Running Free with Linux
By Roman Vichr2005-04-21
A Linux Wireless Access Point: Build or Buy?
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The fundamental question is this: Is a Linux system capable of integrating with other access points in a wireless network to provide connectivity between fixed nodes and roaming wireless clients?
The actual installation of a wireless LAN is not such a big deal; the key lies in what you buy, and whether it is supported under a Linux system. Check to make sure that you have the hardware for a specific operating system driver. Keep in mind that if you are looking for high performance (that is, gigabit data transfer rates), wireless is the wrong choice for you; even the latest standards offer transmission rates under 100 Mbps. Roaming presents a different challenge: maintaining a connection between various access points. To help with this, set up the Wired Equivalent Privacy (WEP) key to detect an access point that you can connect to. Keep things down to earth and remember that the main purpose of an access point is to be a bridge; that is, it should route packets from one network to another.
This article focuses on the various options and tools offered to manage these access points. Basically, you're choosing whether to use tools with or without wireless extensions. (Wireless extensions is the name of a generic API that allows a driver to inform the user about space configuration and statistics specific to common wireless LANs.) For a step-by-step guide to installing access point software under Linux, read the article Building a wireless access point on Linux. For a good overview of the wireless network structure and bridging between LAN and WLAN, read the Linux Wireless Access Point HOWTO (see Resources for more information).
No Wireless Extensions
There are several technologies available that can help you set up Linux wireless networking without using wireless extensions, including Linux-IrDa and BlueZ, the Linux Bluetooth stack. Both are well described in Linux wireless networking. Another choice is called ROSE (Radionet Open Source Environment; see Resources for more information), a platform for building an 802.11 wireless access point. It is distributed both as an open source package and as a commercial version with documentation. ROSE's advantage is that it can be compiled for any platform (for example, MIPS, x86, ARM, PowerPC). Supported WLAN cards are based on Intersil Prism Chipset 2/2.5/3. It also offers support for 5 GHz frequencies. Here are additional ROSE features:
• Access point development kit.
• 802.11 protocols and security, MAC address filtering, IPv4 routing, firewall, RADIUS, QoS, bridging, NAT, DHCP. Additional drivers like 802.11a/g/h are also supported.
• The ROSE builder is written in Python for good support and maintainability (Python language version 2.x with standard Python libraries -- 2.1 and 2.2 have been tested, plus working C compiler, gzip).
Essentially, by using ROSE (with the Linux 2.4 kernel) and an 802.11 Intersil Prism-based WLAN card, you can build a Linux-based wireless access point.
Wireless Signal Strength
If you want to be an informed user, you should know how to estimate the strength of a wireless signal. The following is a simplified formula reflecting radio frequency signal loss for the 2.45 GHz frequency band used at 802.11b/g access points:
SignalLoss = 40 + 20•log (d) ;Here, SignalLoss is expressed in dB and d is distance expressed in meters. This formula is simplified, as it does not take into account the loss of signal due to filtering by wall material; however, it gives you a quick idea about wireless signal coverage.
Choosing Wireless Extensions
The beauty of wireless extensions is that a single set of tools can support all the variations of wireless LANs, regardless of their type (as long as the hardware driver supports the wireless extensions). Another advantage is that these parameters can be changed on the fly without restarting the driver (or Linux).
The set of Linux tools that manipulate wireless extensions are generally referred to as the wireless tools. They use a textual interface and are rather simplified. The main ones used in Linux implementations are:
• iwconfig: Manipulates the basic wireless parameters
• iwlist: Initiates scanning and list frequencies, bit-rates, and encryption keys
• iwspy: Retrieves per-node link quality
• iwpriv: Allows for manipulation of wireless extensions specific to a Wi-Fi driver
• ifrename: Allows name interfaces based on various static criteria
Tutorial Pages:
» A Variety of Tools and Projects Help Get Wireless Linux Off the Ground
» A Linux Wireless Access Point: Build or Buy?
» Friendly Tools
» Interoperability Issues
» Linux's Wireless Future
» Resources
First published by IBM DeveloperWorks
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