ITU-T X.25：公共数据网络接口标准

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"ITU-T X.25是国际电信联盟（ITU）电信标准化部门发布的一份推荐标准，主要用于规范公共数据网络中数据终端设备（DTE）与数据电路终端设备（DCE）之间的接口，适用于通过专用电路连接到公共数据网络的分组模式终端。这个标准在1996年进行了更新，并被纳入ITU-T的X系列推荐标准中，该系列涵盖了数据网络和开放系统通信的多个方面，包括服务、接口、传输、信令、切换、网络管理、维护和行政安排等。" ITU-T X.25标准详细规定了数据通信网络中的分组交换技术，它是早期广域网（WAN）应用中广泛使用的协议。该协议允许远程终端通过公用数据网络进行通信，提供了一种可靠的、面向连接的服务，支持错误检测和纠正、流量控制以及网络拥塞管理。在X.25的框架下，DTE（如计算机）和DCE（如调制解调器或网络接口卡）之间的接口定义了物理层和数据链路层的协议。物理层通常遵循EIA/TIA-232或V.24等接口标准，而数据链路层则由X.25协议自身定义，它包括三个子层：LAP-B（Link Access Procedure Balanced），LLC（Logical Link Control）和PDU（Packet Data Units）。LAP-B处理帧的传输，LLC提供服务接口到上层协议，PDU则是数据传输的基本单元。 X.25协议提供了以下主要功能： 1. **错误控制**：通过差错检测和纠正机制，确保数据的正确传输。 2. **流量控制**：通过窗口机制控制发送方的数据速率，避免网络拥塞。 3. **网络服务**：提供了虚电路（VC）服务，每个虚电路对应一个独立的数据流，确保数据按照发送顺序到达。 4. **呼叫控制**：定义了建立、修改和释放虚电路的规程。 5. **多路复用**：允许多个虚电路同时存在于同一物理连接上。此外，X.25还支持动态带宽分配，可以根据网络条件和需求调整每个虚电路的带宽。然而，随着TCP/IP的普及，X.25的使用逐渐减少，因为它相对复杂且效率较低。尽管如此，X.25在某些特定领域，如工业控制系统和远程终端连接，仍然有其应用价值。 X.25标准的完整系列还包括其他相关推荐，如X.21定义了DCE与DTE之间的物理接口，X.29定义了控制和管理信息的交换，X.31则涉及拨号访问服务。这些推荐共同构成了公共数据网络及其接口的一个全面规范体系。

8 Recommendation X.25 (10/96)

TABLE 2-1/X.25

Frame formats – Basic (modulo 8) operation

TABLE 2-2/X.25

Frame formats – Extended (modulo 128) operation

TABLE 2-3/X.25

Frame formats – Super (modulo 32 768) operation

Bit order of transmission 12345678 12345678 1 to

16 to 1 12345678

Flag Address Control FCS Flag

01111110

8 bits

bits

FCS

16 bits

01111110

Bit order of transmission 12345678 12345678 1 to

16 to 1 12345678

Flag Address Control Information FCS Flag

01111110

8 bits

bits

Info

N bits

FCS

16 bits

01111110

FCS Frame Check Sequence

32 for frame formats that contain sequence numbers; 8 for frame formats that do not contain sequence numbers.

Bit order of transmission 12345678 12345678 12345678 16 to 1 12345678

Flag Address Control FCS Flag

01111110

8 bits

FCS

16 bits

01111110

Bit order of transmission 12345678 12345678 12345678 16 to 1 12345678

Flag Address Control Information FCS Flag

01111110

8 bits

Info

N bits

FCS

16 bits

01111110

FCS Frame Check Sequence

Bit order of transmission 12345678 12345678 1 to

16 to 1 12345678

Flag Address Control FCS Flag

01111110

8 bits

bits

FCS

16 bits

01111110

Bit order of transmission 12345678 12345678 1 to

16 to 1 12345678

Flag Address Control Information FCS Flag

01111110

8 bits

bits

Info

N bits

FCS

16 bits

01111110

FCS Frame Check Sequence

16 for frame formats that contain sequence numbers; 8 for frame formats that do not contain sequence numbers.

Recommendation X.25 (10/96) 9

2.2.7.3 Information field

The information field of a frame, when present, follows the control field (see 2.2.7.2 above) and precedes the frame

check sequence field (see 2.2.7.4 below).

For start/stop transmission there shall be eight (8) information bits between the start bit and the stop bit.

When transmitting from the DCE to the DTE, if the information to be inserted in the information field does not have a

number of bits that is multiple of 8, the DCE shall pad this information field with zeros so that the information field will

be octet-aligned.

When transmitting from the DTE to the DCE, the DTE shall transmit only octet-aligned information.

See 2.3.4.9, 2.5.2 and clause 5 for the various codings and groupings of bits in the information field as used in this

Recommendation.

See 2.3.4.9 and 2.4.9.5 below with regard to the maximum information field length.

2.2.7.4 Frame Check Sequence (FCS) field

The notation used to describe the FCS is based on the property of cyclic codes that a code vector such as

1000000100001 can be represented by a polynomial P(x)

1. The elements of an n-element code word are

thus the coefficients of a polynomial of order n – 1. In this application, these coefficients can have the value 0 or 1 and

the polynomial operations are performed modulo 2. The polynomial representing the content of a frame is generated

using the first bit received after the frame opening flag as the coefficient of the highest order term.

The FCS field shall be a 16-bit sequence. It shall be the ones complement of the sum (modulo 2) of:

1) the remainder of x

+ x

divided (modulo 2) by the generator polynomial x

1, where k is the number of bits in the

frame existing between, but not including, the final bit of the opening flag and the first bit of the FCS,

excluding bits (synchronous transmission) or octets (start/stop transmission) inserted for transparency,

and bits inserted for transmission timing (i.e. start or stop bits); and

2) the remainder of the division (modulo 2) by the generator polynomial x

1 of the product of

by the content of the frame, existing between but not including, the final bit of the opening flag and

the first bit of the FCS, excluding bits (synchronous transmission) or octets (start/stop transmission)

inserted for transparency and bits inserted for transmission timing (i.e. start or stop bits).

As a typical implementation, at the transmitter, the initial content of the register of the device computing the remainder

of the division is preset to all 1s and is then modified by division by the generator polynomial (as described above) on

the address, control and information fields; the ones complement of the resulting remainder is transmitted as the 16-bit

FCS.

At the receiver, the initial content of the register of the device computing the remainder is preset to all 1s. The final

remainder, after multiplication by x

and then division (modulo 2) by the generator polynomial x

1 of the

serial incoming protected bits and the FCS, will be 0001110100001111 (x

through x

, respectively) in the absence of

transmission errors.

NOTE – Examples of transmitted bit patterns by the DCE and the DTE illustrating application of the transparency

mechanism and the frame check sequence to the SABM command and the UA response are given in Appendix I.

2.3 LAPB elements of procedures

2.3.1 The LAPB elements of procedures are defined in terms of actions that occur on receipt of frames at the DCE

or DTE.

The elements of procedures specified below contain the selection of commands and responses relevant to the LAPB data

link and system configurations described in 2.1 above. Together, 2.2 and 2.3 form the general requirements for the

proper management of an LAPB access data link.

10 Recommendation X.25 (10/96)

2.3.2 LAPB control field formats and parameters

2.3.2.1 Control field formats

The control field contains a command or a response, and sequence numbers where applicable.

Three types of control field formats are used to perform numbered information transfer (I format), numbered supervisory

functions (S format) and unnumbered control functions (U format).

The control field formats for basic (modulo 8) operation are depicted in Table 2-4.

The control field formats for extended (modulo 128) operation are depicted in Table 2-5.

The control field formats for super (modulo 32 768) operation are depicted in Table 2-6.

2.3.2.1.1 Information transfer format I

The I format is used to perform an information transfer. The functions of N(S), N(R) and P are independent; i.e. each

I frame has an N(S), an N(R) which may or may not acknowledge additional I frames received by the DCE or DTE, and

a P bit that may be set to 0 or 1.

2.3.2.1.2 Supervisory format S

The S format is used to perform data link supervisory control functions such as acknowledge I frames, request

retransmission of I frames, and to request a temporary suspension of transmission of I frames. The functions of N(R) and

P/F are independent; i.e. each supervisory frame has an N(R) which may or may not acknowledge additional I frames

received by the DCE or DTE, and a P/F bit that may be set to 0 or 1.

2.3.2.1.3 Unnumbered format U

The U format is used to provide additional data link control functions. This format contains no sequence numbers, but

does include a P/F bit that may be set to 0 or 1. The unnumbered frames have the same control field length (one octet) in

basic (modulo 8) operation, extended (modulo 128) operation, and super (modulo 32 768) operation.

2.3.2.2 Control field parameters

The various parameters associated with the control field formats are described below.

2.3.2.2.1 Modulus

Each I frame is sequentially numbered and may have the value 0 through modulus minus 1 (where “modulus” is the

modulus of the sequence numbers). The modulus equals either 8, 128 or 32 768 and the sequence numbers cycle through

the entire range.

2.3.2.2.2 Send state variable V(S)

The send state variable V(S) denotes the sequence number of the next in-sequence I frame to be transmitted. V(S) can

take on the values 0 through modulus minus 1. The value of V(S) is incremented by 1 with each successive I frame

transmission, but cannot exceed the N(R) of the last received I or supervisory frame by more than the maximum number

of outstanding I frames (k). The value of k is defined in 2.4.9.6 below.

2.3.2.2.3 Send sequence number N(S)

Only I frames contain N(S), the send sequence number of transmitted I frames. At the time that an in-sequence I frame is

designated for transmission, the value of N(S) is set equal to the value of the send state variable V(S).

2.3.2.2.4 Receive state variable V(R)

The receive state variable V(R) denotes the sequence number of the next in-sequence I frame expected to be received.

V(R) can take on the values 0 through modulus minus 1. The value of V(R) is incremented by 1 by the receipt of an

error-free, in-sequence I frame whose send sequence number N(S) equals the receive state variable V(R).

12 Recommendation X.25 (10/96)

TABLE 2-6/X.25

LAPB control field formats – Super (modulo 32 768) operation

2.3.2.2.5 Receive sequence number N(R)

All I frames and supervisory frames, except SREJ frames with F bit set to 0, shall contain N(R), the expected send

sequence number of the next received I frame. At the time that a frame of the above types is designated for transmission,

the value of N(R) is set equal to the current value of the receive state variable V(R). N(R) indicates that the DCE or DTE

transmitting the N(R) has received correctly all I frames numbered up to and including N(R) – 1.

2.3.2.2.6 Poll/Final bit P/F

All frames contain P/F, the Poll/Final bit. In command frames, the P/F bit is referred to as the P bit. In response frames,

it is referred to as the F bit.

2.3.3 Functions of the Poll/Final bit

The Poll bit set to 1 is used by the DCE or DTE to solicit (poll) a response from the DTE or DCE, respectively. The

Final bit set to 1 is used by the DCE or DTE to indicate the response frame transmitted by the DTE or DCE,

respectively, as a result of the soliciting (poll) command.

The use of the P/F bit is described in 2.4.3 below.

2.3.4 Commands and responses

For basic (modulo 8) operation, the commands and responses represented in Table 2-7 will be supported by the DCE and

the DTE.

For extended (modulo 128) operation, the commands and responses represented in Table 2-8 will be supported by the

DCE and the DTE.

For super (modulo 32 768) operation, the commands and responses represented in Table 2-9 will be supported by the

DCE and the DTE.

For purposes of the LAPB procedures, those encodings of the modifier function bits in Tables 2-4, 2-5 and 2-6 not

identified in Tables 2-7, 2-8 and 2-9 are identified as “undefined or not implemented” command and response control

fields.

The commands and responses in Tables 2-7, 2-8 and 2-9 are defined as follows:

First two octets Next two octets

Control field bits 1 2.... 16 17 18.... 32

I format 0 N(S) P N(R)

S format 1 0 S S X X X X ... X P/F N(R)

U format 1 1 M M P/F M M M

N(S) Transmitter send sequence number (bit 2 = low-order bit)

N(R) Transmitter receive sequence number (bit 18 = low-order bit)

S Supervisory function bit

M Modifier function bit

X Reserved and set to 0

P/F Poll bit when issued as a command, final bit when issued as a response (1 = Poll/Final)

P Poll bit (1 = Poll)

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