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No Strings (Wires) Attached: Wireless LANs, Part II

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on July 22, 2011 by jjrinehart

Wireless = Radio!

Wireless LANs transmit signals across the air rather than across copper wires or fiber optic cable.  For those of you who can remember back far enough before cable television, you may recall seeing antennas sticking up from the back of the set (remember rabbit ears?).  Television stations transmitted one-way signals that reached the television set, were decoded, and turned back into light and sound to entertain the masses.  In the “good old days” you turned a knob on the front of the set to change the channel (frequency) that was being displayed on the screen, and only a couple were usually available, NBC, ABC, and CBS, and maybe PBS.  Understanding the basics of wireless LAN technologies actually start at this point, in getting a better grasp of how radio signals actually act and operate.

Radio signals travel through the air and require both a transmitter and receiver, which are actually separate operations, although at one time the term transceiver identified something that did both.  Just as with human speech (using sound waves), wireless technologies are analog rather than digital.  Digital signals have one of two values, namely Zero (0) or One (1), indicating on or off status of a computer circuit.  Electromagnetic radiation, including radio frequency (RF), transmit information by changing some aspect of these waves, usually termed frequency (the measure of how many waves are repeated per interval), amplitude (strength of the signal), or phase (difference between the wave and some reference point).  All wireless technologies use some form of encoding/modulation to change the signal to communicate the zeroes or ones in order to carry the digital information.  For the sake of simplicity, let’s think of frequencies the way you typically use them: channels on your television or radio.  When you want to receive a different stream of data (for example, ESPN instead of the Opera Channel), you use the remote control to change the frequency from one channel to another.  In the United States, the Federal Communications Commission (FCC) sets the rules for who can use certain frequencies, as well as power levels so that they can coexist.  Some organizations pay the “big bucks” for use of certain frequencies of operation, such as television and radio stations, and cellular telephone companies.  These are referred to as licensed frequencies because they require a valid agreement in place with the FCC in order to use them.  For our purposes, we need not worry about these RF signal families, but rather those that are part of the unlicensed frequencies group.  Since this is a much deeper topic, we will discuss this next time.

– Joe

No Strings (Wires) Attached: Wireless LANs, Part I

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on June 20, 2011 by jjrinehart

Evil Wire Monster! Run!

The evolution of networking has been rapid over the past several decades as computing moved from a single centralized mainframe to a more distributed model with server farms.  Alongside computing has been vast arrays of cable plants, originally consisting of thick coaxial cable, migrating to thin coaxial cable, to the twisted pair cables we all know and love.  However, just as a puppet without strings would be considered a marvel, a network without wires is equally impressive and desirable; this brings us to the subject of wireless LAN’s, or WLANs, as they are affectionately called.

The umbrella IEEE standard for wireless is designated as 802.11, representing a wide family of other standards, protocols, and so forth.  The establishment of the Wi-Fi Alliance (http://www.wi-fi.org/) in 1999 to promote interoperability has not only created widespread awareness of the technology, but has become synonymous with te technology itself (users frequently refer to WLAN’s as “wi-fi networks.”)

It probably sounds overly simplistic to say that the differences between wired and wireless networks are vastly different, but there is more truth to that than simply saying one medium uses wires and the other does not.  There are some similarities, however, that should not be overlooked:

  1. Layer 2 Technology: While they implement it differently, both operate at Layer 2 of the OSI stack.
  2. Communication Between Devices: Both allow for inter-device communication and data transmission.
  3. Frame Formats: While not identical, both use frame formats constructed with a similar anatomy, including headers/trailers, source/destination MAC addresses, etc.

One of the most striking differences between the 802.11 family of wireless standards and their 802.3 relatives has to do with the mechanics of data transmission; Ethernet uses Carrier Sense Multiple Access with Collision Detect (CSMA/CD), and responds to frames which collide in transit, while a WLAN uses CSMA/CA, in which is the stands for avoidance.  If you think of an intersection with cars crashing into one another as analogous of Ethernet, cars constantly swerving to get out of the way would be closer to wireless operation.  Here is the basic process a wireless devices uses to transmit data:

  1. Listen to make sure there is no traffic on the medium (in this case, the channel/air)
  2. Set a random timer and do nothing until it expires
  3. Listen again to make sure that there is no traffic
  4. Send the frame
  5. Wait for an acknowledgement
  6. If there is no acknowledgement, assume the frame was lost and start over at #1

Keep in mind that this is the process for transmitting a single frame, and you don’t have to be a rocket scientist to see the amount of overhead this takes.  The reason?  While you can control things that happen on a wire, you have no control over the air, namely transmitted signals.  Lots more to come!

– Joe