The Internet Encyclopedia (Volume 3)

P1: IML

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

650 VOICE OVERINTERNETPROTOCOL(IP)

Table 2Summary of Compression Algorithm Performance (Source:

Cisco, Understanding Codec Complexity (2002) and Sanford (1999))

Compression Bandwidth (Kbit/s) MOS score Delay (ms)

PCM (G.711) 64 4.4 0.75

ADPCM (G.726) 32 4.2 1

LD-CELP (G.728) 16 4.2 3.5

CS-ACELP (G.729) 8 4.2 10

MPMLG (G.723.1) 6.3 3.9 30

ACLEP (G.7231.1) 5.3 3.5 30

code modulation (DPCM) is another method used to re-

duce the number of bits transmitted without compromis-

ing quality. DPCM only transmits the changes in levels,

rather than absolute levels. Another variation on DPCM

is adaptive pulse code modulation, which can halve the

bit transmission requirements. Current compression al-

gorithms can reduce the 64,000 bit per second transmis-

sion requirement for a voice signal to as low as 8,000

bits per second without incurring a substantial increase in

delay.

Table 2 summarizes some of the most popular voice

compression algorithms used in Internet telephony. The

frame of reference is the G.711 standard. Note that its

MOS (“Mean Opinion Score,” an objective measurement

of the perceived quality of the received voice which ranges

from 0 to 5) is 4.4. Processing delay is negligible. All

other compression systems are more efficient (less than

64 kb/s of bandwidth consumption per voice conversa-

tion) but exhibit additional processing delays, which may

cause additional problems in long, echo-prone circuits.

For wide area networks (WAN), the G.729 standard is one

of the most popular, because it represents a reasonable

compromise between efficiency and delay performance,

at a modest 10 ms. The G.711 is most popular around

the campus, where bandwidth cost is generally not an

issue.

THE TELEPHONY NETWORK

The Communications Network Topology

From a user’s perspective, a telephone network is a big

cloud that connects the originating points for transmit-

ted information with terminating points for information.

A network is typically made up of nodes interconnect-

ing transmission links to form transmission paths for

the transmitted information to follow. The transmission

links in a typical network often use different transmis-

sion modalities (e.g., wire, wireless, fiber). The nodes may

contain electronic devices, such as switches, which estab-

lish a temporary or permanent path to be taken by the

communicated information traveling from one link onto

another link, and/or those nodes may contain conversion

devices that change the modality of the transmitted signal

to allow it to pass between links using different modalities

(e.g., connecting a wired link to a wireless link, an analog

link to a digital link, or a fiber link to a copper wire link).

See Figure 3.

Transmission Links

Most communications, whether digital or analog, are

transmitted over wire, fiber, or wireless links (i.e., the at-

mosphere). Each physical medium has its own physical

characteristics that limit the ability of the transmitted

Multi-Medium Network

Node

Node

Node

Fiber

Link

Microwave

Link

Copper Wire

Link

Access

Link

Access

Link

Network

Interface

Device

(e.g., telephone,

codec, and/or packetizer

Network

Interface

Device

Figure 3: Multimedium network.