Scanning

Before using any network, you must first find it. With wired networks, finding the network is easy: look for the cable or a jack on the wall. In the wireless world, stations must identify a compatible network before joining it. The process of identifying existing networks in the area is called scanning.

Several parameters are used in the scanning procedure. These parameters may be specified by the user; many implementations have default values for these parameters in the driver.

BSSType (independent, infrastructure, or both)

Scanning can specify whether to seek out independent ad hoc networks, infrastructure networks, or all networks.

BSSID (individual or broadcast)

The device can scan for a specific network to join (individual) or for any network that is willing to allow it to join (broadcast). When 802.11 devices are moving, setting the BSSID to broadcast is a good idea because the scan results will include all BSSs in the area.

SSID ("network name")

The SSID assigns a string of bits to an extended service set. Most products refer to the SSID as the network name because the string of bits is commonly set to a human-readable string. Clients wishing to find any network should set this to the broadcast SSID.

ScanType (active or passive)

Active scanning uses the transmission of Probe Request frames to identify networks in the area. Passive scanning saves battery power by listening for Beacon frames.

ChannelList

Scans must either transmit a Probe Request or listen on a channel for the existence of a network. 802.11 allows stations to specify a list of channels to try. Products allow configuration of the channel list in different ways. What exactly constitutes a channel depends on the physical layer in use. With direct-sequence products, it is a list of channels. With frequency-hopping products, it is a hop pattern.

ProbeDelay

This is the delay, in microseconds, before the procedure to probe a channel in active scanning begins. This delay ensures that an empty or lightly loaded channel does not completely block the scan.

MinChannelTime and MaxChannelTime

These values, specified in time units (TUs), specify the minimum and maximum amount of time that the scan works with any particular channel.

Passive Scanning

Passive scanning saves battery power because it does not require transmitting. In passive scanning, a station moves to each channel on the channel list and waits for Beacon frames. Any Beacons received are buffered to extract information about the BSS that sent them.

In the passive scanning procedure, the station sweeps from channel to channel and records information from any Beacons it receives. Beacons are designed to allow a station to find out everything it needs to match parameters with the basic service set (BSS) and begin communications. In Figure 8-2, the mobile station uses a passive scan to find BSSs in its area; it hears Beacon frames from the first three access points. If it does not hear Beacons from the fourth access point, it reports that only three BSSs were found.

Figure 8-2. Passive scanning

 

Active Scanning

In active scanning, a station takes a more assertive role. On each channel, Probe Request frames are used to solicit responses from a network with a given name. Rather than listening for that network to announce itself, an active scan attempts to find the network. Stations using active scanning employ the following procedure for each channel in the channel list:

  1. Move to the channel and wait for either an indication of an incoming frame or for the ProbeDelay timer to expire. If an incoming frame is detected, the channel is in use and can be probed. The timer prevents an empty channel from blocking the entire procedure; the station won't wait indefinitely for incoming frames.

  2. Gain access to the medium using the basic DCF access procedure and send a Probe Request frame.

  3. Wait for the minimum channel time, MinChannelTime, to elapse.

    1. If the medium was never busy, there is no network. Move to the next channel.

    2. If the medium was busy during the MinChannelTime interval, wait until the maximum time, MaxChannelTime, and process any Probe Response frames.

Probe Response frames are generated by networks when they hear a Probe Request that is searching for the extended service set to which the network belongs. At a party, you might look for a friend by wandering around the dance floor shouting out her name. (It's not polite, but if you really want to find your friend, you may not have much choice.) If your friend hears you, she will respondothers will (you hope) ignore you. Probe Request frames function similarly, but they can also use a broadcast SSID, which triggers a Probe Response from all 802.11 networks in the area. (It's like shouting "Fire!" at the partythat's sure to result in a response from everybody!)

One station in each BSS is responsible for responding to Probe Requests. The station that transmitted the last Beacon frame is also responsible for transmitting any necessary Probe Response frames. In infrastructure networks, the access points transmit Beacons and thus are also responsible for responding to itinerant stations searching the area with Probe Requests. IBSSs may pass around the responsibility of sending Beacon frames, so the station that transmits Probe Response frames may vary. Probe Responses are unicast management frames and are therefore subject to the positive acknowledgment requirement of the MAC.

It is common for multiple Probe Responses to be transmitted as a result of a single Probe Request. The purpose of the scanning procedure is to find every basic service area that the scanning station can join, so a broadcast Probe Request results in a response from every access point within range. Any overlapping independent BSSs may also respond.

Figure 8-3 shows the relationship between the transmission of Probe frames and the various timing intervals that can be configured as part of a scan.

Figure 8-3. Active scanning procedure and medium access

In Figure 8-3 (a), a mobile station transmits a probe request to which two access points respond. The activity on the medium is shown in Figure 8-3 (b). The scanning station transmits the Probe Request after gaining access to the medium. Both access points respond with a Probe Response that reports their network's parameters. Note that the second Probe Response is subject to the rules of the distributed coordination function and must wait for the contention window to elapse before transmitting. The first response is transmitted before the minimum response time elapses, so the station waits until the maximum response time has elapsed before collating the results. In areas with a large number of networks, it may be necessary to adjust the maximum channel time so the responses from all the access points in the area can be processed.

Scan Report

A scan report is generated at the conclusion of a scan. The report lists all the BSSs that the scan discovered and their parameters. The complete parameter list enables the scanning station to join any of the networks that it discovered. In addition to the BSSID, SSID, and BSSType, the parameters also include:[*]

[*] The items actually exposed by any particular software vary.

Beacon interval (integer)

Each BSS can transmit Beacon frames at its own specific interval, measured in TUs.

DTIM period (integer)

DTIM frames are used as part of the powersaving mechanism.

Timing parameters

Two fields assist in synchronizing the station's timer to the timer used by a BSS. The Timestamp field indicates the value of the timer received by the scanning station; the other field is an offset to enable a station to match timing information to join a particular BSS.

PHY parameters, CF parameters, and IBSS parameters

These three facets of the network have their own parameter sets, each of which was discussed in detail in Chapter 4. Channel information is included in the physical-layer parameters.

BSSBasicRateSet

The basic rate set is the list of data rates that must be supported by any station wishing to join the network. Stations must be able to receive data at all the rates listed in the set. The basic rate set is composed of the mandatory rates in the Supported Rates information element of management frames, as in Chapter 4.

What s in a Name? (or, the Security Fallacy of Hidden SSIDs)

WEP is not required by 802.11, and a number of earlier products implement only open-system authentication. To provide more security than straight open-system authentication allows, many products offer an "authorized MAC address list." Network administrators can enter a list of authorized client addresses, and only clients with those addresses are allowed to connect.

The SSID is an important scanning parameter. Stations search for an SSID when scanning, and may build a list of SSIDs for presentation to the user. As a unique identifier for a network, the SSID is often given mythic security properties it does not actually possess.

At the dawn of 802.11, the SSID was broadcast in the clear in Beacon frames, right there for the listening. All that was necessary was an 802.11 interface tuned to the right radio channel. When the stone age of 802.11 began, one vendor began to treat the SSID as a valuable security token. By enabling the "closed network" option on that vendor's equipment, the SSID was no longer put in Beacon frames, thus "protecting" the network from attackers. To further "protect" the SSID from prying eyes, access points operating a closed network would not respond to Probe Requests with the broadcast SSID.

Closed networks break passive scanning because the SSID is no longer available for easy collection. In order to prevent a closed network from being completely closed to clients, however, access points must respond to Probe Requests containing the correct SSID. Management frames have no encryption, and the SSID value is right there for the taking in the Probe Request. To be scrupulously correct, the closed network may offer a vanishingly small incremental amount of security because the SSID is only available when stations search for the network, rather than several times per second in Beacon frames.

Hiding an SSID can cause problems with 802.11 management. Although most 802.11 interfaces and their associated drivers can handle hidden SSIDs, not all can. Hiding an SSID is a nonstandard procedure that can cause problems, and does not provide any real security. Leave the SSID in the Beacon frames for interoperability, and use a real security solution like 802.1X if you need it.

 

Joining

After compiling the scan results, a station can elect to join one of the BSSs. Joining is a precursor to association; it is analogous to aiming a weapon. It does not enable network access. Before this can happen, both authentication and association are required.

Choosing which BSS to join is an implementation-specific decision and may even involve user intervention. BSSs that are part of the same ESS are allowed to make the decision in any way they choose; common criteria used in the decision are power level and signal strength. Observers cannot tell when a station has joined a network because the joining process is internal to a node; it involves matching local parameters to the parameters required by the selected BSS. One of the most important tasks is to synchronize timing information between the mobile station and the rest of the network, a process discussed in much more detail in the section "Timer Synchronization," later in this chapter.

The station must also match the PHY parameters, which guarantees that any transmissions with the BSS are on the right channel. (Timer synchronization also guarantees that frequency-hopping stations hop at the correct time, too.) Using the BSSID ensures that transmissions are directed to the correct set of stations and ignored by stations in another BSS.[*] Capability information is also taken from the scan result, which matches the use of WEP and any high-rate capabilities. Stations must also adopt the Beacon interval and DTIM period of the BSS, though these parameters are not as important as the others for enabling communication.

[*] Technically, this is true only for stations obeying the filtering rules for received frames. Malicious attackers intent on compromising network security can easily choose to disobey these rules and capture frames, and most existing product implementations do not correctly implement the filtering rules.

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