Planning and Implementing Wireless LANS By Peter Rysavy Understanding Wireless LANs Before you deploy wireless LANs, it helps to understand how they work. Wireless LANs operate in almost the same way as wired LANs, using the same networking protocols and supporting the most of the same applications. But they have some fundamental differences: While they use the same networking protocols, they use specialized physical and datalink protocols; They integrate into existing networks through access points which provide a bridging function; They let you stay connected as you roam from one coverage area to another; They have unique security considerations; They have specific interoperability requirements; They require different hardware; They offer performance that differs from wired LANs. We will study each of these topics in detail, as well as the IEEE 802.11 standard, how to go about deploying a wireless LAN, and the use of unlicensed wireless links to bridge networks together. Physical and Link Layers Integration with Existing Networks Roaming Security Interoperability Hardware Performance The IEEE 802.11 Standard Deploying Wireless LANs Bridging Networks With Unlicensed Wireless Links Physical and Link Layers In looking at a typical wireless LAN protocol stack diagram, we see that from the network layer up, wireless LANs use the same protocol stack as wired LANs. We review network protocols and applications in Making Network Software and Applications Work . But what about at the lower layers? At the physical layer, the wireless network interface card takes frames of data from the link layer, scrambles the data in a predetermined way, then uses the modified data stream to modulate a radio carrier signal. At the link layer, a key function is to control access to the medium. Unlike Ethernet networks, which are carrier-sense-multiple-access with collision detect (CSMA/CD), most wireless LANs are carriers-sense-multiple-access with collision avoidance (CSMA/CA). Once a node starts sending data, it cannot detect whether another station is also transmitting and so senders rely on a positive acknowledgment from the receiver to indicate that no interference occurred during the transmission. This makes wireless LANs less efficient than wired LANs under heavy loading. You should test your network under typical loading conditions. Integration With Existing Networks While you can operate a wireless LAN as a standalone network, chances are you will want to connect it to your wired infrastructure. This is easily accomplished by using a wireless access point, a small device that bridges wireless traffic to your network. The wireless LAN then appears as one network segment in your overall network. Today you usually need to purchase the access point from the same vendor as the wireless NIC, though with standards such as IEEE 802.11 and industry interoperability initiatives such as the Wireless LAN Interoperability Forum, you will have increasing options to combine equipment from different vendors. Most access points bridge wireless LANs into Ethernet networks, but Token-Ring options are often available as well. Roaming In many wireless LAN applications, you would like users to maintain a continuous connection as they roam from one physical area to another. In doing so, they may well move from the coverage of one access point to another. Nearly all wireless LAN vendors support this kind of roaming through a process by which the mobile nodes automatically register with the new access point. What you will need to consider in your network planning is how your infrastructure network is divided into subnets. If one access point is on one subnet and another access point is on another subnet, traffic will have to cross a router, something that most wireless LAN vendors currently do not support. The two possible solutions are: Connect all access points back to one subnet, which might require extra cabling. Use Mobile IP if your network protocol is IP, as we discuss further below. Sine the IEEE 802.11 standard does not address roaming, you may need to purchase equipment from one vendor if your users need to roam from one access point to another. Security The thought of radio waves propagating in all directions from your network may concern you. Fortunately most wireless LANs provide a number of effective security measures. First of all, spread spectrum radio signals are inherently difficult to decipher without knowing the exact hopping sequences or direct sequence codes used. This at least keeps honest people honest. To protect you against truly determined intruders, most wireless LAN products also offer optional encryption mechanisms. Also, the IEEE 802.11 standard specifies optional security called "Wired Equivalent Privacy," whose goal is that a wireless LAN offer privacy equivalent to that offered by a wired LAN. The standard also specifies optional authentication measures. Interoperability Before the IEEE 802.11 specification, the only interoperability between wireless NICs and access points occurred when vendors worked cooperatively. One such effort is The Wireless LAN Interoperability Forum (WLIF), which was formed by a number of wireless LAN vendors to address interoperability issues. WLIF has published protocol specifications based on Proxim's airlink and also performs certification testing. A number of vendors are also working together to create what is called the Inter-Access Point Protocol (IAPP), which defines how access points communicate with each other. Though IEEE 802.11 lends legitimacy to the industry, by itself it does not guarantee interoperability, since it only standardizes the physical and medium access control layers. The standard does not address issues of roaming or communications between access points which is required for handoffs. And vendors must still work with each other to ensure their IEEE 802.11 implementations interoperate. You should ask your potential vendors which products they work with, regardless of what standards they meet. Also, you should do your own verification testing if you plan on using products from multiple vendors. Hardware Wireless LAN hardware typically comes in the following physical forma ts: PC Card, type 2 format, either with integral antenna which is becoming the norm or with tethered antenna/RF module. ISA Card with external antenna connected by cable. Handheld terminals with integrated radios for vertical market applications such as warehousing. Access points that are standalone devices about the size of a hardcover book. Costs range from $300 to $800 for network cards, with access points costing $1,500 or more. Wireless bridges that link wired LANs together range from $2,000 to as high as $10,000 per node depending on performance. Performance The quoted throughputs of commercially available wireless LANs range from 1 Mbps to 10 Mbps. See the following table for examples of throughput: TYPE OF WIRELSS LAN QUOTED DATA THROUGHPUT IEEE 802.11 standard 1 Mbps or 2 Mbps Open Air Interface as specified by the Wireless LAN Interoperability Forum (based on Proxim frequency-hopping spread-spectrum technology) 1.6 Mbps WaveLAN (Lucent Technologies) and Aironet direct-sequence spread-spectrum products 2.0 Mbps WinData 5.7 Mbps RadioLAN 10 Mbps HiperLAN standard (no commercial products) 24 Mbps You need to be careful with quoted throughputs since they represent best case throughputs. Wireless LANs experience higher error rates than wired LANs, resulting in retransmission of frames. Furthermore, as discussed above in Physical and Link Layers , the collision avoidance mechanism is not as efficient as collision detection used in Ethernet, especially with a large number of users. Based on various tests, you can expect actual throughput for your application to be about half the quoted "over-the-air" throughput. Note also that there is a trade-off between throughput and distance. Higher da ta rates come at the expense of lower maximum distances for reliable operation, which is why many products offer a choice of throughput speeds. The IEEE 802.11 Standard Finally released in 1997 after nearly seven years of development, the IEEE 802.11 standard specifies physical layer and medium access control (MAC) protocols. The MAC constitutes the lower half of the datalink layer in the OSI network model. 802.11 was designed so that to upper levels the network behaves like a standard wired network. To accomplish this the link layer engages in error correcting functions that are not usually employed at the link layer in wired LANs. At the physical layer, 802.11 specifies use of the 2.4-GHz ISM band with both frequency-hopping and direct-sequence spread-spectrum at 1 Mbps with optional 2-Mbps throughput. Power can range from 10 MW to 1 watt. In trying to accommodate every possible variation in technology, the standard specifies an IR physical layer as well. At the datalink layer, 802.11 specifies a MAC protocol based on carrier-sense-multiple-access with collision avoidance and an optional request-to-send and clear-to-send mechanism that allows longer uninterrupted transmissions. The standard also provides for optional time-bounded services such as voice and video communications by allowing access points to control communications using polling methods. The 802.11 standard is an important milestone for the industry and will provide a degree of interoperability, but it is not essential that you use 802.11-compliant equipment for a number of reasons: You may be comfortable purchasing all your equipment from one vendor. Many vendors already provide interoperable equipment. 802.11 only specifies 1-Mbps and 2-Mbps throughput. You may need a wireless LAN that operates at higher speeds. Deploying Wireless LANs Deploying a wireless LAN is relatively straightforward. A t the wireless nodes you must install the wireless network card as well as the appropriate software drivers which your network protocol stacks use to access the wireless card. These drivers come with the card. The difficult part is knowing where to install your access points. The first step is to define exactly where you want coverage, which requires a site survey. Wireless signals can penetrate walls, sometimes several walls, but how well they do so depends on how the building was constructed. You will need to verify that you have coverage where you want it. Different vendors provide different tools to do this, but one approach is to use two portable computers with wireless hardware operating on a point-to-point basis. Using diagnostic software provided by the vendor, you can determine a coverage area for a potential access point by keeping one portable computer fixed and moving around with the other. Check with your vendors as to what tools they provide and what approach they recommend for deploying their access points. Bridging Networks With Unlicensed Wireless Links Wireless LAN technology is also used for bridging conventional wired networks together, which can be particularly useful if two sites have a line of sight with each other and the only wired bridging option involves monthly services charges to a telco or other network provider. Many wireless LAN vendors offer bridging products based on the same spread-spectrum technology as their wireless LANs. Using these is quite straightforward. First you need to determine whether to use omnidirectional antennas, effective to about 100 meters, or directed antennas for greater range. These directed antennas can easily cover distances of over a kilometer and with amplification (which is now allowed under recently relaxed FCC rules) can reach as far as 40 kilometers (25 miles). Specified throughputs range from 1 Mbps to 10 Mbps, depending on the product offering and price. Actual throughput will be less. Testing of 10-Mbps bridges by Network Computing showed throughputs of just over 5 Mbps. After taking into account range and throughput, consider any other bridging requirements you may have, such as support for routing and for specific protocols. For more details, see the article Bridging The Miles With 10 Mbps Spread Spectrum Wireless Networking in the November 1, 1997 issue of Network Computing. Updated January 14, 1998 Print This Page E-mail this URL