Agilent Technologies Auroraduet N1726A, Руководство пользователя, Страница: 412 / 507

Issue 1 - 06/99 A1 - 11
Appendix 1 - Technical Information
Low Layer Compatibility
The calling terminal uses the optional Low Layer
Compatibility (LLC) to indicate to the remote terminal,
the lower layer attributes of the call. For example, it may
indicate:
• user bit rate (if there is an additional DTE interface
on the remote TE)
• duplex type
• modem type
• data framing
• codec type information for voiceband calls.
In practice, this information element is coded as a replica
of the Bearer Capability. LLC information is transparent
to the network - it is only used by the destination
terminal. Most called party equipment ignores the LLC,
as sufficient information is contained in the Bearer
Capability.
If you are involved in troubleshooting on the ISDN, bear
in mind that some networks may modify or discard LLC,
in part or in whole, before sending D channel
information to the called party.
High Layer Compatibility
The optional High Layer Compatibility (HLC)
information element identifies the terminal type at the
higher layers of the OSI 7-layer model.
The simplest example of an HLC is 'Telephony 3.1 KHz'.
This indicates to an ISDN called party, that for a call to
succeed, there must be a telephone handset or
speakerphone available, with two-way real time speech
transfer capability. This HLC is usually combined with a
Bearer Capability of Speech, in the Call Setup message.
Appx1.PM6 17/06/99, 09:54 11