WLAN telephone
A WLAN telephone is essentially a cordless telephone , which differs from conventional cordless telephones (e.g. according to the DECT standard) mainly through the WLAN- based radio technology . Appropriate Internet-compatible protocols that ensure communication between the devices and the corresponding servers are used for this purpose. Most of the WLAN telephones currently on the market work with the following, partly proprietary packet-switching protocols:
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- One of the first standards for VoIP telephony, which has mainly established itself in the corporate environment.
- Common standard for home user products that is also increasingly used in the corporate environment.
- Proprietary protocol primarily intended for home users.
The great advantage of WLAN telephones over other cordless systems is that no special second radio technology is required for voice traffic. Both voice and data can now be transmitted via WLAN. This is very interesting for home use, as only one WLAN base station (hereinafter referred to as AP ) is required. Companies also benefit from not having to set up separate systems for voice and data.
WLAN telephones are not yet very widespread. In the household sector, this is mainly due to the fact that the DECT standard is better suited with its greater range and lower power consumption. The advantages of a WLAN telephone only come into play in larger installations and thus mainly in companies. They manifest themselves in the lower costs and in the simplified administration.
Range
By default, WLAN telephones have a shorter range than DECT devices. It is true that if the AP is conveniently positioned in the middle of the house, it is easily possible to cover a regular apartment with WiFi. However, the first problems already appear in two-story houses or apartments with a solid reinforced concrete ceiling. As a rule, three floors cannot be bridged with a single AP. It should be noted that it is often not enough to check the connection with a notebook or similar. Qualitatively poor WLAN connections can often still be used for Internet use via PC. The user barely notices dropouts of a few seconds while surfing. However, such a connection cannot be used for VoIP purposes; the voice transmission is briefly interrupted completely without warning.
Power consumption
Although the chipsets are constantly being improved, WLAN telephones currently consume significantly more electricity than conventional cordless telephones. Currently available chipsets offer talk times in the range of five hours and standby times of a maximum of two to three days.
safety
One advantage over DECT technology is that very secure encryption methods already exist for WLAN ( TKIP , AES ). Also, 802.1x authentication via certificate is possible if the device or the infrastructure supports. However, the effort that must be made for this type of authentication is so great that it is usually only worthwhile for company networks.
configuration
Since WLAN telephones are basically nothing more than small computers (mostly based on Linux ) and thus have a complete operating system including a network stack, it makes sense to integrate a small web server in order to offer the user a convenient way of using the telephone Configure web browser. This advantage is put into perspective by the somewhat more complex settings that such a phone requires. However, it should only be a matter of time before the manufacturers have simplified the commissioning of the telephones to such an extent that even a layperson can configure it with little problem.
Radiation and health
Due to legal regulations, the transmission power of WLAN devices is limited to 100 milliwatts. However, the value is a peak value. Values averaged over time, as determined for other wireless technologies, are much lower. However, it must be taken into account that these mean values depend on the average data volume. Since the data load can fluctuate greatly with WLAN, it is difficult to give such an average.
Since WLAN works with channels with a bandwidth of 20 MHz, the 100 mW mentioned above are distributed over this relatively wide band. In comparison, DECT transmits with 250 mW in a band that is only 1.7 MHz wide, which is a little less than a tenth of the WLAN bandwidth. A wider spectral distribution is generally considered safer.
Although there are sometimes very heated debates about the dangers of radio radiation , there are currently no reliable findings that prove a risk to people. As a preventive measure, the maximum transmission power of the devices, which can usually be set manually, should be limited to the necessary minimum.
Disruptor
WLAN uses a worldwide unlicensed radio band. This means that this band can also be used by other devices with completely different radio systems. Problems arise in particular from the following devices that also work in this band:
- Microwave ovens
- Bluetooth (PC, mobile phone)
- digital wireless thermometer
- Alarm systems that are operated by radio
- u. Ä.
Most of the devices mentioned do not take an already occupied tape into account. An exception is Bluetooth from specification 1.2.
Special requirements for the access points
A major source of problems arises from the apparent advantage of being able to use any AP. The smooth functioning of a WLAN telephone can only be guaranteed if the device is connected to an AP that supports Wi-Fi multimedia or the official IEEE standard 802.11e . These features are required for the prioritization of the voice data and the throttling of the energy consumption. All modern APs have this capability, but not all parameters are configurable for all devices, but are partially fixed and optimized for normal data connections. a. does not allow an optimal energy saving configuration for WLAN telephones. There are also some subtleties in the WLAN standards (802.11x) that are described imprecisely and are therefore implemented differently by each manufacturer. Under certain circumstances, this leads to considerable problems with telephones that rely on precisely these details and their correct implementation.
Handover
WLAN telephony is particularly suitable for large companies, as the so-called cell handover (also incorrectly referred to as roaming in normal parlance ) offers the possibility of automatically switching from radio cell to radio cell. This property, which is already known from cell phones, offers the possibility, for example, of providing a company site with WLAN across the board and thus enabling calls to be made throughout the site. Technically, this is implemented by many (possibly several hundred) individual APs that are centrally managed by special WLAN controllers.
Since the handover mechanism is an initial component of WLAN, WLAN telephones for domestic use also have the ability to switch to the most powerful AP, which has the appropriate settings for SSID, encryption, etc. This generally makes it possible to compensate for the shorter range of WLAN with additional APs. However, when buying a WLAN telephone, care must be taken to ensure that this function is actually supported, because although a WLAN cell handover works automatically, the manufacturer still needs to make some preparations to ensure that the cell can be changed in a minimum time that is useful for VoIP to accomplish.
When using strong encryption, there is still a minimal delay in handover to the next AP according to the current technical status (2008). This results from the fact that a new key negotiation must take place after each change to a new AP. Proprietary mechanisms exist to accelerate this, but as a rule a short dropout of around 50-100 ms must be expected. Most of the time this does not bother, since part of this delay is absorbed by a so-called jitter buffer and the actual dropout is extremely short. Nevertheless, it is clearly audible. Ideally, times of just under 50 ms are achieved without encryption. However, due to some judgments, it is at least legally risky to operate a WLAN unencrypted. For companies, this is out of the question for data protection reasons.
Thanks to the WLAN standard IEEE 802.11r, which was adopted in 2008, a seamless transition between VoIP calls is ensured. This standard (also called "Fast Basic Service Set Transition") is an important step for WLAN telephony, especially in commercial use.
Individual evidence
- ↑ wlan-telefone.de: New WLAN standard adopted ( memento of the original from December 19, 2008 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
