastructure in place. So a PoE capable Ethernet switch was built into the product. Now what to call this new thing with mysterious internal capabilities and rows of RJ45 Ethernet jacks on the face? But of course -- a WLAN Switch! Well it wasn't long before enough customers had other PoE solutions in place that it made no more sense to build it into the WLAN "switch", and without the PoE capable Ethernet jacks the box no longer looked like an Ethernet switch. Thin AP vendors renamed their mystery products "WLAN Controllers", but the word "switch" lingers on.

    In the case of midspan power over Ethernet, the better term is "midspan equipment" or more loosely "midspan device". Here is text from IEEE:

    IEEE 802.3-2005 section 33.2.1 PSE location

    "PSEs may be placed in two locations with respect to the link segment, either coincident with the DTE/Repeater or midspan. A PSE that is coincident with the DTE/Repeater is an ?¡é?€??Endpoint PSE.?¡é?€?? A PSE that is located within a link segment that is distinctly separate from and between the MDIs is a ?¡é?€??Midspan PSE.?¡é?€?? The requirements of this document shall apply equally to Endpoint and Midspan PSEs unless the requirement contains an explicit statement that it applies to only one implementation. See Figure 33?¡é?€?¡°4.

    "Endpoint PSEs may support either Alternative A or B, or both. Endpoint PSEs can be compatible with 10BASE-T, 100BASE-TX and/or 1000BASE-T.

    "Midspan PSEs shall use Alternative B. Midspan PSEs are limited to operation with 10BASE-T and 100BASE-TX systems. Operation of Midspan PSEs on 1000BASE-T systems is beyond the scope of this standard."

    PSE - Power Sourcing Equipment
    DTE - Data Terminal Equipment
    MDI - Media Dependent Interface
    Alternative A - Power on pairs used or unused by data
    Alternative B - Power on pairs unused by data
    10BASE-T - Uses two pairs for data
    100BASE-TX - Uses two pairs for data
    1000BASE-T - Uses four pairs for data

    An IEEE 802.3 Ethernet repeater regenerates preamble bits and reshapes frame bits as it "repeats" frames at the bit level. Unlike a repeater that buffers only a few octets worth of a repeated frame, an IEEE bridge, commonly known by the more marketable term "switch", typically buffers the entire frame. An IEEE 802.3 Ethernet Midspan PSE passes signals along virtually unchanged and without buffering -- it is neither a repeater nor a bridge.

    Whether you add PoE to your closets via endspan switches or midspan equipment, you will still need more power and cooling to the closet.

    Since midspan PoE and 1000BASE-T are incompatible, midspan PoE will not be with us for long. As gigabit Ethernet becomes the norm, the choices will reduce to endspan PoE or none at all.

    My own enterprise theory is that PoE hungry devices, APs and phone sets, will become integrated in the next few years, sit on desktops, and be within easy reach of the traditional structured cable, power outlets, and air handling we have long been familiar with. PoE will be an important service to provide but it will not necessitate the huge changes some have imagined.

    The huge changes will come only after we have much more fully integrated DC power, AC power, and data cable systems a few decades hence with one jack to rule them all.

    I hope this helps. Thanks. /criss


    Here's an example of Midspans.



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