1. Protocol Details
John Bellardo
UCSD

2. Introduction 802.11 specifies: 802.11b specifies:
Physical Connection (Layer 1):
2.4 GHz 1 & 2 mbit DSSS
2.4 GHz 1 & 2 mbit FH
850 nm to 950 nm IR
Common Data Link (Layer 2)
802.11b specifies:
Extends radio PHY up to 11 mbits

3. Outline Network Components Higher Level Services Medium Access
Authentication, Association, Encryption
Medium Access
Frame Format, CSMA/CA, Ad Hoc, Managed
Power Management

4. Network Components
ESS
BSS
BSS AP DS AP
IBSS

5. Outline Network Components Higher Level Services Medium Access
Authentication, Association, Encryption
Medium Access
Frame Format, CSMA/CA, Ad Hoc, Managed
Power Management

6. Authentication
Each node must authenticate with an AP; authentication optional with non-AP nodes
Two authentication algorithms:
Open System
Shared Key
Open System accepts all nodes (null authentication)

7. Shared Key Authentication
Node sends authenticate message to AP
AP generates challenge string and sends it back in response
Node sends challenge back to AP in a WEP encrypted packet using the shared key
AP validates WEP packet, and if valid node is authenticated. Otherwise the node is not authenticated.

8. Associations (Infrastructure Mode)
Every node must associate with an AP
Association happens after authentication
Node sends association request to AP
AP responds with yes/no and an association ID
AP registers the node with the DS
Authenticated nodes may reassociate at any time
APs may disassociate nodes

9. Wired Equivalent Privacy (WEP)
Designed to provide the same level of security as traditional wired networks
Prepends a random 3 octet string (IV) to the shared key to form a seed value
Uses the seed value to generate a random bit string the length of the message + 4 octets using RC4 from RSA Data Security, Inc.
Appends the 4 octet payload CRC to the payload

10. WEP Cont’d XOR’s the bit string with the payload + CRC
Appends the IV in clear text to he payload
Sets a WEP bit in the MAC frame header
Sends the packet with clear text headers and encrypted payload
Only data and authentication frames may be encrypted

11. Outline Network Components Higher Level Services Medium Access
Authentication, Association, Encryption
Medium Access
Frame Format, CSMA/CA, Ad Hoc, Managed
Power Management

12. General Frame Format Frame Control Sequence Control
Octets:
Frame
Control
Duration/ ID
Sequence
Control
Frame
Body
Address 1
Address 2
Address 3
Address 4
FCS
MAC Header
Frame Control
Bits:
Protocol
Version To DS
From DS
More
Frag
Pwr
Mgt
More
Data
Type
Subtype
Retry
WEP
Order
Sequence Control
Bits:
Fragment Number
Sequence Number

13. CSMA/CA Operation
Immediate access when medium
is free >= DIFS
DIFS
Contention Window
PIFS
Busy Medium
DIFS
SIFS
Backoff-Window
Next Frame
Slot Time
Defer Access
Select Slot and Decrement Backoff as long
As medium is idle
Inter-frame Spacing (short, point, distributed) values (usec):
DSSS: 10, 30, 50 (20 slot)

14. RTS/CTS/ACK Operation
DIFS
Source
RTS
Data
SIFS
SIFS
SIFS
Destination
CTS
ACK
DIFS
Other
NAV (RTS)
Contention Window
NAV (CTS)
Defer Access
Backoff After Defer
• Broadcast doesn’t use RTS/CTS/ACK

15. CFP/CP Medium Allocation
• Requires coordination node in BSS
Delay (due to busy medium)
CFP repetition interval
Foreshortened CFP
CF Period
Contention Period
CF Period
Contention Period
DCF
Busy
Medium
DCF B
PCF B
PCF
Variable Length
B = Beacon Frame
NAV

16. CFP Operation Dx = Frames sent by Point Coordinator
Contention-Free Repetition Interval
Contention-Free Period
SIFS
SIFS
SIFS
PIFS
SIFS CP
Beacon
D1+poll
D2+a+p
D3+a+p
D4+p
CF-end
U1+ack
U2+a
U4+a
PIFS
SIFS
SIFS
No response
to CF-Poll
SIFS
Dx = Frames sent by Point Coordinator
Ux = Frames sent by pooled stations
Reset NAV
NAV
CF_Max_Duration

17. Outline Network Components Higher Level Services Medium Access
Authentication, Association, Encryption
Medium Access
Frame Format, CSMA/CA, Ad Hoc, Managed
Power Management

18. Power Management Requires timer synchronization Infrastructure Mode
IBSS

19. Timer Synchronization
Infrastructure
AP has master timer
Periodically broadcast (in beacons) to BSS
IBSS
All nodes responsible for beaconing at an interval set by node that started IBSS
Nodes adopt greatest timer value
Provides 4 usec accuracy

20. Infrastructure Power Management
Node inform AP of power settings; get beacon time intervals
Traffic indication maps (TIM’s) included in AP beacons and also sent at known intervals
Node polls AP when ready to receive, and for each additional packet as needed
Buffered broadcast / multicast packets delivered after delivery TIM (DTIM) in some beacons

21. IBSS Power Management Packets for power save nodes buffered
Ad hoc traffic information message (ATIM) window established after beacon
ATIMs sent in this window for buffered data
ATIMs require an ACK
Data for all ACK’ed ATIMs sent during data period
All undeliverable data gets re-ATIM’ed during next window