September 2004, Issue 170

Uncomplicated Wireless Networking

---

One of the funniest bits in Monty Python and the Holy Grail (1975) is the knights’ encounter with the killer rabbit. Well, I too have embarked on a noble quest, however, my quest is not for gold. I’m looking for knowledge instead. And I’m not talking about your everyday run-of-the-mill sort of knowledge. I’m searching for 802.11b wireless knowledge. Even though I am not out to save the king, Camelot, and the world from evil, my sojourn also has led me into an encounter with a killer rabbit. Instead of pointy teeth, my killer rabbit is a collection of silicon known as the Rabbit RCM3100 RabbitCore Module. The “killer” in my rabbit doesn’t lie in attitude or teeth, but rather in the application.

A standard RCM3100 prototyping board that contains all of the expected push button switches, LEDs, and connectors supports the RCM3100 RabbitCore module. However, there is one major difference in this spin of the RCM3100 prototyping board: it also carries a plug-in card that allows for the inclusion of a wireless CompactFlash card. In addition, the 802.11b prototyping kit comes with a version of Dynamic C, which includes library support for the PRISM wireless chipset. The business end of the Wi-Fi application kit is shown in Photo 1.

(Click here to enlarge)

Photo 1—The CF card’s interface is relatively simple. The RCM3100 RabbitCore module has plenty of I/O to support the wireless CF card and drive status LEDs and a serial port. The Linksys wireless CF card requires 3.3 VDC, which is supplied by a switcher on the RCM3100 prototyping board.

I’ve decided that this is a good time to delve into the bitstream of Wi-Fi communications. So, I obtained a copy of Netasyst, which is a wireless Sniffer package, to capture the data that the Wi-Fi application kit’s wireless CompactFlash card will be throwing out into the Florida room ether. Let’s begin by attending Wi-Fi 101. 

Wi-Fi BASICS

Wi-Fi is slang for 802.11b wireless communications. 802.11b can be described as a medium access control (MAC), physical (PHY), and link layer control (LLC) combination that operates in the unlicensed 2.4-GHz industrial, scientific, medical (ISM) band at a maximum speed of 11 Mbps. It uses standards set forth by the IEEE. 

The ISM frequency band that most 802.11b devices operate in is called the S-Band, which extends from 2.4 to 2.5 GHz. Operation in this band puts the typical 802.11b device at odds with other devices (e.g., microwave ovens) in that bandwidth. To reduce the interference factor and raise data delivery reliability, 802.11b devices don’t use collision detection schemes (carrier sense multiple access/collision detection, or CSMA/CD) like those found in wired 802.3 networks. Instead, a collision avoidance (CSMA/ CA) approach is implemented. This means that the 802.11b device listens to the ether before attempting to transmit unlike 802.3 devices that collide and then back off for a period of time before trying to gain access to the communications channel again. 

To make the 802.11b CSMA/CA scheme work, a set of timing rules is implemented that allows an 802.11b station to cleanly enter the communications channel, transmit its message, and release the ether to the next 802.11b station that needs to send a message. If an 802.11b station hears traffic, it does not attempt a transmission and performs an exponentially timed back-off procedure. To further enhance data delivery reliability, every transmitted message must be acknowledged by the receiver.

As you well know, there are countless RF demons that can attack a wireless network. The 802.11b hardware and IEEE standards are designed to reduce the wireless network’s susceptibility to external signals that may compromise the network’s data.

The 802.2 LLC layer is common to both the wired 802.3 MAC and 802.3 PHY OSI layers and the wireless 802.11 MAC and 802.11b PHY OSI layers. This kinship in the upper OSI layers allows the 802.11b LAN to play easily with an 802.3 LAN at the LLC layer and higher. This relationship is exhibited in the Wi-Fi application kit because a common TCP/IP library is used to support both the wired and wireless versions of RabbitCore-based development kits.

If your wireless network includes an access point, you’re operating in Managed mode, or Infrastructure mode. 802.11b devices that communicate peer-to-peer are said to be operating in Unmanaged mode, or Ad Hoc mode. You may see these modes described as independent basic service set (IBSS) for ad hoc networks or infrastructure basic service set (BSS) for infrastructure networks. A BSS is simply a number of 802.11b stations communicating with each other using Ad Hoc mode or Infrastructure mode. The word infrastructure in the BSS network mode description is implied and never used in the Infrastructure mode abbreviation for obvious reasons.

In addition to managing some of the network traffic, access points enhance the mobility of stations in a wireless network. An access point usually acts as a bridge between a wireless station and a wired system. When multiple access points are used in a network, each access point must be able to pass the mobile station’s data to and from another mobile station, another access point, or a wired network station. The passing and routing of messages is performed using what is called a distribution system. A distribution system in this sense is a logical component of 802.11 that simply routes messages to their destinations no matter which access point in the network the mobile 802.11b station is using.

A service set identity (SSID) and a channel number are used to distinguish access points. The SSID is usually a human-readable description that can be up to 32 characters in length. An 802.11b station will scan the ether channel by channel to detect an access point to join.

In the U.S., there are 11 valid channels that can be used for 802.11b. Most European countries can use 13 of the 14 available 802.11b channels. If a particular SSID is specified, the 802.11b station will only join the access point with that specific SSID. Otherwise, the 802.11b station can be configured to join the strongest signaled access point it can find.

An 802.11b station can simply listen to the ether (passive scanning) for a beacon from an access point or probe it (active scanning) to detect an in-range access point. A beacon is transmitted by all access points and delivers all of the information that is necessary for an 802.11b station to determine if it can enter a BSS via that particular access point.

After all of the available access points are identified by an 802.11b station, the station can introduce itself and join a BBS via the selected access point. Before the 802.11b station can participate in the BSS, the access point must authenticate it. There are several ways to authenticate an 802.11b station. One method, open-system authentication, is really not authentication at all. The access point simply allows the requesting 802.11b station to come in. If your 802.11b station is WEP-enabled (wired equivalent privacy), the access point could invoke shared-key authentication. Depending on whom you talk to, WEP is like using a spider web to capture a Sherman tank. But using WEP is better than using nothing at all.

You can also instruct the access point to authenticate only certain station MAC addresses. This authentication process is called address filtering. It’s also possible to use the proprietary authentication security method that the access point vendor has implemented. 

No matter how the 802.11b station gets authenticated, after the authentication process is complete, the station can then associate with the access point. Association allows the 802.11b station to use the access point to gain access to the distribution system and thus gain access to the network.

In an ad hoc system, each 802.11b station that will initiate peer-to-peer communication must be set up to the same channel and SSID. No access point is involved with stations configured for ad hoc operation. And because there is no access point involved, there is no authentication or association of the ad hoc stations.