Introduction to IEEE 802.11 Wireless LANs Jon-Olov Vatn
KTH/IMIT, TSLab
Introduction to IEEE 802.11 Wireless LANs Jon-Olov Vatn
KTH/IMIT, TSLab
Outline of presentation
A little about other IEEE wireless technologies
Family of IEEE 802.11 PHY layers
Building networks, Infrastructure or Ad-hoc?
Frame formats, addressing
Medium Access Control
Security
Handover between Access Points (Aps)
More on security if there is time …
Some IEEE Working groups for wireless technologies
802.11 Wireless LAN (WLAN)
The main topic of today
802.15 Wireless Personal Area Network (WPAN)
Bluetooth and other WPANs (higher and lower rates)
802.16 Broadband Wireless Access
First/Last mile (DSL competitor)
Fixed Wireless Access (FWA)
2-66 GHz (licenced), ~128 kbit/s - 20Mbit/s (?)
802.20 Mobile Broadband Wireless Access (MBWA)
Newly formed WG, no standards produced yet
More info on http://grouper.ieee.org/groups/802/dots.html
Family of 802.11 technologies
802.11 (the original)
DSSS & FHSS, 1 & 2 Mbit/s (2.4 GHz)
Infrared
802.11a
54 Mbit/s OFDM at 5-6 GHz
802.11b
5.5 & 11 (& 22) Mbit/s DSSS at 2.4 GHz
802.11g
54 Mbit/s OFDM (and DSSS) at 2.4 GHz
There are a lot of other 802.11 Standards/Task Groups, e.g., ’TGf (Inter-access point protocol), TGi (Enhanced Security) etc.
Infrastructure or Ad-hoc?
IEEE 802.11 specifies two ways to
build networks, infrastructure and ad-hoc.
Infrastructure
Access Points (APs)
Mobile Stations (Mobile STAs)
Ad-hoc
Spontanous networks, no backbone infrastructure, (no APs).
Infrastructure Mode
AP AP STA BSS BSS ESS More (WDS) The 802.11 spec contains a general model with terms such as Access
Points (APs) and Stations (STAs), Basic Service Set (BSS), Extended
Service Set (ESS), Distribution System (DS), Portals etc.
We consider the case where bridging APs are connected with Ethernet.
Network ID (SSID)
Configure your APs and STAs with
appropriate SSID! How do we know that two APs belong to the same ESS?
If a STA is within range of multiple APs on different networks/organizations, how does it know which one to associate with?
Interference between channels
1 2 3 4 5 6 7 8 9 10 11 12 13 Channel center frequencies 5 MHz apart
Signal bandwidth about 20 MHz
(No code division multiplexing)
Number of channels differ
802.11 MAC frame format
Frame Control Duration/ ID Address1 Address4 Sequence control Frame Body FCS Address2 Address3 2 2 2 6 6 6 6 0-2312 4 This is the general frame format. All fields
are not present in all types of frames.
802.11 Addresses
Frame Control Duration/ ID Address1 Address4 Sequence control Frame Body FCS Address2 Address3 Address1: Immediate destination, used by STAs/APs for receive decisions.
Address2: Immediate source.
Address3 and Address4 may be used to hold the address of the final destination and original receiver.
802.11 Addresses (example)
Frame Control Duration/ ID Address1 Sequence control Frame Body FCS Address2 Address3 AP STA1 STA2 1 2 STA1-MAC BSSID STA2-MAC 2 STA2-MAC STA1-MAC BSSID 1 Address3 Address2 Address1 More (adr) immediate destination immediate source final dst/src
Medium Access Control
IEEE 802.11 is sometimes refered to as a
wireless Ethernet. The standard specifies
two medium access control schemes:
DCF, Distributed Coordination Function Similar to an Ethernet
PCF, Point Coordination Function (optional, not used, 802.11 TgE) AP controls medium access by polling.
DCF Medium Access Control
Before a STA/AP transmits, it uses carrier sensing
to verify that the medium is idle for a certain time,
known as the DCF Inter-Frame Space (DIFS).
If the medium goes from busy to idle, how can we avoid that multiple waiting senders start to transmit at the same time?
How can a sender be certain that the message was successfully received?
CSMA/CA + ACK
DIFS Contention
Window SIFS Ack DIFS Backoff
Timer=0 Busy medium Next frame Slot time Example Collision avoidance rather than collision detection:
Wait random time (after DIFS) before transmitting.
Exponential backoff in case of unsuccessful transmissions.
Upon successful reception of a frame, the receiver sends an acknowledge (ACK). Sent after short inter-frame space (SIFS).
Why would there still be collisions?
S R S The backoff timer of multiple senders may reach zero at the same time.
Due to signal attentuation, carrier sensing may not always be effective. Hidden terminal problem.
802.11 Security
What is different compared to a wired LAN?
Easier for attacker to connect
Easier to eavesdrop (sniff) the traffic
Authentication
Encryption + Integrity
Authentication
Figure Before associating with an AP, the STA must
authenticate itself to the AP
Open system authentication (default)
Null authentication!!
List of allowed MAC addresses, RADIUS etc
Shared key authentication
AP sends STA a challenge
Wired Equivalent Privacy
What is ”equivalent”?
32 bit CRC for integrity
40 bit shared key to encrypt data+CRC Lucent introduced a 108 bit WEP key
Key distribution one of many problems.
Handover in 802.11
AP AP STA AP AP No-transition
BSS handover (reassociation)
ESS-handover (IP-level)
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