Denotes a G.726 standardized by the ITU-T based ADPCM codec for compressing speech into digital signals. G.726 is used, among other things, for IP telephony. The standard was adopted in 1990 and holds the older G.721 1984 (32 kbit / s) and G.723 of 1988 ( 24 and 40 kbit / s, not to be confused with the CELP -based G.723.1 ) as its successor standard together.
The method is based on adaptive differential pulse code modulation (ADPCM ).
The codec supports bit rates of 16, 24, 32 and 40 kbit / s
G.726 reached a Mean Opinion Score ( MOS) of about 4.2 for the 40-kbit/s-Variante and about 3.85 in the 32-kbit/s-Variante.
With DECT phones, the 32-kbit/s-Variante is used predominantly. The 16-kbit/s-Variante is hardly used here because of the already very poor transmission quality.
Other use is the codec for international voice network connections for the fixed and mobile network infrastructure. The multiplex method used for this is usually DCME ( Digital Circuit Multiplication Equipment) in accordance with G.763 implemented and used depending on the utilization of international voice traffic, the G.726 codec with 16, 24, 32 and 40 kbit / s These compressions are internationally also in some access networks for connecting PBXs in use.
Data traffic and delay times
For example arises for speech compression at 32 kbit / s in a minute a data volume of 240 kB; a one-hour VoIP call thus gives 14.4 MB of voice data. Not counting here are the protocol data for communication in IP networks that require depends on the number of the data packet rate and the protocol up to 50% of additional bandwidth. In circuit- switched networks, the log data are part of a separate signaling channel.
Delay times in IP networks are dependent on the time of transmission ( transmission delay), the necessary buffering for jitter ( Jitterbuffering ), the number of intermediate nodes and their transmission rate (transmission delay, if it is not cut-through switches are ) and the encoding and decoding ( packetization time) the language used here by the G.726 codec with a corresponding packet rate. In circuit- switched networks, only a delay due to transmission time encoding and decoding occurs.
Problems in practical use with SIP
Although generally to data protocols in the Internet context for many years in network byte order (big endian ) is prescribed as byte order is explicitly not clearly defined in RFC 3551 section 4.5.4 the bit order of the compressed data in RTP packets. Practical implementations such as the AVM Fritz! Box, allow the user therefore define the bit order itself. For example, in the Fritz! Box with the check box " provider supports G726 according to RFC 3551 " the bit order can be switched by the user.
In other cases, such as media gateways from Cisco or Huawei, the bit order is fixed predetermined. Other implementations provide both partially Bitreihenfolgen to, under the name " AAL2 - G726 -32 " and " G726 -32 ". After negotiating a telephony connection in G.726 -32 over SIP between different devices may cause problems with speech intelligibility. Language sounds after decompression in the "wrong " bit order heavily distorted and it is difficult or impossible to understand.