B channel

You can help expand this article with text translated from the corresponding article in German. (September 2017) Click [show] for important translation instructions. Machine translation, like DeepL or Google Translate, is a useful starting point for translations, but translators must revise errors as necessary and confirm that the translation is accurate, rather than simply copy-pasting machine-translated text into the English Wikipedia. Consider adding a topic to this template: there are already 2,052 articles in the main category, and specifying|topic= will aid in categorization. Do not translate text that appears unreliable or low-quality. If possible, verify the text with references provided in the foreign-language article. You must provide copyright attribution in the edit summary accompanying your translation by providing an interlanguage link to the source of your translation. A model attribution edit summary is Content in this edit is translated from the existing German Wikipedia article at [[:de:B-Kanal]]; see its history for attribution. You may also add the template {{Translated|de|B-Kanal}} to the talk page. For more guidance, see Wikipedia:Translation.

This article does not cite any sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "B channel" – news · newspapers · books · scholar · JSTOR (July 2009) (Learn how and when to remove this message)

B channel (bearer) is a telecommunications term which refers to the ISDN channel in which the primary data or voice communication is carried. It has a bit rate of 64 kbit/s in full duplex.

The term is applied primarily in relation to the ISDN access interfaces (PRA or PRI and BRA or BRI), since deeper in the PSTN network an ISDN bearer channel is essentially indistinguishable from any other bearer channel.

Apart from any transmission errors, the purpose of the network is to carry the contents of the B channel transparently between the endpoints of the call. Exceptions to this general principle include:

• If one or more trunks on the route between the endpoints employs Robbed Bit Signalling, this will result in frequent bit errors in the least-significant bit of bytes transported by the channel, effectively limiting the channel to 56 kbit/s.
• If the call has been established as a long-distance voice call, then echo cancellers may be being employed. While these improve the quality of the channel when it is used for voice communications, they render the channel essentially useless for data communications purposes (whether modulated or not).
• International voice calls may include A-law to μ-law conversion, which also impairs the channel when used for data communications purposes.
• Long-distance voice calls may also employ some form of voice compression, typically provided by equipment such as DCME.

In order to assure the usability of the channel for data communications, the following measures are at the disposal of users:

• ISDN terminal equipment can specify the required Bearer Capability when establishing the call.
• If the call is being set up as a voice call between modems, then a 2100 Hz answer tone sent in the backward direction during the initial seconds of the conversation phase of the call will result in any echo cancellers or echo suppressors being disabled for the remainder of the call.

• D channel
• H channel