VoLTEIMS(IP Multimedia Subsystem) SIP(Session Initiation Protocol)

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我是靠谱客的博主瘦瘦往事，最近开发中收集的这篇文章主要介绍VoLTEIMS(IP Multimedia Subsystem) SIP(Session Initiation Protocol)，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

- IMS(IP Multimedia Subsystem)

Please refer to: 3GPP TS23.002TS23.228

It's a kind of service domain in the core netowrk, like PS domain and CS domian. IMS doesn't serve LTE only,also not binding to VoLTE only. Actually, IMS was introduced since WCDMA andpublished in 3GPP Rel5 in 2002.

The structure of IMS domain,

Application layer

Control layer

Access layer

The entities related to IMS areCSCF, MGCF, MRF, etc. CSCF, Call Session Control Function, the entitiesat Control layer.

P-CSCF: Proxy CSCF, it's theinterface between UE and Control layer of IMS, forward the signaling to S-CSCF and I-CSCF

I-CSCF: Interrogation CSCF,mainly to locate the S-CSCF

S-CSCF: Serving CSCF, it'sthe key entity, and responsible for handling registration and servicesignaling.

- SIP(Session Initiation Protocol)

Please refer to: RFC3261

- The structure of SIP protocol

SDP
SIP
TCP	UDP
IP

- SDP(Seesion Descritpion Protocol)

Please refer to: RFC4566

It defines the details of themultimedia telephony service, e.g. media description, type (audio, video, text,etc..) and parameters of these types.

A SIP message is either a requestfrom a client to a server, or a response from a server to a client, so themessage types are,

-Request

-Response

Table 1.1 The basic request	Table 1.2 The mostly used response
INVITE	response code	description
REGISTER	100	Trying
OPTIONS	180	Ringing
ACK	183	Session Progress
BYE	200	OK
CANCEL	401	Unauthorized
	404	Not Found
	486	Busy Here

- SIP message

It consists of Start line, Header, and Body,

Start Line

Header

Body

Start Line,

For request messgae, it contains Method, Request-URI,and SIP-Version.

Method: It is from the above Table 1.1 or other not commonlyused requests.

Request-URI: It is the complete name of the IMS domain

SIP-Version: Usually it's SIP 2.0

Here's an example

For response message, it contains SIP-Version, Status-Code,and Reason-Phrase, Status-Code,

Status-Code: It is from above Table 1.2, responsecode.

Reason-Phrase: It is from above Table 1.2 description

Here's an example,

SIP/2.0 200 OK

Header,

Table 1.3 The most important fields
Fields	abbre	descritpion
From	F	IMPU of the UE who originates the call
To	T	IMPU of the UE who is being called
Via	V	IP address, transport layer protocol, port

IMPU and IMPI

In SIP protocol, IMPI(IPMultimediate Private Identity) and IMPU(IP Multimediate Public Identity) areintroduced, where IMPI is private identification, unique and privacy, it'ssimilar to IMSI. IMPU is public identification, not unique, it's similar toMSISDN.

IMPI must be obtained from theUSIM during attached procedure, the format looks like, e.g.

IMPI=IMSI@ims.mnc007.mcc460.3gppnetwork.org>

Here's an example,

f: <sip:460078411105822@ims.mnc007.mcc460.3gppnetwork.org>;tag=2682882783

t: <sip:460078411105822@ims.mnc007.mcc460.3gppnetwork.org>

CSeq: 535399125 REGISTER

v: SIP/2.0/TCP[2409:8115:2001:31ae:1:1:7113:a929]:5060;branch=z9hG4bK2698913372

Body,

To define the characters of themultimedia telephony service, e.g. AMR/8KHz or AMR-WB/16KHz.

Here's an example,

m=audio 50010 RTP/AVP 104 102 105 96

a=rtpmap:104 AMR-WB/16000/1

a=fmtp:104 mode-change-capability=2;max-red=0

a=rtpmap:102 AMR/8000/1

a=fmtp:102 mode-change-capability=2;max-red=0

...

- SIP Procedures

Basically, it contains REGISTER,INVITE, NEGOTIATE, RELEASE

SDP Offer, it contains the UE A's capability of themultimedia. After UE B receives it, choose the appropriate type of media, thenrespond with 200OK, afterwards, UE/A and UE/B decides the type of themedia(audio or video, etc..),