GB/T 11595-1999 Interface between packet data terminal equipment (DTE) and data circuit-terminating equipment (DCE) connected to a public data network using dedicated circuits
Some standard content:
ICS33.040.01
National Standard of the People's Republic of China
GB/T 11595—1999
idtITU-TX.25:1996
Interface between data terminal equipment (DTE) and data circuit-terminating equipment(DCE)for terminals operatingin the packet mode and connected to public data networksbydedicated circuit
Published on November 11, 1999
Implementation on June 1, 2000
State Administration of Quality and Technical Supervision
GB/T11595—1999
This standard is equivalent to the ITU-TX.25 (1996 edition) standard recommendation. The format and coding used in the standard refer to the format and coding of ISO/8583 (standard recommendation): 1987; the writing format follows the provisions of GB/T1.1-1993. This standard defines the interface between packet-based data terminal equipment (DTE) and data circuit-terminating equipment (DCE) connected to a public data network using a dedicated circuit.
This standard specifies the characteristics and operation of the interface between packet-based data terminal equipment (DTE) and data circuit-terminating equipment (DCE) connected to a public data network using a dedicated circuit. This standard applies to packet-based data terminal equipment (DTE) and data circuit-terminating equipment (DCE) connected to a public data network using a dedicated circuit.
The main contents of this revision of this standard are as follows: The terms are modified as follows:
"throughput" → "throughput",
"entry closed user group" → "closed user group with entry", "exit closed user group" → "closed user group with exit", "octet" → "octet",
"RPOA\-→\ROA";
"release" → "clear".
Chapter 1 adds ×.31 interface (1.4)
Chapter 2 mainly adds "super" mode and selective retransmission mechanism, and deletes LAP protocol. Chapters 3, 4 and 5 mainly remove the registration group of the registration service facility in the online service facility, and modify the format of the address block in the call request and incoming call group. Chapter 6 mainly removes "online facility registration" and so on. Appendices A to G of this standard are standard appendices, and Appendices H, J, K, L, M, N and P are informative appendices.
The standards referenced in this standard are as follows:
GB/T11589—1999
GB/T11593—1989
International user service categories and access types for public data networks and integrated services digital networks (ISDN) (eqvITU-TX.1:1996)
Interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) working synchronously on public data networks (eqvITU-TX.21:1984) GB/T178011—1999 Interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) working synchronously on public data networks (eqvITU-TX.21:1984) The interface between packet-based data terminal equipment (DTE) and data circuit-terminating equipment (DCE) for accessing a packet-switched public data network or circuit-switched public data network to a packet-switched public data network (eqvITU-TX.32:1996)
ITU-TX.31
Support for packet-based terminal equipment in the integrated services data network (ISDN)ITU-TX.213 Definition of network services for open systems interconnection for ITU applicationsITU-TX.301 Description of the general configuration for call control within and between subnets to provide data transmission services This standard was proposed by the Ministry of Posts and Telecommunications of the People's Republic of China. This standard is under the jurisdiction of the Telecommunications Science Research and Planning Institute of the Ministry of Posts and Telecommunications. This standard was drafted by the Data Communication Technology Research Institute of the Ministry of Posts and Telecommunications. The main drafters of this standard are: Chen Xiaohui, Bao Shibo, Hu Lin, Zhang Xiaoxuan, Jin Jun, Huang Liang. This standard is entrusted to the Data Communication Technology Research Institute of the Ministry of Posts and Telecommunications for interpretation. I
GB/T11595—1999
ITU-T Foreword
Since public data networks providing packet-switched data transmission services have been established in many countries, it is necessary to develop some standards to promote international interoperability. ITU-T takes into account: (a) Recommendation X.1 includes user service categories specific to data terminal equipment working in packet mode and specifies access categories, Recommendation X.2 specifies user facilities, Recommendation X.21 and Recommendation X.21bis specify DTE/DCE physical layer interface characteristics, Recommendation X.92 specifies the hypothetical reference connection for packet-switched data transmission services, and Recommendation X96 specifies call progress signaling. (b) Data terminal equipment operating in packet mode will send and receive network control information in packet format; (c) Some data terminal equipment operating in packet mode will use synchronous data circuits interleaved with packets; (d) It is desirable to have a single data circuit connected to the data switching equipment (DSE) for all user equipment; (e) Recommendation X.2 specifies which of the many data transmission services and optional user facilities described in this standard are "basic" and must be provided internationally, and which are not "basic"; (f) In order to use packet-switched data transmission services, an international recommendation for the exchange of control information between DTE and DCE is required;
(g) Regarding the use of packet-switched data transmission services via public switched telephone lines This DTE/DCE interface for access to a telephone network, an integrated services digital network (ISDN) or a circuit-switched public data network is specified in Recommendation X.32. (h) Recommendation X.31 specifies support for packet-based terminal equipment in the integrated services digital network (ISDN); (i) When this standard is used to support the network services specified in Recommendation X.213 | ISO/IEC 8348, the physical layer, data link layer and packet layer are equivalent to the physical layer, data link layer and network layer specified in Recommendation X.200, respectively; (i) This standard includes the features necessary to support all services of Recommendation X.213 | ISO/IEC 8348 and other features; Recommendation X.223 specifies how to use X.25 packet layer protocol to provide OSI connection mode network services; (k) the essential elements of an interface recommendation should be separately specified as: physical layer - mechanical, electrical, functional and procedural characteristics used to establish, maintain and clear the physical link between DTE and DCE; data link layer - link access procedures for exchanging data on the link between DTE and DCE; packet layer - packet format and control procedures used to exchange packets containing control information and user data between DTE and DCE.
Consensus recommendation: For terminal equipment working in packet mode, access to the public data network through dedicated circuits must comply with the following standards.
1) The mechanical, electrical, functional and procedural characteristics used to establish, maintain and clear the physical link between DTE and DCE shall comply with the provisions of Chapter 1, Characteristics of the DTE/DCE Interface;
2) The link access procedures for exchanging data on the link between DTE and DCE shall comply with the provisions of Chapter 2, Link Access Procedures on the DTE/DCE Interface;
3) The packet layer procedures for exchanging control information and user data at the DTE/DCE interface shall comply with the description of the packet layer DTE/DCE interface in Chapter 3;
4) The procedures for virtual calls and permanent virtual circuit services shall comply with the provisions of Chapter 4, Virtual Circuit Service Procedures; 5) The packet format exchanged between DTE and DCE shall comply with the provisions of Chapter 5, Packet Format; 6) The selected user facility procedures shall comply with the provisions of Chapter 6, Selected User Facility Procedures; 7) The selected user facility format shall comply with the provisions of Chapter 7, Format of Facility Field. 1
GB/T11595—1999
Note: This standard specifies the behavior of DCE comprehensively. In addition, a set of minimum requirements is made for DTE. Guidance material on DTE design can be found in ISO standards ISO7776 (data link layer) and ISO8208 (packet layer). This standard does not require the use of these ISO/EC standards. If these ISO standards are used, a note must be made to indicate that their scope exceeds the interface of packet-switched public data networks. It must be noted that the term DTE used in this standard refers to the equipment of the DCE interface. In ISO/EC8208, a distinction is made between DTE and packet-switched private data networks, but in this standard both are considered DTE. Finally, the procedures of this standard can also be selected for occasions where packet mode operation is not used when accessing the public data network through dedicated circuits. In such cases, it is not possible or necessary to use the capabilities specified in this standard in full. For example, for this specific environment, it may be necessary to change the layer 2 addressing procedures in Chapter 2 or the optional user facility procedures in Chapter 6. An example of this approach is ISO/IEC 8881 (in this standard, only packet layer procedures are used and some optional user facilities are added for local area networks). Another example is the use of this standard for the interface between a packet-switched public data network and a packet-switched private data network, with the goal of providing fully transparent services to the DTEs of both networks. In this case, the problem of addressing and optional user facilities needs to be solved correctly. Recommendation X.327 provides a framework for solving this problem.
National Standard of the People's Republic of China
Interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE)for terminals operating in the packet mode and connected to public data networks by dediccated circuit1DTE/DCE interface characteristics (physical layer)
GB/T11595—1999
idtITU-TX.25:1996
Replaces GB/T11595—1989
The competent authority may provide one or more of the following interfaces. For the exact use of the relevant points of these standards, see below. 1.1 X.21 Interfaces
1.1.1 Elements of the DTE/DCE physical interface
The elements of the DTE/DCE physical interface shall conform to 2.1 to 2.5 of Recommendation X.21. 1.1.2 Procedure for entering the operational phase
The procedure for entering the operational phase shall be as specified in 5.2 of Recommendation X.21. When the interface is in states 13S, 13R and 13 of Figure A-3/X.21, the data exchanged on the T and R circuits shall be as specified in the following clauses of this standard. The not ready state described in 2.5 of Recommendation X.21 refers to an inoperative state, which may be interpreted by higher layers as a fault state (see 4.6).
1.1.3 Fault detection and test loops
The fault detection principles shall conform to 2.6 of Recommendation X.21. In addition, the 1=OFF signal may indicate an instantaneous transmission fault signal, and the higher layers may delay for a few seconds before determining whether the interface has failed. The definition of test loops and the principles for maintenance testing using test loops are given in Recommendation X.150. A description of test loops and their use procedures is given in Chapter 7 of Recommendation X.21. It is not possible for the DTE to automatically initiate the No. 2 test loop of the remote terminal DCE. However, some administrations allow the DTE to control a loop equivalent to the No. 2 test loop in the local DSE in order to test the operation of leased lines or subscriber lines, all or part of the DCE or line terminating equipment. If this service is provided, the loop control may be performed manually or automatically as described in Recommendations X.150 and X.21, respectively. 1.1.4 Signal code element timing
Information code element timing shall be in accordance with the provisions of Recommendation X.21, Section 2.6.3. 1.2 X.21bis Interface
1.2.1 Elements of the DTE/DCE Physical Interface
The elements of the DTE/DCE physical interface shall conform to the provisions of 1.2 of Recommendation X.21bis. 1.2.2 Operational Phase
When circuit 107 is in the ON state, and circuits 105, 106, 108, and 109 (if such circuits are provided) are all in the ON state, data interchange on circuits 103 and 104 shall conform to the following clauses of this standard. When circuit 107 is in the OFF state, or any of circuits 105, 106, 108, or 109 (if such circuits are provided) is in the OFF state, it shall be considered an inoperative state and a fault state in higher layers (see 4.6). Approved by the State Administration of Quality and Technical Supervision on November 11, 1999 and implemented on June 1, 2000
1.2.3 Fault detection and test loops
GB/T11595-1999
The description of fault detection principles, test loops and their use procedures shall comply with the provisions of 3.1 to 3.3 of Recommendation X.21bis. In addition, circuits 106 and 109 may enter the OFF state due to momentary transmission failures. The higher layer may delay for a few seconds before determining whether a fault has occurred on the interface.
It is not possible for the DTE to automatically initiate a Type 2 test loop in the DCE at the remote terminal. However, some competent authorities allow the DTE to control the equivalent of a Type 2 test loop in the local DSE in order to test the operation of leased lines or subscriber lines, all or part of the DCE or line terminating equipment. If this service is provided, loop control may be performed manually or automatically as described in Recommendations X.150 and X.21bis, respectively.
1.2.4 Signal Symbol Timing
Signal symbol timing shall be as specified in 3.4 of Recommendation X.21bis. 1.3 V Series Interface
General operation using a V Series modem is the same as specified in 1.2 above. However, details regarding fault detection principles, loop testing, and the use of circuits 107, 108, 113, and 114 refer to the relevant provisions of the V Series. The delay between circuits 105-0N and 106-0N (if provided) is greater than 10 ms and less than 1 ms. In addition, circuits 106 or 109 may be put into the OFF state due to momentary transmission failures or modem retraining. Higher layers may delay for several seconds before determining whether an interface has failed.
1.4 X.31 Interface
1.4.1DTE/DCE physical interface
The DTE/DCE physical interface shall correspond to the R reference point between the DTE and the terminal adapter (TA). The use of the TA function is to enable the operation of the DTE over the ISDN. When access to packet-switched transmission services is via a semi-permanent ISDN connection (i.e., a non-switched B-channel), the functions of this TA are described in clause 7 of X.31. NOTE
1 This type of access is considered to be access to public switched data transmission services over dedicated lines. Non-dedicated access to public switched data transmission services is specified in Recommendations X.32 and X.31.
2 When the packet terminal TE1 conforms to the 1-series Recommendations, the functions of the DTE and TA may be implemented in the same equipment. Therefore, this standard deals with layer 2 and layer 3 operation over semi-permanent B-channels. 1.4.2 Operational phase
The operational phase shall conform to clause 7 of Recommendation X.31. 1.4.3 Maintenance
Maintenance shall be as specified in 7.6 of Recommendation X.31. 1.4.4 Synchronization
Synchronization shall be as specified in clause 7 of Recommendation X.31. 2 Link Access Procedure over the DTE/DCE Interface 2.1 Scope and Application
2.1.1 The Link Access Procedure (LAPB) is described as an element of the data link layer that can be used to interchange data between DCE and DTE on a single physical circuit or, in an optional manner, on multiple physical circuits as specified in user service categories 8 to 11, 26, 30 to 33, 35, 37, 45, 53 and 59 of standard X.1. If it is desired that the effects of circuit failures not interrupt packet layer operation, then optional, reservation-selectable, multiple physical circuit operation (this is called multilink operation) may be required.
The Single Link Procedure (SLP) described in 2.2, 2.3 and 2.4 (LAPB) is used to interchange data on a single physical circuit between DTE and DCE as described in Chapter 1. When optional multi-link operation is used, the Single Link Procedure (SLP) may be used on each physical circuit alone, while the Multi-Link Procedure (MLP) described in 2.5 may be used to interchange data on multiple parallel LAPB data links. In addition, when there is only one physical circuit in the LAPB, the optional multi-link procedure may be used on one LAPB data link by agreement with the competent authority.
2.1.2 The Single Link Procedure (SLP) uses the principles and terminology of the High-Level Data Link Control (HDLC) procedure defined by the International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC). The Multi-Link Procedure (MLP) is based on the principles and terminology of the Multi-Link Control procedure specified by ISO/IEC.
2.1.3 Each transmission facility operates in full duplex. 2.1.4 DCE is compatible with the balanced type of HDLC (BA type) by using the LAPB procedures specified in 2.2, 2.3 and 2.4 of this standard. BA type (LAPB synchronous mode 8) including options 2,8 is a basic service and must be provided by all networks. BA type 2,8, and the addition of option 10.1 (LAPB synchronous mode 128) are considered optional, subscription-optional, extended sequence numbering services and can be used in networks that wish to serve DTE applications that require mode 128 sequence numbering. When HDLC option 10.1 is added, option 3.3 (SREJ) is used instead of option 2 (REJ). If option 3.3 is used, option 2 should not be used. This option 3.3 replacement for option 2 is an optional subscription-optional service. BA types 3.3, 8 and additional option 10.2 (LAPB synchronous modulo 32768) are considered optional, subscription-selectable, super sequence numbering services and may be used in networks that wish to serve DTE applications that require modulo 32768 sequence numbering and multiple selective rejection restoration.
The relationship between sequence numbering and restoration is:
Basic (modulo 8)
REJ restoration
(2.3.5.2.1)
SREJ restoration
(2.3.5.2.2)
Note: If SREJ restoration is used, REJ restoration should not be used. Extended (Mod 128)
Optional at the time of reservation (Note)
Optional at the time of reservation (Note)
Super (Mod 32768)
HDLC Option 15.1 may be added to BA2, 8 or BA2, 8, 10.1, or BA3.3.8, 10.1 to replace synchronous transmission with start-stop transmission. This additional part is an optional, optional service at the time of reservation, and can be used in networks that wish to serve DTEs using start-stop transmission. DTE manufacturers and implementers must be aware that the LAPB synchronous transmission Mod 8 protocol described below is the only protocol that can be used in all networks.
Note: Some networks may continue to support another data link layer protocol called LAP. The definition of LAP has not been modified since 1988. It is planned that all future additions to X.25 will be based on LAPB. Thus, the details of LAP can be found in the X.25 standard of the 1988 Blue Book series (see 2.1.6, 2.22.6 and 2.7). 2.1.5 For those networks that choose to support the basic service (LAPB synchronous transmission, mode 8) and at least one extended LAPB sequence number, SREJ recovery and (or start-stop transmission), the selection of the basic mode or the addition of these optional functions is made at the time of reservation. For each data link procedure, these selections are independent of each other. The selection of extended LAPB sequence number is also independent of the corresponding packet layer procedure. All selections need to be agreed upon with the competent authorities for a period of time. 2.2 Frame structure
2.2.1 Flag sequence
All frames shall begin and end with a flag sequence consisting of 01111110. When sending multiple flag sequences, the DTE and DCE shall send the complete 8-bit flag sequence (see 2.2.4) . A flag can be used as the end flag of a sequence and as the start flag of the next frame at the same time.
2.2.2 Transparency
2.2.2.1 Synchronous transmission
When sending, the DCE or DTE should check the content of the frame between the two flag sequences, including the address, control information and FCS fields, and should insert a "0" bit after all 5 consecutive "1" bit sequences (including the last 5 bits of FCS) to ensure that no 3
GB/T11595—1999
false flag sequences appear. When receiving, the DCE or DTE should check the content of the frame and remove any "0" bit following 5 consecutive "1" bits.
2.2.2.2 Start-stop format Transmission
The control escape octet is used to identify the presence of an octet in a frame using the following transparency procedure. The encoding of this control escape octet is:
Bit Transmission Sequence 12345678
10111110
When transmitting, the DCE or DTE shall examine the contents of the frame between the two flag sequences, including the address, control information, and FCS fields, and after completing the FCS calculation, shall:
1) invert data bit 6 when a flag or control escape octet is present, and 2) insert a control escape octet before the octet generated by the above procedure before transmission. When receiving, the DCE or DTE shall examine the contents of the frame between the two flag sequences, and after receiving After a control escape octet and before FCS calculation:
a) discard the control escape octet, and b) invert data bit 6 to recover the following octet. NOTE: The sender may optionally include additional octets in the transparency procedure. These octets are to be determined. 2.2.3 Transmission Considerations
2.2.3.1 Bit Transmission Order
Addresses, commands, responses, and sequence numbers shall be transmitted low-order bits first (e.g., the first bit of a sequence number transmitted shall be the bit with weight 2°). The order in which the information field bits are transmitted is not specified in Chapter 2. The FCS shall be transmitted to the line starting with the highest order coefficient, which is located in bit position 16 of the FCS field (see Tables 1, 2, and 3). NOTE: In Tables 1 to 9, bit 1 is defined as the low-order bit. 2.2.3.2 Start-stop transmission
For start-stop transmission, each octet is delimited by a start bit and a stop bit. If necessary, the signal hold (continuous logic "1" state) can be used as time fill for the octet. A typical octet transmission is shown in Figure 1. When receiving a frame, the DTE or DCE should check its content and discard its start and stop bits as well as the "1" inserted as time fill between octets. How many byte groups
CGceCce
Bits
How many bits?
The data bits of the bit group are *0\ or 1\
I use the required transmission status (the actual transmission status) to complete (select 1\transmission status)
Teach the group bits (check the total low bits first
Figure 1 Typical 8-bit group transmission (start-stop transmission) 2.2.4 Time filling between frames
Time filling between frames is accomplished by sending continuous flags between frames (see 2.2.1). 2.2.5 Time filling in frames
2.2.5.1 Synchronous transmission
When using synchronous transmission, there is no provision for time filling in frames. 2.2.5.2 Start-stop transmission
In start-stop transmission, in a frame, the next octet group cannot be provided immediately after the previous octet group. This time filling sequence is sent when the transmission is continuous. The time filling between octets is accomplished by sending continuous signal states (logical "1" states) (see 2.2.3.2 above). There is no provision for the time filling in an octet (i.e., the time filling between the start and stop bits). 4
2.2.6 Link channel status
GB/T11595—1999
The link channel defined here refers to the transmission means in one direction. 2.2.6.1 Channel working status
When the DCE is receiving or sending a frame, a discard sequence or a burst time filling sequence (only for start-stop transmission), the input or output channel of the DCE is defined as being in operation. In working state. 2.2.6.2 Channel idle state
When DCE receives or sends continuous "1" state respectively within a period of time, the input or output channel of DCE is defined as being in idle state.
When the idle state appears on the input channel of DCE for too long, the action of DCE is specified in 2.3.5.5. 2.2.6.2.1 Synchronous transmission
For synchronous transmission, the channel idle state exists when the continuous "1" state lasts for at least 15 bits. 2.2.6.2.2 Start-stop transmission
For start-stop transmission, the channel idle state exists when the continuous "1" state lasts for at least ××X bits (the value of XXX must be greater than the reasonable frame time filling time). 2.2.7 Frame Structure
All SLP transmissions are in frames whose format shall conform to the format in Table 1 for basic (modulo 8) operation, Table 2 for extended (modulo 128) operation, and Table 3 for super (modulo 32768) operation. The marker preceding the address field is called the start marker. The marker following the FCS field is called the end marker. The format of these frames does not contain bits (synchronous transmission) or octets (asynchronous transmission) inserted for transparency (see 2.2.2), nor does it contain bits inserted for transmission timing (i.e., start bits or stop bits).
2.2.7.1 Address Field
The address field shall consist of one octet. It identifies the receiver of a command frame and the sender of a response frame. The encoding of this field is described in 2.4.2.
2.2.7.2 Control Field
For modulo 8 (basic) operation, the control field shall consist of one octet. For modulo 128 (extended) operation, the control field shall consist of two octets for the frame format with sequence numbers and one octet for the frame format without sequence numbers. For modulo 32768 (super) operation, the control field shall consist of four octets for the frame format with sequence numbers and one octet for the frame format without sequence numbers. See 2.3.2 for a description of the contents of this field. Table 1
Frame format one
Bit transmission order
Bit transmission order
FCSFrame check sequence
12345678
01111110
12345678
01111110
12345678
8 bits
123 45678
8 bits
Basic operation (modulo 8)
12345678
8 bits
12345678
8 bits
N bits
16 to 1
16 bits
16 to 1
16 bits
12345678|| tt||01111110
12345678
01111110
Bit transmission order
Bit transmission order
FCS frame check sequence
GB/T11595—1999
Table 2 Frame format
12345678
01111110|| tt||12345678
01111110
12345678
8 bits
12345678
8 bits
Extended operation (modulo 128)
bits\
bits*
a) A frame with a sequence number is 16 bits; a frame without a sequence number is 8 bits. Table 3 Frame format
Bit transmission order
Bit transmission order
FCS Frame check sequence
12345678
01111110
12345678
01111110
N bits
Super operation (modulo 32768)
12345678
8 bits
12345678
8 bits
a) A frame with a sequence number is 32 bits, and a frame without a sequence number is 8 bits. 2.2.7.3 Information field
bits*)
N bits
16 to 1
16 bits
16 to 1
16 bits
16 to 1
16 bits
16 to 1
16 bits
12345678
01111110||tt ||12345678
01111110
12345678
01111110
12345678
01111110
The information field (when present) is located after the control field (see 2.2.7.2 above) and before the frame check field (see 2.2.7.4 below).
When used for start-stop transmission, there should be eight information bits between the start and stop bits. When DCE sends to DTE, if the number of information bits to be inserted in the information field is not a multiple of eight, the DCE shall pad the information field with "0" to align the information field to octets. When DTE sends to DCE, the DTE shall only send information that is aligned to octets. For the encoding and combination of the various bits of the information field used in this standard, see 2.3.4.10, 2.5.2 and 5. For the maximum information field length, see 2.3.4.9 and 2.4.9.5 below. 2.2.7.4 Frame Check Sequence (FCS) Field
The notation used to describe FCS is based on the properties of cyclic codes, and a code vector such as 1000000100001 can be represented by the polynomial P(α) = z12 + z5 + 1. Thus, the elements of an n-ary codeword are the coefficients of an n-1 order polynomial. In this application, these coefficients can take the value of "0" or "1", and the polynomial operation is performed in modulo 2. The first bit received after the frame start marker is used as the coefficient of the highest order term to generate a polynomial representing the content of a frame. The FCS field should be a 16-bit bit sequence. It should be the inverse code of the sum of the following two terms (modulo 2): 1) *15+14+13+12+11+10+9+8++6+++3+2++1) is the remainder obtained by dividing the generating polynomial 16+12+1 (modulo 2), where k is the number of bits in the frame between the last bit of the start marker and the first bit of FCS (excluding these two bits), but does not include bits inserted for transparency (synchronous transmission) or octets (start-stop transmission) and for 1) The content of the frame between the last bit of the frame start mark and the first bit of the FCS (excluding these two bits) and its corresponding content, excluding the bits inserted for transparency (synchronous transmission) or the eight-bit group (start-stop transmission) and the bits inserted for transmission timing (i.e. 6
GB/T11595-1999
start bit or stop bit) multiplied by 16, and then (modulo 2) divided by the remainder obtained by the generator polynomial 16+12+5+1. The typical implementation method is: at the sending end, the initial content of the register of the device for calculating the division remainder is preset to all "1", and then the generator polynomial (introduced above) is used to remove the address field, control field and information field to change the content. The inverse of the remainder obtained is sent out as a 16-bit FCS.
At the receiving end, the initial content of the register of the remainder calculation device is preset to all "1", the protected bits and FCS of the serial input are multiplied by 21, and then (modulo 2) are divided by the generating polynomial 16 + 12 + 5 + 1. In the case of error-free transmission, the final remainder is 0001110100001111 (corresponding to 15 to 15). Note: Appendix H gives examples of bit patterns sent by DCE and DTE. These examples are for SABM commands and UA responses to illustrate the application of transparency mechanism and frame check.
2.3 Elements of LAPB procedure
2.3.1 LAPB procedure elements refer to the actions generated when DCE or DTE receives a frame. Next The elements of procedure specified below contain a selection of commands and responses related to the LAPB data link and system configuration described in 2.1 above. 2.2 and 2.3 together constitute the general requirements for the proper management of the LAPB access data link. 2.3.2 LAPB Control Field Formats and Parameters
2.3.2.1 Control Field Formats
The control field contains a command or response and a sequence number (when applicable). Three types of control fields may be used to implement numbered information transfer (I format), numbered supervisory functions (S format), and unnumbered control functions (U format).
The control field format for basic operations (modulo 8) is shown in Table 4. The control field format for extended operations (modulo 128) is shown in Table 5. The control field format for super operations (modulo 32768) is shown in Table 6. 2.3.2.1.1 Information Transfer Formats—1
The I format is used to complete information transfer. The functions of N(S), N(R) and P are independent of each other, that is, each I frame has an N(S), an N(R) and a P bit. DCE or DTE can use N(R) to confirm or not confirm the newly received I frame, and the P bit can be set to "0" or "1".
2.3.2.1.2 Monitoring format
The S format is used to complete the monitoring function of the data link, such as confirming I frame, requesting retransmission of I frame and requesting suspension of I frame transmission. The functions of N(R) and P/F bits are independent of each other, that is, each monitoring frame has an N(R) and a P/F bit. DCE or DTE can use N(R) to confirm or not confirm the newly received I frame, and the P/F bit can be set to "0" or "1". Table 4 LAPB control field format - Basic operation (modulo 8) 1
I format
S format
U format
Transmitter send sequence number (bit 2 = low-order bit) N(R) Transmitter receive sequence number (bit 6 = low-order bit) S Monitor function bit
M Modify function bit
P/F is a polling bit when sent as a command and a stop bit when sent as a response (1 = polling/stop) P Polling bit (1 = polling)
Control word Segment bits
1Format
sFormat
UFormat
GB/T11595—1999
5LAPB control field format
1st octet
N(S)Sender send sequence number (bit 2 = low-order bit)N(R)Sender receive sequence number (bit 10 = low-order bit)SMonitor function bit
MModify function bit
XReserve, set to "0"
Extended operation (modulo 1 28)
2nd octet
P/F is the poll bit when sent as a command and the stop bit when sent as a response (1=polling/stop)P Poll bit (1=polling)
Table 6 LAPB control field format
Bits of control field
I format
S format
U format
First 2 octets
N(S) Sender send sequence number (bit 2 = low order bit)N(R) Sender receive sequence Number (bit 18 = low order bit) S Monitor function bit
M Modify function bit
X Reserved, set to "0"
-Super operation (modulo 32768)
Last 2 octets
P/F is the polling bit when sent as a command and the stop bit when sent as a response (1-polling/stop) P Polling bit (1=polling)
2.3.2.1.3 Unnumbered format - U
The U format is used to provide additional data link control functions. This format has no sequence number, but contains a P/F bit that can be set to "0" or "1". The control field length (1 octet) of the unnumbered frame is the same for basic operation (modulo 8), extended operation (modulo 128), and super operation (modulo 32768).
2.3.2.2 Control field parameters
The various parameters related to the control field format are described below. 2.3.2.2.1 Modulus
Each I-frame is numbered in sequence, and the numbering can range from 0 to the modulus minus 1 ("modulus" is the sequence number modulus). This modulus is 8, 128, or 32768, and the sequence number cycles through the entire modulus range. 2.3.2.2.2 Transmit state variable V(S)
The transmit state variable V(S) represents the sequence number of the next I to be sent in sequence. V(S) can take any value from 0 to the modulus minus 1. The value of V(S) increases by 1 for each successive I frame sent, but its value over the last received information frame or monitoring frame N(R) cannot be greater than the maximum number of I frames to be acknowledged (s). The value will be specified in 2.4.9.6 below. 2.3.2.2.3 Transmit sequence number N(S)
Only I frames contain N(S), which is the transmit sequence number of the frame being sent. When sending a sequenced I frame, the value of N(S) must be equal to the value of the send state variable V(S).3 Sending sequence number N(S)
Only I frames contain N(S), which is the sending sequence number of the frame being sent. When sending an I frame in sequence, the value of N(S) must be equal to the value of the sending state variable V(S).3 Sending sequence number N(S)
Only I frames contain N(S), which is the sending sequence number of the frame being sent. When sending an I frame in sequence, the value of N(S) must be equal to the value of the sending state variable V(S).2 Monitoring format
S format is used to complete the monitoring functions of the data link, such as confirming I frame, requesting retransmission of I frame and requesting suspension of I frame transmission. The functions of N(R) and P/F bits are independent of each other, that is, each monitoring frame has an N(R) and a P/F bit. DCE or DTE can use N(R) to confirm or not confirm the newly received I frame, and the P/F bit can be set to "0\" or "1". Table 4 LAPB control field format - basic operation (modulo 8) 1
I format
S format
U format
Sender's transmission sequence number (bit 2 = low-order bit) N(R) Sender's reception sequence number (bit 6 = low-order bit) S Monitoring function bit
M Modify function bit
P/F is a polling bit when sent as a command and a stop bit when sent as a response (1 = polling/stop) P Polling bit (1 = polling )
Bits of the control field
1Format
sFormat
UFormat
GB/T11595—1999
5LAPB control field format
1st octet
N(S) Sender send sequence number (bit 2 = low-order bit)N(R) Sender receive sequence number (bit 10 = low-order bit)SMonitor function bit
MModify function bit
XReserve, set to "0"
Extended operation (modulo 128)
2nd octet
P/F is a polling bit when sent as a command and a stop bit when sent as a response1=polling/stop)PPoll bit (1=polling Table 6 LAPB control field format
Bits of the control field
I format
S format
U format
First 2 octets
N(S) Sender’s transmission sequence number (bit 2 = low-order bit)N(R) Sender’s reception sequence number (bit 18 = low-order bit)S Monitoring function bit
M Modify function bit
X Reserved, set to “0”
—Super operation (modulo 32768)
Last 2 octets
P/F Poll bit when sent as a command, stop bit when sent as a response (1—polling/stop)P Poll bit (1=polling)
2.3.2 .1.3 Unnumbered Format—U
The U format is used to provide additional data link control functionality. This format has no sequence number, but contains a P/F bit that can be set to either "0" or "1". The length of the control field (1 octet) for unnumbered frames is the same for basic operation (modulo 8), extended operation (modulo 128), and super operation (modulo 32768).
2.3.2.2 Control Field Parameters
The various parameters associated with the control field format are described below. 2.3.2.2.1 Modulus
Each I-frame is sequentially numbered from 0 to the modulus minus 1 (the "modulus" is the sequence number modulo). The modulus is 8, 128, or 32768, and the sequence number cycles through the modulus range. 2.3.2.2.2 Transmit state variable V(S)
The transmit state variable V(S) represents the sequence number of the next I frame to be sent in sequence. V(S) can take any value from 0 to modulus minus 1. The value of V(S) shall be increased by 1 for each successive I frame sent, but its value over the N(R) of the last received information frame or monitoring frame shall not be greater than the maximum number of I frames to be acknowledged (s). The value shall be specified in 2.4.9.6 below. 2.3.2.2.3 Transmit sequence number N(S)
Only I frames contain N(S), which is the transmit sequence number of the frame being sent. When a sequential I frame is to be sent, the value of N(S) shall be equal to the value of the transmit state variable V(S). 82 Monitoring format
S format is used to complete the monitoring functions of the data link, such as confirming I frame, requesting retransmission of I frame and requesting suspension of I frame transmission. The functions of N(R) and P/F bits are independent of each other, that is, each monitoring frame has an N(R) and a P/F bit. DCE or DTE can use N(R) to confirm or not confirm the newly received I frame, and the P/F bit can be set to "0\" or "1". Table 4 LAPB control field format - basic operation (modulo 8) 1
I format
S format
U format
Sender's transmission sequence number (bit 2 = low-order bit) N(R) Sender's reception sequence number (bit 6 = low-order bit) S Monitoring function bit
M Modify function bit
P/F is a polling bit when sent as a command and a stop bit when sent as a response (1 = polling/stop) P Polling bit (1 = polling )
Bits of the control field
1Format
sFormat
UFormat
GB/T11595—1999
5LAPB control field format
1st octet
N(S) Sender send sequence number (bit 2 = low-order bit)N(R) Sender receive sequence number (bit 10 = low-order bit)SMonitor function bit
MModify function bit
XReserve, set to "0"
Extended operation (modulo 128)
2nd octet
P/F is a polling bit when sent as a command and a stop bit when sent as a response1=polling/stop)PPoll bit (1=polling Table 6 LAPB control field format
Bits of the control field
I format
S format
U format
First 2 octets
N(S) Sender’s transmission sequence number (bit 2 = low-order bit)N(R) Sender’s reception sequence number (bit 18 = low-order bit)S Monitoring function bit
M Modify function bit
X Reserved, set to “0”
—Super operation (modulo 32768)
Last 2 octets
P/F Poll bit when sent as a command, stop bit when sent as a response (1—polling/stop)P Poll bit (1=polling)
2.3.2 .1.3 Unnumbered Format—U
The U format is used to provide additional data link control functionality. This format has no sequence number, but contains a P/F bit that can be set to either "0" or "1". The length of the control field (1 octet) for unnumbered frames is the same for basic operation (modulo 8), extended operation (modulo 128), and super operation (modulo 32768).
2.3.2.2 Control Field Parameters
The various parameters associated with the control field format are described below. 2.3.2.2.1 Modulus
Each I-frame is sequentially numbered from 0 to the modulus minus 1 (the "modulus" is the sequence number modulo). The modulus is 8, 128, or 32768, and the sequence number cycles through the modulus range. 2.3.2.2.2 Transmit state variable V(S)
The transmit state variable V(S) represents the sequence number of the next I frame to be sent in sequence. V(S) can take any value from 0 to modulus minus 1. The value of V(S) shall be increased by 1 for each successive I frame sent, but its value over the N(R) of the last received information frame or monitoring frame shall not be greater than the maximum number of I frames to be acknowledged (s). The value shall be specified in 2.4.9.6 below. 2.3.2.2.3 Transmit sequence number N(S)wwW.bzxz.Net
Only I frames contain N(S), which is the transmit sequence number of the frame being sent. When a sequential I frame is to be sent, the value of N(S) shall be equal to the value of the transmit state variable V(S). 8
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