Building An Exchange Unified Messaging Lab (Part 3)

Posted on Posted in 2010, draytek, exchange, firewall, rtp, sip, unified messaging, voicemail

This blog is part of a series on creating a unified messaging lab for Microsoft Exchange Server. Configuring Unified Messaging was not as easy as I thought it would be and there was a lack of information that brought all the settings into one place, and a lot of incorrect information! The series started with Part 1 for the requirements and Part 2 for the initial configuration of AsteriskNOW and FreePBX.

Up until now the changes you have made have been pretty much the same for everyone. Sure, you have set an IP, keyboard and timezone that are different but everything else has been pretty much standard. Now we need to change some Asterisk configuration files to support Exchange Server Unified Messaging.

Configuring Asterisk for Internal and External Calls

As we have chosen to install FreePBX as well, we will edit the configuration files that FreePBX does not control. If you are doing your configuration without FreePBX installed there will be different files to change.

Before we make the changes though, you need to decide a few things. Some of these will be determined by your current environment. The first thing you will need to know is the number of digits in your dialplan. A dialplan is the internal extension number configuration at your office. For example if you dial 1xxx to reach one office and 2xxx to reach another then you have a four digit dialplan and sequences starting 1 and 2 are already reserved. In my lab I am going to use a four digit dialplan where 8xxx is going to be allocated to physical telephone handsets (extensions) and 8000 is going to be the number I call to listen to my voicemail (the Pilot Number) when I am using Exchange 2010 and 8500 when I am using Exchange 2013. Two numbers for voicemail allows me to use two different Exchange labs from one set of SIP phones.

Once you have picked your dialplan you can start to configure the various components of your PBX for your telephone network. These changes include forwarding your pilot number (8000 and 8500 in this blog) to Exchange and configure your telephone extensions.

In Asterisk we need to do these configuration changes by editing the config files. We can do this in a few different ways. We can edit the config files directly in the Linux console (using text editors such as vi), use WinSCP from a Windows PC if you don’t want to edit the files in Linux directly or use FreePBX for some of the changes. You must use FreePBX to change any file that has the FreePBX banner at the top of the config file.

SIP.Conf Changes for NAT and Exchange Server

Firstly, if you have a NAT’ed network you need to tell Asterisk your external IP address. Edit /etc/asterisk/sip_general_custom.conf to contain:

nat=yes
;externip needs to be your public IP
externip=w.x.y.z
;localnet=internal_IP_network/subnet_mask
localnet=192.168.5.0/255.255.255.0

You also need to add the following to the same file:

context = default
bindport = 5060
bindaddr = 0.0.0.0
tcpbindaddr = 0.0.0.0
tcpenable = yes
promiscredir = yes

Amongst these changes some of them tell Asterisk to listen on TCP, bind to all IP addresses and listen on port 5060 for UDP. Exchange Server and Lync Server require TCP support from the IP PBX that they connect to and without these settings Asterisk will only do UDP. Asterisk 1.8 will only listen on 5060 for TCP and there is no config setting to change this. The bindport setting controls the listening port for UDP.

Notice that we changed the sip_general_custom.conf file and not sip.conf. If you did not have FreePBX installed you would make all your changes to Asterisk in the config files and so could edit sip.conf directly. FreePBX overwrites some config files with its settings whenever you click Apply Config in the web GUI. To avoid having your settings overwritten you need to make them to files that are referenced by include statements in the master file.

For this example, if you open sif.conf (in /etc/asterisk) then in the [general] section (where the above edits are needed) you will see #include sip_general_custom.conf. This tells Asterisk to load sip_general_custom.conf as part of sip.conf, and we know that sip_general_custom.conf will not be overwritten by FreePBX because it does not tell us this at the top of the file.

To determine the file that you need to make the change in for other config files open the master file that you need to edit (i.e. sip.conf in this example) and see if there is a FreePBX banner at the top of the file. If not, then edit the file as required. If there is a banner telling you not to make changes then look for the section that your change will be inside (for example in sip.conf above we made our initial changes in the [general] section) and locate the #include statement that follows that section. This statement tells Asterisk the name of additional config files to load and to consider as part of the master file that you are currently reading. Some of these include files contain the FreePBX banner as well but others don’t for example to make changes to the [general] section of sip.conf we will edit sip_general_custom.conf, the custom config file for the general section in the sip.conf file.

RTP.Conf Changes For Your Network

SIP is the protocol that is used to manage connections between the parties involved in the call. RTP is the protocol used to transfer the voice data. You need to edit /etc/asterisk/rtp.conf so that the rtpstart and rtpend values are suitable for your network.

For each call connections will be made to 5060 and two additional ports. These two additional ports need to be sequential, and the odd numbered port will carry RTP data (voice traffic) into your PBX and the even numbered port carries RTCP packets (data about the connection). Outbound SIP/RTP traffic is determined by settings on the other parties PBX, so you typically need to allow all outbound ports from your PBX.

Therefore you need to configure Asterisk to have a start and end range for RTP that is a minimum of two ports (for one concurrent call) and a max of the number of concurrent calls you can make to through your PBX. Your external firewall will need to be configured to publish all these ports to your IP PBX so don’t make the range too big – but equally you need two ports per concurrent call so don’t make the range too small.

The range will always be the higher of the max number of calls your SIP Trunk provider allows and the number of physical handsets you have (plus some overhead to allow for parked calls). So if you have a five call SIP trunk, ten staff members, and 12 handsets you would need to support at least 12 concurrent calls. Therefore configure RTP to start at 10010 and finish at 10034 (two ports for each of the twelve concurrent calls you can support). Then increase it a bit for your sanity!

Edit /etc/asterisk/rtp.conf so:

rtpstart=10110
rtpend=your calculated value

 

Make sure your firewall forwards these ports to this PBX server and if you have other PBX servers ensure that you do not use the same port range. The following shows an example firewall configuration for this PBX. In the picture and in my config files I am using 5065 for SIP as I have two PBX’s and the other is using 5060.

 

image

 

Once we test calls to the outside world, if you start getting “one way traffic” (that is you can be heard but you cannot hear the caller or the reverse) then you need to check your firewall rules.

 

In Part 4 the fun will start. In this part we will configure a few telephone extensions so that we can make internal calls and then configure a SIP Trunk provider so we can make external calls. Part 5 will look at configuring Exchange Server 2010 and Part 6 the same, but for Exchange Server 2013. Part 7 will look at connecting these calls to your Exchange Server when we want to record a voicemail message.

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