X.21
Overview
back
| next | index
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.
back
| next | index
|