In circuit-switched networks, such as the Public Switched Telephone Network, multiple calls are transmitted across the same transmission medium. Frequently, the medium that's used in the PSTN is copper. However, fiber optic cable might also be used.

A circuit-switched network is a network in which there exists a dedicated connection. A dedicated connection is a circuit or channel that's set up between two nodes so they can communicate. After a call is established between two nodes, the connection may be used only by these two nodes. When the call is ended by one of the nodes, the connection is canceled.

Different types or categories of telephone systems found in businesses and organizations include a circuit-based network, an IP-based network, or both. Each type of telephone system has distinct advantages and disadvantages you need to consider when planning and implementing a telephony system.

• Centrex   Centrex is a type of telephone service that telephone companies lease to businesses and organizations. A traditional Centrex telephone system eliminates the need for a business or organization to purchase the telephony hardware used onsite to support the organization's telephone system. Typically, Centrex systems are used by small offices that rent Centrex services from a telephone company on a line-by-line and month-by-month basis. Centrex telephony systems are sometimes used by larger organizations, but are most frequently found in government, public, and private organizations. Centrex frequently uses analog telephone lines for the connections to a business or organization. But it can also use T1-circuits with a demultiplexer onsite to support analog and digital telephones or ISDN lines.
  
  In a Centrex-based telephony system, the telephone company's central office acts as the telephone exchange. It's designed specifically to support the needs of a given organization. The central telephone office routes the calls that originate from inside the company to the appropriate internal or external telephone number. Centrex uses the telephone company’s central office exchange to route internal calls back to an extension. For example, with Centrex, the telephone exchange or telephone company's central office knows which extensions are internal. So an employee who's located within the organization's telephony network can dial another employee in the same telephony network or dial plan by using a 4-digit extension number. When a call is dialed to the internal telephone extension number, it's forwarded to the telephone company’s central office and then routed back to the extension number that initiated the call.
  
  

A variation of a traditional Centrex telephony system is called IP Centrex. In an IP Centrex telephone system, the call is sent through an IP gateway located at a telephone company’s central office or located onsite at a service provider. In this kind of telephone system, the IP gateway translates the call into IP-based data packets that can be sent over the Internet or over a Voice over IP–based network. However, if the call is sent over the Internet, there's typically another IP gateway that receives the call and then translates the call back to a traditional circuit-switched call.

Organizations that currently have a traditional Centrex telephone system in place have to install, deploy, and maintain one or more IP gateways for Unified Messaging to work correctly. Unified Messaging may require that you install, deploy, and maintain IP gateways to work with IP Centrex. Several variables will determine whether you need an IP gateway. These variables include the type of telephones used in your organization (analog, digital, or IP) and the protocols supported by the IP Centrex system.

• Key telephone   In a Key telephone system, the telephone company’s central office is connected to the organization using standard analog or digital telephone lines. A single telephone extension number is connected to multiple telephones so when a call is placed into the organization using this telephone number, all the telephones associated with that line or extension number will ring at the same time.
  
  

With Key telephone systems, individual users share lines across telephones. Therefore, callers won't experience frequent busy signals when they try to call into an organization. Key telephone systems are typically used by small offices where internal call volume is high but external call volume is low.

Key telephone systems have become more sophisticated over time and can work with Unified Messaging if an IP gateway is added. However, some less sophisticated systems may not work even if a supported IP gateway is used.

• PBX   A legacy PBX is a telephony device that switches calls in a telephony or circuit-switched network. A legacy PBX is a PBX that doesn't have a network adapter and can't pass IP packets. Because they can't pass IP packets, some businesses and organizations have replaced legacy PBXs with IP PBXs. For a list of PBXs supported by Unified Messaging, see Telephony Advisor for Exchange 2010.
  
  PBXs are used by most medium- and larger-sized companies. A PBX enables users or subscribers of the PBX to share a certain number of outside lines for making telephone calls considered external to the PBX. A PBX is a much less expensive solution than giving each user in a business a dedicated external telephone line. Telephones, in addition to fax machines, modems, and many other communication devices, can be connected to a PBX.
  
  The PBX equipment is typically installed on an organization's premises and connects calls between the telephones located onsite and the telephone company. A limited number of outside lines, also known as trunk lines, are typically available for making and receiving calls external to the business from an external source such as the PSTN.
  
  To enable a legacy PBX to be used with Unified Messaging, you need to deploy a supported IP gateway. For a list of supported IP gateways, see Telephony Advisor for Exchange 2010.
  
  
• IP PBX   An IP PBX is a PBX that has a network adapter that supports the IP protocol. It's a piece of telephone switching equipment that generally resides in an organization or business instead of being located at a telephone company office. There are two types of IP PBXs: traditional IP PBXs and hybrid IP PBXs. Both traditional IP PBXs and hybrid IP PBXs support the IP protocol for sending voice conversations in packets to VoIP-based telephones. However, hybrid IP PBXs also connects traditional analog and digital telephones.
  
  IP PBXs are frequently easier to administer than legacy PBXs, because administrators can more easily configure IP PBX services using an Internet browser or another IP-based tool. Also, no additional wiring, cabling, or patch panels has to be installed. With an IP PBX, you can move an IP-based telephone by merely unplugging a telephone and plugging it in at a new location. This lets you avoid the costly service calls required to move a telephone from legacy PBX vendors. Additionally, organizations that own an IP PBX don't have to incur the additional infrastructure costs required to maintain and manage separate circuit-switched and packet-switched networks. For a list of IP PBXs supported for Unified Messaging, see Telephony Advisor for Exchange 2010.
  
  Return to top
  
  

Analog PBXs send voice and call signaling information, such as the touch tones of a dialed telephone number, as an analog sound. Therefore, the sound is never digitized. To correctly direct the call, the PBX and the telephone company’s central office have to listen for the signaling information.

Note:

Touchtone is more technically known as dual tone multi-frequency. When a caller presses a key on a telephone keypad, the telephone produces two separate tones: a high frequency tone and a low frequency tone. When a person speaks into the telephone, only a single tone or frequency is emitted. Sending two tones with different frequencies at the same time reduces the possibility that the signaling tones will be interpreted as a human voice or that a human voice will be interpreted as the signaling tones.

Digital PBXs encode or digitize the analog sound into a digital format. Digital PBXs typically encode the voice sounds using a standard industry audio codec like G.711 or G.729. After the digitized voice is encoded, it's sent over a channel by using circuit switching. Circuit switching sets up an end-to-end open connection. It leaves the channel open for the length of the call and for the caller's exclusive use. However, the signaling method that's used by the PBX depends on the manufacturer. PBX manufacturers may have their own proprietary signaling method for call setup. For more information about the audio codecs used, see Understanding Unified Messaging Audio Codecs.

Note:
Digital PBXs can support both digital and analog trunk lines.

In larger organizations, PBXs make it possible for employees in separate physical locations to contact one another by dialing an extension number for a user. This can be done by using a single PBX or may involve multiple PBXs networked together. PBXs at different office locations can be connected to a single transparent circuit-switched network by using T1- or E1-lines. When these lines connect PBXs together, they are frequently known as tie lines. The PBXs communicate with one another across the tie lines using a PBX-to-PBX protocol, such as QSIG. QSIG lets a set of PBXs act as if they are a single PBX.

This kind of PBX environment can also include advanced features, such as call transferring and telephone conferencing. In addition to allowing for advanced features, having two connected PBXs can also save the organization money because long distance charges between employees in the different locations will be reduced. This is because a call made between two employees remains on a tie line between the PBXs and requires that the user dial only an extension number for the other user instead of placing a long distance call.

The following figure illustrates a typical telephony and data network that includes legacy or traditional PBXs.

In a telephony environment that includes a single or multiple analog or digital PBXs, an IP gateway is required between the PBX and the Exchange 2010 computer that has the Unified Messaging server role installed to convert the circuit-based protocols found on a telephony network into the IP-based protocols found on a data network. For more information about IP gateways, see the following topics:

• Understanding Unified Messaging IP Gateways
  
  
• Managing IP Gateways
  
  

For a list of IP gateways supported for Unified Messaging, see Telephony Advisor for Exchange 2010.

Return to top

A standard or traditional IP PBX contains at least a single network interface that connects to a data network using VoIP protocols. It may also contain additional network interfaces or other telephony interfaces that enable it to connect to an existing telephony network such as the PSTN. The connection to the data network allows for communication with other VoIP hosts located on the data network by using IP data packets. These VoIP hosts include other IP PBXs, VoIP-based telephones, IP gateways, and Unified Messaging servers. A traditional IP PBX is doesn't support analog or digital telephones. It supports only VoIP telephones.

The following figure illustrates a typical telephony and data network that includes a traditional IP PBX.

Because the IP PBX can already connect to a data network and can convert the circuit-based protocols from the PSTN to packet-switched VoIP protocols, an IP gateway may not be required to enable communication with UM servers on the data network.

Hybrid IP PBXs can provide analog, digital, and VoIP-based capabilities. If the correct interfaces are installed on an IP PBX and the software that supports multiple types of interfaces is installed correctly, the IP PBX is considered a hybrid IP PBX. An IP PBX hybrid makes it possible to use a mixture of analog, digital, and IP-based telephones.

Most modern IP PBXs can support and provide all three types of voice communication or a traditional IP PBX can be upgraded to a hybrid IP PBX by installing the necessary interfaces and software or firmware updates.

The mixture of analog, digital, and IP-based telephones makes it possible for users in your organization to use many new features and also provides great flexibility in your telephony environment. Using an IP PBX hybrid also allows for a more gradual migration to a completely VoIP-based telephony environment and voice messaging system for your organization.

The following figure illustrates a typical telephony and data network that includes an IP PBX hybrid configuration.

Several factors determine whether an IP gateway will be required when you connect with an UM server. One of these factors is the compatibility of the VoIP protocols used by the IP PBX or hybrid IP PBX and Unified Messaging. If an IP gateway isn't required, it will reduce the complexity of the telephony infrastructure, and the support that you must have for Unified Messaging will be simpler.

Return to top