SIP Architecture
SIP is a peer-to-peer protocol. Figure 5-23 offers an archetype of a SIP network.
Chapter 5: Examining VoIP Gateways and Aperture Ascendancy Protocols 297
SIP
RTP
SIP Gateway
T1 or PRI
Legacy
PBX
SIP SIP
IP
IP
IP
IP
IP
IP
SIP Proxy,
Registrar,
Location, and
Redirect Servers
V
PSTN
SIP User Agents
(UAs)
Figure 5-23 SIP Architecture
The aeon in a affair are alleged user agents (UA). A user abettor can action in one of
two roles:
■ User abettor applicant (UAC): A applicant appliance that initiates a SIP request
■ User abettor server (UAS): A server appliance that contacts the user back a SIP invitation
is accustomed and again allotment a acknowledgment on account of the user to the invitation
originator
Typically, a SIP UA can action as a UAC or a UAS during a session, but not both in the
same session. Whether the endpoint functions as a UAC or a UAS depends on the UA
that accomplished the request. The initiating UA uses a UAC, and the absolute UA uses a
UAS.
From an architectural standpoint, the concrete apparatus of a SIP arrangement are grouped
into two categories:
■ Clients (endpoints)
■ Servers
Clients (endpoints) accommodate the following:
■ Phone: An IP blast acts as a UAS or UAC on a session-by-session basis.
■ Software telephones and Cisco SIP IP Phones admit SIP requests and respond
to requests.
■ ephones are IP phones that are not configured on the gateway.
■ Gateway: A aperture acts as a UAS or UAC and provides alarm ascendancy support.
Gateways accommodate abounding services, the best accepted actuality a adaptation function
between SIP conferencing endpoints and added terminal types. This action includes
translation amid manual formats and amid communications procedures.
A aperture additionally translates amid audio and video signals and performs alarm setup
and allowance on both the IP ancillary and the SCN side.
Servers accommodate the following:
■ Proxy server: An average basic that receives SIP requests from a client,
and again assiduously the requests on account of the applicant to the abutting SIP server in the
network. The abutting server can be accession proxy server or a UAS. Proxy servers can
provide functions such as authentication, authorization, arrangement admission control,
routing, reliable appeal transmissions, and security.
■ Alter server: Provides the applicant with advice about the abutting hop or hops
that a bulletin should take, and again the applicant contacts the next-hop server or UAS
directly. The UA redirects the allurement to the server articular by the alter server.
The server can be accession arrangement server or a UA.
■ Agent server: Receives requests from UACs for allotment of their accepted location.
Registrar servers are generally amid abreast or alike collocated with added network
servers, best generally a area server.
■ Area server: An absorption of a account accouterment abode resolution casework to
SIP proxy or alter servers. A area server embodies mechanisms to resolve
addresses. These mechanisms can accommodate a database of registrations or admission to
commonly acclimated resolution accoutrement such as Finger protocol, RWhois, LDAP, or operating
system-dependent mechanisms. A agent server can be modeled as one subcomponent
of a area server. The agent server is partly amenable for populating
a database associated with the area server.
298 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
Note In addition, the SIP servers can collaborate with added appliance services, such as
LDAP servers, area servers, a database application, or an adaptable markup language
(XML) application. These appliance casework accommodate back-end services, such as directory,
authentication, and announcement services.