IP PBXs use packet-switched rather than circuit-switched technology, and the software can run on standard Intel PC servers. IP PBXs comes in two flavors: PC PBX and LAN PBX. PC PBXs do not leverage the network infrastructure, instead delivering voice traffic purely over traditional telephone wiring. The LAN PBX, however, can route voice and data over your existing network infrastructure--that is, if your network is up to the task of real-time communications.
Although non-real-time voice traffic, such as voicemail, is transmitted in .WAV files over TCP, real-time audio is primarily sent via UDP, which does not retransmit lost packets. Network jitter, delay and congestion can cause dropped voice packets, resulting in stuttering audio. Voice-traffic bandwidths depend on the coding algorithms used to convert analog voice waveforms to a digital stream; they range from 24 Kbps to 80 Kbps. This can add up, depending on the number of simultaneous calls on the network and the available bandwidth left over from critical applications. Be sure you provision sufficient bandwidth for voice traffic or implement QoS (quality of service) guarantees with RSVP (Resource Reservation Protocol), DiffServ (Differentiated Services), or MPLS (Multiprotocol Label Switching). (For more on these protocols, see "Survivor's Guide to Digital Convergence".)
LAN PBXs are implemented in the enterprise in three ways. One approach combines all the voice traffic with data on the network. Utilizing PC servers, these systems can scale to support the required number of users and provide good call control. And once SIP is enabled, call control and management should be greatly improved. LAN PBXs also make it easy to add users and move them throughout the enterprise. End users can log into a phone just as they do on a PC and receive a consistent suite of services. On the downside, these implementations require IP or SIP phones, and voice traffic will reduce available network bandwidth. Also, fax servers still require analog telephone lines.
Other implementations of LAN PBXs separate some or all voice traffic from data traffic, using the LAN for call control and sending voice traffic over standard telephone cabling. The third method uses a separate LAN to carry digitized voice using line-interface units. These implementations require less network bandwidth and will work with standard telephone and fax equipment, but you need to maintain expensive analog or digital voice cards in the UM server to interface with the PBX.
Unified messaging started out as a simple concept: a single store for e-mail, voicemail and faxes. It promised to alleviate some of the pain for those who had to collect messages from multiple e-mail accounts and voicemail boxes on a daily basis or who lacked direct access to fax services.
But most enterprises have not reached this stage of simplicity. Why not? In short, because no one is talking.
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