3CX FAQ: Other Functionalities
What is Echo Cancellation?Before we talk about echo cancellation, we should briefly define “echo” in a telephony context.
What is Echo?Echo, in a telephony context, is a situation where a person’s voice in the outgoing stream from a handset microphone is reflected back to the same person in the incoming return stream to the handset’s loudspeaker.
Echo is typically created in one of 2 ways:
- voice coming through a phone’s loudspeaker gets picked up by the same phone’s microphone (sometimes called acoustic echo)
- electrical signals travelling over a sending wire get picked up be an adjacent receiving wire (sometimes called line echo); this can happen in a few different ways, but is typically to do with the electrical characteristic of the sending, receiving, and intervening devices
How Does it work?Echo cancellation is a simple concept – a voice processing engine analyses an incoming voice stream, and simultaneously monitors the return voice stream prior to transmission. If the engine detects a copy of the incoming voice stream in the return stream, it performs a “mathematical subtraction” on the return stream to remove the copy before sending it out for transmission.
Echo cancellation is sometimes combined with silence detection (and suppression), bringing in some bandwidth conservation benefits as well. However, silence suppression techniques are only really relevant in very limited bandwidth scenarios; as bandwidth becomes more readily available, silence suppression becomes less relevant.
What Does ENUM Mean?ENUM stands for Telephone Number Mapping. Behind this abbreviation hides a great idea: To be reachable anywhere in the world with the same number. Not only that but also via the best and cheapest route. The general idea behind ENUM is to unify the international Public Switched Telephone Network (PSTN), with internet addressing. ENUM takes a phone number and links it to an internet address (URL or IP address) which is published in the DNS system, that can be used in Internet Communications. The owner of an ENUM number can thus publish where a call should be routed to via a DNS entry. What’s more, different routes can be defined for different types of calls – for example you can define a different route if the caller is a fax machine. ENUM does require the specific support from the phone of the caller.
You register an ENUM number rather like you register a domain. At present many registrars and VoIP providers are providing this as a free service.
ENUM is a new standard, and is not that widespread yet. Though it looks to become another revolution in communications and personal mobility.
What Different Types of CODECS are There?A Codec is a device or software capable of encoding or decoding a digital stream or a signal for transmission over a data network. There are video and audio codecs. Codecs are divided into two categories. Lossless codecs and Lossy codecs. Lossless codecs retain all the information contained in the original stream thus preserving the audio/video quality in a signal, while lossy codecs reduce the quality to achieve compression but also use lower data bandwidth.
The following is a list of Codecs that are in common use today:
- GSM – 13 Kbps (full rate), 20ms frame size.
- iLBC – 15Kbps,20ms frame size: 13.3 Kbps, 30ms frame size.
- ITU G.711 – 64 Kbps, sample-based. Also known as A-law/μ-law.
- ITU G719 – 32/48/64/128 Kbps, 28 ms Frame Size.
- ITU G.722 – 48/56/64 Kbps.
- ITU G.723.1 – 5.3/6.3 Kbps, 30ms frame size.
- ITU G.726 – 16/24/32/40 Kbps.
- ITU G.728 – 16 Kbps.
- ITU G.729 – 8 Kbps, 10ms frame size.
- Speex – 2.15 to 44.2 Kbps.
- LPC10 – 2.5 Kbps.
- DoD CELP – 4.8 Kbps.
- VP8 is codec used for the encoding and decoding of high definition video as a either a file or a bit-stream for viewing. The VP8 codec is – in contrast to the H.264 codec – free for use.
- H.264/MPEG-4 Part 10 or AVC (Advanced Video Coding) is currently one of the most commonly used formats for the recording, compression, playback of high definition video. In contrast to VP8, H.264 is not free.