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Why
design a voice network?
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| 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. |
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Companies
with no network
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| 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. |
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Direct Inward
Dialling (DID/DDI)
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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Types
of private circuits
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| 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. |
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| 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. |
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Virtual
private networks (VPN)
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| 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. |
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| 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. |
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| 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 analyse networks based on
these networking technologies to show you the best way of using them in your company. Why not try our Telecom Design
Forum? It is an interactive newsgroup which you can use to exchange ideas about
telecommunications design.
Return to the technical document index
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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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