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

Wireless LAN Unit 2: Wireless Fundamentals

Wireless connections which technology? Areas covered: Basic radio parameters. This area covers the main type of wireless communications. IEEE 802.11b issues. This area covers some of the fundamentals of radio waves. Sample Aironet Configurations This shows some simple configuration examples.

Wireless Access point Wireless Bridge Wireless Client

Broadcast span Defined by broadcast domain

IEEE 802.11b networks

IEEE 802.11 - Wireless IEEE 802.11a. 802.11a deals with communications available in the 5GHz frequency, and has a maximum data rate of 54 Mbps. IEEE 802.11b. 802.11b, 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 802.11g. 802.11g is a proposed standard that hopes to provide 54Mbps maximum bandwidth over a 2.4GHz connection, the same frequency as the popular 802.11b standard. IEEE 802.11c. 802.11c is a group set up to deal with bridging operations when developing access points. IEEE 802.11f. 802.11f is concerned with standardising access point roaming which is involved in making sure that interoperability between access points is guaranteed

IEEE 802.11b Operating Channels: 11 for N. America, 14 Japan, 13 Europe (ETSI), 2 Spain, 4 France Operating Frequency: 2.412-2.462 GHz (North America), 2.412-2.484 GHz (Japan), 2.412-2.472 GHz (Europe ETSI), 2.457-2.462 GHz (Spain), 2.457-2.472 GHz (France) Data Rate: 1, 2, 5.5 or 11Mbps Media Access Protocol: CSMA/CA, 802.11 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)

Available bandwidth Maximum bandwidth (IEEE 802.11b) 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

Maximum bandwidth (IEEE 802.11b) 11Mbps Max bandwidth 5.5Mbps Actual maximum bandwidth 2Mbps 1Mbps 100m 200m 300m 400m

Maximum bandwidth (IEEE 802.11b) 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

IEEE 802.11g networks

Available bandwidth Maximum bandwidth (IEEE 802.11g) 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 802.11g Mbps Modulation 6 BPSK 9 BPSK 12 QPSK 1Mbps 18 QPSK 2Mbps DBPSK 24 16-QAM 36 400m 16-QAM 48 64-QAM 54 64-QAM

Netperf on a 11g link

Windows IP Configuration Ethernet adapter Wireless Network Connection 3: Connection-specific DNS Suffix. : IP Address............ : 192.168.1.100 Subnet Mask........... : 255.255.255.0 Default Gateway......... : 192.168.1.1 C:\>netperf -H 192.168.1.101 -p 1001 TCP STREAM TEST to 192.168.1.101 Recv Send Send Socket Socket Message Elapsed Size Size Size Time Throughput bytes bytes bytes secs. 10^6bits/sec 8192 8192 8192 10.00 9.60 C:\>netperf -H 192.168.1.101 -p 1001 TCP STREAM TEST to 192.168.1.101 Recv Send Send Socket Socket Message Elapsed Size Size Size Time Throughput bytes bytes bytes secs. 10^6bits/sec 8192 8192 8192 10.00 7.60 c:\>netperf -H 192.168.1.101 -p 1001 TCP STREAM TEST to 192.168.1.101 Recv Send Send Socket Socket Message Elapsed Size Size Size Time Throughput bytes bytes bytes secs. 10^6bits/sec 8192 8192 8192 10.00 7.60

IEEE 802.11g example

IEEE 802.11n

802.11n 802.11n 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.

802.11n

MIMO

IEEE 802.11 networks

CSMA/CA and PCF IEEE 802.11 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.

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.

IEEE 802.11 data frame Frame control Duration/ ID Address 1 Address 2 Address 3 Sequence control Address 4 Frame body FCS 2 Bytes 2 6 6 6 2 6 0-2312 4 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.

Frame control Duration/ ID Address 1 Address 2 Address 3 Sequence control Address 4 Frame body FCS 2 Bytes 2 6 6 6 2 6 0-2312 4

IEEE 802.11 networks

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 192.168.0.1 255.255.255.0 (config-if)# exit Antenna connector

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

Fixed network Root # config t (config)# ip default-gateway 192.168.1.254 (config)# exit Repeater

Channel Frequency Channel. If an ad-hoc network is used, then the nodes which communicate must use the same channel. 1 11 6 1 11 6 channel 1 2412 channel 2 2417 channel 3 2422 channel 4 2427 channel 5 2432 channel 6 2437 channel 7 2442 channel 8 2447 channel 9 2452 channel 10 2457 channel 11 2462 channel 12 2467 channel 13 2472 channel 14 2484

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

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.

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? <256-2346> (config-if)# fragment-threshold 700 (config-if)# end Possibly allows for a smoother and fairer transmission.

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

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

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

Span of network L L L

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.

Other Factors

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.

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.

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.

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.

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

RTS CTS RTS Data CTS Data

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 4000. 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

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

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.

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

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.

Maximum bandwidth (IEEE 802.11b) CCK 11Mbps Max bandwidth (config)# int dot11radio0 (config-if)# speed 1.0 5.5Mbps (config-if)# exit (config)# exit DQPSK 2Mbps DBPSK 1Mbps 100m 200m 300m 400m

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.

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

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.