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.Furthermore, an optional data compression procedure, as defined in ITU-T Rec.V.42 bis, may be used in conjunction with the LAPM procedures.Use of any optional procedure requires agreement by both control functions using the mechanisms in 8.10.The negotiation-initiator can choose whether to request use of a particular procedure.The procedure is used only if the negotiation-initiator requests its use and the responder agrees to its use.For selective retransmission purposes, the negotiator-initiator may propose either or both of a) and d) options.The responder may choose not to use selective retransmission (i.e.use REJ); alternatively, if it wants to use selective retransmission, it shall choose only one of the options a) or d).11 Control function-to-control function connectionAn error-corrected connection, with a DLCI value of 63, has been reserved for use as a control function-to-control function connection.The protocol used between two control functions is for further study.12 Encoding of information fields12.1 Information fields in I framesThe encoding of the information field of I frames is determined by the use of the error-corrected connection (e.g.when used to carry data received from the V.24 interface).12.2 Information fields in XID frames12.2.1 GeneralThe general structure of the information field of an XID frame is based on the encoding in ISO/IEC 8885, including Addendum 3, and is shown in Figure 11 below.The information field is composed of a number of subfields.These subfields are a format identifier subfield, zero or more data link layer subfields and, possibly, a user data subfield.Figure 11/V.42 - General format of XID information fieldWhen an octet encoding is shown for any of the subfields, it is shown with the right-most bit being the low-order bit and the bit transmitted first.12.2.1.1 Format identifier subfieldFormat Identifier (FI) subfield is one octet in length and is the first octet of the information field of the XID frame.In general, the FI is encoded such that it can designate 128 different formats standardized by ISO and 128 different formats defined by users.Each ISO-standardized format is associated with a different FI value.The only such format defined at this time is the "general purpose" format.12.2.1.2 Data link layer subfieldsData link layer subfields are used to specify various data link layer characteristics, such as operational parameters.In terms of Figure 11, a data link layer subfield consists of a Group Identifier (GI) one octet in length, a Group Length (GL) two octets in length, and a parameter field (whose length is given by GL).The parameter field, in turn, is similarly decomposed into one or more sets of a Parameter Identifier (PI), a Parameter Length (PL), and a Parameter Value (PV) (the parameter length, however, is only one octet in length).The data link layer subfields, if present, follow in ascending order according to their GI values.12.2.1.3 User data subfieldA GI has been defined to specify a user data subfield used in conjunction with the "general purpose" FI.This subfield, which follows all data link layer subfields as Figure 11 shows, does not contain a GL.The subsequent information is bounded by the frame's FCS field.For the purposes of this Recommendation, this subfield is also divided into sets of PIs, PLs, and PVs.12.2.2 Encoding for negotiation/indication of parameter values and optional proceduresThe information field shall be encoded as specified below.Fields that are not recognized are ignored.Format identifier subfieldFor negotiation/indication of parameter values and optional procedures, the FI subfield shall be encoded as "10000010" to indicate the ISO-standardized "general purpose" FI.Data link layer subfieldsOnly two of the data link layer subfields specified in ISO/IEC 8885 shall be used in conjunction with this Recommendation.These are:a) the "parameter negotiation" subfield (this subfield has a GI value of "10000000"); andb) the "private parameter negotiation" subfield (this subfield has a GI value of "11110000")
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