Networking: A Beginner's Guide
Voice over IP (VoIP)
An important emerging set of IP protocols concerns the transmission of voice and
facsimile information over IP-based networks, called Voice over IP, or VoIP for short
(pronounced "voyp"). VoIP is a protocol that allows analog voice data--for telephone
calls--to be digitized and then encapsulated into IP packets and transmitted over a
network. VoIP can be used to carry voice telephone calls over any IP network, such as
a company's local area network (LAN) or wide area network (WAN), or the Internet.
Sending voice data over IP networks has some very attractive possible payoffs. One
is more efficient use of available connections.
Consider a large company with two main offices. At any given time, hundreds of
voice conversations might be occurring between those two offices. Each traditional
voice connection consumes one DS0 line, capable of carrying up to 56 Kbps of data
if the line were used digitally. Each conversation does not use all of the available
bandwidth on the line. Part of this is because most conversations have a lot of silent
spaces--time between words or sentences, time where one party stops speaking and
the other starts, and so forth. Plus, most conversations, were they encoded digitally,
could be significantly compressed. Add all of this up, and each voice conversation
is likely to use only one-third to one-half of the available bandwidth on a single
If you were able to carry all of these voice conversations digitally, much less bandwidth
would be required. Instead of 100 DS0 lines for 100 conversations, for example, the same
conversations might use up only 25 to 33 DS0 lines if they were digitally packaged. Many
companies can save a significant amount of money by using VoIP.
Another advantage of VoIP is that the connections are packet-oriented. When the
user places a call, a single connection is formed between the caller and the receiver. This
connection is static for the duration of the call. If the conversation were digitized and
sent over a packet-oriented network, however, many possible paths would be available
for each packet, and much more redundancy would be automatically available. For
instance, if some portion of the network between the two points went down, the
packets could still arrive at their destination through an alternate route, just as data
packets do over the Internet. Also, available circuits would be used more efficiently,
allowing more calls to be routed within a particular geographic area.
VoIP also has some disadvantages that you need to consider:
No guaranteed delivery
VoIP does not guarantee delivery of IP packets over
the Internet. For a digital transmission of data, this is no big deal; if a packet
isn't confirmed as being received, it is simply retransmitted. For a real-time
voice conversation, the loss of packets directly inhibits the conversation, and
you can't go back in time to retransmit missing packets.