PT1327 trunking system address 46.4.3 Half/full duplex codeword structure Base station control channels operate either on a dedicated channel basis, on a rotating channel basis with 'Move' commands instruct- ing mobiles to change channel accordingly, on a time shared basis where a channel is cycled between multiple transmitter sites, or with 46.4.5 Call processing a combination of these dependent upon system requirements. Base stations for both control and traffic channel purposes operate on a Once a successful RQS has been made, the called party is polled full duplex basis, mobile units operate using half duplex with with an AHY message on the control channel the called mobile is 1200bit/s FFSK signalling and FFSK data communication. active on at any time. An ACK from the mobile indicates it will accept calls, otherwise alternative messages, such as ACKB indicat- ing the call will be queued in the mobile memory for callback, are 46.4.4 Control channel sent for information to the calling party. The called party is then either alerted for subscriber availability status, or queued for traffic A Control Channel System Codeword (CCSC) is continuously channel availability, both parties receiving a GTC message when a transmitted, giving details of the system's network, area, zone, traffic channel at each site is available. PABX originated calls may control category etc. Mobile units scan the available channels stored be made with a dedicated link between the user PABX and the in a programmed 'preferential hunt list', followed by an optional Trunking System Controller (TSC), likewise with PSTN calls. "comprehensive hunt', until the correct control channel matching Once the mobile is present on the traffic channel, periodic FFSK the unit's personalisation is confirmed. maintenance messages from both the mobile and base may be The CCSC operates on a 'Slotted Aloha' basis, with Aloha mess- processed, indicating user pressed 'on', 'off', and mobile identifi- ages inviting mobile units to register and/or send message request