B channel is a telecommunications term which refers to the ISDN

Integrated Services Digital Network

Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...

 channel

Channel (communications)

In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...

 in which the primary data

Data

The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...

 or voice

Voice message

Voice message refers to a message that could be sent to a destination using voice media. Voice itself could be 'packaged' and sent through the IP backbone so that it reaches its marked 'address'. In a technical sense, the process of sending 'voice packets' is a semi passive way of communication...

 communication

Communication

Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...

 is carried. It has a bit rate

Bit rate

In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....

 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
  Robbed bit signaling
  In communication systems robbed-bit signaling is a scheme to provide maintenance and line signaling services on many T1 digital carrier circuits using Channel Associated Signaling ....
  
  , 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
  Speech encoding
  Speech coding is the application of data compression of digital audio signals containing speech. Speech coding uses speech-specific parameter estimation using audio signal processing techniques to model the speech signal, combined with generic data compression algorithms to represent the resulting...
  
  , typically provided by equipment such as DCME
  Digital Circuit Multiplication Equipment
  Digital Circuit Multiplication Equipment was a type of voice compression equipment that is installed at either end of a long-distance link . The main characteristics of DCME are defined in ITU-T recommendation G.763...
  
  .

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
  Bearer Capability
  Bearer Capability is one of the Information Elements of the Q.931 SETUP message. It is used by the calling party to specify the kind of B channel that is being requested....
  
   when establishing the call.
• If the call is being set up as a voice call between modems, then a 2100 Hz answer tone
  Answer tone
  Answer tone is a feature of wireline modems.The answer tone is the first signal sent by the answering modem after the billing delay. In its most basic form, it is a single continuous tone with a frequency of 2100 Hz ....
  
   sent in the backward direction during the initial seconds of the conversation phase of the call will result in any echo cancellers or echo suppressor
  Echo suppressor
  An echo suppressor is a telecommunications device used to reduce the echo heard on long telephone circuits, particularly circuits that traverse satellite links...
  
  s being disabled for the remainder of the call.