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This page describes the principles behind and advantages of the proper design of voice networks. Its aim is to give the reader a basic grounding in the skills required to design economical carrier networks, private networks or VPN access arrangements.

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Voice network design

Why design a voice network?

Designers of private voice networks have two goals. Firstly, they want to save money. Secondly, they want to ensure that the users of their telephone systems, and perhaps more importantly, the people who call their company, get through on the first call.

The telecommunications industry around the World is undergoing enormous changes. Countries are opening their markets to competition, which is allowing new companies to offer alternative services to their customer. With so many telephone companies (also called PTTs, PTOs, telcos and carriers) offering a wide variety of voice telecommunications services, there is certainly scope for many companies to make significant savings on their telephone bills. However, it is important to understand how much money each type of service can save your company.

Companies with no network

Many companies have offices in more than one town. Staff in each of these offices will almost certainly need to talk to each other by telephone. If these companies do not have private voice networks in place, then these calls will be routed over the PSTN. PSTN stands for Public Switched Telephone Network. It is the basic dialled service offered by almost all telephone companies.

Direct Inward Dialling (DID/DDI)

Calls into a building routed over PSTN are often answered by switchboard operators who then connect them to the required extension. An alternative to this exists which allows incoming calls to be routed directly to extensions. This service is called DID (or DDI in the UK) and enables telephone users to publish their individual direct numbers rather than their company's main number.

The advantages of DDI are that dialling between offices within a company is quicker and callers from outside the company who know the extension number of the person they want to speak to can call that person directly. This also means that companies can save money because they require fewer switchboard operators although a switchboard service is usually retained to some degree to cater for general incoming queries where callers do not know who they need to speak to.

Although PSTN and DDI are very flexible and simple services, more economic solutions exist for companies which make high volumes of calls between their buildings.

Private leased circuits

Private leased circuits (also known as leased lines, private wires, private circuits and tielines) are direct connections between PBXs in two buildings. The two ends of these circuits can be in the same town or can be in different towns, or countries. Callers in one office can call users in another office by dialling a prefix code to access leased circuits and then dialling the users’ extension numbers.

PTTs (telephone companies) charge a fixed rental for each private circuit. However, the calls made on these circuits are free. There is obviously a break-even point in the number of calls made between two offices at which point a private circuit connection between those sites becomes economically attractive.

Another factor in the justification of private wires is the number of connections required. Unless PSTN overflow is used, sufficient circuits must be leased to carry all telephone calls between sites without being blocked. (Blocking is the failure of a call to connect because there are not enough lines). Callers who are blocked hear a permanently busy tone, even if the person they are calling is available.

PSTN overflow

A solution exists which avoids blocking if insufficient private leased circuits have been connected between two sites. Many telephone switchboards (PBXs) can be programmed to provide a more intelligent routing of calls between buildings. PABXs which have been configured for PSTN overflow will attempt to place calls between buildings over a private circuit if one is available. If all private wires are busy, calls will be routed over PSTN, the normal public telephone service. This option only works properly if the destination site has DDI, as previously described, to ensure that PSTN calls can be directly routed to the required extensions.

PSTN overflow avoids callers hearing a busy tone (blocking), but if a network is designed professionally, it can also provide economic benefits. The more calls a private wire is carrying, the more money is being saved. However, if a private wire connection is required between two sites, sufficient lines must be provided to carry all traffic including the occasional peaks in traffic. The result is that some lines are almost constantly in use and so are paying for themselves but a few lines are only used occasionally and become an expensive overhead.

If enough private wires are provided to carry the bulk of the calls, PSTN overflow can be used to route the occasional peak traffic over the public network. The rental saved in removing the peak traffic private circuit easily outweighs the cost of putting calls on the PSTN which charges per minute, but working out the optimum number of private wires between two sites using PSTN overflow is a complicated task.

Tandem switching

Things become more complicated if a company has more than two buildings. Imagine a company with offices in New York, Chicago and Los Angeles. This company wishes to install private wires to route its internal telephone traffic (telephone calls). One option would be to install three routes (groups of lines) linking all sites in a triangle.

That is:

From To
Chicago New York
New York Los Angeles
Los Angeles Chicago

Another option which would almost certainly be cheaper would be to provide the following two routes:

From To
Chicago New York
Chicago Los Angeles

The PBX in New York would be programmed to route all private network calls to the tandem PBX at Chicago. When the call arrived at Chicago, the Chicago PBX decides whether the call needs to be onwardly routed to Los Angeles or can be terminated on a local Chicago extension. The advantage of this set up is clear: fewer private wires will be required between the East and West sides of the U.S.A and, as private wire rental is usually charged on a per distance basis, a cost saving would be realised.

Types of private circuits

Different types of private wires are offered by PTTs. Analogue private wires are basic individual connections between two sites. Digital private wires use a more modern transmission technology which group lines together in multiples of 30 (24 in the USA) and provide a single connection to the PABXs at each end. For situations where a high number of private circuits is required, it can often be cheaper to lease these circuits using a single digital link capable of carrying 30 individual lines because digital private circuits are always cheaper than 30 individual analogue private circuits.

The decision to install a digital private circuit is usually based on a costing exercise although it should also be noted that digital circuits use a more modern technology which is more reliable and usually produces better quality connections.

Network convergence

For companies which operate a wide area data network, opportunities exist to share networking resources. Voice channels have traditionally been derived from TDM (Time Division Multiplexer) network equipment. However, recent improvements in technology are allowing voice communications to be established through ATM, IP and Frame Relay. Nevertheless, most voice over data solutions still involve the derivation of discreet voice channels from a data network which are connected to PBXs using traditional interfaces.

Some networking equipment is now becoming capable of voice switching rather than just the creation of fixed voice channels. Instead of just passing a call set up through the network transparently, a network node can analyse the addressing information of the calls (either a private numbering plan or an international E164 number) and can route the call accordingly.

Effectively, this solution moves the tandem switching function from a PBX to the wide area network, resulting in a reduced number of voice connections, and reduced hardware and transmission costs.

Virtual private networks (VPN)

Many telcos are now offering a modern alternative to private circuits which in many cases has great advantages over private circuits. Virtual Private Networking offers companies the advantages of private networking (with private wires) without the additional overhead of managing such networks. Links are provided between telco's and customer's PABXs in much the same way as a network of private wires except that the VPN network becomes the tandem site.

PABXs are programmed to pass all calls to other buildings in their company onto these VPN lines. The VPN provider then sends these calls to the correct destination based on the number which the callers have dialled. The numbering plan for the company can be programmed into the VPN network by the VPN provider so giving the impression to users that a private network is in place.

The charges made for a VPN service differ between the service providers but generally include elements of private wire costs and PSTN costs. A charge may be made for provision of the service; a charge may be levied per site and a fixed charge per line can be made. In addition to these fixed costs, which are usually lower than private wire costs, charges are made for calls made based on duration. This per minute charging is similar to PSTN but not as expensive. Another important cost advantage of VPN is that service providers are often eager to take calls to off-net locations, that is, locations which are not connected to the VPN and offer advantageous call rates for all calls made on the VPN lines. In this way, VPN can be considered an alternative to PSTN.

It should be noted that modern wide area data networks can provide connections which are similar in nature to VPN in that they can switch calls. This moves the tandem switching functions away from PABXs and reduce the associated hardware costs.

Numbering plans

Numbering or dialling plans have been mentioned in previous paragraphs. A numbering plan is a structured system of extension numbers which extend across some or all sites within a company. It is the means by which one user dials another. It can consist merely of a four or five digit extension number, each site being allocated a unique range of number or it can include a site location code which is usually three digits.

Here are two examples of numbering plans:

Numbering plan with site codes
Chicago 770 1000 to 770 1999
Los Angeles 771 1000 to 771 1999
New York 772 1000 to 772 1999


Numbering plan without site codes
Chicago 1000 - 1999
Los Angeles 2000 - 2999
New York 3000 - 3999

Numbering plans can be programmed into PABXs which use private wires or can be given to a VPN service provider to programme into their network so they know where to route calls. With modern PABXs, numbering plans can even be used when the only service between company sites is PSTN (the public network) hiding the fact that there is no network in place. The advantages of numbering plans should not therefore be used to justify the move to any particular networking strategy.


This page has given you a brief introduction into the many services offered by PTTs for routing telephone calls between your buildings.

Why not try our Telecom Design Forum? It is an interactive newsgroup which you can use to exchange ideas about telecommunications design.

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This document should not be viewed as a consultative document. It is the readers' responsibility to ensure that the most appropriate telecommunications strategy is applied to his or her business. No liability is accepted by the authors for omission or error.
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Last modified: 14 July 2014

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