On the other hand DSLAM it is included in user's cross-country. Considering that that DSLAM has built in сплитеры, to it, except subscribers, connecting lines from PBX the enterprises (streams Е1-2048кбит/с) also can be connected. DSLAM divides streams from subscribers into phone calls which go through automatic telephone exchange to a network of TfOp and a data transmission network (the data, the Internet, an IP-telephony).

On the party of the subscriber located user's сплитер which divides a signal of a telephone system and a signal behind frequency that intended for the ADSL-modem to which the workstation of the end user is directly connected.

Let's consider an example of connection process with SIP-report use (the example is taken from RFC 3261). The set example reflects work of base functions of a telephony and accordingly transfer of text messages does not mention such possibilities as visual communication, etc. - the general principle of work of the report remains invariable.

User Alice (SIP:alice@atlanta.com) causes user Bob (SIP:bob@biloxi.com).
-User Alice sends message INVITE a proxy-server by default (atlanta.com) If to user Alice the IP-address of user Bob was known and he could address to it directly inquiry INVITE in this case could be sent directly caused user.
-The proxy-server sends inquiry INVITE to a server of the caused subscriber (biloxi.com).
-Further the proxy-server of user Bob if necessary defines its current IP-address and sends it message INVITE - at the user phone on what it is informed in the answer 180 (Ringing) starts to call.
-If the caused user has answered a call on inquiry INVITE the answer 200 (OK) is sent.
-The causing user sends message ACK informing caused that it has received the answer to the inquiry INVITE, it sets definitive parametres of connection. At this stage all is ready to a connection establishment under report RTP (Real-time Transport Protocol).
-RTP-connection with in advance co-ordinated parametres is established.
-For the connection end, the finishing user (puts down a reciever) sends inquiry BYE on which the answer 200 (OK) is sent.

While messages of an establishment of connection (INVITE) go between proxies-servers and whether it is not known the caused user is accessible, in reply to INVITE the answer 100 (Trying), informing on attempt of an establishment of connection is sent.

As the proxy-server can establish own connections, its use allows calls to overcome without problems NAT. Also construction of several proxies-servers in one chain that allows to overcome at once a little NAT is possible.