CMUX User Guide

CMUX User Guide 

30268ST10299A Rev. 0 - 13/10/06


30268ST10299A Rev. 0 - 13/10/06 

This document is refered to the following products: 

GM862-GPS 3 990 250 657 GE863-GPS 3 990 250 660 

GM862-QUAD 3 990 250 659 

GM862-QUAD-PY 3 990 250 658 

GE863-OUAD Pb balls 3 990 250 662 

GE863-PY Pb balls 3 990 250 661 

GE864-QUAD 3 990 250 675 

GE864-PY 3 990 250 676 

GE863-QUAD 3 990 250 648 

GE863-PY 3 990 250 650 

Reproduction forbidden without Telit Communications S.p.A. written authorization - All Right reserved page 2 of 24


30268ST10299A Rev. 0 - 13/10/06 

Contents 

1 INTRODUCTION................................................................................................................4 

1.1 Scope of document ................................................................................................................4 

2 APPLICABLE DOCUMENTS ............................................................................................5 

3 TECHNICAL CHARACTERISTICS ...................................................................................6 

3.1 Product architecture ..............................................................................................................6 

3.2 Implementation feature and limitation..................................................................................6 

3.3 AT command interface interactions .....................................................................................6 

4 SERIAL MULTIPLEXER PROTOCOL...............................................................................8 

4.1 CMUX Frame Structure ..........................................................................................................8 

4.2 UIH Control Channel Frame Coding ...................................................................................11 

4.3 UIH Data Channel Frame Coding ........................................................................................13 

4.4 CMUX establishment scenario ............................................................................................14 

5 TELIT SERIAL PORT PC INTERFACE FOR CMUX.......................................................15 

5.1 Interface Specification .........................................................................................................15 

5.2 Scenario ................................................................................................................................15 

5.3 Graphical Interface ...............................................................................................................16 

5.4 Application Setup .................................................................................................................18 

6 INTEGRATOR HINTS......................................................................................................19 

6.1 How to begin .........................................................................................................................19 

6.2 Flow control ..........................................................................................................................20 

6.3 Blocking command ..............................................................................................................20 

6.4 Lookup table for FCS calculation .......................................................................................21 

6.5 Ignored command ................................................................................................................22 

7 AT Commands................................................................................................................23 

8 Document Change Log ..................................................................................................24 



30268ST10299A Rev. 0 - 13/10/06 

1 INTRODUCTION 

1.1 Scope of document 

The scope of the present document is to describe a multiplexing protocol implemented in the Telit 

module. The multiplexing protocol can be used to send any data, SMS, fax, TCP data. 

NOTE: CUMX can be activated only with the interface SELINT 2; this means that for the 

modules GM862-QUAD/PY and GE863-QUAD/PY which start as default in SELINT 0 or SELINT 

1 1 , before activating CMUX, interface should be changed to SELINT 2. 

1 please consult AT Commands Reference Guide 80000ST10025a for more information 


2 APPLICABLE DOCUMENTS 


30268ST10299A Rev. 0 - 13/10/06 

• Digital Cellular Telecommunications Systems (Phase 2+); Terminal Equipment to Mobile Station 

(TE-MS) "Multiplexer Protocol"; ETSI TS 101 369 V7.1.0 (1999-11), GSM 07.10 Version 7.1.0, 

Release 199 


3 TECHNICAL CHARACTERISTICS 


30268ST10299A Rev. 0 - 13/10/06 

3.1 Product architecture 

The Multiplexer mode enables one serial interface to transmit data to four different customer 

applications. This is achieved by providing four virtual channels using a Multiplexer (Mux). 

This is especially advantageous when a fax/data/GPRS call is ongoing. Using the Multiplexer features, 

e.g. controlling the module or using the SMS service can be done via the additional channels without 

disturbing the data flow; access to the second UART is not necessary. 

Furthermore, several accesses to the module can be created with the Multiplexer. This is of great 

advantage when several independent electronic devices or interfaces are used. 

To access the three virtual interfaces, both the GSM engine and the customer application must 

contain Mux components, which communicate over the multiplexer protocol. 

In Multiplexer mode, AT commands and data are encapsulated into packets. Each packet has channel 

identification and may vary in length. 

3.2 Implementation feature and limitation 

• 7.10 CMUX Basic Option used 

• CMUX implementation support four full DLCI (Serial Port) 

• CMUX can operate only at Fixed rate, if AT+CMUX is sent with IPR=0 an Error is returned, 

with a maximum rate of 115200 

• Every instance has its own user profile storage in NVM 

• Independent setting of unsolicited message. 

• In case of GPS product one serial port can be dedicated to NMEA output. 

• Every Instance has its own independent flow control. 

3.3 AT command interface interactions 

The following AT commands would modify the global operation of the system, independently from the 

CMUX instance that is using them. 

• AT#HFMICG 


• AT#HSMICG 

• AT+CMUT 

• AT#STM 

• AT#SHFEC 

• AT#CAP 

• AT+CLVL 

• AT#SRS 

• AT+CRSL 

• AT#SRP 

• AT#NITZ 

• AT+CALM 

• AT#SHFSD 

• AT#DAC 

• AT+CFUN 

• AT+COPS 


30268ST10299A Rev. 0 - 13/10/06 

The AT commands that require to modify a setting would be saved in a global profile. They would be 

reloaded at boot time with relation to the ATY1, ATY0 previously issued on the instance CMUX1 or 

ASC0. 

When in multiplexed mode it is suggested not to used the +CFUN command but the PSC (Power 

saving command) described by GSM 7.10 specification. 



30268ST10299A Rev. 0 - 13/10/06 

4 SERIAL MULTIPLEXER PROTOCOL 

4.1 CMUX Frame Structure 

All information transmitted between the module and the application is based on frame that have the 

following frame Structure 

Flag Address Control Length Information FCS Flag 

Indicator 

1 octet 1 octet 1 octet 1or2 octets Unspecified length 

but integral number 

of octets 

1 octet 1 octet 

Flag Octet 

Each frame begins and ends with a flag octet define as ( binary:11111001 or hexadecimal 0xF9) 

Address Octet 

Is formatted as follow: 

0 1 2 3 4 5 6 7 

EA C/R D L C I 

EA: Extension Bit 

Shall be always 1 in the basic option of the protocol 

C/R: Command Response 

The C/R (command/response) bit identifies the frame as either a command or a response. In 

conformance with the standard HDLC rules, a command frame contains the address of the data link 

connection entity to which it is transmitted while a response frame contains the address of the data 

link connection entity transmitting the frame. 

Command/response Direction C/R value 

Command Application ⎯⎯→ Module 1 

Module ⎯⎯→ Application 0 

Response Application ⎯⎯→ Module 0 

Module ⎯⎯→ Application 1 


Example: 


30268ST10299A Rev. 0 - 13/10/06 

Application take the initiative to initialize the multiplexer (i.e. sends the SABM command at DLCI 0 ) 

and the Module that accepts the initialization of the multiplexer (i.e. sends the UA response at DLCI 0) 

DLCI: Data Link Connection Identifier 

DLCI value identify the Virtual Port inside the Module with the following assignment 

DLCI 

0 Reserved to Control Channel 

1 Virtual Port #1 



4 Reserved for Python debug 

Control Field 

The content of the control field defines the type of frame 

Frame Type 0 1 2 3 4 5 6 7 

SABM (Set Asynchronous 1 1 1 1 P/F 1 0 0 

Balanced Mode) 

UA (Unnumbered 

1 1 0 0 P/F 1 1 0 

Acknowledgement) 

DM (Disconnected Mode) 1 1 1 1 P/F 0 0 0 

DISC (Disconnect) 1 1 0 0 P/F 0 1 0 

UIH (Unnumbered 

Information with Header 

check) 

1 1 1 1 P/F 1 1 1 

P/F is the Poll/Final bit: 

Commands: P=1 

Response: F=1 

SABM (Set Asynchronous Balanced Mode ) 

The SABM command shall be used by the application to start the HDLC Connection. 

The module will answer to this command with an UA Frame. 

UA (Unnumbered Acknowledgement) 

The UA response is sent by the module to inform that a SABM or DISC command was accepted 

DM (Disconnected Mode) 

The DM response is sent by the module to inform that a SABM or DISC command was rejected, this 

can happens if for example a SABM is sent for a DLCI not supported. Or if a DISC is sent to a DLCI 

Address already closed. 



30268ST10299A Rev. 0 - 13/10/06 

DISC (Disconnect) 

The DISC is used to close a previous established connection. If the application send a disc for the 

DLCI 0 (the control channel), all the established channel will be closed. 

The module will answer to this command with an UA Frame. 

UIH (Unnumbered Information) 

Refer to the specific chapter 

Length Indicator 

This Octet specified the length of the information field 

0 1 2 3 4 5 6 7 

E/A L1 L2 L3 L4 L5 L6 L7 

E/A Bit shall be 1 if 7 bit are enough for the len ( len


30268ST10299A Rev. 0 - 13/10/06 

4.2 UIH Control Channel Frame Coding 

DLCI shall be always 0 

Type Length 

Value 

Indicator 

1 octet 1or2 octets n Octet 

Type Octet: 

0 1 2 3 4 5 6 7 

EA C/R T Y P E 


Shall be always 1. 

C/R: 

Identify if is a Command or Response 

Length indicator 

Is code like in the Cmux Frame Structure 

Value 

The number of octet are specified by the Length Indicator and depend on the type of the command as 

follow: 

Multiplexer close down (CLD) 

The multiplexer close down command is used to reset the link into normal AT command mode without 

multiplexing 

Type Len 

3 0 

Test Command (Test) 

The test command is used to test the connection between MS and the TE. The length byte describes 

the number of values bytes, which are used as a verification pattern. The opposite entity shall respond 

with exactly the same value bytes. 

Type Len Value 1 Value2 Value .. Value N 

4 N Any Char Any Char Any Char Any Char 

Modem Status Command (MSC) 

This command is used to send V.24 signal info. 



30268ST10299A Rev. 0 - 13/10/06 

Format without break indication 

Type Len Value 1 Value 2 

6 2 DLCI V24 Octet 

Format with Break Indication 

V24 Octet from Module to Application 

V24 Octet from Application to Module 

Type Len Value 1 Value 2 Value 3 

6 3 DLCI V24 Octet Break 

Octet 

0 1 2 3 4 5 6 7 

1 FC DSR CTS 0 0 RIN DCD 

G 

0 1 2 3 4 5 6 7 

1 FC DTR RTS 0 0 0 0 

Break Octet 

0 1 2 3 4 5 6 7 

1 0 0 0 0 0 0 0 

This octet shall be sent every time a Break signal shall be simulated 

Non Supported Command Response (NSC) 

This response is sent whenever a command type is not supported by the receiving entity. 

Type Len Value 1 

8 1 Command Not 

Supported 

Power Saving Control (PSC) 

This command is used to program the power saving mode of the module when Multiplexer mode. 

AT+CFUN command shall not be used. 

Type Len Value 1 

6 1 Power Saving Mode 

Power Saving Mode: TBD 


4.3 UIH Data Channel Frame Coding 


30268ST10299A Rev. 0 - 13/10/06 

DLCI shall be 1,2,3 or 4 

Length Indicator 

1or2 octets 

User Data 

n Octet 

Type Octet: 

0 1 2 3 4 5 6 7 

EA C/R T Y P E 


Shall always be 1. 

C/R: 

Identify if is a Command or Response 

Length indicator 

Is code like in the Cmux Frame Structure 

User Data 

The Number of data is defined by the Len Indicator 



30268ST10299A Rev. 0 - 13/10/06 

4.4 CMUX establishment scenario 

MSC CMU Establihment 

APPLICATION 

MODULE 

AT+CMUX=0 

OK 

SABM 

(DLCI = 0) 

UA 

((DLCI = 0) 

Control Channel 

Established 

SABM 

(DLCI = 1) 

UA 

((DLCI = 1) 

Virtual Port #1 

Open 

UIH 

(DLCI = 0, MSC on DLCI1 ) 

UIH 

V.24 Signal 

Exchange 

( DLCI=0, MSC, On DLCI=1) 

UIH 

(DLCI = 1) 

UIH 

(DLCI=0, MSC) 



30268ST10299A Rev. 0 - 13/10/06 

5 TELIT SERIAL PORT PC INTERFACE 

FOR CMUX 

5.1 Interface Specification 

To exploit the capabilities of CMUX, Telit developed a tool called Telit Serial Port MUX. It is a PC 

interface able to manage data coming/to be sent from/to CMUX. This target is achieved creating up to 

four serial virtual port on the PC and using a specific communication protocol to manage the 

communication between the real serial COM, changing data with CMUX, and each of the virtual port. 

5.2 Scenario 

Virtual COM 1 

PC TOOL 1 

MODULE 

CMUX 

C 

O 

M 

Virtual COM 2 

Virtual COM 3 

PC TOOL 2 

PC TOOL 3 

P 

C 

Python debug 

PC TOOL 4 

As you can see, you can run 3 different applications using the same CMUX module, for example 3 

HyperTerminal sending commands at the same time to the same module. 



30268ST10299A Rev. 0 - 13/10/06 

5.3 Graphical Interface 

Telit Serial Port Mux application on your PC after installation looks like in the following figure: 

Modem Port Panel: 

• Real port (COM) used to communicate 

with PC 

• Speed (Baud rate) 

• Status 

• Model 

Virtual Port Panel: 

• Virtual port number 

• Status 

• Baud Rate 

• Bytes Transmitted\Received 

• RTS: Request to send 

• DTR: Data Terminal Ready 

• CTS: Clear To Send 

• DCD: Data Carrier Detect 

• DSR: Data Set Ready 

• RI: Ring Indicator 

Tray icon: when Telit Serial Port Mux is working, there is a 

Tray Icon Showing the state of the connection 

• Modem Port Panel: here you can find the information about the modem connected to your PC, 

like: 

1. COM on your PC used to transfer data, selected during the initial setup or selecting the 

Setup voice in the application menu. 

2. COM Speed selected. 



30268ST10299A Rev. 0 - 13/10/06 

3. Connection Status: it can be “idle” or “error” when CMUX is disconnected, “connecting” 

when PC is trying to connect to CMUX and “connected” when CMUX is connected 

successfully. 

4. Indication about the model of the modem connected. 

• Virtual Port Panel: here you can find all the information about the connection using a Virtual 

Com installed on your PC: 

1. Virtual Com number. 

2. Virtual Port Status: it can be “idle”, “idle no DCD”, “Error”, “Opened”. 

3. Baud Rate. 

4. Number of bytes received and transmitted (RX Bytes, TX Bytes). 

5. All the common serial port signals like RequestToSend, DataTerminalReady, 

ClearToSend, DataCarrierDetected, DataSetReady and RingIndicator. 

• Tray Icon: there is a Tray Icon indicating the status of the Serial Port Mux: 

1. CMUX connected: the Tray Icon is blinking. 

2. CMUX disconnected or connecting. 

3. CMUX error. 



30268ST10299A Rev. 0 - 13/10/06 

5.4 Application Setup 

The selection of the number of Virtual ports to be created, like the selection of the real COM to be 

used and its speed is made in the setup section. There are two possibilities to do this, during the tool 

installation and when the tool is running. 

Module Serial Port Panel: 

here you can select the Real COM to 

use and its speed 

Virtual Serial Port Panel: 

here you can create virtual ports 

Virtual Ports created are visible also on the Device Manager: 


6 INTEGRATOR HINTS 


30268ST10299A Rev. 0 - 13/10/06 

6.1 How to begin 

The customer/integrator in order to design its own Multiplexer application must follow these basic 

requirements: 

• The GSM engine supports the basic option and UIH Framing according to GSM 07.10; 

• Character framing must be configured for 8 data bits, no parity and 1 stop bit; 

• Hardware flow control is recommended for use with multiplexer mode. If used, it needs to be 

set before Multiplexer mode is entered; 

If the GSM engine is operated in multiplexer mode, the following restrictions apply: 

• MO and MT circuit-switched data and fax calls are available on any channels; 

• Multiplex mode cannot be started while autobauding (AT+IPR=0) is enabled; 

• XON/OFF flow control is not supported in multiplexer mode. 

When using the following functions, be aware of possible dependencies between the different 

channels. One way of avoiding problems may be to dedicate certain commands/features to one of the 

channels or to assure that the application avoids conflicts. 

• Call control: a voice call can be initiated, answered on any channel and closed only on the 

initialized channel; 

• Phonebook access: if you wish to write the same phonebook entry on two or more different 

channels at the same time, please note that the only the last entry will be stored; 

The following functions or events may be ongoing independently on different channels: 

• Unsolicited Result Codes (URCs) will generally be transmitted to all logical channels with the 

dependency on the issued AT+CNMI; 

• Single quality and cell information can be retrieved on a single channel; 

• Profile configuration commands can be issued and retrieved on any channel, no matter on 

which the command is executed. 


6.2 Flow control 


30268ST10299A Rev. 0 - 13/10/06 

The internal logical flow control represents the existing flow control to the module. For example, if a 

data call is initiated and the customer application transmits data to the module on this channel, the 

module will stop the data transmission from time to time. This happens because the module operates 

with a bandwidth of 9k6 on air, but the customer application uses a larger width. In this case the 

module sends a MSC message with FC-BIT set. After all data stored in the internal buffer have been 

sent, the module will send a second MSC message with FC-BIT reset. As already pointed out, the 

logical flow control operates like RTS/CTS but with FC-BIT on every channel. The RTS/CTS are not 

used for flow control because the traffic on the logical channels may cause a temporary loss of 

bandwidth on another channel. This behavior has no impact on the handshake V.24 lines. 

Hardware flow control is recommended for use with multiplexer. 

The customer application decodes and encodes the data. To prevent loss of data, the application must 

be able process the information about internal flow control (MSC) regulated by the module. Flow 

control information is transmitted within the data flow and contains messages whether or not the 

channel is allowed to send. As of Multiplexer Protocol, the customer application must set RTS (in the 

direction to the module) on channel 1. RTS shall not be switched off to prevent loss of data (control 

data cannot be sent in this case). If flow control is needed, it is recommended to use logical flow 

control on every channel. 

6.3 Blocking command 

With the first versions of the CMUX implementation some AT command will frieze execution of other 

command on the other instance until they complete. Executions of the freeze command restart 

automatically. 

The list of command is: 

• ATA 

• ATD 

• ATO 

• ATH 

• AT#CAMEN 

• AT#TPHOTO 

• AT+CGATT 

• AT+CGACT 

• AT+COPS 

• AT+CREG 

• AT#SEMAIL 

• AT#EMAILD 

• AT#SKTOP 

• AT#SKTD 


• AT#QDNS 

• AT+CAMM 

• All FTP Command 

• All CSURV Command 

• All Supplementary Service Command 

• All Phone Book related command 


30268ST10299A Rev. 0 - 13/10/06 

6.4 Lookup table for FCS calculation 

In order to make easier the FCS calculation the following piece of code is provided. 

static const unsigned char crctable[256] = { //reversed, 8-bit, poly=0x07 

0x00, 0x91, 0xE3, 0x72, 0x07, 0x96, 0xE4, 0x75, 0x0E, 0x9F, 0xED, 0x7C, 0x09, 0x98, 0xEA, 0x7B, 

0x1C, 0x8D, 0xFF, 0x6E, 0x1B, 0x8A, 0xF8, 0x69, 0x12, 0x83, 0xF1, 0x60, 0x15, 0x84, 0xF6, 0x67, 

0x38, 0xA9, 0xDB, 0x4A, 0x3F, 0xAE, 0xDC, 0x4D, 0x36, 0xA7, 0xD5, 0x44, 0x31, 0xA0, 0xD2, 0x43, 

0x24, 0xB5, 0xC7, 0x56, 0x23, 0xB2, 0xC0, 0x51, 0x2A, 0xBB, 0xC9, 0x58, 0x2D, 0xBC, 0xCE, 0x5F, 

0x70, 0xE1, 0x93, 0x02, 0x77, 0xE6, 0x94, 0x05, 0x7E, 0xEF, 0x9D, 0x0C, 0x79, 0xE8, 0x9A, 0x0B, 

0x6C, 0xFD, 0x8F, 0x1E, 0x6B, 0xFA, 0x88, 0x19, 0x62, 0xF3, 0x81, 0x10, 0x65, 0xF4, 0x86, 0x17, 

0x48, 0xD9, 0xAB, 0x3A, 0x4F, 0xDE, 0xAC, 0x3D, 0x46, 0xD7, 0xA5, 0x34, 0x41, 0xD0, 0xA2, 0x33, 

0x54, 0xC5, 0xB7, 0x26, 0x53, 0xC2, 0xB0, 0x21, 0x5A, 0xCB, 0xB9, 0x28, 0x5D, 0xCC, 0xBE, 0x2F, 

0xE0, 0x71, 0x03, 0x92, 0xE7, 0x76, 0x04, 0x95, 0xEE, 0x7F, 0x0D, 0x9C, 0xE9, 0x78, 0x0A, 0x9B, 

0xFC, 0x6D, 0x1F, 0x8E, 0xFB, 0x6A, 0x18, 0x89, 0xF2, 0x63, 0x11, 0x80, 0xF5, 0x64, 0x16, 0x87, 

0xD8, 0x49, 0x3B, 0xAA, 0xDF, 0x4E, 0x3C, 0xAD, 0xD6, 0x47, 0x35, 0xA4, 0xD1, 0x40, 0x32, 0xA3, 

0xC4, 0x55, 0x27, 0xB6, 0xC3, 0x52, 0x20, 0xB1, 0xCA, 0x5B, 0x29, 0xB8, 0xCD, 0x5C, 0x2E, 0xBF, 

0x90, 0x01, 0x73, 0xE2, 0x97, 0x06, 0x74, 0xE5, 0x9E, 0x0F, 0x7D, 0xEC, 0x99, 0x08, 0x7A, 0xEB, 

0x8C, 0x1D, 0x6F, 0xFE, 0x8B, 0x1A, 0x68, 0xF9, 0x82, 0x13, 0x61, 0xF0, 0x85, 0x14, 0x66, 0xF7, 

0xA8, 0x39, 0x4B, 0xDA, 0xAF, 0x3E, 0x4C, 0xDD, 0xA6, 0x37, 0x45, 0xD4, 0xA1, 0x30, 0x42, 0xD3, 

0xB4, 0x25, 0x57, 0xC6, 0xB3, 0x22, 0x50, 0xC1, 0xBA, 0x2B, 0x59, 0xC8, 0xBD, 0x2C, 0x5E, 0xCF 

}; 

static UINT8 CalcFCS( UINT8 *buf, int len) 

{ 

UINT8 FCS=0xFF; 

while (len--) FCS=crctable[FCS^*buf++]; 

return (0xFF-FCS); 

} 

static int CheckFCS( UINT8 *buf, int len) 

{ 

UINT8 FCS=0xFF; 

UINT8 crc; 

while (len--) 

{ 

FCS=crctable[FCS^*buf++]; 

} 



30268ST10299A Rev. 0 - 13/10/06 

/*0xCF is the reversed order of 11110011.*/ 

return (FCS==0xCF); 

} 

6.5 Ignored command 

This command are ignored by the module if the multiplexer is active. 

It is possible to read/write value but they have no effect on the behaviour of the module 

AT+IPR 

AT+IFC 

AT+ICF 

ATS2 • Escape sequence is not handled by the Module but shall be 

handled by the applicator that will send a BREAK signal to the 

module using the MSC command 

ATS12 • Like ATS2 

ATS25 • Module will recognize DTR off as soon as the MSC command is 

received 


7 AT Commands 


30268ST10299A Rev. 0 - 13/10/06 

AT commands related to the CMUX feature are the following: 

command 

+CMUX 

#CMUXSCR 2 

description 

Enable/Disable Multiplexing Mode 

CMUX Script Enable 

For more detailed information please refer to AT Commands Reference Guide 80000ST10025a. 

2 only for the modules with Python feature 



30268ST10299A Rev. 0 - 13/10/06 

8 Document Change Log 

Revision Date Changes 

ISSUE #0 13/10/06 Initial release

CMUX User Guide

Create successful ePaper yourself

Delete template?

Save as template?