From the blog.

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OECG – Chapter 22

593 Words. Plan about 4 minute(s) to read this.

This chapter covers basics of wireless networking.  Standards, terminology, security and signaling are all discussed.  There is little in the way of specific steps or procedures here.  Almost everything is conceptual.  The next chapter covers more of how Cisco does wireless.  So, I think for most of us, this will be a big review, just depending on your background.  Most of what I saw in the chapter, glancing through, was familiar.

Personally, I’ve never done a huge wireless access point deployment (such as in a hospital).  Plus, some of my recent employers have been scared of wireless, largely because of the weakness of WEP encryption.  Their solution was to ban wireless altogether, rather than modify the wireless standard for proper security.  Nonetheless, I’ve still been able to work with a directional microwave point-to-point link, a few Aironet APs, and several Linksys APs.  And I’ve done my share of wardriving, although I never built a directional antenna out of a Pringles can.  (At the last Network/InterOp held in Atlanta I witnessed such a thing at a vendor display.)  Anyway…enough babbling.  My point is that while I’m no wireless expert by any means, I’m no stranger either.  So let’s move on to OECG content…

802.11a

  • Standardized in 1999, not widely deployed.
  • Defines 12 RF channels in the 5GHz spectrum that do not overlap.
  • Uses orthogonal frequency division multicplexing (OFDM).
  • Data rates up to 54Mbps.
  • 12 channels results in the possibility of 12 APs, each supporting their own wireless clients.
  • Ideal for high performance applications.
  • Less congestion in the 5GHz range (thus fewer bit errors), as opposed to 2.4GHz where you compete with microwaves, bluetooth, cordless phones (I went back to 900MHz phones just for this reason), and other wireless LANs.
  • Fewer consumer choices.  Not many 802.11a interfaces available, not commonly deployed when compared to 802.11b or g.

802.11b

  • Standardized in 1999, very widely deployed.
  • Compatible with 802.11g, but not 802.11a.
  • Defines 11 RF channels in the 2.4GHz spectrum that do overlap.  You end up with roughly 3 “clear” channels: 1, 6 & 11.  That assumes you need to support 3 clear channels in the same area.  The problem is that there’s about 84MHz of spectrum, and an 802.11b card will use about 30MHz when sending.
  • Data rates up to 11Mbps.
  • Uses direct sequence spread spectrum (DSSS).
  • Subject to congestion with other consumer devices using the spectrum, thus causing bit errors and subsequent retransmissions.

802.11g

  • Standardized in 2004, widely deployed.
  • Backwards compatible to 802.11b.
    • Running in mixed mode 802.11b/g.
    • 802.11b stations will use Request To Send/Clear to Send (RTS/CTS) frames.  Avoids collisions over the air.
    • The AP will tell all associated endpoints that there’s a “b” client on the wire by setting a bit in beacon frames.
    • With “b” stations present, throughput can be degraded on a “g” LAN by as much as 30% because of the protection mechanisms everyone must use like RTS and CTS frames, adding conversation overhead.
  • Data rates up to 54Mbps.
  • Same channel limitations as 802.11b.
  • Supports an 802.11g-only mode, which can improve throughput with the downside of “b” clients being unable to associate.
  • Dual-mode APs are available, which are actually 2 radios in one AP.  This allows for “b” clients to talk the AP at the same time as “g” clients, but without the “g” clients have to downgrade to “b” style protection mechanisms.

802.11n

  • Still in progress, a standard by 2006 or 2007, according to the book.  Wikipedia says mid- to late-2008 before we see a standard.
  • Data rates above 100Mbps planned.
  • Multiple input multiple output (MIMO) antenna technology that creates multiple channels that are transmitted on at the same time.