Datacommchapter 5 Part3a

PART 3: 5.4: WIRELESS LAN

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5.4 WIRELESS LAN(802.11)

Wireless LANs offer the following productivity, convenience, and cost advantages over traditional wired networks:
Mobility: Wireless LAN systems can provide LAN users with access to real-time information anywhere in their organization.
Installation Speed and Simplicity: Installing a wireless LAN system can be fast and easy and can eliminate the need to pull cable through walls and ceilings.
Installation Flexibility: Wireless technology allows the network to go where wire cannot go.
Scalability: Wireless LAN systems can be configured in a variety of topologies to meet the needs of specific applications and installations.

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Components of 802.11 LAN Four major physical components: Distribution system

Wireless medium

Access point







Station

Stations: Networks are built to transfer data between stations. Stations are computing devices with wireless network interfaces (typically battery-operated laptop or handheld computers) Access points (base stations): Frames on an 802.11network must be converted to another type of frame for delivery to the rest of the world. Devices called access points perform the wireless-to-wired bridging function . 3







wireless medium: To move frames from station to station, the standard uses a wireless medium (Initially, two radio frequency (RF) physical layers and one infrared physical layer were standardized). Distribution: When several access points are connected to form a large coverage area, they must communicate with each other to track the movements of mobile stations. The distribution system is the logical component of 802.11 used to forward frames to their destination. In most commercial products, the distribution system is the backbone network (mostly Ethernet) used to relay frames between access points.

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Access Method








The 802.11 MAC layer provides functionality to allow reliable data delivery for the upper layers over the wireless media. The data delivery itself is based on an asynchronous, best-effort, connectionless delivery of MAC layer data. There is no guarantee that the frames will be delivered successfully. The 802.11 MAC protocol is quite different from that of Ethernet due to the inherent complexity of the wireless environment compared to that of a wired system.

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No CSMA/CD

Wireless LAN cannot implement CSMA/CD for three reasons:
Collision detection implies that the station must be able to send data and receive collision signals at the same time. This implies costly stations and increased bandwidth requirements.
The distance between stations in wireless LANs can be great. Signal fading could prevent a station at one end from hearing a collision on the other end.
Collision may not be detected because of hidden terminal problem. This does not happen in wired LAN because all stations are connected by wire and any collision is heard by all stations.

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Hidden stations problem:
Since not all stations are within radio range of each other,

transmissions going on in one part of a cell may not be received elsewhere in the same cell.
Eg., station C is transmitting to station B. if A senses the channel, it will not hear and falsely conclude that it may now start transmitting to B

A

B

C

i. A & C want to transmit to B ii. C starts transmitting, A cannot hear (out of range ) iii. A then transmits, intefere with C

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Exposed stations problem:
This is an inverse problem.
Eg., station C want to send to D so it listens to the channel. When it

hears a transmission, it falsely concludes that it may not send to C even though it does not interfere with the transmission from B to A

A

B

C

D

i. B transmits to A, while C wants to transmit to D ii. C senses that the channel is busy, and doesn 't transmit

Given these difficulties with detecting collision at a wireless receiver, an access protocol that aimed to avoid collision was developed rather than detect and recover from collision. 8

CSMA/CA












This procedure avoids collisions instead of detecting them. The sender uses one of the persistence strategies. After it finds the line idle, the station waits an IFG, interframe gap (DIFS – distributed interframe space) It then waits another random amount of time. After that, it send the frame and sets a timer. The sender waits for an acknowledgement from the receiver. If the receiver has correctly and completely received a frame, it waits for another IFG (short interframe space, SIFS), and then sends an explicit acknowledgement to the sender. If the sender receives the acknowledgement before the timer expires, the transmission is successful. If the station does not receive an acknowledgement, it knows that an error has occurred (the frame or acknowledgement is lost ). 9

It increments the value of the backoff parameter, waits for a backoff amount of time, and re-senses the medium Source

Destination

Others

DIFS

SIFS

NAV: defer access

Data

Note: The data frame contains a duration field which indicate the Network allocation vector,NAV which indicate the time which others should defer

ACK

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RTS

A

B

C

A requests to send CTS

A

CTS

B

C

B notifies A that it is OK to send

The 802.11 protocol can also use a short Request to Send(RTS) and Clear to Send (CTS) control frame to reserve access to the channel.

Data

A

A sends data frame

B

C

C remains silent

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Before sending a frame, the source station senses the medium by using a persistence strategy with backoff until the channel is idle. If the medium is idle, the station waits for a DIFS before it sends RTS. After receiving the RTS, the destination station waits for a SIFS before sending CTS (indicating that it is ready to receive data). After receiving the CTS, the source station waits for another SIFS before sending the data frame. If the data frame is completely and correctly received, the destination station sends back an acknowledgement after another SIFS. Acknowledgement is needed as the source station does not have any means to check for successful arrival of its data at the destination (In CSMA/CD, the lack of collision implies that the data have arrived safely). 13

Source

Destination

Others

DIFS

RTS SIFS

NAV: defer access

CTS SIFS

Data

SIFS

ACK

Note: NAV is set in both RTS and CTS

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Types of Networks






The basic building block of an 802.11 network is the basic service set (BSS), which is simply a group of stations that communicate with each other Communications take place within an area known as the basic service area BSSs come in two flavors: i. Ad hoc network ii. Infrastructure network

Station

Station

Station

Station Access point

Station

Station

Ad hoc network (BSS without AP)

Station

Station

Infrastructure network (BSS with an AP)

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Ad Hoc Network: Stations in an independent BSS communicate directly with each other and thus must be within direct communication range. The smallest possible 802.11 network is a network with two stations. Typically, ad hoc network are composed of a small number of stations set up for a specific purpose and for a short period of time (short-lived network to support a single meeting in a conference room). They are sometimes referred to as ad hoc BSSs or independent BSS Infrastructure BSS: They are distinguished by the use of access points. Access points are used for all communications in infrastructure networks, including communication between mobile nodes in the same service area. If one mobile station in an infrastructure BSS needs to communicate with a second mobile station, the communication must take two hops. Stations must associate with an access point to obtain network services (logically equivalent to plugging in network cable in Ethernet ) 16




Extended Service Set (ESS): 802.11 allows wireless networks of arbitrarily large size to be created by linking BSSs into an extended service set. An ESS is created by chaining BSSs together with a backbone network. All the access points in an ESS are given the same service set identifier (SSID), which serves as a network “name” for the users Distribution system

Server or gateway

Access point

Access point

Access point

BSS

BSS

BSS

Station

Station

Station

Station

Station

Station

Station

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MAC Frame Format





Frame formats are specified for wireless LAN systems by 802.11. Each frame consists of a MAC header, a frame body and a frame check sequence (FCS). The MAC header consists of seven fields and is 30 bytes long. The fields are frame control, duration, address 1, address 2, address 3, sequence control, and address 4.

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The frame control field is 2 bytes long and is comprised of 11 subfields.

o o o o o

The protocol version field carry the version of the 802.11 standard. Type and subtype fields work together hierarchically to determine the function of the frame. The “To DS” and “From DS” bits indicate that frame is going to or coming from the intercell distribution system The MF bit means more fragments will follow. The Retry bit marks a retransmission of a frame sent earlier. 19

The Power management bit is used by the base station to put the receiver into sleep state or take it out of sleep state. o The more bit indicates that the sender has additional frames for the receiver. o The W bit specifies that the frame body has been encrypted using the WEP(Wired Equivalent Privacy) algorithm. o If the O bit is on, the receiver must process the frame strictly in order. Duration/ID field (2 bytes long). o For data frames, it indicates the time (ms) the channel will be allocated for successful transmission o For control frames, it contains an association, or connection identifier o




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The address fields identify the basic service set, the destination address, the source address, and the receiver and transmitter addresses. Each address field is 6 bytes long. The sequence control field is 2 bytes and is split into 2 subfields: Fragment number (4 bits) - tells how many fragments the MSDU is broken into. Sequence number (12 bits) - indicates the sequence number of the MSDU. The frame body (between 0 and 2312 bytes) contains information based on the type and subtype defined in the FC field The FCS (4 bytes) contains CRC-32 error detection sequence.

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