UBX-13001887 - R12
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Multiplexer
3GPP 27.010 Mux implementation in cellular modules
Application note
Abstract
This document provides basic multiplexer information and a short description of how to implement
a multiplexer driver according to 3GPP TS 27.010. This document serves as a reference to customers
developing proprietary multiplexer drivers for communication with u-blox cellular modules using
several virtual channels simultaneously.
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Document information
Title
Multiplexer
Subtitle
3GPP 27.010 Mux implementation in cellular modules
Document type
Application note
Document number
UBX-13001887
Revision and date
R12
26-Oct-2020
Disclosure restriction
C1-Public
This document applies to the following products:
Product name
LEON-G1 series
LISA-U2 series
SARA-G3 series
SARA-G4 series
SARA-U2 series
LISA-C2 series
TOBY-L2 series
TOBY-R2 series
LARA-R2 series
SARA-R5 series
u-blox or third parties may hold intellectual property rights in the products, names, logos and designs included in this
document. Copying, reproduction, modification or disclosure to third parties of this document or any part thereof is only
permitted with the express written permission of u-blox.
The information contained herein is provided “as is” and u-blox assumes no liability for its use. No warranty, either express or
implied, is given, including but not limited to, with respect to the accuracy, correctness, reliability and fitness for a particular
purpose of the information. This document may be revised by u-blox at any time without notice. For the most recent
documents, visit www.u-blox.com.
Copyright © u-blox AG.
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Contents
Document information ................................................................................................................................ 2
Contents .......................................................................................................................................................... 3
1 Introduction ............................................................................................................................................. 5
1.1 Features and implementations ................................................................................................................ 6
1.1.1 TOBY-L2, TOBY-R2, LARA-R2, SARA-R5, SARA-U2, LISA-U2, SARA-G3, SARA-G4,
LEON-G1 series ................................................................................................................................................. 6
1.1.2 LISA-C2 series ..................................................................................................................................... 7
2 System parameters .............................................................................................................................. 8
2.1 Acknowledgement timer (T1).................................................................................................................... 8
2.2 Maximum frame size (N1) .......................................................................................................................... 9
2.3 Maximum number of retransmissions (N2) .......................................................................................... 9
2.4 Window size (k) ............................................................................................................................................ 9
2.5 Response timer for multiplexer control channel (T2) .......................................................................... 9
2.6 Response timer for wake-up procedure (T3) ....................................................................................... 10
2.7 Multiplexer control channel setup mode (subset) ............................................................................. 10
3 Multiplexer packet format for basic option ................................................................................. 11
3.1 Flag ................................................................................................................................................................ 11
3.2 Address ........................................................................................................................................................ 11
3.3 Control .......................................................................................................................................................... 11
3.3.1 Set Asynchronous Balanced Mode (SABM) command ..............................................................12
3.3.2 Unnumbered Acknowledgement (UA) response .........................................................................12
3.3.3 Disconnected Mode (DM) response ...............................................................................................12
3.3.4 Disconnect (DISC) command ..........................................................................................................12
3.3.5 Unnumbered Information with Header check (UIH) and unnumbered Information (UI)
frames ...............................................................................................................................................................12
3.4 Length indicator ........................................................................................................................................ 13
3.5 Information field ........................................................................................................................................ 13
3.6 Frame checking sequence field (FCS) .................................................................................................. 13
4 AT command / data interface .......................................................................................................... 14
4.1 AT command profiles ............................................................................................................................... 14
4.2 UART lines state during MUX protocol execution .............................................................................. 14
4.2.1 Exceptions for RING and DCD lines on UART. ............................................................................. 15
4.2.2 Break signal in a MSC command packet from TE ...................................................................... 15
5 Linux reference driver ........................................................................................................................ 16
5.1 Gsm0710.c .................................................................................................................................................. 16
5.1.1 Main ..................................................................................................................................................... 16
5.1.2 openDevicesAndMuxMode .............................................................................................................. 17
5.2 Buffer ............................................................................................................................................................ 17
6 Notes for developers .......................................................................................................................... 18
6.1 Enabling multiplexer trace messages .................................................................................................. 18
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6.2 Frame size .................................................................................................................................................. 18
6.3 Flow control on virtual channels ............................................................................................................ 18
6.4 Multiplexer start-up procedure .............................................................................................................. 19
6.5 Multiplexer close-down procedure......................................................................................................... 19
Appendix ....................................................................................................................................................... 20
A Glossary ................................................................................................................................................. 20
B Multiplexer transactions example ................................................................................................. 21
B.1 Multiplexer protocol establishment .......................................................................................................21
B.2 Multiplexer protocol AT command communication .......................................................................... 22
B.3 Multiplexer protocol close-down ............................................................................................................ 22
C Multiplexer debug command .......................................................................................................... 23
C.1 Debug command +UMUX ........................................................................................................................ 23
C.1.1 Defined values ................................................................................................................................... 24
C.2 Debug command +XL1SET="muxtron:<value>" ................................................................................. 24
C.2.1 Defined values ................................................................................................................................... 24
C.3 Debug with +TRACE AT command ....................................................................................................... 25
C.3.1 Defined values ................................................................................................................................... 25
Related documents ....................................................................................................................................26
Revision history ...........................................................................................................................................26
Contact ........................................................................................................................................................... 27
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1 Introduction
☞ The multiplexer protocol is not supported by TOBY-L2 "00" product versions.
This document describes the implementation of the multiplexer protocol for cellular modules. The
following symbols are used to highlight important information within the document:
☞ An index finger points out key information pertaining to integration and performance.
⚠ A warning symbol indicates actions that could negatively impact performance or damage the
device.
u-blox cellular modules support the 3GPP TS 27.010 multiplexer protocol [3]. It is possible to emulate
several virtual connection channels on a single physical interface to access the module concurrently
(e.g. it is possible to read phonebook contacts while receiving GPRS data).
Each channel between the TE and UE is called a Data Link Connection (DLC) and is separately and
sequentially established.
The multiplexer has three operating options:
• basic
• advanced without error recovery
• advanced with error recovery
u-blox cellular modules only support the basic mode.
Module
Control channel
AT commands /
data connection
GNSS
tunneling
SAP (SIM
Access Profile)
Remarks
TOBY-L2
Channel 0 (DLC0)
Channel 1 - 5
Multiplexer control channel as described in
3GPP TS 27.010 [3]
TOBY-R2
Channel 0 (DLC0)
Channel 1 - 5
LARA-R2
Channel 0 (DLC0)
Channel 1 - 5
Channel 61
SARA-R5
Channel 0 (DLC0)
Channel 1 - 32
Channel 43
LEON-G1
Channel 0 (DLC0)
Channel 1 - 5
Channel 6
SARA-G300
Channel 0 (DLC0)
Channel 1 -2
Channel 6
SARA-G310
Channel 0 (DLC0)
Channel 1 - 2
Channel 6
SARA-G340
Channel 0 (DLC0)
Channel 1 - 5
Channel 6
SARA-G350
Channel 0 (DLC0)
Channel 1 - 5
Channel 6
SARA-G4
Channel 0 (DLC0)
Channel 1 - 5
Channel 64
SARA-U2
Channel 0 (DLC0)
Channel 1 - 5
Channel 6
Channel 7
LISA-U2
Channel 0 (DLC0)
Channel 1 - 5
Channel 6
Channel 75
LISA-C2
Channel 0 (DLCI)
Channel 1 (DLCI1)
as AT channel
Channel 2 (DLCI2)
as data channel
Channel 3
(DLCI3)
AT channel is used for all AT commands
except the ATD#777 command, which sets
up a data call. Data channel is used for
data calls; it is only set up by ATD#777.
Table 1: Multiplexer configuration on u-blox cellular modules
1
Not supported by LARA-R204-02B and LARA-R211-02B-00
2
Channel 3 only available in case of single UART configuration or in case the AUX UART is not configured as AT interface
3
Channel 4 only available in case of single UART configuration or in case the AUX UART is not configured as GNSS tunneling
interface
4
Not supported by SARA-G450-00C
5
Not supported by LISA-U200-00S
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1.1 Features and implementations
1.1.1 TOBY-L2, TOBY-R2, LARA-R2, SARA-R5, SARA-U2, LISA-U2,
SARA-G3, SARA-G4, LEON-G1 series
• Multiplexer Basic Option compliant to 3GPP TS 27.010 [3] is supported. All the required features
are supported, within the limits identified in this document. If not specified in the document, the
optional features described in 3GPP technical specification are not supported.
• The multiplexer can operate only at a fixed baud rate. Once the +CMUX AT command is sent and
the autobauding is active (AT+IPR=0, where applicable), a switch to fixed rate is performed as soon
as the multiplexer protocol is successfully activated. It is strongly recommended to enable the
highest baud rate supported by the UE.
• Every instance has its own user profile. For additional information, see section 4.
• Every instance has its own independent flow control, implemented by means of MSC packets (for
more details, 3GPP TS 27.010 [3]).
• Software flow control (XOn/XOff) on the DLCs is not supported and should be disabled.
• The response to only one test command (as specified in 3GPP TS 27.010 [3], section 5.4.6.3.4) is
supported, since it is presumed that, when the connection UE-TE is established, it will not be lost.
This behavior prevents from looping TEST traffic.
• It is recommended to use Hardware Flow Control (RTS/CTS) on the multiplexer physical port. If the
ME power saving is enabled (e.g. AT+UPSV=1) and the UART interface is used this configuration
is mandatory.
• The flow control configuration of the multiplexer physical port is not changed when the multiplexer
protocol is established (the configuration previously selected via the &K or +IFC AT commands is
retained).
• Multiplexer timers and retransmission (T1, T2, T3, and N2, see 3GPP TS 27.010 [3]) have limited
support. For more details, see section 2.
• The multiplexer protocol can be activated over SPI interface (where available).
• No need to use multiplexer protocol with USB interface (where available). USB multiplexing
capability based on multiple CDCs should be used.
• The multiplexer is supported only on UART and SPI interfaces. Only one interface can be used at a
time.
1.1.1.1 Multiplexer Control Channel messages (as specified in 3GPP TS
27.010 [3], section 5.4.6.3)
• Parameter Negotiation (PN) command is not supported.
• Multiplexer Power control and wake-up mechanism via power saving control (PSC) command (as
described in the 3GPP TS 27.010 [3]) is not supported. If power saving is enabled, power saving
information towards the TE is indicated by the module CTS line (module output).
• Flow Control On Command (FCon) is not supported.
• Flow Control Off Command (FCoff) is not supported.
• Remote Port Negotiation (RPN) command is not supported.
• Remote Line Status (RLS) command is not supported.
• Service Negotiation Command (SNC) is not supported.
☞ On LISA-U2 / SARA-G3 / LEON-G1 series, the Hardware Flow Control is forced active on the
multiplexer physical port, when the multiplexer protocol is established. This configuration cannot
be subsequently changed.
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1.1.2 LISA-C2 series
The multiplexer implementation has the following limitations:
• When an AT command is sent on Channel 1, the application must wait for a response prior to
sending the ATD#777 command on Channel 2 and vice versa.
• When a data call is active on Channel 2, the application cannot use any AT commands that require
data services on Channel 1. This includes FTP, UHTTP, and sockets. For more details, see u-blox
C200 AT commands manual [4].
• The +CMUX AT command cannot be used again once the multiplexer has been enabled.
• The UART baud rate cannot be changed after the multiplexer enabling, so the +IPR command
should be used to set the desired baud rate prior to using the +CMUX AT command.
• The multiplexer frame size, N1, must typically be greater than 64; below this size there may be
issues with the host to modem PPP data transfers.
• Multiplexer timers and retransmission (T1, T2, T3, and N2, see 3GPP TS 27.010 [3]) have limited
support.
• Software Flow control (XOn/XOff) on the DLCs is not supported and should be disabled.
• Multiplexer Power Control and wake-up mechanism (as described in the 3GPP 27.010 Technical
Specification) is not supported.
• It is recommended to use Hardware Flow Control (RTS/CTS) on the multiplexer physical port.
• No need to use the multiplexer protocol with USB interface (where available). USB multiplexing
capability based on multiple CDCs should be used.
• The multiplexer is supported only on UART and SPI interfaces. Only one interface can be used at a
time.
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2 System parameters
The +CMUX AT command starts the multiplexer operation. This command enables the multiplexer to
start up the multiplexer control channel. The TE multiplexer initiates this by sending an SABM frame
on DLCI 0.
Once the multiplexer channel is established, other DLCs can be established. The multiplexer may
negotiate the parameters associated with each DLC prior to its establishment or use the default
values.
System parameters T1, N1, N2 and k can be configured by the multiplexer control channel, or the
default values given here can be used. T2, T3 and subset system parameters are set by means of the
+CMUX AT command.
In the basic mode, T1 is only used in case of SABM and DISC commands sent by the module. The SABM
command is never sent, because the DLC establishment procedure can only be initiated by the TE.
The DISC would only be triggered in response to a T2 timeout.
☞ For LISA-U2, SARA-U2, TOBY-R2 and LARA-R2 series, T1 and T2 timers and the retransmission
mechanism are not implemented.
☞ For SARA-R5 series, T1 timer is not implemented.
☞ For LEON-G1, SARA-G3, SARA-U2, LISA-U2, TOBY-L2, TOBY-R2, LARA-R2 and SARA-R5 series,
only the MSC command requires a response, and is therefore the only one that involves T2. On
LEON-G1 and SARA-G3 series, in case retransmission reaches the N2 number, the not responding
DLC is released (i.e. the DCE starts a DISC for the not responding DLC).
☞ In SARA-U2, LISA-U2, TOBY-R2 and LARA-R2 series, instead, T2 is not implemented to avoid
restrictions of the application response time.
☞ For all the modules, T3 is not implemented. This is because the wake-up procedure is not
supported, since the power saving control relies on the underlying hardware (UART, SPI, etc.)
power saving control mechanism.
☞ For more details about command syntax, see u-blox AT commands manual [1] or SARA-R5 series
AT commands manual [5] and AT commands examples application note [2].
2.1 Acknowledgement timer (T1)
The acknowledgement timer governs the time that a station waits for an acknowledgement before
resorting to other actions (e.g. transmitting a frame). The two stations may operate with different T1
values.
☞ For LEON-G1 and SARA-G3 series, this value is fixed at 253.
☞ For LISA-U2, SARA-U2, TOBY-L2, TOBY-R2, LARA-R2, SARA-R5 series, the T1 timer is not
implemented and the requested T1 value is not considered.
☞ For the LISA-C2 series, this timer is not implemented; if a T1 value is set then it will be ignored.
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2.2 Maximum frame size (N1)
The N1 parameter defines the maximum number of octets that may be contained in an information
field. It does not include octets added for transparency purposes.
The default value for the basic mode is 31 octets. The range is from 1 to 1509 octets.
☞ For the LISA-C2 series, this value does not include octets added for transparency purposes. The
default value for this mode is 32 octets. 0 should return error, the valid range goes from 1 to 256
octets, any other value returns error.
2.3 Maximum number of retransmissions (N2)
The N2 parameter defines the maximum number of times that a station re-attempts a procedure
requiring a response. The two stations may operate with a different value of N2.
☞ For LEON-G1, SARA-G3, TOBY-L2, SARA-R5 series, the range is 0-5, with the default value at 3.
☞ For LISA-U2, SARA-U2, TOBY-R2 and LARA-R2 series, the N2 value is not considered, since (as
previously described) the retransmission mechanism is not implemented. This could result in MSC
commands not being received (which cannot be avoided even if retransmission is implemented).
The TE must guarantee correct MSC command reception in all operating scenarios (especially if
the MSC commands are used to carry flow control information).
☞ For the LISA-C2 series, the retransmission mechanism is not implemented; if a value is requested,
then it will be ignored.
2.4 Window size (k)
The window size parameter (k) defines the maximum number of I frames that a DLC can have
outstanding. Since u-blox cellular modules support only the MUX basic mode (which does not
implement the Error Recovery Option) this parameter is not supported.
☞ For LISA-C2 series, this parameter is not supported.
2.5 Response timer for multiplexer control channel (T2)
The T2 timer is the amount of time that the multiplexer control channel waits before re-transmitting
a command.
T2 must be greater than T1.
☞ For LEON-G1, SARA-G3, TOBY-L2, SARA-R5 series, this value is fixed at 254.
☞ For LISA-U2, SARA-U2, TOBY-R2 and LARA-R2 series, the T2 timer is not implemented; the
requested T2 value is not considered.
☞ The LISA-C2 series does not support this parameter; if a value is set, it will be ignored.
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2.6 Response timer for wake-up procedure (T3)
The T3 timer is the amount of time that the transmitting station of a power wake-up command waits
before raising an alarm when no response is received. This parameter is not considered, since the
power wake-up procedure is not supported; instead, the underlying hardware (UART, SPI, etc.) power
saving mechanism is used.
☞ The LISA-C2 series does not support this parameter.
2.7 Multiplexer control channel setup mode (subset)
The subset parameter defines the way in which the multiplexer control channel is set up. A virtual
channel may subsequently be set up differently, but in the absence of any negotiation for the settings
of a virtual channel, the virtual channel shall be set up according to the control channel subset setting.
u-blox implementation does not support subsequent negotiation on a virtual channel basis.
Supported subset values are:
• 0 UIH frames used only
• 1 UI frames used only
☞ SARA-R5, LARA-R2, TOBY-R2, TOBY-L2, LISA-U2, SARA-U2, SARA-G4, LEON-G1, LISA-C2 series
and SARA-G3 "00" and "01" product versions do not support subset value 1.
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3 Multiplexer packet format for basic option
☞ LISA-C2 series only supports Mux Packet Format for the basic operating option.
Frame structure in basic mode:
Flag
Address
Control
Length Indicator
Information
FCS
Flag
1 octet
1 octet
1 octet
1 or 2 octets
Unspecified length but integral
number of octets
1 octet
1 octet
Table 2: Frame Structure for Basic option
Opening and closing flags may appear in the information field of the frame. They cannot be used to
determine the beginning and end of a frame. The Length Indicator for the frame must be used.
☞ In this document and in the 3GPP TS 27.010 [3] the bit fields are represented from the less
significant bit (leftmost one) to more significant (rightmost one): B1(LSB) B2 B3 B4 B5 B6 B7
B8(MSB).
3.1 Flag
Flags (when included in the frame) always have value 0xF9.
3.2 Address
The Address field contains the Data Link Connection Identifier (DLCI), the C/R bit, and the address
field extension bit (always 1). The C/R (command/response) bit identifies the frame as either a
command or a response.
Bit No. 1 2 3 4 5 6 7 8 EA
C/R D L C I
Table 3: Format of Address Field
The DLCI identifies an individual user information stream and it identifies connections between the
TE and UE. Multiple DLCIs are supported, but the number is implementation-specific. The DLCIs are
dynamically assigned.
3.3 Control
The content of the control field defines the type of frame. Table 4 describes the frame control fields.
Frame Type
1 2 3 4 5 6 7 8 Notes
SABM (Set Asynchronous Balanced Mode)
1 1 1 1 P/F 1 0 0
UA (Unnumbered Acknowledgement)
1 1 0 0 P/F 1 1 0
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
UI (Unnumbered Information)
1 1 0 0 P/F 0 0 0 Optional
Table 4: Coding of Control Field
In Table 4, P/F is the Poll/Final bit. The poll (P) bit set to 1 is used by a station to solicit (poll) a response
or sequence of responses from the other station. The final (F) bit set to 1 is used by a station to
indicate the response frame transmitted as the result of a soliciting (poll) command.
The poll/final (P/F) bit serves a function in both command frames and response frames (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).
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☞ For LISA-C2 series modules, every incoming packet will have a parity (Poll) bit set; if the parity bit
is not set, then the packet data will be discarded.
☞ For LISA-C2 series modules, the frame or packet data sent by the MUX channel to the host will
have the parity bit set in all cases.
3.3.1 Set Asynchronous Balanced Mode (SABM) command
The SABM command sets the addressed station to Asynchronous Balanced Mode (ABM) where all
control fields are one octet in length. The station confirms acceptance of the SABM command by
transmission of a UA response at the first opportunity. Upon acceptance of this command, the DLC
sends and receives state variables that are set to zero.
☞ u-blox implementation only supports DLC establishment initiated by the TE. SABM is therefore a
mandatory command that the TE must issue before starting any kind of communication with a
DLC. Repeat the SABM command for each DLC the TE wishes to establish a communication with.
3.3.2 Unnumbered Acknowledgement (UA) response
The UA response is used by the station to acknowledge the receipt and acceptance of SABM and DISC
commands.
3.3.3 Disconnected Mode (DM) response
The DM response reports a status where 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 an SABM command. If a DISC command is received while in disconnected mode a DM
response should be sent.
3.3.4 Disconnect (DISC) command
The DISC command terminates an operational or initialization mode previously set by a command. It
is used to inform one station that the other station is suspending operation and that the station
should assume a logically disconnected mode. Prior to executing the command, the receiving station
confirms the acceptance of the DISC command by the transmission of a UA response.
3.3.5 Unnumbered Information with Header check (UIH) and unnumbered
Information (UI) frames
The information is conveyed using UI or UIH frames. The support of UIH frames is mandatory, while
the UI frames support is optional.
UIH frames are used where the integrity of the information being transferred is of lesser importance
than its delivery to the correct DLCI. UI frames are used when it is important to know that data
received is correct.
For the UIH frame, the FCS is calculated over only the address, control, and length fields. For the UI
frame, the FCS is calculated over all fields (Address, Control, Length Indicator, Information).
The Address field identifies if the UI/UIH frame is a data or a control frame. If the Address refers to
the DLC0, the UI/UIH frame is a command/response (based on C/R bit) frame.
3.3.5.1 UIH (UI) command and response
☞ UI frames are only supported by SARA-G3 "02" product versions.
The UIH (UI) command/response sends information without affecting the V(S) or V(R) variables at
either station. Reception of the UIH (UI) command/response is a non-sequential number verified by
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the data link procedures; therefore, the UIH (UI) frame can be lost if a data link exception occurs during
transmission of the protected portion of the command, or duplicated if an exception condition occurs
during any reply to the command. There is no specified response to the UIH (UI) command/response.
3.4 Length indicator
The length indicator has the following format:
Bit 1 2 3 4 5 6 7 8 E/A
L1
L2
L3
L4
L5
L6
L7
Table 5: Length field, first byte
Bits L1 to L7 indicate the length of the following data field. The default length is 31 bytes.
When the EA bit is set to 1 in an octet, it 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. The total length
of the length field is in that case 15 bits, L1-L15.
The second octet of the length field has the following format (and is only present when the EA field in
the first byte is set to 0):
Bit 1 2 3 4 5 6 7 8 L8
L9
L10
L11
L12
L13
L14
L15
Table 6: Length field, second byte
The length field is always present, even if the data field is empty.
3.5 Information field
The information field is the payload of the frame and carries the user data and any convergence layer
information. The field is octet structured. The information field is only present in UI frames and UIH
frames.
3.6 Frame checking sequence field (FCS)
The Frame Checking Sequence Field (FCS) is a checksum of the payload.
The FCS is the ones complement of the sum (modulo 2) of:
• the remainder of
x
k
(
x
7
+
x
6
+
x
5
+
x
4
+
x
3
+
x
2
+
x
1
+ 1)
divided (modulo 2) by the generator polynomial
x
8
+
x
2
+ 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 start and stop elements (start/stop
transmission), and bits (synchronous transmission) and octets (start/stop transmission) inserted
for transparency, and
• the remainder of the division (modulo 2) by the generator polynomial
x
8
+
x
2
+ x + 1
of the product of
x
8
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 start and stop elements (start/stop
transmission), and bits (synchronous transmission) and octets (start/stop transmission) inserted
for transparency.
In the case of the UIH frame, the contents of the I-field are not included in the FCS calculation. FCS
is calculated based on the contents of the address, control, and length fields only. This means that
only the delivery to the correct DLCI is protected, but not the information. The FCS is calculated in
the normal manner for all other frames in Table 4.
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4 AT command / data interface
4.1 AT command profiles
When using the AT command interface over a multiplexer channel, the following notes should be
considered:
• Every virtual channel has its own AT profile.
• The +UPSV is the only profile field that is shared among all channels (both virtual and physical). A
change of this parameter for any channel applies to all other channels.
• The power-saving configuration selected by means of the +UPSV AT command acts directly on
the physical device and therefore it is effective even if it is issued on a virtual channel. Since the
mode is shared among all the channels (virtual or not), each channel has the capability of
modifying the power saving configuration.
• On LEON-G1, SARA-G3, SARA-G4, SARA-U2, LISA-U2, TOBY-L2, TOBY-R2, LARA-R2, SARA-R5
series modules, &K and +IFC AT commands are applied to the multiplexer virtual devices. This
means that they configure the multiplexer protocol to enable/disable the flow control via MSC
packets on the selected DLC (see section 6.3).
• On LISA-C2 series modules, the +IFC AT command is applied to the multiplexer virtual devices.
This means that they configure the multiplexer protocol to enable/disable the flow control via MSC
packets on the selected DLC. The default value is AT+IFC=2,2.
☞ The implementation on LISA-C2 series only allows hardware flow control.
• On LISA-C2 series modules, the module ignores the +IPR AT command if the multiplexer is active.
It is possible to read/write a value (the “OK” final result code is returned and profile updated), but
it has no effect on the module behavior.
• LEON-G1, SARA-G3, SARA-U2, LISA-U2, TOBY-L2, TOBY-R2, LARA-R2, SARA-R5 series ignore
+ICF and +IPR AT commands if the multiplexer is active. It is possible to read/write a value (the
“OK” result code is returned and profile updated), but it has no effect on the module behavior.
• Using the Software Flow control (XOn/XOff) on virtual channels is not allowed. All the commands
trying to set this mode are refused and a message error is returned.
☞ AT channels are dynamically attached/detached in the multiplexer protocol. Each ATTACH
operation results in a profile reload from the NVM. Therefore, a profile change (and in particular a
change of the +UPSV command parameter) in one open channel due to open/close operations of
another channel may occur. In the same way, when all the virtual channels are closed and the
multiplexer protocol is uninstalled, the channel associated with the physical device is reattached;
this also results in a profile reload.
4.2 UART lines state during MUX protocol execution
As described in 3GPP TS 27.010 [3] the multiplexer protocol implemented in u-blox cellular modules
specifies that the notification of any change in the ITU-T V.24 lines is performed via a proper MSC
packet.
☞ On LISA-U2 and SARA-U2 series modules, after every AT command response sent over an AT
command DLC (channels from 1 to 5), the module sends an MSC command for the same DLC to
TE with RTR flag to 1 (bit 1 of control signal octate), indicating that the DLC is ready to receive data.
This is something redundant, since RTR has not changed during AT command execution. The
3GPP TS 07.10 [3] states about the MSC frame “Every time the signals change, the DTE or DCE
sends this command to indicate the current status of each signal.”
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4.2.1 Exceptions for RING and DCD lines on UART.
To ensure a high level of host responsiveness to module notified events, such as incoming calls or
establishment of a data call, the UART RING and DCD lines have been customized. (An example of a
module notified event is the host in power saving mode being rapidly woken up by an event notification
using a line toggling.)
• LEON-G1, SARA-G3, SARA-G4 and LISA-C2 series modules: if the multiplexer protocol is
activated on the UART interface, the handling of UART RING and DCD physical output (from the
module) lines is implemented. The two lines are physically driven in the same way as without
multiplexer protocol. For these lines both MSC packet sending and physical line switch are
implemented.
• LISA-U2 / SARA-U2 / TOBY-R2 / LARA-R2 series modules: UART RING and DCD output lines are
physically changed in case of incoming voice or data call (RING line toggling) or established voice
call (DCD line to ON for the entire call duration), even if the MUX protocol is installed on UART (and
therefore MSC packet sending is implemented as well).
This means that in any conditions (MUX installed or not on UART), the UART DCD line notifies the
voice call establishment. In all the other scenarios (e.g. data call), if MUX is activated on UART,
DCD line change is only notified to the TE via a MSC packet on the relevant DLC.
• SARA-R5 series modules: if the multiplexer protocol is activated on the UART interface for MT
SMS related URCs, the RING line setting follows the +CNMI AT command configuration. Its
change is only notified to the TE via a MSC packet on the relevant DLC (where the URC is sent).
For all the other URCs, only the physical UART RING line is driven (no MSC packet sent to TE).
4.2.2 Break signal in a MSC command packet from TE
On a DLC used as AT command interface (e.g. channel from 1 to 5 in section 1 for LEON-G1,
SARA-G340, SARA-G350, SARA-G4, SARA-U2, LISA-U2, TOBY-L2, SARA-R5, TOBY-R2 and LARA-R2
series modules), the break signal octet reception in a MSC command frame from TE causes the
module to act as in case of escape sequence reception (e.g. +++) in case of established data call.
See 3GPP TS 27.010 [3], section “5.4.6.3.7 Modem Status Command (MSC)” for details about the
meaning and generation of the break signal octet.
See u-blox AT commands manual [1] or SARA-R5 series AT commands manual [5] for details about
the escape sequence detection effect on an active data call (i.e. AT interface in online data state).
☞ The module does not consider the length of break in units of 200 ms (L1-L4 in Table 8 of 3GPP TS
27.010 [3]). Only B1 bit is evaluated.
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5 Linux reference driver
Below is a brief description of a multiplexer host driver for Linux systems. This can be used as a
reference for developing a proprietary multiplexer driver, since most parts of the code are machine
independent.
5.1 Gsm0710.c
DLC 2
DLC n
write_frame
handle_command
buffer
Virtual ports
ussp_send_data
DLC 1
MUX driver
Serial port
ussp_rec_data
5.1.1 Main
The main program performs the following:
1. Parses the command line arguments. It sets some global variable and perhaps calls the “usage”
function to show the help.
2. It goes to background or not (in the daemonize() function) depending on command line
parameters.
3. Allocates memory for the structures used by the driver.
4. Starts the Mux protocol and initializes virtual ports. This part is handled by
“openDevicesAndMuxMode” function (see section 5.1.2).
5. Continuously reads data from the TE serial port and writes it to the buffer
(gsm0710_buffer_write), then extracts from it the single frames and forwards the payload
(extract_frames) to the correct virtual channel (ussp_send_data) or performs a control action
(handle_command). It also reads data from the virtual channels and writes it to the TE serial port
(ussp_recv_data).
6. When terminated, it frees the allocated resources and closes the virtual ports and the Mux
protocol.
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5.1.2 openDevicesAndMuxMode
This part is Linux dependent; it creates symbolic links for the virtual ports and configures their
parameters (“open_pty” function).
It opens and configures the physical serial port in “open_serialport” function.
Once the port is open it sends a simple “AT” command and waits for the OK answer, then it sends the
Mux activation command (AT+CMUX=….), this part is handled by “initGeneric” function.
When the Mux protocol is established, it opens the virtual channels.
5.2 Buffer
This file contains general utility functions to manage data from/to the TE and to parse/from mux
packets. These functions are platform independent. The functions that are mainly used are:
• gsm0710_buffer_init: Allocates memory for a new buffer and initializes it
• gsm0710_buffer_destroy: Frees allocated memory
• gsm0710_buffer_write: Writes data to the buffer
• gsm0710_buffer_get_frame: Gets a frame from buffer
• destroy_frame: Destroys a frame (free the memory allocated by gsm0710_buffer_get_frame)
• make_fcs: Calculates frame check sequence from given characters
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6 Notes for developers
6.1 Enabling multiplexer trace messages
☞ SARA-G4 and LISA-C2 series do not support this feature.
For debugging purposes, a special trace of the multiplexer protocol can be enabled.
LEON-G1 / SARA-G3 series
LISA-U2 / SARA-U2 / TOBY-R2 / LARA-R2 series
TOBY-L2 series
Description
AT+XL1SET="muxtron"
AT+UMUX=1,11,255
AT+TRACE=127,,"MUX=1"
Enable the trace
AT+XL1SET="muxtroff"
AT+UMUX=0
AT+TRACE=127,,"MUX=0"
Disable the trace
☞ On LISA-U2, SARA-U2, TOBY-R2, LARA-R2 series the +UMUX AT command offers a higher
granularity in the multiplexer trace activation. See appendix C.1 for a complete command
description.
This trace provides information (in form of ASCII output) about the multiplexer packets passing
on the UART and on the DLCs.
☞ On LEON-G1 “07” product versions and onwards and SARA-G3 modules, the “muxtron:<value>”
command is available. See appendix C.2 for the description.
☞ On TOBY-L2 series the +TRACE AT command allows configuring the MUX protocol logging. MUX
logging is disabled by default, and the setting is not persistent. Issue the AT+TRACE command
on the same AT terminal. See appendix C.3 for a complete command description.
☞ On SARA-R5 series, the +TRACE AT command allows configuring the MUX protocol logging. By
default, MUX logging is enabled while MUX frame dump is not; the setting is persistent. The
configuration requires the use of a binary filter that can be provided on demand.
6.2 Frame size
If the host driver sends packets bigger than the Maximum Frame Size (for more details see
section 2.2), the module ignores the packets without returning any error.
6.3 Flow control on virtual channels
The flow control of the virtual channel is implemented in terms of MSC packets with the FC bit. If the
application processor sets the FC bit to 1 for a particular DLC, the TE does not send data to the
application processor for that DLC until FC returns to 0 for the same DLC. The TE has limited
resources for buffering data, so if the DLC is involved in large data transfers (for example downloading
data through a GPRS connection) a buffer overflow may occur if the time between FC=1 and FC=0 is
too long; in this case data may be lost and there is no error indication.
The application processor should avoid (if possible) the use of this feature or keep the time interval
with the FC=1 as small as possible.
☞ LISA-C2 series only supports hardware flow control.
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6.4 Multiplexer start-up procedure
As described in the section 2, once the multiplexer channel is established, other DLCs can be
established. This is up to the TE (see the section 3.3.1), which shall issue a SABM command before
start any kind of communication with a DLC.
☞ On TOBY-L2 series, SABM frame shall be sent with a time interval of 5 s. Moreover, the TE shall
establish all the required DLCs at the “same time”, namely before performing any AT commands
communication over the DLCs.
6.5 Multiplexer close-down procedure
Even if the multiplexer close-down is a symmetric procedure, in the u-blox cellular modules it is always
requested by the TE.
The host should trigger the close-down procedure when all the DLCs are in an idle state that is when
there is neither any pending AT command response not ongoing data connections.
The multiplexer close-down command (CLD) is used to reset the link into normal AT command mode
without multiplexing.
Once the command to close down is received the multiplexer will close down each DLC via a DLC
release command (DISC). The release of a DLC may be initiated by either station by the transmission
of a DISC frame with the P-bit set to one. Confirmation of the DLC release is signaled by the other
station sending a UA frame with the F-bit set to 1. Once the DLC has been released the stations enter
disconnected mode for that particular DLC. If the station receiving the DISC command is already in a
disconnected mode it will send a DM response.
When all the DLCs (except the DLCI 0 - multiplexer control channel) are closed down (disconnected
mode) the multiplexer that initiated the close-down procedure will send a close-down message on the
multiplexer control channel. When this message is acknowledged both stations will revert to the nonmultiplexed mode.
☞ u-blox implementation also supports a clear down procedure where the TE sends a CLD before
having close each DLC with a DISC command.
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Appendix
A Glossary
Abbreviation
Definition
3GPP
3rd Generation Partnership Project
ASCII
American Standard Code for Information Interchange
AT
AT Command Interpreter Software Subsystem, or attention
ABM
Asynchronous Balanced Mode
C/R
Command/Response
CDC
Communications Device Class
CDMA
Code Division Multiple Access
CTS
Clear To Send
DCD
Data Carrier Detect
DISC
Disconnect
DLC
Data Link Connection
DLCI
Data Link Connection Identifier
DM
Disconnected Mode
EA
Extended Address
FCON
Flow Control ON
FCOFF
Flow Control OFF
FCS
Frame Check Sequence
FTP
File Transfer Protocol
GNSS
Global Navigation Satellite System
GPRS
General Packet Radio Service
LSB
Least significant bit
MSB
Most Significant Bit
MSC
Modem Status Command
MUX
Multiplexer
NVM
Non Volatile Memory
P/F
Poll/Final
PC
Personal Computer
RAM
Random Access Memory
RTR
Ready To Receive
RTS
Request To Send
SABM
Set Asynchronous Balanced Mode
SMS
Short Message Service
SPI
Serial Peripheral Interface
TE
Terminal Equipment
UA
Unnumbered Acknowledgement
UART
Universal Asynchronous Receiver-Transmitter
UE
User Equipment
UI
Unnumbered Information
UIH
Unnumbered Information with header Check
USB
Universal Serial Bus
Table 7: Explanation of the abbreviations and terms used
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B Multiplexer transactions example
The following example illustrates the procedure to establish two DLCs (DLC0 (control) and DLC1 (AT
command channel)). After establishment, an example of AT command issuing/response over DLC1
follows.
B.1 Multiplexer protocol establishment
From host driver SABM frame on DLCI0:
From host driver UIH frame on DLC0, modem status command:
From host driver SABM frame on DLCI1:
From host driver UIH frame on DLCI1, modem status command:
From modem UA (answer to SABM) on DLCI0
From modem UIH frame on DLC0, modem status command:
From modem UA (answer to SABM) on DLCI1
From modem UIH frame on DLC1, modem status command:
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
03
3F
01 - 1C
F9
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
03
EF
0B
E3 07 07 8C 01
18
F9
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
07
3F
01 - DE
F9
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
07
EF
0B
E3 07 07 8C 01
DA
F9
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
03
73
01 - D7
F9
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
01
EF
0B
E1 07 07 8C 01
79
F9
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
07
73
01 - 15
F9
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
05
EF
0B
E3 07 07 8C 01
BB
F9
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B.2 Multiplexer protocol AT command communication
From host driver UIH frame on DLCI1, “AT+CPIN?”:
From modem UIH frame on DLC1, “+CPIN: READY”:
B.3 Multiplexer protocol close-down
From host driver DISC frame on DLC1:
From modem UA frame on DLC1:
From host driver CLD command frame on DLC0:
From modem CLD response frame on DLC0:
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
07
EF
13
61 74 2B 63 70 69 6E 3F
0D
C8
F9
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
05
EF
21
0D 0A 2B 43 50 49 4E
3A 20 52 45 41 44 59 0D
6E
F9
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
07
53
01 - 3F
F9
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
07
73
01 - 15
F9
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
03
EF
05
C3 01
F2
F9
Flag
Address
Control
Length Indicator
Information
FCS
Flag
F9
01
EF
05
C1 01
93
F9
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C Multiplexer debug command
☞ SARA-G4 and LISA-C2 series do not support multiplexer debug command.
These icons are used to indicate applicability to the related products:
• : LEON-G1 series
• : SARA-G3 series
• : LISA-U2 series
• : SARA-U2 series
• : TOBY-L2 series
• : TOBY-R2 series
• : LARA-R2 series
C.1 Debug command +UMUX
The +UMUX command allows setting the multiplexer protocol’s trace (debug) capabilities. For each
DLC both a global and a local trace level can be set.
☞ The command is used only for testing / debugging purpose.
Type
Syntax
Response
Example
Set
AT+UMUX=<action>[,<p1>][,<p2>]
[,<p3>][,<p4>][,<p5>][,<p6>][,
<p7>][,<p8>][,<p9>][,<p10>]
OK
AT+UMUX=1,4,255
OK
Read
AT+UMUX?
+UMUX: <action>,<p1>,<p2>,<p3>
,<p4>,<p5>,<p6>,<p7>,<p8>,<p9>
,<p10>
OK
+UMUX: 1,5,255,0,0,0,0,0,0,0,0
OK
Test
AT+UMUX=?
+UMUX: (list of supported
<actions>),(list of supported
<p1's>),(list of supported
<p2's>),(list of supported
<p3's>),(list of supported
<p4's>),(list of supported
<p5's>),(list of supported
<p6's>),(list of supported
<p7's>),(list of supported
<p8's>),(list of supported
<p9's>),(list of supported
<p10's>)
OK
+UMUX: (0,1),(0-5,11),(0-
65535),,,,,,,,
OK
LEON-G1
SARA-G3
LISA-U2
SARA-U2
TOBY-L2
TOBY-R2
LARA-R2
LISA-U2
SARA-U2
TOBY-R2
LARA-R2
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C.1.1 Defined values
Parameter
Type
Description
<action>
Number
Operation to be performed:
• 0 (factory-programmed value): MUX trace disabled
• 1: MUX trace enabled
<p1>
Number
Global trace level:
• 0: no global trace
• 1-2: low trace load
• 3: auxiliary trace (data dump)
• 4: statistics trace
• 5: power saving trace
• 11: error detection trace
All the trace capabilities for a <p1> value are included if higher values are
selected.
<p2>
Number
Bitmask representing the list of the local trace capabilities over the
available DLCs. The bit position corresponds to the corresponding DLC. If
the trace capabilities must be activated the corresponding bit must be set
to 1.
<p3>-<p10>
Number
Extended trace options (not supported)
The read command always returns 0 for these parameters.
C.2 Debug command +XL1SET="muxtron:<value>"
☞ Not supported by LEON-G1 "06" and previous version.
The +XL1SET command allows setting the multiplexer protocol’s trace (debug) capabilities, with a
configurable trace level.
☞ The command is used only for testing / debugging purpose.
☞ The multiplexer protocol debug information requires that trace capability is activated on the
module; trace data must be collected on the serial interface configured as debug interface. If the
trace data is filtered, keep the ASCII trace and the “mux” process traces active.
Type
Syntax
Response
Example
Set
AT+XL1SET="muxtron:<value>"
OK
AT+XL1SET="muxtron:1"
OK
C.2.1 Defined values
Parameter
Type
Description
<value>
Number
Level of trace:
• 1: base MUX trace (equivalent to the “muxtron” command) enabled
• 2: extended MUX trace (enables the MUX frames dump) enabled
LEON-G1
SARA-G3
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C.3 Debug with +TRACE AT command
☞ The +TRACE AT command setting for debug is not stored in the NVM, so every power cycle turns
off this feature.
Type
Syntax
Response
Example
Set
AT+TRACE=127,,"MUX=<umode>"
OK
AT+TRACE=127,,"MUX=1"
OK
C.3.1 Defined values
Parameter
Type
Description
<umode>
Number
Level of trace:
• 0: MUX log disabled
• 1: MUX log enabled (only basic information)
• 2: MUX log enabled (full information without data dump)
• 3: MUX log enabled (full information with data dump)
TOBY-L2
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Related documents
[1] u-blox AT commands manual, UBX-13002752
[2] u-blox AT commands examples application note, UBX-13001820
[3] 3GPP TS 27.010 3rd Generation Partnership Project; Technical Specification Group Terminals;
Terminal Equipment to User Equipment (TE – UE) multiplexer protocol (Release 1999)
[4] u-blox LISA-C200 AT commands manual, UBX-13000621
[5] u-blox SARA-R5 series AT commands manual, UBX-19047455
☞ For regular updates to u-blox documentation and to receive product change notifications, register
on our homepage (www.u-blox.com).
Revision history
Revision
Date
Name
Comments
0
22-May-2011
lpah
Initial release
1
25-Nov-2011
lpah
Extended to include LISA-U2 series. Added section 4.2.
2
28-Mar-2012
mben
Extended "muxtron" command description
Added SAP channel for LISA-U200-01x / LISA-U230-01x and further versions
3
19-Jul-2012
mben
Improved the timers and retransmission mechanism
4
22-Feb-2013
mben / acub
Some improvements. Added description for CDMA modules
Last revision with docu number WLS-CS-11002
A
29-May-2013
mben
Extended to include SARA-G300 / SARA-G310 series
R07
16-Jun-2014
mben
Extended to include SARA-G340 / SARA-U2 series
R08
15-Apr-2015
mben
Extended the applicability to TOBY-L2 except for TOBY-L200-00S /
TOBY-L210-00S
R09
08-Mar-2016
mben
Extended UI frames handling description
R10
04-Jul-2017
mben
Extended the document applicability to TOBY-R2 / LARA-R2 series
R11
19-Jun-2020
alos / asci
Some improvements to Multiplexer close-down procedure and to TOBY-L2
seriers Multiplexer start-up procedure. AddedTOBY-L2 multiplexer debug
command by means of +TRACE AT command. Obsolete FW75 product
removed.
R12
26-Oct-2020
mben / fvid
Extended document applicability to SARA-R5 and SARA-G4 series.
Multiplexer - Application note
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Contact
For complete contact information, visit us at www.u-blox.com.
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