For decades PBXes (Private Branch Exchanges, small phone switches) were used in various businesses as central parts of their phone systems, providing various functionality such as communications between company's phones, voice mail, interface with the phone network, support of extensions for incoming calls and routing of internal calls through private lines between offices. As technology advanced, functionality of those systems expanded and included interactive menus, queuing and routing of customers' calls, and various forms of interaction with applications running on servers. VoIP technology not only drastically reduced the cost of those systems but also reduced the set of standards that have to be implemented to provide compatibility with phones and networks, allowing greater compatibility and expanding the number of vendors and providers of telephony equipment, software and services.
Currently various phone systems available on the market are mostly designed for small and medium-sized offices and include some usual PBX functionality, limited by the number of included interfaces and by performance of those systems' CPUs. Various systems have different kinds of interfaces, sometimes all of them in one device or interoperable set of devices:
Even though there are various telephony products and solutions on the market, it is always possible for a new product to compete with them by providing features and capacity demanded by the users if it can not be easily added to existing solutions without significant redesign or increased cost. Complexity and high cost of telephony components usually prevent users from building their own systems from off the shelf hardware and software components, so it is necessary to provide a full hardware+software solution that includes all components necessary to perform the desired functionality after being installed and connected to pre-existing equipment. The user should be able to provide the list of equipment that should be supported and type of the connections, and get a device or set of devices that can be directly connected to his network and provide all necessary services. If the set of equipment or lines expands demanding some additional type of interface, the upgrade procedure should involve adding a device (server with interface cards, or adapter) that easily integrates into the system.
Digium produces a full line of telephony interface PCI cards including PSTN/POTS, T-1 and PRI. While T-1 and PRI cards have dimensions suitable for most of 1u servers, 24-port PSTN/POTS boards are full length (12") oversized (top of the card protrudes above the bracket) boards that can't fit into most of the available 1u cases. TD-44 servers have sufficient space for one card with those dimensions, and an additional special riser should allow to install a T-1 card. So a single 1u box will have up to 24 analog lines and 24 channels to the phone company.
If used for direct calls (no transcoding or scripting), it is reported that a dual single-core server can safely pass 300-500 simultaneous calls, so this configuration will be able to handle 100% saturation of all lines while running at 10% of its call capacity. If the phone system is not a call center, and should not support 100% saturation with outside calls, it should be possible to add IP phones with additional extensions. In an office setup, assuming 50% peak usage of phones, it should be possible to add 24 more IP phones.
T-1 and PRI cards with four ports are half-length full height (same height as the bracket), so TD-44 server can have up to two of those cards given that no other PCI cards should be installed. A call center system with two quad T-1 or PRI cards in a single TD-44 server will support up to 192 outside calls, still reaching only 60-70% of its call capacity if used by the same number of IP phones (a call center setup), and should support up to 300 IP phones in a typical office.
In the configuration with two servers one server can provide 24 POTS interfaces for local phones, and another one with two quad T1 or PRI cards provide up to 192 channels like in the previous example. Assuming that 24 channels serve company office, there are 168 channels left for a call center equipped with IP phones, still using up to 60-70% of call capacity on the server with T1 cards and 10% of call capacity on the server with analog card. If necessary, spare capacity of the second server can be used to run conferences, scripts, databases and various computer telephony applications that may be helpful in the operation of that call center.
2u TD-88 server should support at least one quad T-1 or PRI card, and one 24-port analog card, providing 96 outside lines and supporting 24 analog phones plus 72 (call center) to 168 (office) IP phones.
The above examples show that TD-44 and TD-88 servers with telephony hardware, while providing the same channel capacity as existing telephony gateways (not servers or self-contained PBXes) with comparable pricing and physical dimensions, have a significant amount of spare CPU capacity, and the load can be distributed between multiple CPUs/cores to ensure that telephony-related processes' timing will not be affected by the load from other applications. Two Gigabit Ethernet interfaces should allow servers to act as a cluster, proxying and routing calls between a large number of servers, and distributing the more CPU-intensive task such as transcoding and conferencing within a cluster. Even without network-attached storage or external enclosures servers can provide hundreds of gigabytes for voice mail, databases, activity logs, etc., allowing to create small self-contained systems that act as intelligent PBXes and host computer telephony applications.
High performance and possibility to host complex applications allow to provide a pre-made solutions for various office PBX and call center problems. For example, all companies with a large number of internal extensions can benefit from a directory lookup application, plugged into a LDAP server. A web server can provide the local caller an interface that would allow him to transfer or conference a call while displaying other parties' and potential parties' status and history of the call, thus allowing to make a decision before even trying to communicate with someone other than the party on the phone, and avoiding common problems such as circular transfers, transfers to voice mail, attempts to invite unavailable parties into a conference with a large number of people waiting, etc.
A more complex yet very effective application that can run on this kind of server may be an integration of customer support queue with a CRM/support ticket tracking system. A customer waiting on the phone may be offered a way to enter some information into a web form by entering his ID or, if he is not identified, by providing him a temporary request number that he can enter on the company's web site and fill a form that will be presented to the agent once the call will be answered. This dual-media session established when the caller was waiting in the queue may be bidirectional -- an application may allow agent to pass a URL to the customer that will appear as a link in the form, thus eliminating the need to spell out long strings of text over the phone.
More "traditional" applications such as interactive menus, web interface to voice mail, general interface to policy-based or traditional queue-based call routing, agent status monitor, call topology recording and even some forms of analysis should be included as well. Despite significant differences in the business logic implemented in telephony-related applications, there are large parts of them that fall into common patterns, and as long as external web services API is provided, this system can easily host those common pieces of applications while the rest of business logic is implemented elsewhere or is handled manually through provided interface.
Large configurations may require hardware redundancy that can be provided by a separate device that physically switches outside lines between active and spare servers. IP telephony failover should be implemented in software. Small offices that lack QoS-capable routers and use external VoIP services may be better served by a pre-configured office QoS router provided with the system.