R2 signaling is a channel associated signaling (CAS) system developed
in the 1960s that is still in use today in Europe, Latin America, Australia,
and Asia. R2 signaling exists in several country versions or variants in an
international version called Consultative Committee for International Telegraph
and Telephone (CCITT-R2). The R2 signaling specifications are contained in
International Telecommunication Union Telecommunication Standardization Sector
(ITU-T) Recommendations Q.400 through Q.490.
E1 R2 signaling is an international signaling standard that is common
to channelized E1 networks. E1 R2 signaling is supported on the Cisco AS5200,
Cisco AS5300, and Cisco AS5800 series access routers. E1 R2 signaling was
introduced to the Cisco 2600/3600 series routers in Cisco IOS® Software Release
12.1.2XH and 12.1(3)T and later. This support is now available on Cisco 3700
Note: R2 signaling is not supported on the Cisco MC3810 router.
E1 R2 signaling support allows the Cisco AS5x00s and Cisco
2600/3600/3700 series routers to communicate with a central office (CO) or
private branch exchange (PBX) trunk and act as a tie-line replacement. Although
R2 signaling is defined in the ITU-T Q.400-Q.490 recommendations, there are
many variations in how R2 is implemented. (Various countries have chosen to
implement R2 differently.) The Cisco implementation of R2 signaling on routers
is able to address this issue to accommodate most countries.
There are no specific requirements for this document. However,
knowledge of CAS signaling is an added advantage. Refer to
CAS (R2, E&M, FXS, FXO) for more information on CAS
This document does not use any specific hardware or software versions.
It mainly discusses the theory of E1 R2 signaling. However, E1 R2 signaling is
currently supported on these Network Modules:
Refer to the
Technical Tips Conventions for more information on document
R2 signaling operates across E1 digital facilities. The E1 digital
facilities carrier runs at 2.048 Mbps and has 32 time-slots. E1 time-slots are
numbered TS0 to TS31, where TS1 through TS15 and TS17 through TS31 are used to
carry voice, which is encoded with pulse code modulation (PCM), or to carry 64
kbps data. This image shows the 32 time-slots of an E1
An E1 carrier can use a multiframe structure within a Super Frame (SF)
format or it can run in a non-multiframe mode without cyclic redundancy check
(CRC). The SF format contains sixteen consecutive frames numbered 0 to 15.
Time-slot TS16 in frame 0 is used for SF alignment, and TS16 in the frames that
remain (1 through 15) is used for CAS trunk signaling. TS16 uses four status
bits, designated as A, B, C, and D, for signaling purposes. This multiframe
structure is used for CRC, or error checking. This 16-frame multiframe
structure (SF) allows a single 8-bit time slot to handle the line signaling for
all 30 data channels. This diagram illustrates the E1 SF
The two elements to R2 signaling are line signaling (supervisory
signals) and interregister signaling (call setup control signals). Most country
variations in R2 signaling are with the interregister signaling
You can use line signaling, which uses TS16 (bits A, B, C, and D), for
supervisory purposes such as handshaking between two offices for call setup and
termination. In the case of CCITT-R2 signaling, only bits A and B are used (bit
C is set to 0 and bit D is set to 1). For two-way trunks, the supervision roles
for forward and backward signaling vary on a call-by-call basis. This table
illustrates the R2 supervision signal, transition, and direction used on
Note: An idle state is denoted when A=1 and B=0.
A,B: 1,0 to 0,0
A,B: 0,0 to 1,0
Seizure Acknowledgment (ACK)
A,B: 1,0 to 1,1
A,B: 1,1 to 0,1
A,B: 0,1 to 1,1
A,B: 0,1 to 1,0
Line signaling is defined with these types:
R2-Digital—R2 line signaling type ITU-U Q.421,
typically used for PCM systems (where A and B bits are used).
R2-Analog—R2 line signaling type ITU-U Q.411,
typically used for carrier systems (where a Tone/A bit is
R2-Pulse—R2 line signaling type ITU-U Supplement 7,
typically used for systems that employ satellite links (where a Tone/A bit is
Note: R2-Pulse reflects the same states as the analog signaling. But the
analog signal is a steady state (continuous signal), while the pulsed signal
stays on for only a short duration. Pulsed is just a single pulse to reflect
the state change.
R2 Signaling Configuration and Troubleshooting for more information on
how to configure line signaling.
The concept of address signaling in R2 is slightly different than that
used in other CAS systems. In R2 signaling, the exchanges are considered
registers and the signaling between these exchanges is called interregister
signaling. Interregister signaling uses forward and backward
in-band multifrequency signals in each time slot to
transfer called and calling party numbers, as well as the calling party
Note: Some countries use two-out-of-six in-band dual tone multifrequency
(DTMF) instead of forward and backward in-band multifrequency signals.
Multifrequency signals used in interregister signaling are divided in
forward signal groups (I and II), and backward signal groups (A and B).
Interregister signaling starts after the "Seize-ACK" of the line. This diagram
and table illustrate forward and backward signal
Forward Signal Groups
Backward Signal Groups
Represent the called party number or dialed
Dialed Number Identification Service (DNIS)/Automatic Number
Identification (ANI) digits.
I-1 to I-10 are digits 1 to 10.
I-15 is the end of
Represent the calling party category.
II-1 is subscriber without priority.
II-2 to II-9 are subscriber with priority.
II-11 to II-15 are spare for national
Indicate if the signaling ended or if a particular forward
signal is required.
Used to acknowledge and convey signaling
A-1 is send next digit.
A-3 is address-complete, changeover to reception of Group-B
A-4 is congestion.
A-5 is send calling party category.
A-6 is address complete, charge, setup, speech conditions.
Sent by the terminating switch to acknowledge a forward
signal, or to provide a call charging and called party
Used to acknowledge Group-II forward signals. This is always
preceded by an address-complete signal A-3.
B-3 is subscriber line busy.
B-4 is congestion.
B-5 is unallocated number.
B-6 is subscriber line free charge.
These interregister group sequence rules are used to identify the group
to which the signal belongs:
The initial signal received by the incoming exchange is a Group I
Outgoing exchanges consider backward signals as Group A
Group A signals received by outgoing exchanges are used to identify
whether the next signal is a Group B signal.
Group B signals always indicate an end-of-signaling
There are three types of interregister signaling:
R2-Compelled—When a tone-pair is sent from the
switch (forward signal), the tones stay on until the remote end responds (sends
an ACK) with a pair of tones that signals the switch to turn off the tones. The
tones are compelled to stay on until they are turned off.
R2-Non-Compelled —The tone-pairs are sent (forward
signal) as pulses so they stay on for a short duration. Responses (backward
signals) to the switch (Group B) are sent as pulses. There are no Group A
signals in non-compelled interregister signaling.
Note: Most installations use the non-compelled type of interregister
R2-Semi-Compelled—Forward tone-pairs are sent as
compelled. Responses (backward signals) to the switch are sent as pulses. It is
the same as compelled, except that the backward signals are pulsed instead of
Note: Do not use compelled signaling on slow (satellite) links. The call
setup time is too large because of distance delays.
Most country-specific variations of R2 signaling are seen in
interregister signaling. Unique E1 R2 signaling parameters for specific
countries and regions are set when you issue the
command, followed by the
R2 Signaling Configuration and Troubleshooting for more information on
configuration of interregister signaling and