Wlan Networks

Introduction

Since the early 1990's there has been a tremendous growth in the wireless community. The modern idea of wireless networking was first introduced in the late 60's at the University of Hawaii with the ALOHANET. The ALOHANET was used to connect the different campus locations on the Hawaiian Islands. "The history of wireless networking stretches farther back than you might think. It was over fifty years ago, during World War II, when the United States Army first used radio signals for data transmission. They developed a radio data transmission technology, which was heavily encrypted. It was used quite extensively throughout the campaign with the US and her allies. This inspired a group of researchers in 1971 at the University of Hawaii to create the first packet based radio communications network. ALOHNET, as it was named, was essentially the very first wireless local area network (WLAN)." [2]

Figure 1 - Simple WLAN using 802.1x from Lecture13 notes.

Frequency-hopping spread spectrum

The implementation of a wireless LAN is done with the use of radio-frequency technology. The two technologies that are used in wireless LANs that include frequency-hopping and direct-sequence spread-spectrum. "Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver."[3] This technology was developed during WWII by the Army Signal Corps in a system called SIGSALY. SIGSALY was initially used for communication between Roosevelt and Churchill; this was not known for some years because the development of SIGSALY was classified top secret.[8] Under this type of communication more bandwidth is used than the usual transmission in which only a single frequency is used. This increased bandwidth is due to the signal being spread over a greater number of frequencies. FHSS is the process of hopping from one frequency to another by the entire list of host in the network. Under this process "a narrow-band carrier is shifted in discrete increments of frequency. The frequency shift is based on a pattern generated by a code sequence that spreads transmission over a wide frequency band". [6] The order of the frequencies that are used is then repeated once the entire list has been used. If a frequency is already being used by another device that frequency be eliminated which helps to eliminate interference.

Direct-sequence spread-spectrum

The direct-sequence spread-spectrum method is performed by altering the message using a digital code, which increases the size of the message. "The code bits represent a redundant bit's pattern generated by the direct-sequence spread-spectrum technology that is applied to each information bit to be transmitted. This bit pattern is referred to as a chip or chipping code"(Held, pg. 232). There is a requirement of additional bandwidth because of this increase in message size. An example of this would be a message of 101 and a chipping code of 010. The resulting transmission of the message would be 101010101, with addition modulo 2. The process of obtaining this would be to take the first bit of the message and add it to each bit of the chipping code modulo 2, which would give us 101. The process is continues for each of the remaining message bits to give us the transmitted message of 101010101. This process adds redundancy to the message that allows the receiver to recover bits if some are lost or inverted during the transmission process without the user having to send the message again. This process is also secure if the chipping code is not known.

802.11 Wireless Networks

Network and data security is of the highest importance for nearly all users whether they are using a business network or their home network. Maintaining a secure network is a constant battle and adding wireless nodes to the network make this task almost impossible. While the implementation of the wireless standard by Institute of Electrical and Electronics Engineers (IEEE) known as 802.11 does provide for a variety of measures and other measures not included in 802.11 are used in an attempt to secure the wireless network and the data integrity they are not perfect. [9] The three main methods that are used to try and secure the 802.11 implementation are the use of the Extended Service Set ID (ESSID), Media Access Center (MAC) access list and the Wired Equivalent Privacy (WEP) encryption scheme. The IEEE has several approved protocols under 802.11 and others that are in development.

Protocol Release Date Op. Frequency Throughput (Typ) Data Rate (Max) Modulation Technique Range (Radius Indoor)

Depends, # and type of walls Range (Radius Outdoor)

Loss includes one wall

Legacy

1997 2.4 GHz 0.9 Mbit/s 2 Mbit/s ~20 Meters ~100 Meters

802.11a

1999 5 GHz 23 Mbit/s 54 Mbit/s OFDM

~35 Meters ~120 Meters

802.11b

1999 2.4 GHz 4.3 Mbit/s 11 Mbit/s DSSS

~38 Meters ~140 Meters

802.11g

2003 2.4 GHz 19 Mbit/s 54 Mbit/s OFDM

~38 Meters ~140 Meters

802.11n

June 2009[4]

(est.) 2.4 GHz

5 GHz 74 Mbit/s 248 Mbit/s MIMO

~70 Meters ~250 Meters

802.11y

June 2008[4]

(est.) 3.7 GHz 23 Mbit/s 54 Mbit/s ~50 Meters ~5000 Meters

Figure 2. 802.11 protocol list from Wikipedia [5].

Extended Service Set ID

The first method that administrators have for maintaining security is the use of the Extended Service Set ID (ESSID) sometimes just called SID for short. Anyone trying to connect to a wireless network must know the ESSID can access the

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