GSM协议详解：初学者必备的DCE-DTE接口与SMS/CBS技术规范

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GSM协议（Global System for Mobile Communications）是一套全球广泛使用的数字蜂窝电话通信系统标准，特别适合初学者深入理解。ETSITS100585V7.0.1（1999-07）文档详细阐述了GSM Phase 2+阶段的协议规范，重点关注数据终端设备（DTE-DCE）接口在短消息服务（SMS）和细胞广播服务（CBS）中的应用。GSM 07.05版本7.0.1（1998年发布）是这一系列标准的重要组成部分。这份技术规范提供了关于如何在移动通信网络中实现短信和细胞广播功能的清晰指导，包括如何建立DTE（如手机）与DCE（如基站或交换机）之间的连接。它定义了数据传输的帧结构、编码方式、寻呼机制以及用户认证流程等关键要素，确保了在全球范围内的一致性和互操作性。关键词包括"数字蜂窝通信系统"、"全球移动通信系统（GSM）"和"欧洲电信标准化协会（ETSI）"，这些都是理解GSM协议核心概念的关键。文档提供了一个联系地址，以便用户获取最新版本或报告可能存在的错误，同时强调了版权规定，禁止未经许可的复制。对于希望进入移动通信行业或者从事相关技术研发的人来说，掌握GSM协议不仅限于理论学习，还需要通过实践操作来熟悉其实际应用。通过研究这份标准，开发者可以设计出兼容性强、性能稳定的通信设备和服务，推动无线通信技术的发展。此外，随着移动互联网的兴起，GSM协议后续的演进版本如GPRS、EDGE、UMTS等也值得进一步关注，它们都是基于GSM基础之上发展起来的新一代移动通信标准。

ETSI

ETSI TS 100 585 V7.0.1 (1999-07)11(GSM 07.05 version 7.0.1 Release 1998)

2.1.2 Returning from SMS/CBS Block Mode To Default Mode

When the terminal equipment wishes to return to default mode from SMS/CBS mode, it shall issue the command 'END

SMS MODE', described in subclause 2.4.1.11. The mobile termination shall respond with 'OK' (or 0) to indicate that the

DTE/DCE interface has returned to default mode. The TE shall change back to default mode whether or not such a

response is received.

The TE may also indicate that it has exit from the SMS/CBS mode through the use of CT 108/2 (see subclause 1.1)

If an incoming data call arrives while the DTE/DCE interface is set to SMS/CBS mode, then the mobile termination may

autonomously issue the 'END SMS MODE' indication (subclause 2.4.2.11) and revert to default mode in order to

connect the data call through the TAF.

The MT may exit from SMS/CBS mode autonomously if the power to the MT is switched off and then on again. In

addition, the MT manufacturer may provide MMI to change the mode back to the default mode. In the latter case, the

MT shall issue the 'END SMS MODE' indication (subclause 2.4.2.11) and exit the SMS/CBS mode immediately.

The MT may also indicate that it has exit from the SMS/CBS mode through the use of CT 107 and

CT 109 (see subclause 1.1).

A BREAK condition in either direction at the DTE/DCE interface shall cause the TE and the MT to exit from the

SMS/CBS block mode and return to the default mode.

In the event where the TE or the MT find themselves unable to recover from a protocol error then either entity may exit

the SMS/CBS mode using any of the mechanisms described above. Confirmation of default mode operation will be

achieved through the use of AT commands and responses.

2.2 Protocol description

The communication path between the MT and the TE across the DTE/DCE interface should be quite reliable if it uses a

short wire link. However, to ensure that the low error rate does not cause malfunction, the following error protection

scheme is provided.

Each message sent from the MT to the TE or vice-versa consists of a data block (DATA) and block check sum (BCS,

see figure 2.2.1). In the following description the notation DLE, STX, NUL and ETX refer to control characters having

the values 10 02 00 and 03 hexadecimal respectively.

<-----------------

DATA

-----------------> <-

BCS

DLE STX Message content DLE ETX BCS BCS

10H 02H 10H 03H MSB LSB

Figure 2.2.1/GSM 07.05: Format of DTE/DCE interface messages

The data block consists of a start transmission sequence, set to 00010000 00000010 (10 02 hex), the message content as

defined below and an end transmission sequence, set to 00010000 00000011 (10 03 hex). The least significant bit of

each octet is always transmitted first.

The block check sum is calculated at the transmitter by adding all of the octets in the message content modulo 65536.

Each bit of the 16-bit result is then inverted, and 1 is added to the answer.

During transmission of the message content and the BCS octets, any occurrence of the value 10 hex (DLE) shall result in

an additional 'stuffing' octet of value 00 hex (NUL) being transmitted immediately following the octet containing 10 hex.

This is to ensure that the start and end markers are unambiguous. The receiver shall remove stuffing octets by discarding

any octet of value 00 hex (NUL) which immediately follows an octet of value 10 hex (DLE).

After removal of any stuffing octets, the receiver can check the BCS by adding all of the octets in the message content

and the 16-bit BCS modulo 65536. The correct result is 0000 hex. If any message is received with an incorrect BCS,

then the message is discarded. No response is sent over the DTE/DCE interface, but an indication may be provided to

higher layers within the receiving entity.

ETSI

ETSI TS 100 585 V7.0.1 (1999-07)12(GSM 07.05 version 7.0.1 Release 1998)

The transmitter shall only send DLE when it is followed by STX, NUL or ETX. Therefore, if the receiver sees a DLE

followed by anything else then the receiver shall assume that some data has been lost, and shall start to search for the

start marker. An unexpected end marker at the receiver shall also result in a search for a start marker. A start marker

shall always be treated as the start of a new block, regardless of which state the receiver is in.

Examples of state diagrams for a block receiver to implement this procedure are given in Annex B, together with an

example of coding and decoding a message.

Only one Command/Response transaction shall be permitted at any one time from any sending or receiving entity. It

shall however be possible for a Command/Response transaction from one entity to be initiated even if there is a

Command/Response transaction in progress from the other entity.

If an immediate response is expected to a message sent over the DTE/DCE interface, then the sending entity shall wait

10 seconds. If no response is received within this time, the sending entity shall repeat the message. The message shall be

repeated a maximum of 3 times, after which the sending entity shall exit from the SMS/CBS mode and provide an error

indication to the user.

If a message cannot be understood by the receiving entity even though it has a correct BCS, then it shall return an

UNABLE TO PROCESS message with cause value 'Command not understood'. The receipt of an UNABLE TO

PROCESS message should not in itself initiate re-transmission although re-transmission may take place due to the

timeout mechanism described earlier since an UNABLE TO PROCESS is deemed to be an invalid response. The

‘Cause’ may however be referred to a higher layer. An UNABLE TO PROCESS shall not be sent as the result of an

incorrect BCS.

2.3 Requesting messages already held in the Mobile

Termination

The TE may request the MT to provide SMS or CBS messages already stored. The TE will either request all messages,

or request a list of messages and subsequently ask for specific messages.

At the start of the SMS/CBS mode session, the MT shall number all messages contiguously, starting with message

number 1. These "Short Message References" are only valid for a single SMS/CBS MODE session and should not be

confused with the GSM 03.40 TP-Message-Reference. Each message retains its Short Message Reference for the

duration of the SMS/CBS mode session. New messages will normally be given the lowest previously-unused Short

Message Reference. However, if all Short Message References have been used then the MT may reallocate Short

Message References previously allocated to now-deleted messages.

Short Message Reference 0 signifies that there are no messages in the MT. The value of 0 is used under the following

conditions:

- When an INSERT SMS command is used to transfer an SM over the air interface and not store it in the MT then

the MT will return a Short Message Reference of 0 in the REQUEST CONFIRMED response and the ensuing

INSERT SMS COMPLETE / INSERT SMS FAILURE indications.

- For Class 0 SM’s which are not stored in the MT

- For TE specific SM’s which are not stored in the MT

If Message number 0 is requested by the TE, the MT will always return an error cause, but will also include the highest

valid Short Message Reference (see subclause 2.3.2.1 below).

2.3.1 Requesting List Of Messages

The TE may request the MT to provide a list of SMS and CBS messages currently stored in the mobile termination.

This is achieved by the LIST REQUEST command (subclause 2.4.1.1). The MT divides the messages stored into groups

of 5 (called pages) and transfers the first 5 in a MESSAGE LIST response (subclause 2.4.2.1) containing message

references allocated by the MT, plus the relevant header information described in GSM 03.40/04.11 and GSM

03.41/04.12.

If there are no messages stored in the MT, then the MESSAGE LIST response shall be empty.

ETSI

ETSI TS 100 585 V7.0.1 (1999-07)13(GSM 07.05 version 7.0.1 Release 1998)

The TE may then request further groups of up to 5 messages by repeating the LIST REQUEST command for pages 2,3,

and so on. The MT will indicate that there are no more pages by responding with an empty MESSAGE LIST response.

2.3.2 Requesting Transfer Of Messages

The TE may request the transfer of one or more messages by means of the commands described below. The MT does

not delete messages which have been transferred. Messages can only be deleted by the DELETE MESSAGE command

(subclause 2.4.1.9).

2.3.2.1 Requesting Transfer Of A Specific Message

The TE may request the MT to transfer a specific message by sending the GET MESSAGE command (subclause

2.4.1.2), including the appropriate message reference. The MT will provide the full message including header in a

MESSAGE response (subclause 2.4.2.2). If the message reference is unallocated, then the GET MESSAGE FAILURE

response is returned with cause 'No such message' and the highest valid Message Reference (subclause 2.4.2.3).

2.3.2.2 Requesting Transfer Of All Messages

The TE may request the MT to transfer all messages by sending the GET FIRST MESSAGE command (subclause

2.4.1.3), followed by the appropriate number of GET NEXT MESSAGE commands (subclause 2.4.1.4).

The MT shall be able to transfer all messages one-by-one, starting with the 'first' and continuing with the 'next'. The

precise ordering of the messages is left to the MT implementation.

If the MT exits from SMS/CBS mode for any reason, then this information need not be retained.

On receipt of the GET FIRST MESSAGE command, the MT shall set a pointer to the first message, and transfer this

message using the MESSAGE response as described in subclause 2.3.2.1.

On receipt of the GET NEXT MESSAGE command, the MT shall move the pointer to the first available message after

the last message transferred (using either GET FIRST MESSAGE, GET MESSAGE or GET NEXT MESSAGE), and

transfer this message using the MESSAGE response as described in subclause 2.3.2.1.

If the MT receives a GET NEXT MESSAGE command when all messages have been transferred to the TE, or there are

no messages stored in the MT, then the GET MESSAGE FAILURE response shall be provided with the cause 'No such

message' (see subclause 2.4.2.3).

If the TE receives an out of sequence message then it shall attempt to transfer the missing message using the GET

MESSAGE command before continuing with GET NEXT MESSAGE. If this attempt fails with the cause 'no such

message', it means that the message has been deleted, or it has been lost due to a failure at the MT.

The MT includes a LAST SHORT MESSAGE REFERENCE in the GET MESSAGE FAILURE response. This is so

that the TE can detect whether or not the last short message was received in error.

If the MT receives a GET NEXT MESSAGE command prior to receiving a GET FIRST MESSAGE or GET

MESSAGE command, then it shall continue as if the command had been GET FIRST MESSAGE (i.e. provide the 'first'

message and continue with the 'next' on receipt of the subsequent GET NEXT MESSAGE command).

2.3.3 Requesting Diversion Of Incoming Messages

The TE may request the MT to transfer SMS or CBS messages directly from the air interface to the DTE/DCE interface,

by the following procedures. If messages are diverted then they are not stored in the MT. If messages are diverted and

there is no communication path to the TE (e.g. because it has been disconnected), the diversion shall be cancelled.

2.3.3.1 Requesting SMS Messages

The TE may request an indication of arrival of incoming SMS messages, or the direct transfer of incoming SMS

messages.

ETSI

ETSI TS 100 585 V7.0.1 (1999-07)14(GSM 07.05 version 7.0.1 Release 1998)

The TE requests new SMS messages by the TRANSFER INC SMS command (subclause 2.4.1.5). This command will

be sent with parameters indicating whether all incoming SMS messages are to be transferred, or only those indicated as

being for the TE.

The MT shall confirm receipt of this command with a REQUEST CONFIRMED message provided there is memory

available to store SM's in the ME or the SIM. If there is no memory available, the MT shall respond with 'unable to

process' with a cause value No memory.

The MT shall transfer incoming messages by the INC MESSAGE indication (subclause 2.4.2.4).

For an INC MESSAGE which contains a Short Message (SMS) info element id, the TE shall acknowledge receipt of the

INC MESSAGE with an ACKNOWLEDGE MESSAGE (subclause 2.4.1.12). The MT should not send another INC

MESSAGE which contains a Short Message (SMS) info element id to the TE whilst it is waiting for an

ACKNOWLEDGE MESSAGE.

In the event of the MT not receiving an ACKNOWLEDGE MESSAGE within a time specified by the MT manufacturer

the MT shall exit the SMS mode automatically after 'n' attempts to send the INC MESSAGE (where n is a number

specified by the MT manufacturer). The MT should attempt to store the unacknowledged SM or Status Report

(contained in the INC MESSAGE) in the MT or on the SIM as appropriate.

The ACKNOWLEDGE MESSAGE sent from the TE to the MT must not delay the MT sending the RP-ACK defined in

GSM 03.40 (to the SC) for longer than the RP-ACK timeout specified in GSM 04.08.

The TE requests the cessation of incoming message transfer by the same command, indicating no incoming messages.

The transfer of messages will automatically cease on exit of the SMS/CBS mode. Transfer shall not recommence until a

new request is issued by the TE.

2.3.3.2 Requesting CBS Messages

The TE may request the transfer of all cell broadcast messages directly from the air interface to the DTE/DCE interface.

This is achieved by the use of the TRANSFER INC CBS message (subclause 2.4.1.7).

The MT shall confirm receipt of this command with a REQUEST CONFIRMED message.

After receipt of this command, the MT shall transfer all CBS pages as they arrive on the air interface, using the INC

MESSAGE indication (subclause 2.4.2.4).

While the CBS pages are being transferred, any other indication or response required to be sent to the TE will take

precedence over the CBS pages. However, the MT shall not interrupt the transfer of a page to send other information

within the SMS/CBS mode (ie. the MT shall wait until a page boundary).

The transfer of messages will automatically cease on exit of the SMS/CBS mode. Transfer shall not recommence until a

new request is issued by the TE.

2.3.3.3 Requesting indication of message arrival

If the TE requires an indication of incoming message arrival, the INDICATE INC SMS command (subclause 2.4.1.6)

shall be used.

The MT shall confirm receipt of this command with a REQUEST CONFIRMED message.

After receipt of this command, the MT shall indicate all incoming messages in the specified categories (unless they are

directly transferred) with the MESSAGE ARRIVED indication (subclause 2.4.2.5). This indication shall be of the same

format as the MESSAGE LIST response described in subclause 2.3.1.

The TE shall acknowledge receipt of the MESSAGE ARRIVED with an ACKNOWLEDGE MESSAGE. (subclause

2.4.1.12). The MT should not send another MESSAGE ARRIVED to the TE whilst it is waiting for an

ACKNOWLEDGE MESSAGE.

In the event of the MT not receiving an ACKNOWLEDGE MESSAGE within a time specified by the MT manufacturer

the MT shall exit the SMS mode automatically after ‘n’ attempts to send the MESSAGE ARRIVED (where n is a

number specified by the MT manufacturer). The MT should attempt to store the unacknowledged SM or Status Report

in the MT or on the SIM as appropriate.

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