Sunsea AIoT SIMCom SIM800 Series Installation and operating instructions

SIM800 Series_Multiplexer
_Application Note
SIMCom Wireless Solutions Limited
Building B, SIM Technology Building, No.633, Jinzhong Road
Changning District, Shanghai P.R. China
Tel: [phone]
[e-mail]
www.simcom.com
GPRS Module

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Document Title:
SIM800 Series_Multiplexer_Application Note
Version:
1.04
Date:
2020.06.15
Status:
Released
GENERAL NOTES
SIMCOM OFFERS THIS INFORMATION AS A SERVICE TO ITS CUSTOMERS, TO SUPPORT
APPLICATION AND ENGINEERING EFFORTS THAT USE THE PRODUCTS DESIGNED BY SIMCOM.
THE INFORMATION PROVIDED IS BASED UPON REQUIREMENTS SPECIFICALLY PROVIDED TO
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FOR ADDITIONAL RELEVANT INFORMATION, INCLUDING ANY INFORMATION THAT MAY BE IN THE
CUSTOMER’S POSSESSION. FURTHERMORE, SYSTEM VALIDATION OF THIS PRODUCT
DESIGNED BY SIMCOM WITHIN A LARGER ELECTRONIC SYSTEM REMAINS THE RESPONSIBILITY
OF THE CUSTOMER OR THE CUSTOMER’S SYSTEM INTEGRATOR. ALL SPECIFICATIONS
SUPPLIED HEREIN ARE SUBJECT TO CHANGE.
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THIS DOCUMENT CONTAINS PROPRIETARY TECHNICAL INFORMATION WHICH IS THE PROPERTY
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ARE FORBIDDEN WITHOUT EXPRESS AUTHORITY BY SIMCOM. OFFENDERS ARE LIABLE TO THE
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CHANGE WITHOUT NOTICE AT ANY TIME.
SIMCom Wireless Solutions Limited
Building B, SIM Technology Building, No.633 Jinzhong Road, Changning District, Shanghai P.R. China
Tel: [phone]
Email: [e-mail]
For more information, please visit:
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For technical support, or to report documentation errors, please visit:
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Copyright © 2020 SIMCom Wireless Solutions Limited All Rights Reserved.

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About Document
Version History
Version
Date
Owner
What is new
1.00 2013-10-28 Ming.yang New version
1.01 2014-06-30 Haibing.chen Chapter Scope, Add projects
Chapter 2.6,Add note
1.02 2016-11-17 Wenjie.lai Scope
1.03 2019-08-12 Yulong.zheng Modify format of UA frame
1.04 2020-06-15 Yizhe.Tan
/Wenjie.Lai
Change the style
Scope
This document presents the AT command of multiplexer operation and application examples.
This document can apply to SIM800 series modules with multiplexer function.

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Contents
About Document ..................................................................................................................................3
Version History ......................................................................................................................................3
Scope ....................................................................................................................................................3
Contents................................................................................................................................................4
1Introduction ...................................................................................................................................6
1.1 Purpose of the document............................................................................................................6
1.2 Related documents.....................................................................................................................6
1.3 Conventions and abbreviations...................................................................................................6
2Multiplexer Introduction ...............................................................................................................8
2.1 Multiplexer Design Purpose........................................................................................................8
2.2 Architecture Diagram ..................................................................................................................8
2.3 Restrictions.................................................................................................................................9
3SIMCOM Multiplexer Protocol....................................................................................................10
3.1 Transmission Frame Structures................................................................................................10
3.1.1 Opening and Closing Flag Field.....................................................................................10
3.1.2 Address Flag Field .........................................................................................................10
3.1.3 Address Flag Field ......................................................................................................... 11
3.1.4 Length Field ................................................................................................................... 11
3.1.5 Information Field ............................................................................................................12
3.2 Frame Type...............................................................................................................................12
3.2.1 SABM.............................................................................................................................12
3.2.2 UA..................................................................................................................................12
3.2.3 DISC...............................................................................................................................13
3.2.4 DM..................................................................................................................................13
3.2.5 UIH.................................................................................................................................13
3.2.6 UI ...................................................................................................................................13
3.3 DLC Establishment...................................................................................................................13
3.4 Closing Down DLC....................................................................................................................14
3.5 Control Channel........................................................................................................................14
3.6.1 PSC................................................................................................................................15
3.6.2 CLD................................................................................................................................15
3.6.3 Test.................................................................................................................................15
3.6.4 MSC...............................................................................................................................16
3.6.5 FCoff ..............................................................................................................................16
3.6.6 FCon ..............................................................................................................................17
3.6.7 PN,NSC,RPN,RLS,SNC.................................................................................................17

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3.7 Data Channel............................................................................................................................17
3.8 About Flow Control ...................................................................................................................17
3.9 Samples for Frame Structure....................................................................................................18
3.10 Transmission Bit Sequence ......................................................................................................19
4MUX Examples.............................................................................................................................20
4.1 Establish Channels...................................................................................................................20
4.2 Frame Transmission .................................................................................................................22
4.3 Power Saving Mode and Wake Up ...........................................................................................22
4.4 Flow Control..............................................................................................................................23
4.5 Dealing with the Wrong Frame..................................................................................................24
4.6 Closing down Multiplexers ........................................................................................................24

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1 Introduction
1.1 Purpose of the document
Based on module AT command manual, this document will introduce SIMCom multiplexer protocol and the
technical details of how to make use of it.
Developers could understand and develop application quickly and efficiently based on this document.
1.2 Related documents
[1] SIM800 Series AT Command Manual
[2] Digital cellular telecommunications system (Phase 2+)
[3] Terminal Equipment to Mobile Station (TE-MS) multiplexer protocol (GSM 07.10 version 7.1.0 Release
1998)
1.3 Conventions and abbreviations
Abbreviation
Description
DLC Data Link Connection
DLCI Data Link Connection Identifier
RLS Remote Line Status Command
SABM Set Asynchronous Balanced Mode
UA Unnumbered Acknowledgement
DM Disconnected Mode
DISC Disconnect (DISC) command
UIH
Unnumbered information with header check (UIH) command and
response

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UI Unnumbered Information command and response
PSC Power Saving Control
CLD Multiplexer Close Down
MSC Modem Status Command
TE Terminal Equipment
MS Mobile Station
FC Flow Control
RTC Ready To Communicate
RTR Ready To Receive
IC Incoming Call Indicator
DV Data Valid
PN Parameter Negotiation
FCon Flow Control On Command
FCoff Flow Control Off Command
NSC Non Support Command
RPN Remote Port Negotiation
RLS Remote Line Status Command
SNC Service Negotiation Command
TE Terminal Equipment
MS Mobile Station

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2 Multiplexer Introduction
2.1 Multiplexer Design Purpose
A device using GPRS or GSM data may wish to receive and transmit multiple streams of data
simultaneously. These are Command data (AT commands), GPRS data and GSM circuit switched data
(CSD). These streams are essentially independent to one another.
As to the non-multiplexer device, it is so inefficient to deal with only one kind or one channel of data steam
during a period of time. Therefore, SIMCom multiplexer is designed with GSM0710 standard to separate
transmission device layer into several logic channels (DLC) in order to transmit data simultaneously. Each
channel has its own buffer management and flow control mechanism.
2.2 Architecture Diagram
SIMCOM multiplexer architecture diagram is as following：
SIMCom Multiplexer
Transmission Device Layer (Serial Port)
D
L
C
1
D
L
C
2
D
L
C
3
D
L
C
4
Client Application
Multiplexed
data
streams

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SIMCOM Multiplexer is established upon system transmission device layer (Commonly serial port). Data
streams are addressed with DLCI value and encapsulated in frames based on GSM 0710 protocol (Chapter
2, SIMCOM Multiplexer Protocol) and transmitted through interface provided by transmission device layer.
2.3 Restrictions
Error Recovery Mode is not supported
PN, NSC, RPN, RLS, SNC message frames are not supported,
All the system parameters defined in GSM 0710 are set to default as following table
Parameter Value Comment
T1
(Acknowledgement Timer) 100 milliseconds Time that a station will wait for an acknowledgement before resorting to other
action
N1
(Maximum Frame Size)
255 Maximum number of octets that that may
be contained in an information field
N2
(Maximum number of
retransmissions)
3 Not used
T2
(Response Timer for multiplexer
control channel)
300 milliseconds Not used
T3
(Response Timer for wake-up
procedure)
10 seconds Amount of time the transmitting station of a
power wake-up command waits before
raising an alarm when no response is
received
K
(Window Size)
N/A Not used
UI Frames are not supported
Only supports GSM 0710 Basic Option

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3 SIMCOM Multiplexer Protocol
SIMCom Multiplexer protocol provides a data transmission mechanism by establishing DLC between TE
and MS. Several DLC can be set up. Each one is independent to one another and has its own management
of buffer and flow control. All information transmitted between the TE and MS is conveyed in frames.
3.1 Transmission Frame Structures
The frame structure is composed of an opening and a closing flag, an address field, a control field, a Length
field, an information field and FCS field. Please see following table.
Opening
Flag
Address
Field
Control
Field
Length
Field
Information
Field
FCS
Field
Closing
Flag
1 byte 1 byte 1 byte 2 byte Multi-byte 1 byte 1 byte
3.1.1 Opening and Closing Flag Field
Each frame begins and ends with a flag sequence octet which is defined as a constant bit pattern 0xF9.
3.1.2 Address Flag Field
The address field consists of a single octet. It contains the Data Link Connection Identifier (DLCI), the C/R
bit and the address field extension bit (EA) as following table.
Bit
1 2 3 4 5 6 7 8
EA CR DLCI DLCI DLCI DLCI DLCI DLCI
The range of the address field may be extended by use of the EA bit. When the EA bit is set to 1 in an octet,
it signifies that this octet is the last octet of the address field. When the EA bit is set to 0, it signifies that
another octet of the address field follows. SIMCom multiplexer only supports one address octet so the EA
bit is always set to 1.

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The C/R (command/response) bit identifies the frame as either a command or a response.
The DLCI is used to identify an individual data stream as well as channels between TE and MS. Multiple
DLCIs shall be supported but the number is implementation-specific. The DLCIs are dynamically assigned.
3.1.3 Address Flag Field
The content of the control field defines the type of frame. The control fields of the frames used in the present
document are described in the following table.
Bit HEX[1] Frame Type Comment
1 2 3 4 5 6 7 8
1 1 1 1 P/F 1 0 0 0x2F SABM Set Asynchronous Balanced Mode
1 1 0 0 P/F 1 1 0 0x63 UA Unnumbered Acknowledgement
1 1 1 1 P/F 0 0 0 0x0F DM Disconnected Mode
1 1 0 0 P/F 0 1 0 0x43 DISC Disconnect
1 1 1 1 P/F 1 1 1 0xEF UIH Unnumbered Information with Header check
1 1 0 0 P/F 0 0 0 0x03 UI Unnumbered Information (Not supported)
Note
：
Hex value does not count the bit 5 value.
3.1.4 Length Field
This field is present only in case when basic option is activated.
Bit
1 2 3 4 5 6 7 8
EA L1 L2 L3 L4 L5 L6 L7
The L1 to L7 bits indicates the length of the following data field.
The range of the length field may be extended by use of the EA bit. When the EA bit is set to 1 in an octet, it
is signifies that this octet is the last octet of the length field. When the EA bit is set to 0, it signifies that a
second octet of the length field follows. SIMCom multiplexer only supports two length octets so the EA bit is
always set to 0.
Note
：
Length field should always be contained in each frame even though information field is empty.

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3.1.5 Information Field
The information field is the payload of frame and carries the user data information (e.g. AT Command and
PPP data packet). The field is octet structured. The information field is only present in UIH frames.
3.2 Frame Type
3.2.1 SABM
SABM is command frame and shall be used to establish DLC between TE and MS. Please refer to 3.1 for
more details.
3.2.2 UA
UAframe is the response to SABM or DISC frame. Please see following diagram and refer to 3.1 and 3.6 for
more details.
MS
SIMCom
Multiplex
er
TE
Client
Receiver
SABM (Set up DLC 1)
UA (Response)
DISC (Close DLC 1)
UA (Response)

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3.2.3 DISC
DISC is command frame and shall be used to close down DLC. Prior to acting the command, the receiving
station shall confirm the acceptance of the DISC command by the transmission of a UA response. Please
see the diagram above and refer to 3.6 for more details.
3.2.4 DM
The DM response frame shall be used to report a status whether the station is logically disconnected from
the data link. When in disconnected mode no commands are accepted until the disconnected mode is
terminated by the receipt of a SABM command. If a DISC command is received while in disconnected mode
a DM response should be sent. Please refer to 3.1 for more details.
3.2.5 UIH
The UIH command/response shall be used to send user data at either station. Please refer to 3.2 for more
details.
3.2.6 UI
Not support.
3.3 DLC Establishment
The establishment of a DLC will be initiated by the TE.
TE wishing to establish a DLC transmits a SABM frame with the P-bit set to 1. The address field contains
the DLCI value associated with the desired connection. If MS is ready to establish the connection it will reply
with a UA frame with the F-bit set to 1. If MS is not ready or unwilling to establish the particular DLC it will

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reply with a DM frame with the F-bit set to 1. Please refer to 3.1 for more details.
3.4 Closing Down DLC
The release of a DLC will be initiated from by the transmission of a DISC frame with the P-bit set to 1.
Confirmation of the DLC release is signaled by MS sending a UA frame with the F-bit set to 1. Once the
DLC has been released the MS enter disconnected mode for that particular DLC. If MS receiving the DISC
command is already in a disconnected mode it will send a DM response. Please refer to 3.6 for more
details.
3.5 Control Channel
Multiplexer control channel is the basic channel which is used to establish DLC, launch power saving, wake
up from power saving and implement flow control mechanism.
Control channel is the first channel established at the initiation of the multiplexer between the TE and MS
and it has the DLCI value 0.
UIH message frame is transmitted through control channel. All UIH message frame conform to the following
format.
Type
Length
Value 1
Value 2
……
Value n
Each box in the table represents a field of minimum size one octet.
The first type field octet has the following format:
1
2
3
4
5
6
7
8
EA
C/R
T1
T2
T3
T4
T5
T6
The EA bit is an extension bit. It is set to 1 in the last octet of the sequence. In other octets EA is set to 0.
SIMCom multiplexer only supports one octet is transmitted. So EA is always set to 1.
The C/R bit indicates whether the message is a command or a response.
The T bits indicate the type coding. Each command has a unique pattern of bit sequence. This means that a
single-octet type field can encode 63 different message types. Only single octet message types are defined
in the present document. Please refer 2.5.1 to 2.5.6 for more details.
The length field octet has the following structure:

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3.6.1 PSC
Message type coding octet:
1
2
3
4
5
6
7
8
EA
C/R
0
0
0
0
1
0
Hex value is 0x43(Command), 0x41(Response)
The EA bit is always set to 1.
The length field in PSC message frame is 0. It has no value octet.
3.6.2 CLD
Message type coding octet:
1
2
3
4
5
6
7
8
EA
C/R
0
0
0
0
1
1
Hex value is 0xC3 (Command), 0xC1 (Response)
The EA bit is always set the 1.
The length field in CLD message frame is 0. It has no value octet.
3.6.3 Test
Message type coding octet:
1
2
3
4
5
6
7
8
EA C/R 0 0 0 1 0 0
Hex value is 0x23 (Command), 0x21 (Response).
The EA bit is always set the 1.
The test command is used to test the connection between MS and the TE. The length byte describes the
number of value bytes, which are used as a verification pattern. The opposite entity shall respond with
exactly the same value bytes.

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3.6.4 MSC
MSC message frame is designed to convey virtual V.24 control signals. It has one mandatory control signal
byte and an optional break signal byte.
MSC shall be sent prior to any user data after a creation of a DLC.
Message format is:
Type Length DLCI V.24 control signals Break signals (Optional)
Message type coding octet:
1
2
3
4
5
6
7
8
EA C/R 0 0 0 1 1 1
Hex value is 0xE3 (Command), 0xE1 (Response).
The EA bit is always set the 1.
The C/R bit is used to indicate if it is a Modem Status Command or Modem Status Response.
In a Modem Status Command it is the status of the sender’s own V.24 signals that shall be sent, but in a
Response it is copy of the V.24 signals that are received from the Command frame that shall be returned.
The DLCI field identifies the specific DLC to which the command applies. EA bit are always set to 1.
V.24 control signals format is:
1
2
3
4
5
6
7
8
EA FC RTC RTR reserved(0) reserved (0) IC DV
Break signals is set to 0x01.
3.6.5 FCoff
Message type coding octet is:
1
2
3
4
5
6
7
8
EA C/R 0 0 0 1 1 0
Hex value is 0x63 (Command), 0x61 (Response).
The length byte contains the value 0 and there are no value octets.

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3.6.6 FCon
Message type coding octet is：
1
2
3
4
5
6
7
8
EA C/R 0 0 0 1 0 1
Hex value is 0xA3 (Command), 0xA1 (Response).
The length byte contains the value 0 and there are no value octets.
3.6.7 PN,NSC,RPN,RLS,SNC
Not Supported.
3.7 Data Channel
SIMCOM multiplexer data channels shall be used to transmit user data streams such as AT command data,
GPRS data and GSM CSD data streams.
Data channels shall be established after and only after control channel (DLCI 0) connected.
Please refer to 3.1 for more details.
If the customer wants to transmit large data streams such as PPP, the DLCI 1 is recommend strongly
because the buffer of DLCI 1 (16K Byte) is more than other channel (1K Byte).
3.8 About Flow Control
SIMCOM multiplexer supports software flow control and can not perform hardware flow control mechanism.
Software flow control is implemented by GSM 0710 MSC, FCoff and FCon message frame.
MS will send MSC message to TE with FC bit set to 1 in V.24 control signals when refuse to accept frames.
NOTE

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Whereas, set to 0 to inform recovery of receiving frames.
TE will send MSC message to MS with FC bit set to 1 in V.24 control signals when refuses to accept frames.
Whereas, set to 0 to inform recovery of receiving frames. When receiving MSC, MS will feed back MSC
response to indicate recover data transmission.
TE also can send Fcoff message to MS when refuses accept anything except control messages on DLC 0.
After this, MS will stop sending any frames through all the data channels except control channels. Control
channel is still alive and free to send any control message. Whereas, sends FCon to recover transmission.
When receiving Fcoff or FCon message, MS will feed back Fcoff or FCon response.
The difference between MSC and Fcon, Fcoff is that the former only flow controls one of the data channels,
and the latter controls all the data channels except controls channel.
Please refer to 3.4 for more details.
3.9 Samples for Frame Structure
Sample 1：
F9
03
3F
01
1C
F9
Opening Flag
Address Field
Control Field
Length Field
FCS
Closing Flag
Header
DLCI 0
SABM Frame
0, no information filed
Tail
This sample is a SABM frame to open DLCI 0.
Sample 2：
F9
05
EF
09
41 54 49 0D
58
F9
Opening
Flag Address Field Control Field Length Field Information Field FCS Closing Flag
Header DLC 1 UIH Frame 4 AT Command
"ATI<CR>”
Tail
This sample is a UIH frame to transmit AT command "ATI<CR>".
Sample 3：
F9
01
EF
0B
E3 07 07 0D 01
79
F9
Opening
Flag
Address
Field
Control
Field
Length
Field
Information Field FCS Closing Flag
Header DLC 0 UIH Frame 5 MSC Message,
length 3 Tail
This sample is a MSC message carried in UIH frame to transmit V2.4 signal 0x0D.

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3.10Transmission Bit Sequence
Transmission is based on 1 start bit, 8 data bits, 1 stop bit, and no parity.

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4 MUX Examples
4.1 Establish Channels
Step 1
：
Launch Multiplexer
No Step
Data
Direction
Hex Comment
TE<——>MS
1
TE launches MS
multiplexer function by
AT command ——> 61 74 2B 63 6D 75
78 3D 30 0D 0D 0A
4F 4B 0D 0A 0D 0A AT+CMUX=0<CR><LF>
MS feed back response <—— 61 74 2B 63 6D 75 78 3D 30 0D 0D 0A
4F 4B 0D 0A 0D 0A
AT+CMUX=0<CR><LF>O
K<CR><LF><CR><LF>
Host need quickly send
the establish DLC0 frame,
otherwise the client will
exit MUX state.
Step 2
：
Establish DLC 0
No Step
Data
Direction
Hex Comment
TE<——>MS
1
TE requests to
Establishes control
channel DLCI 0, using
SABM frame
——> F9 03
[note 2]
3F
[note 3]
01 1C F9 SABM Frame
MS feeds back UA
[note 1]
for receiving SABM and
accepts to create DLCI
0
<
—— F9 03 73 01 D7 F9 UA Frame
Step3
：
Establish DLC 1, 2