Service set (802.11 network)

Service Set designated by 802.11 of the Institute of Electrical and Electronics Engineers, all devices on a wireless network. In this structure emerge some related terms.

Basic Service Set

A Basic Service Set (BSS ) is caused by synchronization of basic parameters of multiple devices, which begins one of the devices by sending a proposal in the format START and others can accept the format JOIN. Through JOIN only the possible data transfer rates are substantially confirmed.

Which gives the two types of BSS, is determined by whether connections to other networks such other BSS are created:

• An Independent Basic Service Set ( IBSS ) is a BSS with no connection to other networks, a self-contained network. An IBSS may be an ad hoc network.

• An Extended Service Set (ESS ) is a BSS with connecting to other networks of other standards such as Ethernet. An ESS is an infrastructure network.

The starting device continuously transmits a clock signal, and in an IBSS, each device is equally responsible for this task, so that such a BSS may persist without the starting device.

Basic Service Set Identification

The Basic Service Set Identification ( BSSID ) refers to any BSS clearly. Either corresponds to the MAC address of the wireless access points or alternatively generated as a random number.

Service Set Identifier

A Service Set Identifier ( SSID ) is a name of your choice of a service sets, by which it is addressed.

A wireless access point can support multiple SSID at the same time, what became more important with the introduction of authorization for WLANs. Such multiple SSID provide a wireless access point to provide multiple virtual local area networks.

An SSID can be up to 32 bytes long and include calibrate accordingly up to 32 ASCII characters. If a device a data frame with empty SSID, wireless access points send as response each SSID, they support. From these responses, the device can generate a list that allows the user to select a service sets.

SSID and ESSID

By assigning different network identifiers can also be different wireless LANs operate in the same place. The SSID is set in the base station. To a plurality of base stations of a network to link ( OSI Layer 2), one of these sets the same SSID; It is then referred to as ESSID (Extended SSID).

Clients ( that is, the radio participants) enter the ESSID on either explicitly, or trying to "*" to join with the ESSID any network. In the latter case, it depends on the configuration of the base station determines whether the client is allowed to register on the network.

Hidden networks

As a security measure the transmission (broadcast beacon ) the SSID can be turned off in some base stations. Prior to accession, it is then necessary, they must fill in a password possibly used. However, the effect of safety is rather low by concealing; this method only lowbrow opportunity burglars can be kept away from the radio network.

In an actively used the WLAN SSID can be determined quickly. Once an attacker eavesdropping on a client that is using a wireless sniffer when joining the network, so they are even the hitherto suppressed SSID. In order not to have to wait for this process, the attacker can throw with fake data packets targeted clients from the network. Since they log on then usually automatically, he learns so in no time the oppressed SSID. The passive network sniffer Kismet for example has no problems, up capture the SSID.