WLAN. Right, Gentlemen. The aim of our presentation is to give you detailed information on the topic WLAN (Wireless Local Area Network).

Transcription

1 WLAN Right, Gentlemen. The aim of our presentation is to give you detailed information on the topic WLAN (Wireless Local Area Network). Our presentation is split into the following key areas. First I will give you a short introduction. Secondly we want to cover the IEEE Standard and its features. For Example we want to discuss the topics Topology, then the layer model of the IEEE Standard, of course the Security of a WLAN and in addition to this we want to show you an example for a WLAN and the adjustments, which are necessary. So let s turn to the first point Features. Features Let s focus on the advantages and disadvantages of a WLAN. First I want to consider the Advantages. The biggest advantage compared to a normal LAN is the comfortable flexibility within the range. The next advantage of a WLAN is that no cabling has to be done. Everytime a system has advantages also disadvantages appear. So let s consider the Disandvantages: An important fact is the low bitrate of this Network. Moreover the lack of security doesn t improve the use. Another disprofit is the limited frequency range because every country has its own frequency range the WLAN is used in. International agreements will be defined in future. The last disadvantage is the limited range. The range of a WLAN is about 30 to 60 meters. The next point is the Aim of an perfect WLAN: A WLAN should be designed transparent because it must be posssible that the Layers of a WLAN can work with the Layers of a normal LAN for example. Another need is that a WLAN should be applicable worldwide. As I said earlier before this depends on the international agreements for example for the frequency. This was a short introduction and I rather suggest moving on to the next point The IEEE Standard. IEEE Standard There are many different standards for example the IEEE for CSMA/CD- based networks, therefore there is also a standard for WLANs so called IEEE The Standard defines the Layer 2, the MAC- Layer, and the Layer 1, the Physical- Layer. For a better understanding I would like you to look at the slide show. Here you can see the main parts of the IEEE standard. file:///d /Refs/_To%20Do/13_01_06/Tag%202/WLANEnglisch(1).html (1 von 9) :43:21

2 The MAC- Layer includes the medium access and the function of encoding data, which is very important when the air-interface is used. Another function of the MAC- Layer is the fragmentation (Fragmentierung) of oversized data packets. The procedure of fragmentation is used for the adaption of the packet size on the current error rate without affecting (beeinflussen) higher layers. The Physical- Layer is split into the PLCP, the Physical Layer Convergence Protocol, and the PMD, the Physical Medium Dependent. The function of the PLCP is the check of the channel condition (Kanalzustand) and it offers an interface which is independent from the medium. The PMD controls the actual coding of the data. Parallel to these layers there are management- layers. The MAC- Management controls the roaming, which is the change of different access points (Zugangspunkten), the synchronization of the system and moreover it controls the actual transfer state. The Physical- Management chooses the transfer channel. There is another management- layer the station management, which operates parallel to the other layers. The function of the Station Management is the coordination of all the layers. I think you have already got an impression about the IEEE Standard so let me recommend turning to the next topic the Topology. Network Topology There a two possible types of a WLAN the Ad-hoc- and the Infrastructure- Network. Ad-Hoc-Network: The mobile users communicate together directly. The Topology of such an Ad-hoc- Network is called Independent Basic Service Set (IBSS). Each Station is an own communication cell. Only within their range they can communicate. Basically Infrared or Bluetooth is used for the Ad-hoc Network because the use of IEEE in this content is more difficult than with the other two systems. Infrastructure-Network: The Infrastructure- Network provides the communication between a remote station (Endgerät) and an Access Point (Basisstation). The Topology of such an Infrastructure- Network with an Access Point is called Basic Service Set (BSS). If the range is too short, a second Access Point can extend the range. Ranges of 100 to 300 meters can be achieved. This Topology is called Extended Service Set (ESS). The Access Point also allows the communication with the wired LAN. If the Access Point is used in this way, it works like a bridge. The result is that the Access Point can filter protocolls which are not used. The next point is giving you a summary of the networks. I want to cover an example of a network, which includes everything I have mentioned before. Therefore, ma I draw your file:///d /Refs/_To%20Do/13_01_06/Tag%202/WLANEnglisch(1).html (2 von 9) :43:21

3 attention to the slideshow. Example: The Example shows a network with two Access Points, three remote stations and a connection to the wired network. A station (STA) is a terminal equipment with access functions to the WLAN and with a radio contact to the Access Point. An Access Point (AP) is a device which is integrated in the WLAN and in the Distribution System (DS). A DS connects the Access Points with each other and offers a connection to the wired LAN via a portal. The APs, the DS and the Portal have a bridge function as I said earlier before. A group of STAs which communicates over the same AP is called Basic Service Set (BSS). With the help of the DS a Extended Service Set (ESS) can be designed. Within this ESS dates can be transmitted transparent because the DS is working like a bridge and so the DS forwards the dates to the right receiver. In this content the receiver can be the Portal or one of these Access Points. Before becoming too specific with the Network topology let s turn to the next point The Physical Layer. Physical Layer Originally there are three alternatives in the IEEE Standard. Two radio based (funkbasiert) methods the FHSS and the DSSS and one not radio based, the Infrared method. The data rate is about one to two Mbit/s. Spread Spectrum A big problem concerning the radio transmission is, that frequency-independent Fading or narrowband disturbing signals can delete a certain narrowband part of a signal. Therefore a method is needed to avoid this deletion and this deletion is called spread spectrum (Bandspreizverfahren). The hidden agenda (Hintergedanke) is that a narrowband signal gets spread to a broadband signal and so other narrowband disturbing signal can t delete the signal. In this spread spectrum method the bandwidth R of a signla is getting spread to a bandwidth G. The bandwidth G can be up to times wider than the bandwidth R. There are two spread spectrum methods. So let s focus on the first one the FHSS. FHSS Frequency Hopping Spread Spectrum: The process of the FHSS is the continuous frequency hopping. The change of the file:///d /Refs/_To%20Do/13_01_06/Tag%202/WLANEnglisch(1).html (3 von 9) :43:21

4 frequency is done in a certain sequence. This sequence assures (sichern) that each transmitter has another frequency. This alternative is used in the older gadgets. The minimum of frequency hops is 2,5 hops per second. DSSS Direct Sequence Spread Spectrum: The other spread spectrum method is the DSSS. This alternative is used in the latest devices. The spreading is achieved with the logical connection of the signal with a casual numeric order (Zufallszahlenfolge). Moreover the code of the casual numeric order must have some special features. This code is the barker code. Now I will explain the procedure of the DSSS. 1.) In the first part you can see the normal signal which should be transmitted. 2.) The Signal gets spread by the use of the logical connection with this barker code. 3.) The next step is the transmission of the spread signal. During the transmission narrowband and broadband faults disturb the signal. 4.) The next picture shows the reversion of the spreading. With the help of the identic casual numeric order the spread signal becomes narrowband again and the faults become broadband. 5.) The last step is the filtering of the original narrowband signal. The next point is the MAC- Layer. My colleague Martin Schwarzbauer will continue with this very interesting topic. Please stay tuned. Good afternoon gentlemen. As Mister Eder has mentioned I am going to give you information about the MAC-Layer. So let s start: MAC-Layer The MAC-Layer provides us with two services and 3 media access methods, which can be used together. The services are: asynchronous data service: This service is standard. It allows us to send data packets on best-effort base. This means that you can compare it with the fixed network (Festnetz) and it supports multicast and broadcast. time limited data service: This service can be used optional and it gives us a time guarantee for de transmission of data. As an obligatory version of media access the CSMA/CA was defined. The media access is named DFWMAC (Distributed Foundation Wireless MAC) The three types of media access are: DFWMAC DCF with CSMA/CA: file:///d /Refs/_To%20Do/13_01_06/Tag%202/WLANEnglisch(1).html (4 von 9) :43:21

5 This type of media access is standard. To avoid collisions this version uses parallel to CDMS/CD a back-off mechanism and acknowledge-bits. I ll give you an exact description of this type later in my presentation. DFWMAC DCF with RTS/CTS:(request to send and clear to send) This type of media access avoids problems with hidden end devices. DFWMAC PCF: (Point Coordination Function) The access point asks every station if they want to send data, this is called polling. With this method can t appear collisions and the time limits could be kept. Only the access point differ about the order of the scan und he is the only which can send whenever he wants. Now let s focus on my next point the Prioritisation: How and when the communications are set to the medium says the priority. There is no guarantee that the prioritisation was accomplished. (soft priorities). There are 3 different waiting times and they say when you can access the free medium. These three types are: SIFS: Short Inter Frame Spacing This is the shortest waiting time and has consequently the highest priority. High priorities have acknowledge Bits and answers to a send request. PIFS: Point Coordination Function Inter Frame Spacing This is the middle priority. Middle priorities have questions to an access point in the PCFaccess mode. DIFS: Distributed Coordination Function Inter Frame Spacing DIFS have the longest waiting times and the lowest priority. Low priority data is an access request in the asynchronous data service. Now I want to tell you in detail how DFWMAC-DCF with CSMA/CA works: CSMA/CA can in contrast to CSMA/CD avoid collisions but it can t find or correct them. How does it work?: The station which wants to send data listens to the medium. When the medium is free for the period of a DIFS the station can send his data. But when the medium is busy the station has to wait a DIFS period and after this period the station delays the time with a random time and start the Back-off Timer. This random time is called back-off time. When another station accesses the medium during the back-off time the back-off timer is stopped. The back-off timer starts again when the medium is free again. When the back-off timer executes the station can put his data to the medium. Originaldokument enthält an dieser Stelle eine Grafik! Original document contains a graphic at this position! The receiver of the packets has to approve the reception with an acknowledge bit. When the file:///d /Refs/_To%20Do/13_01_06/Tag%202/WLANEnglisch(1).html (5 von 9) :43:21

6 sending station doesn t receive this acknowledge it start to transmit the packet again and again till he got the acknowledge or the maximum of the repeat-counter was reached. Originaldokument enthält an dieser Stelle eine Grafik! Original document contains a graphic at this position! Now let s focus on the frame format: In this slide you can see the general frame body of a MAC frame. The frame control contains the version, type detection and so on. The duration ID contains the sending time for this packet. The sequence control holds a consecutive number to detect multiple transmitted packets. After the data array follows a 32 bit long checksum in the CRC method. MAC packets can be sent between mobile stations and inside the DS (Distribution System) so you need up to four addresses in one packet. Address one is read from every station in the reception area. With this address the station can decide if the packet is for it or another station. The second address describes the physical transmitter, this is important to send an acknowledge back to the transmitter. Address three and four are for the logical assignment of packets. Now let me demonstrate you 4 examples. You can see them here on this slide: Example 1: the packet is sent between two mobile stations without DS. Transmitter and receiver addresses and the BSS identifier are required. (Ad-Hoc network) Example 2: a packet is sent from a transmitter over the DS to the receiver. The BSSID stands on the second place and the logic receiver address on the third place. Example 3: A mobile station sends the packet to the DS and the DS sends this packet to the receiver. On the first place stands the physical receiver address and the logical receiver address on the third place Example 4: Is packet is sent inside two DS. The first and the second address represent receiver and transmitter inside the DS an address 3 and 4 are the logical receiver and transmitter addresses. file:///d /Refs/_To%20Do/13_01_06/Tag%202/WLANEnglisch(1).html (6 von 9) :43:21

7 MAC-Management The following function groups have been standardised: synchronisation: The job is to find networks, for standing there or to synchronize the internal clocks. The access point sends for this reason the so called beacon signal to every mobile station. This signal contains further management information. power management: The power management contains function which can influence the readiness of the receiver or the transmission power. When a mobile station is used in the battery mode it is important keep the stations only ready to receive when data would be sent. For this reason exists defined sleep-models when a station has to receive data. association / reassociation: After the connection of a mobile station with the access point a association is build. When you use roaming ( later in my presentation I will explain it to you what this means) the association has to be rebuild and this is called reassociation. management infortmation base (MIB) Every parameter about the actual state of the mobile station is managed in the MIB. As I have mentioned before I will tell you what Roaming is: Roaming: When a mobile station recognizes that the transmission quality is getting bad it tries to find another access point with a better quality. This is called roaming. In addition the mobile station scans the neighbourhood for a compatible access point. The access point could be identified by the beacon signal. Every mobile station can send an active proof und wait for answers, because every access point in the neighbourhood is able to answer. Now let s turn to one of the most important points the SECURITY: Security: Wired Equivalent Privacy, shortly named WEP, is part of the wireless LAN standard IEEE and it is a safety standard. It contains FHSS, DSSS and RC4 algorithm for encryption. The encryption is only 40 bit or 104 bit long so that the code is easy to decode, because the RC4 and the CRC are mathematically seen as unsafe. For this reason they developed WEPplus, WPA and Fast Packet Keying. The follower of the WEP is the new safety standard i. He has a higher security level und is now not to decrypt. file:///d /Refs/_To%20Do/13_01_06/Tag%202/WLANEnglisch(1).html (7 von 9) :43:21

8 Another way to encrypt data is on the IP-Layer. For example by usage the IPsec or a VPN tunnel. Shortly I want to tell you a few things about WPA: WPA stands for Wi-Fi Protected Access an is an encryption method. WPA gives further safety because it uses dynamic keys which are based on a Temporal Key Integrity Protocol, called TKIP. WPA uses the key only for initialisation und after the initialisation a session key is used. There are two methods of the key management: Managed Key: The access identifier is managed on a central server Pre-Shared Keys When you use Pre-Shared Keys every user of the network uses the same password. When this password is to short or to easy to guess this is a point where WPA could be hacked. Finally I want you to tell shortly a few thing about SSID: SSID stands for service set identifier. SSID is nothing other than the network name of a wireless LAN based on the standard IEEE Every WLAN holds this SSID and it is free configurable and it is used to identify clearly the WLAN. The SSID is up to 32 Bits long and ii is set in the access point. The 32 Bits are added unencrypted in front of every packet. On every mobile station you can select SSID ANY and this means that every reachable access point should send an SSID broadcast so that the mobile station can choose which access should be used. Thank you Martin. My next point is the Extension of IEEE Standard. Extension of IEEE Standard As a result of the success of the IEEE Standard, new Standards were realised. Basically the aim was the improvement of the bitrate. I would like to give you a short overview of the extensions (Erweiterungen). On the slide show you can see a really short overview. You see the improvement of the bitrate. The bitrate of the IEEE Standard is about 2 Mbit/s. The first improvement realised a bitrate of 11 Mbit/s. And so on the bitrate climbed up to 54 Mbit/s at the IEEE a and g Standard. file:///d /Refs/_To%20Do/13_01_06/Tag%202/WLANEnglisch(1).html (8 von 9) :43:21

9 Of course, there are many more extensions but covering of all the extension would blow up (sprengen) the presention. You will be able to read it in the summary which you get. Before coming to the last point of our presentation I will give you a short Confrontation between WLAN and other radio technologies. The Confrontation On this slide you can see different LAN- radio access techniques. The confrontation contains of course the WLAN, the Hiperlan, Bluetooth and many more. Particularly I want to cover the differences of WLAN and Bluetooth. In the first place it must be said, that WLAN has a higher bitrate as opposed to that of Bluetooth. Also the WLAN has a higher range. The only advantage of Bluetooth is the simpleness. Ad- hoc networks can be achieved easier than with WLAN. In my opinion WLAN is the better solution realising an ad-hoc network. In this content the 3G- mobile radio must be mentioned. With regard to WLAN the 3Gmobile radio system doesn t only contain the telephone part, with the help of this system also a communication between two or more computers can be achieved. But 3Gtechnology has a lower bitrate than the WLAN technology. So, to conclude, WLAN is really the best solution realising a wireless connection between two terminal equipments. An exact listing of the features of the WLAN and the UMTS is printed on the summary you will get. file:///d /Refs/_To%20Do/13_01_06/Tag%202/WLANEnglisch(1).html (9 von 9) :43:21