Chapter 6

Describe Contention

- Devices contend, or compete, with each other to use the network medium - Any device can attempt to transmit a message at any time. However, if two devices send frames at the same time, a collision results and the frames become unintelligible. One way to prevent network collisions is to employ the same principles that people use in polite conversation: first, listen to make sure no one else is talking. If someone is talking, then wait; if no one else is talking, then go ahead and speak

Some of the types of management frames include

1. Authentication frame 2. Association request frame 3. Association response frame 4. Beacon frame 5. Deauthentication frame 6. Disassociation frame 7. Probe request frame 8. Probe response frame 9. Reassociation request frame 10. Reassociation response frame

What are the three different wireless LAN service set configurations?

1. Basic service set 2. Extended service set 3. Independent basic service set

The distributed coordination function defines two procedures:

1. Carrier Sense Multiple Access with Collision Detection 2. Request to Send/Clear to Send

Why can't CSMA/CD be used for wireless networks?

1. Difficult to detect collisions 2. Hidden node problem

What 4 fields are required in all MAC frames?

1. Frame Control 2. Duration/ID 3. Address 1 4. Last field (FSS)

What are the 3 types of MAC frames?

1. Management frames 2. Control frames 3. Data frames

What are the 2 types of authentication?

1. Open system authentication 2. Shared key authentication

When discovering a WLAN, what 2 types of discovery processes are there?

1. Passive scanning 2. Active scanning

Information regarding the MAC frame formats generally focuses on?

1. The data units 2. The issues that involve interoperability

The advertised BSA of an 802.11b network states that a station can be up to ____ away from the AP

375 feet (114 meters)

If a wireless device roams into a different cell it sends a ____ to the new AP

reassociation request frame

Instead of using CSMA/CD, the 802.11 standard uses an access method known as the ____

Distributed Coordination Function (DCF)

____ is used when frames must be retransmitted

Extended IFS space (EIFS)

IEEE 802.11 standard specifies that all WLAN features are implemented in the...

PHY and MAC layers

Because wireless LANs cannot limit access to the RF signal by walls or doors, wireless ____ requires the wireless device (and not the individual user) to be authenticated prior to being connected to the network

authentication

The OSI model uses the term ____ to describe the sets of data that move through the OSI layers

data unit

Describe the Aggregate MAC Protocol Data Unit (A-MPDU)

- A data unit that allows multiple MPDUs to be aggregated together. - Like A-MSDU, the MPDUs within the single A-MPDU must be addressed to the same receiver

Describe Association

- Once the AP verifies that the SSID of the station matches that of the wireless network (open system authentication) or that the challenge texts match (shared key authentication), then the wireless device is authenticated - It is the process by which the station "registers" with the AP so that the AP reserves memory space in the AP and establishes an association ID for it. The AP sends an association response frame that includes information such as the association ID and supported data rates

Describe the Point Coordination Function (PCF)

- The access point serves as the polling device or "point coordinator." It queries each device in an orderly fashion to determine if the device needs to transmit. The point coordinator begins by sensing the medium, just as all other devices do, after a SIFS time gap during which an ACK was transmitted - However, whereas the other devices must wait through the duration of the DIFS time gap, the point coordinator has to wait only through the PIFS time gap. Because the PIFS is shorter than the DIFS time gap, the point coordinator will gain control of the medium before any other devices - If the point coordinator hears no traffic at the end of the PIFS time gap, then it sends out a beacon frame to all stations. One field of this frame contains a value that indicates the length of time that PCF (polling) will be used instead of DCF (contention), and stations store it in their NAV field. After the stations receive this beacon frame they must stop any transmission for that length of time. The point coordinator then sends out another frame to a specific station, granting it permission to transmit one frame to any destination. If it has nothing to send, then that station returns a null data frame to the point coordinator

Describe Control Frames

- The second type of MAC frame - After the connection among the stations and AP is established, control frames provide assistance in delivering frames that contain the data by controlling access to the medium

Describe Active Scanning (Discovering a WLAN)

- The station first sends out a management probe request frame on an available channel - This probe request frame can be a directed probe that contains a specific SSID that the device is searching for (only APs with a matching SSID will reply with a probe response) or it can be broadcast probe with a null value as the SSID (all APs will respond) - Like the beacon frame, the probe response frame has the information the station needs to connect to the wireless LAN

The MTU of an Ethernet 802.3 frame is ____

1,500 bytes

The body of a beacon frame includes the following:

1. Beacon interval (tells the amount of time between beacon transmissions) 2. Timestamp 3. SSID 4. Supported rates 5. Parameter sets 6. Capability information

MTU of an 802.11 frame is ____

2,304 bytes

The A-MSDU increases the MTU to 7,935 while A-MPDU allows up to ____

64k bytes

____ is used when setting priorities to different types of transmissions

Arbitration IFS (AIFS)

A ____ has the flexibility to connect users to wired networks

BSS

____ is the standard interval between the transmission of data frames

Distributed Coordination Function IFS (DIFS)

An ____ provides the greatest range of functions: it allows more users to cover a broader wireless area as well as connect to external networks

ESS

One way to roam between APs that are separated by a router is to implement ____

Mobile IP

A ____ specifies the data that will be sent to the peer protocol layer at the receiving device instead of that being sent to a lower layer level (an SDU). Thus the PDU at a given layer is the SDU of the layer below

Protocol Data Unit (PDU)

An ____ allows an AP to allocate resources for a wireless device

association request frame

An AP regularly transmits a ____ to announce its presence and send wireless information to all devices that are within range

beacon frame

The ____ is normally sent once every 100 milliseconds (ms), although it can be modified

beacon interval

A second way in which CSMA/CA reduces collisions is by using explicit ____

frame acknowledgment

Devices 1 and 2 are within range of the AP but not within range of each other. If Device 2 "listens" and hears no traffic, it might assume there are no transmissions taking place, while actually Device 1 is already transmitting. This is known as the ____

hidden node problem

A third channel access method, which is also optional like PCF, is the ____

hybrid coordination function (HCF)

What is the PLCP Service Data Unit (PSDU)?

The data unit created when the MPDU is sent to the Physical Layer Convergence Procedure (PLCP) sublayer in the Physical Layer (Layer 1)

What is the purpose of a PLCP frame?

To essentially to establish synchronization between the receiving device and the incoming frame, inform the device about the number of bytes in the frame, and tell what speeds the sending device supports. MAC frames are put into PLCP frames btw

The 802.11 standard provides an option that can be used when collisions occur due to a hidden node. This option is known as virtual carrier sensing or the ____ protocol. It is the alternative CSMA/CA and fixes the hidden node problem

Request to Send/Clear to Send (RTS/CTS)

A ____ describes a specific unit of data that has been passed down from a higher OSI layer to a lower layer but has not yet been encapsulated by that lower layer

Service Data Unit (SDU)

Describe the Aggregate MAC Service Data Unit (A-MSDU)

- A data unit that allows multiple MSDUs to be combined (aggregated) together - The 802.11n added this feature to reduce the amount of overhead transmitted and thus increase overall throughput - All MSDUs within the single A-MSDU must be addressed to the same receiver

Describe the Basic Service Set Identifier (BSSID)

- A separate identifier that is the media access control (MAC) address of the AP - Is included in the header of frames that are transmitted by the AP and stations for a variety of identification purposes

Describe the HT 20 MHz Protection Mode (Mode 2)

- All of the stations in the HT 20 MHz Protection Mode are 802.11n HT device - If a 20-MHz-only HT device associates to a 20/40-MHz AP, then protection must be used to prevent the 20-MHz-only station from transmitting at the same time

Describe the Hybrid Coordination Function (HCF)

- Allows for different types of wireless traffic to be given different levels of priority. The foundation of HCF is that it schedules access to the channel by allocating transmission opportunities (TXOP) to the stations - Each TXOP has a starting time and a maximum duration, which is the time interval the station has the medium all to itself. This means that a station with TXOP can transmit multiple data frames without entering the backoff procedure. This reduction in overhead means that more frames can be sent in a given period of time

Describe the HT Mixed Mode (Mode 3)

- Both 802.11n and 802.11a/b/g devices can interoperate in the same BSA in HT Mixed Mode - Although this enables backwards compatibility, it does so at a price. The HT 802.11n devices must transmit a legacy format preamble followed by an HT format preamble. The legacy preamble tells the 802.11a/b/g devices to avoid transmitting over the HT frames that are sent to and from the 802.11n devices - Although sending two preambles adds additional overhead, it still allows the HT stations to take advantage of HT features

Describe an Extended Service Set (ESS)

- Comprised of two or more BSS networks that are interconnected. By using multiple APs an ESS can accommodate additional users over a wider area - In this set a group of APs are configured with the same SSID (logical network name) to create a single distributed WLAN. Because these APs form a single logical network they must exist in one Layer 2 broadcast domain so that when any station sends a message it goes to all other stations in the WLAN - When a user carrying a wireless device enters into the range of more than one AP the station will choose an AP based on signal strength or packet error rates - Once that station is accepted by the AP it "tunes" to the radio channel at which the AP is transmitting. The station continues to survey the appropriate radio frequencies at regular intervals to determine if a different AP can provide better service. If it finds one (perhaps because the user has moved closer to it) then the station associates with the new AP (this process is called a handoff), tuning to the different radio frequency of the new AP. To the user it is seamless because there has not been an interruption of service. This is known as Layer 2 roaming

Describe the Basic Service Set (BSS)

- Defined as one or more wireless client devices (called stations or STAs) that are served by a single AP. These devices send all transmissions to the AP as well as receive transmissions from it - Although by definition it is not required that the AP be connected to another network, practically speaking the BSS would have limited functionality if the AP were not connected: the stations would only be able to communicate between each other but not to any other devices or networks outside the BSS - Must be assigned a unique identifier to differentiate it from other WLANs. This "logical network name" is known as the Service Set Identifier (SSID) and is created by the administrator of the WLAN

Describe Passive Scanning (Discovering a WLAN)

- Depends upon the AP "advertising" itself - At regular intervals, the AP sends a beacon frame to both announce its presence and to provide the necessary information for wireless stations wanting to join the network. This process is known as beaconing - The beacon frame provides the "pulse" of the wire- less network and is an orderly means for stations to establish and also maintain communications

Describe Enhanced Distributed Channel Access (EDCA)

- Divides transmissions into four different access category (AC) classes: 1. Background (AC_BK) 2. Best Effort (AC_BE) 3. Video (AC_VI) 4. Voice (AC_VO) - Traffic that has been "tagged" with a higher priority AC (such as AC_VO) is given priority over traffic with a lower priority (like AC_BK)

Describe Block Acknowledgment

- IEEE 802.11n adds a feature known as block acknowledgment, which is necessary for A-MPDU aggregation - The block acknowledgment mechanism in 802.11n supports multiple MPDUs in an A-MPDU. When an A-MPDU from one station is received and errors are found in some of aggregated MPDUs, the receiving node sends a block ACK only acknowledging the correct MPDUs. The sender then will only retransmit nonacknowledged MPDUs

Describe the HT Greenfield Mode (Mode 0)

- In Greenfield Mode all of the stations in the BSS or ESS are 802.11n devices operating at the same HT speed with the same parameters - If an 802.11a/b/g device should roam into this BSA it would not be able to access the WLAN

Describe the Independent Basic Service Set (IBSS)

- It is a wireless network that does not use an AP and thus cannot connect to another network. In this type of network, which is also known as peer-to-peer or ad hoc mode, wireless devices communicate directly between themselves - Useful for quickly and easily setting up a wireless network anywhere that users need to share data between themselves but do not need a connection to the Internet or an external network

Describe Mobile IP

- Provides a mechanism within the TCP/IP protocol to better support mobile computing - Computers are given a home address, which is a static IP number on their home network. The computer also has a home agent, which is a forwarding mechanism that keeps track of where the mobile computer is located. When the computer roams to another network (called the foreign network) a foreign agent provides routing services to the mobile computer. The foreign agent assigns the mobile computer a new—yet temporary—IP number. This new IP number is known as the care-of address. The computer then registers the care-of address with its home agent - When a frame is sent to the computer's home address, the home agent intercepts the frame. It then encapsulates (or tunnels) that frame into a new frame with the care-of address as the destination address. It then redirects it to the foreign agent, which sends it on to the computer now located on the foreign network

Describe Distributed Coordination Function (DCF)

- Specifies that a modified procedure known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) be used - Whereas CSMA/CD is designed to handle collisions when they occur, CSMA/CA attempts to avoid collisions altogether - CMSA/CA has all stations wait a random amount of time after the medium is clear. This significantly reduces the number of collisions

Describe the HT Nonmember Protection Mode (Mode 1)

- The HT Nonmember Protection Mode is the "legacy" mode of transmitting - All stations—including 802.11n—use the 802.11a/b/g format to ensure backwards compatibility - None of the HT enhancements are utilized

Describe Management Frames

- Used to set up the initial communications between a device and the AP (for infrastructure mode) or between stations (for ad hoc mode), and then maintain the connection. - This is necessary because of the dynamic nature of a WLAN in which a station needs to locate an AP in its territory and then access the WLAN as well as eventually disconnect from the network, all of which requires multiple exchanges of frames - The format of a management frame contains a few fields: 1. The Frame control field contains information such as the current version number of the standard and if encryption is being used 2. The Duration field contains the number of microseconds needed to transmit; this value will vary depending upon which mode of wireless transmission is being used 3. The Address 1 (Destination address) and Address 2 (Source address) fields contain the addresses of the receiving and sending devices, respectively 4. The Sequence control field contains the sequence number for the packet and packet fragment number

Describe HCF Controlled Channel Access (HCCA)

- Uses polling along with centralized scheduling that is controlled by the AP (called the hybrid coordinator or HC) - Each station that requires priority of its frames sends that information to the HC, which then assigns a TXOP to the station. Each station is polled and allowed to transmit its packets until its TXOP duration elapses - Yet unlike PCF, the HC can start a polling period at different times

It is important for different systems to be able to understand each other, that is, to have interoperability. The primary areas of interoperability for WLANs are in two particular areas:

1. 802.11 interoperating 802.3 frame types 2. High-throughput (HT) devices (IEEE 802.11n HT_ interoperating with non-HT slower speed wireless networks (802.11a/b/g)

The MAC layer plays a role in several functions in a WLAN. These functions can be broken into three classifications:

1. Discovering a WLAN 2. Joining the WLAN 3. Transmitting on a WLAN

The IEEE 802.11 standard specifies three procedures for transmitting on the WLAN:

1. Distributed Coordination Function 2. Point Coordination Function 3. Hybrid Coordination Function

What are the 2 access methods specified by the Hybrid Coordination Function (HCF)?

1. Enhanced Distributed Channel Access (EDCA) 2. HCF Controlled Channel Access (HCCA)

The differences in frame size can be easily addressed to provide interoperability between the networks. The three major options are:

1. Fragmentation 2. Jumbo frames (a network interface adapter can be configured to accept frames that are larger than 1,500 bytes yet less than 9,000 bytes—this is called jumbo frame support) 3. Lowest common denominator (most wireless devices automatically set the default MTU at the lowest common denominator of 1,500 bytes)

Because of the significant differences between high throughput (HT) 802.11n and non-HT 802.11a/b/g, an 802.11n AP can tell 802.11n wireless devices to change "on the fly" to one of four HT Operation Modes in order to interoperate with slower devices. These modes are:

1. HT Greenfield Mode (Mode 0) 2. HT Nonmember Protection Mode (Mode 1) 3. HT 20 MHz Protection Mode (Mode 2) 4. HT Mixed Mode (Mode 3)

IEEE wireless standards follow the Open Systems Interconnection (OSI) model with some modifications. The IEEE has divided the Data Link layer into two sublayers:

1. Logical Link Control (LLC) sublayer 2. Medium Access Control (MAC) sublayer

List the types of Interframe Spaces (IFS)

1. Short IFS 2. Point Coordination Function IFS 3. Distributed Coordination Function IFS 4. Extended IFS 5. Arbitration IFS 6. Reduced IFS

What is the MAC Service Data Unit (MSDU)?

A data unit that contains data from Layers 3-7 along with LLC data. It is usually created when the Network Layer (Layer 3) sends data to the LLC sublayer of the Data Link Layer (Layer 2)

____ is used for immediate response actions such as ACK and has the highest level of priority

Short IFS (SIFS)

Explain how frame acknowledgment works

An acknowledgment frame (abbreviated ACK) is sent by the receiving device back to the sending device to confirm that the data frame arrived intact. If the ACK frame is not returned a problem is assumed to have occurred and the data frame is transmitted again. This explicit ACK mechanism handles interference and other radio-related problems

The physical area of radio frequency (RF) coverage provided by the AP of a BSS is called a?

Basic Service Area (BSA)

Why is it important that the 802.11 frames of a wireless network interoperate with an 802.3 Ethernet network?

Because these wireless and wired networks share a common IEEE 802 foundation, the frames share a similar format

How does Shared Key Authentication work?

Both the AP and the station are given the same key value in advance. (In other words, they "share" the key value.) The station first sends an authentication frame to the AP, and the AP responds with an authentication frame that contains a block of text known as the challenge text. The station must encrypt the text with its key value and return it to the AP in an authentication frame. The AP will then decrypt what was returned with its own key to see if it matches the original challenge text. If it does, the AP sends an authentication frame signifying the result of the authentication. It is based on the fact that only preapproved wireless devices have been given the shared key

The ____ is used when 802.11g devices are mixed with 802.11b devices together

CTS-to-self

The IEEE 802.3 Ethernet standard specifies contention with this type of "politeness" as its channel access method. Known as ____, it specifies that before a networked device starts to send a frame it should first listen on the wire (called carrier sensing) to see if any other device is currently transmitting. If it senses traffic, it waits until that traffic is finished. If it hears no traffic, then the device can send its frame. However, what if two devices simultaneously listen, hear nothing on the cable, and then both start to send at exactly the same time? A collision would still result. It handles the collisions by having the two stations responsible for the collision wait a random amount of time (the backoff interval) before attempting to resend

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

The two limitations of a BSS—a relatively small number of users being confined to a small geographical area—can be overcome with an?

Extended Service Set (ESS)

The ____ is used with 802.11n devices in a mixed environment with 802.11a/b/g device

HT Dual-CTS Protection

What happens if you increase or decrease beacon interval?

Increasing the beacon interval (and thus decreasing the number of beacon frames transmitted) can decrease wireless network traffic but may result in problems when roaming because timely information about availability of the network can be missed. Decreasing the beacon interval (and increasing the rate of beacons) makes the roaming process faster but increases network traffic and thus decreases overall throughput

What does Dynamic Rate Switching allow for?

It allows a station to remain connected albeit at a slower speed. This means that a wireless mobile user on the edge of the BSA will have a slower connection speed than a user closer to the AP

What does the Medium Access Control (MAC) sublayer do?

It appends physical addresses to the frame. The functions that are performed at the MAC sublayer involve different frame formats and types

What is one weakness of the IEEE 802.11 standard?

It does not specify how a handoff should take place. Because roaming between APs of different vendors can sometimes be a problem, some industry experts recommend that all APs in an ESS be from the same vendor

Explain how Mobile IP is useful

It enables a host to be identified by a single IP number even as it moves from one network to another. This movement is seamlessly achieved without the intervention or the knowledge of either the mobile user or the sending computer. In fact, it was the first protocol to offer transparent mobility

Why is the Distribution System (DS) needed?

It is how an AP determines what communication needs to take place with other APs in the ESS or with the wired network. It decides if it is necessary to exchange frames for stations in their own BSSs, to exchange frames with a wired network (typically through the Ethernet switch to which each AP is connected), or to forward frames to another BSS (to follow stations as they roam from one BSS to another)

Explain how Open system authentication works

It is virtually a "handshake" between the AP and station in which the station establishes its identity (but the AP does not)

What happens when a wireless device discovers the WLAN?

It next requests to join the network. This is a two-fold process known as authentication and association

What does the Logical Link Control (LLC) sublayer do?

It provides a common interface, reliability, and flow control

If a router separates the APs and each AP resides in a separate subnet, this may cause problems. When a user roams from one AP coverage area to another then a new IP address must be assigned. Connectivity can be temporarily lost and running applications may have to be restarted. This is called ____

Layer 3 roaming

Within the Data section of the PLCP frame is the ____

MAC frame

What is the difference between Maximum Transmission Unit (MTU) and Maximum Segment Size (MSS)?

MTU is not the same as the TCP value maximum segment size (MSS). The MSS name is actually misleading because it refers to the maximum amount of data that a segment can hold and does not include the TCP headers. Whereas MTU is the size of the entire packet, MSS is the size of the payload. In some consumer broadband routers and game consoles the parameter that is called MTU is in fact MSS

____ is the basic (and the default) method of authentication. After discovering the network through passive scanning or active scanning and receiving the necessary information, the wireless device sends an association request frame to the AP, which contains information such as the WLANs SSID and the data rates that the device can support. After receiving the association request, the AP then responds with an association response frame, which contains either an acceptance or rejection notice

Open system authentication

The 802.11 standard provides for an optional polling function known as ____

Point Coordination Function (PCF)

____ is the time used by a device to access the medium after it has been asked and then given approval to transmit

Point Coordination Function IFS (PIFS)

The RTS/CTS protocol imposes significant overhead upon the WLAN with the transmission of its RTS and CTS frames. The RTS/CTS protocol is especially taxing when short data packets are being transmitted. For this reason the 802.11 standard allows that when the RTS/CTS option is invoked, short data packets may still be transmitted without RTS/CTS. This is known as the ____. Only packets that are longer than the RTS threshold are transmitted using RTS/CTS

RTS threshold

Used by 802.11n devices, ____ reduces the amount of "dead space" required between OFDM transmissions, yet is restricted to Greenfield deployments

Reduced IFS (RIFS)

What is the MAC Protocol Data Unit (MPDU)?

The data unit that is formed when the LLC then sends MSDU to the MAC sublayer where the MAC header information is then added. It is also known as an IEEE 802.11 frame

What is the PLCP Protocol Data Unit (PPDU)?

The data unit when the PSDU is then passed to the Physical Medium Dependent (PMD) sublayer by adding a header and other data to it

What is the difference between Passive and Active Scanning?

The difference between passive scanning and active scanning essentially comes down to which device initiates the discovery. In passive scanning, the AP starts the process by sending out a frame that says "Here I am," while in active scanning the station sends out a frame that says, "Is anybody out there?"

Even though the Basic Service Area (BSA) is the geographical limit of the RF signal, why is it not the sam as the practical coverage area of a Basic Service Set (BSS)?

This is because there are several factors that can impact the practical distance a station can be away from the AP and still have an acceptable throughput. These factors include: 1. Obstruction 2. Number of users 3. Applications 4. Distance from AP

What happens in Polling?

With this method, each device is polled, or asked, in sequence if it wants to transmit: if the answer is yes, then it is given permission to transmit while all other devices must wait, whereas if the answer is no, then the next device in sequence is polled. Polling effectively prevents collisions because every device must wait until it receives permission before it can transmit. However, polling can have an impact on performance

Once a wireless device is authenticated, the final step is to be accepted into the wireless network. This is known as ____?

association

When an AP accepts or rejects a wireless device it sends an ____

association response frame

An ____ is used by the access point in determining whether to accept or reject a wireless device from entering the network

authentication frame

Because the wireless medium is shared, rules for cooperation among the wireless stations are necessary. These different ways of sharing are called channel access methods. One type of channel access method is known as _____

contention

The third type of MAC frame is the ____, which carries the information to be transmitted to the destination device

data frame

A device sends a ____ to another device if it wants to end a communication

deauthentication frame

A wireless device sends a ____ to another device if it wishes to end the connection

disassociation frame

The means by which multiple BSS networks in an ESS share information is through the ____

distribution system (DS)

The ____ can be a wired network to which the APs are connected, a wireless radio within the APs, or even a special purpose device that interconnects the APs and provides the required distribution services

distribution system media

Both BSS and ESS operate in what is called ____ since stations communicate through an AP

infrastructure mode

The 802.11 standard also defines different ____ or "time gaps." These are standard spacing intervals between the transmissions of the data frames

interframe spaces (IFS)

One area of difference between 802.11 and 802.3 is the frame size, known as ____. Another difference is the number of address fields

maximum transmission unit (MTU)

The contention channel access method, in which any computer can attempt to transmit a message at any time, is the basis for CSMA/CA. Another type of channel access method is ____

polling

A device sends a ____ when it needs to obtain information from another device

probe request frame

After it receives a probe request frame, a device will respond with a ____ containing capability information, supported data rates, etc

probe response frame

After it receives a reassociation request frame, an access point sends a ____ containing an acceptance or rejection notice to the wireless device requesting reassociation

reassociation response frame

The APs in an Extended Service Set (ESS) should be positioned so that the cells overlap to facilitate the movement between cells known as ____

roaming

A ____ is all of the devices that are associated with an 802.11 WLAN

service set

With wireless CSMA/CA, the amount of time that a station must wait after the medium is clear is called the ____

slot time

A wireless configuration that is used to connect the APs is called a ____

wireless distribution system (WDS)