Analysis of key technical issues of IMS-based network convergence

Analysis of key technical issues of IMS-based network convergence

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The emergence of Softswitch technology has led to the adoption of Softswitch technology for the integration of core control layers. However, due to the large differences in fixed and mobile Softswitch functions and protocols, Softswitch has become the focus of control layer integration.
The possibility is reduced. With the emergence of SIP-based IMS technology in mobile networks, it is consistent with the application of Fixed-network broadband Softswitch Based on SIP. The original intention of introducing IMS to mobile networks is to provide mobile multimedia services,
However, IMS adopts the same Protocol (SIP) and similar architecture as fixed broadband Softswitch, and its system has advantages such as independence from access and support for user roaming, this is implemented using IMS.
Network convergence brings unprecedented opportunities.

　
IMS technology was initially introduced in 3gppr5. IMS has received high attention from the industry for its unique access independence and complete separation of business and control. Currently, 3GPP is researching
IMS is applied to the mobile field. At the same time, tispan and ITU-T are actively promoting the Technology Research Based on IMS in the fixed network application and in the network fusion field. International
Carriers in China and China have also paid great attention to the IMS technology, and have thought that IMS will become one of the important technical directions of network convergence and core control layer development.

From the current research
According to the progress, the IMS standard for mobile application is mature, but the technology standard for IMS-based network convergence is still in the development stage. Because wired and wireless networks provide network bandwidth, terminal authentication, location information, and
Resource management and other aspects are different. tispan expands IMS in these aspects to achieve fixed access. However, differences between fixed and mobile IMS in terms of service capabilities, differences in system architecture, and differences in protocols
Exceptions and other issues will become a key issue for IMS to achieve fixed and mobile network convergence.

Differences in business capabilities

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

　
The IMS defined by tispan not only supports presence, instant messaging, location services, videophone and other multimedia services, but also supports PSTN/ISDN simulation services and Imitation
Real business. The IMS network adopts a completely isolated architecture of business and control, which requires the application server to provide the business logic of the PSTN/ISDN simulation service and simulation service.

Tispan has begun to develop a series of PSTN simulation business requirements and specifications, such as call transfer and call persistence, and expand the SIP Protocol to implement PSTN simulation business as needed.

　
The PSTN simulation service is intended for fixed-network pots end users. It requires the IMS network to provide users with services that are no different from those of PSTN. This puts forward higher requirements for IMS, requiring IMS not only
Implements call transfer, call wait, and other PSTN services, and also provides users with the same service awareness. Therefore, IMS must make more extensions for PSTN simulation service capabilities. For example, the PSTN network can
Different dial-up voices are played for users based on different supplementary services applied by users. However, due to the full separation of control and business, IMS controls the functional entities of the control layer (agcf) I do not know the business applied by the user, so there is no
This method controls the dialing tone that the gateway sends to the user. If IMS implements this service capability of PSTN/ISDN simulation, it needs to expand between the PSTN/ISDN simulation service application server and agcf.
To transmit business-related information.

From the above analysis, we can see that IMS needs more expansion to support PSTN simulation service capabilities for fixed network pots users.
And these extensions increase the differences between fixed IMS and mobile IMS. The difference in service capabilities leads to the difference in the fixed network and mobile IMS system architecture and protocols, which will affect the implementation of fixed and mobile IMS.
Key factors in the degree of dynamic convergence.

Differences in architecture

Tispan directly references the architecture of 3gppr6ims, and
On this basis, it expands and supports the access of Fixed-network broadband users and the access of Fixed-network narrowband users (POTS) to implement PSTN/ISDN simulation and simulation services. Bandwidth users and fixed bandwidth narrowband applications
Different user access methods and service capabilities lead to different fixed network IMS defined by tispan:

-- Tispan: IMS for Broadband User Access. The IMS architecture is basically the same as that of ppims, mainly including the addition of Nass and RACs) provides access control, location management, and QoS Control for fixed network users. At the same time, the specific functions of P-CSCF and other entities are extended.

-- Tispan implements IMS for fixed-network pots user access. Tispan defines the system architecture for PSTN simulation Service Based on IMS, that is, IMS can access narrowband users, broadband users and mobile users, and achieve wide and narrow band convergence based on IMS.

Differences in the SIP protocol

The protocols used in the IMS architecture include sip, H.248, diameter, and cops, but the main call control protocol is sip. The fixed network broadband Softswitch also uses the SIP protocol. Therefore, the convergence of the SIP Protocol becomes a key factor for network convergence.

The broadband domains of fixed Softswitch currently use the ietf sip protocol (rfc3261). IMS uses the IETF-defined SIP protocol as the Call Control Protocol Based on Mobile features and needs, an extension of the ietfsip protocol is proposed. The ietfsip protocol of 3GPP in R6 is mainly used in the following aspects.

-- Ietfrfc3261 defines the identity authentication for the SIP terminal through digest authentication. 3gppims authenticates user access using aka authentication, which is used in the wireless field. Therefore, digest authentication is extended to carry the parameters required for aka authentication.

-- IMS requires QoS for user services. rfc3312 defines the mechanism for QoS resource reservation during the establishment of SIP sessions.

-- Rfc3455 defines multiple private extension header fields (p-header) for 3GPP ). For example, p-associated-Uri is used by the S-CSCF to indicate to ue a list of common signs (impu) that the user is authorized to use during registration.

-- Rfc3311 defines the message and command for SIP update. UE can change the call parameters.

-- Due to limited bandwidth resources for wireless access, rfc3320 defines that forced compression of SIP messages is supported between pcscf and UE.

-- Due to the delay of wired and wireless transmission, 3GPP modified some timer defined in the ietfsip protocol.

　
Tispan defines the IMS for accessing fixed users. Currently, the extension of the SIP protocol to support fixed access includes the following aspects: for the p-access-Network-Info header domain
Supports WLAN, XDSL, and other access methods. extends the p-charging-vector header and adds the BRAS
Address parameters, authentication token, and DSL bearer information parameters. For siptimer extension, 3gppr6 modifies the timer duration of the air interface, such as the Basic timer T1.
The length of MS is changed to 2 s, but the fixed access mode is adopted, it is recommended that the P-CSCF select the timer length according to the access mode; the SIP protocol is extended to support call transfer and other business features
.

Although both the fixed and mobile networks use the IMS architecture and SIP protocol, the fixed and mobile integration with IMS is applicable to different access methods and different
There are still differences in service requirements and extensions of the SIP protocol. Therefore, for the IMS architecture that is applied to the integration of fixed and mobile broadband domains, the SIP protocol capability should support both fixed and mobile access providers.
And must support the capability set for implementing the PSTN/isdnsimulation service. Therefore, it is still necessary for the unified IMS system to support multiple sip parameters and SIP capabilities.
For further verification through experiments.

User Authentication Problems

In 3gppims, mobile users are required to use the AKA authentication method.
The Digest authentication method is extended to carry aka authentication parameters. IMS users need to support isim cards. isim stores users' public and private user logos and authentication
For USIM users, the terminal is required to generate public and private user tags based on user information in USIM.

However, for fixed terminals, there is no UICC card for fixed terminals. fixed terminals usually only have public user signs (e.164 number or sipuri) without private user signs. AKA authentication is not supported, therefore, if IMS uses a unified authentication method for fixed mobile terminals, it is necessary to expand the fixed terminal or require IMS to support different authentication methods.

　
At present, tispan has stipulated that IMS recommends isim/USIM-based aka authentication for mobile and fixed users. For terminals without isim/USIM
Sipdigest authentication method. However, a fixed terminal is required to have private user signs, public user signs, domain names belonging to the network, and other parameters for authentication.

Therefore
The following solutions can be used for terminal authentication: for new fixed terminals, the terminals must support isim cards and authentication methods; for existing SIP terminals, after migrating to the IMS network,
Software must be installed on such terminals to generate private user tags using the software. sipdigest authentication can be used. An adapter is used to access all SIP terminals without UICC cards.
The adapter generates private user signs and authentication parameters for each terminal for aka authentication.

Encoding Method Problems

Currently, fixed network VOIP networks and terminals support the following encoding methods: Speech Encoding: g.711, g.729 and g.723; video encoding: H.261, H.263 and MPEG-4.

The coding and decoding modes supported by mobile networks and terminals are as follows: AMR; H.263 and MPEG-4.

　
Currently, fixed networks and mobile networks support different encoding methods. Therefore, if you use IMS networks to achieve network convergence, You need to redefine the encoding/decoding method. The fixed and mobile terminals must have the same
Otherwise, a specific gateway device is required to support codec conversion. Encoding/decoding conversion may reduce the quality of speech. Therefore, the network should be converted as little as possible, which requires fixed and mobile
Try to use the same encoding method for the terminal.
IMS has become a technology development trend recognized by the industry as the core control layer fusion, and operators have made high efforts to integrate IMS-based networks.
Note: Some integrated operators have started IMS experiments and hope to integrate fixed and mobile networks through a unified IMS network. Currently, the implementation of fixed access in IMS is still in the research stage.
It is a key factor for IMS to achieve fixed and mobile network convergence.

Author: zhangyuan, Beijing Research Institute, China Telecom Co., Ltd.

Source: China Information Industry Network-people's post and telecommunications news

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