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

Figure 2.5 Mixed enterprise voice systems using legacy PBX, the PSTN, and VoIP.

Firewalls and network address translators (NATs) are not shown here for

simplicity, but are discussed in Chapter 9, “SIP Security” and in Chapter 10,

“NAT and Firewall Traversal.” SIP servers placed in both public IP network

domains of ISPs and in private enterprise networks can, however, perform

many functions for end users, as shown in Table 2.8.

Large enterprises must often have a long and smooth transition from the

installed base of PBX systems and new SIP phones, as well as existing connec-

tivity to the PSTN. This is also illustrated in Figure 2.5.

Table 2.8 Functions Performed by SIP Servers

Register SIP phones and other SIP devices

Register individual end users for access to their services

Register end-user preferences

Perform Authentication, Authorization, and Accounting (AAA) for end users

Look up the address of the other endpoint

Route call requests to the appropriate server

Route to devices according to user preferences

Support user mobility across networks and devices

Register, filter, and publish information about presence

Inform users of call progress, success, or failure

Communicate requests for QoS to other network elements

Telephone

Switch

SIP

Server

Third-Party

SIP Server

Enterprise

GWY

Network

GWY

SIP

Phone

PBX

CAS

Enterprise Network ISP Network PSTN

SIP SIP SS7

Q.931

PBX

Phone

PSTN

Phone

30 Chapter 2