|
|
|
|
 |
 |
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 (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. |
|
|
| 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. |
|
|
| 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. |
|
|
| 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 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. 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
|
| 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. | | |
