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ITU Q.931 is specified as the protocol for Layer 3 of the D channel. The protocol messages and its rules for exchange are derived from the DSS1 protocol suite.
The I series specifications define support of the end terminal (end equipment) protocol as it is applied to the Cisco routers. The Q series specifications define the equipment from the network point of view, as supported by the central switch.
These data link layer protocols are not implemented in the NT equipment. They are implemented only in the end equipment, and are used to communicate with the local switch.
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