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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
For APRICOT, Feb 27, 2001 - 3
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
For APRICOT, Feb 27, 2001 - 4
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
For APRICOT, Feb 27, 2001 - 5
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
For APRICOT, Feb 27, 2001 - 7
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
For APRICOT, Feb 27, 2001 - 8
• 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.
For APRICOT, Feb 27, 2001 - 9
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
For APRICOT, Feb 27, 2001 - 10
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
For APRICOT, Feb 27, 2001 - 12
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
For APRICOT, Feb 27, 2001 - 13
SIP Architectural Model
Private IP Network
Public IP Network
SIP Agent
RegistrarProxyServer
RedirectServer Location
Server
SIP AgentUAS
UAC
UACUAS
For APRICOT, Feb 27, 2001 - 14
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
For APRICOT, Feb 27, 2001 - 15
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
For APRICOT, Feb 27, 2001 - 16
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.
For APRICOT, Feb 27, 2001 - 17
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
For APRICOT, Feb 27, 2001 - 18
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”
For APRICOT, Feb 27, 2001 - 19
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
For APRICOT, Feb 27, 2001 - 20
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.
For APRICOT, Feb 27, 2001 - 21
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
For APRICOT, Feb 27, 2001 - 22
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
For APRICOT, Feb 27, 2001 - 23
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
For APRICOT, Feb 27, 2001 - 24
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.
For APRICOT, Feb 27, 2001 - 25
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
For APRICOT, Feb 27, 2001 - 26
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
For APRICOT, Feb 27, 2001 - 27
Thank You!