Author: Bill Buchanan. Wireless LAN. Unit 2: Wireless Fundamentals

Transcription

1 Wireless LAN Unit 2: Wireless Fundamentals

2 Wireless connections which technology? Areas covered: Basic radio parameters. This area covers the main type of wireless communications. IEEE b issues. This area covers some of the fundamentals of radio waves. Sample Aironet Configurations This shows some simple configuration examples.

3 Wireless Access point Wireless Bridge Wireless Client

4

5 Broadcast span Defined by broadcast domain

6 IEEE b networks

7 IEEE Wireless IEEE a a deals with communications available in the 5GHz frequency, and has a maximum data rate of 54 Mbps. IEEE b b, or Wi-Fi, is the standard that is most commonly used in wireless LAN communications. It has a maximum bandwidth of 11Mbps, at a frequency of 2.4GHz. IEEE g g is a proposed standard that hopes to provide 54Mbps maximum bandwidth over a 2.4GHz connection, the same frequency as the popular b standard. IEEE c c is a group set up to deal with bridging operations when developing access points. IEEE f f is concerned with standardising access point roaming which is involved in making sure that interoperability between access points is guaranteed

8 IEEE b Operating Channels: 11 for N. America, 14 Japan, 13 Europe (ETSI), 2 Spain, 4 France Operating Frequency: GHz (North America), GHz (Japan), GHz (Europe ETSI), GHz (Spain), GHz (France) Data Rate: 1, 2, 5.5 or 11Mbps Media Access Protocol: CSMA/CA, Compliant Range: 11Mbps: 140m (460 feet) 5.5Mbps: 200m (656 feet) 2Mbps: 270m (885 feet) 1Mbps: 400m (1311 feet) RF Technology: Direct Sequence Spread Spectrum Modulation: CCK (11Mps, 5.5Mbps), DQPSK (2Mbps), DBPSK (1Mbps)

9 Available bandwidth Maximum bandwidth (IEEE b) CCK 11Mbps Max bandwidth CCK - Complementary Code Keying DQPSK - differential quadrature phase shift keying BPSK - biphase shift keying 5.5Mbps CCK DQPSK 2Mbps DBPSK 1Mbps 100m 200m 300m Distance 400m

10 Maximum bandwidth (IEEE b) 11Mbps Max bandwidth 5.5Mbps Actual maximum bandwidth 2Mbps 1Mbps 100m 200m 300m 400m

11 Maximum bandwidth (IEEE b) Available throughput 8Mbps Too many errors causes the TCP window to close, and reduce throughput. 6Mbps 2Mbps Linear increase in actual throughput against required throughput More collisions and errors are occurring, thus data frames are being deleted, causing wasted bandwidth. 2Mbps Required data throughput 4Mbps 8Mbps 10Mbps

12 IEEE g networks

13 Available bandwidth Maximum bandwidth (IEEE g) 64-QAM 54Mbps QAM Quadrature Amplitude Modulation CCK - Complementary Code Keying DQPSK - differential quadrature phase shift keying BPSK - biphase shift keying 100m 16-QAM 200m 24Mbps QPSK 300m Distance g Mbps Modulation 6 BPSK 9 BPSK 12 QPSK 1Mbps 18 QPSK 2Mbps DBPSK QAM m 16-QAM QAM QAM

14 Netperf on a 11g link

15 Windows IP Configuration Ethernet adapter Wireless Network Connection 3: Connection-specific DNS Suffix. : IP Address : Subnet Mask : Default Gateway : C:\>netperf -H p 1001 TCP STREAM TEST to Recv Send Send Socket Socket Message Elapsed Size Size Size Time Throughput bytes bytes bytes secs. 10^6bits/sec C:\>netperf -H p 1001 TCP STREAM TEST to Recv Send Send Socket Socket Message Elapsed Size Size Size Time Throughput bytes bytes bytes secs. 10^6bits/sec c:\>netperf -H p 1001 TCP STREAM TEST to Recv Send Send Socket Socket Message Elapsed Size Size Size Time Throughput bytes bytes bytes secs. 10^6bits/sec

16 IEEE g example

17 IEEE g example

18 IEEE n

19 802.11n n Frequency: Max: Range: 2.4 GHz or 5 GHz 540 Mbit/s Same as 11b MIMO (Multiple-in, multiple-out) Sends information on two or more antennas. These signals then reflect off objects, creating multiple paths creating multiple paths. Normally these cause interference and fading, but with MIMO they carry different information, which are recombined on the receiving side.

20 802.11n

21 MIMO

22 IEEE networks

23 CSMA/CA and PCF IEEE can use two mechanisms for shared access: CSMA/CA. CSMA/CA is, like standard Ethernet (IEEE 802.3) a contention-based protocol, but uses collision avoidance rather than collision detection. It would be impossible to use collision detection as a radio wave is always either sending or receiving and can never do both at the same time. The nodes will thus not be able to listen on the channel while they are transmitting. Point Coordination Function (PCF). This is an optional priority-based protocol, which provides contention-free frame transfer for transmission of time-critical data, such as realtime video or audio. With this, the point coordinator (PC) operates in the wireless access point and identifies the devices which are allowed to transmit at any given time. Each PC then, with the contention-free (CF) period, the PC polls each of the enabled PCF to determine if they wish to transmit data frames. No other device is allowed to transmit while a another node is being polled. Thus, PCF will be contention-free and enables devices to transmit data frames synchronously, with defined time delays between data frame transmissions.

24 CSMA/CD Listen for no activity 1 ACK ACK time-out 2 2 Node has gone. Data frame has collided with another Data frame corrupted with noise.

25 IEEE data frame Frame control Duration/ ID Address 1 Address 2 Address 3 Sequence control Address 4 Frame body FCS 2 Bytes Frame control. This contains control information. Duration/ID. This contains information on how long the data frame will last. Address fields. This contains different types of address, such as an individual address of group addresses. The two main types of group addresses are broadcast and multicast. Sequence control. This identifies the sequence number of the data frames, and allows the recipient to check for missing or duplicate data frames. Frame body. This part contains the actual data. The maximum amount is 2312 bytes, but most implementations use up to 1500 bytes. FCS (Frame Check Sequence). This is a strong error detection code.

26 Frame control Duration/ ID Address 1 Address 2 Address 3 Sequence control Address 4 Frame body FCS 2 Bytes

27 IEEE networks

28 dot11radio0 (or d0) con bvi 1 port is used to configure both ports with the same address e0 (or fa0) # config t (config)# int bvi1 (config-if)# ip address (config-if)# exit Antenna connector

29 Fixed network Root # config t (config)# int dot11radio0 (config-if)# station role root (config-if)# station role repeater (config-if)# end Repeater

30 Fixed network Root # config t (config)# ip default-gateway (config)# exit Repeater

31 Channel Frequency Channel. If an ad-hoc network is used, then the nodes which communicate must use the same channel channel channel channel channel channel channel channel channel channel channel channel channel channel channel

32 Channel Frequency Channel. If an ad-hoc network is used, then the nodes which communicate must use the same channel channel channel channel channel channel channel channel channel channel channel channel channel channel channel (config)# int dot11radio0 (config-if)# channel 7 (config-if)# no shutdown (config)# int fa0 (config-if)# no shutdown

33 Fragmentation threshold Fragmentation threshold. This can be used to split large data frames into smaller fragments. The value can range from 64 to 1500 bytes. This is used to improve the efficiency when there is a high amount of traffic on the wireless network, as smaller frames make more efficient usage of the network. Data packets are split into 1500 byte data frames (MTU) The large data frames may allow nodes to hog the airwave

34 Fragmentation threshold Fragmentation threshold. This can be used to split large data frames into smaller fragments. The value can range from 64 to 1500 bytes. This is used to improve the efficiency when there is a high amount of traffic on the wireless network, as smaller frames make more efficient usage of the network. Data frames are fragmented into smaller frames Possibly allows for a smoother and fairer transmission.

35 Network settings Fragmentation threshold. This can be used to split large data frames into smaller fragments. The value can range from 64 to 1500 bytes. This is used to improve the efficiency when there is a high amount of traffic on the wireless network, as smaller frames make more efficient usage of the network. Data frames are fragmented into smaller frames # config t (config)# int dot11radio0 (config-if)# fragment-threshold? < > (config-if)# fragment-threshold 700 (config-if)# end Possibly allows for a smoother and fairer transmission.

36 Infrastructure or ad-hoc Network type. This can either be set to an infrastructure network (which use access points, or wireless hubs) or Ad-hoc, which allows nodes to interconnect without the need for an access point. Ad-hoc Infrastructure

37 Infrastructure or ad-hoc Infrastructure SSID defines the connected nodes SSID defines the connected nodes Ad-hoc

38 Infrastructure or ad-hoc Infrastructure SSID defines the connected nodes SSID defines the connected nodes Ad-hoc # config t (config-if)# dot11 ssid fred (config-ssid)# guest-mode (config-ssid)# exit (config)# int dot11radio0 (config-if)# ssid fred (config-if-ssid)# end

39 Span of network L L L

40 Network settings Authentication algorithm. This sets whether the adapter to use an open system (where other nodes can listen to the communications), or uses encryption (using either a WEP key, or a shared key). # config t (config)# dot11 ssid fred (config-ssid)# authentication? client LEAP client information key-management key management network-eap leap method open open method shared shared method (config-ssid)# authentication open (config-ssid)# exit (config)# exit Authentication is a key issue, and will be covered later in the module. At present the authentication is open, so that any user and device can connect without authenticating itself.

41 Other Factors

42 Network settings Preamble mode. This can either be set to Long (which is the default) or short. A long preamble allows for interoperatively with 1Mbps and 2Mbps DSSS specifications. The shorter allows for faster operations (as the preamble is kept to a minimum) and can be used where the transmission parameters must be maximized, and that there are no interoperatablity problems. Preamble this is sent before the start of the data transmission so that nodes can detect that it is about to transmit.

43 Network settings Preamble mode. This can either be set to Long (which is the default) or short. A long preamble allows for interoperatively with 1Mbps and 2Mbps DSSS specifications. The shorter allows for faster operations (as the preamble is kept to a minimum) and can be used where the transmission parameters must be maximized, and that there are no interoperatablity problems. # config t (config)# int dot11radio0 (config-if)# preamble-short (config-if)# end Preamble this is sent before the start of the data transmission so that nodes can detect that it is about to transmit.

44 Hidden node problem These nodes cannot hear each other. The hidden node problem occurs when two nodes transmit to an access point, but they are not in communication range, thus their signals can collide, and cause errors.

45 Network settings (cont.) RTS/CTS threshold. The RTS Threshold prevents the Hidden Node problem, where two wireless nodes are within range of the same access point, but are not within range of each other. As they do not know that they both exist on the network, they may try to communicate with the access point at the same time. When they do, their data frames may collide when arriving simultaneously at the Access Point, which causes a loss of data frames from the nodes. The RTS threshold tries to overcome this by enabling the handshaking signals of Ready To Send (RTS) and Clear To Send (CTS). When a node wishes to communicate with the access point it sends a RTS signal to the access point. Once the access point defines that it can then communicate, the access point sends a CTS message. The node can then send its data.

46 Hidden node problem RTS (Ready To Send) RTS (Ready To Send) CTS (Clear To Send) Data transmitted

47 RTS CTS RTS Data CTS Data

48 Hidden node problem RTS threshold RTS threshold determines the data frame size that is required, in order for it send an RTS to the WAP. The default value is RTS (Ready To # config Send) t (config)# int dot11radio0 (config-if)# rts threshold 8000 (config-if)# end CTS (Clear To Send) RTS (Ready To Send) Data transmitted

49 Hidden node problem RTS retries RTS Retries defines the number of times that an access point will transmit an RTS before it stops sending the data frame. Values range from 1 to 128. # config RTS (Ready t To (config)# Send) int dot11radio0 (config-if)# rts retries 10 (config-if)# end RTS (Ready To Send) CTS (Clear To Send) Data transmitted

50 Power management The higher the transmitting power, the wider the coverage. The power of the access point and also of the client are important as they will define the coverage of the signal, and must also be within the required safety limits.

51 Power management # config t (config)# int dot11radio0 (config-if)# power? (config-if)# power local? (config-if)# power local 30 The higher the (config-if)# power client 10 transmitting power, (config-if)# speed? the wider the coverage. (config-if)# speed 1.0 (config-if)# exit (config)# exit The power of the access point and also of the client are important as they will define the coverage of the signal, and must also be within the required safety limits. [1.0] [11.0] [2.0] [5.5] [basic-1.0] [basic-11.0] [basic-2.0] [basic-5.5] range throughput

52 Power management Power saving modes: CAM (Constant awake mode). Used when power usage is not a problem. PSP (Power save mode). Power is conserved as much as possible. The card will typically go to sleep, and will only be awoken by the access point, or if there is activity. FastPSP (Fast power save mode). This uses both CAM and PSP, and is a compromise between the two.

53 Maximum bandwidth (IEEE b) CCK 11Mbps Max bandwidth (config)# int dot11radio0 (config-if)# speed Mbps (config-if)# exit (config)# exit DQPSK 2Mbps DBPSK 1Mbps 100m 200m 300m 400m

54 Maximum associations A particular problem in wireless networks is that the access point may become overburdened with connected clients this could be due to an attack, such as DoS (Denial of Service), or due to poor planning.

55 Maximum associations For example: Max bandwidth = 25Mbps Av rate = 0.5Mbps Max. associations = 50

56 Maximum associations # config t (config)# dot11 ssid fred (config-ssid)# max? <1-255> association limit (config-ssid)# max 100 (config-ssid)# exit (config)# int dot11radio0 (config-if)# ssid fred (config)# exit A particular problem in wireless networks # show dot11 association is that the access point may become # show dot11 statistics overburdened client-traffic with connected clients... # show dot11 adjacent-ap... this could be due to an attack, such as DoS (Denial of Service), or due to poor planning.