CSE 453: Wireless Networks Lecture 6: Introduction to Wireless LANs Fall 2014
Aug 21, 2015
Outline
Introduction
Overview of 802.11 Networks Wireless LAN Architecture
Wireless LAN standards
Media Access Problem
The IEEE 802.11 MAC standard MAC Sublayer
Access Procedures
DCF and PCF in detail
DCF Performance
Frame Aggregation
Radio Propagation and Interference Receiver Sensitivity, Capture Threshold
Interference Models
2
Overview of 802.11
• IEEE 802
– a series of specification for wireless LAN technologies.
– Focused on the lowest two layers of OSI model.
• MAC component– Set of rules to determine how to access the medium and send
data
• Physical Component– Details of transmission and reception
Wireless LAN Pros and Cons
Pros Flexibility – Place your device anywhere in you house
Easy Setup – No cable, connectors.
Cost – Initially higher, afterwards no cost for new wiring.
Cons Speed – Lower compared to wired. 100Mbps can be achieved
using 802.11n hardware.
Radio Interference – Cordless phones, microwaves, and ham radios interfere the communication
Distance – Signal strength degrades exponentially over distance
Security – Radio signal broadcasts in all directions, anyone within range can tune in.
5
WLAN Architecture
Infrastructure Wireless stations communicate with a Central Coordinator
named as Access Point (AP) The coverage area of an AP is referred to as Basic Service
Set (BSS) Coverage area of other Access Points is called Extended
Service Set (ESS)
Ad-hoc Distributed Coordination (Local)
6
Infrastructure Architecture
APs are used to coordinate BSS
STAs communicate with AP within BSS
AP to AP communication through Wired Network
7
Ad-hoc Architecture
Distributed Coordination by each STA within their vicinities
No AP present, Hop-by-Hop communication
8
Components of 802.11 LANs
• Components of IEEE 802.11 LANs are– Stations are computing devices with
wireless network interfaces. Typically, the stations are battery-powered laptop and handheld computers, but they could also be fixed workstations.
– Access points perform the wireless-to-wired bridging function, although they have a number of other functions.
– Distribution system (DS) is the mechanism by which APs exchange frames with one another and with wired networks.
– Wireless medium is used for signal transmission. Several physical layers are defined and the architecture allows multiple physical layers to be developed to support the 802.11 MAC.
802.11 LANs Building blocks - BSS
• Basic Service Set (BSS)– a set of stations that
communicate with one another.
• Two types of BSS1.Independent BSS (IBSS)
• When all of the stations in the BSS are mobile stations and there is no connection to a wired network
• called ad hoc networks
802.11 LANs Building blocks - BSS
2. Infrastructure BSS
– all of the mobile stations in the BSS communicate with the
– The AP provides both the connection to the wired LAN and the local relay function for the BSS.
802.11 LANs Building blocks - ESS
• Extended Service Set (ESS) – is a set of infrastructure BSSs, where the APs communicate among
themselves • to forward traffic from one BSS to another • to facilitate the roaming of mobile stations between the BSSs.
– The APs perform this communication via the DS. – ESS is the highest-level abstraction supported by 802.11 networks. – Roaming between different ESSs is not supported by IEEE 802.11 and
must be supported by a higher-level protocol
IEEE 802.11 Network Services
• 9 services defined by the 802.11 architecture.
• Divided into two groups
– station services
• implemented in every 802.11 station, including AP stations.
– distribution services
• provided between BSSs – may be implemented in an AP or in another special-purpose
device attached to the distribution system.
IEEE 802.11 Network Services
• Distribution– Service used by stations to exchange MAC frames when the
frame must traverse the DS to get from a station in one BSS to a station in another BSS.
• Integration– Frame delivery to an IEEE 802 LAN outside the wireless network.
• Association– Used to establish a logical connection between a mobile station
and an AP. This connection is necessary in order for the DS to know where and how to deliver data to the mobile station.
• Reassociation– Enables an established association to be transferred from one
AP to another, allowing a mobile station to move from one BSS to another.
IEEE 802.11 Network Services
• Disassociation– Removes the wireless station from the network.
• Authentication– Establishes identity prior to establishing association.
• Deauthentication– Used to terminate authentication, and by extension,
association.
• Privacy– Provides protection against eavesdropping.
• MSDU delivery– Delivers data to the recipient.
Wireless LAN Standards
802.11a Speed 54Mbs Frequency used is 5 gigahertz (GHz)
802.11b Speed 11Mbs Frequency used is 2.4 gigahertz (GHz)
802.11g Speed 54Mbs Frequency used is 2.4 gigahertz (GHz)
16
Wireless LAN Standards
New and Future Technology
Multiple-Input Multiple-Output (MIMO) Allows present technology to achieve greater throughput with the present standards
802.11n going to be the new standard for LAN: Throughput may reach 540 Mbps (PHY)
10 times faster than 802.11a or 802.11g
100 Mbps at MAC Service Access Point (SAP)
A better operating distance
Shall include the MIMO technology
802.11s going to be the new standard for Mesh Networks:
All features up to the 802.11n
With Layer–2 Routing (Router to Router)
With Deterministic Access to the channel (MDA)
17
Wireless Nodes/Stations18
STAs either can transmit or receive at any instant
As the receiving circuit is inactive when transmit, STAs cannot detect collisions from the transmitting end (CSMA/CD is not possible)
Basic Access idea is: Transmit packets if and only if there is no signal in the channel.
Wireless Tx-Rx19
One STA acts asTransmitter, one or moreSTAs receives
Transmitter injects theelectro-magnetic signalinto the wireless channel
The signal follows radiopropagation rules andattenuates over distance
If the signal strength at areceiver is high enough toreceive and decode, thepacket is received by thereceiver.
Wireless Tx-Rx 20
Signal Level at Receiver Radiated (Transmitted) Signal Strength
Channel Quality (Attenuation Factor)
Distance between the Transmitting and Receiving STAs
Receiver Sensitivity The minimum level of the signal strength
for receiving a packet
Receiver cannot receive and extract a packet if the received signal is below this level
Collision (Simple Illustration)21
Receiver R is supposed to receive signal from T as the received signal is above the threshold
Receiver R can/cannot receive from T’ depending on signal strength
When they arrive simultaneously at R, the combined level is also above the sensitivity, however, R cannot receive either