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Standards Based Wireless Networking with Linux

Brian D. Mathews - Harris Semiconductor
Jo-Ellen F. Mathews - AbsoluteValue Systems
Mark S. Mathews - AbsoluteValue Systems

1. Introduction

Wireless Local Area networks (WLANs) have been employed to add mobility features to office and campus networks since the late 1980s. This article presents a discussion of the current state of WLAN technology and some of the products available.

Physically, there are two ways to implement wireless LANs: infrared and radio. Since radio is currently the most popular choice, we will restrict this discussion to radio wireless LANs.

WLANs are a distinct category of products and technology that must be differentiated from Metropolitan Area Networks (MANs) and Wide Area Networks (WANs). Examples of wireless MANs are Ricochet, Ardis, and RAM Mobile Data which provide city-wide coverage for low bit-rate data services. The most visible wireless WAN system is the cellular telephone system which can be used for data services just as telephone land lines can be used with a modem for data services. However, the bandwidth limitations when using switched cellular technology are severe, and data connections generally are not tolerant of the extended drop-outs that can occur. Conversely, WLANs are generally accepted to be 1Mbps links or above (although a few drop into the 100's of Kbps), short range (100's of meters) technologies which do not need to support vehicular mobility (high speed handoffs) or wide area coverage. What they do provide is the wireless equivalent of a LAN for file sharing, remote database access, file server access, internet access, e-mail, and all the other applications which operate over LANs, only the user is no longer tethered to an RJ45 wall jack. Note also that there are a few applications of WLAN technology which operate over great distances (1-20 miles) but they are not generally regarded as WLANs, but as wireless bridges or point-to-point data links.

2. A Historical Perspective

WLAN products were initially developed to support vertical market applications, i.e. sold as components of a specific solution to a specific problem where user and station mobility were required. You may be familiar with one or more of the following WLAN vertical applications:

Hertz rental car portable check-in terminals,
Wal-Mart inventory and pricing terminals, and
Hospital portable patient information terminals.

In each of these applications, the WLAN was deployed to support the specific application. The WLAN was not a part of the general purpose LAN infrastructure. More recently, particularly because of recent steps towards WLAN standardization, WLANs are being deployed to support general purpose LANs.

2.1. RF, the FCC, and WLAN limitations

In most (if not all) countries, devices that emit radio signals are closely regulated by the government. Such regulation includes WLAN products. Here, in the United States, the regulating agency is the Federal Communications Commision or FCC. The FCC licenses the use of the radio spectrum by various users, usually on a per frequency band basis. In most bands, for any particular user, there are two licenses required: a use license and an equipment license. The use license is required for the individual end-user of the transmitting equipment. The equipment license is usually obtained by the manufacturer. There are a few small frequency bands where a license for the end user is not required, but only if certain rules are observed regarding transmitted power and signal modulation. An example of this license-free radio usage is found in cordless telephones. These telephones use frequency bands (48-49MHz and 902-928MHz) allocated by the FCC for unlicensed usage and use very low power to restrict their range and the potential for interference.

In modern wireless LANs, the frequency band most commonly used is 2.4GHz to 2.485GHz. Within this band, no end-user license is required as long as the transmitted power is no more than 1 Watt and the transmitter uses a 'Spread Spectrum' transmission technique. The 1 Watt power restriction serves to limit the range where one radio may interfere with another. The 'Spread Spectrum' requirement is intended to make the WLAN signal appear as background noise to a narrowband or 'narrow spectrum' receiver.

In actual practice, the FCC restrictions limit the range of most wireless LAN devices to approximately 100 meters indoors and 1000 meters outdoors (line-of-site). As we'll see, these range limitations have a significant impact on the architecture of wireless LANs.

2.2. IEEE 802.11 - A standard is born

Until the summer of 1997, there were no standards for wireless LAN products. Each vendor defined the protocols and signaling for their own products, and these proprietary products did not interoperate with one another. In 1990, the Institute of Electrical and Electronics Engineers (IEEE) formed a working group, identified as 802.11, to standardize wireless LAN signaling and protocols.

The standard developed by working group 802.11 was accepted by the IEEE board during the summer of 1997 and became IEEE standard 802.11-1997. The new standard defines three different physical implementations (signaling techniques and modulations), a Media Access Control function, and a Management function. The three physical implementations are:

direct sequence spread spectrum radio (DSSS) in the 2.4 GHz band,
frequency hopping spread spectrum radio (FHSS) in the 2.4 GHz band, and
infrared light (IR).

All of the implementations support data rates of 1 Mb/s and, optionally, 2 Mb/s. The 802.11 working group is currently considering additions to the standard that will provide higher data rates (5.5 and 11 Mb/s) in the 2.4 GHz band and additions that will allow wireless LANs to operate in a 5 GHz band. Acceptance of the standard for higher data rates in the 2.4 GHz band is expected in October of 1998.

2.3. Growth of a horizontal market

Wireless LANs have traditionally been employed in vertical markets. Until recently, WLANs haven't been widely deployed as a general purpose LAN solution. There are three primary reasons for this:

single vendor proprietary products,
high cost, and
low throughput.

These three factors have led most LAN designers to simply ignore the benefits of WLAN technology in general purpose LAN environments. Fortunately, the development of a standard will mitigate, if not eliminate, all of these issues. With these issues out of the way, WLANs are likely to become a more widely accepted element of Local Area Network design.

Standardizing network technologies has always led to greater deployment. The LAN market has shown again and again how products in non-standardized categories have slow adoption rates. By standardizing the signaling and protocols for WLANs, true multi-vendor solutions will be possible. Standardization forces vendors to compete for the same customers in the same markets, and this competition will inevitably drive prices down. Wireless LAN cards are currently (4th Qtr 1998) priced at approximately $500-$700 retail. Given current component and manufacturing costs, there is no reason why those prices won't erode to $150-$200. Additionally, increasing chip-level integration (e.g. putting the functionality of 3 chips into 1 chip) in the semiconductor products used to manufacture WLAN cards will also continue to drive down the costs associated with manufacturing. The standard will also press WLAN manufacturers to improve the performance of their products. By leveling the playing field and introducing multi-vendor installations, WLAN vendors will be forced to look at improving throughput and adding more management tools to their products in an effort to differentiate themselves from their competitors.

Another aspect that will continue to drive an increase in the use of WLANs is the explosive growth in the area of mobile computing. Many users are replacing their desktop computers with mobile equivalents. Examples are: notebooks, palmtops and Personal Digital Assistants (PDAs). The next step for these users is to demand network connectivity to accompany their mobility and the best solution to meet that demand is WLAN.

Unfortunately, given 100 meter range of wireless LAN technology, simple point to point implementations won't be acceptable in most installations. To support larger offices and campuses, an architecture providing several hundred or several thousand meters of seamless coverage is required. Fortunately, the IEEE 802.11 standard defines most, but not all, of the elements to support such an architecture.

3. IEEE 802.11 WLAN Components

The IEEE 802.11 standard defines a number of elements that are necessary for a scalable, secure wireless LAN. Depending on the size and requirements for the WLAN, some of these items are optional. This section introduces and defines the items and terminology associated with 802.11 based wireless LANs.

Within any 802.11 wireless LAN, there are three possible elements:

one or more wireless stations (STAs),
one or more wireless access points (APs) - [optional], and
only one Portal - [optional].

The simplest 802.11 WLAN consists of just wireless stations. All of the stations are within range of each other and they only communicate amongst themselves. A more complicated arrangement consists of a collection of wireless stations using one or more central access points to coordinate their communications. In the latter case, the stations don't have to be in range of each other, but they do have to be in range of an access point. If two stations wish to communicate and they are within range of different access points, then the access points will forward the station traffic to each other. The most complicated arrangement arises when there are multiple access points and we want the wireless stations to have the following capabilities:

change access points with no loss of connection (roaming), and
communicate with nodes on an existing wired network.

For roaming, this model requires that access points communicate management information among themselves. One such item of management information is identifying which stations are currently within range of each access point. This communication is required so that the forwarding function mentioned above can continue to operate when a station changes from one access point to another. To communicate with nodes on a wired network, a Portal is required.

For stations to maintain their connections when changing between APs, they must retain their OSI layer 3 address (e.g. IP address). However, it is possible that the APs reside on different subnets of the wired network. Therefore, a Portal is required to act as the gateway between all APs and the wired network. Whenever an AP receives a frame from a wireless station that is destined for a wired node, that AP will forward the frame to the Portal. It is then the Portal's responsibility to forward the frame to the wired node. Additionally, all frames sent from wired nodes will initially be received by the Portal. The Portal then sends those frames to the appropriate AP for delivery to the destination wireless station.

3.1. Wireless Stations

The most common architectural element of a standard 802.11 network is the wireless station referred to in the standard as a STA. Each of these stations contains a network interface card (NIC) implementing an 802.11 standard physical (PHY) layer, a media access control (MAC) layer, and a management function. Each NIC (or STA) is identified by a 48 bit address (or a broadcast or multicast adrress) coded in the same format as ethernet. The NIC provides the capability to:

detect an existing WLAN,
join or synchronize with the WLAN,
authenticate with the WLAN,
transmit frames to other stations in the WLAN,
receive frames from other stations in the WLAN, and
encrypt/decrypt frames being transmitted or received.

The radio medium is shared among many different stations using an algorithm called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). CSMA/CA is similar to ethernet's CSMA/CD (collision detection), but considerably more complicated. To implement CSMA/CA, the 802.11 MAC function transmits timing information allowing a station to 'reserve' the medium for a particular period of time, thus preventing many possible collision scenarios.

Normally, there are many STAs in any given wireless network. They communicate with each other by transmitting frames with the addreess of the destination STA encoded in the frame header. Like ethernet, all stations within the network hear the frame, but only the destination STA accepts the frame. The receiving STA will transmit an acknowledgement back to the originating station. Given the uncertainties of the wireless medium, positive acknowlegement allows the transmitting STA to know whether the frame was received by the destination STA.

A collection of stations within range of each other, communicating only among themselves, is the simplest WLAN architecture. To build more complicated networks, additional elements are required.

3.2. Access Points

For some WLANs, access points (APs) are used to provide a central (or 'point') coordination function for wireless LANs. APs are also referred to as 'base stations'. Physically, an access point provides a 'center point' for a collection of stations. In the presence of an access point, STAs no longer communicate with one another directly. All frames are transmitted to the AP, and the AP transmits them to their destined wireless stations. Since the AP is retransmitting all frames, the STAs are no longer required to be in range of one another. The only requirement is that the stations be within range of the AP. In the figure, station A is sending a frame to station B. Station A first sends the frame to the AP, and the AP retransmits the frame to station B.

Access points can also be employed to build WLANs that are physically larger than the range of one station. Since all traffic within a group passes through the access point for that group, the access point has the ability to inspect the destination address for each and every frame. If the access point identifies a frame that is destined for a STA that is not within the local group, the AP can forward that frame to the AP 'closest' to the destination STA. This functionality can also be applied to support 'roaming'.

Roaming occurs when a station moves out of the range of one AP and reconnects with a different AP. When a station moves out of range of an AP, that station will begin scanning for a new AP. Scanning is a function performed by the station. The scanning function can be performed actively or passively. When actively scanning, a station begins transmitting 'Probe' frames. When an AP hears a 'Probe' frame, it responds with a 'Probe Response' frame that contains information about the AP. The station collects 'Probe Response' frames from as many APs as possible and then selects an AP. When passively scanning, a station simply listens for traffic from any AP that might be nearby. Again, the station collects information about as many APs as possible and then selects one. Once a new AP has been selected, the station notifies that AP and becomes 'associated' with that AP. Traffic destined for the roaming station will now be sent to the new AP.

To support the capability for stations to change APs, the access points have to communicate among themselves. The most important item of information that must be transmitted between APs is a table containing the addresses of all the stations associated with each AP. The 802.11 standard does not specify what kind of protocols or networks will be used to transmit this inter-AP information. The IEEE intentionally left inter-AP communications out of the 802.11 standard. This is largely due to the fact that OSI layer 3 and 4 protocols are needed to implement inter-AP communication. Generally, the 802 family of standards focuses on layer 1 and 2 elements.

Unfortunately, the lack of standards for inter-AP communications leads to proprietary products again. Currently, vendors who are shipping 802.11 compliant equipment have their own inter-AP communication protocols.

3.3 Portals

In most situations, WLANs will be integrated with a wired LAN. The WLAN component responsible for this integration is the Portal. The Portal provides the one entry/exit point for frame data to/from the WLAN. A frame being sent from a wireless station to a wired node first reaches the AP for that station, then the AP forwards the frame to the Portal, and the Portal forwards the frame to the wired node. A frame being sent from a wired node to a WLAN station is first received by the Portal, then sent to the appropriate AP, and the AP forwards the frame to the destination WLAN station.

To support the proper delivery of frames to the APs for particular wireless stations, Portals must also be able to receive the management information exchanged between APs. Unfortunately, this means that Portals are no more standardized than APs.

4. Assembling Wireless LANs

At this point, we've discussed all of the primary architectural elements used to construct IEEE 802.11 standard wireless LANs. This section will look at different ways to assemble these pieces to build a variety of WLANs.

The 802.11 standard discusses two types of WLANs: Ad Hoc networks and Infrastructure WLANs.

4.1 Ad Hoc Wireless LANs

The 802.11 standard defines an 'Ad Hoc' network as "a network composed solely of stations within mututal communication range of each other via the wireless medium'. The standard goes on further to more completely define 'Ad Hoc' networks as 'Independent Basic Service Sets' or IBSSs. An IBSS is a specific form of a Basic Service Set or BSS. A Basic Service Set (BSS) is a collection of stations that are either strictly within range of one another and communicating amongst themselves without an AP, or a group of stations associated with one AP. An IBSS is a BSS where stations communicate without an AP. Each BSS and IBSS is identified by a BSS identifier (BSSID) that is formatted the same as a WLAN NIC address (a 48 bit number). Each frame transmitted within an 802.11 WLAN includes the destination BSSID. This is the basic way that separate wireless LANs are distinguished from one another.

As implied above, Ad Hoc networks are the simplest form of 802.11 networks. Ad Hoc networks are convenient for temporary workgroups or for small installations where the management capabilities of Access Points aren't necessary. Ad Hoc networks can usually be rapidly constructed simply by activating a collection of stations within range of one another.

A common misconception is that Ad Hoc networks cannot be connected to wired networks. This is not the case. Using the capabilities of network protocols outside the realm of the 802.11 standard, Ad Hoc networks can be connected to wired networks. The only restriction is that there can be only one IBSS and no roaming is possible. A simple example of this is that of a router (or host configured as a router). If the router has one interface that is a wireless NIC which is a member of an IBSS and another interface that is a wired NIC such as ethernet, then the router will be able to forward OSI layer three data between the two interfaces as long as the appropriate subnetwork numbers have been assigned to stations on the wired and wireless networks.

4.2 Infrastructure Wireless LANs

Infrastructure wireless LANs, according to 802.11, are any wireless LANs that include elements other than stations. The simplest infrastructure WLAN consists of a single AP and its associated stations. This collection is also considered to be a BSS. The BSSID for AP centered BSSs is equal to the NIC address of the AP. Since all access to the BSS is controlled through the AP, infrastructure networks are generally more secure than IBSSs.

Larger infrastructure networks can be constructed by using more than one AP centered BSSs. Multiple BSSs are collected together into an Extended Service Set or ESS. ESSs are identified using a 32 character string (the ESSID) defined by the network administrator when the ESS is established.

5. Linux and Wireless LANs

At the time of this writing, the only Linux driver available for 802.11 compatible hardware is for the Harris PRISM wireless LAN card. The PRISM card is a 2 Mb/s DSSS PCMCIA card developed as a reference design by Harris Semiconductor. The driver currently supports only IBSS (Ad Hoc) mode.

An Open Source development project is under way to develop software that will allow Linux systems to serve as the access points (APs) and Portal for large scale WLAN systems. One of the primary goals of this project is to provide an Open Source reference design for the protocols to support inter-AP communications and AP to portal communications. It is our sincere hope that WLAN product vendors will use this project as a template for their own products, thus developing the Open Source implementation into a standard defining those elements the IEEE chose not to define.

Appendix A. Further Reading

Appendix B. Acronyms

ACK = Acknowledgment
AP = Access Point
BSA = Basic Service Area
BSS = Basic Service Set
BSSID = Basic Service Set Identification
CF = Coordination Function
CTS = Clear To Send
DSSS = Direct Sequence Spread Spectrum
ESA = Extended Service Area
ESS = Extended Service Set
FH = Frequency Hopping
FHSS = Frequency Hopping Spread Spectrum
IBSS = Independent Basic Service Set
MAC = Medium Access Control
MIB = Management Information Base
PDU = Protocol Data Unit
PHY = Physical (Layer)
RTS = Request To Send
SSID = Service Set Identifier
STA = Station
WEP = Wired Equivalent Privacy
WM = Wireless Medium

Appendix C. Glossary

Access Point (AP): Any entity that has station functionality and provides access to the distribution services, via the wireless medium (WM) for associated stations.
Ad hoc network: An ad hoc network is a network composed solely of stations within mutual communication range of each other via the wireless medium. An ad hoc network is typically created in a spontaneous manner. The principal characteristic of an ad hoc network is its limited temporal and spatial extent. These limitations allow the act of creating and dissolving the ad hoc network to be sufficiently straightforward and convenient so as to be achievable by non-technical users of the network facilities (i.e. no specialized 'technical skills' are required with little and/or no investment of time or additional resources required beyond the stations which are to participate in the (ad hoc) network). The term �Ad Hoc� is often used as slang to refer to an Independent BSS (IBSS).
Authentication: The service used to establish the identity of one station as a member of the set of stations authorized to associate with another station.
Basic Service Area (BSA): The conceptual area within which members of a Basic Service Set may communicate.
Basic Service Set (BSS): A set of stations that may directly communicate with one another.
Broadcast: The Broadcast address is a unique Multicast address that specifies all stations.
Coordination Function (CF): The logical function which determines when a station operating within a Basic Service Set is permitted to transmit and may be able to receive Protocol Data Units (PDUs) via the wireless medium.
Extended Service Area (ESA): The conceptual area within which members of an Extended Service Set may communicate. An Extended Service Area is larger or equal to a Basic Service Area and may involve BSSs in overlapping, disjoint or both configurations.
Extended Service Set (ESS): A set of one or more interconnected Basic Service Sets which appear as a single seamless network.
Frame: A data link layer "packet" which contains the header and trailer information required by the physical medium. That is, network layer packets are encapsulated to become frames. (Thanks to FOLDOC)
Independent Basic Service Set (IBSS): A BSS which forms a self contained network containing only stations, and in which no AP is present.
Infrastructure: All of the elements required to tie a collection of BSSs into and ESS: a collection of APs, the network the APs use to communicate among themselves, and optionally, a Portal.
Mobile Station: A mobile station uses network communications while in motion.
Multicast: A frame that is simultaneously transmitted to more than one destination.
Point Coordination Function (PCF): A class of possible coordination functions where the coordination function logic is active in only one station in a BSS at any given time that the network is in operation.
Portable Station: A portable station is one that may be moved from location to location, but only uses network communications while at a fixed location.
Portal: The logical point at which frames from a non-802.11 LAN enter the infrastructure.
Privacy: The service used to prevent the content of messages from being read by other than the intended recipients.
Station (STA): Any device which contains an 802.11 conformant MAC and PHY interface to the wireless medium.
Wired Equivalent Privacy (WEP): The optional cryptographic confidentiality algorithm specified by 802.11 used to provide data confidentiality which is subjectively equivalent to the confidentiality of a wired LAN medium that does not employ cryptographic techniques to enhance privacy.
Wireless Medium (WM): The medium used to implement the transfer of PDUs between peer PHY entities of a wireless LAN.