Ijctt v8p127

International Journal of Computer Trends and Technology (IJCTT) – volume 8 number 3– Feb 2014

WLan Architecture Ishu Mittal1, Mr. Anshul Anand2

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(Computer Science, Shri Baba Mastnath Engg. College/ Maharashi Dayanand University,Rtk, India (Computer Science, Shri Baba Mastnath Engg. College/ Maharashi Dayanand University,Rtk, India)

ABSTRACT : It is the review paper of Architecture of Wireless local area networks. In this paper we are discussing the architecture of wlan. A wireless LAN (WLAN) is a local area network based on wireless technology. Most modern local area networks now employ some wireless network infrastructure because it allows existing networks to be extended without the expense of additional cabling, and allows users of portable network devices to maintain connectivity with the network as they move around. Wireless networks can be set up quickly and can be configured in either ad hoc or infrastructure mode. In ad hoc mode, two or more portable computers can communicate with each other in the wireless equivalent of a peer-topeer network. There is no connection to a wired network and a wireless access point is not required. In this paper I also explaining the advantages, architecture, layers, frame format of wireless Local Area Networks.

Keywords

Introduction, Advantages, Architecture, Access Points,BSS, ESS, Frame Format, Layers,.

INTRODUCTION A wireless local area network (WLAN) links two or more devices using some wireless distribution method (typically spread-spectrum or OFDM radio), and usually providing a connection through an access point to the wider Internet. This gives users the ability to move around within a local coverage area and still be connected to the network.

Most modern WLANs are based on IEEE 802.11 standards,

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ADVANTAGES

It is easier to add or move workstations.

It is easier to provide connectivity in areas where it is difficult to lay cable.

Installation is fast and easy, and it can eliminate the need to pull cable through walls and ceilings.

Access to the network can be from anywhere within range of an access point.

Portable or semi permanent buildings can be connected using a WLAN.

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Although the initial investment required for WLAN hardware can be similar to the cost of wired LAN hardware, installation expenses can be significantly lower.

When a facility is located on more than one site (such as on two sides of a road), a directional antenna can be used to avoid digging trenches under roads to connect the sites.

In historic buildings where traditional cabling would compromise the façade, a WLAN can avoid the need to drill holes in walls.

Long-term cost benefits can be found in dynamic environments requiring frequent moves and changes

WIRELESS NETWORK

There are two major approaches today for deploying WLAN networks in the enterprise.

Both approaches require Wireless 802.11 based Access Points (APs) and some method for managing these network elements. However, the two approaches have some basic philosophical differences which can have a major impact on deployment costs, security and manageability.

The first architecture to be presented is the socalled “Centralized” WLAN Architecture.

The Centralized Architecture requires one or more servers or special purpose switches to be deployed in conjunction with Wireless Access Points. In the Centralized approach, all wireless traffic is sent through the WLAN switch.

Another approach is the “Distributed” Access Point WLAN Architecture. The Distributed Architecture adheres closely to the principles of the IEEE 802.11 standard. In the Distributed approach, APs have built-in WLAN security, layer 2 bridging, and access control features. Depending on the number of APs required, Centralized Management may be required. Distributed AP vendors may provide Centralized Management tools or the APs may be managed by the existing Network Management Infrastructure.

The AP is connected directly to the trusted wired infrastructure and “extends” the wired network by providing wireless connections to wireless client devices.

One of the advantages of the Distributed or Wireless Extension approach is that the wireless traffic load is literally distributed across the APs and does not depend on a centralized element to

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International Journal of Computer Trends and Technology(IJCTT) – volume X Issue Y–Month 2014 process all of the wireless traffic. In the Centralized approach, loss of the WLAN Switch results in loss of the wireless network, whereas with the Distributed architecture, there is no single point of failure. From a performance point of view, the Distributed Architecture is superior from a performance/efficiency point of view.

This is because the Centralized approach requires all wireless packets to be processed by the centralized WLAN Switch whereas in the Distributed Architecture, the packets are handled by the APs and only management traffic needs to go to and from a central point. Centralized Architectures tend to be difficult to scale because each WLAN switch can only handle a limited number of APs. 4.

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WLAN ARCHITECTURES

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Station (STA) terminal with access mechanisms to the wireless medium and radio contact to the access point.

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Access Point (or Base Station) station integrated into the wireless LAN and the distribution system.

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propagation. When all of the stations int the BSS are mobile stations and there is no connection to a wired network, the BSS is called independent BSS (IBSS). IBSS is typically short-lived network, with a small number of stations, that is created for a particular purpose. When a BSS includes an access point (AP), the BSS is called infrastructure BSS. When there is a AP, If one mobile station in the BSS must communicate with another mobile station, the communication is sent first to the AP and then from the AP to the other mobile station. This consume twice the bandwidth that the same communication.

While this appears to be a significant cost, the benefits provided by the AP far outweigh this cost. One of them is, AP buffers the traffic of mobile while that station is operating in a very low power state.

Extended Service Set (ESS):- A ESS is a set of infrastructure BSSs, where the APs communicate among themselves to forward traffic from one BSS to another and to facilitate the movement of mobile stations from one BSS to another. The APs perform this communication via an abstract medium called the distribution system (DS). To network equipment outside of the ESS, the ESS and all of its mobile stations appears to be a single MAC-layer network where all stations are physically stationary. Thus, the ESS hides the mobility of the mobile stations from everything outside the ESS. Distribution System:- the distribution system (DS) is the mechanism by which one AP communicates withanother to exchange frames for stations in their BSSs, forward frames to follow mobile stations from one BSS to another, and exchange frames with wired network.

Infrastructure wireless LAN is a term often referred to wireless LANs that deploy AP, with the infrastructure being the APs along with wired Ethernet infrastructure that connects APs and router, hub or switch

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Basic Service Set A BSS is a set of stations that communicate with one another. A BSS does not generally refer to a particular area, due to the uncertainties of electromagnetic

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International Journal of Computer Trends and Technology(IJCTT) – volume X Issue Y–Month 2014

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transforms them into RF signals for the wireless media by using carrier modulation and FHSS techniques.

The PLCP preamble is used to acquire the incoming signal and synchronize the receiver‘s demodulator.

LAYERS

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1. Physical layer The PHY is the interface between the MAC and wireless media, which transmits and receives data frames over a shared wireless media. The PHY provides three levels of functionality: First, the PHY layer provides a frame exchange between the MAC and PHY under the control of the physical layer convergence procedure(PLCP) sublayer.

Secondly, the PHY uses signal carrier and spread spectrum modulation to transmit dataframes over the media under the control of the physical medium dependent (PMD) sublayer. Thirdly, the PHY provides a carrier sense indication back to the MAC to verify activity on the media.

1.1 DSSS The DSSS uses the 2.4 GHz frequency band as the RF transmission media. Data transmission over the media is controlled by the DSSS PMD sublayer as directed by the DSSS PLCP sublayer.

The DSSS PMD takes the binary bits of information from the PLCP protocol data unit (PPDU) and transforms them into RF signals for the wireless media by using carrier modulation and DSSS techniques. .

1.1.1 DSSS PLCP Sublayer The PLCP protocol data unit (PPDU) is unique to the DSSS PHY layer. The PPDU frame consists of a PLCP preamble, PLCP header, and MAC protocol data unit (MPDU). The PLCP preamble andPLCP header are always transmitted at 1 Mbps, and the MPDU can be sent at 1 Mbps or 2 Mbps.

1.2The Frequency Hopping Spread Spectrum (FHSS) Data transmission over the media is controlled by the FHSS PMD sublayer as directed by the FHSS PLCP sublayer. The FHSS PMD takes the binary bits of information from the whitened PSDU and

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1.3 IR PHY Layer The IR PHY is one of the three PHY layers supported in the standard. The IR PHY differs from DSSS and FHSS because IR uses nearvisible light as the transmission media. IR communication relies on light energy,which is reflected off objects or by line-of-sight. The IR PHY operation is restricted to indoor environments and cannot pass through walls, such as DSSS and FHSS radio signals. Data transmission over the media is controlled by the IR PMD sublayer as directed by the IR PLCP sublayer.

The PLCP preamble, PLCP header, and PSDU make up the PPDU. The PLCP preamble and PLCP header are unique to the IR PHY. The PLCP preamble is used to acquire the incoming signal and synchronize the receiver prior to the arrival of the PSDU. The PLCP header contains information about PSDU from the sending IR PHY. The PLCP preamble and PLCP header are always transmitted at 1Mbps and the PSDU can be sent at 1 Mbps or 2 Mbps.

2. Mac Layers 2.1 Mac Layer1 Traffic services 1.Asynchronous Data Service (mandatory) 2.exchange of data packets based on “best-effort” 3.support of broadcast and multicast 4.Time-Bounded Service (optional) 5.implemented using PCF (Point Coordination Function) Access methods 6.DFWMAC-DCF CSMA/CA (mandatory) 7.collision avoidance via randomized „back-off“ mechanism 8.minimum distance between consecutive packets 9.ACK packet for acknowledgements (not for broadcasts) 10.DFWMAC-DCF w/ RTS/CTS (optional) 11.Distributed Foundation Wireless MAC 12.avoids hidden terminal problem 13.DFWMAC- PCF (optional) 14.access point polls terminals according to a list 2.2 Mac Layer2 Priorities 1.defined through different inter frame spaces 2 .no guaranteed, hard priorities 3. SIFS (Short Inter Frame Spacing) 4.highest priority, for ACK, CTS, polling response PIFS (PCF IFS)

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International Journal of Computer Trends and Technology(IJCTT) – volume X Issue Y–Month 2014 5.medium priority, for time-bounded service using PCF 6.DIFS (DCF, Distributed Coordination Function IFS) 7.lowest priority, for asynchronous data service

3. The 802.11 MAC protocol To deal with problems of hidden station and exposed station problems 802.11 supports two modes of operations.

Address 3 field contains the MAC address of the router to which AP is connected Sequence number helps to distinguishing between a newly transmitted frame and there transmission of a previous frame. The duration value field is used when transmitting station reserves the channel for the time to transmit data frame and ACK. Frame control fields type and subtype are used to distinguish the association, RTS, CTS, ACK, and data frames. The to and from fields are used to define the meaning of the address fields which meanings change depending whether it is an ad hoc or infrastructure network.

3.1 DCF(Distributed coordination Function): does not use any kind of central control.

VI. 3.2 PCF(Point Coordination Function): uses the base station to control all activity in its cell.

V. THE 802.11 FRAME

CONCLUSIONS

In this review paper we studied the architecture of WLAN how it works .how two stations communicate with one another. WLAN offers user mobility, Users can access files, network resources, and the Internet without having to physically connect to the network with wires. Users can quickly install a small WLAN for temporary needs such as a conference, trade show, or standards meeting. WLANs are now becoming a viable alternative to traditional wired solutions. For example, hospitals, universities, airports, hotels, and retail shops are already using wireless technologies to conduct their daily business operations.

REFERENCES. [1] Yalemzewd Negash, Hailu Ayele, “Power Control in

Fig 3

spontaneously

The first two bytes of the IEEE 802.11 header are taken up by the frame control field, which consists of a number of sub-fields that contain information about the frame, such as the protocol version, frame type, whether power management is active, and so on. The address fields carry the MAC address of the source and destination devices, as well as that of one or more access points. The sequence control field is used for ordering message fragments, and for the identification of any duplicated frames. The variable-length payload can carry user data or control and management information, depending on the frame type. The Frame Check Sequence occupies the last 4 bytes of the frame, and is used for error detection purposes.

Address 1 field holds the MAC address of the station that is suppose to receive the frame.

Address 2 field holds the MAC address of station that sends data.

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Inter national

deployedWireless Conference

LANs”,

on

A dvance d

Communication Technology, ICACT, 2011 .

[2] Ashish Raniwala, Tzi-cker Chiueh “Architecture and Algorithms for an IEEE 802.11-Based Multi-Channel Wireless Mesh Network” [3]Mustafa Ergenergen@eecs.berkeley.edu “IEEE 802.11 Tutorial” June 2002 [4] http:// wikpedia.org//wiki Wireless networks [5] Jochen Schiller ”Wirless LAN.pdf” [6] Shou-Chih Lo, Guanling Lee, and Wen-Tsuen Chen, Fellow, IEEE “An Efficient Multipolling Mechanism for IEEE 802.11 Wireless LANs” [7] Tomasz Kurzawa “IEEE _802_11.PDF”,Frame format [8] http://wikipedia.org//wiki frame format [9] Tom Karygiannis and Les Owens “Wireless Network Security”

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