User Manual

SIP-Service-Domain owner to provide its services to its subscribers. Sometimes, the
SIP Registration Server will be used for distinguishing purpose.
There are three kinds of the server:
z SIP Proxy Server; it relays call signaling, i.e. acts as both client and server
operating in a transactional manner, i.e., it keeps no session state
z SIP Redirect Server; it redirects caller to other servers
z SIP Registrar; it accepts registration requests from users and maintains user’s
whereabouts at a Location Server.
1.3 VoIP-Friendly Netw orking Conc e pts
There are many common features from regular router which will block your VoIP
communications, or make your network environment very VoIP-unfriendly. Here are
some topics which you need to take care of, or might be helpful in making a
VoIP-friendly network.
1.3.1 NAT, Network Address Translation
One of the most common features which router provides in order to provide Internet
access to all the computers in your LAN environment is “NAT”. In the simplest case,
NAT changes the source IP address of a packet received from a device to another IP
address before forwarding the packet towards its destination. When the response
comes back, NAT translates the destination address back to the device's IP address
and forwards it to the device.
NAT routers are commonly used to translate private (or internal) IP addresses in
packet headers to public (or external) IP addresses and vice versa. A NAT router
maps a private IP address & port; pairs it to a public IP address & port and whenever
the NAT router receives a packet with that public IP address & port, it knows how to
reroute the packet back to the private IP address & port.
The WIFI-ATA must know the public IP address which you get from your ISP in order
to provide its services. If there is a NAT router between the WIFI-ATA and your
broadband modem then your WIFI-ATA probably has a private IP address. In this case,
the SIP session will not be able to establish properly, and you must configure the NAT
router to forward traffic with the correct IP address & SIP port number to solve this
problem.
4