Talk:High-Level Data Link Control
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What is "loop-mode" vs. "point-to-point" vs. "multidrop" HDLC?
- To answer your questions...
Multidrop HDLC
This is the original SDLC mode, which Normal Response Mode was designed for. There is one master ("primary station") and a large number of slaves ("secondary stations"). the primary's transmitted data is received by all the secondaries, and all of the secondary transmitters are connected together to the primary's receiver, although at most one transmitter is switched on at a time. The primary gives each secondary in turn permission to talk by sending it a frame (commonly an RR S-frame) with P=1. The secondary transmits all the data it has, then ends with a frame with F=1. When the primary sees this, it knows that station has stopped talking, and can poll the next station in turn.
Point-to-point
In this mode, there is only one secondary. You can use NRM, but you cal also permit the secondary to transmit whenever it likes, without waiting for permission from the primary. The simplest adaptation leads to so-called Asynchronous Response Mode, which still preserves a primary/secondary distinction. A more complex version is Asynchronous Balanced Mode, where the two ends are symmetrical.
SDLC loop mode
This is associated with IBM's original SDLC, as it was not part of the HDLC spec. Here, rather than sharing a broadcast but, the primary and the secondaries are daisy-chained, each one transmitting to the next one in line. Normally, stations retransmit the data they receive, delayed by one bit time.
The primary transmits the data it wants, followed by a closing flag and an "end of poll" pattern 0xFE (7 consecutive 1 bits). When a secondary sees this, and has data to send, it converts the 7th 1 into a 0, making it a normal flag character, and sends its data, ending with another EOP. This goes to the next secondary, and so on. finally, it gets back to the primary, which begins a new cycle.
All communication is still to or from the primary, so it does not retransmit the data it receives, which is a copy of the data it transmits followed by the data sent by each of the secondaries in turn.
This permits very rapid arbitration for permission to send, basically one bit time per station on the loop, rather than having to have the primary poll each secondary in turn. There is a provision for devices to join and leave the ring, enabling a bypass relay that can maintain connectivity even when they are switched off.
Too technical
Now I'm a techie, but while studying for the ICND1, I came here.Now this article seems to get heavy into some of the protocol specifications, but we really need to see some basics here. What is the "claim to fame" for HDLC, How does it compare to PPP ect. And we need it in english. Sephiroth storm (talk) 14:05, 18 October 2012 (UTC)
Framing
The description of the NRZI protocol in this page is the exact opposite of the description contained in the NRZI page (both the english and the italian one). I suppose the correct one is the last one, but I'm not sure. Plus, I'd rather avoid committing grammar errors since I'm not english. — Preceding unsigned comment added by 79.8.87.157 (talk) 11:42, 12 November 2013 (UTC)