Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

years to the 0.1 percent to 1.0 percent range, and the delay is close to the speed
of light. We believe, therefore, the Internet “as is” to be adequate for high-
quality real-time communications, as long as congestion in the access net-
works is avoided.
Besides delay and packet loss that are relevant to transmission impairments,
occasional route flappingon the Internet can also impair real-time communica-
tions, though proper router maintenance procedures can reduce route flap-
ping, since human interventions for maintenance are quoted to produce 80
percent of the network problems. BGP route flap damping is described in [12].

IP Network Layer Services

Though the terms “Internet service” or “IP service” have many marketing con-
notations, IP-level services in the technical sense really refer to the level
of quality of service (QoS) provided. Figure 5.2 shows the spectrum of IP
services.
Most Internet and IP users are familiar only with the best-effort type of ser-
vice. Best-effort service is shown at the far left of the spectrum in Figure 5.2.
Best-effort service can provide adequate QoS for interactive communications
as long as there is no traffic congestion on any of the links between the respec-
tive endpoints. Best-effort IP service, however, cannot provide assurance that
QoS will be maintained at all times during a session, since congestion may
affect packet delay and packet loss in an unpredictable manner for media
packets. Rare route flapping events as mentioned previously can also be a
source of impairments.
Readers should not assume that best-effort service might not be adequate
for IP communications. Daily use of public VoIP services and of SIP telephones
on the public Internet by the authors have convinced us that, with adequate
broadband access (such as cable or DSL for home use), best-effort service pro-
vides high-quality telephony equal or better than that on the PSTN, though no
guarantees can be provided. Emerging fiber to the home and broadband wire-
less will also support better than PSTN voice services.

Figure 5.2 Range of IP and Internet services

Best effort,

unsignaled

Assured by

typed of service

Guaranteed by

per-flow reservation

Pure packet switching:
Most scalable

Circuit switching:

Least scalable

Assured by

aggregate reservation

Prioritized by

type of service

Real-Time Internet Multimedia 87