This article focuses on basic analog voice port configuration. When implementing a VoIP network, there are a number of different components that need to be designed and configured in order to make the whole solution work correctly. One of these many components, the voice port, is located at the intersection of the network between the packet based network and the traditional telephony network. Along with physically connecting together these two different network types, a voice port is responsible for correctly translating voice calls and signaling.
As the name suggests, analog voice ports transport the voice information between devices using analog methods. The three main types of analog voice port types include Foreign Exchange Office (FXO), Foreign Exchange Station (FXS), and Ear and Mouth (E&M) (also called rEceive and TransMit or Earth and Magnet).
FXO & FXS Voice Port Types
Most people are actually quite familiar with these interface types as they have existed in most homes for the last several decades. The FXS voice port type is used to connect to traditional telephones and fax machines, the telephone itself is actually a FXO device; FXO and FXS voice ports are always paired together. A FXS voice port provides ring, voltage and dial tone to the end FXO device. An FXO voice port is usually used to connect a device to a Public Switch Telephone System (PSTN) Central Office (CO) or to a Public Branch Exchange (PBX), when an E&M voice port is not available. Both FXS and FXO voice ports indicate on and off hook status and telephone line seizure information by one of two signaling methods: loop start or ground start.
Loop Start Signaling Method
The loop start signaling method is more common and is typically used by residential phone lines. When a voice port is configured with loop start signaling, the device (telephone) closes the circuit loop that signals the CO voice port to provide dial tone; an incoming call is signaled on the CO by supplying a predefined voltage on the line. The loop start signaling method has one main disadvantage in that it has no method of preventing both sides of the connection from attempting to seize the line at the same time; this condition is referred to as glare. Because of this, loop start signaling is typically not used on high demand circuits.
Ground Start Signaling Method
The ground start signaling method is typically used when connecting lines between PBX’s in a business setting, where high demand could result in multiple occurrences of glare. When a voice port is configured with ground start signaling, the device (PBX) grounds one side of the circuit that tells the CO to provide dial tone. Ground start signaling provides a method to detect off hook and trunk seizure that allows positive recognition of connects and disconnects. This ability makes ground start signaling the preferred method between PBX’s when not using an E&M voice port type.
E&M Voice Port Type
The E&M voice port type is used only to connect different switches together and not to connect to end user equipment. The E&M voice port type uses separate voice and signaling paths. With an E&M circuit, there is a signaling side and a trunking side; the signaling side is usually represented by a PBX and the trunking side is typically represented by a router. There are five different physical configurations that can be used when setting up an E&M circuit, these are referred to as types I through V. Cisco supports types I, II, III and V with type I being the most used in the US and type V being the most used outside the US. E&M circuits can also be configured in either a “two-wire” or “four-wire” configuration, this refers to the number of wires that are used to transmit and receive voice.
Like FXO and FXS voice port types, there are a number of different methods that can be used to indicate on hook / off hook status and line seizure. With E&M the available types include the following.
- immediate-start
- wink-start
- delay-start
Immediate-start
With immediate-start, the calling side of the connection seizes the line by going off hook on the E-lead and address information is sent using dual-tone multifrequency (DTMF) digits. Immediate start signaling is vulnerable to glare just like loop-start signaling.
Wink-start
Wink-start signaling is more commonly used and is the default type used. With wink-start signaling, the calling side of the connection seizes the line by going offhook on the E-lead, then waits for a temporary offhook pulse (“wink”) from the other end on the M-lead before address information is sent.
Delay-start
With delay-start signaling, the calling side of the connection seizes the link by going offhook on the E-lead, then waits for a specified amount of time (delay) before checking to see if the called side is on-hook. If the called side is onhook, address information is sent with DTMF digits, if the called side is not onhook, the calling side continues to wait and monitor until the called side goes onhook.
Analog Voice Port Configuration
The basic configuration of an analog voice port is not that hard to complete as long as a basic knowledge of port operation exists. Because of their slightly different port attributes, this section will show the commands available for FXO/FXS and E&M voice ports separately.
FXO/FXS
As with all configurations, the user must start in global configuration mode; the command syntax for this is shown below:
router#configure terminal
Once in global configuration mode, the user must enter the configuration mode for the specific voice port that is to be configured; the command syntax for this is shown below:
router(config)#voice-port slot/port
The next configuration step is to set the access signaling type to be used on the voice port; the command syntax for this is shown below:
router(config-voiceport)#signal {loop-start | ground-start}
The next step in the configuration sets the locale. The locale selected determines the voice port call progress tones and other locale specific parameters; by default the locale is set to us. The command syntax for this is shown below:
router(config-voiceport)#cptone locale
At this point, the configuration of the FXO and FXS voice ports diverge a little as some parameters are set on either one or the other port type. For the FXO port type, the next step is to set the dialing method for outgoing calls. The command syntax for this is shown below:
router(config-voiceport)#dial-type {dtmf | pulse}
For the FXS port type, the next step is to set the ring frequency, this must match the connected telephony equipment. The command syntax for this is shown below:
router(config-voiceport)#ring frequency frequency
The final basic step in the configuration of the FXO port type includes specifying the number of rings that are required to be seen on a router before a call is answered. The command syntax for this is shown below:
router(config-voiceport)#ring number number
The final basic step in the configuration of the FXS port type includes specifying the ring pattern to be used; by default a pattern is used based on the configuration of the locale. The command syntax for this is shown below:
router(config-voiceport)#ring cadence {[pattern01 | pattern02 | pattern03 | pattern04 | pattern05 | pattern06 | pattern07 | pattern08 | pattern09 | pattern10 | pattern11 | pattern12] | [define pulse interval]}
E&M
As with all configurations the user must start in global configuration mode, the command syntax for this is shown below:
router#configure terminal
Once in global configuration mode, the user must enter the configuration mode for the specific voice port that is to be configured; the command syntax for this is shown below:
router(config)#voice-port slot/port
The next configuration step is to set the access signaling type to be used on the voice port; the command syntax for this is shown below:
router(config-voiceport)#signal {wink-start | immediate-start | delay-dial}
The next step in the configuration sets the locale, the locale selected determines the voice port call progress tones and other locale specific parameters; by default the locale is set to us. The command syntax for this is shown below:
router(config-voiceport)#cptone locale
The next configuration step specifies the number of wires to be used for voice transmission on the port. The command syntax for this is shown below:
router(config-voiceport)#operation {2-wire | 4-wire}
The final step specifies the type of E&M interface that is connected to the voice port; by default the E&M type is set to 1. The command syntax for this is shown below:
router(config-voiceport)#type {1 | 2 | 3 | 5}
Summary
While the configuration of different voice ports in itself is not all that challenging, what is really important is having a good understanding of the different voice port types and how they operate. Once their operations are understood, their configuration becomes fairly simple, and typically is with most equipment. Hopefully the information in this article clarifies the differences between various voice port types, and helps in your basic understanding of analog voice port configuration.