The Internet Encyclopedia (Volume 3)

P1: IML

Morris WL040/Bidgoli-Vol III-Ch-53 September 15, 2003 12:23 Char Count= 0

654 VOICE OVERINTERNETPROTOCOL(IP)

Figure 6: VOIP network.

Figure 7 shows a generic signaling progression for es-

tablishing a VOIP call that might occur using H.323-type

centralized call control arrangement. First, the calling

telephone dials the called telephone’s number (1). The call-

ing phone forwards the dialed telephone number (address

signaling) to a VOIP call controller—which is a special

purpose server. That call controller does a lookup (2) in

a database for determining the IP address to reach the

called telephone. If the called telephone is on the IP net-

work, a call setup signal is routed to the called telephone

to ring the called telephone (4).

While the called telephone is ringing, a ringing signal

is sent back to the calling telephone (3), which telephone,

in turn, generates a ringing sound in the caller’s earpiece.

When and if the called party answers the telephone, a se-

ries of signals to set up the channel path are returned to

the calling and called telephones (5, 6, 7).

A UDP/IP communications path is then set up between

the calling and called telephone (8). The digitized voice

is transported using the user datagram protocol (or UDP)

in the transport layer, with two protocols, RTP (real time

protocol) and RTCP (real time control protocol), rather

than TCP—which is often used by non-voice data. UDP

is connectionless (e.g., packets can take different routes)

and can transport data packets without acknowledging

their receipt. UDP is nonstop with less address informa-

tion overhead. The tradeoff of using a UDP path is lower

reliability than TCP. UDP packets may be dropped or ar-

rive out of order, but if they do arrive they do with less

delay. This is a good tradeoff for voice communications,

which is highly tolerant to dropped packets, but relatively

intolerant to delay.

RTP over UDP provides packet sequence numbering,

so out-of-order and/or missing packets are detectable at

Calling

Telephone

Called

IP Network Telephone

IP Router

Call

Control

Gateway

1. Dial
   2. Look Up
   Dialed #


2. Generate
   Audible Ring


3. Ring


4. Off-
   Hook


5. Open
   Channel


6. Open
   Channel


7. RTP
   (Voice Stream)

IP Packets

Voice

Mail

Figure 7: VOIP H.323-type signaling system using central call control.