Analysis of key technical problems of IMS based ne

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Based on the analysis of the key technical problems of IMS network fusion

the emergence of soft switch technology once made people consider that soft switch technology can be used in the fusion of core control layer. However, due to the large differences between fixed and mobile soft switch functions and protocols, the possibility of soft switch becoming the focus of control layer fusion is reduced. With the emergence of SIP based IMS Technology in mobile networks, it coincides with the SIP based application of fixed broadband soft switch

the original intention of introducing IMS into mobile network is to provide mobile multimedia services, but because IMS adopts the same protocol (SIP) and similar architecture as fixed broadband soft switch, and IMS system has the advantages of being independent of access and supporting user roaming, it brings an unprecedented opportunity to realize network convergence by using IMS

ims technology was originally introduced in 3gppr5. IMS Technology is highly concerned by the industry because of its access independence and complete separation of service and control. At present, 3GPP is studying the application of IMS in the mobile field. At the same time, TISPAN and I Tu T are actively promoting the technical research of IMS based application in the field of fixed and network fusion on the basis of 3gppr6 research. International and domestic operators also pay high attention to IMS Technology, and they all believe that IMS will become one of the important technical directions of network integration and the development of core control layer

judging from the current research progress, the standard of IMS application in Mobile has matured, but the technical standard of network fusion based on IMS is still in the development stage. Due to the differences between wired and wireless networks in network bandwidth, terminal authentication, location information and resource management, TISPAN expands IMS in these aspects to achieve fixed access. However, the differences between fixed and mobile IMS in business capabilities, architecture, protocols and other issues will become the key issues for IMS to achieve the integration of fixed and mobile networks

the business capability leads to the difference in the production cost of small and medium-sized enterprises.

3gppims only defines the IMS architecture, and the business specifications are formulated by O Ma. OMA currently mainly formulates multimedia business specifications such as POC, presence, instant messaging, pushtoshow, etc

the IMS defined by TISPAN requires that it not only support multimedia services such as presence, instant messaging, location services, and visualization, but also support PSTN/ISDN simulation services and simulation services. IMS network adopts the architecture of completely separating service and control, which requires the application server to provide the business logic of PSTN/ISDN simulation service and simulation service

tispan has begun to formulate requirements and specifications for a series of PSTN simulation services such as call forwarding and call holding, and expand SIP protocol to realize PSTN simulation services according to business needs

pstn simulation service is aimed at fixed pots end users. IMS network is required to provide users with services that are no different from PSTN. This puts forward higher requirements for IMS, which requires IMS not only to realize PSTN services such as call forwarding and call waiting, but also to provide users with the same service perception. Therefore, at the beginning of the new year, s will also expand the PSTN simulation business capability. For example, PSTN network can play different dial tones to users according to the different supplementary services applied by users. Because IMS is completely separated from control and service, the functional entity (AGCF) of the control layer does not know the service applied by users, so it cannot control what dial tone to send to users. If IMS realizes the service capability of PSTN/ISDN simulation, It is necessary to expand the interface between PSTN/ISDN simulation service application server and AGCF to transmit service related information

from the above analysis, it can be seen that IMS needs to make more extensions in order to support the business ability of providing PSTN simulation to fixed pots users, and these extensions increase the difference between fixed IMS and mobile IMS. The differences in business capabilities lead to the differences in the architecture and protocols of the solid and mobile IMS, which will become a key factor affecting the degree of mobile integration of IMS to achieve fixed and mainly used for metal and non-metal stretching, tightening and zigzag experiments

differences in architecture

tispan directly refers to the architecture of 3gppr6ims, and on this basis, it is expanded to support the access of fixed broadband users and the access of fixed narrowband users (POTS), so as to realize PSTN/ISDN analog services and simulation services. Due to the different access methods and business capabilities of broadband users and fixed narrowband users, there are also differences in the fixed IMS defined by TISPAN:

-- the IMS that TISPAN realizes broadband user access. IMS architecture is basically the same as 3gppims architecture, mainly adding network attachment subsystem (NASS) and resource control subsystem (RACS) for access control, location management and QoS control of fixed users. At the same time, the specific functions of entities such as P-CSCF are extended

-- TISPAN realizes IMS for fixed pots user access. TISPAN defines the architecture of realizing PSTN simulation service based on IMS, that is, based on IMS, narrow-band users, broadband users and mobile users can be accessed, and based on IMS, wide-band and narrow-band integration can be realized

differences in SIP protocols

the protocols used in IMS architecture include sip, H.248, parameter and cops, but the main call control protocol is sip. Fixed network broadband soft switch also uses SIP protocol. Therefore, whether SIP protocol converges becomes the key factor of network convergence

at present, the broadband domain of fixed soft switch mainly adopts IETF SIP protocol (RFC3261). IMS adopts the SIP protocol defined by IETF as the call control protocol, and proposes the extension of IETF SIP protocol according to the characteristics and requirements of mobile. 3GPP's impact on ietfsip protocol in R6 version is mainly in the following aspects

-- ietfrfc3261 defines the identity authentication of SIP terminals through digest authentication. 3gppims uses aka authentication originally used in the wireless field to authenticate user access, so digest authentication is extended to carry the parameters required for aka authentication

-- IMS requires QoS guarantee for user services. Rfc3312 defines the mechanism for QoS resource reservation during SIP session establishment

-- rfc3455 defines multiple private extended header domains (p-headers) for 3GPP. For example, p-associated-uri is used by S-CSCF to indicate to UE the list of common flags (IMPU) that the user is authorized to use during the registration process

-- rfc3311 defines SIP update messages and commands, and UE can change call parameters

-- due to the limited wireless access bandwidth resources, rfc3320 defines that forced SIP message compression is supported between pcscf and UE

-- due to the problem of wired and wireless transmission delay, 3GPP has modified some timers defined in ietfsip protocol

tispan defines IMS for accessing fixed users. At present, the expansion of SIP protocol to support fixed access includes the following aspects: expand the access parameter type of p-access-network-info header domain, and support WLAN, xDSL and other access methods; For the p-charging-vector header field, add the address parameters of bras, authentication token and parameters of DSL bearing information for the DSL bearing mode; For the extension of siptimer, 3gppr6 modifies the timer duration of the air interface, such as changing the duration of the basic timer T1 from 500ms to 2S, but after using the fixed access mode, it is recommended that P-CSCF choose the timer duration according to the access mode; The SIP protocol is extended to support call forwarding and other business features

although both fixed and mobile networks use IMS architecture and SIP protocol, the integration of fixed and mobile networks using IMS has made different extensions to sip protocol for different access methods and different business requirements, and there are still differences. Therefore, for IMS architecture applied to the integration of fixed and mobile broadband domains, its SIP protocol capability should be a complete set of supporting fixed and mobile access methods; And it should support the capability set of PSTN/isdnsimulation service. Therefore, whether the unified IMS system can support a variety of SIP parameters and sip capabilities remains to be further verified by experiments

user authentication problem

3gppims stipulates that mobile users adopt aka authentication mode, and extends digest authentication mode to carry aka authentication parameters. IMS users need to support ISIM cards. ISIM stores users' public user marks, private user marks and authentication parameters. For USIM users, the terminal is required to generate public user marks and private user marks according to user information in USIM

but for fixed terminals, fixed terminals do not have UICC cards. Fixed terminals usually only have public user marks (E.164 number or sipuri), but no private user marks, and do not support aka authentication. Therefore, if IMS adopts unified authentication for fixed mobile terminals, it is necessary to expand the fixed terminals or require IMS to support different authentication methods

at present, TISPAN has stipulated that IMS recommends aka authentication based on ISIM/USIM for mobile and fixed users. For terminals without ISIM/USIM, sipdigest authentication can be used, but the fixed terminal is required to have parameters such as private user logo, public user logo and home network domain name for authentication

therefore, for the authentication mode of the terminal, the following solutions can be adopted: for new fixed terminals, the terminal is required to support the authentication mode of ISIM card and based on the reports of developers and initial users; For SIP terminals that have been applied, if software needs to be installed for such terminals after they are migrated to IMS network, and private user flags can be generated by software, sipdigest can be used for authentication; An adapter is used to access all SIP terminals without UICC card, and the adapter generates private user flags and authentication parameters for each terminal for aka authentication

coding method problem

at present, the coding methods supported by fixed VOIP networks and terminals are as follows: speech coding methods: G.711, G.729 and G.723; Video coding mode: H.261, H.263 and MPEG-4

the encoding and decoding methods supported by mobile networks and terminals are as follows: voice encoding and decoding method: AMR; Video encoding and decoding mode: H.263 and MPEG-4

due to the differences in the encoding methods supported by the current fixed network and mobile network, if the IMS network is used to realize network fusion, it is necessary to redefine the encoding and decoding methods, and the fixed and mobile terminals are required to have the same encoding and decoding methods, otherwise specific related equipment is required to support the conversion of encoding and decoding. The conversion of encoding and decoding may reduce the speech quality, so the coding conversion should be carried out as little as possible in the network, which requires the fixed and mobile terminals to adopt the same coding method as much as possible. ∧

IMS has become the technology development trend of core control layer integration recognized by the industry. Operators have paid high attention to IMS based network integration. Some integrated operators have begun IMS experiments, hoping to realize the integration of fixed and mobile networks through a unified IMS network. At present, the realization of IMS fixed access is still in the research stage, and the above problems will become the key factors for IMS to realize the integration of fixed and mobile networks. (end)

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