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



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.