SIP phone
SIP phones are a type of phone that is based on Voice over IP and uses the Session Initiation Protocol (SIP) for this. The conversation is sent over the Internet in individual data packets . SIP telephones can be implemented as a single device (hardphone), telephone adapter plus classic telephone or as a software solution ( softphone ) on a PC or smartphone . There are both wired (mostly Ethernet) and cordless (mostly WLAN or DECT ) variants.
Features and functions
The hardphone solution can be operated like an ordinary telephone and looks like it. In addition to telephone numbers , SIP addresses (also called URIs ) can be selected on SIP telephones , which are shaped exactly like e-mail addresses and have the scheme prefix sip: for example sip: info@muster.de . If the SIP address of the call partner is known, this can be B. sip: info@muster.de can be entered directly in the telephone instead of the number. The prerequisite is that the phone or the software supports the entry of an alphanumeric SIP address.
If the SIP client , i.e. a hard or soft phone, supports the ENUM look-up procedure, after dialing the phone number, the SIP client checks at a registration office whether there is a SIP URI entry for the selected phone number. If this is the case, the conversation is established with the stored SIP URI entry; this is referred to as a real ENUM look-up (public ENUM look-up). The conversation therefore runs entirely as an IP conversation. An ENUM look-up can also be carried out by a VoIP provider, but not all providers support this function. Some providers limit this to their own members or to members of their partner providers; this is not referred to as a real ENUM look-up (private ENUM look-up). This means that a call can be billed as a normal landline call, even though a SIP phone was dialed. With the ENUM look-up, the selected E.164 phone number is converted into a corresponding ENUM domain. This domain is searched for NAPTR resource records via the resolver . The evaluation of these resource records results in one or more URIs under which the desired service of the specified domain or telephone number can be addressed.
The LAN can be used for the transmission of telephone calls , which means that separate wiring of telephone lines is unnecessary and costs can be saved. With Power over Ethernet (PoE) power cable required fall in addition to office phones off.
SIP telephony is designed for worldwide use and therefore usually also uses international address formats for national use. For example, the connection could always be reached under the same telephone number anywhere in the world . A geographic telephone number for London can be assigned to a SIP telephone , but the SIP telephone registers itself in Stuttgart , for example . If the London number is called, the SIP telephone answers in Stuttgart. This mobility functionality is used in IMS- based multimedia networks of the next generation.
A selection of so-called codecs allows the transmission of speech in different quality levels. This can range from cell phone quality with the GSM codec to hi-fi quality with wideband codecs such as iLBC and the G.722 standard, which is also understood by UMTS end devices. See also HD telephony .
disadvantage
Since the end of 2004, German VoIP providers are no longer allowed to assign geographical numbers to their customers if they do not have a place of residence in this number range. To compensate for this, a separate number range with the area code 032 has been made available since early 2005 . The same applies to Austria, where the number ranges 0720 subscriber numbers for location- independent numbers and 0780 subscriber numbers for convergent services (ENUM) .
A very restrictive firewall can make configuration difficult and sometimes makes it completely impossible to use. A consistent and high voice quality can, however, thanks to QoS i. d. Usually guaranteed even if there is a high level of utilization by other services.
- (110, 112 ) - local service center: Many providers can only use emergency numbers to a limited extent and often do not go to the nearest service center, but to one specified by the provider.
- (110, 112) - Röchelruf : If the caller can no longer speak (so-called Röchel calls), the transmission of the address data to the office is particularly important. In the case of nomadic use of the VoIP connection, the customer must tell the provider his location so that he can forward the address in the event of an emergency. Many providers cannot update the emergency call address. If you take your VoIP connection with you when you are abroad, there is no longer any responsibility for the German emergency call services. It is not always possible to encrypt signaling and payload (e.g. voice) , which makes eavesdropping much easier. Remedy - if available - can be provided by SRTP (voice, video), ZRTP (key exchange ) and TLS ( signaling ).
Synonyms
- SIP user agent (UA)
- SIP client
- SIP terminal
Individual evidence
- ↑ www.ilbcfreeware.org ( Memento of May 13, 2008 in the Internet Archive )