Chapter 9. Flashcards
How many sublayers is the 802.11 data-link layer divided into? What are they?
2 sublayers.
The upper portion is the IEEE 802.2 Logical Link Control (LLC).
The bottom portion of the data-link layer is the media access control (MAC) sublayer.
What is the MSDU? What kind of 802.11 frame carries the MSDU payload?
The MSDU contains data from the LLC and layers 3-7. A simple definition of the MSDU is that it is the data payload that contains the IP packet plus some LLC data.
Only 802.11 data frames carry an MSDU payload in the frame body.
What is the max size for an MSDU payload?
2,304 bytes plus an overhead for encryption.
What is an MPDU?
When an MSDU (MAC Service Data Unit), is sent to the MAC sublayer of the OSI the MAC header information is added to the MSDU to identify it, THE MSDU is now encapsulated in a MAC PROTOCOL DATA UNIT (MPDU)
What are the 3 basic components to of an MPDU? What do they do?
A MAC Header, Frame Body, Frame Check Sequence.
A MAC Header : Frame control information, duration information, MAC addressing, and sequence control information are all found in the MAC header. Furthermore, QoS data frames contain specific QoS control information.
A Frame Body : The frame body component can be variable in size and contains information that is different depending on the frame type and frame subtype. The MSDU upper layer payload is encapsulated in the frame body. The MSDU layer 3–7 payload is protected when using encryption.
Frame Check Sequence (FCS). The FCS comprises a 32-bit cyclic-redundancy check (CRC), that is used to validate the integrity of received frames.
What are the 2 layers of the Physical layer?
The upper portion is known as the physical convergence procedure sublayer (PLCP), and the lower portion is known as the physical medium dependent (PMD) sublayer
What does the Physical Layer Convergence Procedure (PLCP) do?
The PLCP prepares the frame for transmission by taking the frame from the MAC sublayer and creating the PLCP Protocol Data Unit (PPDU).
What does the PMD sublayer do?
Modulates and transmits the data as bits that is just recieved from the PLCP sublayer above it.
What is the only difference between the PLCP and a MPDU?
They are the exact same thing, except which layer of the OSI model they are on. The PLCP is located on the physical layer and the MPDU is located on the data layer.
What does the PLCP do to the PSDU to prepare it for transmission?
The PLCP adds a preamble and PHY header
to the PSDU.
What do preambles do?
The preamble is used for synchronization between transmitting and receiving 802.11 radios.
What is the purpose of an integration service?
This transports the MSDU payload in the data frame from an 802.11 wireless frame format into an 802.3 frame to ride on the ethernet medium.
What is the max size for 802.3 Ethernet frames with a payload? What is the max size for VLAN payload?
802.3 frame size is 1,518 with a max payload of 1,500.
1522 with a data payload of 1504 bytes.
What is an individual MAC address located in the 802.11 frame?
Individual addresses are assigned to unique stations on the network (also known as a unicast address).
What is a Group Address MAC address located in the 802.11 frame?
A multiple destination address (group address) could be used by one or more stations on a network. There are two kinds of group addresses.
What are the 2 kinds of group addresses located in an 802.11 frame header?
Multicast-Group Address An address used by an upper-layer entity to define a logical group of stations is known as a multicast-group address.
Broadcast address : A group address that indicates all stations that belong to the network is known as a broadcast address. A broadcast address, all bits with a value of one, defines all stations on a local area network.
How many MAC addresses can an 802.11 frame have? How many are typically used?
4 max.
3 typically used,
What is the source address (SA) used for in an 802.11 frame header?
The MAC address of the original sending station is known as the SA. The source address can originate from either a wireless station or the wired network
What is the destination address (DA) used for in an 802.11 frame header?
The MAC address that is the final destination of the layer 2 frame is known as the DA. The final destination may be a wireless station or could be a destination on the wired network such as a server or a router.
What is the transmitter address (TA) used for in an 802.11 frame header?
The MAC address of an 802.11 radio that is transmitting the frame onto the half-duplex 802.11 medium is known as the TA
What is the receiver address (TA) used for in an 802.11 frame header?
The MAC address of the 802.11 radio that is intended to receive the incoming transmission from the transmitting station is known as the RA.
What is the Basic Service Set Identifier (BSSID) used for in an 802.11 frame header?
This is the MAC address that is the layer 2 identifier
of the basic service set (BSS). The BSSID is the MAC address of the AP’s radio or is derived from the MAC address of the AP’s radio if multiple basic service sets exist.
What are management frames for?
Management frames are used by wireless stations to join and leave the basic service set (BSS).
What IOS layer information does an 802.11 management frame carry?
Only layer 2, frame with an important field called the information elements.
What are 802.11 control frames? What is the only thing that they contain?
They assist with the delivery of the data frames, and are transmitted at one of the basic rates. Control frames are also used to clear the channel, acquire the channel, and provide unicast frame acknowledgements. They contain only header information.
What are 802.11 data frames?
They carry the actual data that is passed down from the higher protocols. The layer 3 – 7 MSDU payload is normally encrypted for data privacy reasons.
What are beacons and their purpose?
The AP of a basic service set sends the beacons while the clients listen for the beacon frames. Each beacon contains a timestamp, which client stations use to keep their clocks synchronized with the AP. Because so much of successful wireless communications is based on timing, it is imperative that all stations be in sync with each other.
What is a time stamp in a beacon frame?
Synchronization information
What are some things that are included in a beacon frame?
- Time Stamp
- Spread spectrum information
- Channel information
- Data Rates
- Service Set Capabilities.
- SSID
- Traffic indication Map.
- QoS capabilities
- Robust Security Network Capabilities (TKIP or CCMP)
- Vendor Proprietary Info
What is located in the QoS capabilities section in a beacon frame?
Quality of service and Enhanced Distributed Channel Access (EDCA) information
How many times a second is a beacon frame transmitted?
10 times per second.
What is passive scanning?
The client station listens for the beacon frames that
are continuously being sent by the APs
What does a client station do when it hears the same SSID from multiple stations?
If the client station hears beacons from multiple APs with the same SSID, it will determine which AP has the best signal, and it will attempt to connect to that AP.
How does active scanning work? (DETAILED)
In active scanning, the client station transmits management frames known as probe requests. These probe requests either can contain the SSID of the specific WLAN that the client station is looking for or can look for any SSID.
If a directed probe request is sent, all APs that support that specific SSID and hear the request should reply by sending a probe response.
The information that is contained inside
the body of a probe response frame is the same information that can be found in a beacon
frame, with the exception of the traffic indication map (TIM). Just like the beacon frame, the probe response frame contains all of the necessary information for a client station to learn about the parameters of the basic service set before joining the BSS
What is a directed probe request?
A probe request with the specific SSID information.