eap Mailing List: Re: EAP WG Last Call on Network Discovery andSelectionProblem Document

[Bernard Aboba (bernard_aboba]

Found some typos and some additional material that needs to change:

Change:

   As noted in [Velayos], the IEEE 802.11 Beacon mechanism does not
   scale well; with a Beacon interval of 100ms, and 10 APs in the
   vicinity, approximately 32 percent of an 802.11b AP's capacity is
   used for beacon transmission.

To:
   The 802.11 Beacon is sent only at a rate within the basic rate
   set, which typically consists of the lowest supported rate, or
   perhaps the next lowest supported rate.  As a result, "virtual AP"
   mechanisms that require a separate Beacon for each "virtual AP"
   do not scale well.

   For example, with a Beacon interval of 100 Time Units (TUs) or
   102.4 ms (9.8 Beacons/second), twenty 802.11b "virtual APs"
   each announcing their own Beacon of 170 octets would result
   in a channel utilization of 37.9 percent.   The calculation can be
   verified as follows:

1. A single 170 octet Beacon sent at 1 Mbps will utilize the channel
   for 1360us (1360 bits @1Mbps)

2. Adding 144us for the Physical Layer Convergence Procedure (PLCP)
    long preamble (144 bits @1Mbps), 48us for the PLCP header
   (48 bits @1 Mbps),  10us for the Short  Interframe Space (SIFS),
   50us for the Distributed Interframe Space (DIFS), and 320us for the
   average minimum Contention Window
   (CWmin/2 * SlotTime = 32/2 * 20 us)  implies that a single
   Beacon will utilize an 802.11b channel for 1932us.

3.  Multiply the channel time per Beacon by 196 Beacons/second, and we
    obtain a channel utilization of 378672us/second = 37.9 percent.

In Section 4, change:

  o  Studies such as [MACScale] and [Velayos], demonstrate that the
     IEEE 802.11 Beacon/Probe Response mechanism has substantial
     scaling issues, and as a result a single physical access point is
     in practice limited to less than a dozen virtual APs on each
     channel with IEEE 802.11b.

To:

  o  Studies such as [MACScale] and [Velayos], as well as the calculations
     described in Section 2.1 demonstrate that the
     IEEE 802.11 Beacon/Probe Response mechanism has substantial
     scaling issues in situations where a new Beacon is used for each
     "virtual AP".  As a result a single channel is in practice limited to 
less
     than twenty Beacon announcements with IEEE 802.11b.

In Appendix A, delete:

      The simulations presented in [Velayos] appear to confirm this
      conclusion; with a Beacon interval of 100 ms, once more than 8
      virtual APs are supported on a single channel, more than 20% of
      bandwidth is used for Beacons alone.  This would indicate a limit
     of approximately 20 virtual APs per physical AP.

BTW, I noticed a few other typos:

In Section 4, change:

  o  Work is underway in IEEE 802.1, IEEE 802.21 and the IEEE 802.11u
     to provide enhanced discovery functionality.  Similarly, IEEE
     802.1af has discussed addition of network functionality to IEEE
     802.1X. However, neither IEEE 802.1ab nor IEEE 802.1af is likely
     to support fragmentation of advertisement frames, so that the
     amount of data that can be transported will be limited.

To:

  o  Work is underway in IEEE 802.1, IEEE 802.21 and the IEEE 802.11u
     to provide enhanced discovery functionality.  Similarly, IEEE
     802.1af has discussed addition of network discovery functionality to 
IEEE
     802.1X. However, neither IEEE 802.1ab nor IEEE 802.1af is likely
     to support fragmentation of network advertisement frames, so that the
     amount of data that can be transported will be limited.