Some of the parameters for FXO/FXS interfaces also apply to E&M interfaces. However, additional parameters are required to support the different E&M's types, wiring configurations, and off-hook detection schemes.
Page 153 ! hostname vp_ex_1 ! ... ! voice-port 0/0/0 ! voice-port 0/0/1 ! voice-port 0/1/0 ring frequency 50 ! voice-port 0/1/1 ring frequency 50 ! voice-port 1/0/0 dial-type pulse signal ground-start ! voice-port 1/0/1 ! interface Ethernet2/0 no ip address shutdown ! interface Ethernet2/1 no ip address shutdown ! interface Serial2/0 ip address 192.168.1.1 255.255.255.0 ! interface Serial2/1 no ip address shutdown !
Figure 6-4
FXO voice port configuration example—router configuration.
The wide range of configuration possibilities and larger variance in vendor implementations make it unlikely that the default values will work for all parameters. This section presents the basic configuration parameters for E&M interfaces.
Off-hook signaling for analog E&M interfaces is explained in detail in Chapter 3. The three different methods, wink start, immediate start, and delay start, are all configurable and should be matched with the device to which the router is connecting. The voice port command which accomplishes this is the following:
signal { wink-start | immediate | delay-dial }
where the default is wink-start.
Page 154 The dial method can be set to touch-tone (DTMF) or pulse and should match the expectations of the system to which the router is connecting. The appropriate voice-port command is
dial-type { dtmf | pulse }
where dtmf stands for Dual Tone Multiple Frequency dialing, which is more commonly known as touch tone. DTMF is the default.
The details of call progress tones are standardized within geographic regions. Cisco routers support several regional call progress tones. Unlike FXS/FXO interfaces, regional support is limited to a few regions. These are configured with the following voice-port command:
cptone region
where region can be: australia, brazil, china, finland, france, germany, japan, northamerica, or unitedkingdom.
E&M interfaces can work in 2-wire or 4-wire configurations. The number refers to the number of wires used for voice transmission, not the total number of wires used on the interfaces. The following voice-port command is used to establish the number of wires to be used for the interface.
operation { 2-wire | 4-wire}
where 2-wire is the default.
The different physical wiring and signaling methods used for E&M are grouped into 5 types, E&M I through V. These types are discussed in detail in Chapter 3. The router should be configured to comply with the type of E&M signaling used by the external system it is
connecting to. Cisco currently supports all types except for type IV. The voice-port command used to set the E&M type is
type { 1 | 2 | 3 | 5 }
where type I is the default and the rest are dynamically configurable with the command above. The terminating impedance of the voice port must be set properly to prevent undue echo. The PBX or other system which is connecting to the router should specify the impedance required. Mismatched impedance settings will cause voice signals to be reflected at the interface and cause a significant amount of echo. PBX systems and central office
Page 155 switches in the United States usually run at 600 real ohms (600 r). The two ports on a VIC must be set to the same impedance and the ports must be reset by shutting it down and reenabling it before a new impedance setting will take effect. The voice-port command for setting the impedance follows:
impedance {600c | 600r | 900c | complex1 | complex2 }
where
600c represents 600 ohms complex 600r represents 600 ohms real 900c represents 900 ohms complex complex1 and complex2 are as written The default value is 600r.
Private-line-automatic-ring-down (PLAR) circuits can also be set up over E&M interfaces. Voice ports configured for PLAR automatically connect to a fixed destination as soon as the off-hook condition is detected. PLAR is configured using the following command:
connection plar string
where string is the phone number or dial string of the destination.
EXAMPLE
In this example of an E&M Voice Port Configuration, Figure 6-5 depicts a PBX connected to a 3640 which uses a company's IP infrastructure to transport voice traffic to the central site. The PBX requires 4-wire E&M type V with wink start and specifies an impedance of 900c. Figure 6-6 is a configuration excerpt from the 3640 router connected to the PBX.
Figure 6-5
Page 156 ! hostname vp_ex_2 ! ... ! voice-port 0/0/0 operation 4-wire type 5 impedance 900c ! voice-port 0/0/1 operation 4-wire type 5 impedance 900c ! voice-port 0/1/0 operation 4-wire type 5 impedance 900c ! voice-port 0/1/1 operation 4-wire type 5 impedance 900c ! ! interface Ethernet1/0 ip address 172.16.1.0 255.255.255.0 ! interface Ethernet1/1 no ip address shutdown ! interface Serial1/0 ip address 192.168.1.1 255.255.255.0 ! interface Serial1/1 no ip address shutdown ! Figure 6-6
E&M voice port configuration example router configuration excerpt.
Additional Voice-Port Parameters
In addition to the basic parameters presented above, additional voice-port parameters allow for the tweaking of voice-port sensitivity, timing values, and for the enabling of echo cancellation. The adjustment of these parameters helps fine tune the voice-port interface to better interact with the connected telephony device. The following parameters are configurable on all analog
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Gain and Loss Parameters
To control the input sensitivity of the voice port, the input gain parameter can be adjusted. It amplifies or reduces the voice signal based upon the factor specified. Input gain can increase the signal by as much as 14 decibels and decrease it by as much as 6 decibels. Adjusting the input gain may be required to maintain consistent volume levels throughout the voice network. The voice-port command for adjusting the input gain is
input gain n
where n is an integer between -6 and 14 representing the desired decrease or increase in decibels. The default is 0.
The voice-enabled routers also allow for control over the output levels of voice traffic through a voice port. Reducing the voice signal at the output port allows for an otherwise standard signal to be reduced in order to adapt to a specific system receiving the signal from that port. The
command which controls the level at which signals are reduced at the output voice port is
output attenuation n
where n is an integer from 0 to 14 and the default value is 0. Note that the output level cannot be increased.
Echo Cancellation
Echo caused by impedance mismatches impacts the perceived quality of voice traffic and can render the connection unusable. Echo is essentially the electrical voice signal reflected back from an end system. To counteract echo, an echo canceler can be placed inline between the codec transmitting to the voice port and the end system. Echo cancelers operate by storing samples of voice signals in one direction and waiting for the reflection of the stored signal to be received on the return path. At this point the canceler combines the echoed signal with the inverse of the memorized signal and effectively eliminates the echo. The length of time which an echo canceler remembers traffic patterns is sometimes called its coverage. To enable echo cancellation and adjust its coverage, the following voice-port commands are used:
echo-cancel enable
and
Page 158
echo-cancel coverage time
where time is in milliseconds and configurable to 16, 24, or 32 ms. The default value is 16 ms.
To enable voice ports to interoperate with a wide array of systems, various timing parameters are configurable on a per-voice-port basis. These parameters control timing for the transmission and duration of both dialed digits and signaling transitions.
Dialing Specific Timers
The amount of time a voice port waits for digits to be dialed after the connected device goes off-hook can be adjusted. To change this timeout from the default value of 10 seconds, the following voice-port command is used:
timeouts initial n
where n is the desired timeout in seconds. Acceptable values are from 0 to 120 seconds.
The amount of time the voice port will wait for the next digit to be dialed is configurable as well. If the timeout is exceeded, a tone is sent to the user and the call is terminated. The default of 10 seconds can be changed using the following voice-port command:
timeouts interdigit n
where n is the desired timeout in seconds. Acceptable values are from 0 to 120 seconds. When configured for DTMF dialing, the duration of individual digit tones and the time in between subsequent digits can be configured. The voice-port commands used for this are
timing digit m
timing inter-digit n
where m and n are in milliseconds between 50 and 100 ms and the default for each is 100 ms. When configured for pulse dialing, the duration of the individual pulses is configurable and so is the time that the voice port waits in between digits. Note that with pulse dialing, digits are comprised of mul-
Page 159 tiple pulses. The default-time pulse duration is 20 ms and the default time in between digits is 500 ms. The appropriate voice-port commands to modify these attributes are
timing pulse-digit m
timing pulse-inter-digit n
where m and n are in milliseconds; m is configurable between 10 and 20 ms and n is configurable between 100 and 1000 ms. FXS interfaces do not support these commands since they do not perform outbound dialing.