LAN telephony integrates voice and data over the same medium, enabling automated call distribution, voice mail, and interactive voice response, as well as voice calls and teleconferencing between workstations. However, LAN telephony need not be confined only to LANs. Like data, voice can also go out over wide-area data networks; in some cases, it may not involve a local-area component at all. One of the most common uses of LAN telephony will be in the corporate intranet environment, where it is referred to as IP telephony.
Applications
With LAN telephony, users working away from their offices—at home or in a hotel—can use a single phone line to carry both data and voice traffic. Users dial up to access the corporate intranet, which is equipped and engineered to carry real-time voice traffic. Such a system provides an integrated directory view, enabling remote users to locate individuals within the corporation for voice- or e-mail connection in a unified way. Likewise, phone callers (internal or external to the corporation) can locate the mobile workers connected to any part of the intranet. Thus, LAN telephony allows users to work seamlessly from any location.
By using the LAN-based conferencing standards, transparent connectivity of different terminal equipment can be achieved; the media used by conference participants is limited only by what is supported by their terminal equipment. Connectivity to room-based conference systems or analog telephone can be achieved by means of gateways, which perform the required protocol and media translations. The building block of LAN telephony is the international H.323 standard, which specifies the visual telephone system and equipment for packet-switched networks. H.323 is an umbrella standard that covers a number of audio and video encoding standards. Among these standards is H.225 for voice packetization, which is based on the Internet Engineering Task Force’s (IETF) Real Time Protocol (RTP) specification and the H.245 protocol for capability exchange between terminals.
On the sending side, the uncompressed audio/video information is passed to the encoders by the drivers, then given to the audio/video application program. For transmission, the information is passed to the terminal management application, which may be the same as the audio/video application; the media streams are carried over RTP/UDP, and the call control is performed using H.225 to H.245/TCP.
Gateways provide the interoperability between H.323 and the public switched telephone network (PSTN), as well as networks running other teleconferencing standards such as H.320 for ISDN, H.324 for voice, and H.310/H.321 for ATM. An example H.323 deployment scenario involves terminals interconnected in the same local area by a switched LAN. Access to remote sites is provided by gateways, routers, or integrated gateway/router devices. The gateways provide communication with H.320 and H.324 terminals remotely connected to the ISDN and PSTN, respectively. H.323-to-H.323 communication between two remote sites can be achieved with routers that directly carry IP traffic over the Point-to-Point Protocol (PPP) running on ISDN. For better channel efficiency, gateways can translate H.323 streams into H.320 to be carried over ISDN lines, and vice versa.
Vendor Implementation
NBX Corporation, now a unit of 3Com, is among the growing number of vendors offering LAN telephony systems. The company’s NBX 100 Communication System leverages the ubiquity of Ethernet—once customers install NBX 100, they have a LAN infrastructure for connecting PCs, printers, servers, and other network devices. NBX 100 employs a pack-etizing technology that delivers high-quality voice reliably over the Ethernet. The system includes a network call processor for call control, voice mail, auto attendant, browser administration, and connectivity to the PSTN and WAN/Internet.
With connectivity to the WAN/Internet, NBX 100 also supports the growing demand for telecommuter offices. Using a standard multiprotocol router (ISDN, ATM, or frame relay), remote users gain access to the full suite of NBX 100 voice communication features, just as if they were in the office headquarters. Simultaneously, remote users also gain access to all the resources on their company’s LAN, including e-mail, file servers, and intranet/Internet access.
Last Word
Unlike traditional PBXs, a LAN-based telephony system has no central point of failure. Calls are routed between remote locations over a company’s wide-area data network, providing significant savings on longdistance calls, especially if the company has offices in many countries. Using a switched 100-Mbps Ethernet, network engineers can design telephone networks with essentially unlimited capacity. When the need arises for more workstations (extensions), another Ethernet switch is added. With H.323, these systems offer a high degree of interoperability. These systems can be administered locally through a Windows graphical user interface or remotely through a Web browser.
