IMS and Next Generation Networking
Tuesday, 6. November 2007, 18:02:10
First, let's define IMS, IMS or IP Multimedia Subsystem is an architectural computer networking framework or an integrated network for telecommunications carriers that uses the IP protocol as its foundation for packetized voice, video and data in purpose to deliver internet protocol (IP) multimedia to mobile users. On other words, IMS is a general-purpose, open industry standard for voice and multimedia communications over packet-based IP networks. It is a core network technology, that can serve as a low-level foundation for technologies like Voice over IP (VoIP), Push-To-Talk (PTT), Push-To-View, Video Calling, and Video Sharing. IMS is based primarily on SIP (Session Initiation Protocol).
And, Next Generation Networking (NGN) is a packet-based network able to provide Telecommunication Services to users and able to make use of multiple broadband, QoS-enabled transport technologies and in which service-related functions are independent of the underlying transport-related technologies, seamlessly blends the public switched telephone network (PSTN) and the public switched data network (PSDN), creating a single multiservice network. Rather than large, centralized, proprietary switch infrastructures, this next-generation architecture pushes central-office (CO) functionality to the edge of the network. The result is a distributed network infrastructure that leverages new, open technologies to reduce the cost of market entry dramatically, increase flexibility, and accommodate both circuit-switched voice and packet-switched data. It enables unfettered access for users to networks and to competing service providers and services of their choice. It supports generalised mobility which will allow consistent and ubiquitous provision of services to users.
The NGN is characterised by the following fundamental aspects:
Relationship between IMS and NGN
IMS itself is part of architectural evolutions in telecommunication core which is call as NGN, the idea behind IMS is to eventually move all voice and multimedia communication (mobile and fixed) to flexible, packet-based technologies derived from Internet protocols. It is intended to eventually replace all circuit-based technologies currently used in mobile networks. Next Generation Networks are based on Internet technologies including Internet Protocol (IP) and Multiprotocol Label Switching (MPLS).
In a NGN there is a more defined separation between the transport (connectivity) portion of the network and the services that run on top of that transport. This means that whenever a provider wants to enable a new service, they can do so by defining it directly at the service layer without considering the transport layer - i.e. services are independent of transport details. Increasingly applications, including voice, will tend to be independent of the access network (de-layering of network and applications) and will reside more on end-user devices (phone, PC, Set-top box).
Wireless System Evolution
In relationship with IMS and NGN, let's take a look to wireless system evolution:
First generation: Almost all of the systems from this generation were analog systems where voice was considered to be the main traffic. These systems could often be listened to by third parties. some of the standards are NMT, AMPS, Hicap, CDPD, Mobitex, DataTac.
Second generation: All the standards belonging to this generation are commercial centric and they are digital in form. Around 60% of the current market is dominated by European standards. The second generation standards are GSM, iDEN, D-AMPS, IS-95, PDC, CSD, PHS, GPRS, HSCSD, and WiDEN.
Third generation: To meet the growing demands in the number of subscribers (increase in network capacity), rates required for high speed data transfer and multimedia applications, 3G standards started evolving. The systems in this standard are basically a linear enhancement of 2G systems. They are based on two parallel backbone infrastructures, one consisting of circuit switched nodes, and one of packet oriented nodes. The ITU defines a specific set of air interface technologies as third generation, as part of the IMT-2000 initiative. Currently, transition is happening from 2G to 3G systems. As a part of this transition, lot of technologies are being standardized. From 2G to 3G: 2.75G - EDGE and EGPRS, 3G - CDMA 2000,W-CDMA or UMTS (3GSM), FOMA, 1xEV-DO/IS-856, TD-SCDMA, GAN/UMA. Similarly from 3G to 4G: 3.5G - HSDPA, HSUPA, Super3G - HSOPA/LTE.
Fourth generation: According to the 4G working groups, the infrastructure and the terminals of 4G will have almost all the standards from 2G to 4G implemented. Even though the legacy systems are in place to be adopted in 4G for the existing legacy users, going forward the infrastructure will however only be packet based, all-IP. Also, some proposals suggest having an open platform where the new innovations and evolutions can fit. The technologies which are being considered as pre-4G are used in the following standard version: WiMax, WiBro, 3GPPLong Term Evolution and 3GPP2 Ultra Mobile Broadband.
4G system will be able to provide a comprehensive IP solution where voice, data and streamed multimedia can be given to users on an "Anytime, Anywhere" basis, and at higher data rates than previous generations. There is no formal definition for what 4G is; however, there are certain objectives that are projected for 4G.
These objectives include: that 4G will be a fully IP-based integrated system. This will be achieved after wired and wireless technologies converge and will be capable of providing 100 Mbit/s and 1 Gbit/s speeds both indoors and outdoors, with premium quality and high security. 4G will offer all types of services at an affordable cost.
IPv6
IPv6 have significant role in Next Generation Networking and IMS, which IPv6 will be adopt by those architectural network to achive the purpose of all packet-based network. Unlike 3G, which is based on two parallel infrastructures consisting of circuit switched and packet switched network nodes respectively, 4G will be based on packet switching only. This will require low-latency data transmission.
It is generally believed that 4th generation wireless networks will support a greater number of wireless devices that are directly addressable and routable. Therefore, in the context of 4G, IPv6 is an important network layer technology and standard that can support a large number of wireless-enabled devices. By increasing the number of IP addresses, IPv6 removes the need for Network Address Translation (NAT), a method of sharing a limited number of addresses among a larger group of devices.
In the context of 4G, IPv6 also enables a number of applications with better multicast, security, and route optimization capabilities. With the available address space and number of addressing bits in IPv6, many innovative coding schemes can be developed for 4G devices and applications that could aid deployment of 4G networks and services.
Architecture
IMS's general architecture:
1. Access Network
The user can connect to an IMS network in various ways, all of which use the standard Internet Protocol (IP). Direct IMS terminals (such as mobile phones, personal digital assistants (PDAs) and computers) can register directly on an IMS network, even when they are roaming in another network or country (the visited network). The only requirement is that they can use IPv6 (also IPv4 in early IMS) and run Session Initiation Protocol (SIP) user agents. Fixed access (e.g., Digital Subscriber Line (DSL), cable modems, Ethernet), mobile access (e.g. W-CDMA, CDMA2000, GSM, GPRS) and wireless access (e.g. WLAN, WiMAX) are all supported. Other phone systems like plain old telephone service (POTS -- the old analogue telephones), H.323 and non IMS-compatible VoIP systems, are supported through gateways.
2. Core Network
a. Home subscriber server
The Home Subscriber Server (HSS), or User Profile Server Function (UPSF), is a master user database that supports the IMS network entities that actually handle calls. It contains the subscription-related information (user profiles), performs authentication and authorization of the user, and can provide information about the user's physical location. It is similar to the GSM Home Location Register (HLR) and Authentication Centre (AUC).
An SLF (Subscriber Location Function) is needed to map user addresses when multiple HSSs are used. Both the HSS and the SLF communicate through the DIAMETER protocol.
b. User Identities
Normal 3GPP networks use the following identities:
IMSI is a unique phone identity that is stored in the SIM. To improve privacy, a TMSI is generated per geographical location. While IMSI/TMSI are used for user identification, the IMEI is a unique device identity and is phone specific. The MSISDN is the telephone number of a user.
IMS also requires IP Multimedia Private Identity (IMPI) and IP Multimedia Public Identity (IMPU). Both are not phone numbers or other series of digits, but Uniform Resource Identifier (URIs), that can be digits (a tel-uri, like tel:+1-555-123-6789) or alphanumeric identifiers (a sip-uri, like sip:blu3c4t@example.com). There can be multiple IMPU per IMPI (often a tel-uri and a sip-uri). The IMPU can also be shared with another phone, so both can be reached with the same identity (for example, a single phone-number for an entire family).
The HSS user database contains, the IMPU, IMPI, IMSI, and MSISDN and other information.
c. Call/Session Control
Several roles of Session Initiation Protocol (SIP) servers or proxies, collectively called Call Session Control Function (CSCF), are used to process SIP signalling packets in the IMS.
1. A Proxy-CSCF (P-CSCF) is a SIP proxy that is the first point of contact for the IMS terminal. It can be located either in the visited network (in full IMS networks) or in the home network (when the visited network isn't IMS compliant yet). Some networks may use a Session Border Controller for this function. The terminal discovers its P-CSCF with either DHCP, or it is assigned in the PDP Context (in General Packet Radio Service (GPRS).
2. A Serving-CSCF (S-CSCF) is the central node of the signalling plane. It is a SIP server, but performs session control too. It is always located in the home network.
3. An I-CSCF (Interrogating-CSCF) is another SIP function located at the edge of an administrative domain. Its IP address is published in the Domain Name System (DNS) of the domain (using NAPTR and SRV type of DNS records), so that remote servers can find it, and use it as a forwarding point (e.g. registering) for SIP packets to this domain. The I-CSCF queries the HSS using the DIAMETER Cx interface to retrieve the user location (Dx interface is used from I-CSCF to SLF to locate the needed HSS only), and then routes the SIP request to its assigned S-CSCF. Up to Release 6 it can also be used to hide the internal network from the outside world (encrypting part of the SIP message), in which case it's called a THIG (Topology Hiding Inter-network Gateway). From Release 7 onwards this "entry point" function is removed from the I-CSCF and is now part of the IBCF (Interconnection Border Control Function). The IBCF is used as gateway to external networks, and provides NAT and Firewall functions (pinholing).
d. Application Servers
Application servers (AS) host and execute services, and interface with the S-CSCF using Session Initiation Protocol (SIP). An example of an application server that is being developed in 3GPP is the Voice call continuity Function (VCC Server). Depending on the actual service, the AS can operate in SIP proxy mode, SIP UA (user agent) mode or SIP B2BUA (back-to-back user agent) mode. An AS can be located in the home network or in an external third-party network. If located in the home network, it can query the HSS with the DIAMETER Sh interface (for a SIP-AS) or the Mobile Application Part (MAP) interface (for IM-SSF).
e. Media Servers
The MRF (Media Resource Function) provides media related functions such as media manipulation (e.g. voice stream mixing) and playing of tones and announcements.
Each MRF is further divided into a Media Resource Function Controller (MRFC) and a Media Resource Function Processor (MRFP).
f. Breakout Gateway
A BGCF (Breakout Gateway Control Function) is a SIP server that includes routing functionality based on telephone numbers. It is only used when calling from the IMS to a phone in a circuit switched network, such as the Public Switched Telephone Network (PSTN) or the Public land mobile network (PLMN).
g. PSTN Gateways
A PSTN/CS gateway interfaces with PSTN circuit switched (CS) networks. For signalling, CS networks use ISDN User Part (ISUP) (or BICC) over Message Transfer Part (MTP), while IMS uses Session Initiation Protocol (SIP) over IP. For media, CS networks use Pulse-code modulation (PCM), while IMS uses Real-time Transport Protocol (RTP).
h. Media Resources
Media Resources are those components that operate on the media plane and are under the control of IMS Core functions. Specifically, Media Server (MS) and Media gateway (MGW).
NGN Interconnection
There are two types of NGN Interconnection:
1. Service oriented Interconnection (SoIx): The physical and logical linking of NGN domains that allows carriers and service providers to offer services over NGN (i.e. IMS and PES) platforms with control, signalling (i.e. session based), which provides defined levels of interoperability. For instance, this is the case of "carrier grade" voice end/or multimedia services over IP interconnection. "Defined levels of interoperability" are dependent upon the service or the QoS or the Security, etc.
2. Connectivity oriented Interconnection (CoIx): The physical and logical linking of carriers and service providers based on simple IP connectivity irrespective of the levels of interoperability. For example, an IP interconnection of this type is not aware of the specific end to end service and, as a consequence, service specific network performance, QoS and security requirements are not necessarily assured. This definition does not exclude that some services may provide a defined level of interoperability. However only SoIx fully satisfies NGN interoperability requirements.
An NGN interconnection mode can be direct or indirect. Direct interconnection refers to the interconnection between two network domains without any intermediate network domain.Indirect interconnection at one layer refers to the interconnection between two network domains with one or more intermediate network domain(s) acting as transit networks. The intermediate network domain(s) provide(s) transit functionality to the two other network domains. Different interconnection modes may be used for carrying service layer signalling and media traffic.
NGN Architectural :
Summary
The Internet, underscored by IP, has not only affected every business, information systems department and software publisher, but it has changed world communications forever. And, Next Generation Network is concept for architectural evolutions in telecommunication and access network which IMS is part of that architectural concept. IMS become important architectural key in Next Generation Network to bring "everything on IP", in order to provide converged data and voice services.
And, Next Generation Networking (NGN) is a packet-based network able to provide Telecommunication Services to users and able to make use of multiple broadband, QoS-enabled transport technologies and in which service-related functions are independent of the underlying transport-related technologies, seamlessly blends the public switched telephone network (PSTN) and the public switched data network (PSDN), creating a single multiservice network. Rather than large, centralized, proprietary switch infrastructures, this next-generation architecture pushes central-office (CO) functionality to the edge of the network. The result is a distributed network infrastructure that leverages new, open technologies to reduce the cost of market entry dramatically, increase flexibility, and accommodate both circuit-switched voice and packet-switched data. It enables unfettered access for users to networks and to competing service providers and services of their choice. It supports generalised mobility which will allow consistent and ubiquitous provision of services to users.
The NGN is characterised by the following fundamental aspects:
- Packet-based transfer.
- Separation of control functions among bearer capabilities, call/session, and application/service.
- Decoupling of service provision from transport, and provision of open interfaces.
- Support for a wide range of services, applications and mechanisms based on service building blocks (including real time/streaming/non-real time services and multi-media).
- Broadband capabilities with end-to-end QoS and transparency.
- Interworking with legacy networks via open interfaces.
- Generalised mobility.
- Unfettered access by users to different service providers.
- A variety of identification schemes which can be resolved to IP addresses for the purposes of routing in IP networks.
- Unified service characteristics for the same service as perceived by the user.
- Converged services between Fixed and Mobile networks.
- Independence of service-related functions from underlying transport technologies.
- Support of multiple last mile technologies.
- Compliant with all Regulatory requirements, for example concerning emergency communications and security/privacy, etc.
Relationship between IMS and NGN
IMS itself is part of architectural evolutions in telecommunication core which is call as NGN, the idea behind IMS is to eventually move all voice and multimedia communication (mobile and fixed) to flexible, packet-based technologies derived from Internet protocols. It is intended to eventually replace all circuit-based technologies currently used in mobile networks. Next Generation Networks are based on Internet technologies including Internet Protocol (IP) and Multiprotocol Label Switching (MPLS).
In a NGN there is a more defined separation between the transport (connectivity) portion of the network and the services that run on top of that transport. This means that whenever a provider wants to enable a new service, they can do so by defining it directly at the service layer without considering the transport layer - i.e. services are independent of transport details. Increasingly applications, including voice, will tend to be independent of the access network (de-layering of network and applications) and will reside more on end-user devices (phone, PC, Set-top box).
Wireless System Evolution
In relationship with IMS and NGN, let's take a look to wireless system evolution:
First generation: Almost all of the systems from this generation were analog systems where voice was considered to be the main traffic. These systems could often be listened to by third parties. some of the standards are NMT, AMPS, Hicap, CDPD, Mobitex, DataTac.
Second generation: All the standards belonging to this generation are commercial centric and they are digital in form. Around 60% of the current market is dominated by European standards. The second generation standards are GSM, iDEN, D-AMPS, IS-95, PDC, CSD, PHS, GPRS, HSCSD, and WiDEN.
Third generation: To meet the growing demands in the number of subscribers (increase in network capacity), rates required for high speed data transfer and multimedia applications, 3G standards started evolving. The systems in this standard are basically a linear enhancement of 2G systems. They are based on two parallel backbone infrastructures, one consisting of circuit switched nodes, and one of packet oriented nodes. The ITU defines a specific set of air interface technologies as third generation, as part of the IMT-2000 initiative. Currently, transition is happening from 2G to 3G systems. As a part of this transition, lot of technologies are being standardized. From 2G to 3G: 2.75G - EDGE and EGPRS, 3G - CDMA 2000,W-CDMA or UMTS (3GSM), FOMA, 1xEV-DO/IS-856, TD-SCDMA, GAN/UMA. Similarly from 3G to 4G: 3.5G - HSDPA, HSUPA, Super3G - HSOPA/LTE.
Fourth generation: According to the 4G working groups, the infrastructure and the terminals of 4G will have almost all the standards from 2G to 4G implemented. Even though the legacy systems are in place to be adopted in 4G for the existing legacy users, going forward the infrastructure will however only be packet based, all-IP. Also, some proposals suggest having an open platform where the new innovations and evolutions can fit. The technologies which are being considered as pre-4G are used in the following standard version: WiMax, WiBro, 3GPPLong Term Evolution and 3GPP2 Ultra Mobile Broadband.
4G system will be able to provide a comprehensive IP solution where voice, data and streamed multimedia can be given to users on an "Anytime, Anywhere" basis, and at higher data rates than previous generations. There is no formal definition for what 4G is; however, there are certain objectives that are projected for 4G.
These objectives include: that 4G will be a fully IP-based integrated system. This will be achieved after wired and wireless technologies converge and will be capable of providing 100 Mbit/s and 1 Gbit/s speeds both indoors and outdoors, with premium quality and high security. 4G will offer all types of services at an affordable cost.
IPv6
IPv6 have significant role in Next Generation Networking and IMS, which IPv6 will be adopt by those architectural network to achive the purpose of all packet-based network. Unlike 3G, which is based on two parallel infrastructures consisting of circuit switched and packet switched network nodes respectively, 4G will be based on packet switching only. This will require low-latency data transmission.
It is generally believed that 4th generation wireless networks will support a greater number of wireless devices that are directly addressable and routable. Therefore, in the context of 4G, IPv6 is an important network layer technology and standard that can support a large number of wireless-enabled devices. By increasing the number of IP addresses, IPv6 removes the need for Network Address Translation (NAT), a method of sharing a limited number of addresses among a larger group of devices.
In the context of 4G, IPv6 also enables a number of applications with better multicast, security, and route optimization capabilities. With the available address space and number of addressing bits in IPv6, many innovative coding schemes can be developed for 4G devices and applications that could aid deployment of 4G networks and services.
Architecture
IMS's general architecture:
1. Access Network
The user can connect to an IMS network in various ways, all of which use the standard Internet Protocol (IP). Direct IMS terminals (such as mobile phones, personal digital assistants (PDAs) and computers) can register directly on an IMS network, even when they are roaming in another network or country (the visited network). The only requirement is that they can use IPv6 (also IPv4 in early IMS) and run Session Initiation Protocol (SIP) user agents. Fixed access (e.g., Digital Subscriber Line (DSL), cable modems, Ethernet), mobile access (e.g. W-CDMA, CDMA2000, GSM, GPRS) and wireless access (e.g. WLAN, WiMAX) are all supported. Other phone systems like plain old telephone service (POTS -- the old analogue telephones), H.323 and non IMS-compatible VoIP systems, are supported through gateways.
2. Core Network
a. Home subscriber server
The Home Subscriber Server (HSS), or User Profile Server Function (UPSF), is a master user database that supports the IMS network entities that actually handle calls. It contains the subscription-related information (user profiles), performs authentication and authorization of the user, and can provide information about the user's physical location. It is similar to the GSM Home Location Register (HLR) and Authentication Centre (AUC).
An SLF (Subscriber Location Function) is needed to map user addresses when multiple HSSs are used. Both the HSS and the SLF communicate through the DIAMETER protocol.
b. User Identities
Normal 3GPP networks use the following identities:
- International Mobile Subscriber Identity (IMSI)
- Temporary Mobile Subscriber Identity (TMSI)
- International Mobile Equipment Identity (IMEI)
- Mobile Subscriber ISDN Number (MSISDN)
IMSI is a unique phone identity that is stored in the SIM. To improve privacy, a TMSI is generated per geographical location. While IMSI/TMSI are used for user identification, the IMEI is a unique device identity and is phone specific. The MSISDN is the telephone number of a user.
IMS also requires IP Multimedia Private Identity (IMPI) and IP Multimedia Public Identity (IMPU). Both are not phone numbers or other series of digits, but Uniform Resource Identifier (URIs), that can be digits (a tel-uri, like tel:+1-555-123-6789) or alphanumeric identifiers (a sip-uri, like sip:blu3c4t@example.com). There can be multiple IMPU per IMPI (often a tel-uri and a sip-uri). The IMPU can also be shared with another phone, so both can be reached with the same identity (for example, a single phone-number for an entire family).
The HSS user database contains, the IMPU, IMPI, IMSI, and MSISDN and other information.
c. Call/Session Control
Several roles of Session Initiation Protocol (SIP) servers or proxies, collectively called Call Session Control Function (CSCF), are used to process SIP signalling packets in the IMS.
1. A Proxy-CSCF (P-CSCF) is a SIP proxy that is the first point of contact for the IMS terminal. It can be located either in the visited network (in full IMS networks) or in the home network (when the visited network isn't IMS compliant yet). Some networks may use a Session Border Controller for this function. The terminal discovers its P-CSCF with either DHCP, or it is assigned in the PDP Context (in General Packet Radio Service (GPRS).
- it is assigned to an IMS terminal during registration, and does not change for the duration of the registration
- it sits on the path of all signalling messages, and can inspect every message
- it authenticates the user and establishes an IPsec security association with the IMS terminal. This prevents spoofing attacks and replay attacks and protects the privacy of the user. Other nodes trust the P-CSCF, and do not have to authenticate the user again
- it can also compress and decompress SIP messages using SigComp, which reduces the round-trip over slow radio links
- it may include a Policy Decision Function (PDF), which authorizes media plane resources e.g. quality of service (QoS) over the media plane. It's used for policy control, bandwidth management, etc. The PDF can also be a separate function
- it also generates charging records.
2. A Serving-CSCF (S-CSCF) is the central node of the signalling plane. It is a SIP server, but performs session control too. It is always located in the home network.
- it uses DIAMETER Cx and Dx interfaces to the HSS to download and upload user profiles ? it has no local storage of the user. All necessary information is loaded from the HSS.
- it handles SIP registrations, which allows it to bind the user location (e.g. the IP address of the terminal) and the SIP address
- it sits on the path of all signaling messages, and can inspect every message
- it decides to which application server(s) the SIP message will be forwarded, in order to provide their services
- it provides routing services, typically using Electronic Numbering (ENUM) lookups
- it enforces the policy of the network operator
there can be multiple S-CSCFs in the network for load distribution and high availability reasons. It's the HSS that assigns the S-CSCF to a user, when it's queried by the I-CSCF.
3. An I-CSCF (Interrogating-CSCF) is another SIP function located at the edge of an administrative domain. Its IP address is published in the Domain Name System (DNS) of the domain (using NAPTR and SRV type of DNS records), so that remote servers can find it, and use it as a forwarding point (e.g. registering) for SIP packets to this domain. The I-CSCF queries the HSS using the DIAMETER Cx interface to retrieve the user location (Dx interface is used from I-CSCF to SLF to locate the needed HSS only), and then routes the SIP request to its assigned S-CSCF. Up to Release 6 it can also be used to hide the internal network from the outside world (encrypting part of the SIP message), in which case it's called a THIG (Topology Hiding Inter-network Gateway). From Release 7 onwards this "entry point" function is removed from the I-CSCF and is now part of the IBCF (Interconnection Border Control Function). The IBCF is used as gateway to external networks, and provides NAT and Firewall functions (pinholing).
d. Application Servers
Application servers (AS) host and execute services, and interface with the S-CSCF using Session Initiation Protocol (SIP). An example of an application server that is being developed in 3GPP is the Voice call continuity Function (VCC Server). Depending on the actual service, the AS can operate in SIP proxy mode, SIP UA (user agent) mode or SIP B2BUA (back-to-back user agent) mode. An AS can be located in the home network or in an external third-party network. If located in the home network, it can query the HSS with the DIAMETER Sh interface (for a SIP-AS) or the Mobile Application Part (MAP) interface (for IM-SSF).
- SIP AS: native IMS application server
- IM-SSF: an IP Multimedia Service Switching Function interfaces with Customised Applications for Mobile networks Enhanced Logic (CAMEL) Application Servers using Camel Application Part (CAP)
e. Media Servers
The MRF (Media Resource Function) provides media related functions such as media manipulation (e.g. voice stream mixing) and playing of tones and announcements.
Each MRF is further divided into a Media Resource Function Controller (MRFC) and a Media Resource Function Processor (MRFP).
- The MRFC is a signalling plane node that acts as a SIP User Agent to the S-CSCF, and which controls the MRFP with a H.248 interface
- The MRFP is a media plane node that implements all media-related functions.
f. Breakout Gateway
A BGCF (Breakout Gateway Control Function) is a SIP server that includes routing functionality based on telephone numbers. It is only used when calling from the IMS to a phone in a circuit switched network, such as the Public Switched Telephone Network (PSTN) or the Public land mobile network (PLMN).
g. PSTN Gateways
A PSTN/CS gateway interfaces with PSTN circuit switched (CS) networks. For signalling, CS networks use ISDN User Part (ISUP) (or BICC) over Message Transfer Part (MTP), while IMS uses Session Initiation Protocol (SIP) over IP. For media, CS networks use Pulse-code modulation (PCM), while IMS uses Real-time Transport Protocol (RTP).
- A Signalling Gateway (SGW) interfaces with the signalling plane of the CS. It transforms lower layer protocols as Stream Control Transmission Protocol (SCTP, an Internet Protocol (IP) protocol) into Message Transfer Part (MTP, an Signalling System 7 (SS7) protocol), to pass ISDN User Part (ISUP) from the MGCF to the CS network.
- A Media Gateway Controller Function (MGCF) does call control protocol conversion between SIP and ISUP and interfaces with the SGW over SCTP. It also controls the resources in an MGW with an H.248 interface.
- A Media Gateway (MGW) interfaces with the media plane of the CS network, by converting between RTP and PCM. It can also transcode when the codecs don't match (e.g. IMS might use AMR, PSTN might use G.711).
h. Media Resources
Media Resources are those components that operate on the media plane and are under the control of IMS Core functions. Specifically, Media Server (MS) and Media gateway (MGW).
NGN Interconnection
There are two types of NGN Interconnection:
1. Service oriented Interconnection (SoIx): The physical and logical linking of NGN domains that allows carriers and service providers to offer services over NGN (i.e. IMS and PES) platforms with control, signalling (i.e. session based), which provides defined levels of interoperability. For instance, this is the case of "carrier grade" voice end/or multimedia services over IP interconnection. "Defined levels of interoperability" are dependent upon the service or the QoS or the Security, etc.
2. Connectivity oriented Interconnection (CoIx): The physical and logical linking of carriers and service providers based on simple IP connectivity irrespective of the levels of interoperability. For example, an IP interconnection of this type is not aware of the specific end to end service and, as a consequence, service specific network performance, QoS and security requirements are not necessarily assured. This definition does not exclude that some services may provide a defined level of interoperability. However only SoIx fully satisfies NGN interoperability requirements.
An NGN interconnection mode can be direct or indirect. Direct interconnection refers to the interconnection between two network domains without any intermediate network domain.Indirect interconnection at one layer refers to the interconnection between two network domains with one or more intermediate network domain(s) acting as transit networks. The intermediate network domain(s) provide(s) transit functionality to the two other network domains. Different interconnection modes may be used for carrying service layer signalling and media traffic.
NGN Architectural :
Summary
The Internet, underscored by IP, has not only affected every business, information systems department and software publisher, but it has changed world communications forever. And, Next Generation Network is concept for architectural evolutions in telecommunication and access network which IMS is part of that architectural concept. IMS become important architectural key in Next Generation Network to bring "everything on IP", in order to provide converged data and voice services.
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