7 min read

Local telephony interfaces

Having set the tones, announcements, and language so that your users feel at home, the next task is to physically connect our Asterisk to the outside (telephony) world.

Fortunately, the RJ-45 is ubiquitous as far as Ethernet connectivity is concerned, so attention can be focused on traditional telephony connections.

Analog

It is a fairly safe bet that whatever equipment you choose to connect your analog lines and phones to Asterisk, whether you have gone with an ATA or a Digium card, the termination will be with an RJ-11 socket (with the middle two pins used for the pair).

This means that the lead that plugs into that RJ-11 socket becomes a mission-critical accessory, which, in a number of cases, is not shipped with your equipment.

You need to be absolutely sure that the lead you use has the right pin to pin connections as well as the right connectors at each end.

This may seem like stating the obvious, but it is included as a result of a Digium support case where a customer in the UK reported that the analog card they were using was faulty. The case got to the stage where the support team remotely accessed the machine in the UK and established that there appeared to be an unexpected voltage on one of the legs of the telephone line. When someone questioned the patch lead connecting the card to the PSTN, only then was it established that the customer had just picked up a lead that had the right ends and plugged it in, without worrying about whether the electrical connections were right. It turned out that the lead was probably for some old modem and it cross-connected some of the pins.

Of course, physical connectivity is essential, so is ensuring that the pinouts are correct. In the left part of the following picture, we can see the type of lead in question. It has an RJ-11 at one end, and a BT (British Telecom, the main telco in the UK) plug at the other. To the right is a picture of the two ends of the lead that would be required to connect to an analog telephone line in Korea. This has been included to give an idea about the diversity of connectors that are used around the world.

asterisk1.4-02

From a regulatory standpoint, it should be pointed out that only analog telephony cards and ATAs with the appropriate approvals should be connected to the telephone network in a given country.

The main learning point here is to ensure that you consider physical connectivity as part of your localization planning.

Digital

Digital telephony interfaces will be in two groups—basic rate access (otherwise known as BRI or ISDN2), and primary rate access (otherwise known as PRI, E1, T1, or ISDN, and sometimes ISDN30 in countries where 30 voice channels are used).

BRI is almost universally connected using an RJ-45 patch lead, which will look like a standard Ethernet patch, but if more closely inspected, it becomes apparent that only four wires are connected.

PRI connectivity started being implemented using two BNC connectors (one for transmit and one for receive), but is more commonly connected using RJ-45s these days.

Note that a PRI patch lead with RJ-45s at each end will be different from a BRI patch cable, which as mentioned, is different from an Ethernet patch. So, label your cables!

In countries such as the UK, you would not expect to find signaling systems with telco presentation on co-axial connections, but many countries (for example, those found in South America) still utilize R2 signaling, which may well be presented using co-ax.

Most PRI apparatus (including telephony cards) will use RJ-45 sockets these days. Therefore, it may be possible that in some countries you are presented with a situation where the telco is giving you two BNC connectors, but your equipment wants an RJ-45 plug. This is simply remedied by using a balun (so named because it is connecting a balanced interface, the RJ-45 end, to an unbalanced interface, the BNC end). Here is a picture:

asterisk1.4-03

When I say “simply remedied”, that is as long as you have thought of it in advance, and ordered these inexpensive, yet very useful adapters. If you have not thought of it prior to implementation and find yourself in need of a balun, be prepared to wait to get them shipped. These are not items you will find at the corner store.

Localizing caller ID signaling on Digium analog interfaces

Given the work we have already identified how to localize your Asterisk telephony solution in the previous article, it will not come as a complete surprise to learn that the method used to send caller ID information over analog lines varies from country to country.

Fortunately, Asterisk is capable of recognizing many different types of caller ID signaling. This should not be a problem as long as you remember to set up the correct type for the country where the system will be connected.

All caller ID settings are made in chan_dahdi.conf (formerly zapata.conf), which is found in the /etc/asterisk/ directory.

When you look into this file, the only lines you see concerning caller ID are:

usecallerid=yes
hidecallerid=no

Asterisk will be using its default type of caller ID signaling, which is North American. You may not even see these lines, as this is the default position.

In order to change things, you will need to add some extra lines to the file. The number of lines you add will depend on the type of signaling you want to work with, and whether you are connecting to analog phones, or lines, or both.

Shown next, is the kind of entry you would expect to see for a connection to an analog telephone line, with the entries that change the caller ID signaling highlighted:

context=from_outside
signalling=fxs_ks
usecallerid=yes
hidecallerid=no
cidsignalling=v23 ;BT CallerID presentation signalling method
cidstart=polarity ;Indication of CallerID data starting
callerid=asreceived
callwaiting=no
channel => 4

The two highlighted lines are equipping Asterisk to recognize the caller ID as it is sent in the UK over BT (British Telecom) analog lines. The callerid=asreceived line is not part of the change and serves to propagate the caller ID information through Asterisk. Therefore, it will show up on connected extensions when they are called.

What we saw just now covers the case where we want to interpret incoming caller ID data. However, we also need to look at the sending of caller ID to any analog phone that we have connected to our Asterisk. If the analog phones are connected using ATAs, it is those devices that will handle the caller ID. If the phones are connected to a Digium card, then we are back in chan_dahdi.conf.

context=users
signalling=fxo_ks
callerid="David Duffett"
cidsignalling=v23 ;BT CallerID presentation signalling method
cidstart=polarity ;Indication of CallerID data starting
sendcalleridafter=2;The number of rings before sending data
mailbox="5001"
callwaiting=yes
threewaycalling=yes
transfer=yes
channel => 1

In the code that we saw just now, the third highlighted line is particular to only a few caller ID signaling systems, and instructs Asterisk to send the caller ID data after (in this case) two rings. This is for systems where the data is sent between rings. It is the kind of system used in the UK.

A final note on caller ID:Do make sure that your customer actually has caller ID enabled on their telephone line before you run around worrying that Asterisk is not recognizing it. There was a guy who spent a good amount of time analyzing why the caller ID was not showing up in Asterisk by changing settings, changing them back, and so on before asking the customers if they actually had caller ID on their line. The response: “No, we never use it”. Needless to say, it’s the kind of mistake you make only once or twice.


Subscribe to the weekly Packt Hub newsletter

* indicates required

LEAVE A REPLY

Please enter your comment!
Please enter your name here