PBX for LAN telephony

LAN telephony is understood as a telephone communication organization in which only one signal wiring remains (in addition to power) in the building with a local computer network, either computer network (for example, Ethernet) or telephone. In this case, both the computers and telephone sets are connected to the same socket.

For the organization of telephone communications in these conditions, use alternative PBX (also called pseudo-PBX or un-PBX) – these are PBXs based on personal computers (PCs). Those un-PBXs that use the same signal wiring to transmit telephone signals and data can be classified as PBX for LAN telephony.

Systems of this class are divided into the following three main categories:

  • Distributed LAN-PBX. These systems consist of several elements: telephone servers, gateways, and telephone switches.
  • Integrated communication systems (all-in-one-box) – specialized devices with built-in OS and application software.
  • PBX with IP gateway – traditional PBX, where IP gateway cards are installed for IP communication.

The companies came to LAN-telephony in different ways. Those that dealt with the equipment supply for computer networks, proposed so-called distributed solutions that can be used at the offices without PBX in the usual sense (Cisco, etc.). Companies specializing in the production of traditional PBXs began to equip them with IP gateways – cards that are inserted into those PBXs that are already being manufactured and are in operation (Avaya, Ericsson, Siemens, Alcatel, Samsung, etc.). The same companies began to offer integrated communication systems.

All these systems are based on a common signal transfer and have a centralized intelligence, concentrated in the core of the system – a server or a PBX processor.

Distributed systems

In the LAN-telephony systems with distributed switching, the switching process is carried out in the local network. All such systems contain switches, IP telephones, gateways, packages with the voice in them do not necessarily go through the central core; Packets receive authorization and a route from the intelligent core and are routed to the phone or gateway. In such systems, the principle of distributed switching is implemented – it is implemented not in the switching field, but in the network itself. In this case, the call service servers are connected to the LAN and used to establish and disconnect traditional telephone connections, as well as to perform additional functions, for example, voice mail functions. As a rule, after establishing a connection in such systems, the servers are disconnected, and the subscribers’ phones exchange voice packets directly.


The overwhelming majority of local networks around the world implement some kind of Ethernet technology (typical network structure with distributed LAN-PBX based on Ethernet technology). However, if you need to quickly deploy a telephone and computer network in a building where there is only telephone wiring, it is advisable to dwell on distributed solutions for local ATM networks. In this case, high quality of telephone communication will be provided, but the network performance will be significantly lower than when using Ethernet. At the same time, telephone sets (analog or digital – depending on which telephone network is actually installed in the building) are connected directly to the network, and computer network equipment – via ATM modems. However, most often the ATM-enterprise class is implemented on wiring type twisted pair of a fairly high category, and the choice of this technology is dictated by the structure of traffic data, voice, and video.

Integrated communication system network

An integrated communication system (sometimes called an integrated LAN-PBX) combines the functions of the following devices: LAN switch, IP router, remote access server, VoIP gateway and PBX. In integrated communication systems (IP-PBX), switching is mainly based on PBX.

In this case, the PBX is supplemented, as a rule, with a LAN switch and IP router, therefore it is called an integrated communication system. The connection of switching is carried out by a telecommunication server in accordance with the list of services available to a particular user

The main advantage of such systems in comparison with distributed ones is greater functionality, and with systems based on upgrading existing PBXs by installing additional VoIP gateways – also in greater transparency.

Products in this category are represented by 3Com, Alcatel, Avaya, Samsung and Nortel Networks. Products from small companies such as Merlot Communications, Network Alchemy, Praxon and Vertical Networks are also available


When choosing the hardware and software solutions for organizing a LAN-telephony in your office, it makes sense to be guided by the following considerations

If your office already deploys Ethernet and telephone networks, then you probably have a PBX installed by a reputable manufacturer. In this case, it is advisable to dwell on an intermediate solution – to acquire an IP-gateway and install it in PBX.

If you are mastering a new building and installing a new wire, you will need an advanced distributed solution or an integrated communication system. However, in terms of one workplace, this would appear to be more expensive than a PBX with an IP gateway.

An integrated communications system is preferable to a distributed network solution if users are accustomed to receiving a variety of telephone services provided by traditional PBXs (call transfer, follow me, parking, pick-up, announcement, etc. – about 200 functions).

Distributed LAN-PBX supports only the basic functions of PBX (usually 10-20, at any rate, no more than 25 basic functions), but in some cases, this is not enough (for example, in distributed systems, as a rule, there is no barge in – operator intrusion into the conversation). In addition, there may be problems when connecting to a city exchange.

If you have already decided to implement the most suitable solution, when selecting a supplier, you should consider factors such as ease of configuration, the ability to receive updated software, scalability, diversity, and functions. If the main thing is reliability and fault tolerance, pay special attention to the possibility of clustering equipment provided by Cisco Systems. Some vendors also use redundancy of the main components (for example, in the Alcatel OmniPCX 4400).

If your company has a corporate ATM network, the opportunities for choosing PBX are dramatically reduced. Before choosing a specific supplier, it is advisable to analyze the above factors.

Even when a decision is made to deploy the equipment of a particular company, it will not be superfluous to use LAN phones from different manufacturers. The vast majority of such devices are considered compatible, but this applies only to IP-devices. In fact, this compatibility should be checked in practice. The equipment of 3Com company is practically incompatible with telephone sets from other manufacturers.

When choosing the hardware and software, one should take into account such factor as the cost per line or port, including the cost of the telephone set. The analysis showed that for this indicator the proposed solutions differ significantly – from $ 110 (Samsung) to $ 1000 (Nortel Networks and Cisco Systems).

The specific value of the cost depends heavily on the package and delivery conditions, which are determined by the contract.

The expediency of switching to the active use of IP-telephony in corporate networks is not in anyone’s particular doubt. However, when acquiring and deploying a corporate IP telephony system, you should first think about what you need it for. The fact is that leaving one wire instead of two, you can get a system that has less reliability, although more functionality. Therefore, evaluate in advance all the possible consequences of such a responsible step.

Standards and Algorithms

When creating LAN-telephony systems, two basic standards are used – H.323 and SIP. By now, the ITU H.323 standard has been most widely used, while developing it was the task of “adapting” telephone equipment to work in computer networks. This standard is close to traditional signaling systems (with circuit switching based on the Q.931 protocol), so it is most widely used in systems developed by traditional telephone equipment suppliers. It became the first standard protocol of IP-telephony.

The SIP standard was developed by the IETF group, it implements a simpler, internet-based approach to HTTP-based signaling. Therefore, it was the companies that traditionally produced equipment for computer networks that began to use it. However, everyone expects this standard to have a great future in LAN telephony, as it reflects the main trend – the “Internetization” of telecommunications.

To date, the SIP protocol is rapidly spreading, including through the efforts of SIPNET. Some functions, such as conference calling, are supported by the SIP protocol better than by H.323. Therefore, at the present time it is necessary to choose a solution supporting both these protocols, and in the future, perhaps, it is worth switching to using the SIP protocol. Some companies also use their own proprietary protocols (for example, Cisco’s Skinny Gateway Protocol, which it opened for use by other companies).

As for voice coding algorithms, the most popular conventional PCM is without compression (G.711) and G.723 compression mechanisms (5.3 / 6.3 Kbps) and G.729 (8 Kbps).

Telephone sets

Currently, the employee’s workplace can be equipped with voice communication in one of the following ways:

  • Phone for direct connection to a LAN outlet. In the case where Ethernet is implemented in the LAN, it is a digital Ethernet phone.
  • Multimedia personal computer (with a microphone and headphones or a speaker), equipped with telephone functions and connected to the LAN. The disadvantage of such an end device is typical for any personal computer – it, for example, can “hang” and communication will be interrupted.
  • A conventional analog telephone connected to the LAN.

Often, LAN phones are called IP telephones. They should not be confused with system telephones – digital telephones manufactured by the same company as the rest of the telephone equipment, or by a terminal.