AT&T 7500 series, 7100 series, 2500 series, 7200 series, 7400 series Digital Communications Protocol

D-16COMMUNICATIONS PROTOCOLS

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∙CD

∙CA.

Also, the update messages support three special control functions. They are:

∙BRK — remote break indicator

∙RLR — remote loop request

∙ACK — remote loop acknowledge

Transmission errors are detected by checking the CRC checksum on receive. Errors are corrected by retransmitting the errored block(s).

Data Mode 3

Capabilities:

—Can be used with a 64-kbps (B-type) physical channel

—Full-duplex operation

—Synchronous transmission of user data (slave timing)

—Bit-rate independent, T1 compatible

—May be circuit-switched or virtual-circuit switched

Applications:

The primary use of mode 3 is to multiplex multiple logical links (virtual channels) onto a single ‘‘high- usage’’ physical channel. The method of multiplexing involves the use of the LAPD link layer protocol.

Description:

Mode 3 converts data (at standard data rates) to a format which can be transmitted over a DS0 physical channel. Mode-3 data is segmented into HDLC-type blocks. On the signaling channel either MOS or BOS is used; while the bearer channel uses LAPD at layer 2, and X.25 at layer 3. Each block consists of a flag, address field(s), control field(s), a variable length information field, and then followed by a CRC checksum.

User data is encoded into mode 3 format by a special type of data module. Transmission errors are detected by checking the CRC checksum on receive. Errors are corrected by retransmitting the errored block(s).

Digital Communications Protocol

Digital Communications Protocol is a standard that defines communications along a 3-channel line. The DCP channel structure consists of 2I+1S channel. Each I channel provides for 64 kbps of voice/data communication, while the S channel provides 8 kbps of signaling communication. DCP protocol is similar to the ISDN basic rate interface (BRI). In fact, AT&T created DCP in anticipation of the ISDN-BRI standard. Many of the telephone terminals that interface with the System 75, System 85, and DEFINITY® Generic 1 and 2 systems use this protocol for communications.