WLAN
Distribute Coordination Function (DCF)
•
•
•
Distributed access protocol
Contention-Based
Uses CSMA/ CA
–
•
Uses both physical and virtual carrier sensing.
Suited for ad hoc network and ordinary
asynchronous traffic
Networks: Wireless LANs
2
Physical Carrier Sensing
• Station senses the channel when it wants to send.
• If idle, station transmits.
– Station does not sense channel while transmitting.
• If the channel is busy, station defers until idle and
then transmits.
• Upon collision, wait a random time using binary
exponential backoff.
Networks: Wireless LANs
3
Virtual Channel Sensing in CSMA/CA
Figure 4-27. The use of virtual channel sensing using CSMA/CA.
• C (in range of A) receives the RTS and based on information in
RTS creates a virtual channel busy NAV(Network Allocation
Vector).
• D (in range of B) receives the CTS and creates a shorter NAV.
Networks: Wireless LANs
4
Virtual Channel Sensing in CSMA/CA
• “virtual” implies source station sets duration field
in data frame or in RTS and CTS frames.
• Stations then adjust their NAV accordingly!
Networks: Wireless LANs
5
Point Coordinated Function (PCF)
– Alternative access method on top of DCF
– Centralized access protocol
– Contention-Free
– Works like polling
• PCF uses a base station to poll other stations to see if
they have frames to send.
• Base station sends beacon frame periodically.
• No collisions occur.
– Suited for time bound services like voice or
multimedia
DCF and PCF Co-Existence
• Distributed and centralized control can co-exist using
InterFrame Spacing.
• SIFS (Short IFS) :: is the time waited between packets
in an ongoing dialog (RTS,CTS,data, ACK, next frame)
• PIFS (PCF IFS) :: when no SIFS response, base station
can issue beacon or poll.
• DIFS (DCF IFS) :: when no PIFS, any station can attempt
to acquire the channel.
• EIFS (Extended IFS) :: lowest priority interval used to
report bad or unknown frame.
Networks: Wireless LANs
7
Interframe Spacing in 802.11.
DIFS (MAX) > PIFS > SIFS (MIN)
SIFS - Used for immediate response actions e.g ACK, CTS
PIFS - Used by centralized controller in PCF scheme
DIFS - Used for all ordinary asynchronous traffic
Networks: Wireless LANs
8
RTS-CTS-DATA-ACK
Security
• In theory, spread spectrum radio signals are inherently
difficult to decipher without knowing the exact
hopping sequences or direct sequence codes used
• The IEEE 802.11 standard specifies optional security
called "Wired Equivalent Privacy" whose goal is that a
wireless LAN offer privacy equivalent to that offered by
a wired LAN. The standard also specifies optional
authentication measures.
Security
• WEP – A protocol to protect link-level data during
wireless transmission between clients and access
points.
• Services:
– Authentication: provides access control to the network by
denying access to client stations that fail to authenticate
properly.
– Confidentiality: intends to prevent information
compromise from casual eavesdropping
– Integrity: prevents messages from being modified while in
transit between the wireless client and the access point.