Wireless networks

Stacy Phelps

CET 751

Summer 2002

 


Wireless network technology permits the use of personal computing devices in areas that are not served or can only be poorly served by traditional wired network configurations. This technology is well-suited to public areas and classroom locations in which the number of potential users or physical space configuration makes installation of a wired network difficult or impossible.

In the past, the use of wireless network sites has been difficult due to the lack of standardization in wireless network equipment, the relatively slow speed of wireless network data transmission, and security concerns. With the IEEE 802.11 wireless data transmission specification has helped improve the standardization of equipment which has allowed wireless network interface cards from one vendor to work with the infrastructure equipment provided by another, to a certain extent. Compatibility issues do exist among components manufactured by competing vendors and these must be accounted for in the design of any wireless network.

Many access points (AP) allow you to control access based on the MAC address of the NIC attempting to associate with it. If the MAC address of your NIC isn't in the table of the AP, you won't associate with it. Even though it's true that there are ways of spoofing a MAC address that's been sniffed out of the air, it takes an additional level of sophistication to spoof a MAC address. The downside of deploying MAC address tables is that if you have a lot of AP, maintaining the tables in each AP could be time consuming. Some higher-end, enterprise-level AP have mechanisms for updating these tables across multiple AP of the same brand.

IEEE 802.11b is the most common and established wireless network protocol in use today, referred to as the IEEE 802.11b standard. The 802.11b standard defines, among other things, the radio frequency bandwidth wireless signals can use, throughput rates over that signal, and how wireless endpoints communicate with one another.

802.11b signals function in the 2.4000 GHz to 2.4835 GHz range, and have a maximum theoretical throughput of 11Mbps although testing suggests that actual throughput is more like 4-6Mbps and can even step down to 5.5Mbps, 2Mbps, and 1Mbps to allow a more robust signal. 802.11b uses only Direct Sequence Spread Spectrum (DSSS) radio signaling, as opposed to Frequency Hopping Spread Spectrum (FHSS), which was part of the original 802.11 specifications. DSSS allows for greater throughput, but is more susceptible to radio signal interference. Interestingly, many DSSS-based 802.11-based products are interoperable with current 802.11b networks, but only at 802.11's 2Mbps or 1Mbps. Wireless endpoints have a coverage area that depends on antenna strength and the ability and clarity of the local environment to transmit radio signals typically ranging from 75 to 150 feet.

Multiple 802.11b endpoints connect wirelessly through an access point (AP) to the wired network. In the simplest version of this scenario, an AP forms an association with one or more wireless clients and acts as a bridge between them and the wired Ethernet network. This is referred to as a Basic Service Set, or BSS. The AP also handles network synchronization tasks that make the wireless client able to behave as if it's on the wired network, such as forwarding broadcasts.

Basic Service Set (BSS)

The further away a client station is from the AP, the weaker the perceived radio signal becomes. As mentioned above, throughput rates step down systematically as a result, slowing down performance in favor of a more robust signal. In order to increase the range and coverage of the wireless network, you can add more strategically placed APs to the environment to increase density. This is referred to as an Extended Service Set (ESS), and is defined as two or more APs that connect to a specific wired Ethernet LAN and their associated wireless clients.

Extended Service Set (ESS)

To implement we will need to figure out who needs access to the network and what network resources should each user type be able to access? How many users require access in total, and how many are expected to be accessing the wireless points simultaneously? And what are their bandwidth requirements? The physical nature for access to the wireless network. Will users be moving around a lot, or will wireless communication be used in a warehouse environment where users are riding around in fast moving equipment? The level of security is needed and how will authentication of clients to the wireless network and to other network resources be achieved. 

With knowledge of the anticipated numbers of users, the anticipated bandwidth consumption, and some idea of roaming frequency, we can match the wireless network requirements to the physical site. This is accomplished with a site survey, that involves taking a look at the physical layout of the site and determine the optimal placement and density of APs to maximize client connectivity and bandwidth.

The next step is to determine the number of APs needed, and where to place them and provide optimal wireless coverage. An important goal is to ensure that users who are roaming from area to area have adequate coverage and bandwidth, while not over placing AP. The site survey also helps determine the areas where coverage quality will suffer or where signal loss occurs because of interference. Make sure that coverage areas slightly overlap each other so there are no gaps in connectivity.

Dell Overlapping Coverage

If there are many users in a small area of space, the 11Mbps throughput limitation of an AP capacity will quickly be exhausted. By situating APs so that coverage areas overlap the aggregate throughput to clients can be in excess of 11Mbps. Individual users won't experience an increase in speed above 11Mbps.  Though the total throughput of multiple clients will exceed 11Mbps, depending on the number of APs used.

Overlapping for Thruput

A site survey can identify conditions that will cause deterioration in signal strength through path loss, multi-path loss, or interference with other radio transmitters. Path loss occurs when the signal strength between transmitter and receiver, or AP and endpoint, gets attenuated with increasing distance. In short, the farther away from the AP you are, the weaker the signal, and the lower the throughput becomes. Other things that affect path loss are ceilings, walls, or cubicles, and in particular, the materials used in their construction. Radio signals pass through drywall with moderate difficulty, but not at all well through solid steel. Water absorbs a lot of radio signal, so watch out for fish tanks or other obvious water manifestations. Less obvious are plant foliage and people, both of which can severely affect 802.11b performance. For outdoor wireless implementation complete a site survey when plants are fully leaved--otherwise you'll find signal loss come spring.

Dry outside walls also have caused path loss when wet from rain.  Another pitfall to look out for is the effect of tinted glass on signal strength. Tinted glass tends to absorb radio waves, effectively reducing signal strength and bandwidth. To circumvent these signal-robbing environmental factors, you'll want to increase coverage and density of AP in affected areas.

Multi-path loss occurs when radio signals arrive at the AP out of order because they followed multiple paths from the source to the destination. The result is multiple signals that are slightly out of alignment with one another. This causes additional computational overhead for the AP, as it must rebuild the signals properly, which is done as part of the 802.11b specification. Minimizing the number of objects that block the radio signal helps reduce path loss and multi-path loss.

Using diversity antennas that come standard with most wireless network equipment can also reduce the amount of multi-path loss. Diversity antennas contain two antenna elements at the base station. The antennas have a little physical distance between them that improve signal strength by dissipating the negative effects of multi-path loss. Another strategy to increase signal strength is to use high-gain antennas or to buy APís from vendors that offer a BNC antenna connector option. With BNC connectors, you can mix and match antenna types to suit your environment, improving signal strength.

Another option is to use unidirectional antennas to target areas of poor coverage. A unidirectional antenna has a single, well-defined direction of maximum gain, radiating most of its power in one direction.

Radio signal interference happens when other devices operate in the same frequency range as 802.11b, and can cause a degradation of network performance. 2.4GHz cordless phones, microwave ovens, and Bluetooth networking devices all operate in the 2.4GHz range used by 802.11b. If they broadcast at the same time as a network device, the resulting packet loss from collisions can cause a performance hit because of data retransmission. APs should be placed such that interference from other transmitters is minimized. This is accomplished by either moving APs further away from the source or by increasing the number of APs in the given area.

Even if a simultaneous broadcast as described above doesn't happen, performance can be affected because 802.11b devices have a mechanism that causes them to wait until the air is clear to transmit, to avoid packet loss. Wired Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) to tell when two devices have transmitted at the same time, generating a collision. When that happens, both devices wait a brief, random amount of time and retransmit, but that only works because Ethernet devices can simultaneously send and listen on the wire.

Wide-open spaces provide the highest level of coverage. Areas with many walls, furniture, plants, and other obstacles such as devices broadcasting in 802.11b's bandwidth, can significantly decrease the effective range and performance of an AP.

APs should be placed in overhead areas to maximize range. Once it has been determined optimal placement of APs, you'll want to test the resultant radio coverage by walking around with a laptop and testing signal strength at various locations. If there are gaps in radio coverage more APs will need to be added or reorient physically to provide adequate coverage. Coverage should mimic workflow. Coverage in a conference rooms where clients experience few AP switchovers due to lack of roaming will be easily attained. Whereas in a warehouse environment, roaming between APs becomes more difficult as the mobility speed increases.  Another type would be to try placing AP s in the center of rooms versus on a window shelf to radiate the signal around instead of losing the signal outside.

Ensuring adequate security should be primary concerns. There are several security mechanisms built into the 802.11b standard, but these mechanisms provide only basic security protection that will not be adequate for enterprise and corporate deployment.

The 802.11 standard defines a multi-step method of establishing network connectivity between clients and AP. This process uses a series of broadcast and directed commands that enable the wireless endpoints to identify, authenticate and associate with each other. The process of connecting a wireless client to a network is initiated when the client broadcasts probes on all radio frequency channels used by 802.11b.

The network administrator should periodically survey the site to see if any unknown AP appear. With the declining pricing of AP, it's not hard to imagine that a department might run out to an electronics store buy a couple of NICs and an AP, and plug it into the corporate network. All of the hard work to secure the wireless network could be wasted if an unknown AP were plugged into the network behind the firewall.

The 802.11b standard includes a provision for encryption called WEP (Wired Equivalent Privacy). Depending on the manufacturer and the model of the NIC card and AP, there are two levels of WEP commonly available. One based on a 40-bit encryption key and 24-bit Initialization Vector (64-bit encryption and generally considered insecure) and a 104-bit key plus the 24-bit IV (128 bit encryption.).

Don't buy AP or NICs that only support 64-bit WEP. Some low-end products only support 64-bit (40 bit key) WEP, and as you know by now, even 128-bit WEP is universally considered not very secure. Note that some NICs may only require a driver upgrade to attain 128-bit WEP capability.

As the AP get deployed they should be located towards the center of your building rather than near the windows. Plan coverage to radiate out to the windows, but not beyond. If the AP are located near the windows, a stronger signal will be radiated outside the building making it easier for unauthorized people to gain access to the network.


 

Works Cited

Angel, Jonathan. Look Ma No Cables. Network Magazine. November 6, 2000. http://www.networkmagazine.com/article/NMG20001106S0004.

Delio, Michelle. Wireless Networks in Big Trouble. http://www.wired.com/news/wireless/0,1382,46187,00.html.

Liddle, Deborah and Smitton, Stuart. WIRELESS NETWORKS. http://www.ukoln.ac.uk/public/earl/issuepapers/wireless.html

Salkever, Alex. Wireless Networks: Open Doors for Bad Guys. SEPTEMBER 11, 2001. http://www.businessweek.com/bwdaily/dnflash/sep2001/nf20010911_0545.htm.

Stone, Amey. Wi-Fi: It's Fast, It's Here -- and It Works. April 1, 2002. http://www.businessweek.com/technology/content/apr2002/tc2002041_1823.htm.

Uskela, Sami. Security in Wireless Local Area Networks. 1997. http://www.tml.hut.fi/Opinnot/Tik-110.501/1997/wireless_lan.html#Introduction.

Vamosi, Robert. A hacker's dreamland: wireless networks. March 27, 2002. http://zdnet.com.com/2100-1107-869370.html.