Yealink SIP-T48G Administrator's Manual page 17

preferential to use this method when not using an application layer firewall. Application
layer firewalls like to know what applications are flowing though which ports and it is
possible to use content types of other applications other than the one you are trying to
let through what has been denied.
User agent server (UAS)
UAS is a server that hosts the application responsible for receiving the SIP requests from
a UAC, and on reception it returns a response to the request back to the UAC. The UAS
may issue multiple responses to the UAC, not necessarily a single response.
Communication between UAC and UAS is client/server and peer-to–peer.
Typically, a SIP endpoint is capable of functioning as both a UAC and a UAS, but it
functions only as one or the other per transaction. Whether the endpoint functions as a
UAC or a UAS depends on the UA that initiates the request.
This section introduces SIP-T48G, SIP-T46G, SIP-T42G, SIP-T41P , SIP-T29G, SP-T27P ,
SIP-T23P/G and SIP-T21(P) E2 IP phone models. These IP phones are endpoints in the
overall network topology, which are designed to interoperate with other compatible
equipments including application servers, media servers, internet-working gateways,
voice bridges, and other endpoints. These IP phones are characterized by a large
number of functions, which simplify business communication with a high standard of
security and can work seamlessly with a large number of SIP PBXs.
SIP-T48G, SIP-T46G, SIP-T42G, SIP-T41P , SIP-T29G, SIP-T27P , SIP-T23P/G and SIP-T21(P) E2 IP
phones provide a powerful and flexible IP communication solution for Ethernet TCP/IP
networks, delivering excellent voice quality. The high-resolution graphic display
supplies content in multiple languages for system status, call log and directory access.
IP phones also support advanced functionalities, including LDAP , Busy Lamp Field, Sever
Redundancy and Network Conference.
The following IP phone models are described:
SIP-T48G

SIP-T46G

SIP-T42G

SIP-T41P

SIP-T29G

SIP-T27P

SIP-T23P/G

SIP-T21(P) E2

IP phones comply with the SIP standard (RFC 3261), and they can only be used within a
network that supports this model of phone.
Product Overview
3