CCNA Wireless Chapter 7 answers

MAC layer provides 3 services, which are
1) Asynchronous data service
2) Security services
3) Media access control

With asynchronous data service…
Peer LLC’s exchange MSDU’s

With security services, there are 3 options:
Authentication, WEP or 802.11i

Every frame contains 3 things:
1) MAC header
2) Variable length frame body
3) FCS

The frame body is specific to…
1 of the 3 frame types in 802.11

How long are CRC’s?
4 bytes

The 3 802.11 frame types are:
1) Data
2) Control
3) Management

What do control frames do in 802.11?
Send CTS/RTS, ACK etc.

What do management frames do in 802.11?
Send associations and beacons

Management frames are ______________ to upper layer protocols
Not forwarded

802.11 is a CSMA/CA network, meaning it uses physical and virtual carrier sense mechanisms

Virtual carrier sense mechanisms….
Announce the use of media (NAV)

NAV stands for:
Network allocation vector

The distributed coordination function simply means the exchange of…
RTS’s and CTS’s

Both RTS’s and CTS’s contain a…
Duration field

The duration field is…
The time to tx data frame, ack frame, and inter fame spaces

Four diff. types of IFS’s:
1) SIFS: short inter frame space
2) PIFS: point coordination function inter frame space
3) DIFS: distributed coordination function inter frame space
4) EIFS: extended inter frame space

What does PCF do?
Creates contention free access to wireless network for tx devices, only available in infrastructure networks because AP is the polling master and central point of this optional method of access

The MAC layer can intentially _________ MSDU’s
reorder

When the MAC layer is reordering MSDU’s, what type of frames get priority?
unicast

The frame control field in the MAC header is how long
2 bytes

The protocol version field would be what for the current version?
00

The type bits would be set to what for each type of MAC frame
1) 00 – mgmt
2) 01- ctrl
3) 10 – data

The subtype field in the frame control field consists of…
Additional info about whatever frame type has been specified in the type section

The last half of the frame control field is…
1 byte of flags

If the destination is the distribution system, the To DS bit would be set to…
1

If the source is the distribution system, the From DS bit would be set to…
1

If a wireless distribution system is in play, the From and To DS bits are set to
1

If an ad-hoc network is in play, the From and To DS bits are set to
0

If the More flag is set to 1, what does this mean?
It means the MPDU is too large

What does 802.11 do with MPDU’s which are too large
It can fragment them like TCP does

If the retry flag is set to 1…
The frame has been transmitted before

If the power management flag is set to 1, the station is in
Power save mode

If the more data flag is set to 1….
The station is telling the receiver that the transmission is not complete and it should not enter power save mode

If the protected frame flag is set to 1…
The body is encrypted

If the order flag is set to 1…
An upper layer protocol requested strict order transmission, meaning the MAC layer will not reorder anything

The duration field has 2 functions:
1) Remaining duration
2) AID (association ID)

In the duration field, if bit 15 is 0 the field is…
The remaining duration

In the duration field, if bit 15 is 1 the field is…
The AID of the txing station

There are ___ address fields in an 802.11 frame
4

Data frames use how many address fields in an 802.11 frame?
3 or 4

Control and management frames can use as little as 1 address field in an 802.11 frame

The first 3 bytes of a MAC addr =
The OID of the vendor

The last 3 bytes of a MAC addr =
The unique ID of the MAC entity

5 types of addr’s in 802.11:
1) BSSID (ID’s boundaries)
2) Src
3) Dst (final dst)
4) Tx addr (last to tx)
5) Rx addr (next to rx)

The sequence control field has 2 subfields which are:
1) Fragment # (4 bits)
2) Frame seq # (12 bits)

The max frame body size for unencrypted frames is…
2304 bytes

The max frame body size for encrypted frames is…
2346 bytes

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