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It is called Poll when part of a command set by the primary station to obtain a response from a secondary station , and Final when part of a response set by the secondary station to indicate a response or the end of transmission.
In all other cases, the bit is clear. The bit is used as a token that is passed back and forth between the stations. Only one token should exist at a time. The secondary only sends a Final when it has received a Poll from the primary.
The primary only sends a Poll when it has received a Final back from the secondary, or after a timeout indicating that the bit has been lost. In NRM, possession of the poll token also grants the addressed secondary permission to transmit.
The secondary sets the F-bit in its last response frame to give up permission to transmit. It is equivalent to the word "Over" in radio voice procedure. In these modes, the secondary need not wait for a poll to transmit, so the final bit may be included in the first response after the poll. If no response is received to a P bit in a reasonable period of time, the primary station times out and sends P again. When operating as a combined station, it is important to maintain the distinction between P and F bits, because there may be two checkpoint cycles operating simultaneously.
A P bit arriving in a command from the remote station is not in response to our P bit; only an F bit arriving in a response is. N R provides a positive acknowledgement for the receipt of I-frames from the other side of the link.
N R operates the same way whether it is part of a command or response. A combined station only has one sequence number space. N S , the sequence number of the sent frame[ edit ] This is incremented for successive I-frames, modulo 8 or modulo Depending on the number of bits in the sequence number, up to 7 or I-frames may be awaiting acknowledgment at any time.
I-Frames user data [ edit ] Information frames, or I-frames, transport user data from the network layer. In addition they also include flow and error control information piggybacked on data. The sub-fields in the control field define these functions. The least significant bit first transmitted defines the frame type. The S-frame control field includes a leading "10" indicating that it is an S-frame.
Or a 4-bit padding field followed by a 7-bit sequence number. The first least significant 2 bits mean it is an S-frame. Send this packet if you need to send a packet but have no I frame to send.
A primary station can send this with the P-bit set to solicit data from a secondary station. A secondary terminal can use this with the F-bit set to respond to a poll if it has no data to send. Can be used like RR with P bit set to solicit the status of a secondary station Can be used like RR with F bit set to respond to a poll if the station is busy.
Sent in response to an observed sequence number gap; e. Optional to generate; a working implementation may use only RR. Not supported by all HDLC variants. U-Frames[ edit ] Unnumbered frames, or U-frames, are primarily used for link management, although a few are used to transfer user data. They exchange session management and control information between connected devices, and some U-frames contain an information field, used for system management information or user data.
The first 2 bits 11 mean it is a U-frame. In a few cases, the same encoding is used for different things as a command and a response. Briefly, there are two non-operational modes initialization mode and disconnected mode and three operational modes normal response, asynchronous response, and asynchronous balanced modes with 3-bit or 7-bit extended sequence numbers.
Disconnected mode DM response When the secondary is disconnected the default state on power-up , it sends this generic response to any poll command frame with the poll flag set except an acceptable mode setting command. It may alternatively give a FRMR response to an unacceptable mode set command. The secondary acknowledges with UA. Any unacknowledged frames are lost. The primary should do so promptly, but may delay long enough to ensure all pending frames are acknowledged.
Set initialization mode SIM command This rarely-implemented command is used to perform some secondary-specific initialization, such as downloading firmware.
What happens in initialization mode is not otherwise specified in the HDLC standard. It sent in lieu of DM if the secondary requires initialization. These frames may be used as part of normal information transfer. Unnumbered information UI This frame command or response communicates user data, but without acknowledgement or retransmission in case of error. Only a configurable-length prefix "header" of the frame is covered by the CRC polynomial; errors in the rest of the frame are not detected.
Unnumbered poll UP command This command solicits a response from the secondary. With the poll bit set, it acts like any other poll frame, without the acknowledgement that must be included in I or S frame. With the poll bit clear, it has a special meaning in normal response mode: the secondary may respond, even though it has not received the poll bit.
This is rarely used in HDLC, but was used in the original IBM SDLC as a substitute for the lack of asynchronous response mode; where the communication channel could handle simultaneous responses, the primary would periodically send UP to the broadcast address to collect any pending responses.
The error flags are: W: the frame type control field is not understood or not implemented. X: the frame type is not understood with a non-empty information field, but one was present. Y: the frame included an information field that is larger than the secondary can accept. Z: the frame included an invalid receive sequence number N R , one which is not between the previously received value and the highest sequence number transmitted.
V: the frame included an invalid send sequence number N S , greater than the last number acknowledged plus the transmit window size. This error is only possible if a transmit window size smaller than the maximum has been negotiated.
The error flags are normally padded with 0 bits to an 8-bit boundary, but HDLC permits frames which are not a multiple of a byte long. This is a possible alternative to sending a new mode set command, which resets both sequence numbers. It is acknowledged with UA, like a mode set command. This is normally done before sending a mode set command. IBM Systems Network Architecture defined one format for the information field, in which the most significant bit of the first byte is clear 0 , but HDLC implementations normally implement the variant defined in ISO , which has the most significant bit of the first byte set 1.
Defined in other standards[ edit ] There are several U frames which are not part of HDLC, but defined in other related standards. It had a 1-byte payload which specified a non-standard test mode for the secondary. Even numbers disabled the mode, while odd numbers enabled it.
A payload of 0 disabled all test modes. The secondary normally acknowledged a configure command by echoing it in response. A secondary which received no frames at all for a long time would begin sending a stream of beacon responses, allowing a unidirectional fault to be located. Link configurations can be categorized as being either: Unbalanced, which consists of one primary terminal, and one or more secondary terminals. Balanced, which consists of two peer terminals. The three link configurations are: Normal Response Mode NRM is an unbalanced configuration in which only the primary terminal may initiate data transfer.
The secondary terminals transmit data only in response to commands from the primary terminal. The primary terminal polls each secondary terminal to give it an opportunity to transmit any data it has. Asynchronous Response Mode ARM is an unbalanced configuration in which secondary terminals may transmit without permission from the primary terminal.
However, there is still a distinguished primary terminal which retains responsibility for line initialization, error recovery, and logical disconnect. Asynchronous Balanced Mode ABM is a balanced configuration in which either station may initialize, supervise, recover from errors, and send frames at any time.
An additional link configuration is Disconnected mode. This is the mode that a secondary station is in before it is initialized by the primary, or when it is explicitly disconnected. In this mode, the secondary responds to almost every frame other than a mode set command with a "Disconnected mode" response. The purpose of this mode is to allow the primary to reliably detect a secondary being powered off or otherwise reset. If the request is rejected it sends DM disconnect mode frame.
Functional extensions options [ edit ].
High-Level Data Link Control
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