IEC 13239:2002, which replaces all of those standards. Derivatives have since appeared in innumerable standards. HDLC framing techniques but adds a protocol field frame relay protocol architecture pdf the standard HDLC header.
Those links have no mechanism to mark the beginning or end of a frame, so the beginning and end of each frame has to be identified. Each frame begins and ends with a frame delimiter. A frame delimiter at the end of a frame may also mark the start of the next frame. A sequence of 7 or more consecutive 1-bits within a frame will cause the frame to be aborted. When no frames are being transmitted on a simplex or full-duplex synchronous link, a frame delimiter is continuously transmitted on the link. Some protocols allow the 0-bit at the end of a frame delimiter to be shared with the start of the next frame delimiter, i.
For half-duplex or multi-drop communication, where several transmitters share a line, a receiver on the line will see continuous idling 1-bits in the inter-frame period when no transmitter is active. Since the flag sequence could appear in user data, such sequences must be modified during transmission to keep the receiver from detecting a false frame delimiter. The receiver must also detect when this has occurred so that the original data stream can be restored before it is passed to higher layer protocols. This can be done using bit stuffing, in which a “0” is added after the occurrence of every “11111” in the data. When the receiver detects these “11111” in the data, it removes the “0” added by the transmitter. Any time that 5 consecutive 1-bits appear in the transmitted data, the data is paused and a 0-bit is transmitted. This ensures that no more than 5 consecutive 1-bits will be sent.
The receiving device knows this is being done, and after seeing 5 1-bits in a row, a following 0-bit is stripped out of the received data. In the latter case, the frame receive procedure, depending on state, is generally either aborted or restarted. 6 bit times during transmission of data, and one transition per 7 bit times during transmission of flag, so the receiver can stay in sync with the transmitter. 8, and bit-stuffing is inconvenient. If either of these two octets appears in the transmitted data, an escape octet is sent, followed by the original data octet with bit 5 inverted.
Address, Control, and Information fields. It provides a means by which the receiver can detect errors that may have been induced during the transmission of the frame, such as lost bits, flipped bits, and extraneous bits. However, given that the algorithms used to calculate the FCS are such that the probability of certain types of transmission errors going undetected increases with the length of the data being checked for errors, the FCS can implicitly limit the practical size of the frame. After either receiving a negative acknowledge packet or timing out waiting for a positive acknowledge packet, the sender can retransmit the failed frame. FCS could readily be computed by simple, fast circuitry or software. There are three fundamental types of HDLC frames. In addition they can also include flow and error control information piggybacked on data.
Some U-frames contain an information field, depending on the type. Final is a single bit with two names. It is called Poll when set by the primary station to obtain a response from a secondary station, and Final when set by the secondary station to indicate a response or the end of transmission. In all other cases, the bit is clear. 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.
In ARM and ABM, the P bit forces a response. In these modes, the secondary need not wait for a poll to transmit, so need not wait to respond with a final bit. If no response is received to a P bit in a reasonable period of time, the primary station times out and sends P again. F bit and not yet acknowledged will never arrive, and so should be retransmitted. 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. F bit arriving in a response is.
I-frames from the other side of the link. This is incremented for successive I-frames, modulo 8 or modulo 128. Depending on the number of bits in the sequence number, up to 7 or 127 I-frames may be awaiting acknowledgment at any time. In addition they also include flow and error control information piggybacked on data.
The sub-fields in the control field define these functions. F is 0, the two forms are exactly equivalent. Supervisory Frames, or ‘S-frames’, are used for flow and error control whenever piggybacking is impossible or inappropriate, such as when a station does not have data to send. The S-frame control field includes a leading “10” indicating that it is an S-frame. If 7-bit sequence numbers are used, there is also a 4-bit padding field. The first 2 bits mean it is an S-frame.
F bit and a receive sequence number as described above. The 2-bit type field encodes the type of 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. Acknowledge some packets and request no more be sent until further notice.