This page contains the answers to the questionnaire in chapter 4 (Wireless LAN)
All answers have been held as short as possible and require an
understanding and study of the corresponding chapter of the book.
Chapter 4: Wireless LAN
Devices communicating in an Ad-Hoc network exchange their data directly with each other. There is no central point in the network, all devices are equal. This mode is used if no WLAN Access Point is available and data needs to be exchanged between two or more devices. The disadvantage of this mode is that each device has to be configured manually. This includes the IP configuration and wireless LAN settings like for example encryption. In the BSS (Basic Service Set) mode on the other hand, an access point is used. Data is not exchanged directly between client devices. Instead each data packet is first sent to the access point and from there to the final destination. This has the disadvantage that the maximum speed is cut in half compared to an Ad-hoc network. The advantage on the other hand, is an increased coverage area of the network, as distant devices still can communicate with each because they are still in range of the access point. In addition, the access point usually contains a DHCP server which automatically configures the IP stack of other devices in the network when they first register. Furthermore, the access point usually also acts as a bridge to a fixed line network (e.g. the Internet) and fixed line Ethernet client devices.
A wireless LAN access point is usually equipped with a DHCP server to automatically configure end user devices. In addition, an Access Point is usually also equipped with one or more Ethernet sockets for fixed line Ethernet devices (bridging functionality). Furthermore, many access points act as routers for cable- or DSL modems or even include this functionality. Thus, only a single device is needed to connect fixed and wireless devices with each other and the Internet.
In an Extended Service Set (ESS), several access points are used which are interconnected via an Ethernet cable (distribution system). All Access Points broadcast the same SSID which enables wireless clients to roam between them. This way, the coverage area of the wireless network can be increased.
The SSID is the Service Set ID and is used by client devices to identify a wireless network. This way, several independent wireless networks can be operated at a single location. The user typically configures a device by entering the SSID which is then stored in the configuration. Thus, the device automatically remembers which network it should attach to when it is switched on again. The SSID is broadcast in beacon frames which the access point broadcasts several times a second.
A mobile device can use the power save mode in order to conserve energy while no data is transferred. In order to enter this mode an empty frame has to be sent by the mobile device to the access point, which has the power save bit set to ‘1’ in the header of the frame. Afterwards, the mobile device deactivates its transceiver in order to conserve energy. The access point in turn starts to buffer incoming packets for the device, should there be any during its sleep period. From time to time, the device activates its transceiver again to check the Traffic Indication Map (TIM) which is included in a beacon frame to see if there is incoming data waiting to be delivered. If there is no data, the transceiver is deactivated again and the TIM is checked again after the next sleep period. In case data is available, the mobile device exits the sleep mode and polls the access point for the queued frames.
Acknowledgement frames are used as transmission on the air interface is much more volatile then over cables. By sending an acknowledgement frame the receiver informs the sender that the packet was received correctly. If no acknowledgement frame is sent or if it is lost the frame is automatically retransmitted.
The 802.11g standard uses the RTC/CTS mechanism as older 802.11b devices are unable to detect frames which have been sent by using the new modulation and coding schemes offered by the ‘g’ standard. This ensures that older devices do not perceive the channel as free when a frame with an unknown modulation and coding scheme is in the process of being sent. In addition, the RTC/CTS mechanism is also used to avoid the ‘hidden-station’ problem.
First address: sender, second address: receiver, third address: MAC address of the access point. This is required as a frame is not delivered directly to the destination in a BSS setup but always via the access point.
The PLCP header of a WLAN frame is always sent at a data rate of 1 MBit/s. This ensures that even distant devices are able to receive this part of the frame correctly. The PLCP header also contains information on the transfer speed, the modulation and the channel coding used for the main part of the frame.
The theoretical top speed of an 802.11g network is 54 MBit/s. As the frame headers are always sent at a speed of 1 MBit/s, however, the actual top speed is lower. Furthermore, all frames have to pass through the access point which cuts the speed in half if both sender and receiver of a frame are wireless devices. In addition, all frames have to be acknowledged which further reduces the speed. Thus, the top speed that can be achieved between two wireless devices in an 802.11g network is around 1.200 kByte/s.
The Distributed Coordination Function (DCF) is a decentralized approach to control access to the air interface. Collisions on the air interface are seldom but possible as there is no central instance. Furthermore, such an approach is also not able to ensure a certain access time to the air interface and delay. Applications such as Voice over IP, however, highly depend on constant delay times. While a WLAN network is only lightly loaded, this approach is less of a problem. In highly loaded networks on the other hand, voice quality can be degraded.
One of the weaknesses of the WEP encryption algorithm is the use of the same key for all devices. As the key has to be distributed to all users of the network, potential intruders have the possibility to obtain the key by fraud. Furthermore, certain parts of the encrypted payload header of each frame are known as it is identical in each frame. In combination with the variant of the RC-4 algorithm used for encrypting the frame, this fact can be exploited to break the encryption by collecting a high number of frames and then applying this knowledge on them. A rough estimation shows that an attacker has to collect about 1.5 GByte of data to be able to break the WEP key.