hdlc protocol

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FRAME CHARACTERISTICS Transparency ● ● Allows to transmit any bit pattern Principle (between two [non included] flags) Emission: inserts 1 "zero" after 5 consecutive "ones" Reception: suppresses every "zero" following 5 consecutive "ones" Frame inter-filling ● ● Maintains the synchronization Continuous emission of flags Frame abandon ● Forces the receiver to ignore a frame ● Sequence of M bits: M ≥ 7 GET/INT/LOR/RIP 8 of 33 October 15, 2007
TRANSPARENCY The aim of transparency is to transmit any bit pattern between two flags. HDLC uses a special bit pattern (01111110) as a delimiter. This special pattern is called a flag. A receiver enters the reception state (and remains in this state) when it receives any bit pattern different from the delimitation pattern. It quits the reception state when it detects a new flag. However, the delimitation pattern can be found anywhere between two real flags: ● it can be an ASCII character (tilde; "~") whose binary coding is 01111110 ● it can be the association of two adjacent bytes giving the illusion of a flag. For instance, let us consider the two following bytes: 10010011 and 11110011. When these byte are put together, we obtain the following binary stream: 1001001111110011 where a "flag" seems to appear. We absolutely must remove these pseudo-flags when sending a frame. On the reception side, we will have to restore the original bit stream. The principle to do that is simple, and is applied only between two flags. On the sender side, we systematically add one "0" after five consecutive "1"s. On the receiver side, each time we receive five consecutive "1"s, we apply the following algorithm: 1. if the next bit is a "0", this bit is immediately and systematically removed 2. if the next bit is a "1", there are two possibilities: a) the next bit after this "1" is a "0": we are detecting a real flag. This is the end of the frame; b) the next bit after this "1" is still a "1": we are detecting an abandon sequence. The frame must be discarded. GET/INT/LOR/RIP 9 of 33 October 15, 2007
Page 1 and 2: HDLC PROTOCOL Michel GARDIE INT/LOR
Page 3 and 4: HDLC ISO 3309 Data communication -
Page 5 and 6: FRAME STRUCTURE Definition Data and
Page 7: FRAME FIELDS Information field ●
Page 11 and 12: ELEMENTS OF PROCEDURE The elements
Page 13 and 14: FRAME TYPES Information ● The inf
Page 15 and 16: SUPERVISION FUNCTIONS F A C Info FC
Page 17 and 18: SUPERVISION FRAMES RNR RECEIVER NOT
Page 19 and 20: EXTRA FUNCTIONS LAPB (LINK ACCESS P
Page 21 and 22: RULES OF RESTART Some error conditi
Page 23 and 24: RULES OF RESTART RESTART AFTER TIME
Page 25 and 26: ADDRESS FIELD The address field is
Page 27 and 28: FLOW CONTROL DTE W = 3 NETWORK DLDT
Page 29 and 30: EXCHANGE MODES DEFINITIONS INFORMAT
Page 31 and 32: EXCHANGE MODES SYMMETRIC POINT-TO-P
Page 33: CREDITS This document was written w

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