VoIP
Background
T1 lines allow
for transmission of voice at 1.544 megabits per second (Mbps). A
T1 line carries twenty-four 64-Kbps channels. The mechanism to
place multiple calls on the T1 line is known as TDM (time division
multiplexing). TDM connections, known as circuit-switched
connections, always follow the same path through the network and
consume the same amount of bandwidth, whether there is useful data to
be transmitted
or not. For example, during a silent pause on a phone call, 64
Kbps of data
are still being transmitted in each direction. From an economic
viewpoint, this
is obviously inefficient.
Packet-switched networks like the Internet have significant advantages
over circuit-switched networks.
For example, packets can take different routes through the network,
making them resilient in the face of network failures. Also, the
only bandwidth used
is what's required to send the data.
Voice transmission involves both bearer and signaling
components. Bearer refers to content -- the actual voice -- while signaling refers to information necessary to setup and teardown a call: DNIS, ANI, on-hook, off-hook, etc.
With voice, tolerance for delays is low. All VoIP traffic is
transmitted using the Realtime Protocol (RTP) which, like TCP, wraps
UDP, adding a sequencing component so packets may be received in the
correct order.
A set of G-series recommendations by the ITU (International
Telecommunication
Union) comprise the common codecs (voice encoding/decoding algorithms),
e.g. G.711, which encodes at 64 Kbps, and G.729, which encodes
at 8 Kbps. Each makes its own tradeoffs for quality, compression,
etc.
There are primarily two types of signaling protocols: H.323, derived from the PSTN protocols, and SIP, inspired by HTTP.
ENUM allows VoIP to re-use DNS for telephone numbers.
Quality factors:
- Packet loss (2-5% is perceptible)
- Delay (150 ms is perceptible)
- Jitter (the variation in packet arrival
times)
- Codecs (intrinsic delay, varying tolerance for loss)
- Bandwidth (the RSVP reservation protocol,
RFC 2750).
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