802.11 Protocol Details John Bellardo UCSD

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 802.11 radio PHY up to 11 mbits

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

Network Components ESS BSS BSS AP DS AP IBSS

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

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)

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.

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

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

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

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

General Frame Format Frame Control Sequence Control Octets: 2 2 6 6 6 2 6 0-2312 4 Frame Control Duration/ ID Sequence Control Frame Body Address 1 Address 2 Address 3 Address 4 FCS MAC Header Frame Control Bits: 2 2 4 1 1 1 1 1 1 1 1 Protocol Version To DS From DS More Frag Pwr Mgt More Data Type Subtype Retry WEP Order Sequence Control Bits: 4 12 Fragment Number Sequence Number

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): 802.11 DSSS: 10, 30, 50 (20 slot)

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

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

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

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

Power Management Requires timer synchronization Infrastructure Mode IBSS

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

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

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