The Internet Encyclopedia (Volume 3)

P1: IML

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

VOICE OVERINTERNETPROTOCOL 657

Calling

Telephone

Called

Telephone

IP Network

IP Router

or Switch

Call

Control

Gateway

2. Look Up
   Dialed #


3. Generate


4. Call

4b. Off-

Hook

5. Open


6. Open
   Channel


7. RTP
   (Voice Stream)

IP Packets

Voice

Mail

PSTN

4a.

Ring

SS7*

7a. Call

Setup

* SS7 connection if Gateway

Is Carrier Provided

7.Call Setup

1. Dial

Audible Ring Channel

Figure 10: VOIP signaling internetworking with public switched network (PSTN).

a phone on a PSTN network. A device, called a gate-

way, is used to translate signaling messages across the

VOIP/PSTN network boundary and to transform the jit-

tery voice packets on the IP side of the gateway into a

synchronous stream of voice data information (if there is

a digital voice circuit on the other PSTN side of the gate-

way) or an analog voice signal (if there is an analog voice

circuit on the PSTN side of the gateway).

On the IP side, the VOIP signaling to the gateway looks

much the same as the signaling that would be done to

another end device on the IP network.

ENUM, the Fully Interoperable

Numbering Plan

ENUM is a new standard for numbering plans that

would allow seamless telephone number addressing be-

tween conventional and VOIP telephony. See, e.g., Neustar

(2003) and IETF, Telephone Number Mapping (ENUM)

(2003). It unifies Internet and conventional telephone ad-

dressing schemes by mapping E.164 (i.e., conventional

telephone) numbers to a URL Internet (and SIP)-friendly

format. With ENUM, a single global digital identifier sys-

tem can serve equally subscribers attached to the PSTN

or the Internet. The same identifier can be used to identify

multiple devices, such as plain telephones, fax, voicemail,

and data services, regardless of whether they are on the

PSTN or public Internet, thus conserving scarce E.164

numbers and domain name resources.

As an example, a telephone number 1-305-599-1234

would map onto the URL 4.3.2.1.9.9.5.5.0.3.1.E164.arpa.

A DNS query on this domain name would return a number

of records, each listing a specific service registered to the

owner of the E.164 number. Devices and services attached

anywhere on the public Internet or PSTN could be regis-

tered to this universal, fully portable, single number iden-

tifier of their registered owner, allowing mobility around

both the PSTN and the public Internet.

The ENUM standards raise new issues of privacy, se-

curity, and administration. Also, final agreement between

Internet and traditional telecom industry represented by

the ITU is still pending.

Quality of Service Issues

Transmission and routing can introduce effects that de-

grade the quality of VOIP. VOIP signals are not immune

to the deficiencies of the facilities that transport and route

them. These deficiencies can cause packet loss, jitter, and

delay.

Packet loss refers to the loss of packets containing some

of the voice samples during transmission. The loss might

be caused by high bit error rates in the UDP transmission

channel, misrouted packets, and/or congestion causing

intermediate routing devices to drop packets. Because

voice transmission is very sensitive to delay, the reassem-

bly of the voice signal at the receiving end of the call

cannot typically wait for the retransmission of erroneous

or misrouted packets. However, voice transmission has

a high tolerance for packet loss—mainly because the ul-

timate receiver of the signal (the human ear and/or in-

tervening CODEC) does a good job of interpolating (i.e.,

filling in the gap) where a packet has been lost. There is a

limit to how much packet loss can be tolerated. That limit