Voice over IP

Broadband modems

Assuming users already have broadband, the type of modem they have is important.

A home or office modem should be Network Address Translation (NAT)-enabled. NAT is a protocol used by a device such as a firewall, router or computer, and provides a mapping between internal IP addresses and public IP addresses in real time. The modem may be NAT-enabled with a DHCP server in it (Dynamic Host Configuration Protocol is for assigning dynamic IP addresses to devices on a network). In most cases, when the ATA is plugged in, it will boot up and automatically seek out the softswitch of the service provider to which it is connected. More on softswitches later, but for now it is important to note that the softswitch is owned by the VoIP service provider and will direct phone calls to where they are supposed to be going.

If users have an old broadband account, it is likely that their modem may not be up to scratch for voice delivery.

Users must look for these requirements in a modem:

It must terminate L2TP or PPTP: some ISPs have a point-to-point session used for authentication. If a point-to-point session goes from user's ISP to the software on their PC, they are in trouble. This is because as soon as they plug in their ATA, their ISP is going to see the packets coming from their ATA and say "you are not authenticated". If the modem authenticates, then everything behind it will get through. Some of the older and cheaper modems don't do L2TP because of additional cost.

Ethernet on the modem: USB is inferior to Ethernet because it means the modem needs to be connected to the PC to talk. In this case, users would be limited to a soft phone.

CPU power: Modems are CPU based. A slow CPU means it can only do so many packets per second. A VoIP phone call transmits at 100 packets per second (50 packets per second each way). That is the sort of performance that every phone call is going to require of a modem (and ATA). Therefore a modem must be able to handle that traffic.

Soft phones

Soft phones, as their name suggests, are not hardware devices. The purely software services, such as FreshTel, Skype or BroadBand Phone, require users to install small software applications, or clients, on their PC to run the service. In this case, users need to connect a USB/FireWire handset or microphone headset to a PC to talk.

The key point to note here is that users need to have their PC turned on at all times to make and receive calls, so if there is a power outage, you aren't able to use the service.







Codecs Description Sampling Rate (Khz) Bitrate (Kbps) MOS
G.711 Pulse Code Modulation (PCM) 8 64 3.65
G.722 7kHz sub channels 16 64 3.6
G.722.1 Coding for low frame loss systems 16 24/32 n/a
G.723.1 Dual-rate speech coder 8 5.3/6.3 3.8
G.726 Adaptive Differential Pulse Code Modulation (ADPCM) 8 16/24/32/40 4.3
G.729 Conjugate-Structure Algebraic-Code-Excited Linear-Prediction (CS-ACELP) 8 8 4.02
GSM Global system for mobile communications 8 13 2.5
iLBC Internet low bit rate codec 8 13.3 3.7
LPC-10 Linear-predictive codec 8 2.4 2.3
Speex Multivairable rate, with Narrow and Wide band operation 8/16/32 2.15-24.6 (Narrow Band), 4-44.2 (Wide Band) n/a
This table lists well known codecs used to compress/decompress human speech signals. Sampling rate shows the range of sound frequencies sampled; the bit rate determines the quality of the sound reproduced by the codec, where a higher bit rate allows for better sound quality to be transmitted, bearing in mind a high bit rate will utilise more bandwidth. The Mean Opinion Score (MOS) is a numerical measure (a rating out of 5) of the quality of speech at the receiving end of a phone line.

Source: Symbio Networks















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PC World Staff

PC World

1 Comment

Robert

1

There was an article about this being used in the Voice Over industry on http://www.allstarvoices.com

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