A private voice network can save companies money on telephone bills and ensure callers can reach the desired extension on the first call. Designing such a network involves choosing between direct inward dialing (DID), private leased circuits, public switched telephone network (PSTN) overflow, or virtual private networks (VPN). The optimal solution depends on factors like call volumes, number of sites, and whether a wide area data network exists that could incorporate voice services. Numbering plans are also important to integrate different sites on a private network or VPN.

1. Topic :
Voice Networking

2. 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.

3. 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.

4. 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.
 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.

5.  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.

6. 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.

7.  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.

8. 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.

9.  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.

10. 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.
From To
Chicago New York
New York Los Angeles
Los Angeles Chicago
That is:
Another option which would almost certainly be cheaper would be to provide the
following two routes:
From To
Chicago New York
Chicago Los Angeles

11. 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.

12. 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.

13. 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 analyze 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.

14. 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.

15.  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.

16. 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

17. 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.

18. Conclusion
This page has given you a brief introduction into the many
services offered by PTTs for routing telephone calls between your
buildings. Westplan can easily analyze networks based on these
networking technologies to show you the best way of using them in your
company.