X.21 is a state-driven protocol running full duplex at 9600 bps to 64 Kbps with subscriber networks. It is a circuit-switching protocol using Synchronous ASCII with odd parity to connect and disconnect a subscriber to the public-switching network.
The data-transfer phase is transparent to the network. Any data can be transferred through the network after Call Establishment is made successfully via the X.21 protocol. The call-control phases which are used were defined in the CCITT (now ITU) 1988 “Blue Book” Recommendations X.1 – X.32.

Signals Provided

The signals of the X.21 interface are presented on a 15-pin connector defined by ISO Document 4903. The electrical characteristics are defined in CCITT Recommendations X.26 and X.27, which refer to CCITT Recommendations V.10 and V.11.

X.21 provides eight signals:

Signal Ground (G) –
This provides reference for the logic states against the other circuits. This signal may be connected to the protective ground (earth).
DTE Common Return (Ga) – Used only in unbalanced-type configurations (X.26), this signal provides reference ground for receivers in the DCE interface.
Transmit (T) – This carries the binary signals which carry data from the DTE to the DCE. This circuit can be used in data-transfer phases or in call-control phases from the DTE to DCE (during Call Connect or Call Disconnect).
Receive (R) – This carries the binary signals from DCE to DTE. It is used during the data-transfer or Call Connect/Call Disconnect phases.
Control (C) – Controlled by the DTE to indicate to the DCE the meaning of the data sent on the transmit circuit. This circuit must be ON during data-transfer phase and can be ON or OFF during call-control phases, as defined by the protocol.
Indication (I) – The DCE controls this circuit to indicate to the DTE the type of data sent on the Receive line. During data phase, this circuit must be ON and it can be ON or OFF during call control, as defined by the protocol.
Signal Element Timing (S) – This provides the DTE or DCE with timing information for sampling the Receive line or Transmit line. The DTE samples at the correct instant to determine if a binary 1 or 0 is being sent by the DCE. The DCE samples to accurately recover signals at the correct instant. This signal is always ON.
Byte Timing (B) – This circuit is normally ON and provides the DTE with 8-bit byte element timing. The circuit transitions to OFF when the Signal Element Timing circuit samples the last bit of an 8-bit byte. Call-control characters must align with the B lead during call-control phases. During data- transfer phase, the communicating devices bilaterally agree to use the B lead to define the end of each transmitted or received byte. The C and I leads then only monitor and record changes in this condition when the B lead changes from OFF to ON, although the C and I leads may be altered by the transitions on the S lead. This lead is frequently not used.

X.21 Protocol Operation

As stated previously, X.21 is a state protocol. Both the DTE and DCE can be in a Ready or Not-Ready state.
The Ready state for the DTE is indicated by a continuous transmission of binary 1’s on the T lead. The Ready state for the DCE is continuous transmission of binary 1’s on the R lead. During this continuous transmission of Ready state, the control leads are OFF.
During the Not-Ready state, the DCE transmits binary 0’s on the R lead with the I lead in the OFF state.
The DTE Uncontrolled Not-Ready is indicated by transmission of binary 0’s with the C lead in the OFF state. The DTE Uncontrolled Not-Ready state signifies that the DTE is unable to accept calls due to an abnormal condition.
The DTE Controlled Not-Ready state sends a pattern of alternating 1’s and 0’s on the T lead with the C lead OFF. This state indicates that the DTE is operational, but unable to accept incoming calls.
The characters sent between the DTE and DCE during call-control phases are International Alphabet 5 (IA5), defined by CCITT Recommendation V.3. At least two Sync characters must precede all sequences of characters sent between the DTE and DCE to establish 8-bit byte synchronization between the transmitter and the receiver. If the Byte Timing (B) lead is used, these Sync characters must align with the B lead timing signals.

Procedure for a DTE Placing a Call

The following procedure is used when a DTE places a call:

Call Request:
The DTE will transmit continuous 0’s on the T lead with the C lead in the ON state to indicate a desire to make a call.
Proceed to Select:
If the DCE is prepared to receive information, it will send continuous plus (+) characters on the R lead with the I lead in the OFF state. This state is maintained until the selection information is completed. The Proceed to Select signal must be sent within 3 seconds of the Call Request signal being sent by the DTE.
Selection Signal Sequences:
As indicated, the DTE is transmitting to the DCE during Call Request with the C lead ON. After the DCE has sent the Proceed to Select signal, the Selection Signal must start within 6 seconds and be completed within 36 seconds.
The Selection Signal will be either a Facility Request block, an Address block, or a Facility Registration/Cancellation block. If the DTE wishes to terminate the Selection Signal, it sends a plus (+) character.
A Facility Request block consists of a code followed by a backslash (/) separator and then a parameter value. If multiple Facility Requests are used, they are separated by commas.
The Address block may be one or more addresses separated by commas. This may be either a full network address or an abbreviated address (which would start with a period).
A Facility Registration/ Cancellation block will be one or more signals separated by backslashes (/). These consist of Facility Codes, Indicator, Address and Parameter. Multiple FR/Cs may be separated by commas. The end of the FR/Cs are indicated by a minus (-) followed by a plus (+).

During the Selection Sequences, the network will continue to transmit plus (+) characters followed by Call Progress signals. The Call Progress signal is a value or set of values separated by commas (,) and terminated with a plus (+). The values indicate if a call has been successful or if it has failed, and the reason for the failure.
This signal must be sent from the DCE to the DTE within 20 seconds of the end of the Selection Sequence and may be followed by DCE-provided information. The DCE also passes this information to the DTE being called, which will detail who is making the call.
When the network has established a connection between two DTEs, it will signal the calling DTE with a Ready for Data signal by setting the I lead to ON. The two DTEs are now connected until the call is cleared by one or the other.

DTE in the Ready State Receiving an Incoming Call

The DTE will signal Ready state with continuous binary 1’s on the T lead with the C lead in the OFF state. The DCE responds with a Ready state via continuous binary 1’s on the R lead and the I lead in the OFF state.
When a calling DTE wants to establish a connection to another DTE, the DCE will signal the called DTE with continuous BEL characters. The called DTE accepts by changing the C lead to the ON state. The DCE indicates to the called DTE who is calling and indicates that a connection is established by changing the I lead to the ON state.
When both DTEs have entered the Ready for Data state, the Data Transfer state is entered. The DCE or either DTE may terminate the call by signalling a CLEAR. If either DTE clears the call, it will send continuous 0’s on the T lead and set the C lead to OFF. The DCE responds with Clear Confirmation by sending continuous 0’s on the R lead and setting the I lead to OFF.
After sending a Clear Confirmation, the DCE will signal Ready state within 2 seconds. The clearing DTE must respond with Ready state within 100 milliseconds.
The cleared DTE will receive Cleared signal by the DCE and must send a Clear Confirmation to the DCE within 2 seconds. The DCE will signal Clear Confirmation within two seconds and must receive a Ready state from the cleared DTE within 100 milliseconds.

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