SIP Architecture

SIP (Session Initialization Protocol) specifies a VoIP architecture that allows the product developers and service providers the flexibility to develop and deploy VoIP services in the SIP phone, the SIP server or in both devices.  

SIP VoIP services in a distributed SIP installation are concentrated in the SIP phones.  Modern SIP phones, like the SIP 4G phones4G phones with SIP software installed, are capable of providing most relevant phone services that a centralized signaling system like the legacy SS7 can. See SIP phone products and their phone services.

SS7 is the name of the signaling system currently being used by all voice carriers, wired or wireless.

Distributed SIP 

Distributed SIP architecture

In a distributed SIP model as shown in the above drawing, SIP phone is independently connected to the Internet or any IP enabled network. The network needs not be informed about the SIP phone being online, nor does it create new services for the SIP phone to work.  The SIP phone can make VoIP calls to another SIP phone directly if it knows the phone's IP address or DNS name.

The distributed model of SIP architecture is flexible and scalable because the SIP phones are end-devices of an IP network. The SIP phone users can connect or disconnect the phones without impacting the network or the network services. And as long as the IP network is scalable, so will the number of SIP phones.

With this end-to-end architecture, users can experiment and compare VoIP voice quality, call features and services provided by different SIP phones without impacting other SIP phones or the network operation and services.  

The distributed SIP deployment allows the phone users the power to select the phone with the right services they need and it also creates the initiatives for the SIP phone developers to invent more applicable services and promote them to consumers directly.

Follow this link to download Free SIP software for different hardware and operating systems.  This list also includes SIP software for mobile devices, smartphones and 4G phones of different operating systems: iOS, Android, and others.

Centralized SIP

Centralized SIP architecture

This drawing shows a SIP centralized model in which SIP phones register with SIP servers.  The SIP server will forward the incoming call to the phone using the its IP address that was collected during the registration process.

The centralized SIP server can be a single point of failure.  Such failure causes the registered phones not reachable.  The reasons for server failure include: service outage, software errors, overload, Internet hacker attacks and other security problems.