Internet
telephony (broadband telephony) and VOIP
Since
Internet telephony (also called broadband telephony or VOIP – voice-over-IP) is
increasingly being offered as an integral part of many ADSL providers service offerings
(as a cheaper alternative to ‘normal’ telephony), it seems relevant to provide
a brief overview of the typical VOIP services, what you will need and where the
pitfalls are:
Factor
to be considered |
Remarks |
Basic
Requirements for VOIP |
4
In order that a VOIP (Internet
telephony) service can work at all, at least three elements are required as
described below: 4
The VOIP service provider must
provide a VOIP server (usually nowadays a SIP server) 4
The VOIP user must be equipped
with a suitable VOIP telephone or other VOIP client device (e.g. SIP client) 4
An Internet or other IP-network
path must exist between VOIP caller and called destination |
Calling
from VOIP to the ‘normal’ or ‘legacy’
telephone network |
4
This is technically possible,
provided that the VOIP service provider/network operator has installed a media gateway (MGW), as well as the
appropriate network interconnection 4
For certain types of network
interconnection to the public telephone network, the VOIP network operator
may need to be registered as a public telecommunications operator |
The SIP
Server (alternatively
H.323 or MGCP server) |
4
The SIP server is the VOIP
equivalent of a telephone exchange. This arranges for the establishment of call
connections between VOIP caller and the called party (destination address) 4
Alternatively, an H.323 or MGCP
server could be provided instead of a SIP server. This are alternative
protocols to SIP. However, SIP is becoming the predominant protocol |
VOIP
telephone handset or softclient |
4
In order to make a VOIP call,
the VOIP user must be in possession either of a VOIP telephone or a
VOIP-software-client running on his or her PC 4
Alternatively, a telephone-to-VOIP adaptor (correctly
called an IAD – integrated access device) may be used (such a device may be
built-in to your DSL modem) 4
In order to avoid speech quality
problems (in particular bad echo heard by your VOIP counterpart), it is advisable
when using a VOIP softclient to use a headset and microphone (rather than the
speakers of your computer) |
Telephone
addresses used for VOIP |
4
Different VOIP service providers
use different types of addresses for making VOIP calls 4
Skype, for example, uses a
special Skype user name. All well and good if only other Skype users are to
call you, but not so good if you wish to be generally reachable under this
‘name-like’ address 4
Increasingly common is the
allocation by VOIP service providers of standard telephone numbers (so-called
E.164 numbers). Such a number allows a VOIP user to be called from any public
telephone 4
Remember that the format and
area code of the number you are allocated will affect the charge which
callers incur when telephoning you. |
Advantages
of VOIP |
4
The main advantage is the
reduced cost of making calls (often free calls to other VOIP users and very
cheap calls to normal telephone numbers) 4
Increasingly, VOIP service providers
are offering flat rate monthly usage charges for unlimited numbers of VOIP
calls 4
Another advantage is the ability
to use your VOIP telephone account as a ‘nomadic’ telephone line: anywhere
you can connect to the Internet you can be reached under the same VOIP
telephone number and make calls on your own account. |
VOIP
voice quality |
4
This can vary dramatically from
one VOIP provider to another 4
Ask your VOIP service provider
about the average service quality he achieves. Ask for his MOS (mean opinion
score). MOS is rated on a scale of 1-5. The public fixed telephone network
gets a score around 3 to 4, while mobile networks score a little lower. An
MOS of 3 or more for a VOIP network means that the voice quality is at least
as good as the telephone network 4
If your VOIP provider can’t tell
you his MOS, then he probably hasn’t bothered to set up a proper QOS
monitoring and management framework yet 4
Check which voice codecs are
used and the bitrate used to carry voice. A bitrate lower than 64 kbit/s will
mean voice quality lower than the telephone network 4
Ask your VOIP provider also for
his IP network quality of service targets for signal latency, jitter and
packet loss, as well as the planned backbone capacity in kbit/s per
connection. The target values should be: latency < 90ms, jitter < 20
ms, packet loss < 1% and minimum 64 kbit/s per connection. 4
The best VOIP connection quality
is achieved with a DSL connection, when the DSL modem prioritises voice ahead
of data – particularly on the bandwidth-limited upstream direction. 4
The best quality is thus
achieved on DSL (rather than dial-up connections) by DSL modems with built-in
VOIP integrated access devices (IADs) 4
Soft-client solutions and
peer-to-peer solutions (e.g. Skype) may achieve reasonable quality but are
inadequate when any data (e.g. from Internet surfing) has to share the same
Internet access line, since they are unable to prioritise the voice for
better QOS (quality of service) |
VOIP
quality: delay and echo |
4
Because the Internet imposes variable
and sometimes relatively long delays on the messages it carries, VOIP calls
can be subjected to longer delays than we are used to on normal telephone
connections 4
The delay itself can be
disturbing, since your correspondent may appear to take an unusually long
time to answer questions. Your may think he hasn’t heard and start talking
again, only to find you’re talking over his reply. 4
The delay can lead to unusually
long times to establish calls and unreliable establishment of calls. 4
Furthermore, the delay can lead
to echo (hearing your own voice again – somewhat time delayed) 4
Special measures may be
necessary to eliminate echo. One such measure is to use a headset rather than
the speakers of your computer. This avoids the possibility of the loudspeaker
noise being returned via your microphone and thus causing echo. The delay
itself cannot easily be eliminated. |
Security
and Fraud risks of VOIP |
4
Much is made by some operators (e.g.
Skype) about the benefits of signal encryption when using VOIP. Maybe you
feel this is important when choosing your VOIP service provider and VOIP
devices 4
But for most purposes the
chances of overhearing are pretty remote and the dangers probably not that
great. (There has never been encryption in the fixed telephone network, and
the encryption in mobile networks is primarily to prevent the chance of radio
overhearing). 4
I personally reckon there is a
far greater need to be concerned about fraud. If someone steals your VOIP
username and password, they can make calls from anywhere and run up your
account. Ask your VOIP provider what precautions are in place to minimise
this risk. |
Telephone
numbers which are difficult to reach or unavailable from VOIP phones |
4
Many VOIP operators are unable
to provide access from VOIP callers to various categories of telephone
numbers: 4
Calls to emergency services
(fire, police ambulance) are rarely available 4
Calls to service numbers (e.g.
800, 900) and operator services may be unavailable 4
Calls to mobile phones may not
be possible, be of poor quality and/or expensive 4
Calls to VOIP users of other
service providers may not be possible if there is not a ‘peering’ agreement |
Other
things you should be aware of |
4
Fax: may not work on a VOIP network – check with
your selected VOIP service provider 4
Calling Line Identity (CLI): is
offered on both outgoing and incoming calls by some VOIP operators, but is
not always 100% reliable 4
ENUM: this is a database of VOIP users. By participating
in the ENUM service, your VOIP operator is able to ensure the widest possible
access to VOIP users worldwide – using the most efficient connection path. 4
Lawful Interception: Currently, many VOIP operators do
not yet conform to telecom laws requiring them to enable ‘lawful
interception’ of telephone calls by public enforcement agencies such as the
police. However, this situation can be expected to change shortly. But
assuming you’re not a criminal, you have nothing to fear. 4
Number portability: while you are now able to ‘take
your telephone number with you’ should you change from one public fixed
telephone network operator to another, you may have difficulty porting your
existing fixed network number to a VOIP service and in number portability of
a VOIP number from one provider to another. |