Signalling and Control In IP Networks: H.248, H.323 and SIP

Signalling and Control In IP Networks - H.248, H.323 and SIP

For APRICOT, Feb 27, 2001 - 2

Signaling Standards

• Media Gateway Control Signalling– H.248/Megaco

• Call Signalling– SIP and SIP-T– H.323

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Network View

SG

MG

MGC SG

MG

MGC

SIPUser Agent

H.323Endpoint

PSTN PSTN

Call signalling

Media gateway control signalling

Media flows

SIP-T

SIP

H.323 callsignallingH.248

MegacoH.248Megaco

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H.248/MEGACO Overview

• MEdia GAteway Control Protocol [RFC3015]

• H.248 is ITU-T reference for the same protocol• Protocol for controlling telephony gateway and

terminals (IP Phones)• Basis for Vendor Independent Network

deployment

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H.248/Megaco evolution

SGCP MGCP

Megaco/H.248

I-RFC 2705

MGCP released as Informational RFC

(Oct 99)

MGCP proposal by merging IPDC and

SGCP(Telcordia & Level 3)

Consensus between IETF and ITU on Megaco Protocol

(March 99)

Lucent submits MDCPto ITU-T SG16

(Nov 1999)

IETFRFC 3015

MDCP(proposal)

IPDC

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Megaco connection modelBased on 3 concepts:Termination

– Identifies an end point for media flows– Implements Signals, and generates Events– Can appear in at most one context.– Permanent (provisioned) terminations can exist

outside a context

Context– Defines communication between Terminations,

acts as a mixing bridge– Contains 1 or more Terminations– Supports multiple streams

Stream– A context can have multiple streams, each

typically for a medium, e.g. audio, video, etc– The MGC specifies which streams a given

termination supports

MG

Ta

Tb

Cn

Tc

Td

I2

O2=I1+I3

I1

I3

O3=I1+I2O1=I2+I3

Simple, powerful connection/resource model

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• Provides control for manipulating terminations and contexts.

• Current Command Set:

Megaco/H.248: Commands

Command Initiator DescriptionAdd MGC Adds a termination to a context.

Modify MGC Modifies a termination’s properties, events, andsignals.

Move MGC Moves a termination from one context to another.Subtract MGC Removes a termination from its context.

AuditValue MGC Returns current state of properties, events, signals,and statistics.

AuditCapabilities MGC Returns all possible values for terminationproperties, events, and signals allowed by an MG.

Notify MG Informs MGC of event occurrence(s).ServiceChange MGC Takes or places a termination(s) out of or in service.

MG For registration and restart; notifies MGCtermination(s) will be taken out of or returned toservice.

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Packages and profiles

• Packages:– Extension mechanism to define new termination and MG

behavior– Adds detailed application semantics to the protocol, e.g. a

package can define the events for DTMF signaling

• Profiles:– Define particular applications of Megaco/H.248 – Created by industry organizations, e.g. TIA– Specifies which packages are to be supported and other

protocol options

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Megaco/H.248 Standards Status

• Standardization of Base Protocol — DONE– Decided as ITU-T Recommendation H.248 in June 2000– Published as IETF Megaco RFC 3015, in Nov 2000– Continued work on bug fixes for the base protocol– First issue of H.248 Implementor’s Guide approved, Nov 2000

• Package definition — Rapid Progression– “Starter Kit” (Annex E) defined in base protocol– Additional packages defined as Annexes F, G, J and K to H.248 in Nov 2000– More ongoing in both IETF and ITU

• Megaco/H.248 Interoperability — Second Event Just Completed!– First multi-vendor interoperability event was great success (Aug 2000)

• 18 participants including MGCs, MGs, and Test equipment• Ad-hoc group formed to test Megaco/H.248 implementations and collect feedback

on specification issues• Line to Line and Line to Trunk calls with and without digit collection completed

with bearer path– Second event just occurred last week (Feb 2001)

For APRICOT, Feb 27, 2001 - 11

SIP overview

• SIP (Session Initiation Protocol – IETF RFC 2543)

• Application-layer signaling protocol for creating, modifying and terminating sessions with one or more participants

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SIP overview

• Light-weight generic signaling protocol

• Used to initiate sessions and invite members to a session

• Text-based protocol (good for prototyping)

• Syntax is textual and based on HTTP

• There have been several bake-offs with different vendors demonstrating interoperability of basic calls

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SIP Architectural Model

Private IP Network

Public IP Network

SIP Agent

RegistrarProxyServer

RedirectServer Location

Server

SIP AgentUAS

UAC

UACUAS

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SIP messages• Message consists of initial line, headers and body

• There are two types of SIP message– Request – Response

• Requests are always initiated by a UAC function– First line contains the method being invoked, e.g. INVITE– RFC 2543 methods include INVITE, ACK, BYE,

REGISTER, CANCEL, OPTIONS• Responses are generated by servers

– First line contains the response code• Headers provide information needed to process or route the message

• Body contains Session Description Protocol (SDP) describing media flows or other materials such as encapsulated ISUP messages.

• New methods and header types can be added at any time without changing the protocol

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SIP status

• Base protocol is RFC2543— Presently being revised – RFC2543bis is under discussion

• Standardized in the IETF as RFC 2543 in March 99 (now being further refined in the SIP working group).

– RFC 2543 just covers basic functionality. There are several related internet drafts covering services.

• Has rapidly growing industry momentum– Intense efforts underway to develop service-specific

extensions

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SIP-T• SIP-T = Session Initiation Protocol – Telephony

– previously referred to as SIP+ or SIP BCP-T– a collection of internet drafts that extend SIP to support

inter-Media Gateway Controller (MGC) communications.– SIP-T is an interface agreement on a collection of

standards as opposed to a separate protocol

• SIP-T describes how to interwork SIP and ISUP

• SIP-T directly negotiates a media connection between gateways. Endpoint information is carried in SDP (Session Description Protocol) which can describe both IP and ATM endpoints.

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SIP-T purpose

PSTN bridging (PSTN - IP - PSTN)PSTN Signaling is carried transparently over the IP network

PSTN to IP interworking

MGC 1 MGC 2

Proxy

Proxy

SIP enabled network

IP network

SS7 network SS7

network

MGC

Proxy

Proxy

SIP enabled network

IP network

SS7 network

SIPagent

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SIP-T Technical Approaches

• SIP-T uses two approaches:1. Map ISUP message contents to fields in the

SIP header for interworking with pure SIP agents

2. Encapsulate ISUP message within SIP message body for “PSTN bridging”

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SIP-T Implementation

•Three major extensions required:— INFO method extension to the base protocol in addition to others

• Session initiation and teardown is not enough• Mid-Call events• More complex services enabled

— MIME Type addition• Standard method of encapsulating legacy signaling• Simplifies Inter-working:

- Local variant - Interconnect variant (LCD)

— ISUP to SIP Mapping

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SIP–T status• Work on SIP-T was initiated by the International

SoftSwitch Consortium (ISC) in early 1999.

• SIP-T is still a work in progress. The ISC is continuing to develop profiles for SIP and telephony interworking.

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H.323 Overview• Packet-based multi-media communications system• It includes several protocols

– H.225.0 RAS (registration, admission, status …)– H.225.0 Call Signalling – H.245 Logical channel signalling and media control– RFC 1889 RTP/RTCP for media transport – H.450.x Supplementary services – H.225.0 Annex G Inter-domain registration and

billing information exchange • The original VoIP protocol suite• Whole System Architecture

– Provides Interoperability– Transport independence – Platform and application independence– Multipoint support

• Primarily used in corporate networks

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H.323 overview continued • Comprised of several protocols:

System Control User InterfaceDataVideoAudio

Lower Layers VaryIPUDP or TCPUDP

RTP/RTCP

Call Control

RASH.245Control

H.225T.120H.261H.263

G.711G.722G.723G.728G.729

AAL5

ATM

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Gatekeeper

MCU Gateway

Endpoints• can make or receive calls• Realized by terminals and logically

present in Gateways

Gatekeeper• address translation (IP, telephone)• admission control• cannot generate or terminate calls

Gateway• Interworking with

• other multimedia terminals• GSTN

Multipoint Control Unit (MCU)• Support for multipoint conferences• Always contains a MC• Optionally contains an MP

Terminals• PCs, IP phones, set-

top boxes• Audio• Video (optional)• Data (optional)

H.323 elements and entities

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H.323 current status

• H.323 version 4—Includes many major changes—A significant number of contributions from Nortel

Networks—Approved in November 2000

• H.245 and H.225.0 also updated

• Currently working on version 5. No release date specified.

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H.323/SIP comparison summaryH.323 SIP

Stds Body • ITU-T SG-16 •IETF SIP, MMusic, ...

Properties •Complex, monolithic design•Difficult to extend & update•Based on H.320 conferencing and ISDN Q.931 legacy (“Bell headed”)

•Limited potential beyond telephony•Some QoS built in… CODEC types specified

•Higher degree of interoperability

•Modular, simplistic design•Easily extended & updated•Based on Web principals (“Internet-friendly”)

•Readily extensible beyond telephony

Status w.r.t. end device

•H.450.x series provides minimal feature set only, pure peer approach

•Adding ( mixed peer/stimulus approach soon poor architecture)

•Slow moving

•No real end-device features std, yet•Many options for advanced telephony features (need to make specific choices)

•Astounding progress, velocity

Industry acceptance

•Established now, primarily system level•Few if any H.323-base telephones•End-user primarily driven by Siemens, Microsoft (NetMeeting), Intel

•Rapidly growing industry momentum, at system and device level

•Growing interest in SIP-Phones and soft clients, products appearing

SIP is anticipated long-term winner, but H.323 networks will remain for some time, and are a source of revenue in the international market

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For More Information

IETF:– IETF home page

http://www.ietf.org/– Internet-draft search engine

http://search.ietf.org/search/brokers/internet-drafts/query.html– RFC search engine

http://www.rfc-editor.org/rfcsearch.html– Megaco WG charter

http://ietf.org/html.charters/megaco-charter.html– Megaco documents repository

ftp://standards.nortelnetworks.com/megaco/ftp://standards.nortelnetworks.com/megaco/docs/latest/

ITU:– ITU home page

http://www.itu.int/ITU-T/index.html– SG-16 document repository (H.323, H.248)

ftp://standard.pictel.com/avc-site– Most Megaco/H.248 related documents should also be available through

Megaco WG, above

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Thank You!