Negotiation of the low layer protocols (in octets 5,6 or 7) may be conducted between the terminating and originating terminals by means of the Low Layer Compatibility IE. This IE is transferred transparently over the network.

If it is considered that negotiation of the low layer protocols may be required then this IE is sent with the Setup message during call establishment. If the Out-band negotiation bit is selected then negotiation can take place. This is implemented by including the required low layer user (B channel) protocol information in octets 5 (Layer 1), 6 (Layer 2), or 7 (Layer 3) in the Low Layer Compatibility IE sent by the terminating side to the originating side with the Connect Message. Octets 3 and 4 containing information transfer attributes must be identical to those sent in the BCC IE with the Setup message. This negotiation process is allowed because the protocols employed in the low layers are, unlike the transfer attributes, independent of the Bearer Services.

In the case where CAPI is used as the API, the default B channel protocol employed is 64 kb/sec with HDLC framing for Layer 1, X75 for Layer 2 and Transparent for Layer 3.