This chapter provides
information about peer-to-peer transport of rEceive and transMit (E and M)
signals and voice data over MPLS on the Cisco ASR 903 Router.
E and M Signaling
E and M signaling
defines a trunk circuit side and a signaling unit side for each connection
similar to the data circuit-terminating equipment (DCE) and data terminal
equipment (DTE) reference type. The analog E and M interface functions as the
signaling unit side and it expects the other side to be a trunk circuit.
E and M controller
indexes are populated in IfTable, the IfTable is defined in IF-MIB.
Type IV is not supported on Cisco gateways.
only support CAS in-band signaling.
index in the IfTable does
the information of the following:
Transmit Unit (MTU)
Information About Voice Port
The following section provides an overview of voice port configuration.
Voice Port Configuration
Voice ports are found
at the intersections of packet-based networks and traditional telephony
networks, and they facilitate the passing of voice and call signals between the
two networks. Physically, voice ports connect a router or access server to a
line from a circuit-switched telephony device in a PBX or the PSTN.
configuration for voice ports describes the type of connection being made and
the type of signaling to take place over this connection. In addition to the
commands for basic configuration, there are also commands that provide
fine-tuning for voice quality, enable special features, and specify parameters
to match those of proprietary PBXs.
Voice ports on routers and access servers emulate physical telephony
switch connections so that voice calls and their associated signaling can be
transferred intact between a packet network and a circuit-switched network or
device. For a voice call to occur, certain information must be passed between
the telephony devices at either end of the call. This information is referred
to as signaling. The devices at both ends of the call segment must use the same
type of signaling.
The devices in the packet network must be configured to convey
signaling information in a way that the circuit-switched network can
understand. The devices must also be able to understand signaling information
received from the circuit-switched network.
Voice ports on
routers and access servers physically connect the router or access server to
telephony devices such as telephones, fax machines, PBXs, and PSTN central
office (CO) switches. These devices may use any of several types of signaling
interfaces to generate information about on-hook status, ringing, and line
voice-port hardware and software need to be configured to transmit and receive
the same type of signaling being used by the device with which they are
interfacing so that calls can be exchanged smoothly between the packet network
and the circuit-switched network.
The signaling interfaces discussed in this document include foreign exchange office (FXO), foreign exchange station (FXS),
and receive and transmit (E&M), which are types of analog interfaces. Some digital connections emulate FXO, FXS, and E&M interfaces,
and they are discussed in "FXS and FXO Interfaces" and the E and M Interfaces section. It is important to know which signaling method the telephony side of the connection is using, and to match the router
configuration and voice interface hardware to that signaling method.
The next three illustrations show how the different
signaling interfaces are associated with different uses of voice ports. In the
"FXS Signaling Interfaces" figure, FXS signaling is used for end-user telephony
equipment, such as a telephone or fax machine. The "FXS and FXO Signaling
Interfaces" figure shows an FXS connection to a telephone and an FXO connection
to the PSTN at the far side of a WAN; this might be a telephone at a local
office going over a WAN to a router at headquarters that connects to the PSTN.
In the "E&M Signaling Interfaces" figure, two PBXs are connected across a
WAN by E&M interfaces. This illustrates the path over a WAN between two
geographically separated offices in the same company.
E and M Interfaces
connect telephone switches to one another; they do not connect end-user
equipment to the network. The most common form of analog trunk circuit is the
E&M interface, which uses special signaling paths that are separate from
the trunk’s audio path to convey information about the calls. The signaling
paths are known as the
E-lead and the
M-lead. The name
thought to derive from the phrase
although it could also come from
The history of these names dates back to the days of telegraphy, when the CO
side had a key that grounded the E circuit, and the other side had a sounder
with an electromagnet attached to a battery. Descriptions such as
adopted to help field personnel determine the direction of a signal in a wire.
E&M connections from routers to telephone switches or to PBXs are
preferable to FXS/FXO connections because E&M provides better answer and
Like a serial port,
an E&M interface has a data terminal equipment/data communications
equipment (DTE/DCE) type of reference. In telecommunications, the
trunking side is
similar to the DCE, and is usually associated with CO functionality. The router
acts as this side of the interface. The other side is referred to as the
like a DTE, and is usually a device such as a PBX. Five distinct physical
configurations for the signaling part of the interface (Types I-V) use
different methods to signal on-hook/off-hook status, as shown in the table
below. Cisco voice implementation supports E&M Types I, II, III, and V.
Table 1. EandM Wiring and Signaling
Battery Lead Configuration
referenced to ground
referenced to ground
Feed for M,
connected to -48V
Return for E,
galvanically isolated from ground
referenced to ground
referenced to -48V
The physical E&M
interface is an RJ-48 connector that connects to PBX trunk lines, which are
classified as either two-wire or four-wire. This refers to whether the audio
path is full duplex on one pair of wires (two-wire) or on two pair of wires
(four-wire). A connection may be called a four-wire E&M circuit although it
actually has six to eight physical wires. It is an analog connection although
an analog E&M circuit may be emulated on a digital line.
For more information on digital voice port
configuration of E&M signaling, see the "DS0 Groups on Digital T1/E1 Voice
Ports" section in the "Configuring Digital Voice Ports" chapter .
PBXs built by
different manufacturers can indicate on-hook/off-hook status and telephone line
seizure on the E&M interface by using any of the following three types of
is the simplest method of E&M access signaling. The calling side seizes the
line by going off-hook on its E-lead and sends address information as dual-tone
multifrequency (DTMF) digits
(or as dialed pulses on Cisco 2600 and
Cisco 3600 series routers) following a short, fixed-length pause.
Wink-start is the
most commonly used method for E&M access signaling, and is the default for
E&M voice ports. Wink-start was developed to minimize glare, a condition
found in immediate-start E&M, in which both ends attempt to seize a trunk
at the same time. In wink-start, the calling side seizes the line by going
off-hook on its E-lead, then waits for a short temporary off-hook pulse, or
"wink," from the other end on its M-lead before sending address information.
The switch interprets the pulse as an indication to proceed and then sends the
dialed digits as DTMF or dialed pulses.
signaling, the calling station seizes the line by going off-hook on its E-lead.
After a timed interval, the calling side looks at the status of the called
side. If the called side is on-hook, the calling side starts sending
information as DTMF digits; otherwise, the calling side waits until the called
side goes on-hook and then starts sending address information.
E and M Interface
In Transmission Only
(TO) mode configuration, CESoP is configured without CAS support to transport
voice data using T1/E1CESoP PW. Field Programmable Gate Arrays (FPGA)
configures the port to Transmission Only and port is always on off-hook mode.
In this mode,
signaling is not expected to provide through router.
E and M Signaling
Start dial supervision
is the line protocol that defines how the equipment seizes the E&M trunk
and passes the address signaling information such as dual tone multifrequency
(DTMF) digits. There are three main techniques used for E&M start dial
Start-This is the most basic protocol. In this technique, the originating
switch goes off-hook, waits for a finite period of time (for example, 200 ms),
then sends the dial digits to the far end.
Wink Start—Wink is
the most commonly used protocol. In this technique, the originating switch goes
off-hook, waits for a temporary off-hook pulse from the other end (which is
interpreted as an indication to proceed), then sends the dial digits.
Delay Dial—In this
technique, the originating side goes off-hook and waits for about 200 ms, then
checks to see if the far end is on-hook. If the far end is on-hook, it then
outputs dial digits. If the far end is off-hook, it waits until it goes
on-hook, then outputs dial digits.
Codec Complexity for Analog
codec stands for
codec is a particular method of transforming analog voice into a digital bit
stream (and vice versa) and also refers to the type of compression used.
Several different codecs have been developed to perform these functions, and
each one is known by the number of the International Telecommunication
Union-Telecommunication Standardization Sector (ITU-T) standard in which it is
defined. The various codecs use different algorithms to encode analog voice
into digital bit-streams and have different bit rates, frame sizes, and coding
delays associated with them. The codecs also differ in the type of perceived
voice quality they achieve.
Select the same type
of codec as the one that is used at the other end of the call. For instance, if
a call was coded with a G.711 codec, it must be decoded with a G.711 codec.
Circuit Emulation Service
(CEM) is a technology that provides a protocol-independent transport over IP
Networks. It enables proprietary or legacy applications to be carried
transparently to the destination, similar to a leased line.
The Cisco ASR 903 Series Router E & M IM supports pseudowire types
that utilize CEM transport, Circuit Emulation Service over Packet-Switched
Network in T1 and E1 modes to transport Analog Signals and Voice traffic over
The voice port interfaces that connect the router or access server to E
& M lines pass voice data and signaling between the packet network and the
For more information
about the Circuit Emulation Service over Packet-Switched Network on Cisco ASR
903 Series Router, see Chapter 16 Configuring Pseudowire.
Channel Associated Signaling (CAS) is also referred to as Robbed Bit
Signaling. In this type of signaling, the least significant bit of information
in a T1 signal is "robbed" from the channels that carry voice and is used to
transmit framing and clocking information. This is sometimes called "in-band"
signaling. CAS is a method of signaling each traffic channel rather than having
a dedicated signaling channel (like ISDN). In other words, the signaling for a
particular traffic circuit is permanently associated with that circuit. The
most common forms of CAS signaling are loopstart, groundstart, Equal Access
North American (EANA), and E&M. In addition to receiving and placing calls,
CAS signaling also processes the receipt of Dialed Number Identification
Service (DNIS) and automatic number identification (ANI) information, which is
used to support authentication and other functions.
Each T1 channel carries a sequence of frames. These frames consist of
192 bits and an additional bit designated as the framing bit, for a total of
193 bits per frame. Super Frame (SF) groups twelve of these 193 bit frames
together and designates the framing bits of the even numbered frames as
signaling bits. CAS looks specifically at every sixth frame for the timeslot's
or channel's associated signaling information. These bits are commonly referred
to as A- and B-bits. Extended super frame (ESF), due to grouping the frames in
sets of twenty-four, has four signaling bits per channel or timeslot. These
occur in frames 6, 12, 18, and 24 and are called the A-, B-, C-, and D-bits
The biggest disadvantage of CAS signaling is its use of user bandwidth
in order to perform signaling functions.
How to Configure Analog Voice Ports
Configuring CEM Group and Parameters on Analog E and M Voice Ports
Perform this task to configure cem group and basic parameters:
Enables privileged EXEC mode.
Enter your password if prompted.
Router# configure terminal
Enters global configuration mode.
controller voice-port slot/ subslot/ port
voice-port slot/ subslot/ port
Router(config)# controller voice-port 1/0/0
Enters voice-port configuration mode.
The show voice port summary command enters into the controller for further parameter configuration.
example shows how to configure xconnect.
Router# configure terminal
Router(config)# interface CEM0/1/0
Router(config-if)# cem 0
Router(config-if-cem)# xconnect 188.8.131.52 1 encapsulation mpls
Verifying E & M
The following show commands you can use to verify the E and M
show controllers voice-port : This command shows
how to configure voice port interface.
Router# show controllers voice-port 0/3/0
voice-port 0/3/0 - (A900-IMA6EM) is up
Encapsulation : None
Type : Type I
Operation : 2-Wire
Signalling : wink-start
Companding : u-law
show interface cem : This command shows how to
Router# show interface CEM0/3/0
CEM0/3/0 is up, line protocol is up
Hardware is Circuit Emulation Interface
MTU 1500 bytes, BW 64 Kbit/sec, DLY 0 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation CEM, loopback not set
Keepalive not supported
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/375/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/0 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 unknown protocol drops
0 output buffer failures, 0 output buffers swapped out
show cem circuit : This command shows how to
configure CEM circuit.
Router# show cem circuit 0
CEM0/3/0, ID: 0, Line: UP, Admin: UP, Ckt: ACTIVE
Controller state: up, T1/E1 state: up
Idle Pattern: 0xFF, Idle CAS: 0x8
Dejitter: 8 (In use: 0)
Payload Size: 32
Framing: Framed (DS0 channels: 1)
CEM Defects Set
Signalling: No CAS
RTP: No RTP
Ingress Pkts: 0 Dropped: 0
Egress Pkts: 0 Dropped: 0
CEM Counter Details
Input Errors: 0 Output Errors: 0
Pkts Missing: 0 Pkts Reordered: 0
Misorder Drops: 0 JitterBuf Underrun: 0
Error Sec: 0 Severly Errored Sec: 0
Unavailable Sec: 0 Failure Counts: 0
Pkts Malformed: 0 JitterBuf Overrun: 0
show xconnect all : This command shows the output
of xconnect command.
Router# show xconnect all
Legend: XC ST=Xconnect State S1=Segment1 State S2=Segment2 State
UP=Up DN=Down AD=Admin Down IA=Inactive
SB=Standby HS=Hot Standby RV=Recovering NH=No Hardware
XC ST Segment 1 S1 Segment 2 S2
UP pri ac CE0/3/0:0(CESoPSN Basic) UP mpls 184.108.40.206:1 UP
UP pri ac CE0/3/1:0(CESoPSN Basic) UP mpls 220.127.116.11:2 UP
UP pri ac CE0/3/2:0(CESoPSN Basic) UP mpls 18.104.22.168:3 UP
UP pri ac CE0/3/3:0(CESoPSN Basic) UP mpls 22.214.171.124:4 UP
UP pri ac CE0/3/4:0(CESoPSN Basic) UP mpls 126.96.36.199:5 UP
UP pri ac CE0/3/5:0(CESoPSN Basic) UP mpls 188.8.131.52:6 UP
show ip int brief : This command shows the
usability status of interfaces configured for IP.
Router# show ip int brief
Interface IP-Address OK? Method Status Protocol
GigabitEthernet0/4/0 184.108.40.206 YES NVRAM up up
GigabitEthernet0 220.127.116.11 YES NVRAM up up
CEM0/3/0 unassigned YES unset up up
CEM0/3/1 unassigned YES unset up up
CEM0/3/2 unassigned YES unset up up
CEM0/3/3 unassigned YES unset up up
CEM0/3/4 unassigned YES unset up up
CEM0/3/5 unassigned YES unset up up
References for Configuring E and M Signals
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