Signalling and Control
In IP Networks
- H.248, H.323 and SIP
Signaling Standards
• Media Gateway Control Signalling
– H.248/Megaco
• Call Signalling
– SIP and SIP-T
– H.323
For APRICOT, Feb 27, 2001 - 2
Network View
SIP-T
SG
SG
MGC
MGC
SIP
SIP
User Agent
PSTN
H.323 call
signalling
PSTN
H.248
Megaco
H.248
Megaco
H.323
Endpoint
MG
MG
Call signalling
Media gateway control signalling
Media flows
For APRICOT, Feb 27, 2001 - 3
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 - 4
H.248/Megaco evolution
MGCP proposal by
merging IPDC and
SGCP
(Telcordia & Level 3)
IPDC
MGCP
SGCP
Lucent submits MDCP
to ITU-T SG16
(Nov 1999)
I-RFC 2705
MGCP released as
Informational RFC
(Oct 99)
MDCP
(proposal)
Consensus between IETF and
ITU on Megaco Protocol
(March 99)
Megaco/H.248
IETF
RFC 3015
For APRICOT, Feb 27, 2001 - 5
Megaco connection model
Based 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
I1
MG
Context
– Defines communication between Terminations,
O1=I2+I3
acts as a mixing bridge
– Contains 1 or more Terminations
– Supports multiple streams
Stream
Cn
O2=I1+I3
Tb
I2
Ta
Tc
O3=I1+I2
I3
Td
– 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
Simple, powerful connection/resource model
For APRICOT, Feb 27, 2001 - 7
Megaco/H.248: Commands
• Provides control for manipulating terminations and contexts.
• Current Command Set:
Command
Initiator
Description
Add
Modify
MGC
MGC
Move
Subtract
AuditValue
MGC
MGC
MGC
AuditCapabilities
MGC
Notify
ServiceChange
MG
MGC
MG
Adds a termination to a context.
Modifies a termination’s properties, events, and
signals.
Moves a termination from one context to another.
Removes a termination from its context.
Returns current state of properties, events, signals,
and statistics.
Returns all possible values for termination
properties, events, and signals allowed by an MG.
Informs MGC of event occurrence(s).
Takes or places a termination(s) out of or in service.
For registration and restart; notifies MGC
termination(s) will be taken out of or returned to
service.
For APRICOT, Feb 27, 2001 - 8
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 - 9
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 - 10
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 - 11
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 - 12
SIP Architectural Model
Public IP
Network
SIP
Agent
UAC
UAS
Proxy
Server
Redirect
Server
Registrar
Private IP
Network
Location
Server
SIP
Agent
UAC
UAS
For APRICOT, Feb 27, 2001 - 13
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 - 14
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 - 15
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 - 16
SIP-T purpose
Proxy
PSTN bridging (PSTN - IP PSTN)
SIP enabled
network
PSTN Signaling is carried transparently
over the IP network
MGC 1
MGC 2
IP network
Proxy
SS7
network
SS7
network
Proxy
SIP enabled
network
PSTN to IP interworking
MGC
SIP
agent
IP network
SS7
network
Proxy
For APRICOT, Feb 27, 2001 - 17
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 - 18
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 - 19
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 - 20
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 - 21
H.323 overview continued
• Comprised of several protocols:
Audio
G.711
G.722
G.723
G.728
G.729
Video
H.261
H.263
RTP/RTCP
AAL5
UDP
ATM
Data
T.120
System Control User Interface
H.225
H.245
Call
RAS Control
Control
UDP or TCP
IP
Lower Layers Vary
For APRICOT, Feb 27, 2001 - 22
H.323 elements and entities
Terminals
Gatekeeper
• PCs, IP phones, settop boxes
• Audio
• Video (optional)
• Data (optional)
Gatekeeper
• address translation (IP, telephone)
• admission control
• cannot generate or terminate calls
Endpoints
MCU
Gateway
• can make or receive calls
• Realized by terminals and logically
present in Gateways
Gateway
Multipoint Control Unit (MCU)
• Support for multipoint conferences
• Always contains a MC
• Optionally contains an MP
• Interworking with
• other multimedia terminals
• GSTN
For APRICOT, Feb 27, 2001 - 23
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 - 24
H.323/SIP comparison summary
H.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
• H.450.x series provides minimal feature
set only, pure peer approach
• Adding ( mixed peer/stimulus approach
soon poor architecture)
• Slow moving
• Established now, primarily system level
• Few if any H.323-base telephones
• End-user primarily driven by Siemens,
Microsoft (NetMeeting), Intel
• Modular, simplistic design
• Easily extended & updated
• Based on Web principals (“Internetfriendly”)
• Readily extensible beyond telephony
Status
w.r.t. end
device
Industry
acceptance
• No real end-device features std, yet
• Many options for advanced telephony
features (need to make specific
choices)
• Astounding progress, velocity
• 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 - 25
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 - 26
Thank You!
For APRICOT, Feb 27, 2001 - 27