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Overview of Telephony Concepts and Components


Applies to: Exchange Server 2007 SP3, Exchange Server 2007 SP2, Exchange Server 2007 SP1, Exchange Server 2007

Topic Last Modified: 2007-11-14

If you are planning and deploying Microsoft Exchange Server 2007 Unified Messaging (UM) on your network, you must broaden your understanding and knowledge of Unified Messaging and telephony networks. This topic provides an overview of telephony infrastructure concepts and components and will help you plan and deploy a server that is running Exchange 2007 Unified Messaging.

In earlier versions of Microsoft Exchange, the Exchange administrator's main responsibility was managing e-mail messages and, sometimes, managing a network infrastructure.

However, the earlier versions of Exchange did not have unified messaging capabilities. The Exchange Server version 5.5, Exchange 2000 Server, and Exchange Server 2003 administrator had to focus only on the Exchange environment and the network infrastructure and relied heavily on telephony consultants to manage their telephony environment and infrastructure.

To successfully deploy a Unified Messaging server, you must have a good understanding of basic telephony concepts and telephony components. After you gain a good understanding of telephony basics, you can successfully integrate Exchange 2007 Unified Messaging into an Exchange 2007 organization.

To successfully deploy an Exchange 2007 Unified Messaging server in an Exchange organization, the Exchange administrator must become knowledgeable about data networking concepts and telephony terminology and concepts. This topic provides an overview of networking and telephony components and concepts that you must have to understand Unified Messaging. These include the following:

  • Circuit- and packet-switched networks

  • Private Branch eXchange (PBX)

  • Internet Protocol Private Branch eXchange (IP PBX)

  • Voice over Internet Protocol (VoIP)

  • IP gateways

In circuit-switched networks, such as the Public Switched Telephone Network (PSTN), multiple calls are transmitted across the same transmission medium. Frequently, the medium that is 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 is set up between two nodes so that 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.

PSTN is a grouping of the world's public circuit-switched telephone networks. This grouping resembles the way that the Internet is a grouping of the world's public IP-based packet-switched networks.

There are two basic types of circuit-switched networks: analog and digital. Analog was designed for voice transmission. For many years, the PSTN was only analog, but today, circuit-based networks such as the PSTN have transitioned from analog to digital. To support an analog voice transmission signal over a digital network, the analog transmission signal must be encoded or converted into a digital format before it enters the telephony WAN. On the receiving end of the connection, the digital signal must be decoded or converted back into an analog signal format.

There are advantages and disadvantages to circuit-switched networks. Circuit-switched networks have several disadvantages. Circuit-switched networks can be relatively inefficient, because bandwidth can be wasted. This is not the case when VoIP is used on a packet-switched network. VoIP shares the available bandwidth with all other network applications and makes more efficient use of the available bandwidth. Another disadvantage to circuit-switched networks is that you have to provision for the maximum number of telephone calls that will be required for peak usage times and then pay for the use of the circuit or circuits to support the maximum number of calls.

Circuit switching has one big advantage over packet-switched networks. In a circuit-switched network when you use a circuit, you have the full circuit for the time that you are using the circuit without competition from other users. This is not the case with packet switched networks.

Synchronous Digital Hierarchy (SDH) has become the primary transmission protocol for most PSTN networks. SDH is carried over fiber optic networks.

Packet switching is a technique that divides a data message into smaller units that are called packets. Packets are sent to their destination by the best route available, and then they are reassembled at the receiving end.

In packet-switch networks such as the Internet, packets are routed to their destination through the most expedient route, but not all packets traveling between two hosts travel the same route, even those from a single message. This almost guarantees that the packets will arrive at different times and out of order. In a packet-switched network, packets (messages or fragments of messages) are individually routed between nodes over data links that may be shared by other nodes. With packet switching, unlike circuit switching, multiple connections to nodes on the network share the available bandwidth.

With circuit switching, all packets go to the receiver in order and along a single path.

Packet-switched networks exist to enable data communication on the Internet throughout the world. A public data network or packet-switched network is the data counterpart to the PSTN.

Packet-switched networks are also found in such network environments as LAN and WAN networks. A WAN packet-switched environment relies on telephone circuits, but the circuits are arranged so that they retain a permanent connection with their end point. In a LAN packet-switched environment, such as with an Ethernet network, the transmission of the data packets relies on packet switches, routers, and LAN cables. In a LAN, the switch establishes a connection between two segments only long enough to send the current packet. Incoming packets are saved to a temporary memory area or buffer in memory. In an Ethernet-based LAN, an Ethernet frame contains the payload or data portion of the packet and a special header that includes the media access control (MAC) address information for the source and destination of the packet. When the packets arrive at their destination, they are put back in order by a packet assembler. A packet assembler is needed because of the different routes that the packets may take.

Packet-switched networking has made it possible for the Internet to exist and, at the same time, has made data networks—especially LAN-based IP networks—more available and widespread.

A legacy PBX is a telephony device that acts as a switch for switching calls in a telephony or circuit-switched network.

A legacy PBX is a PBX that cannot pass IP packets. In many businesses, legacy PBXs have been replaced by IP PBXs.

A PBX is a telephony device that is 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 that are 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. Telephone sets, in addition to fax machines, modems, and many other communication devices, can be connected to a PBX.

The PBX equipment is typically installed at a business's premises and connects calls between the telephones located and installed in the business site. A limited number of outside lines, also known as trunk lines, are typically available for making and receiving calls that are external to the business from an external source such as the PSTN.

Internal business calls made to external telephone numbers by using a PBX are made by dialing 9 or 0 in some systems followed by the external number. An outgoing trunk line is automatically selected to complete the call. Conversely, the calls placed between users within the business do not ordinarily require special dialing digits or use of an external trunk line. This is because the internal calls are routed or switched by the PBX between telephones that are physically connected to the PBX.

In medium- and larger-sized businesses, the following PBX configurations are possible:

  • A single PBX that supports the whole business.

  • A grouping of two or more PBXs that are not networked or connected to each other.

  • A grouping of two or more PBXs that are connected together or networked.

An Exchange 2007 Unified Messaging dial plan can span more than one PBX and one IP gateway.

An IP PBX is a Private Branch eXchange (PBX) that supports the IP protocol to connect phones by using an Ethernet or packet-switched LAN and sends its voice conversations in IP packets. A hybrid IP PBX supports the IP protocol for sending voice conversations in packets, but also connects traditional analog and digital circuit-switched Time Division Multiplex (TDM) telephones. An IP PBX is telephone switching equipment that resides in a private business instead of the telephone company.

IP PBXs are frequently easier to administer than legacy PBXs, because administrators can easily configure their IP PBX services by using an Internet browser or another IP-based utility. Plus, no additional wiring, cabling, or patch panels must be installed. With an IP PBX, moving an IP-based telephone is as simple as unplugging a telephone and plugging it in at a new location, instead of the costly service calls to move a telephone from legacy PBX vendors. Additionally, businesses that own an IP PBX do not have the additional infrastructure costs that are required to maintain and manage two separate circuit-switched and packet-switched networks.

Voice over Internet Protocol (VoIP) is a technology that contains hardware and software that enables people to use an IP-based network as the transmission medium for telephone calls. In VoIP, voice data is sent in packets by using IP instead of by traditional circuit transmissions or the circuit-switched telephone lines of the PSTN. An IP gateway that you connect to your IP network uses VoIP to send voice data packets between an Exchange 2007 Unified Messaging server and a PBX system.

An IP gateway is a third-party hardware device or product that connects a legacy PBX to your LAN. The IP gateway lets the PBX system communicate with your Exchange 2007 Unified Messaging server that is running IP.

The IP gateway can also connect to PBX systems that use VoIP instead of PSTN circuit-switched protocols.

Exchange 2007 Unified Messaging relies on the gateway's abilities to translate or convert TDM or telephony circuit-switched based protocols like ISDN and QSIG from a PBX to IP- or VoIP-based protocols like Session Initiated Protocol (SIP), Real-Time Transport Protocol (RTP), or T.38 for Real-Time Facsimile Transport. The IP gateway is integral to the functionality and operation of Unified Messaging.

After you install the IP gateway, you must create an IP Gateway object in the Active Directory directory service to represent the IP gateway. After you have created a UM IP Gateway object, the Unified Messaging server associated with the UM IP gateway will send a SIP OPTIONS request to the IP gateway to ensure that the IP gateway is responsive. If the IP gateway does not respond to the SIP OPTIONS request from the Unified Messaging server, the Unified Messaging server will log an event with ID 1088 stating that the request failed. To resolve this issue, ensure that the IP gateway is available and online and that the Unified Messaging configuration is correct.

For more information about IP PBX and PBX configurations, see Understanding PBX and IP PBX Configurations.

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