| 摘要 |
<p>In an integrated services digital network (ISDN) a telephone subscriber is physically connected to a telephone exchange by a digital signal subscriber loop which provides in time division multiplex (TDM), two B channels and a D channel. The two B channels are used for voice and data at a bit rate of sixty-four kilobits per second each. The D channel is used for packet data and for telephone signalling and supervision at a bit rate of sixteen kilobits per second. An ISDN D channel handler, in an exchange termination (ET) collects D channel data from and distributes D channel data to various ISDN subscriber lines. A frame processor, in the D channel handler, receives D channel information as it occurs in each of the receive channels. It directs D channel data to a receive buffer storage location where it is accessible by a translator. The translator recognizes information in the data which relates to supervisory and signalling functions, and passes the information to a central controller in the ET. Likewise, signaling and supervisory information destined for an ISDN subscriber line is translated into ISDN compatible data which is stored in transmit buffer storage locations. The frame processor subsequently pulls this data from the storage locations, and formats it according to CCITT standard, before transmitting it on the appropriate transmit D channel of the TDM bit stream to the subscriber. Packet data received in the D channel is recognized after is has been stored in the receive buffer storage location and then is immediately queued for transmission via the frame processor and a digital transmission liink to a separate packet network. Packet data from the separate packet network is received from the digital transmission link and stored via the frame processor in buffer storage locations. Subsequently, the information bits of each stored data packet are formatted according to the CCITT standard by the frame processor, and transmitted to the intended subscriber on the appropriate D channel.</p> |
