The X.21 Interface
The X.21 Interface
The X.21 Interface
21 Interface
CCITT X21 is a physical and electrical interface that uses two types of circuits:
balanced (X.27N.1 1) and and unbalanced (X.26N.10). CCITT X.21 calls out the DA-
15 (also know by DB-15) connector.
The physical interface between the DTE and the local PTT-supplied DCE is defined
in ITU-T recommendation X.21. The DCE provides a full-duplex, bit-serial,
synchronous transmission path between the DTE and the local PSE. It can operate
at data rates from 600bps to 64Kbps. A second standard, X.21bis has been defined
for use on existing (analogue) networks. An X.21bis is a subset of EIA-232D/V.24
therefore allowing existing user equipment to be readily interfaced using this
standard. It should perhaps be emphasized here that V24 defines the data terminal
equipment interface to the modem and is not concerned with the interface between
the modem and the line itself. The modems themselves therefore form part of the
conceptual physical connection. The V24 interface is thus independent of both
modulation technique and data throughput rate.
The X.21 interface protocol is concerned only with the set-up and clearing operations
between DTE and DCE associated with each call. The control of the ensuing data
transfer is the responsibility of the link layer.
X21 Overview
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 X21 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.
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.
Electrical Characteristics.
Data signaling rates of 9600 bit/s and below. X.27 (= V. 11) & X.26 (= V. 10)
[V.11 specifies an interface circuit with a differential, balanced signal from transmitter
to receiver which may accommodate an optional DC offset voltage. This
approximates EIA-4221
X.21 Overview
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:
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.
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.
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.
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 DTE's have entered the Ready for Data state, the Data Transfer state is
entered. The DCE or either DTE may terminate the call by signaling 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.
Signal Element Timing = TC & RC (see note 2) Byte timing: rarely used.
DTE signal element timing : even more rarely used. Not supported.
X.21
General
The X.21 interface was recommended by the CCITT in 1976. It is defined as a digital
signaling interface between customers (DTE) equipment and carrier's equipment
(DCE). And thus primarily used for telecom equipment.
All signals are balanced. Meaning there is always a pair (+/-) for each signal, like
used in RS422. The X.21 signals are the same as RS422, so please refer to RS422
for the exact details.
Functional Description
As can be seen from the pinning specifications, the Signal Element Timing (clock) is
provided by the DCE. This means that your provider (local telco office) is responsible
for the correct clocking and that X.21 is a synchronous interface. Hardware
handshaking is done by the Control and Indication lines. The Control is used by the
DTE and the Indication is the DCE one.
Cross-cable pinning