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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. |
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| Westplan helps you sort out the choices and control costs |
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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. Westplan can help you analyse each
of the solutions and put you in a much stronger position when considering your options and
negotiating with PTTs. |
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Westplan brings the ability to
understand and control telephone costs to all telephone administrators. Many readers will
be familiar with the principles of voice communications and voice network design. Others
may be newer to voice communications and if you are one of these readers, Westplan can be just as powerful a tool for you. However, you may
find it useful to read this chapter which introduces the basic principles of network
design and explains the different services on offer. |
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| Companies with no networks |
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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. |
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| DID ( DDI) is an inbound service offered by some telcos |
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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. |
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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. |
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| After a fixed rental is paid, calls on a PW are free |
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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 and Westplan will
show you whether such connections can be cost-effective for your company. |
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| Call blocking results from not having enough lines |
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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. |
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| Overflow prevents blocking |
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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. |
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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 - a task which Westplan automates! |
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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: |
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| From |
To |
| Chicago |
New York |
| New York |
Los Angeles |
| Los Angeles |
Chicago |
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Another option which would almost certainly be cheaper would
be to provide the following two routes: |
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| From |
To |
| Chicago |
New York |
| Chicago |
Los Angeles |
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| Overflow prevents blocking |
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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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Unfortunately, calculating the number of private wires
required between sites when tandem switching is involved is difficult and is further
complicated if PSTN overflow is used. Westplan carries out
this analysis quickly and easily for you. |
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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. |
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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. Westplan can show you if it would be cheaper to provide digital lines between
your buildings or whether a few analogue lines would be more economical. |
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| Voice and data integration |
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| Deriving discreet voice channels from a wide area data network |
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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. |
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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. The sizing of such overlay arrangements can easily be carried out using
Westplan. |
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| Moving the tandem switching function to a wide area network |
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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. |
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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. Westplan can also analysed these situations
by treating the wide area network as a separate network locations with connections to each
of the other sites. |
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| Virtual Private Network (VPN) |
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| The advantages of private networking without the management
overheads |
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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. |
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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. |
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| VPN calls are charged per minute and there are also fixed service charges |
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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. |
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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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The decision to adopt a VPN strategy is a difficult one and
involves considering many factors. The cost analysis for using VPN (and tandem switching
using backbone networks) can be aided with the use of Westplan. |
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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: |
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| Numbering plan with site codes |
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| Chicago |
770 1000 to 770 1999 |
| Los Angeles |
771 1000 to 771 1999 |
| New York |
772 1000 to 772 1999 |
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| Numbering plan without site codes |
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| Chicago |
1000 - 1999 |
| Los Angeles |
2000 - 2999 |
| New York |
3000 - 3999 |
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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 Westbay
Engineers Ltd. for omission or error. |
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| Last modified: 05 July 1999 |
Copyright © 1999. All rights reserved. |
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