communication protocol X –25
BSNL TTA JE Communication Protocol X –25
Full PDF: X –25
What is communication protocol X –25?Answer: X.25 is the CCITT’s recommendation for the interface between a DTE and DCE over a Public Data Network. Generally, X.25 covers layers 1 to 3 of the ISO communication model, but the term is used here to refer specifically to packet layer 3. X.25 is carried within the Information Field of LAPB frames.
Video Lecture on protocol X –25:
The X.25 is illustrated here in relation to the OSI model:Click the protocols on the map to see more details.
Command
LAPB frame types are defined as commands or responses depending on the address field and the direction of the frame. A command frame from the DTE has an address of 1; a command frame from the DCE has an address of 3.
LAPB frame types are defined as commands or responses depending on the address field and the direction of the frame. A command frame from the DTE has an address of 1; a command frame from the DCE has an address of 3.
Control frames
All LAPB frame types except for information frames.
All LAPB frame types except for information frames.
Control packets
All X.25 packet types except for DATA packets.
All X.25 packet types except for DATA packets.
D bit
Delivery confirmation bit of the X.25 packet header. Used to indicate whether packet layer acknowledgements have local (0) or global (1) significance between the DTE and DCE. This is not a universally accepted bit and is generally set to 0.
Delivery confirmation bit of the X.25 packet header. Used to indicate whether packet layer acknowledgements have local (0) or global (1) significance between the DTE and DCE. This is not a universally accepted bit and is generally set to 0.
Information frame
A specific LAPB frame type that is used to transport data across the link, inside the information field of the frame. The information field contains the X.25 packet header and data according to the standards in the X.25 layer 3 protocol.
A specific LAPB frame type that is used to transport data across the link, inside the information field of the frame. The information field contains the X.25 packet header and data according to the standards in the X.25 layer 3 protocol.
LAPB
Link Access Procedure-Balanced protocol. The CCITT’s adaptation of the layer 2 HDLC protocol for the X.25 interface. The LAPB address field, in contrast to HDLC, only has two permissible values.
Link Access Procedure-Balanced protocol. The CCITT’s adaptation of the layer 2 HDLC protocol for the X.25 interface. The LAPB address field, in contrast to HDLC, only has two permissible values.
LCN
Logical Channel Number. Together with the LGN (in the X.25 packet header), identifies the actual logical channel number of the DTE-DCE link. The LCN is an 8-bit field and consequently represents a number between 0 and 255.
Logical Channel Number. Together with the LGN (in the X.25 packet header), identifies the actual logical channel number of the DTE-DCE link. The LCN is an 8-bit field and consequently represents a number between 0 and 255.
LGN
Logical Channel Group Number. Together with the LCN (in the X.25 packet header), identifies the actual logical channel number of the DTE-DCE link. The LGN is a 4-bit field and consequently represents a number between 0 and 15, inclusive.
Logical Channel Group Number. Together with the LCN (in the X.25 packet header), identifies the actual logical channel number of the DTE-DCE link. The LGN is a 4-bit field and consequently represents a number between 0 and 15, inclusive.
M bit
More data bit. A bit contained within the X.25 data packet which is set by layers higher than layer 3 to inform the destination DTE that more data will follow in the next packet. Using the M bit, packets can be logically grouped together to convey a large block of related information.
More data bit. A bit contained within the X.25 data packet which is set by layers higher than layer 3 to inform the destination DTE that more data will follow in the next packet. Using the M bit, packets can be logically grouped together to convey a large block of related information.
Modulo
Window Size. Represents the maximum number of frames (layer 2) or packets (layer 3) that can be left outstanding and unacknowledged for a DTE after transmission. In Modulo 8, sequence numbers can be represented by the digits 0 to 7 and are encoded in 3 bits of information. In Modulo 128, sequence numbers can be represented by the digits 0 to 127 and are encoded in 7 bits of information.
Window Size. Represents the maximum number of frames (layer 2) or packets (layer 3) that can be left outstanding and unacknowledged for a DTE after transmission. In Modulo 8, sequence numbers can be represented by the digits 0 to 7 and are encoded in 3 bits of information. In Modulo 128, sequence numbers can be represented by the digits 0 to 127 and are encoded in 7 bits of information.
N(R)
A frame sequence number used in both LAPB information and supervisory frames in order to indicate to the transmitter the status of sent information frames. The meaning of N(R) is dependent on the type of the frame and can include acknowledgements, missed frames or a busy state. A 3-bit N(R) field is used to identify sequence numbers for a modulo 8 transmission; a 7-bit field is used for modulo 128.
A frame sequence number used in both LAPB information and supervisory frames in order to indicate to the transmitter the status of sent information frames. The meaning of N(R) is dependent on the type of the frame and can include acknowledgements, missed frames or a busy state. A 3-bit N(R) field is used to identify sequence numbers for a modulo 8 transmission; a 7-bit field is used for modulo 128.
N(S)
A frame sequence number used only within an LAPB information frame to identify each frame sent to the receiver. A 3-bit N(S) field is used to identify sequence numbers for a modulo 8 transmission; a 7-bit field is used for modulo 128.
A frame sequence number used only within an LAPB information frame to identify each frame sent to the receiver. A 3-bit N(S) field is used to identify sequence numbers for a modulo 8 transmission; a 7-bit field is used for modulo 128.
P(R)
A packet sequence number used in the X.25 data packets in order to indicate to the transmitter the status of sent data packets. The meaning of P(R) is dependent on the type of the packet and can include acknowledgements, missed frames or a busy state. A 3-bit P(R) field is used to identify sequence number of modulo 8 transmission; a 7-bit field is used for modulo 128.
A packet sequence number used in the X.25 data packets in order to indicate to the transmitter the status of sent data packets. The meaning of P(R) is dependent on the type of the packet and can include acknowledgements, missed frames or a busy state. A 3-bit P(R) field is used to identify sequence number of modulo 8 transmission; a 7-bit field is used for modulo 128.
P(S)
A packet sequence number used only within an X.25 data packet in order to identify each packet sent to the receiver. A 3-bit P(R) field is used to identify the sequence number of modulo 8 transmission; a 7-bit field is used for modulo 128.
A packet sequence number used only within an X.25 data packet in order to identify each packet sent to the receiver. A 3-bit P(R) field is used to identify the sequence number of modulo 8 transmission; a 7-bit field is used for modulo 128.
P/F bit
The Poll/Final bit is set within the LAPB information frame by the transmitter to insist on an immediate response from the receiver. The receiver always turns on the bit in its response to a command from the sender with a poll bit set.
The Poll/Final bit is set within the LAPB information frame by the transmitter to insist on an immediate response from the receiver. The receiver always turns on the bit in its response to a command from the sender with a poll bit set.
Q bit
A bit within the X.25 packet header used to signify X.25 control packets used by asynchronous PADs. Mainly used as a "qualified" proprietary protocol within the packet.
A bit within the X.25 packet header used to signify X.25 control packets used by asynchronous PADs. Mainly used as a "qualified" proprietary protocol within the packet.
Response
LAPB frame types are defined as commands or responses, depending on the address field and the direction of the frame. A response frame is specified as a frame with either an address of 1, (from the DCE), or an address of 3 (from the DTE).
LAPB frame types are defined as commands or responses, depending on the address field and the direction of the frame. A response frame is specified as a frame with either an address of 1, (from the DCE), or an address of 3 (from the DTE).
Sequence number
A unique number given to specific frames and packets to ensure that they are received and interpreted in the correct order. The bit size of the sequence number field is dependent on the respective modulo (maximum window size) of the layer and determines the maximum number of frames or packets that can be outstanding by the transmitter at any given time.
A unique number given to specific frames and packets to ensure that they are received and interpreted in the correct order. The bit size of the sequence number field is dependent on the respective modulo (maximum window size) of the layer and determines the maximum number of frames or packets that can be outstanding by the transmitter at any given time.
Supervisory frames
A collection of LAPB frames used to control information flow, request retransmissions and acknowledge information frames. Receive Ready (RR) and Reject (REJ) are two examples.
A collection of LAPB frames used to control information flow, request retransmissions and acknowledge information frames. Receive Ready (RR) and Reject (REJ) are two examples.
Unnumbered frame
A collection of LAPB frames used to provide additional data-link control functions such as link initialization and disconnection, link reset after unrecoverable errors, and rejection of invalid frames. They are called unnumbered because they do not contain frame sequence numbers. Disconnect (DISC) and Unnumbered Acknowledgement (UA) are two examples.
A collection of LAPB frames used to provide additional data-link control functions such as link initialization and disconnection, link reset after unrecoverable errors, and rejection of invalid frames. They are called unnumbered because they do not contain frame sequence numbers. Disconnect (DISC) and Unnumbered Acknowledgement (UA) are two examples.
X.25
The CCITT’s recommendation for the interface between a DTE and DEC over a Public Switched Telephone Network (PSTN). Generally, X.25 covers layers 1 to 3 of the ISO model, but the term is used in this document to refer specifically to packet layer 3.
The CCITT’s recommendation for the interface between a DTE and DEC over a Public Switched Telephone Network (PSTN). Generally, X.25 covers layers 1 to 3 of the ISO model, but the term is used in this document to refer specifically to packet layer 3.
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