802.11 Standard Comparison and Security

Wireless Brief History.

Wireless devices have been a part of everyday life since the early 1980’s.  Wireless devices communicate with one another without the use of any cabling or physical connection.  We use these devices every time we use a remote control to turn on the television or make a call from a cordless or cellular phone.  Much of today’s technology has come from the evolution of the cellular phone network.  Cellular phone communication is the number one application for wireless technology.  The expansion of the Internet has also affected the use of wireless communications.  Businesses everywhere are implementing wireless as the medium of communication on their networks.

IEEE and 802.11.

To insure compatibility of software and hardware, manufacturers must follow specific standards.  The standards allow devices from different manufacturers and vendors to communicate.  The Institute of Electrical and Electronic Engineers (IEEE) defines the standards implemented in these areas of technology.  This paper will discuss the standards for wireless communications, how wireless communications work and will define some of the security issues that have surfaced since the implementation of the wireless local area network (WLAN). The IEEE bases wireless communications on the 802.11 standard.  There are currently two supplements to the 802.11 standard, 802.11a and 802.11b (802.11g is still being finalized).  Other improvements are still being developed, but have not reached the level of IEEE research.

802.11.

802.11 was designed in June of 1997 specifically to support applications that required a higher rate of data across a wireless network.  It was intended for wireless transmissions to communicate at a rate of 1 to 2 Mbps.  802.11 operates in the 2.4 GHz band.  This band is known as the Industrial, Scientific, and Medical (ISM) band.  It is heavily used by electronic products and therefore has a high amount of interference.  This makes transmitting high-end applications like streaming video or voice difficult due to a limited amount of bandwidth.  The 802.11 standard was implemented to place specifications on the parameters of both layers 1 and 2 of the OSI model. Layer 1 can use either a frequency hopping spread spectrum (FHSS) system with 2 or 4 Gaussian frequency-shift keying modulation or direct sequence-spread spectrum (DSSS) system with differential binary phase-shift keying or differential quadrature phase-shift keying base band modulation.  The third alternative for transmission on the physical layer is using an infrared transmission system, but this paper will keep within the scope of radio frequency transmissions. Layer 2 protocols are responsible for maintaining shared medium access.  802.11 stipulates carrier sense multiple access with collision avoidance (CSMA/CA).  The CSMA/CA protocol determines when a node can transmit.  The node will “listen” to the medium to make sure the medium is free.  If the medium is busy the node will wait a specified amount of time before attempting to transmit again.  Once the medium is clear the source node will transmit a ready to send packet, the destination node will reply with an acknowledgement.  Within the acknowledgement is header information that lets the source node know what parameters to stay within while sending its data payload.  The source node responds acknowledging the destination nodes instructions in its header and data packets follow the header accordingly.  This is what’s known as the “three-way-handshake”.  The protocol insures the source node is notified when the destination node is busy, thus minimizing collisions within the network.   

802.11b.

802.11b passed IEEE tests in 1999 and is intended to be an extension to 802.11 using DSSS.  It supports higher data rates than 802.11 at 5.5 to 11 Mbps and many businesses have implemented it on their networks.  802.11b also operates in the 2.4 GHz band.  Competition for bandwidth in this range with other products such as cordless phones, microwaves, and other networks makes 802.11b vulnerable to interference.  The bandwidth of a spread-spectrum channel is 22 MHz; the ISM band has only three non-overlapping channels 25 MHz apart.  802.11b uses hopping mode for three non-overlapping channels at 10 MHz apart. Using the 2.4 GHz band for transmission gives 802.11b a higher range.  Typically 802.11b will perform at ranges of up to 300 feet using a minimal number of access points.  802.11b is a good choice for networks located in a warehouse, store or any expansive business with sparsely populated users.  The fewer users competing for an access point’s bandwidth the better the performance of the network.  For companies with users who do not use high-end applications, 802.11b is a popular choice.

802.11a.

802.11a passed IEEE tests in September of 1999.  Although it is costly and expensive it has much more to deliver for businesses that require high amounts of bandwidth.  802.11a operates in the 5 GHz band, which is known as the unlicensed national information infrastructure (UNII) band.  The standard can use 300 MHz of bandwidth because the spectrum is divided into three smaller bands.  The first 100 MHz is restricted to a maximum output of 50 mW.  The second 100 MHz has 250 mW of output and the third 100 MHz has a maximum output of 1.0 W. 802.11a uses orthogonal frequency division multiplexing (OFDM).  The standard specifies eight non-overlapping channels in the lower two bands, each divided into 52 sub-carriers.  The upper band has four non-overlapping channels.  Modulation methods depend on the rate of the data being supported by channel conditions between source and destination.  There are four modulation methods used by 802.11a, they are BPSK, QPSK, 16-QAM, and 64-QAM.  Figure 1 represents OFDM sub-channels.

802.11a can deliver data rates as high as 54 Mbps.  The drawback to 802.11a is range.  The higher operating frequency gives 802.11a a range of about 60 feet.  To implement this standard in a large area would require a larger number of access points.  Densely populated areas with users competing for the same access point would make the decision of choosing 802.11a or b an easier choice.  If a business requires high performance to send video, voice, or large images/files then 802.11a would be the logical choice and worth the extra expense.

802.11g.

802.11g is scheduled for approval by May of 2003.  802.11g will expand 802.11b’s data rates to 54 Mbps within the same 2.4 GHz band using OFDM (orthogonal frequency division multiplexing). 802.11g will perform in the 2.4 GHz band using 1/3 of that band to transmit its signal.  Just like 802.11b this will decrease the number of AP’s that will not overlap to three.  This creates problems with channel assignment in heavily populated areas that cover expansive regions.  The answer to this problem is lowering the power of the AP’s.  802.11b users can upgrade to 802.11g, but they will need to decrease the range of their current AP’s or provide new AP’s to handle the high data rates.  To supply backward compatibility 802.11b technology will still interface with 802.11g technology.  “…the 802.11 Task Group is looking to iron out about 100 remaining editorial and technical questions at the next meeting of the group in early July.”  [McGarvey]  One of these questions is, “how will 802.11g deal with RF interference?” Currently, the problem with interoperability between 802.11a and b has caused the need for improvements.  An engineering company in London has developed a dual 802.11a/b chipset.  This new chip will allow an end user device to sense if the access point is using 802.11a or 802.11b.  Vice versa, the access point can also send out 802.11a/b allowing any end user to communicate accordingly.

Wireless Local Area Networks.

Devices connect to the network using a Network Interface Card (NIC).  A NIC carries the devices MAC address.  This identifies the device on the network.  In layer two, Address Resolution Protocol (ARP) converts the MAC address to an address recognized by the network, an IP address.  This allows the device to communicate with other devices on the network.  In order to connect to the network without any cabling, a device must have a wireless NIC installed.  The wireless NIC is used to communicate with other devices within range or with an access point.  An AP communicates with devices equipped with wireless network adaptors and connects to the wired network via an RJ-45 port.  AP devices have an average range of 300 feet, this area is known as a cell.  Users can move within the cell freely without being disconnected from the network.