IMS (IP Multimedia Subsystem) has been described from 3GPP Rel-5. Since lots of LTE carriers even including 3GPP2 related ones have been heading for IMS deployment, several similar terms/concepts confuses us for a while. I can break down what IMS related terms e.g. SIP, VoIP, VoLTE, and etc. mean and compare confusing terms below.
The above picture shows us the related terms and indicates difference between VoIP and VoLTE simply. Before understanding what IMS is, personally I recommend to learn how SIP works in protocol perspective.
SIP is a signalling protocol, widely used for controlling multimedia communication sessions such as voice and video calls over IP networks. It works in conjunction with several applications layer protocols to do services. Media identification and negotiation is achieved with SDP (Session Description Protocol) and transmission of media streams employs RTP (Real-time Transport Protocol).
SIP network defines several network elements such as User Agent, Proxy server, Registrar, Redirect server, and Gateway to PSTN (Public Switched Telephone network). And SIP has been developed and standardized in RFC3261 under IETF.
IMS is an architecture framework under IP multimedia services. IMS uses IETF protocols and one of typical protocols is SIP. This is not intended to work as application servers, but to inter-work from wireless/wireline terminals through access networks to application servers in service layer.
Access networks can be variable depending on what kinds of terminals are used and it could be mobile/cellular access (e.g. LTE, UMTS), wireless access (e.g. WLAN), fixed access (e.g. DSL). Other phone systems like PSTN (Public Switched Telephone network) and non IMS-compatible VoIP system are supported through gateways. Application servers could be variable depending on services such as OIR (Originating ID Restriction)/TIR (Terminating ID Restriction), Supplementary Services, Conference, and etc.
The core component of IMS is CSCF (Call Session Control Function), which is divided into Proxy-CSCF, Serving-CSCF, and Interrogating-CSCF. It refers to HSS during authentication procedure and also uses gateways in order to interwork with non-IMS network.
The term VoIP covers any phone calls made over the Internet as opposed to traditional PSTN (Public Switched Telephone network). This service has been deployed with IP phone as well as various applications on PC e.g. Skype, Lync, Hangout.
Unlikely "naked" or "free" VoIP, the term VoLTE supplies subscribers with VoIP services over LTE mobile access network through IMS. SPS (Semi-Persistent Scheduling) for saving cellular network resource in VoLTE calls is used in LTE. And also in IMS, CSCF (Call Session Control Function)s and extra charging elements are required.
First of all, since operators got involved in this and described in 3GPP specifications, more features are added on pure naked SIP network. IMS defines specific network components and CSCF (Call Session Control Function)s are most critical ones in there.Some of other network components are allowed to interwork with wireless core network. So QoS can be managed per user for various multimedia with charging and emergency calls can be handled. Based on my learning here is comparison chart below.
The above picture shows us the related terms and indicates difference between VoIP and VoLTE simply. Before understanding what IMS is, personally I recommend to learn how SIP works in protocol perspective.
SIP (Session Initiation Protocol)
SIP is a signalling protocol, widely used for controlling multimedia communication sessions such as voice and video calls over IP networks. It works in conjunction with several applications layer protocols to do services. Media identification and negotiation is achieved with SDP (Session Description Protocol) and transmission of media streams employs RTP (Real-time Transport Protocol).
SIP network defines several network elements such as User Agent, Proxy server, Registrar, Redirect server, and Gateway to PSTN (Public Switched Telephone network). And SIP has been developed and standardized in RFC3261 under IETF.
IMS (IP Multimedia Subsystem)
IMS is an architecture framework under IP multimedia services. IMS uses IETF protocols and one of typical protocols is SIP. This is not intended to work as application servers, but to inter-work from wireless/wireline terminals through access networks to application servers in service layer.
Access networks can be variable depending on what kinds of terminals are used and it could be mobile/cellular access (e.g. LTE, UMTS), wireless access (e.g. WLAN), fixed access (e.g. DSL). Other phone systems like PSTN (Public Switched Telephone network) and non IMS-compatible VoIP system are supported through gateways. Application servers could be variable depending on services such as OIR (Originating ID Restriction)/TIR (Terminating ID Restriction), Supplementary Services, Conference, and etc.
The core component of IMS is CSCF (Call Session Control Function), which is divided into Proxy-CSCF, Serving-CSCF, and Interrogating-CSCF. It refers to HSS during authentication procedure and also uses gateways in order to interwork with non-IMS network.
VoIP (Voice over IP) vs. VoLTE (Voice over LTE)
The term VoIP covers any phone calls made over the Internet as opposed to traditional PSTN (Public Switched Telephone network). This service has been deployed with IP phone as well as various applications on PC e.g. Skype, Lync, Hangout.
Unlikely "naked" or "free" VoIP, the term VoLTE supplies subscribers with VoIP services over LTE mobile access network through IMS. SPS (Semi-Persistent Scheduling) for saving cellular network resource in VoLTE calls is used in LTE. And also in IMS, CSCF (Call Session Control Function)s and extra charging elements are required.
Difference between IMS and non-IMS services
First of all, since operators got involved in this and described in 3GPP specifications, more features are added on pure naked SIP network. IMS defines specific network components and CSCF (Call Session Control Function)s are most critical ones in there.Some of other network components are allowed to interwork with wireless core network. So QoS can be managed per user for various multimedia with charging and emergency calls can be handled. Based on my learning here is comparison chart below.
