IP Centrex and Unified Messaging

Collaboration, Variants, Requirements, Selection Criteria

by Sibylle Klein | serVonic GmbH | Public Relations


After the outsourcing of software solutions like e.g. UM-solutions into the cloud, the PBX is the next in line: With IP Centrex, the PBX now can also be used on-demand – the technologies Centrex and Voice over IP allow this. With Centrex, what means Central Office Exchange, a virtual or hosted PBX is available: The features of a PBX are provided for private individuals or companies by means of elements of a public telecommunication network. There is no hardware at the company itself. IP Centrex adopts the principle of Centrex into the Voice over IP technology. By this, PBX and software solutions such as unified messaging solutions can be used from the cloud commonly.


A standard-based unified messaging solution like serVonic’s IXI-UMS can collaborate with all kinds of PBX‘s
A standard-based unified messaging solution like serVonic’s IXI-UMS can collaborate with all kinds of PBX‘s

A Team - UM and TK

Unified messaging unifies all sorts of messages – e-mails, faxes, voice mails, short messages (SMS) – within one user interface. When a company already uses a messaging system such as Microsoft Exchange or IBM Domino, an add-on unified messaging that is integrated into the messaging system via Connectors is suitable. It adds UM-services like Fax, Voice and SMS and makes use of the available messaging architecture with MTA, data bases and clients. The user sends, receives and administers all sorts of messages via the regular e-mail inbox, e.g. Microsoft Outlook or IBM Notes. The collaboration with the PBX plays a central role in the field of unified messaging. Inbound as well as outbound UM-messages like faxes, short messages (SMS) or voice mails are exchanged with the PSTN by means of the PBX. The unified messaging solution works behind the PBX and processes the inbound or outbound messages accordingly. Besides, it does not matter for a standard-based solution like serVonic’s IXI-UMS, whether the PBX is ISDN-based or VoIP-based, a hybrid PBX or an IP Centrex.

The Collaboration

It depends on the respective solutions on the one hand and on the installation constellation on the other hand, on the basis of which protocols and interfaces IP Centrex and the unified messaging solution collaborate. Solutions with proprietary interfaces and protocols communicate with each other without any problems; however, the company depends on one manufacturer. When a company deploys solutions that are based on common standards, it does not depend on one manufacturer and can choose software solutions according to its requirements now and in the future. When a company, for example, switches over to another messaging system, the components messaging system, UM-server and PBX or IP Centrex continue to collaborate smoothly with each other and the new system is seamlessly integrated into the ITK-structure. This means when all the solutions involved are based in standards, two different constellations of the collaboration between UM-server and IP Centrex remain: All the components – messaging system, UM-server and IP Centrex – are used on-demand or the messaging- and UM-server are installed on-premises.

Messaging server, serVonic’s IXI-UMS Unified Messaging Server and unified messaging in the cloud communicate via the standards SIP and T.38 or SoftFax
Messaging server, serVonic’s IXI-UMS Unified Messaging Server and unified messaging in the cloud communicate via the standards SIP and T.38 or SoftFax

IP Centrex and Unified Messaging in the Cloud

When all the components – messaging server, UM-server and PBX – are in the cloud, UM-server and IP Centrex communicate via standards common in Voice over IP, namely SIP, T.38 or SoftFax. SIP means Session Initiation Protocol and is a standard for a signaling protocol, developed by the IETF (Internet Engineering Task Force). This protocol can establish, modify and terminate sessions with two or more participants. SIP takes care of the communication between UM-server and IP Centrex. T.38 is a protocol for the transmission of faxes via IP-connections described by the ITU-T. Faxes here are transmitted between fax server and IP-PBX not – as in the regular PSTN – as audio data, but packaged according to the protocol T.38. SoftFax is functioning via the G.711 Passthrough procedure. G.711 is a guideline of the ITU-T for the digitalization of analog audio signals. Here, the fax is transferred between fax server and PBX as audio data stream. T.38 as well as SoftFax allow the fax transmission here.
An example: When the user wants to send a fax, he creates a message with his e-mail client with the respective fax addressing. After having clicked on the “Send”-button, the UM-message is passed on to the UM-server, e.g. serVonic’s IXI-UMS, via the underlying cloud messaging system, e.g. Microsoft Office 365, IBM SmartCloud or Google Apps. The UM-server checks the permissions, converts the message into fax-capable format and passes it on to the IP Centrex via SIP and T.38 or SoftFax and then transmitted into the PSTN. When a fax comes in for the user, the order is vice versa: First of all, the call from the PSTN arrives at the IP Centrex. By means of the addressing, the IP Centrex detects that this is a connection request for serVonic’s IXI-UMS Unified Messaging Server and passes it on via the respective standards SIP and T.38 or SoftFax. The UM-server processes the message accordingly and transfers it to the messaging server, which delivers the message via the MTA to the e-mail inbox of the respective user. The reception and sending of SMS via fixed lines realized via the G.711 Passthrough procedure: Short messages (SMS) can be exchanged on the basis of the ETSI ES 201 912 - 1 norm.  [more]


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Expert Comment

by Jochen Klein,CEO
at serVonic GmbH


The IT- and PBX-world grow together more and more; with IP Centrex, we have virtual PBX in the cloud at our disposal. Despite this conciliation, we should not forget that the IT- and PBX-world still function in another way and that especially the messaging solutions like messaging system and UM-server collaborate fundamentally closer than PBX and UM-server. Our IXI-UMS Unified Messaging Server solution, for example, is seamlessly integrated into the SMTP-based messaging system used by the company: The UM-messages are transported via the MTA of the messaging system and die IXI-UMS user administration is handled in the data base intended for this purpose, e.g. in the Active Directory. In the mailbox data base, not only the e-mails, but also the UM-messages (faxes, SMS and voice mails) are deposited. This deep integration offers some advantages: The need of hardware remains lower, all the messages are always up to date, no inconsistencies can occur. When the user accesses his messages from on the way and processes them, for example, he directly accesses his mailbox and not a replication of it.  [more]

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