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12.9 LAYER 3 MESSAGES ON THE Um INTERFACE 383Call Confirmed. This message is sent by a called MS, indicating that it is processingthe received Setup message. The message has no counterpart in Q.931.Call Proceeding. This message is sent by the network, on calls originated by aMS. It acknowledges the Setup message and indicates that the connection isbeing set up.Connect. On calls originated by a MS, the message is sent by the network. Oncalls terminating at a MS, it is sent by the MS. It indicates that the called partyhas answered.Connect Acknowledge. This message acknowledges the receipt of a connectmessage.Disconnect. This indicates that the MS user, or the other party in the call, has disconnected.It requests the release of the connection. The message is sent by the MSor the network, depending on which party has cleared. It includes the cause IE,which indicates the disconnect reason (e.g., normal call clearing).Release. This message is sent in two situations. It either acknowledges a receiveddisconnect message or indicates a network-initiated release. In the latter case, themessage includes a cause IE, which indicates why the call setup has failed: forexample, unassigned called number received, called party busy, or networkfailure. The message indicates that the originating entity is about to release the connectionat its end.Release Complete. The message acknowledges a Release message. It indicatesthat its originator will release the connection at its end.Setup. This message is sent by a MS or the network on, respectively, calls originatingor terminating at the MS. The IEs that may be included are outlined below.. Bearer capability. Mandatory in direction MS ! MSC; optional in the otherdirection. This is a compound IE, which includes several information items,for example:Radio channel requirement (full rate, half rate).Information transfer capability (speech, unrestricted digital information,3.1-kHz audio, group 3 facsimile).. Called party address. Mandatory in direction MS ! MSC; optional in the otherdirection.. Calling party address. Optional in both directions.. Called and calling party subaddresses. Optional in both directions.. High- and low-layer compatibility. Optional in both directions.. User–user information. Optional in both directions.